Grain boundary hardening vs softening in nanocrystalline CuZn30

Nanocrystalline (NC) and ultrafine-grained (UFG) materials are known for increased mechanical strength compared to their coarse grained (CG) counterparts. However, at elevated temperatures softening as well as enhanced strain rate sensitivity can be observed in NC and UFG materials. In this study the different effects of temperature, strain rate and strain on the microstructure and mechanical strength of NC and CG CuZn30 are investigated. High pressure torsion (HPT) is used to produce bulk samples of nanocrystalline CuZn30. Tensile, compression and nanoindentation tests are employed to investigate the mechanical properties at temperatures up to 300 °C. This study shows the strengthening effect of GBs at RT and the softening effect at elevated temperatures. It is shown that the microstructure is thermomechanically stable at temperatures up to 150 °C and that the softening at these temperatures is fully reversible. At higher temperatures, a transient regime can be observed where the NC material softens further, but deformation driven grain growth leads to coarsening. The Coble creep, Blum-Zeng and Figueiredo-Langdon models are compared to experimental data for the strain rate sensitivity exponent m. ... mehrThe best agreement was found for the Figueiredo-Langdon model, which fits well for the stable temperature regime from RT up to 200 °C.