Local Mechanical Property Evolution During High Strain-Rate Deformation of Tantalum

Damage within ductile metals is often linked to local heterogeneities. In ductile metals, damage typically occurs after plastic deformation, which evolves the microstructure and its properties in ways that are not easily measured in situ. This is particularly true for materials subject to dynamic loading. Here, we use a combination of spherical nanoindentation testing and electron microscopy to quantify changes in local dislocation slip resistance as a function of grain orientation in polycrystalline tantalum subjected to high strain-rate deformation. A nanoindentation data analysis technique is used to convert spherical nanoindentation data into stress–strain curves. This technique works with microstructural characterization at the indentation site and involves two steps: (1) determination of the functional dependence of the indentation yield strength (Yind) on the crystal orientation in the undeformed condition, and (2) use of nanoindentation and EBSD measurements on the deformed samples to determine changes in the local slip resistance. In this work, undeformed Ta had indentation yield values that varied by as much as 40% depending on the crystal orientation. The dynamically deformed Ta displayed a large variance in the strain hardening rates as a function of grain orientation. Soft grains (those with low Taylor Factor) were found to harden significantly more as compared to hard grains (those with a high Taylor Factor). These data are discussed in terms of grain interactions where the hard grains impose additional work on neighboring soft grains due to constraint at the boundaries.