DIVISION OF THE HARDNESS OF MOLYBDENUM INTO RATE-DEPENDENT AND RATE-INDEPENDENT COMPONENTS

The hardness, H, and rate sensitivity of the hardness, m = partial derivative ln H/partial derivative ln epsilon(eff)\x(p), where epsilon(eff) is an effective strain rate and x(p) 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 rn 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.