Evolution of Ultra-Fine Grains in AZ31 and AZ61 Mg Alloys during Multi Directional Forging and Their Properties

H. Miura, X. Yang and T. Sakai
Department of Mechanical Engineering and Intelligent Systems, UEC Tokyo
(The University of Electro-Communications), Tokyo 182-8585, Japan

AZ31 and AZ61 Mg alloys were multi directionally forged (MDFed) during decreasing temperature from 623K to 423K to cumulative strain of SigmaDeltaepsilon=4.8 by Deltaepsilon=0.8 pass strain at a strain rate of 3 x 10-3 s-1. In both Mg alloys, the average grain size gradually decreased with increasing cumulative strain. After straining to SigmaDeltaepsilon=3.2, i.e., after 4 passes of MDF, ultra fine grained (UFG) microstructures with average grain size of 1 microm were uniformly evolved (Fig. 1). However, tendency of the grain refinement appeared more accelerated or enhanced in the AZ61 Mg alloy than in the AZ31one. By prolonged straining, the grains became further finer. The AZ61 Mg alloy MDFed to SigmaDeltaepsilon=4.0 showed quite high hardness over 1.2 GPa, while that of the AZ31 Mg alloy was 850 MPa at SigmaDeltaepsilon=4.8 (Fig. 2). In spite of the same grain size, AZ61 Mg alloy exhibited much higher strength and ductility compared with those of AZ31 Mg alloy. The thermal stability of the UFGed AZ61 alloy was much higher than that of the AZ31 Mg one (Fig. 3).

[Published in Materials Transactions, Vol. 49, No. 5 (2008), pp. 1015 1020.]

Fig. 1 Microstructure evolved by multi directional forging during decreasing temperature of AZ61 Mg alloy of (a) epsilon=0, (b) SigmaDeltaepsilon=1.6, (c) SigmaDeltaepsilon=3.2, and of AZ31 Mg alloy of (d) epsilon=0, (e) SigmaDeltaepsilon=1.6, (f) SigmaDeltaepsilon=3.2.

Fig. 2 Room temperature hardness change depending on cumulative strain in the AZ31 and 61 Mg alloys processed by multi directional forging. 

Fig. 3 Microstructural change by annealing test at 473 K fort various periods of time of the AZ31 and AZ61 Mg alloys multi directionally forged (MDFed) to SigmaDeltaepsilon=3.2: AZ31Mg (a) as MDFed and (b) annealed at 473 K for 103 s, AZ61 Mg alloy (c) as MDFed and (d) annealed at 473 K for 9 x 102 s and further at 523 K for 9 x 102 s.