ＡＺ９１マグネシウム合金の機械的性質に及ぼす凝固組織の影響

Abstract

An AZ91 alloy was preparded by a gravity casting process with either constant grain size or constant cooling rate. The as-cast alloys show good mechanical properties with decreasing dendrite arm spacing (DAS) or grain size. These phenomena are possibly explaned by the fact that cracks do not easily propagate in the brittle eutectic Mg17Al12 compounds due to the fine compounds. The ultimate tensile strength (UTS), 0.2% proof stress (PS) and elongation (Ef) at fracture of the T6 treated alloys increase with decreasing grain size. But the UTS and Ef of the alloy remain almost constant with decreasing DAS, while the PS increases with decreasing DAS. These phenomena in the T6 treated alloys are explained by assuming that (1) the stress concentration at a grain boundary is higher with larger grain size, (2) the density of grain boundaries which act as obstacles during crack propagation is higher with smaller grain size and (3) the homogenization is easily achieved in a shorter solution heat-treatment time with a small DAS.