Enhancing tensile/compressive response of magnesium alloy AZ31 by integrating with Al2O3 nanoparticles

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Abstract

AZ31 nanocomposite containing Al2O3 nanoparticle reinforcement was fabricated using solidification processing followed by hot extrusion. The Al2O3 nanoparticle reinforcement was isolated prior to melting by wrapping in Al foil of minimal weight (<0.50 wt% with respect to AZ31 matrix weight). The AZ31 nanocomposite exhibited slightly smaller grain and intermetallic particle sizes than monolithic AZ31, reasonable Al2O3 nanoparticle distribution, non-dominant (0 0 0 2) texture in the longitudinal direction unlike monolithic AZ31, and 30% higher hardness than monolithic AZ31. Compared to monolithic AZ31, the AZ31 nanocomposite exhibited higher 0.2%TYS, UTS, failure strain and work of fracture (WOF) (+19%, +21%, +113% and +162%, respectively). Also, compared to monolithic AZ31, the AZ31 nanocomposite exhibited higher 0.2%CYS and UCS, similar failure strain, and higher WOF (+5%, +5%, −4% and +11%, respectively). Inclusive of crystallographic texture changes, the effect of Al2O3 nanoparticle integration on the enhancement of tensile and compressive properties of AZ31 is investigated in this paper.

Introduction

Magnesium and aluminium are commonly used light metals in weight-critical structural applications (for example, automotive and aerospace). Mg is about 35% lighter than Al and both have similar melting points and strengths. Mg has the disadvantage of limited ductility attributed to its HCP structure, while Al is more ductile given its FCC structure. Also, Mg has a lower elastic modulus (40–45 GPa) than Al (69.6 GPa) [1]. Traditional alloying can be used to increase strength and ductility of Mg [2]. Additionally, based on the use of discontinuous reinforcement, many properties of Mg have been improved beyond the limits of alloying [3]. In recent years, three methods that have been tried to improve the strength, ductility and modulus of Mg are: (a) use of various oxide nanoparticles as well as carbon nanotubes for improving strength and ductility [4], [5], [6], (b) use of metallic particles such as Ti and Mo for improving ductility [7], [8], [9] and (c) use of micron size ceramic particulates for improving strength and modulus [10], [11]. AZ31 is a very commonly used Al-containing (or Zr-free) Mg alloy in the world today. It is characterized by: (a) low cost, (b) ease of handling, (c) good strength and ductility and (d) resistance to atmospheric corrosion. Recently, AZ31 has been surface-reinforced with SiC microparticulates [12], C60 molecules [13], and multi-walled carbon nanotubes [14], using the friction stir processing technique. In these studies, good dispersion and hardening of the base matrix at the surface were reported. Similar findings along with grain refinement were also reported for AZ31 reinforced with SiC and B4C microparticulates using gas–tungsten arc (GTA) with simultaneous reinforcement powder feeding processing technique [15], [16], [17]. Defect-free and adherent particle–matrix interface has been reported in the AZ31/SiC microcomposite [16], [17]. TiNi shape memory alloy (SMA) fibers have been incorporated in AZ31 matrix without significant interfacial reaction using pulsed current hot pressing (PCHP) [18]. The yield stress and elongation in the AZ31/TiNi microcomposite increased with temperature (strength significantly exceeded that of AZ31 matrix), as a consequence of residual compressive stress in the AZ31 matrix due to phase change induced shrinkage of the TiNi fiber. Recently, researchers added Al2O3 nanoparticles to AZ31 using disintegrated melt deposition (DMD) [19], [20]. Here, the tensile ductility and strength of AZ31 were significantly increased and compromised, respectively [21]. However, open literature search has revealed that no successful attempt has been made to simultaneously increase tensile strength and ductility of AZ31 with Al2O3 or any other nanoparticles, using a high volume production spray-deposition based solidification processing technique.

Accordingly, one of the primary aims of this study was to simultaneously increase tensile strength and ductility of AZ31 with Al2O3 nanoparticles. Another aim of the present study was to evaluate the compressive properties of AZ31/Al2O3 nanocomposite considering end applications requiring compressive loading. DMD followed by hot extrusion was used to synthesize the AZ31/Al2O3 nanocomposite.

Section snippets

Materials

In this study, AZ31 rod (nominally 2.50–3.50 wt%Al, 0.60–1.40 wt%Zn, 0.15–0.40 wt%Mn, 0.10 wt%Si, 0.05 wt%Cu, 0.01 wt%Fe, 0.01 wt%Ni, balance Mg) supplied by Alfa Aesar (Massachusetts, USA) was used as the matrix material. AZ31 rod was sectioned to shorter pieces. All oxide and scale surfaces were removed using machining. All surfaces were washed with ethanol after machining. Al2O3 nanoparticles (50 nm size) supplied by Baikowski (Japan) was used as the reinforcement phase.

Primary processing

Monolithic AZ31 was cast

Macrostructural characteristics

No macropores or shrinkage cavities were observed in the cast monolithic and nanocomposite materials. No macrostructural defects were observed for extruded rods of monolithic and nanocomposite materials.

Microstructural characteristics

Microstructural analysis results revealed that average grain size reduced while aspect ratio increased in the case of nanocomposite as shown in Table 1 and Fig. 2a and b. Intermetallic particle size and roundness ratio were smaller in the nanocomposite than in the monolithic material. X-ray

Synthesis of monolithic AZ31 and AZ31/Al2O3 nanocomposite

Synthesis of monolithic and nanocomposite materials, the final form being extruded rods, was successfully accomplished with: (a) no detectable metal oxidation, (b) no detectable reaction between graphite crucible and melts. The inert atmosphere used during DMD was effective in preventing oxidation of the Mg melt. No stable carbides of Mg or Al formed due to reaction with graphite crucible.

Microstructural characteristics

Microstructural characterization of extruded samples is discussed in terms of: (a) grain and intermetallic

Conclusions

  • 1.

    Monolithic AZ31 and AZ31/1.5 vol%Al2O3 nanocomposite can be successfully synthesized using the DMD technique followed by hot extrusion.

  • 2.

    Compared to monolithic AZ31, tensile strength of AZ31/1.5 vol%Al2O3 was enhanced. Compared to monolithic AZ31, compressive strength of AZ31/1.5 vol%Al2O3 was slightly increased. This can be commonly attributed first to AZ31/1.5 vol%Al2O3 not exhibiting (0 0 0 2) dominant texture in the longitudinal direction, unlike monolithic AZ31.

  • 3.

    Compared to monolithic AZ31, tensile

Acknowledgement

Authors wish to acknowledge NUS research scholarship for PhD candidature of M. Paramsothy for supporting this research.

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