Strain-rate history effects in polycrystalline molybdenum and niobium

Abstract

Specimens of annealed polycrystalline molybdenum and niobium have been strained in compression at temperatures of 200, 250 and 292 K, the strain rate being suddenly increased during plastic flow at a strain of about 8%. The initial strain rate was varied between 1.7 × 10 −4 and 0.5 s −1 and the strain-rate ratio was varied between 3 and 6 × 10 5, the maximum strain rate being about 100 s −1. In each test the applied stress and the incremental stress caused by the strain-rate change were recorded as functions of time. The results are compared with those of earlier tests conducted at constant strain rates, and it is shown that for both materials the flow stress is dependent on strain-rate history. An approximate analysis is made of the heat loss during deformation at constant strain rate, so that corrections can be made for the temperature rises caused by plastic work. After making these corrections, it is found that significant differences (up to an order of magnitude) exist between the strain rates obtained at a given stress level, depending on the previous strain history. These differences vary in a complex manner with the initial and final strain rates, and it seems possible that a change in the rate-controlling process occurs between the low and high rates in the range covered.