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Severe Plastic Deformation on Magnesium and its
Alloys
Materials Science and Engineering
Severe Plastic Deformation on Magnesium and its
Alloys
Submitting a Master’s thesis of Materials Science & Engineering
July 2018
Syed Hasnain Abbas
July 2018
Vice Chair Prof. Shin, Kwang Seon (Seal)
Examiner Prof. Choi, In-suk (Seal)
iv
Abstract
This study deals with the effect of extrusion temperature on ZX10, a high strength
low alloy (HSLA) biodegradable material, from 180oC to 300oC at 40oC interval to
study the impact on corrosion and mechanical properties in 3.5% NaCl saturated with
Mg(OH)2. The effect of addition of neodymium (Nd) and calcium (Ca) in magnesium
has also been investigated at fixed extrusion temperature of 300oC. All the results
were then compared with commercially pure magnesium processed under similar
conditions.
characterized by optical microscopy (OM), x-ray diffraction (XRD), scanning
electron microscope (SEM) and electron backscattered diffraction (EBSD),
respectively. The electrochemical corrosion behavior was investigated by
potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)
while immersion study was conducted using fishing line specimen arrangement to
simultaneously measure corrosion rates from weight loss (Pw) and hydrogen evolution
(PH) after 168 hours. The mechanical properties including tensile and compression
properties were measured using a 2-ton Unitech R&B machine at fixed strain rate of
2 x 10-4.
As the extrusion temperature decreases, fine dynamic recrystallized (DRX) grains
(~1.67μm) and elongated coarse un-dynamic recrystallized (unDRX) grains turned
out to be more dominant. The binary neodymium alloy precipitates metastable
Mg12Nd and thermodynamically stable Mg41Nd5 phases while ternary Mg-Nd-Ca
alloys precipitated Mg2Ca and Mg41Nd5 phases. The Mg12Nd phase is known to act
as weak cathode phase and is better for corrosion resistance.
Plastic deformation is a convenient method to change the microstructure and improve
corrosion behavior of materials. Since magnesium and ZX10 extruded at 180oC has
the best mechanical and corrosion properties in as-extruded condition, they were
v
subjected to isothermal severe plastic deformation processes, such as screw rolling
(SR) and multi-directional forging (MDF) at 300oC.
In case of ZX10, by observing screw rolled sample at 300oC, it is found that the
corrosion rate was reduced appreciably with increase of yield strength. However,
MDF increased the grain size along with increased porosity and Mg-Zn-Ca phase
leading localized attack. When ternary Mg-Nd-Ca alloys were subjected to screw
rolling, the corrosion resistance of EX33 was increased by impressively 78% at the
expense of strength when compared with as-extruded condition. This elevated
temperature decline in yield strength and increment of corrosion resistance could be
associated with proportions of unDRX grains and reduction of second phases
(galvanic cells).
Finally, the corrosion rates of studied materials were plotted on a corrosion model
recently proposed in the literature. The calculated and predicted data values were
found to be in good agreement for most materials.
Key words: magnesium alloys, mechanical properties, corrosion behavior,
isothermal severe plastic deformation, multi-directional forging, screw rolling, grain
size.
vi
Contents
1.1.1. Alloying Elements.............................................................................. 2
1.1.3. Grain Size ........................................................................................... 9
1.1.4. Current Density ................................................................................ 10
1.1.5. Texture Effect .................................................................................. 13
1.3 Research Objectives ................................................................................. 17
2.1 Extrusion .................................................................................................. 18
2.4 Mechanical properties .............................................................................. 20
2.5 Corrosion Measurements ......................................................................... 20
2.6 Microstructure & Phase Analysis ............................................................ 21
Chapter 3: Results & Discussion ............................................................................. 22
3.1 Effect of Extrusion Temperature and ISPD on Properties of Mg ............ 22
3.1.1 Microstructures ................................................................................ 22
3.1.4 Summary .......................................................................................... 32
3.2 Effect of Extrusion Temperature and ISPD on Properties of ZX10 ........ 33
3.2.3 Mechanical Properties ...................................................................... 38
3.2.4 Corrosion Measurements ................................................................. 40
3.3.3 Mechanical Properties ...................................................................... 51
3.3.4 Corrosion Measurements ................................................................. 53
4.2 UTS vs Corrosion vs Extrusion Temperature .......................................... 58
4.3 Yield Stress vs Yield Asymmetry vs Extrusion Temperature ................. 61
4.4 Corrosion Analysis ................................................................................... 63
Chapter 5: Conclusion .............................................................................................. 66
Chapter 6: References .............................................................................................. 68
7.1 Classification of Different Medium Based on Ionic Concentration ......... 75
7.2 Alloys Properties in Hanks ...................................................................... 76
7.3 Alloys Properties in SBF.......................................................................... 77
7.5 Extrusion Conditions ............................................................................... 79
7.7 Oxide Morphologies ................................................................................ 81
ix
Figures
Fig. 1: Cross-section images of corroded surfaces of HT-AZ31 Sheet samples after
6 days in 5wt.% NaCl solution [21] ......................................................................... 8
Fig. 2: Dependence of Corrosion Rate on grain size: (a) hot-rolled AZ31 sheet
samples with and without heat treatments immersed in 5 wt.% NaCl solution [22]
............................................................................................................................... 11
Fig. 3: Volume percent Intermetallic vs Corrosion Rate for HPDC Mg-RE alloys
where RE: Ce/La/Nd/Y [22] .................................................................................. 11
Fig. 4: Schematic representation of the MDF process [34] .................................. 19
Fig. 5: Temperature distribution (°) of screw rolled sample after one pass [35]
............................................................................................................................... 19
Fig. 6: The OM images of magnesium extruded at (a) 180oC, (b) 220oC, (c) 260oC,
(d) 300oC; (e) screw rolled at 300oC and; (f) MDFed at 300oC ............................. 24
Fig. 7: XRD Analysis of CP magnesium in as-extruded and as-SPD condition .... 25
Fig. 8: (a) Tensile and (b) Compressive tests of magnesium extruded from 180oC to
300oC and as-SPD at 300oC ................................................................................... 27
Fig. 9: Polarization and EIS data in terms of (a) Nyquist, (b) Bode, (c) Tafel plots
for CP magnesium in as-extruded and as-ISPD condition after 1.5 hours immersion
in saturated 3.5 wt. % NaCl solution ..................................................................... 29
Fig. 10: (a) Hydrogen evolution and (b) corrosion rate of as-extruded and as-SPD
magnesium after 168 hours of immersion in saturated 3.5 wt. % NaCl solution.
