Rate-dependent hardening due to twinning in an ultrafine-grained magnesium alloy

An ultrafine-grained (UFG) ZK60 Mg alloy with an average grain size of ∼1.0 μm was processed by extrusion at relatively low temperature (488 K) with a high area reduction ratio (∼25). The mechanical behavior of the UFG Mg alloy is investigated over strain rates spanning nearly eight decades (10−4–104 s−1). The stress–strain responses in the quasi-static (∼10−4 s−1) and high rate (104 s−1) regimes exhibit the characteristic sigmoidal profile that is a signature of {101¯2}〈101¯1¯〉 extension twinning. Further, this sigmoidal profile is accentuated at high rates, suggesting a rate effect of twinning induced hardening. X-ray diffraction (XRD) and analysis of the as-received and deformed microstructures indicate the occurrence of twinning even at the quasi-static rates of loading. This observation is contrary to some of the theoretical predictions that suggest suppression of twinning in Mg below critical grain sizes much larger than in the present work. From the XRD analysis we infer that the twin volume fraction increases with increasing applied strain rate. Transmission electron microscopy observations of the tested specimens reveal high density non-basal dislocations that may result from the activation of these slip systems following twinning-induced lattice reorientation.