Grain size and void formation in Mg alloy AZ31

Highlights

• An explanation for how grain size impacts on the strain to failure of AZ31 alloy.

• Grain size controls void size, not rate of void nucleation.

• Void fraction at fracture is similar in coarse and fine-grained materials.

• Failure at twin boundary and second-phase particle are void nucleation mechanisms.

• Failure at second-phase particle becomes the dominant with grain size reduction.

Abstract

The present study examines void formation during tensile loading in coarse and fine-grained AZ31 magnesium alloy using tensile testing, scanning electron microscopy and micro X-ray tomography techniques. Reducing the grain size from 30 to 4.5 μm doubles the total tensile elongation. At failure, the fine-grained material displays a higher volumetric number density of voids compared to the coarse-grained counterpart. Large voids with lower sphericity are considerably more prevalent in the coarse-grained material. Depending on the grain size, two different void nucleation mechanisms could be distinguished; failure at twin or grain boundaries or at second-phase particles. The dominant mechanism in the fine-grained material is failure at second-phase particles. The void volume fractions at failure were comparable in both materials. We propose that the present results can be understood in terms of the effect of grain size on the rate at which the void fraction grows with strain. The larger voids formed in the large grain sized samples lead to a more rapid increase in void fraction with strain and thus failure ensues at lower strains.

Introduction

Magnesium and its alloys have higher specific bending stiffness and strength compared to many steel and aluminium alloys [1]. However, magnesium has poor low room-temperature formability due to a limited number of active slip systems in its hexagonal close packed (HCP) structure. This has restricted the use of magnesium in sheet-formed components [2]. To enhance the ductility of magnesium alloys, both grain refinement and weakening of the texture have shown promise [[2], [3], [4], [5], [6], [7], [8], [9]]. Grain refinement is attractive because it simultaneously provides strength and ductility improvement [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. The increased ductility has been associated with a reduced tendency for twinning [17,18] and an increased activation of non-basal slip systems [19,20]. However, the mechanisms by which grain refinement improves ductility remain unclear, particularly in relation to development of voids and damage accumulation during straining.

It has been reported that in magnesium alloys, the tensile fracture surface is frequently characterized by microvoids [17,21] or micro-cracks [22] as well as macroscopic shear localizations. In the absence of second-phases, void nucleation occurs at twin and grain boundaries in commercially pure magnesium [23]. Even in magnesium alloys containing second-phase particles, twin-sized voids have been observed inside twin-like regions [17,22,24,25]. There is a strong belief that double twining plays an important role as nucleation sites of voids [3,17,21,22], since this leads to local regions highly susceptible to basal slip. Second-phase particles also have been reported to be an important void nucleation site in magnesium alloys [22,[24], [25], [26], [27], [28]]. In Ref. [25] the authors investigated the impact of stress triaxiality on void nucleation in AZ31 and found that the contribution of second-phase particles in damage formation increased with stress triaxiality [25]. However, the impact of grain size on void formation was not investigated and remains unclear.

The present study aims to understand the correlation between tensile elongation and void formation in magnesium alloys by evaluating the differences in failure behaviour between samples of coarse and fine-grained alloy AZ31.