............................................................................................................................... 30
Fig. 11: SEM images showing surface morphologies of extruded and screw rolled
Mg samples after immersion in saturated 3.5 wt. % NaCl solution for 168 hrs. ... 31
Fig. 12: The OM images of ZX10 extruded at (a) 180oC, (b) 220oC, (c) 260oC, (d)
300oC; screw rolled at (e) 300oC and; MDFed at (f) 300oC ................................... 35
Fig. 13: Solidification behavior calculated by JMatPro 7.0 [41] ........................... 36
Fig. 14: XRD analysis of ZX10 in as-extruded and as-SPD condition .................. 37
Fig. 15: (a) Tensile and (b) Compressive tests of ZX10 extruded from 180oC to
300oC and as-SPD at 300oC ................................................................................... 39
x
for ZX10 alloys in as-extruded and as-ISPD condition after 1.5 hours immersion in
saturated 3.5 wt. % NaCl solution.......................................................................... 41
ZX10 after 168 hours of immersion in saturated 3.5 wt. % NaCl solution. ........... 42
ZX10 alloy after immersion in saturated 3.5 wt. % NaCl solution for 168 hrs. .... 43
Fig. 19: Microstructure of EX series alloy after extrusion and isothermal screw
rolling. (a-d) shows OM before etching, (e-h) shows OM after etching, (i-l) after
EBSD ..................................................................................................................... 47
Fig. 20: SEM/EDX and mapping of EX series alloys (a) EX33, (b) EX36, (c)
EX33SR and, (d) EX36SR, ‘+’ indicates edx results in table, rectangle indicates
mapping and SR denotes isothermal screw rolling at 300oC ................................. 48
Fig. 21: Solidification behavior calculated by JMatPro 7. [41] ............................ 49
Fig. 22: XRD Analysis of ‘E-series’ in as-extruded and as-screw rolled conditions50
Fig. 23: (a) Tensile and (b) Compressive tests of ‘E series’ extruded at 300oC and
as-SPD at 300oC ..................................................................................................... 52
for E series alloys in as-extruded and as-ISPD condition after 1.5 hours of
immersion in saturated 3.5 wt. % NaCl solution. .................................................. 54
E-series alloys after 168 hours of immersion in saturated 3.5 wt. % NaCl solution
............................................................................................................................... 55
E-series alloys after immersion in saturated 3.5 wt. % NaCl solution for 168 hrs .
............................................................................................................................... 56
Fig. 27: Effect of alloying on corrosion and mechanical properties of magnesium
extruded at 300oC ................................................................................................... 59
Fig. 28: The corrosion and mechanical properties of magnesium and its alloys (a)
extruded between 180oC and 300oC, (b) ISPD at 300oC ........................................ 60
xi
Fig. 29: Relationship between yield strength and yield asymmetry (a) at extrusion
temperature between 180oC and 300oC, (b) ISPD at 300oC................................... 62
Fig. 30: The relationship between calculated and predicted corrosion rates [44] .. 65
Fig. 31: Ionic concentration of Hanks, Simulated body fluid (SBF) and Plasma in
terms of ionic concertation in millimoles per Litre (mmol/L) ............................... 75
Fig. 32: Effect of alloying elements in as-cast and as-extruded condition on
instantaneous corrosion rate (mm/y) in Hanks solution and respective mechanical
properties (yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%)) ...................................................................................................... 76
instantaneous corrosion rate (mm/y) in SBF and respective mechanical properties
(yield strength (YS, MPa), ultimate tensile strength (UTS, MPa), elongation (%))
............................................................................................................................... 77
instantaneous corrosion rate (mm/y) in NaCl and respective mechanical properties
............................................................................................................................... 78
Fig. 35: Most commonly studied extrusion ratios and temperature, (b) extrusion
temperature effect on corrosion rate [38, 42, 55] ................................................... 79
Fig. 36: Inverted Pole Figures (IPZ) of selected alloys. Note the twinning in screw
rolled (SR) samples ................................................................................................ 80
xii
Table
Table 1. Corrosion Rates of Nd-Zn-Zr, Nd-Zn alloys after immersion in 5 wt.% NaCl
for 3 days [17] ........................................................................................................... 6
Table 2. Electrochemical Parameters of Mg-RE Cathode Phase [1] ....................... 12
Table 3. Physical and Chemical Properties of Mg & RE Elements [1] ................... 12
Table 4. Corrosion Potentials of Mg-Zn-xNd(Gd) [1] ............................................. 15
Table 5. Composition and Processing Conditions of Pure Magnesium ................... 23
Table 6. Grain size and mechanical properties of magnesium in as-extruded and as-
ISPD condition ......................................................................................................... 27
Table 7. Corrosion rates of magnesium in terms of hydrogen evolved and weight loss
in as-extruded and as-ISPD condition ...................................................................... 30
Table 8. Composition and Processing Conditions of Mg-1Zn-0.3Ca (ZX10) ......... 34
Table 9. Solidification and Phase Calculations for Mg-1Zn-0.3Ca (ZX10) alloys .. 36
Table 10. Grain size and mechanical properties of ZX10 in as-extruded and as-ISPD
condition .................................................................................................................. 39
Table 11. Corrosion rates of ZX10 in terms of hydrogen evolved and weight loss in
as-extruded and as-ISPD condition .......................................................................... 42
Table 12. Composition and Processing Conditions of ‘E-series’ alloys .................. 46
Table 13. Average sizes of DRXed and unDRXed grains (μm) from Fig. 19 ......... 46
Table 14. Solidification and Phase Calculations for ‘E Series’ alloys ..................... 49
Table 15. Grain sizes and mechanical properties of EX alloys in as-extruded and as-
ISPD condition ......................................................................................................... 52
Table 16. Corrosion rates of E series alloys in terms of hydrogen evolved and weight
loss in as-extruded and as-ISPD condition .............................................................. 55
Table 17. Total fraction of second phases ................................................................ 64
Table 18. Volta potentials of second phases in this study [43, 44, 54] .................... 64
1
Chapter 1: Introduction
In biomedical industry where implants, such as plates, screws and pins are used to
repair bone fractures, a subsequent surgical procedure to remove these implants from
the human body after the tissues have healed, is not always necessary. This is because
repeated surgery generally increases health risks to the patient [1]. The widely used
bio-inert metals (Ti alloys, stainless steels etc.) may induce a ‘stress-shielding’ effect
owing to their much higher elastic modulus, compared to that of bones, leading to a
risk of secondary fracture of bones. Stress shielding occurs when a bone is shielded
by an implant from carrying a load. As a result, the bone tends to weaken over time,
resulting in more damage. To minimize the effect of stress shielding on bones while
still retaining enough strength, a soft lightweight and degradable (in physiological
environment) metal (implant) is desirable.
