Strain rate sensitivity of nanolayered Cu/X (X=Cr, Zr) micropillars: Effects of heterophase interface/twin boundary

Manipulation of internal features, e.g., layer/twin interfaces, is standard practice in engineering structural materials to tailor their properties. In the present work, we fabricated nanotwinned (NT)- and nanocrystalline (NC)-Cu/X (X=Cr, Zr) nanolayered micropillars (NLs) with equal layer thickness (h) spanning from 5 to 125 nm. The microcompression methodology was employed to investigate the layer/twin interfaces effects on plastic characteristics of nanolayered materials at different strain rates. Experimental results clearly unveil that the Cu/X NTNLs exhibit a significant increase in strength and in strain rate sensitivity (SRS) by introducing nanotwins, in comparison with the Cu/X NCNLs. The non-monotonic evolution of SRS with h observed in the Cu/X NTNLs is explained by a competition between the monotonically increased interfaces density and the decreased twin boundaries density with reduction in feature size h. Unlike the monotonically enhanced SRS of Cu/X NCNLs, the SRS of Cu/X NTNLs first decreases and subsequently increases with reducing h. A phenomenological model is proposed to rationalize these experimental findings and highlight the microstructural feature size effects on the rate-limiting processes of metallic materials.