Nanocrystalline metals go ductile under shear deformation

Nanocrystalline Pd90Au10 (grain size <10 nm) has been deformed under dominant shear and superimposed compression to large plastic strains (ɛ> 20 %). Taking stress strain curves at different strain rates (10−4<ɛ˙<100) allowed us to extract the shear activation volume and strain rate sensitivity as a function of plastic strain, which are of the order of 4b3 and 0.035, respectively. Particularly, the plastic strain-dependent shear activation volume evolves through a maximum value, which relates to yielding. Analyzing the strain hardening rate Θ as a function of applied stress enables to determine strain rate dependent macroyield stresses. Scaling of the applied stress with the macroyield stress yields in addition also the strain rate dependent onset stresses of microyield. Surprisingly, strain hardening associated with interfacial deformation modes dominates the stage of microplasticity, whereas, in the macroplastic regime the strain hardening rate saturates at strains >10% to approach a more or less generic value of Θ/G = 0.015.

► We show that nc metals manifest large uniform strain when tested under shear.
► We plot strain-rate sensitivity and activation volume vs plastic strain.
► A method to extract the yield stress of nanocrystalline metals is discussed.
► Three distinct deformation stages and their onset stresses are identified.