Magnesium (Mg) is the most abundant element, second only to sodium (Na) in the
hydrosphere and the fourth most abundant cation in living organisms (human body:
Ca > K > Na > Mg) [2]. It exerts a large variety of biological functions. Mg is
abundant in the human body, and bone tissue stores approximately half of the total
physiological Mg [3]. This element is also vital to metabolism processes in almost all
enzymatic systems involved in DNA processing as an essential cofactor, and a key
component of the ribosomal machinery that translates the genetic information
encoded by mRNA into polypeptide [4]. Moreover, researches show that Mg has
stimulatory effects on the formation of new bone [5]. It was also demonstrated that
biodegradable Mg screws have had a low rate of complications, including avascular
necrosis and nonunion. In addition, Mg and its alloys possess considerable potential
as fixation implants for orthopedic applications. The reconsideration of Mg alloys as
biomaterials is mainly to improve corrosion resistance.
2
1.1 Factors Controlling Corrosion Performance
To improve corrosion, it is important to know the factors that control corrosion.
1.1.1. Alloying Elements
It is well known fact that the addition of alloying elements plays crucial role in
controlling corrosion and mechanical properties by inducing defects in the matrix.
The choice of elements in magnesium for invitro environment to improve corrosion
performance is limited due to potential electrochemical corrosion reactions between
matrix and reinforcements. This part describes the alloying elements and their effects
while the next part will describe the effect of defects and intermetallic in terms of
potential difference.
1.1.1.1 Calcium
Calcium (Ca) has lower density 1.55 g/cc than magnesium (1.74 g/cc) and it is very
cost effective. In terms of biocompatibility, Ca is one of the major elements in bones
causes healing and growth and second largest cations in human body contributing to
osteosynthesis phenomenon [6].
Li Z [7] studied the effect of binary alloy Mg-xCa (x=1,2,3 wt.%) in as-cast, as-rolled
and as-extruded condition. In as-cast state, the mechanical properties (YS, UTS,
Elong) deteriorates with increasing Ca content with Mg-2Ca being optimum. There
are generally two phases α-Mg and Mg2Ca [8]. The latter phase has higher corrosion
rate due to increased cathode-to-anode ratio. After hot processing, the grain size
further reduced contributing positively towards mechanical and corrosion properties.
Between as-rolled and as-extruded samples, hot extruded sample’s ultimate tensile
strength (UTS: 240 MPa) and elongation (El: 10.70 %) of Mg-1Ca is superior than
as-rolled.
When 0.3wt.% Ca is added to extruded Mg-1Zn alloy (ZX10, wt.%), very fine
microstructure (grain size < 2μm) with outstanding mechanical properties, high
strength-ductility, as well as low mechanical anisotropy had been reported. Hofstetter
J [9] referred it as high-strength-low-alloyed (HSLA) Mg. It also possesses superior
3
implant applications.
Wang L [10] studied the effect of Ca (0, 0.3, 0.6, 0.9 wt.%) addition on mechanical
and biocorrosion of as-cast ZK30 (Zn: 3, Zr: 0.3 wt.%). Beyond 0.6 wt.% of Ca, there
is no obvious grain refinement and hydrogen precipitation at cathode is not further
inhibited. It is important to balance grain size effect and negative effect of Ca2Mg6Zn3
for optimum mechanical (UTS: 200 MPa, El: 19%) and corrosion properties (0.26
mm/y, mass loss method) of ZK30 alloys.
1.1.1.2 Strontium
Calcium and Strontium (Sr) belongs to group II with Sr being slightly soluble (0.108
wt.%) in magnesium. Sr is also a component of human bone and has been known to
promote growth of osteoblasts and prevent bone resorption [11]. The maximum daily
average intake of Sr is 4 mg/day.
Binary alloys of Mg-xSr (x: 0.3-2.5 wt.%) has been studied for cardiovascular and
other orthopedic applications and to understand how these alloys biodegrade in
physiological conditions [12]. The average corrosion rate of Mg-0.3Sr and Mg-0.5Sr
is ~2.5 and 2 mg/day/cm2 respectively. Consequently, Mg-0.5Sr has lowest
degradation rate (0.01 mg/day < 4 mg/day) in SBF due to less micro-galvanic cells
between Mg-matrix and Mg17Sr2.
Due to biocompatibility of Ca and Sr, Bornapour [13] added 0.3 wt.% Ca in Mg-0.3Sr
and found the corrosion rate decrease by 90% because of (1) third element effect; (2)
Globular Ca/Sr-rich phases. Additionally, Mg-0.3Sr-0.3Ca has better as-cast
mechanical properties (YS: 52MPa, UTS:107 MPa, El: 8.8%) than Mg-0.3Sr (YS:
44MPa, UTS: 98 MPa, El: 4.0%). However, Mg-0.5Sr-0.6Ca increases the corrosion
rate due to significant micro galvanic effect. Moreover, Sr-substituted hydroxyapatite
is also beneficial leading to slower biocorrosion with enhancing cell growth, their
proliferation and healing around bone implants.
