Optimization of cryogenic mechanical alloying parameters to synthesize ultrahard refractory high entropy materials. (15th November 2021)

• Record Type: Journal Article
• Authors: Smeltzer, Joshua A.; Burton, Mari-Therese; Hornbuckle, B. Chad; Giri, Anit K.; Darling, Kristopher A.; Harmer, Martin P.; Marvel, Christopher J.
• Is Part Of: Materials & design. Volume 210(2021)
• Publication Date: 2021-11-15
• Subjects: High entropy alloy -- Nanocrystalline -- Mechanical alloying -- Scanning transmission electron microscopy -- Impurity phase identification -- Hardness mechanisms; Materials -- Periodicals; Engineering design -- Periodicals; Matériaux -- Périodiques; Conception technique -- Périodiques; Electronic journals; 620.11
• DOI: 10.1016/j.matdes.2021.110070
• Languages: English
• ISSNs: 0264-1275

Abstract

Graphical abstract: Highlights: Four MoNbTaW alloys were synthesized by varying milling media and cryogenic fluid Non-metallic impurity concentrations varied between 0.07 and 3.8 wt% Impurity phases contributed to variations in hardness of 5 GPa and a peak of 17 GPa. Sluggish diffusion was determined to be a dominant thermal stability mechanism Grain size and impurity phases were the primary contributors to the high hardness Abstract: 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 concentrations, microstructural evolution, and microhardness. Four derivatives of a Mo25 Nb25 Ta25 W25 high entropy alloy were mechanically alloyed using tool steel or tungsten carbide milling media with either liquid N2 or Ar. Alloys were annealed for 100 h at 1000 °C or 1200 °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.