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

The heat evolution of stress-induced structural disorder, DeltaH(s)(epsilon), of a Zr55Al10Ni5Cu30 bulk metallic glass (BMG) during compressive 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, eta(epsilon), is qualitatively consistent with the enthalpy evolution of the structural disordering, DeltaH(s)(epsilon), but not with the temperature change, DeltaT(epsilon). It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, -log eta equivalent to-log eta /eta(N), is found to be proportional to the DeltaH(s) and the slope of DeltaH(s)(-log eta) can be estimated to 400 J/mol, where eta(N) is the Newtonian viscosity. On the other hand, the temperature raise, DeltaT(epsilon), is pronouncedly delayed as compared with the eta (epsilon) and DeltaH(s)(epsilon) at the transition, but is determined by a product of stress and plastic strain-rate, sigma.epsilon(p), and is nearly proportional to it at the steady state. The slope of DeltaT(sigma.epsilon(p)) can be estimated to 5.2x10(-2) K mol/W. (C) 2003 American Institute of Physics.