Optimization of cryogenic mechanical alloying parameters to synthesize ultrahard refractory high entropy materials

Cryogenic mechanical alloying is a viable method to synthesize nanostructured alloys that exhibit improved mechanical properties without using highly contaminating, process control agents. However, cryogenically milled alloys still contain impurities introduced from the milling media and cryogenic fluid, and it is unclear how these milling parameters can be tailored to optimize alloy design. Here, milling media and cryogenic fluid were systematically varied and studied to quantify differences in impurity con-centrations, microstructural evolution, and microhardness. Four derivatives of a Mo25Nb25Ta25W25 high entropy alloy were mechanically alloyed using tool steel or tungsten carbide milling media with either liquid N-2 or Ar. Alloys were annealed for 100 h at 1000 degrees C or 1200 degrees C. Aberration-corrected scanning transmission electron microscopy (STEM) was primarily used to characterize as-milled and annealed specimens, and Vicker's microindentation was used to compare hardness. Depending on the milling parameters, total impurity concentrations varied between 12 and 44 at.%, different impurity nitrides or carbides were identified in annealed specimens, and differences in hardness of up to 5 GPa were measured amongst the alloys. Overall, milling with LN2 led to the precipitation of ultrahard nitride phases that when combined with optimized heat treatments, improved the hardness beyond 17 GPa. (C) 2021 The Authors. Published by Elsevier Ltd.

Automated summary

Cryogenic mechanical alloying is a feasible method to synthesize nanostructured alloys with improved mechanical properties. By systematically varying milling parameters, it was found that using liquid nitrogen for milling can induce the precipitation of ultra-hard nitride phases, leading to hardness improvement beyond 17 GPa.