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

Heat evolution of stress-induced structural disorder ΔHs(ε) of a Zr55Al10Ni5Cu30 bulk metallic glass (BMG) during compressive constant ram-velocity deformation in the glass transition region (Tg = 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 ΔHs(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 ΔHs. Because ΔT is determined by relationship between the power of the frictional heating and heat convection to the sample-surroundings, ΔHs and η can be positively controlled with varying thermal conductivity D of the surrounding materials, especially after the transition period.