Determination of Time-Dependent Plastic Properties of Metals by Indentation Load Relaxation Techniques

Abstract

Load relaxation testing has been demonstrated to be useful for characterizing the time dependent plastic properties of metals. However, for testing of small material volumes, such as thin film metallizations, thin films, and contact surfaces, conventional load relaxation techniques cannot be used. For such applications an indentation test offers an attractive means for obtaining data necessary for materials characterization. This work shows that an indentation load relaxation test is experimentally feasible for thin film testing. Experiments on brass and beryllium copper samples with or without a gold/nickel plating illustrate different relaxation properties of the substrates and the surface layers. Furthermore, results of experiments on some fcc metals suggest rather simple relations between the conventional uniaxial load relaxation (LR) test and the indentation load relaxation (ILR) test.

Scaling, dimensional analysis, and indentation measurements

“…An example is the impression creep test using a cylindrical flat punch for examining metals and polymers [194][195][196][197][198][199] (see [199] for a comprehensive review on impression creep tests). With the advent of nanoindentation, measurements of strain rate and temperature effects in thin films and nanostructured materials became possible [200][201][202][203][204][205]. These measurements are important for a number of industrial applications, including interconnects for electronic devices, materials for lead-free solder joints, and metals exhibiting superplasticity for low cost forming of complex parts, as well as for fundamental understanding of deformation mechanisms at small length scales.…”

“…Examples include the use of constant loading rate, _ F; constant displacement rate, _ h; or the keeping of parameters such as _ h=h or _ F=F constant. The ''indentation strain rate'' is usually defined as _ h=h [200][201][202]. The strain-rate dependence of a measured property, such as hardness, is then expressed in terms of _ h=h.…”

“…This definition allows quantitative comparison between conventional creep or relaxation tests and indentation creep tests. It has been found that the creep exponent obtained from these macro-and micro-tests are comparable for metals [200][201][202] and ceramics [204,205] where the power-law creep model applies. A number of theoretical and numerical studies of indentation in strain-rate dependent solids have also appeared recently in the literature [206][207][208][209].…”

Analysis of indentation creep

“…As the hardness drops, the rate of penetration decreases, and in this way the experiment sweeps out a spectrum of hardness versus indentation strain rate, the slope of which on a log-log scale gives m H . Other methods for measuring m H include indentation load relaxation, 10 in which the depth is held fixed and the load is allowed to relax; indentation rate-change, in which the specimen is first loaded at one rate and then partially unloaded, then reloaded at a second rate 11 ; and constant indentation strain rate that measures the hardness at fixed _ L=L, where L is the load. 12 In the Appendix, we construct an analysis to handle creep and load relaxation.…”

Estimating the slip resistance from spherical nanoindentation and orientation measurements in polycrystalline samples of cubic metals

“…Among the different experimental techniques explored thus far in literature, instrumented indentation (Doerner and Nix, 1986;Oliver and Pharr, 1992;Oliver and Pharr, 2004) exhibits tremendous potential as a low cost, high throughput, approach because of its capability to probe quickly multiple local volumes in a small sample. Indentation has been applied to study deformation behavior in a wide variety of material systems ranging from metals (Hannula et al, 1985;Mayo and Nix, 1988), polymers (Anand and Ames, 2006), and composites (Chen et al, 2010). Advances in instrumented indentation has facilitated studies of dislocation source activation (Nair et al, 2008;Zbib and Bahr, 2007), mechanical characterization of grain boundaries (Vachhani et al, 2016), slip lines in Ni-based single crystal supperalloy (Sabnis et al, 2012;Sabnis et al, 2013), and geometrically necessary dislocations produced by gradients of slip (Dahlberg et al, 2014).…”