Accepted Manuscript

Microstructure, phase composition and mechanical properties of new, low cost Ti-Mn-Nb alloys for biomedical applications

Shima Ehtemam-Haghighi, Hooyar Attar, Matthew S. Dargusch, Damon Kent

PII: S0925-8388(19)30556-0

DOI: https://doi.org/10.1016/j.jallcom.2019.02.116

Reference: JALCOM 49548

To appear in: Journal of Alloys and Compounds

Received Date: 31 August 2018

Revised Date: 8 February 2019

Accepted Date: 9 February 2019

Please cite this article as: S. Ehtemam-Haghighi, H. Attar, M.S. Dargusch, D. Kent, Microstructure,phase composition and mechanical properties of new, low cost Ti-Mn-Nb alloys for biomedicalapplications, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.02.116.

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Microstructure, phase composition and mechanical properties of new, low cost

Ti-Mn-Nb alloys for biomedical applications

Shima Ehtemam-Haghighi a,b,c*, Hooyar Attar b, Matthew S. Dargusch b,c, Damon Kent a,b,c

aSchool of Science and Engineering, University of the Sunshine Coast, Maroochydore DC, QLD 4558, Australia

bCentre for Advanced Materials Processing and Manufacturing, School of Mechanical and Mining Engineering,

The University of Queensland, St Lucia, Queensland, 4072, Australia

cARC Research Hub for Advanced Manufacturing of Medical Devices

* Corresponding author

Email address: sehtemam@usc.edu.au

Abstract

A group of new Ti-7Mn-xNb alloys with Nb contents varying from 0-10 wt% were fabricated

by conventional press and sinter powder metallurgy processing from blended elemental

powders. The effect of Nb content on the sintering and phase stability as well as

microstructure and mechanical characteristics of the alloys was investigated. Microstructural

studies and phase analysis showed a two-phase microstructure composed of α and β phases in

all alloys. However, addition of Nb up to 10 wt% enhanced the stability of the β phase

hindering the transformation to α during furnace cooling and increasing the proportion of

retained β. Sintering and densification of the Ti-7Mn-xNb alloys was enhanced by the fast

diffusing Mn and negatively impacted by the slow diffusing Nb due to the reliance of solid

sintering processes on diffusion. Nonetheless, the relative densities of the Ti-7Mn-xNb alloys

with up to 10 wt% Nb were higher than that of the sintered CP-Ti. The compressive strength

and hardness of the alloys varied in the range of (1842-2127 MPa) and (341-375 Hv)

respectively. It was also observed that with increasing Nb contents, the elastic modulus

decreased, while compressive strain increased due to stabilization of a greater proportion of

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the β phase. The results showed that the low-cost Ti-7Mn-xNb alloys possess superior

properties to those of CP-Ti and a number of other Ti based alloys developed for biomedical

implant applications.

Keywords: Titanium alloy; Phase stability; Microstructure; Mechanical property; Powder

metallurgy

1. Introduction

Titanium (Ti) and its alloys have been used extensively for biomedical applications due to

their outstanding combination of properties. In comparison to other conventional metallic

biomaterials such as 316L stainless steel and cobalt-chromium alloys, they possess higher

specific strength, lower elastic modulus, better corrosion resistance and superior

biocompatibility [1-4]. Among Ti-based alloys, commercially pure Ti (CP-Ti) and Ti-6Al-4V

are the most commonly used biomaterials. However, they present some concerns such as the

release of Al and V ions from the Ti-6Al-4V alloy which may cause long-term health

problems including neurological disorders and osteomalacia [5]. Additionally, the elastic

modulus mismatch between implants made from CP-Ti and Ti-6Al-4V (104 and 110 GPa,

respectively) and that of the surrounding bone (10-30 GPa) can lead to stress shielding, where

the bone is not subject to normal loads essential for maintaining its strength, density and a

healthy structure, leading to bone resorption and premature failure of the implant [6-10].