4
1.1.1.3 Zirconium
Human body contains, on average 250mg of zirconium (Zr) with intake requirements
of 4.15mg/day. As such there is no adverse biological effect of Zr.
The improvement of corrosion resistance by Zr addition is probably due to: (1) high
Zr content at grain center with respect to grain boundaries; (2) reduction of cathodic
activity of Zr-phases; (3) grain refinement ability of Zr provides more continuous
layers of corrosion resistant intermetallic RE phases around grain boundaries,
decelerating corrosion between grain [14].
Zr, being effective grain refiner, its addition lead to equiaxed single-phase (α-Mg)
solid solution alloy (grain size: 34.7μm) improving mechanical and corrosion
properties [15]. Comparatively, mechanical properties of Mg-1.5Zn-0.6Zr (wt.%) and
AZ91D are: YS: 83 & 97 MPa; UTS: 168 & 151 MPa; and El: 9.1 & 2.7 %,
respectively. The lower yield strength of Mg-1.5Zn-0.6Zr is due to the absence of
second phase. On the other hand, corrosion rate by mass loss technique (0.304 mm/y)
of as-cast alloy is almost half of AZ91D (0.726 mm/y) when tested in Hank’s solution
for 7 days.
1.1.1.4 Rare Earth Elements
Rare earth elements are very reactive because of their low electronegativity, become
positive ions (+3 valency) while remaining stable in alkali. It is due to this reason, RE
exist as stable compounds in magnesium alloys and in nature. RE-compounds formed
have very low electrode potentials which help lower the overall potentials of cathode
phase, consequently the corrosion current when combined with high potential
elements. The AlRE phase has a lower potential than β-AlMg, therefore the addition
of RE elements can improve the anticorrosion capability of AZ series alloys.
The RE advantages include high melting point and can be alloyed by solid solution
strengthening. However, the anticorrosion capability of Mg-RE alloys varies
substantially depending on the alloying components and their structures.
5
1.1.1.4.1 Magnesium-Neodymium Alloys
The maximum solubility of Nd in Mg is 3.6 wt,% but decreases with temperature
from 0.8 ~1.0wt.% at room temperature. Therefore, the optimum content is between
3 and 4 wt.%. In Mg-Nd alloys, Nd forms Mg12Nd phase depending upon processing
condition, which is similar to Mg24Y5 and Mg-Y-Nd phases. It is a high temperature
reinforced phase since the melting point of Mg12Nd is between those of the Mg24Y5
and Mg-Y-Nd phases at 545oC. The Mg12Nd solidifies after the α-Mg matrix and
therefore distributes at the crystal grain boundaries. After solid solution treatment,
almost all of them can be solubilized in the matrix. Moreover, Mg12Nd is a weak
cathode and its potential is close to that of Mg24Y5.
Zr is known to be effective grain refiner whereas Zn, when added in small amounts,
contribute towards castability. Zhang X [16] compared the corrosion behavior of
AZ31, WE43 and JDBM (Mg-3.09Nd-0.22Zn-0.44Zr) in SBF for 10 days. JDBM has
almost 57% more and twice corrosion resistance than of AZ31 and WE43,
respectively. It is due to low cathode/anode surface ratio and lower potential
difference and therefore low degree of local corrosion.
1.1.1.4.2 Magnesium-Neodymium-Zirconium Based Alloys
Zr forms insoluble precipitates with impurities (especially Fe & Ni), so enrichment
of Zr in grain center leads to a higher resistance in this zone. On the other hand, Mg-
RE alloys can also result in higher purity and corrosion resistance. In different heat
treatment conditions, corrosion resistance is T4>T6>F (Table 1.) [17]. The reason is
galvanic corrosion rate relates strongly to morphology of cathode phase like Mg12Nd
such as in Mg-Gd alloy.
6
Table 1. Corrosion Rates of Nd-Zn-Zr, Nd-Zn alloys after immersion in 5 wt.%
NaCl for 3 days [17]
7
1.1.1.5 Detrimental Alloying in Magnesium
Aluminum (Al) is known to slow down the biodegradation rate of magnesium. In
solutions, Al releases ions which could easily combine with inorganic phosphates,
leading to a lack of phosphate in the human body, and an increased concentration of
Al ions in the brain. This may lead to Alzheimer’s disease. Severe hepatotoxicity has
also been detected after the administration of RE elements such as cerium,
praseodymium and yttrium [18].
Although, zinc (Zn) in trace amount (15mg/day) is essential to human body, the main
drawback is Zn’s biocompatibility is it releases Zn2+ in chlorides (especially HCl)
forming ZnCl2, known to damage stomach parietal cells [19] . Wan studied the
surface nanomechanical performance and corrosion behavior Mg–Ca alloys ion-
implanted with zinc. They concluded zinc ion implantation (dose:
0.9 × 1017 ions/cm2) is not a favorable for biomedical Mg-Ca as it deteriorates
corrosion resistance [20].
The crystallographic defects, such as grain boundaries, dislocations, and twins, are
usually corroded preferentially because of their higher chemical activity. If the
intermetallic is distributed randomly and discontinuously, the corrosion of the matrix
is accelerated through galvanic effects. For example, after 6 days of immersion in
5wt.% NaCl, the exposed grain boundaries and twins of heat-treated AZ31 sheet
sample appear to be dissolved slightly deeper as shown in Fig. 1 [21]. Therefore, it is
important to understand how these crystallographic defects and those intermetallic
precipitates influence corrosion performance.
The primary driving force for magnesium alloys corrosion reaction is the varied
electrode potentials on microscale alloy surface. The varied potentials can be
attributed to the multi-phase alloy system as well as the crystal defects. It is known
that the potential at the boundary of two phases is lower than that in crystal matrix. A
deep and wide phase boundary makes inter crystalline corrosion liable to occur.
