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Effects of mechanical alloying conditions on the properties of Mg-based nanomaterials

Kamil Kowalski*, Tomasz Kachlicki, Mieczysław JurczykInstytut Inżynierii Materiałowej, Politechnika Poznańska, *kamil.kowalski@put.poznan.pl

Various methods can be used to improve properties of Mg-based materials. These modifications include alterations of the chemical composition or micro-structure modification. Nanostructured Mg-based alloy powders were produced by mechanical alloying method. The effect of different milling conditions on the microstructure evolution during mechanical alloying of Mg-type alloys with a nominal composition Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr was stud-ied. Bulk nanostructured Mg-type materials were finally obtained by the application of powder metallurgy. The evolution of microstructure and mechanical properties of Mg-based alloys were investigated. Compared to microcrystalline magnesium synthesized samples exhibit higher microhardness. This effect is directly associated with structure refinement and obtaining a nanostructure. Data concerning corrosion of nanostructured Mg-type alloys are scarce. In this paper, the corrosion behaviour of synthesized bulk Mg-based alloys containing different elements were investigated by immersion test in 0.1 M NaCl. The potentiodynamic corrosion test results indicate that addition of alloying elements shifts the corrosion potential to the less negative values. Nanocrystalline Mg-based alloy with enhanced mechanical and corrosion properties can be used in the automotive and marine industries.

Key words: magnesium, mechanical alloying, nanostructure.

WPŁYW PARAMETRÓW TECHNOLOGICZNYCH MECHANICZNEJ SYNTEZY NA WŁAŚCIWOŚCI MATERIAŁÓW NA BAZIE MAGNEZU

Różne metody mogą być stosowane do poprawy właściwości materiałów na bazie magnezu. Modyfikacje te obejmują między innymi zmiany w składzie chemicznym i fazowym. Metodą mechanicznej syntezy przygotowano nanokrystaliczne proszki stopu na bazie magnezu. Sprawdzono wpływ różnych warunków mielenia na zmiany mikrostruktury stopów o składzie Mg-4Y-5,5Dy-0,5Zr oraz Mg-1Zn-1Mn-0,3Zr. Lite próbki wytworzono metodą metalurgii proszków. Zbadano zmiany mikrostruktury i właściwości mechanicznych w trakcie mielenia. W porównaniu z mikrokrystalicznym magnezem wytworzone nanokrystaliczne stopy charakteryzują się większą twardością i modułem Younga, co ma bezpośredni związek z rozdrobnieniem ziaren i uzyskaniem struk-tury nanokrystalicznej. W pracy przeprowadzono badania odporności korozyjnej wytworzonych nanokrystalicznych stopów magnezu w 0,1 M roztworze NaCl. Potencjodynamiczne próby wykazały, iż dodatek pierwiastków stopowych powoduje przesunięcie potencjałów korozyjnych (EC) w kierunku bardziej dodatnich wartości. Wyprodukowane nanokrystaliczne stopy na bazie magnezu mogą znaleźć zastosowanie w przemyśle okrętowym i motoryzacyjnym.

Słowa kluczowe: magnez, mechaniczna synteza, nanostruktura.

Inżynieria Materiałowa 5 (207) (2015) 229÷232DOI 10.15199/28.2015.5.5© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Nanocrystalline materials produced by the application of non-equi-librium processing techniques, such as mechanical alloying (MA) or severe plastic deformation (SPD) demonstrate novel properties compared to conventional (microcrystalline) materials [1, 2]. Na-nomaterials can include metals, ceramics, and composite materials, and they demonstrate novel properties compared to conventional (microcrystalline) materials because of their nanoscale features.

Mechanical alloying is a powder processing technique that ena-bles the production of nanomaterials starting from mixed elemental powders in the right proportion followed by loading the powder mixture into the mill along with the milling balls. During the me-chanical alloying process, the powder particles are repeatedly flat-tened, cold welded, fractured and rewelded. The milled nanocrys-talline powders are finally compacted and heat-treated to obtain the desired microstructure and properties [1, 3].

Magnesium-based materials have recently become of interest in various applications due to their low densities and good mechanical properties [4, 5]. The disadvantages of Mg-based alloys are their poor properties at elevated temperature and their high reactivity and poor corrosion resistance. One of the most common used methods that allow to improve the mechanical properties of Mg alloys is the modification of its chemical composition. Alloying elements have been studied for developing Mg-type alloys with good mechanical and corrosion properties. Three groups of magnesium alloys can be distinguished [6, 7]: i) alloys containing Mn, Zn (Mg–Mn–Zn), ii) alloys containing RE (Rare Earth), Mg–Zn–RE, Mg–Y–RE–Zr (WE43) and iii) new ultralight alloys containing Li (Mg–Li–Al).

The most common addition to magnesium based materials are Zn and Mn [8÷10]. Their influence on the corrosion rate of Mg depends on solute concentration. Zinc can improve the corrosion resistance and mechanical properties of magnesium alloys due to a solid solution hardening mechanism. Manganese does not affect the mechanical property of magnesium alloy, but can improve their corrosion resistance by bonding iron and other heavy-metal ele-ments into relatively harmless intermetallic compounds. It has been shown that when Mn is added to Mg, the corrosion contribution from the impurity of Fe is rendered inactive because Mn atoms sur-round the Fe atoms and act as local cathodes [11].