Therefore, in recent years, attention has been directed toward development of new, more

biocompatible Ti alloys with mechanical properties better suited to implant applications for

replacement of hard tissue, particularly lower elastic moduli in combination with higher

strength [11]. These new Ti alloys are predominantly β-type alloys which typically include

significant proportions of expensive and scarce alloying elements such as Ta, Zr, Nb and Hf

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with high melting points and high densities such as Ti-70Ta, Ti-29Nb-13Ta-4.6Zr and Ti-

42Nb [12-14]. The high melting points of these alloying elements introduces increased

difficulty to the processing of biocompatible Ti alloys and make the alloys more prone to

compositional segregation which is detrimental to their mechanical properties and

performance [15, 16]. Additional post-fabrication processes such as high temperature

homogenising heat treatments and/or thermoplastic processing are required to alleviate or

eliminate segregation [16] which adds to their expense. Therefore, due to the high content of

costly alloying elements and difficulty in alloy fabrication, many of the recently developed Ti

alloys are not competitive with current commercial Ti alloys [15]. Thus, increasing attention

has been paid to design and development of Ti alloys containing greater proportions of

common low-cost, low melting points elements such as Mn, Fe, Sn and reduced content of

expensive, scarce metals [15, 17].

As stated, Mn is a low cost and strong β-stabilising alloy element which can be used to

develop high strength Ti alloys [18]. Mn is also an essential element for maintenance of

regular body and brain function. In addition, the cytotoxicity and cell viability of Ti-Mn

alloys with Mn content up to 13 wt% are comparable to that of CP-Ti [18]. Therefore, a Mn

concentration of less than 13 wt% can be used to produce biocompatible Ti alloys. Nb is

another important biocompatible β-stabiliser and its addition to Ti alloys contributes to

reducing their elastic modulus as well as enhancing their corrosion resistance due to

development of stable bio-inert Nb2O5 surface oxide layer which suppresses the dissolution

of Nb ions [19, 20]. Therefore, there is potential to utilise Mn in conjunction with Nb to

develop more biocompatible, lower cost Ti alloys for biomedical implant applications.

To the authors’ best knowledge, there are limited studies addressing the properties of Ti-Mn

based alloys [18, 21]. Therefore, this research aims to study a series of new Ti-Mn-Nb alloys

prepared by conventional powder metallurgy (PM) from blended elemental powders. As a

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near-net shape process, PM offers considerable cost advantages, providing in the order of a

50% reduction in processing costs compared to other conventional techniques such as ingot

based processing routes due to fewer processing steps, limited need for post-fabrication

machining and less material wastage [22]. Additionally, this method is suited to preparation

of complex alloy compositions, containing alloying elements with significantly different

melting points (such as Mn and Nb in this study), which are otherwise difficult to produce by

conventional liquid metallurgy based approaches [23]. In the present study, a group of Ti-

7Mn-xNb alloys is fabricated using the most flexible and cheapest PM route (i.e. cold

pressing of the blended elemental powders in conjunction with furnace sintering) [24] and the

effect of Nb contents on the phase stability, sintering response, microstructure and

mechanical properties of the alloys is investigated. This study demonstrates that the Ti-Mn-

Nb based alloys are suited to press and sinter PM fabrication and possess a good combination

of mechanical properties for biomedical implant applications.

2. Experimental

Ti powder (average particle size < 50 µm), Mn powder (average particle size ≤ 10 µm) and

Nb powder (average particle size < 50 µm) were used as raw materials. Fig. 1 shows the

typical morphology of the Ti, Mn and Nb elemental powders. Powder mixtures with nominal

compositions of Ti-7Mn-xNb (x = 0, 3, 7 and 10 wt%) were blended in a GlenMills Turbula

T2F mixer for 2 h. The mixed powders were then uniaxially cold pressed under a fixed

pressure of 550 MPa using a Carver 12 Ton Manual Hydraulic press to prepare green

cylindrical compacts. The green components were subsequently sintered in a Carbolite high

temperature tubular furnace (STF 15) under protection of high purity argon atmosphere.

Samples were heated up to 1170 ºC with a heating rate of 4 ºC/min and held isothermally for

8 h before furnace cooling at the rate of 4 ºC/min. For comparison compacts were also

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prepared from the Ti powder only (CP-Ti) using the same compaction and sintering

conditions.

Fig. 1. SEM images showing the morphology of the starting powders: (a) Ti, (b) Nb and (c) Mn.

