Cellular microstructure and plasticity of lead at high temperature

Abstract

These experiments were designed to test some aspects of a recent theory of plastic flow in strain hardened metals, in which thedistribution of dislocations in the hardened micro-structure is a principal variable. In particular, the theory predicts that this distribution determines the shape of the initial portion of the high strain rate stress-strain curve (ideally, the time-independent curve). Abrupt yielding is associated with aregular distribution of dislocations and gradual yielding with anonregular distribution. Thus, from measurement of this curve at any instant, say during high temperature creep deformation, a distribution parametert _v and a dislocation density parameter T_y can be determined. The strain measurements were made on a recording oscilloscope following the application of small, high-rate stress pulses to lead samples suffering creep at various high temperatures. Analysis of the results indicates that T_v/T_y increases strongly with increasing creep temperature. That is, the structure is less regular at high temperature or equiva-lently, as a preliminary structural study suggests, more cellular. In turn, this increasing cellularity may correlate with the increasing temperature and strain rate sensitivity of the flow stress at high temperature, previously measured. The theory predicts this correlation.