8
Fig. 1: Cross-section images of corroded surfaces of HT-AZ31 Sheet samples after
6 days in 5wt.% NaCl solution [21]
9
Depending upon the solubility of alloying elements, the second phases form. When
this second phase(s) precipitate(s) after solidification, due to the contact with matrix,
a potential difference is created. The current starts to flow from higher Fermi energy
level to lower Fermi energy level until thermodynamic equilibrium has been
maintained. Balancing of Fermi energy level does not indicate the electric potentials
are also equal. The electric potential is controlled by its work function. Elements
which have low work function values than pure Mg (3.66) are Ca (2.8), Ce (3.1), Nd
(3.2) and Y (3.1). The resultant second phase forms become anode with respect to
matrix due to lower potential than the matrix and hence, preferentially corrode.
Inorder to measure the surface potential between the matrix and intermetallic,
Scanning Kelvin Probe Force Microscope (SKPFM) is used which measures the
relative potential through difference of work function. What is implied from the
relative potential measured from phases in isolation from say, SKPFM, is that a phase
more positive than the matrix will be polarized cathodically at the open circuit
potential of an alloy in which it populates, and therefore will likely act as a local
cathode. SKPFM also provides an indication of expected relative driving force for
microgalvanic couples (ex-situ) or changes in the modified work function in the
presence of corrosion or surface layers (in-situ).
1.1.3. Grain Size
As described in above section, the presence of intermetallic particles and crystal
defects can accelerate the corrosion of Mg alloys. The crystallographic defects and
intermetallic particles can evolve with grain size during a surface deformation or heat
treatment process and finally influence the corrosion performance of a Mg alloy.
Therefore, grain size is not an independent factor and almost always evolve with
crystal defects and intermetallic particles.
When the grain size of AZ31 is large (e.g. >10 micron), the corrosion rate is relatively
high; but if the grain size is small (e.g. <100 micron) the corrosion rate increases with
decreasing size. This non-monotonous relationship between the grain size and
corrosion rate of AZ31 suggests that not only the density of crystal defects but also
10
the amount of precipitated particles are responsible for corrosion behavior. The
detrimental effect of crystallographic defects (dislocations, grain boundaries and
twins) on corrosion performance interprets the slightly worsened corrosion resistance
when grain is small (<10 micron). After heat-treatment, or in case of squeeze casting,
while grains grow significantly, the number of particles also increases. The later
variation deteriorates the corrosion performance of AZ31, Fig. 2 [21].
1.1.4. Current Density
Corrosion of magnesium alloys leads to reduced mass (macroscopically) and
corrosion current (microscopically). Cathode current determines the corrosion current
and in effect controls the corrosion rate.
Besides the α-Mg matrix, binary alloys, that is Mg-Ce, Mg-La, Mg-Nd and Mg-Y,
contain only the Mg12Ce, Mg12La, Mg3Nd and Mg12Y5 phases. Fig. 3 shows Mg-La
& Mg-Nd has gradual slope while Mg-Ce & Mg-Y has steeper slope between the
intermetallic volume percentage and the corrosion current [22]. The current density
on the surface of the cathode phase is an indicator for hydrogen-evolution efficiency,
which is positively related with the corrosion sensitivity of the magnesium alloy. The
polarization resistance, a barometer for the extent of cathode polarization, is a
parameter for the catalytic activity in the hydrogen-evolution reaction on the cathode
surface. Zero polarization resistance means non-polarized while ∞ resistance
represents complete polarization. Lower cathode current density means higher
polarization resistance, which is more beneficial to the anticorrosion properties.
The current density depends on the potential difference between the cathode phase
and the matrix. The matrix potential also changes with the matrix content. It can be
seen from Table 2 that the Mg12Y5 phase, which has the lowest polarization resistance,
will generate a strong corrosion current even in the absence of a high potential
difference between the RE phase and the matrix [22].
11
Fig. 2: Dependence of Corrosion Rate on grain size: (a) hot-rolled AZ31 sheet
samples with and without heat treatments immersed in 5 wt.% NaCl solution [22]
Fig. 3: Volume percent Intermetallic vs Corrosion Rate for HPDC Mg-RE alloys
where RE: Ce/La/Nd/Y [22]
Table 2. Electrochemical Parameters of Mg-RE Cathode Phase [1]
Table 3. Physical and Chemical Properties of Mg & RE Elements [1]
13
Previous studies show that the corrosion rate strongly depends on crystallographic
orientations [23]. In order to determine this dependency, single crystal studies are
usually carried out to study effect of specific orientations. However, this methodology
limits the corrosion studies to few crystal planes which does not necessarily provide
understanding of the entire system. In corrosion studies, the grain orientations vary
significantly only few degrees difference in crystallographic orientations.
Heat treatment can effectively alter the microstructure without changing the
composition of an alloy. A properly controlled heat treatment will not significantly
change the grain orientation of a hot-rolled AZ31 sheet. However, the heat treatment
changes the grain size also effecting corrosion. The factors to be considered while
studying the effect of crystallographic orientations on corrosion are as follows:
For one type of alloy,
1. Effects of crystal structure on
a. Atomic bonding between nearest neighbors,
b. Coordination, the number of dangling bonds
c. Resultant surface energy
Specifically, for magnesium and its alloys:
1. Mg is a system which corrodes in [Cl-] solutions far from equilibrium. There has
been conflicting data on orientation dependence of Mg corrosion. One study
claims near normal <0001> directions with respect to sample surface are prone
to corrosion in NaCl environments containing CrO3. While, another study
14
examined (0001) planes as more corrosion resistant for Mg in 0.1N HCl. The
Mg (0001) surface exhibited pitting corrosion susceptibility at open circuit
potential (OCP) while the (10-10) and (11-20) surfaces were passive at open
circuit and pitting only initiated at potentials slightly anodic to their OCP in
similar environment [24].
2. In AZ31, the corrosion occurs slowly on closely packed plane but the presence
of secondary phases also effects [25].
3. Another study found in two Mg grains that the one grain near (0001) (between
0001 and 10-10) had higher corrosion rate than the twin containing grain close
to (11-20), when the oxide film was thin in NaCl and saturated Mg(OH)2
solution [26].
16
1.2 Severe Plastic Deformation
Since, the strength coefficent values of magnesium is four times than that of
aluminum, utilizing severe plastic deformation mechanisms is an effective way for
grain refinement. Several SPD techniques, e.g. screw rolling (SR, PSW), equal
channel angular pressing (ECAP), high-pressure torsion (HPT), multidirectional
forging (MDF), accumulative roll bonding (ARB) etc., lead to the development of
ultrafine-grained (UFGed) structures in a whole volume of the products. However,
MDF and SR seems very attractive especially for commercialization where relatively
large samples can be processed.
