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(ε), of a Zr₅₅Al₁₀Ni₅Cu 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(ε), is qualitatively consistent with the enthalpy evolution of the structural disordering, ΔH_s(ε), but not with the temperature change, ΔT(ε). It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, -logη̃ -logη/η_N, is found to be proportional to the ΔH_s and the slope of ΔH_s(-logη̃) can be estimated to ∼ 400 J/mol, where η_N is the Newtonian viscosity. On the other hand, the temperature raise, ΔT(ε), is pronouncedly delayed as compared with the η(ε) and ΔH_s(ε) at the transition, but is determined by the product of stress and plastic strain-rate, σ· ε̇_p, and is nearly proportional to it at the steady-state. The slope of ΔT(σ· ε̇_p) can be estimated to 5.2×10^-2 K mol/W.