Division of the hardness of molybdenum into rate-dependent and rate-independent components
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K. B. Yoder Materials Science Program, The University of Wisconsin at Madison, 1509 University Avenue, Madison, Wisconsin 53706 (Received 13 December 1993; accepted 17 June 1994)
The hardness, H, and rate sensitivity of the hardness, m = d In H/d In eeff \Xp, where eeff 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 denned, 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.
I. INTRODUCTION A classical approach for investigating deformation mechanisms is to examine them from a kinetic perspective, exploring the effects of strain rate and temperature on the flow stress. The flow stress is measured under a simple state of loading such as tension, compression, or shear. With hard materials, where such an approach becomes impractical due to the onset of fracture, the effects of deformation rate and temperature on the hardness (e.g., Meyer hardness) might be used instead. To explore this possibility, we have constructed a high-resolution, load- and depth-sensing indentation tester capable of operating throughout a range of temperatures centered about room. 12 Ultimately, the purpose of this device is to help us explore mechanisms of plastic flow in hard materials at low homologous temperatures, especially hard materials deposited as thin films. Indentation tests have been shown to provide meaningful power law exponents and activation energies for high-temperature creep,3"8 but only qualitative comparisons have been demonstrated in the case of materials at low homologous temperatures.9'10 Therefore, in this article, we explore the effects of temperature and indentation rate on the hardness of a bcc refractory metal, molybdenum, in order to examine more carefully the degree to which these effects carry useful information. Molybdenum is chosen as a model material because its flow behavior at low temperatures, as identified based on experiments performed under uniaxial loading, has long been familiar to materials scientists. Recently, the2524
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J. Mater. Res., Vol. 9, No. 10, Oct 1994
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oretical treatments of the time-dependent response of materials to indentation at low temperature have been presented, 1112 including the possibility that transport of matter away from the indenter via surface diffusion becomes important if the indents
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