Strain-Rate and Temperature Dependences of Deformation Behavior of AZ61Mg Alloy Processed by Multi-directional Forging U

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I.

INTRODUCTION

MAGNESIUM (Mg) alloys have great potential as structural materials owing to their high specific strength. Mg alloys are the lightest metals for practical use. The practical applications of Mg alloys have been, however, limited because of their low actual strength and poor ductility due to their hexagonal-close-packed crystal structure. Numerous numbers of studies have been carried out to improve the mechanical properties of Mg alloys. One of the most promising approaches is to enhance their ductility and strength through grain refinement. To obtain an ultrafine-grained (UFGed) structure (grain size in the submicron range), various

C. WATANABE and R. MONZEN are with the Faculty of Mechanical Engineering, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, 920-1192, Japan. Contact e-mail: chihiro@ se.kanazawa-u.ac.jp R. UEJI is with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan. H. MIURA is with the Department of Mechanical Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi, 441-8580, Japan. Manuscript submitted April 10, 2017. Article published online August 28, 2017 5368—VOLUME 48A, NOVEMBER 2017

severe plastic deformation methods have been applied to Mg alloys.[1–4] Miura and colleagues reported that UFGed AZ31Mg,[5,6] AZ61Mg[6,7] and AZ91Mg[6] alloys produced by multi-directional forging (MDFing) exhibited extremely high strength with reasonable ductility even at ambient temperature. Strength-ductility balances of the UFGed AZ series Mg alloys fabricated by MDFing are comparable to those of precipitation-hardened 2000 series Al alloys. In spite of such attractive mechanical properties of the UFGed Mg alloys, their nature has not been widely investigated yet. Recently, several investigators found that the strainrate dependence of flow stress of various FCC metals became stronger as grain size decreased.[8–11] If this is also the case for HCP Mg alloys, UFGed Mg alloys are really attractive as structural materials for transportation equipment because of their high-energy absorption in dynamic straining together with their superior specific strength. In the present study, AZ61Mg alloy specimens with different grain sizes processed by MDFing under decreasing temperature conditions were tensile-tested under various strain rates from 105 to 103 s1 at some temperatures to examine the strain rate and temperature dependences of deformation behavior.

METALLURGICAL AND MATERIALS TRANSACTIONS A

II.

EXPERIMENTAL

Rods of a commercial hot-extruded AZ61Mg alloy were annealed at 773 K (500 C) for 2 hours to obtain a homogeneous microstructure and an average grain size of approximately 20 lm. The annealed rods were cut into rectangular pieces with a dimension of 31 9 21 9 14 mm3 (aspect ratio of 2.22:1.49:1). The pieces were then processed by multi-directional forging (MDFing) under decreasing temperature conditions. The MDFing processing was conducted using an Amsler-type universal testing machine at an initial strain rate of 103 s