Influence of grain size on microstructure, mechanical properties and strain hardening behavior of Mg-0.3Ca (wt.%) alloy

Abstract

Mg-0.3Ca (X03, wt.%) binary alloy with different grain sizes were obtained by low-temperature extrusion and subsequent annealing heat treatment. The tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation to failure (EL) of the as-extruded X03 binary alloy with an ultra-fine grain size of 0.7 μm was 361 MPa, 370 MPa and 10.6%, respectively. The exceptional high TYS of 361 MPa was mainly due to the strengthening from ultra-fine dynamic recrystallized grains with segregation of Ca atoms at grain boundaries. The high energy barrier for dislocation emission from grain boundaries pinned by segregated Ca atoms increases the tensile stress required to activate dislocation nucleation. With increasing average grain sizes from 0.7 μm to 6.0 μm after annealing treatment, the TYS of the X03 alloy was decreased obviously from 361 MPa to 94 MPa, while the EL increases from 10.6% to 29.6%. Abnormal strain softening was observed during tensile test of the fine-grained samples with grain size less than 2 μm. The mechanism of strain softening may be mainly caused by the decrease of <a> type dislocations density. The alloy with average grain size larger than 2 μm attains more space to store dislocations, thus shows obvious strain hardening, which contributes to high elongation to failure.