Softening and heating behaviors during the nonlinear viscous flow in a Zr-based bulk metallic glass

Heat evolution of stress-induced structural disorder ΔH_s(ε) of a Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass (BMG) during compressive constant ram-velocity deformation in the glass transition region (T_g = 680 K) was deduced from in situ measurements of temperature change ΔT(ε) of deforming samples, where ε is true strain. At transition from the linear to nonlinear viscoelasticity, i.e., a softening phenomenon, viscosity η(ε) does not change consistently with ΔT(ε) but with ΔH_s(ε). Therefore, it is concluded that the stress-induced structural disorder triggered this softening phenomenon. The viscous frictional heating of the softened alloy brings about the increase in ΔT, which also influences ΔH_s. Because ΔT is determined by relationship between the power of the frictional heating and heat convection to the sample-surroundings, ΔH_s and η can be positively controlled with varying thermal conductivity D of the surrounding materials, especially after the transition period.