The relative density of the sintered compacts was calculated as below [25]:

� = �

��× 100% (1)

where is the density of the sintered sample measured using Archimedes’ principal, � is the

alloys nominal theoretical density estimated using the below equation [25]:

� =�

(�%��)�(

�%��)�(

�%��) (2)

where �%, �% and �% are the weight fraction of the constituent alloying elements and �,

� and � are their corresponding theoretical densities. The density measurements were

repeated three times for each alloy composition.

For microstructural studies, specimens were prepared using standard metallographic

procedures and then etched with Kroll’s solution (5 vol% HF, 30 vol% HNO3 and 65 vol%

H2O). The microstructure of the specimens was characterised with a Hitachi Tabletop

Scanning Electron Microscope (SEM, TM3030). The phase constitutions of the sintered

alloys were identified by X-Ray diffraction (XRD) using Bruker D8 Advance X-Ray

Diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA. The

integrated areas for α and β peaks in the XRD patterns were determined using the peak-fitting

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program, Fityk. Based on the integrated areas, the volume fraction (��) of these phases was

estimated in the similar manner to Ref. [26] as below:

��,� =��

����� (3)

��, =��

����� (4)

where ��,� and ��, are the volume fractions and �� and � are the total integrated areas

corresponding to α and β phases, respectively [11].

From the XRD patterns, the lattice parameter, !, of the β phase was estimated using the

following equation [18]:

! = "

# $%&'× √ℎ# + +# + ,# (5)

where - is the X-ray wavelength, . is Brag angle and (ℎ, +, ,) is the miller indices of the

diffraction plane. ! was determined by extrapolation of the variation of the lattice parameter

as a function of the Nelson-Riley parameter (/0$#1$%&'

+/0$#1'

) [27].

To determine the mechanical properties, compression and hardness tests were carried out at

room temperature. Cylindrical specimens with the height-to-diameter ratio of 1.8 were

prepared from the sintered alloys. Compression tests were conducted using an Instron 5584

mechanical tester with a crosshead speed of 0.001 mm/s according to the standard DIN

50106. Mechanical properties including elastic modulus, yield strength, compressive strength

and strain were determined from tests performed in triplicate for each alloy composition.

Vickers hardness measurements on the polished samples was carried out using a Struers

Duramin hardness tester with a load and dwell time of 981 mN and 10 s, respectively. The

Vickers hardness values reported represent the average of at least 10 measurements.

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3. Results and discussion

3.1. Phase analysis and microstructural studies

The morphological characteristics of the initial powders have considerable influence on the

properties of PM processed Ti alloys [28]. The starting powders consist of blended elemental

mixtures of Ti, Mn and Nb powders with irregular shapes as shown in Fig. 1. The irregular

powder morphologies facilitate mechanical interlocking during uniaxial cold pressing

resulting in enhanced green strengths for the compacted powders. Typically higher green

strengths result in better sintering and higher strengths for the final sintered part [29].

Fig. 2 shows XRD spectra obtained from the as-sintered Ti-7Mn-xNb alloys. No obvious

diffraction peaks from remnant elemental Mn and Nb were detected in the XRD patterns,

indicating their complete diffusion into the Ti matrix during sintering. As can be seen, all of

the Ti-7Mn-xNb alloys are comprised of mixtures of the body-centred cubic (bcc) β and

hexagonal closed-packed (hcp) α phases. This is a desirable outcome as biphasic Ti alloys

generally present a good balance of properties including high yield strength, good fracture

toughness and desirable corrosion resistance [30]. Based on the XRD profiles, the volume

fraction of the phases present in the alloys were estimated and are summarised in Table 1. Ti-

7Mn alloy contains the lowest proportion of the β phase (��, = 39%) and the highest amount

of α phase (��,� = 61%). When 3 or 7 wt% Nb is added, the intensity of β peaks, and hence

the proportions of the β phase are increased, whereas the intensity of the α peaks and

correspondingly the α phase volume fraction is decreased. Further increase in the Nb content

to 10 wt% leads to further reduction in the intensity of α peaks and retention of the highest

proportion of the β phase (��, = 70%) in the as-sintered alloy.

Table 1 also presents the values for lattice parameter (a) of the β phase. The results show that

increasing levels of Nb addition lead to proportionate increases in the lattice parameter of the

β phase from a = 3.219 Å for the Ti-7Mn alloy up to 3.250 Å for the Ti-7Mn-10Nb alloy.