1.2.1 Multidirectional Forging (MDF)
In the context of grain refinement described above, multi directional forging (MDF)
is a process in which simple plane strain condition is used to deform FCC and HCP
materials without changing its dimensions. As a result, Zn-Al finds their applications
in sleeve bearings, thrust washers, house couplings & connectors, pressure tight
housings and many other structural applications. The zinc aluminum alloys are known
for their excellent castability and wear resistance. When Zn-Al sample is cast, it
shows dendritic structure leading to heterogenous mechanical properties, negative
effect on ductility [27]. The addition of other alloying elements produces several
metastable phases which have negative effect on dimensional instability.
Miura H [28] studied the evolution of ultra-fine grains in AZ31 and AZ61 by MDF
under decreasing temperature conditions from 350oC to 150oC at a strain rate of 3 x
10-3 s-1. The strength level reported for average of 1-micron grain structure of AZ31
and AZ61 are 850MPa and 1.2GPa, respectively, at cumulative strain of 4.0 and 4.8.
1.2.2 Screw Rolling (SR)
The current economic crisis and trade wars, there is an increasing pressure on leading
economies to reduce labor costs and minimizing the processing time of products
between manufacturers and supplier. Hence, the need for lean production is becoming
more evident. Screw rolling is one such processing technology suitable for small
diameter but hard to deform materials such as high carbon steels, high temperature
17
nickel-cobalt superalloys and white cast iron. The material is deformed in helicoid
motion at a fixed distance of the rollers, creating bulk macroshear. The final structure
consists of small individual isotropic particles with improved physicomechanical
properties, most notably plastic and ductile properties [29].
Sheremetyev V processed shape memory alloy, Ti-18Zr-14Nb, by screw rolling for
bone implants. The results show excellent fatigue properties in conjunction with 35%
of ductility [30]. Akopyan TK recently investigated the effect of screw rolling on Al-
Ni and Al-Ca alloys with up to 2.5 times improved strength than as-cast or annealed
samples [31].
The properties of magnesium alloys can also be improved by employing screw rolling
[32]. Diez M improved the yield strength of pure magnesium to 116 MPa under
decreasing temperature condition with 13% total elongation [33].
1.3 Research Objectives
Due to increased demands in aerospace, automotive, biomaterials and electronics
industry, it is vital to study effect of processing parameters on properties of
magnesium and its alloys. Therefore, following are the main objectives of the current
study:
a. Thorough review on corrosion medium on properties of magnesium alloys:
effect of ionic concentration on corrosion and strength properties
b. Prepare Al-free magnesium alloys with fine microstructure (<2μm)
c. Systematic investigation on effect of extrusion temperature on strength and
corrosion properties of ZX10 and Mg-Nd/Ca alloys
d. Analyze the effect of isothermal severe plastic deformation temperature,
300oC
f. Propose optimum corrosion-strength-ductility
g. Fit corrosion data in the corrosion model recently proposed in the literature
18
2.1 Extrusion
Pure magnesium (Mg, 99.8%) ingot was melted in electric resistance furnace under
anti-oxidizing gas atmosphere (1 vol.% SF6 + 99 vol.% CO2) in a low carbon steel
crucible with diameter 80mm at 750oC. Then, zinc (Zn), calcium (Ca) and
neodymium (Nd) were added according to alloy design. After 30 minutes of stirring,
Mg and alloys were water quenched. Afterwards, the as-cast ingots were machined
into cylindrical samples with dimensions Φ75 x 150 mm2 . The machined billets were
then heat treated before carrying out indirect extrusion at fixed extrusion ratio of 9
and constant ram speed.
2.2 Multidirectional Forging (MDF)
The MDF samples were prepared by machining the center of extruded bar into
rectangular cubes with dimensions, 17 x 17 x 28 mm3. These samples were heated at
300oC in the furnace to carry out isothermal forging. The principle of MDF is
presented in Fig. 4. Accordingly, after each pass the sample was rotated 90o and
abraded to recover deformed surface. Instron-type 5582 machine was used to carry
out forging at an initial strain rate of 3 × 10-3 s-1 with cumulative strain of ∑ = 2.7.
The applicable strain by MDF is theoretically unlimited when the sample is not
fractured. The first forging axis was parallel to the extrusion direction.
19
Fig. 4: Schematic representation of the MDF process [34]
Fig. 5: Temperature distribution (°) of screw rolled sample after one pass [35]
20
2.3 Screw rolling (SR)
The as-extruded bar was cut 70 mm length for three-roll mill, Fig. 5. Prior to planetary
milling, rods were heated in an air furnace for 30 minutes to carry out isothermal
rolling at 300oC. After six passes, the final diameter obtained was 15 mm at a mill
speed of 80 mm/s. The rolling direction was also similar to the extrusion direction.
2.4 Mechanical properties
The mechanical properties were characterized by means of tensile and compression
tests at a strain rate of 2 × 10-4 s-1. A 2-ton Unitech R&B machine was utilized to test
tensile specimen with gauge length 12 mm and 1 mm thickness while the dimensions
of compression samples were 3 x 3 x 4.5 mm3. An average of five specimens for each
sample was taken at room temperature.
2.5 Corrosion Measurements
Specimen for corrosion studies were machined from the center of extruded and
deformed samples by spark-erosion wire cutting with an exposed surface parallel to
final processing direction.
The properties of as-extruded, as-MDF and as-screw rolled samples were measured
by electrochemical, weight loss and hydrogen evolution measurements in 3.5 wt.%
NaCl saturated with Mg(OH)2 at 25 + 1oC. The experimental conditions were similar
as described by Arthanari S [36]. The dimensions of all corrosion samples were kept
constant, 10 x 10 x 2 mm3 sliced from biscuit (ASTM E8). Prior to corrosion
characterization, the samples were ground on abrasive silicon carbide (SiC) with
granularity 2000. Finally, samples were cleaned in an ultrasonic bath using acetone
and blow dried with hot air. The corrosion rate (mm/y) was calculated by using
following equations [37]:
Hydrogen Evolution, PH = 2.088 VH /At (2)
Polarization, Pi = 22.85 icorr (3)
21
Where ‘ΔW’ is the difference of masses before and after immersion, ‘A’ is the area
of the specimen (cm2), ‘t’ is the time (d), icorr (mA/ cm2) is the current density obtained
from tafel plots.
