Indentation power-law creep of high-purity indium
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I.
INTRODUCTION
THE techniques for the measurement of time-dependent phenomena can be conveniently divided into two classes: (1) broad-band, quasistatic, or creep techniques, where the load, stress, or strain rate is held constant for a period of time while the response of the material is measured; and (2) frequency-specific dynamic techniques, where the load or stress is varied at a single frequency and the response of the material is measured. Although standard bulk-testing techniques exist for these types of measurements, in many of today’s technologies, the volume of material of interest may be on such a scale that these techniques become impractical. While, in some instances, specimens may be prepared in a form that allows testing with modified uniaxial techniques, these preparation processes are often tedious or may very well alter those properties that are of interest. It then becomes necessary to find an alternate means for mechanical characterization. In this work, a variation of the broad-band, quasistatic technique described earlier will be utilized to measure time-dependent plasticity or indentation creep at several temperatures. The goal of this study of indentation creep is to explore the dependency of the indentation hardness of high-purity indium on the variables of strain rate and temperature, as well as to investigate the existence of a steady-state behavior in indentation creep using a Berkovich indenter. II.
REVIEW OF DEPTH-SENSING INDENTATION CREEP LITERATURE
Since the flow stress of a material is a function of strain rate and temperature, the hardness of a material should be expected to vary in an analogous way.[1] This fact has led to numerous studies attempting to correlate hardness with the variables of time (or indentation strain rate) and temperature. A brief survey of this body of work follows.
B.N. LUCAS, Director of Analytical Services, and W.C. OLIVER, Vice President, are with the Nano Instruments Innovation Center, MTS Systems Corporation, Oak Ridge, TN 37830. Manuscript submitted April 27, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
A. Effect of Stress (Stress Exponent for Creep) There are four types of tests that have been employed using depth-sensing indentation systems to gain insight into the relationship between indentation strain rate and hardness: indentation load relaxation (ILR) tests,[2,3] constant rate of loading (CRL) tests,[4,5] constant-load indentation creep tests,[6–11] and impression creep tests.[12,13] All of these tests have drawn analogies between hardness and flow strength, as well as between the uniaxial strain rate and the indentation strain rate, i.e., H 5 C1 s
[1]
«z u 5 C2 «z i
[2]
and
In an indentation test, the dynamics of deformation are very different than those occurring in the previously described uniaxial creep test. The deformed volume of material under the indenter is continually expanding to encompass previously undeformed material. As the material strains under the indenter, the material underneath the indenter is very often like
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