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This can ascribed to the substitutional alloying of Nb with a larger atomic radius (2.08 Å)

than that of Ti (2 Å) [25], so that the lattice parameter increases due to substitution of Ti

atoms by Nb in the β solid solution. The increase in the lattice parameter is evident in the

slight shift of the XRD peaks to lower angles with increasing levels of Nb addition (Fig. 2).

Fig. 2. XRD patterns of the sintered Ti-7Mn-xNb alloys.

Table 1. Volume fraction (Vf) of α and β phases, lattice parameter (a) of β phase and relative density of the sintered Ti–7Mn–xNb alloys and CP-Ti

Alloy α phase 23 (%)

β phase 23 (%)

β phase a (Å)

Relative density (%)

Ti-7Mn 61 39 3.219 96.31 ± 0.07 Ti-7Mn-3Nb 52 48 3.234 96.23 ± 0.06 Ti-7Mn-7Nb 39 61 3.246 96.12 ± 0.07 Ti-7Mn-10Nb 30 70 3.250 95.90 ± 0.11 CP-Ti 100 - - 95.16 ± 0.10

The relative density of the sintered Ti-7Mn-xNb alloys and CP-Ti is summarized in Table 1.

As evident, the relative density of the Ti-7Mn is higher than that of CP-Ti and slightly

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reduces (by around 0.4%) with increment of the Nb content from 0 to 10 wt%. It has been

reported that different alloying elements diffuse into the Ti matrix at different rates [23]. Mn

is a fast diffusing element and its diffusion rate is higher than that of the self-diffusion of Ti

in the β phase [31], thus Mn can improve the solid state sintering response of Ti alloys, which

is reliant on diffusion, promoting enhanced densification [32]. In contrast, Nb is a slow

diffuser and has a lower rate of diffusion than that of the self-diffusion of Ti in the β-Ti

matrix [31], so the addition of Nb tends to reduce densification of the Ti alloys [33].

Therefore, the degree of densification of the Ti-7Mn-xNb alloys is dependent on two

counteracting effects induced by the relative diffusivities of the Mn and Nb alloying

additions. In this study, when 7 wt% Mn is added to Ti, the highest final sintered relative

density of 96.31% was achieved. However, as the Nb content was increased to 10 wt%, the

relative density of the alloys was slightly decreased and reaches the minimum of 95.90% in

Ti-7Mn-10Nb alloy, although this is still higher than the relative density obtained for the CP-

Ti. The size of the solute elemental powders also influences the final density of the sintered

alloys. Finer alloying powders require shorter diffusion distances for alloying elements to

dissolve into the Ti matrix, thereby leading to more rapid dissolution of the particles and

enhanced sintering [33]. Hence, to accelerate sintering and enhance the densification of the

alloys, Mn with an average particle size of less than 10 µm was selected to partly compensate

for the effects of the larger and lower diffusivity Nb particles. It should be noted that the Nb

particle size (< 50 µm) used in this work is smaller than that used in other studies [33, 34].

Additionally, it was previously reported that the addition of 10 wt% Nb to CP-Ti reduced the

relative density of the alloy by around 2% [33]. In contrast, the addition of the same

proportion of Nb (10 wt%) to the Ti-7Mn alloy in this study led to reductions in the relative

density by ~0.4%.

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SEM images of the microstructures of the Ti-7Mn-xNb alloys are presented in Fig. 3. In

accordance with the XRD results, all of the alloys display a two-phase composition consisting

of the α and β phases which appear as dark and lightly contrasting grey areas in the

microstructures, respectively. There are also residual pores represented by regions of black

contrast which are typically located along the β phase grain boundaries. Additionally, no

remnant Nb and Mn powder particles can be distinguished indicating their complete

dissolution during sintering. In fact, homogenous distributions of the alloying elements is a

key factor to guarantee reliable and consistent properties [29]. As can be seen in Fig. 3(a), the

Ti-7Mn alloy consists of large irregularly shaped primary α plates, mainly located at prior β

grain boundaries, as well as α and β lamellae which are observed within light grey contrasting

regions in the microstructure. With the addition of 3 wt% Nb (Fig. 3(b)), the α phase becomes

finer and its proportion decreases. Increasing the Nb content to 7 wt% enhances the β phase

stability and leads to further reductions in the α phase volume fraction as evident in Fig. 3(c).

