Heating and structural disordering effects of the nonlinear viscous flow in a Zr55Al10Ni5Cu30 bulk metallic glass

Heat evolution of stress-induced structural disorder, ΔH s (e), of a Zr 55 Al 10 Ni 5 Cu 30 bulk metallic glass (BMG) during constant ram-velocity deformation at the glass transition region ( T g = 680 K) was deduced from in-situ measurements of temperature change of the deforming sample. At the transition from the linear to nonlinear viscoelasticity, the behavior of viscosity change with strain, η(e), is qualitatively consistent with the enthalpy evolution of the structural disordering, ΔH s (e), but not with the temperature change, Δ T (e). It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, -log n ≡ −log η/η N , is found to be proportional to the ΔH s and the slope of ΔH s (–logn) can be estimated to ∼ 400 J/mol, where η N is the Newtonian viscosity. On the other hand, the temperature raise, ΔT (e), is pronouncedly delayed as compared with the η (e) and ΔH s (e) at the transition, but is determined by the product of stress and plastic strain-rate, σ·e p , and is nearly proportional to it at the steady-state. The slope of ΔT (σ·e p ) can be estimated to 5.2×10 -2 K mol/W.