High-strain-rate superplasticity due to Newtonian viscous flow in La55Al25Ni20 metallic glass

We have investigated the deformation behavior of a La55Al25Ni20 (at%) metallic glass that has a wide supercooled liquid region of 72 K before crystallization. The glassy solid below the glass transition temperature exhibited non-Newtonian viscosity, and the supercooled liquid revealed a Newtonian viscosity that transferred to the non-Newtonian viscosity with increasing strain rate. The supercooled liquid exhibited a high-strain-rate superplasticity due to the Newtonian viscous flow that has a strain-rate sensitivity exponent (m value) of unity. The metallic glass exhibited large elongations of more than 1000% at strain rates ranging from 10-4 to 100 s-1 and at relatively low temperatures of about 0.7 Tm, and retained the ductile nature without crystallization even after the deformation. The maximum elongation to failure was about 1800% at a strain rate of 1.7 × 10-1 s-1 and at 503 K (0.71 Tm) under a flow stress of about 40 MPa. The elongation was restricted by the transition to non-Newtonian viscosity and crystallization. We succeeded in establishing the constitutive formulation of the flow stress in the supercooled liquid region. Its formulation was expressed very well by a stretched exponential function σflow = D qq exp (H*/RT) [1 - exp(E/{qq exp(H**/RT) }0.82)]. The superplasticity of the La55Al25Ni20 metallic glass was superior to that of the Zr65Al10Ni10Cu15 metallic glass. The metallic glass, moreover, had many advantages in the superplastic deformation, as compared with superplastic polycrystalline materials.