2.6 Microstructure & Phase Analysis
The microstructures evolved after extrusion, MDF and screw rolling were analyzed
perpendicular to the final processing direction. The specimens were ground, polished
and etched using picral. Finally, the average grain size was measured by using image
analysis software.
The corroded morphology after electrochemical and immersion tests was analyzed by
scanning electron microscopy (JSM-6360) equipped with an energy dispersive X-ray
spectroscopy (EDS). The accelerating voltage range 15.0 kV and working distance of
10 mm is kept for all samples.
X-Ray Diffraction (XRD) was carried in D8-Bruker machine with 2θ scan range from
10o to 90o at a scan rate of 0.03o for investigating the formed phases.
22
of Mg
The first goal of the present work is to study effect of extrusion temperature on
commercially pure magnesium at a constant ratio. The properties of pure magnesium
are very important especially in biomaterials and in magnesium batteries due to
economic viability in latter when compared with doped-magnesium batteries [38].
Due to absence of any second phase, the grain size plays the primary role in deriving
the properties. Additionally, the isothermal severe plastic deformation at 300oC was
carried out on low grain size samples to study its effect on corrosion and mechanical
properties.
3.1.1 Microstructures
The commercially pure magnesium ingots were extruded from 180oC to 300oC at a
fixed extrusion ratio of 9. The chemical compositions determined by spark emission
spectroscopy and extrusion conditions are listed in Table 5. Fig. 6 shows the optical
microstructure of magnesium in as-extruded and as-SPD state. As the extrusion
temperature increases, the grain size increases due to the growth of recrystallized
grains suggesting the linear relationship. Since, Mg180 possessed the smallest grain
size, isothermal rolling and forging at 300oC were carried out to see the effect on
microstructure. The microstructure of screw rolled samples showed several
deformation twins whereas MDF sample shows a more homogenous microstructure.
The grain sizes are listed in Table 6.
23
Material
Temp (oC)
HT (oC/h)
kg/cm3 - 340 516 548 - 402 oC
Mg
180
350/12
220 64 99.60 - 0.01 0.05 0.03 - 0.31
260 55 99.87 - 0.04 0.03 0.02 0.01 0.03
300 48 99.90 - 0.03 0.02 - - 0.05
24
Fig. 6: The OM images of magnesium extruded at (a) 180oC, (b) 220oC, (c) 260oC,
(d) 300oC; (e) screw rolled at 300oC and; (f) MDFed at 300oC
25
Fig. 7: XRD Analysis of CP magnesium in as-extruded and as-SPD condition
26
3.1.2 Mechanical Properties
The tensile and compression tests were carried out at room temperature on
magnesium samples extruded between 180oC and 300oC. The tensile strength of
the as-extruded samples has no significant difference but both tensile and
compressive elongation decreases with extrusion temperature.
To optimize the mechanical properties of pure magnesium, Mg180 was subjected
to isothermal screw rolling and MDF at 300oC. The screw rolled samples shows
the drastic increase in strength due to increased dislocation density. However, the
tensile/compressive slope of MDF samples indicates the softening of the matrix
even though the grains were reduced from 9.27 to 7.13 μm. This can be explained
on the basis of the fact that the grain boundary diffusion coefficient of Mg is large
enough to cause the deformation at higher temperature causing grain boundary
sliding [39]. To overcome, Mabuchi concluded an increase strain rate value can
decrease grain boundary sliding.
The results are summarized in Fig. 8 and Table 6.
27
Fig. 8: (a) Tensile and (b) Compressive tests of magnesium extruded from 180oC to
300oC and as-SPD at 300oC
Table 6. Grain size and mechanical properties of magnesium in as-extruded and as-
ISPD condition
Designation Grain
Size (μm)
28
3.1.3 Corrosion Measurements
The instantaneous corrosion rate was measured by means of electrochemical tests,
Fig. 9, while as-extruded magnesium was immersed over a period of seven days in a
saturated NaCl solution to determine corrosion rate by hydrogen evolution and weight
loss method, Fig. 10.
In as-extruded condition, Mg 180 has the lowest corrosion rate (PH) of 2.96 mm/y
when grain size was 9.27 μm. To further enhance the properties, Mg180 was
subjected to ISPD.
In Nyquist plot, the capacitive loop at the high frequency region indicates the charge
transfer reaction on the sample surface and electrolyte, and the dimension of
capacitance loop determines the charge transfer resistance. In Fig. 9a, the diameter of
high frequency capacitance loop of screw rolled sample is larger than that of the
extruded and MDF, indicating that the charge transfer resistance has remarkably
decreased after screw rolling. The bode plot, Fig. 9b, shows a drop for all samples in
low frequencies except MDF which suggests resistance of the surface film against
breakdown [3]. This improved resistance is attributed to grain refinement being
principally confined to controlling the rate of anodic reactions and having little role
in altering the rate at which cathodic reactions can be sustained [4]. The homogeneity
of microstructure achieved by MDF forms an effective passivation decreasing
corrosion rate of magnesium to 0.74 mm/y in terms of hydrogen evolved, Fig. 10.
Also, the current density (μA/cm2) decrease from 93 to 32 from tafel analysis, Fig.
9c, confirms the corrosion resistance trend of Mg180 MDF sample.
Fig. 11 shows the corroded surfaces after immersion of screw rolled and MDFed
samples in saturated NaCl. The surface of MDF sample was corroded uniformly and
wrinkled with some pin holes and small shallow areas.
29
for CP magnesium in as-extruded and as-ISPD condition after 1.5 hours immersion
in saturated 3.5 wt. % NaCl solution
30
magnesium after 168 hours of immersion in saturated 3.5 wt. % NaCl solution.