At this composition, the prior β grain boundaries, which are covered by finer α (αGB), become

clearly observable. In addition, some finer α lamellae in the vicinity of prior β grain

boundaries known as intergranular α as well as α lamellae within the β grains known as

intragranular α can be seen. The αGB formed on β grain boundaries in the early stages of

cooling (to temperatures below the β transus), act as a precursor for nucleation and growth of

intergranular α lamella [35, 36]. These lamella side plates are produced by branching from

the αGB. The intragranular α lamellae nucleate and grow inside the β grains [36]. Finally,

when the Nb content reaches 10 wt%, the β phase is more stabilised and the α phase

concentration and size is further reduced as can be seen in Fig. 3(d).

Based on the above results it can be deduced that increasing levels of Nb to the base Ti-7Mn

alloy promotes β phase stabilisation and makes the β to α phase transformation during

furnace cooling more difficult, resulting in a decrease in the proportion and size of the α

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plates and an increased β phase fraction. The hampering effect of the β-stabilising elements

which causes fewer α nuclei to form during cooling has also been reported in other studies

[37-39].

Fig. 3. SEM images showing the microstructure of the sintered Ti-7Mn-xNb alloys: a) Ti-7Mn, b) Ti-7Mn-3Nb, c) Ti-7Mn-7Nb and d) Ti-7Mn-10Nb.

Fig. 4 presents the microstructure of the sintered Ti-7Mn-10Nb alloy at a higher

magnification and the elemental Ti, Mn and Nb distributions are revealed in corresponding

energy dispersive spectroscopy (EDS) maps. Dark areas in the Mn and Nb EDS maps clearly

reveal a deficit of these elements in these regions. In contrast, the same regions are richer in

Ti as revealed by the Ti EDS map. On this basis and from additional EDS spot analysis, it

was found that β phase is enriched with Mn (9.1 ± 0.9 wt%) and Nb (10.0 ± 0.6 wt%)

elements, whilst the α phase contains higher Ti content (97.2 ± 0.4 wt%) and less Mn and Nb

(0.4 ± 0.3 wt% and 2.4 ± 0.5 wt%, respectively). This observation confirms that the Mn and

Nb act as β-stabilisers and have higher solubility in β than α.

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Fig. 4. SEM image of the sintered Ti-7Mn-10Nb alloy at a higher magnification and EDS maps indicating Mn and Nb enrichment in the β phase.

Further microstructural investigations also revealed the existence of some lighter contrasting

regions in the microstructure of the Ti-7Mn-10Nb alloy typically surrounded by the β phase.

An SEM image of one of these brighter contrasting features and the corresponding EDS line

scan analysis across the region are shown in Fig 5. As can be seen in Fig. 5(a), there is no

clear boundary between the bright zone and the surrounding β phase. Nonetheless, the EDS

line scan (Fig. 5(b)) along the yellow line indicates a higher content of Nb in the brightly

contrasting region with the bright contrast due to a relatively higher concentration of heavier

atomic weight Nb, suggesting that diffusion of Nb into the Ti matrix is incomplete. However,

Mn shows a homogeneous distribution, signifying its complete diffusion into the matrix. EDS

spot analyses revealed that the concentration of Nb in the brightly contrasting Nb rich region

is almost 27 wt% which is higher than the nominal 10 wt% Nb content of the alloy.

Nevertheless, the bright regions detected in our study do not contain a predominant

proportion of Nb in contrast to similar brightly contrasting features detected in a Ti-10Nb-

3Mo PM alloy microstructure [38]. This can be related to the lower sintering time (3 h) used

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in the other study and also to an absence of the high diffusivity Mn which led to the higher

concentrations of the undissolved Nb in the Ti-10Nb-3Mo alloy microstructure.

Fig. 5. Typical SEM image of an area with higher Nb content observed in the (a) sintered Ti-7Mn-10Nb alloy and b) corresponding EDS line scan analysis.

3.2. Mechanical properties

Compressive strength is a key factor in determining the suitability of implant materials since

bone tissues are primarily exposed to compressive stresses [40]. Fig. 6 displays the typical

compressive engineering stress-strain curves for the as-sintered Ti-7Mn-xNb and CP-Ti

alloys. The plots show that addition of Nb influences the mechanical properties of the alloys.

