Time Dependent Indentation Testing At Non-Ambient Temperatures Utilizing the High Temperature Mechanical Properties Micr
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ABSTRACT Time dependent indentation data for pure indium from -100 'C to 75 'C is presented. The properties reported include hardness, indentation strain rate, stress exponent and apparent activation energy for creep. These properties were measured using a depth-sensing indentation system capable of performing experiments between -100 TC and 300 'C in ultra-high vacuum. In addition, by employing laser interferometric techniques, this system can obtain displacement data with time constants as low as 50 ns. This allows the investigation of the material response to very fast stress changes over an extremely wide range of strain rates. The adverse and beneficial dynamic effects of step-loading the indenter into the surface of the material will be discussed. Initial results obtained from this type of experiment show that it is possible to obtain energy dissipation or damping information from the material being studied. INTRODUCTION Depth-sensing indentation testing involves the forcing of an indenter of known geometry into a planar surface while continuously recording the load on the indenter, the displacement of the indenter into the surface, and the time of the experiment. The resulting load-displacement-time data can then be used to determine certain mechanical properties of the material. The most commonly measured properties are the Young's modulus,(1], the hardness[I,2], and certain time-dependent properties of the material such as the stress exponent for creep or the strain rate sensitivity[3-5].1 The equipment used for this study is capable of data acquisition rates as high as 1.9xl05 s and test temperatures between -100 TC and 300 TC. This unique combination of capabilities allows further investigation into the materials dynamic deformation response and into the mechanisms actually controlling the rate of deformation. It is well documented that hardness can be related to the uniaxial flow strength[5,6]. Recent work has shown that the indentation strain rate(t=+4)[3] can be related to the uniaxial strain rate[7]. These relationships together with the ability to change the test temperature allows one to determine the activation energy for rate controlling mechanisms by tabulating indentation strain rate data as a function of test temperature at a constant hardness.2 It should be noted in this study that the hardness is defined as the load divided by 24.5ht where ht is the total depth of the indentation. The work presented here represents data acquired for pure indium during the initial testing and characterization of the new High Temperature Mechanical Properties Microprobe(HTMPM). Although the data presented here is from a bulk material, the results of this work show tremendous possibilities for future work on thin film materials. EXPERIMENTAL EQUIPMENT AND TECHNIQUES All of the experiments discussed in this work were performed with the new High Temperature Mechanical Properties Microprobe (HTMPM) at the Oak Ridge National Laboratory. The HTMPM is a low load, ultra-high vacuum (UHV), high temperature, continuous
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