Till now, the mechanical alloying method and the powder metal-lurgy process for the fabrication of bulk Ti-type and Ni-free austen-itic stainless steel materials with a unique microstructure have been developed [12, 13]. These projects provided the first evidence for an enhancement of the properties due to the nanoscale structures in consolidated Ti and Ni-free austenitic stainless steel alloys.

The aim of this research is to prepare ultrafine Mg powders and nanocrystalline Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr alloys with enhanced mechanical and corrosion properties. The effect of different milling conditions on the microstructure evolution during mechanical alloying of Mg-type material is studied. The properties of Mg-based nanomaterials have not been previously investigated.

2. EXPERIMENTAL

Mechanical alloying was performed under an inert argon atmos-phere using a SPEX 8000 Mixer Mill. The starting materials includ-ed elemental powders of magnesium (Alfa Aesar, 44 μm, 99.8%),

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zinc (Sigma Aldrich, 149÷595 μm, 99%), manganese (Sigma Al-drich, 44 μm, 99%), zirconium (Ciech Poland, 350 μm, 95.0% pu-rity). Powders of yttrium and dysprosium (about 100 μm in size) were made from the bulk samples (99.9% purity, Alfa Aesar) by filing in argon atmosphere. The vial was loaded and unloaded in a Labmaster 130 glove box under a high-purity argon atmosphere. Round bottom stainless vials were used. A weight ratio of hard steel balls (12 mm diameter) to powder weight ratio equaled 10:1, 20:1 and 40:1. The mixed powders were mechanically alloyed for 36 h. In order to prevent adhesion of the powder to the vial and dissipate the heat the milling process was stopped every 15 min (DM — dis-continuous milling).

The mixture containing MA Mg-type powders was uniaxially pressed at a compacting pressure of 400 MPa. The resulting green compacts were typically 8 mm in diameter and 3 mm in height. Pre-pared compacts were sintered under a protective argon atmosphere at 450°C for 2 h. The microstructure of the samples during different processing stages was investigated using a PANalytical Empyrean XRD with CuKα1 radiation. The crystallite size was calculated by using Scherrer method. Scanning Electron Microscopy (SEM) was used to characterize the synthesized materials. The TEM im-ages were recorded using a Philips CM 20 Super Twin microscope, which provides a 0.24 nm resolution at an acceleration voltage of 200 kV. The Vickers microhardness of the bulk samples was meas-ured using a microhardness tester by applying a load of 300 G for 10 s on the polished surfaces of the samples. Young’s modulus (in-dentation modulus — EIT) was evaluated using a CSM Instruments nanoindenter with a Berkovich diamond tip [14]. The Solartron 1285 potentiostat was applied for corrosion measurements in 0.1 M NaCl solution. The corrosion potentials (EC) and corrosion current densities (IC) were estimated from the Tafel extrapolations of the corrosion curves, using CorrView software.

3. RESULTS AND DISSCUSION

The Mg-based alloys were prepared by mechanical alloying tech-nique. X-ray diffraction was employed to examine the effects of milling process on the produced Mg samples. During the MA pro-cess, the intensity of the Mg Bragg reflections decreases with in-creasing milling time. The microstructure that forms during MA consists of layers of the starting material. The lamellar structure is increasingly refined during further mechanical alloying.

The XRD patterns in Figure 1 reveal the microstructure evo-lution of Mg powder during DM milling at ambient temperature for 8 h. The development of a pronounced broadened peaks at 2θ = 25÷75° already after 8 h of DM is typical for the formation of phases with nanometer-sized crystallites and high defect concentra-tion in the lattice — from 0.242° for (101) line before milling to 0.415° after 8 h of milling (Fig. 2). The fraction of the Mg powder is oxidized during the mechanical alloying process. After 8 h of DM MgO-phase is visible. SEM investigations revealed that the milled Mg and Mg-1Zn-1Mn-0.3Zr powders for 8 h with BPR = 10:1 con-sists of many small heavily deformed and welded together particles with sizes up to 100 μm (Fig. 3), whereas powders prepared with BPR = 40:1 have a quite broad particle size distribution ranging from a few micrometer to approximately 10 μm. For DM for 36 h Mg-4Y-5.5Dy-0.5Zr and Mg-1Zn-1Mn-0.3Zr alloy powders, ac-cording to the Scherrer method for XRD profiles, the average size of crystallites was of the order of 24 and 73 nm, respectively. Figure 4 shows the TEM micrograph of the mechanically alloyed for 36 h and annealed at 450°C for 2 h Mg-1Zn-1Mn-0.3Zr powdered sam-ple. Nanoparticles with the size below 50 nm on average are shown.