Furthermore, all alloys exhibit a combination of high strength and large strain indicating they

do not have major defects which could cause premature failure.

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Fig. 6. Compressive engineering stress-strain curves of the sintered Ti-7Mn-xNb alloys and CP-Ti.

The compression properties of the alloys and CP-Ti are presented in Fig. 7. It can be

observed that addition of 7 wt% Mn to CP-Ti increases the yield strength (45 = 1005 MPa)

and ultimate compressive strength (46�7 = 2127 MPa) of the alloy through solid solution

strengthening. Conversely, with increasing levels of Nb additions, the 45 and 46�7 of the

alloys gradually decreases. It has been reported that with increasing Nb content in Ti alloys,

the bonding force of the β phase crystal lattice decreases [19] and thus, the strength of the

alloys reduces accordingly. Therefore, the Nb both increases the proportion of the β phase

and acts to reduce its lattice bonding strength. This contributes to decreases in 45 and 46�7

with increasing levels of Nb in the Ti-7Mn-xNb alloys, with the lowest values observed for

the Ti-7Mn-10Nb (45 = 842 MPa, 46�7 = 1842 MPa). However, it should be noted that all of

the Ti-7Mn-xNb alloys present 45 and 46�7 values higher than that of the sintered CP-Ti

(602 MPa and 1647 MPa respectively). A high 45 is desirable for implant materials as it

improves their capacity to resist permanent deformation and prevent premature failure [41].

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Fig. 7. Compressive mechanical properties of the sintered Ti-7Mn-xNb and CP-Ti alloys.

Fig 7(a) also displays the compressive strain (8) of the Ti-7Mn-xNb alloys as well as that of

the sintered CP-Ti. As can be seen, 8 of the alloys are lower than that of the CP-Ti (8= 43%)

which is likely due to the solid solution hardening effect of Mn. However, upon increasing

the Nb content, 8 slightly increases and is highest for the Ti-7Mn-10Nb alloy (8 = 34%). This

is presumably due to increased stabilisation and volume fractions of the β phase which

contributes to increase 8 [42].

Fig. 7(b) displays the elastic modulus (E) of the alloys. Ti-7Mn alloy possesses lower E than

that of CP-Ti. Upon increasing the Nb content, E slightly reduces and reaches 87 GPa in Ti-

7Mn-10Nb alloy due to an increased β phase content [43].

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Fig. 7(b) also shows the variation in Vickers hardness (9:) values of the alloys. The 9: of

the Ti-7Mn-xNb alloys shows the same trend to that observed for 45 and 46�7. The highest

9: was observed for Ti-7Mn alloy (9: = 375) due to the solid solution hardening induced by

Mn. It also has the lowest β phase content compared to the Ti-7Mn-xNb alloys. However, as

the Nb is increased, the 9: of the alloys decreases, although they remain higher than that of

CP-Ti. The decrease in 9: with increasing Nb additions is ascribed to increased proportions

of the softer β phase [39].

Classical theories of wear state that hardness is a key parameter in controlling wear resistance

and usually a hard material presents better wear resistance [44]. Therefore, based on the

hardness values, it is anticipated that all of the Ti-7Mn-xNb alloys will exhibit better wear

resistance than CP-Ti. For biomedical implants, a higher wear resistance reduces the

propensity for generation of wear particles during service which can otherwise cause

inflammatory responses and/or loosening of the implant necessitating further surgical

intervention [44-46].

It has been reported that the presence of porosity reduces the effective elastic modulus of a

component. It can also lead to local stress concentrations, which thereby decrease the

strength, hardness and ductility of the alloys [18]. However, in this study, due to only limited

reductions in the relative density (Table 1) or minor increases in the porosity with increasing

Nb additions, this influence is fairly consistent across the range of Ti-7Mn-xNb alloys. It

should be noted that the observed trends in the mechanical properties of the studied alloys are

similar to those reported for other similar alloy systems [19, 38, 42], whereby increasing Nb

contents in Ti-Nb and Ti-Nb-Mo alloys resulted in reduced strength, hardness and elastic

modulus and increased ductility.

