Threshold stresses in amorphous alloys—I. Flow

Abstract

The stress dependence of homogeneous flow in amorphous FeNiPB has been examined in tensile stress relaxation tests. The strain rate sensitivity m = [ d( lnγ)]/[ d( lnτ)] is found to be unity in an intermediate stress range and greater than unity at both high and low stress levels. The transition from linear stress-strain rate behavior ( m = 1) to a non-linear relationship ( m > 1) at high stress has previously been observed and explained by transition state theory. This new observation of linearity breakdown ( m > 1) at low stresses requires the introduction of a threshold stress and appropriate modification of the prior analysis. A physical interpretation of the stress threshold based on flow induced disordering will be discussed. The dependence of the threshold stress and the volume strain element on temperature and annealing time is presented. The implications of this new model on the transition from homogeneous to shear band flow is also discussed.