Microstructure and mechanical properties of multi-directionally forged AZ91 magnesium alloy

Abstract

AZ91 magnesium alloy was multi-directionally forged (MDFed) under decreasing temperature condition from 673 to 413 K up to cumulative strain of ΣΔε=6.4 at a strain rate of 3×10−3 s−1. The average grain size decreased gradually with cumulative strain. After straining to ΣΔε=6.4 (i.e., after 8 passes of MDF) , equi-axed ultrafine grains (UFGs) with average size of about 0.45 μm were uniformly evolved. The hardness increased gradually up to 1.2 GPa with decreasing grain size. While the yield stress was also raised up to 480 MPa, the ductility was not so changed, i.e., 20% at max and about 10% in average. The total strain to fracture in compression was slightly poorer than that obtained by tensile test. The achieved high strength of the MDFed AZ91 magnesium alloy seemed to be induced by strengthening by the effects of grain refinement and distribution of fine precipitates. However, it is revealed that the relatively lower ductility of the UFGed AZ91 magnesium alloy compared with UFGed AZ31, AZ61 magnesium alloys should be induced by coarse precipitates remained.