Table 7. Corrosion rates of magnesium in terms of hydrogen evolved and weight
loss in as-extruded and as-ISPD condition
Sample Hydrogen
Volume (ml/cm2)
62
(a)
(b)
Fig. 29: Relationship between yield strength and yield asymmetry (a) at extrusion
temperature between 180oC and 300oC, (b) ISPD at 300oC
63
slips dominates at higher temperatures. Yin (2009) demonstrated that high frequency
of twinning under basal texture in Mg-3Al-1Zn leads to tension compression yield
asymmetry [53].
Hence, we can say when the grain size reaches to a transition threshold, the yield
asymmetry value can be minimized, ZX180 forged at 300oC, with the dominant
deformation mechanisms change from twinning to slip.
4.4 Corrosion Analysis
Ahmad B [44] presented a corrosion model to predict corrosion rate by estimating
phases fraction, grain sizes, volta potential difference between matrix and second
phase. To corroborate the findings of current work, the area fraction of all second
phases and their respective volta potentials are shown in Table 17 and Table 18,
respectively.
From the data plot, Fig. 30, of calculated and predicted corrosion rates, the findings
of most materials were found to be in good agreement.
64
Sample C)o( Condition ∑f A,i
∑f A,i
ISR 0.0606 9.0975
Table 18. Volta potentials of second phases in this study [43, 44, 54]
Alloy (wt.%) Intermetallic Volta Potential (mV)
Mg-0.6Ca-0.5Mn Ca-Mg-Zn 350
Mg-2.6Nd Mg-Nd 150
Mg-4Y-2.2Nd Mg12Nd 25
Mg-3.5Zn-0.7Ca-0.5Mn Mg2Ca -130
65
Fig. 30: The relationship between calculated and predicted corrosion rates [44]
66
To achieve optimum corrosion-mechanical properties, warm and hot extrusion of
magnesium & its alloy and subsequent isothermal severe plastic deformation have
been studied. The current industrial demand to produce alloys for warm extrusion can
be achieved by low alloying and modification of processing parameters to achieve
balance of desired properties, as achieved in the case of screw rolled ZX10, for
instance.
The linear relationship between corrosion rates and extrusion temperatures in
magnesium is a result of localized corrosion, breaking of passivation film and
increase dissolution of black areas rich with MgO, Mg(OH)2 and, impurities. The
more homogenous microstructure, as after forging of Mg180, can form a thick
passivation of corrosion products over prolonged period which in turn increase
corrosion resistance.
The ZX10 alloys shows a very fine microstructure (1.677μm) for a technical wrought
alloy even in as-extruded condition. A good combination of mechanical properties
such as high strength and high ductility after screw rolling at 300oC has been achieved.
The increased porosity and presence of Mg-Zn-Ca phase, which is less noble than the
matrix, severed the corrosion rate of MDF samples.
For both Mg and ZX10, the decreased strength after MDF could be associated with
softening of matrix because of lower strain rate employed and high deformation
temperature causes grain boundary sliding (high diffusion coefficient value of Mg)
resulting in decreased strength.
When Mg-3Nd was immersed in harsh chloride environment and corrosion rate was
higher than previously reported in SBF. However, the corrosion resistance role of
Mg12Nd phase was found to be consistent with the literature. The volta potential of
Mg12Nd is only ~25mV than that of Mg-matrix making it a weak cathode phase.
67
The addition of Ca in Mg-Nd is found to be suitable for mechanical properties in as-
extruded condition. However, the corrosion properties need optimization by
decreasing unDRX fraction and promoting formation of Mg12Nd phase. When EX
series alloys were subjected to screw rolling, the corrosion resistance of EX33 was
increased by impressively 78% than as-extruded state. This elevated temperature
decline in yield strength and increment of corrosion resistance could be associated
with increased grain size and reduction of second phases (galvanic cells). The results
are consistent with previous reported data.
To design magnesium alloys for structural applications, the yield asymmetry can be
lowered by decreasing extrusion temperature, for example ZX10 series. Further
decline was achieved by forging ZX180 at 300oC resulting in yield asymmetry value
of 0.4 (minimum). The opposite trend of Mg and ZX10 yield asymmetry values can
be explained on the basis of precipitates in latter, grain size difference and texture
evolution with dominant deformation mechanisms changing from twinning to slip,
mainly prismatic slip.
Finally, the corrosion rates of studied materials in this work were plotted on a
corrosion model, recently proposed in the literature. For most alloys, a good
agreement between calculated and predicted values was obtained.
68
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Concentration
Fig. 31: Ionic concentration of Hanks, Simulated body fluid (SBF) and Plasma in
terms of ionic concertation in millimoles per Litre (mmol/L)
76
7.2 Alloys Properties in Hanks
instantaneous corrosion rate (mm/y) in Hanks solution and respective mechanical
properties (yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%))
77
7.3 Alloys Properties in SBF
instantaneous corrosion rate (mm/y) in SBF and respective mechanical properties
78
7.4 Alloys Properties in NaCl
instantaneous corrosion rate (mm/y) in NaCl and respective mechanical properties
79
(a)
(b)
Fig. 35: Most commonly studied extrusion ratios and temperature, (b) extrusion
temperature effect on corrosion rate [38, 42, 55]
80
7.6 EBSD of Selected Alloys
Fig. 36: Inverted Pole Figures (IPZ) of selected alloys. Note the twinning in screw
rolled (SR) samples
Chapter 1: Introduction
1.1.1. Alloying Elements
2.6 Microstructure & Phase Analysis
Chapter 3: Results & Discussion
3.1 Effect of Extrusion Temperature and ISPD on Properties of Mg
3.1.4 Summary
3.2 Effect of Extrusion Temperature and ISPD on Properties of ZX10
4.2 UTS vs Corrosion vs Extrusion Temperature
4.3 Yield Stress vs Yield Asymmetry vs Extrusion Temperature
4.4 Corrosion Analysis
Chapter 5: Conclusion
Chapter 6: References
7.2 Alloys Properties in Hanks
7.3 Alloys Properties in SBF
7.4 Alloys Properties in NaCl
7.5 Extrusion Conditions
7.7 Oxide Morphologies