The formation of the bulk samples was achieved by annealing the MA powders in a high purity argon atmosphere (99.99%) at 450°C for 2 h. The XRD analysis of the bulk Mg-4Y-5.5Dy-0.5Zr sample revealed the presence of hexagonal type structure (a = 3.210 Å and c = 5.210 Å) with some content of MgO phase. The high deforma-tion during the MA leads to the large dislocation density and forma-

Fig. 1. XRD pattern of Mg after 0 h (a), 1 h (b) and 8 h (c) of the MA processRys. 1. Widma rentgenowskie Mg po 0 h (a), 1 h (b), 8 h (c) mechanicznej syntezy

Fig. 2. Changes of crystallite sizes of DM magnesium with BPR 40:1Rys. 2. Zmiana wielkości krystalitów Mg w czasie DM dla BPR 40:1

tion of the nanometer size subgrains. Nanograined materials with a very high number of atoms on the surface have a large surface energy. Thus they exhibit entirely different behaviour compared to micron sized grains.

The Vickers hardnesses for the bulk nanostructured Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr alloys reached 88 and 90 HV0.3, respectively, and are almost two times greater than that of pure microcrystalline Mg metal (50 HV0.3), (Tab. 1). This ef-fect is directly associated with structure refinement and obtaining a nanostructure. The indentation modulus EIT was measured, too (Tab. 1). The higher average values of EIT (51.59 and 51.48 GPa) characterize the Mg with ultrafine grains and nanostructured Mg-4Y-5.5Dy-0.5Zr alloy, respectively.

Potentiodynamic polarization curves of microcrystalline mag-nesium and bulk Mg-1Zn-1Mn-0.3Zr alloy with nanostructure are shown in Figure 5 (Tab. 2). The corrosion potential EC of

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Addition of high amounts of RE elements shifts corrosion poten-tial to the less negative value. The existence of RE accelerates the kinetics of MgH2 formation, which prevents further dissolution of magnesium [15].

4. CONCLUSIONS

Nanostructured alloy powders alloy of the nominal compositions Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr were produced by mechanical alloying. Alloying conditions such as BPR and milling time affects the microstructure and the morphology of powders. Upon changed milling conditions, Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr al-loys with different crystal sizes were obtained. Mechanical alloying in a SPEX shaker mill at ambient temperature results in formation of a major fraction of a nanocrystalline α-Mg phase. Optimal BPR for milled magnesium-based materials reach 40:1 with combina-tion of discontinuous milling (15 min milling and 15 min break). Upon sintering bulk Mg-based samples with hexagonal phase were produced. An enhancement of the properties due to the nanoscale structure in a consolidated bulk Mg-based alloys was observed. The bulk Mg-4Y-5.5Dy-0.5Zr, Mg-1Zn-1Mn-0.3Zr nanostructured alloys would offer new structural and functional properties for in-novative products in technical applications.

ACKNOWLEDGEMENT

The work was financed by National Science Centre Poland un-der the decision No. DEC-2013/11/B/ST8/04394.

Fig. 3. Morphology of powders after 8 h DM of Mg for BPR = 10:1 (a)

and BPR = 40:1 (b) and Mg-1Zn-1Mn-0.3Zr for BPR = 10:1 (c) and BPR = 40:1 (d)Rys. 3. Morfologia proszku Mg po 8 h DM dla BPR = 10:1 (a) i BPR = 40:1 (b) oraz Mg-1Zn-1Mn-0.3Zr dla BPR = 10:1 (c) i BPR = 40:1 (d)

Fig. 4. TEM image of Mg-1Zn-1Mn-0.3Zr alloy powderRys. 4. Obraz TEM proszku Mg-1Zn-1Mn-0,3Zr

Table 1. Crystal sizes, microhardness and Young modulus of studied samplesTabela 1. Wielkość ziaren, mikrotwardość i moduł Younga badanych materiałów

Sample d, nm HV0.3 EIT, GPa

Mg-4Y-5.5Dy-0.5Zr 24 88±4 51±2

Mg-1Zn-1Mn-0.3Zr 73 90±8 48±3

Mg (ultra fine grain) 150 91±2 52±3

Mg (microcrystalline) 40 000 50±3 38±7

Fig. 5. Potentiodynamic curves of sintered Mg-1Zn-1Mn-0.3Zr alloy

(a) and microcrystalline Mg (b)Rys. 5. Krzywe potencjodynamiczne dla spiekanego stopu Mg-1Zn-1Mn-0.3Zr (a) i mikrokrystalicznego Mg (b)

Table 2. Mean values of corrosion current densities and corrosion po-tentials of studied samplesTabela 2. Średnia wartość prądów i potencjałów korozyjnych badanych materiałów

Sample IC, A/cm2 EC, vs SCE V

Mg-4Y-5.5Dy-0.5Zr 4.838·10–4 –1.555

Mg-1Zn-1Mn-0.3Zr 1.620·10–4 –1.777

Mg (ultra fine grain) 1.970·10–4 –1.660

Mg-4Y-5.5Dy-0.5Zr alloy shifted to the nobler direction. Corrosion current density IC decreases from 1.97·10–4 to 1.62·10–4 A·cm–2, which shows a good corrosion resistance of the nanostructured Mg-1Zn-1Mn-0.3Zr sample compared with microcrystalline Mg.

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