Examination of the failed samples after compression testing shows that they fracture into two

distinct pieces after reaching 46�7. Fig. 8 shows SEM fractographs of the Ti-7Mn-10Nb

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alloy. It reveals that the fracture surface is oriented at about 45º to the compressive loading

axis which is characteristic of compressive shear failure in ductile materials (Fig. 8(a)) [47].

A typical fracture surface of the alloy at higher magnification (Fig. 8(b)) shows that the

former interface between powder particles cannot be distinguished indicating that the sintered

sample achieved full metallurgical bonding. In addition, a mixture of smooth and rough zones

are observable across the fracture surface. The presence of a high proportion of rough regions

is indicative of predominantly ductile fracture [48]. Also, some micro-cracks can be seen

which may have originated from the pores in the alloy microstructure acting as stress

concentrators during compression testing.

Fig. 8. Typical SEM fractographs of the Ti-7Mn-10Nb alloy after compression testing; (a) macroscopic view of fracture profile and (b) microscopic view of fracture interface.

The Ti-7Mn-xNb alloys exhibit yield strengths (45 = 842-1005 MPa) comparable to those of

other conventional Ti alloys (758-1117 MPa) used for biomedical application [49]. In

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addition, the Ti-7Mn-xNb alloys exhibit comparable or lower elastic moduli than some β-

type Ti alloys produced by PM techniques such as Ti-12Mo-6Zr-xFe (x = 1, 2, 3, 4 wt%) (E

= 93-105 GPa) [50], Ti-6Mn-3Mo (E = 87 GPa) [21] and Ti-35Nb-10Ta-3Fe (E = 98 GPa)

[51]. Furthermore, although the strain to failure of the Ti-7Mn-xNb alloys is lower than that

of CP-Ti, the values are higher than other Ti alloys such as Ti-10Nb-3Mo (8 = 29%) [38], Ti-

6Al-4V ELI (8 = 24%) [18], Ti-13Nb-(0-6)Zr (8 = 20-26%) [52] and Ti-(2-25)Cu (8 = 15-

28%) [53]. Therefore, this study demonstrates that by using β-stabilising Mn which is a

widely available and low cost element which imparts considerable strengthening to the alloy,

along with Nb, also an effective β-stabiliser which reduces the β phase bond strength and

increases the corrosion resistance of the Ti alloys, a good combination of high strength and

ductility as well as relatively low elastic modulus can be achieved. Moreover, considering the

high melting temperature of Nb which makes the fabrication of Ti alloys by traditional

casting methods difficult, the PM process used in this study offers a suitable, cost-effective

route for production of the net-shape Ti-Mn-Nb alloy components for biomedical implant

applications.

4. Conclusions

In this work, a series of new Ti-Mn-Nb alloys were fabricated using a press and sinter

powder metallurgy approach from blended elemental powders. The effect of Nb content on

the sintering, phase stability, microstructure and mechanical properties of Ti-7Mn-xNb alloys

were evaluated. The XRD and microstructural examinations revealed that all alloys consisted

of β and α phases, the proportions of which depend on the levels of Nb. Increasing Nb

contents enhanced the stability of β phase and decreased the proportion and size of the α

plates. The alloys exhibited higher relative densities than that of CP-Ti due to the fast

diffusion of Mn in Ti which aids solid state sintering. The mechanical properties obtained

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from hardness and compression tests showed that the Ti-7Mn-xNb alloys present higher

hardness, compressive and yield strengths as well as lower elastic moduli than the widely-

used CP-Ti. It is concluded that the press and sinter powder metallurgy approach can be

employed to successfully fabricate Ti-Mn-Nb alloys and the newly-developed alloys are

promising materials for biomedical implant applications.

Acknowledgements

The Authors acknowledge the support of Queensland Centre for Advanced Materials

Processing and Manufacturing (AMPAM). The Authors would like also to acknowledge the

support of the Australian Research Council through the ARC Research Hub for Advanced

Manufacturing of Medical Devices (IH150100024) and the ARC Hub for transforming

Australian industry through Additive Manufacturing (IH130100008).

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Highlights

• Low-cost Ti-Mn-Nb alloys were produced by conventional press and sinter.

• Phase and microstructural evolution induced by Nb addition is elucidated.

• Relationship between microstructure and mechanical properties is established.

• Ti-Mn-Nb alloys show desirable mechanical properties for biomedical applications.