High mechanical strength and good plasticity of ZrAlNiCu bulk glassy alloys containing icosahedral or bcc β-Zr plus hexagonal ω-Zr phases prepared by annealing

Abstract

Zr-rich Zr_70‒74Al_7‒7.5Ni_17‒20Cu_2‒2.5 bulk metallic glasses (BMGs) with high yield strength (σ_y) of 1261–1316 MPa and large plastic strains of 8.4–11.3 % were formed in the as-cast state. The phase transition of these BMGs upon heating occurs through two stages of G → G + IQ → Zr₂Ni + Zr₅Al₃ for the 70–72 %Zr alloys and three stages of G → G + β-Zr + ω-Zr → G + β-Zr + ω-Zr + Zr₂Ni → α-Zr + Zr₂Ni + Zr₅Al₃ for the 74 %Zr alloy. The partially transformed rods exhibit σ_y and plastic strain of 1318 MPa and 1.3 % for the 70 %Zr alloy and 1411 MPa and 3.9 % for the 74 %Zr alloy, respectively. It is noticed that the mixed phase alloys keep good plasticity and exhibit higher σ_y. These features are different from the previous results that the annealing-induced mixed phase alloys become always brittle and exhibit lower σ_y without plastic strain. The novel improvement of mechanical properties is due to the combination of homogeneous dispersion of β-Zr and ω-Zr phases with a size of 15–20 nm, good plasticity of β-Zr and ω-Zr phases, the existence of high density of faults in ω-Zr phase, and good plasticity of the residual glassy phase with Zr-rich compositions. The distinct increase in σ_y for the G + crystal composites is opposite to all the previous results where σ_y decreases significantly by the coexistence of crystalline phases. The present novel results are encouraging for future extension of application fields of BMGs.