Division of the hardness of molybdenum into rate-dependent and rate-independent components

Abstract

The hardness, H, and rate sensitivity of the hardness, m = ∂ ln H/∂ ln ∊eff|xp, where ∊eff is an effective strain rate and xp the plastic depth, are measured in molybdenum at room and low temperature (160 and 170 K) using as-received and annealed specimens. Based on these measurements it is found that H separates into two components: one depending on indentation rate and temperature, and the other depending on the starting state of the material. An activation volume is defined, v∗ = 9kT/mH, which falls within the range of values derived from other experimental techniques. The values of m obtained from indentation creep, indentation load relaxation, and indentation rate-change experiments agree closely with each other provided a consistent analysis is used. The results of these experiments suggest that the rate- and temperature-dependence of the hardness can be used to discriminate between strengthening mechanisms at low temperature.