Strain-rate and temperature dependent optimum precipitation sizes for strengthening in medium-entropy alloys

Abstract

The (FeCoNi)₈₆Al₇Ti₇ medium-entropy alloy (MEA) with varying sizes and fixed volume fraction of coherent L1₂ precipitates was fabricated, and the effects of precipitation size on mechanical properties at varying strain rates and temperatures were investigated experimentally. An optimum precipitation size for precipitation strengthening can be always observed for the experimental curves under different strain rates and temperatures. The dominant precipitation mechanism under dynamic conditions is found to be transited from the dislocation-shearing mechanism to the Orowan dislocation-looping mechanism with increasing precipitation size. A novel theoretical model was developed to consider the effects of strain rate and temperature on the precipitation shearing strengthening and the Orowan looping strengthening. The predicted precipitation strengthening curves as a function of precipitation size by the newly-developed model are observed to be well consistent with the experimental results under different strain rates and temperatures. The optimum precipitation size for the strongest precipitation strengthening is found to be strain-rate and temperature dependent, and shift to higher values with increasing strain rate and decreasing temperature, as predicted by the theoretical model and validated by the experimental results.