Effect of hierarchical multimodal microstructure evolution on tensile properties and fracture toughness of rapidly solidified Mg–Zn–Y–Al alloys with LPSO phase
Soya Nishimoto, Yuta Koguchi, Michiaki Yamasaki, Yoshihito Kawamura
Abstract
High fracture toughness Mg96.75Zn0.85Y2.05Al0.35 alloys featuring a multimodal microstructure were developed by rapidly solidified (RS) ribbon-consolidation processing. A non-heat-treated alloy that was prepared by consolidating the as-quenched ribbons exhibited a high tensile yield strength of ∼500 MPa, an elongation of ∼5%, and a plane-strain fracture toughness (KIc) of ∼9.5 MPa m1/2. Microstructure of the non-heat-treated alloy consists of two matrices: coarse-worked grains with a high kernel average misorientation (KAM) (group 1) and ultra-fine dynamically recrystallized (DRXed) grains with an intermediate KAM (group 2). The formation of ultra-fine DRXed grains with the precipitation of fine LPSO phase strengthens the alloy, but causes work-softening. The heat-treated alloy that was prepared by consolidating the RS ribbons heat-treated at 738 K exhibited a good balance of a high KIc (∼15 MPa m1/2), a reasonable yield strength (∼400 MPa), and a large elongation (∼12%) with a positive work-hardening rate. The optimized pre-consolidation heat-treatment produces multimodal microstructure having three matrices; a third group of the fine DRXed grains with a low KAM was formed in addition to groups 1 and 2. The formation of hierarchical multimodal structure in the grain size and KAM evokes strain-hardening, which has a positive effect on the fracture toughness.