Role of stacking fault energy (SFE) on the high strain rate deformation of cold sprayed Cu and Cu?Al alloy coatings

Cold spray deposition involves a unique combination of high strain rate and moderate to high strain deformation of powder particles that contrasts it from conventional severe plastic deformation techniques. In the present work, a systematic study of the microstructure and micromechanical properties of cold sprayed Cu and Cu?Al alloys, which are known for significant variation in stacking fault energy with small alloying addition, is presented. The deposition process results in significant grain refinement (<50 nm) and profuse deformation twinning with very fine twins (<20 nm) in the Cu?Al alloys. The theoretical predictions of the extent of grain refinement using a dislocation-based model agree well with the experimental observations for the alloys, even in the presence of significant twinning. The inherent discrete nature of the cold spray coating deposition results in significant heterogeneity and hence extensive hardness mapping has been performed to establish structureproperty correlations at the micrometer length scale. Furthermore, the various factors influencing the hardness of the coatings are analytically determined and it was found that the strengthening contribution from the boundary (grain/twin) is much lower than that determined from Hall-Petch type relationship in case of the alloys that showed significant twinning. A twin mediated deformation mechanism in the presence of fine twins, which subsequently de-twin, is found to explain the lower boundary strengthening.