Microstructural evolution of wear-resistant FeCrB and FeCrNiCoB coating alloys during high-energy mechanical attrition

Mechanical milling/attrition provides a convenient scope of simulating the microstructural changes encountered by wear-resistant coating alloys subjected to deformation under high frequency and high-intensity impact loading or accelerated wear condition. In the present study, the microstructural evolution of two commercial coating materials, FeCrB (Armacor M) and FeCrNiCoB (Armacor C), in the course of low- and high-intensity mechanical attrition, was monitored by X-ray diffraction and high-resolution transmission electron microscopy. While low-intensity milling leads to marginal grain refinement but no change in phase-aggregate in FeCrB, similar mechanical attrition causes boride precipitation in FeCrNiCoB alloy powder. On the other hand, attrition in high-intensity mill leads to formation of a nanocrystalline extended solid solution of ferrite in FeCrB and nanocrystalline two-phase mixture of boride and austenite in FeCrNiCoB, respectively. Furthermore, continued milling seems to produce partial solid-state amorphization, particularly in FeCrB. On isothermal heating, the milled powders (both FeCrB and FeCrNiCoB) develop identical two-phase nanocrystalline aggregate signifying large-scale solute redistribution and nearly the same level of metastability of the deformed microstructure. Thus, it is concluded that the microstructure and hence wear resistance of FeCrB and FeCrNiCoB coatings is likely to undergo significant changes during high load and high strain rate impact or wear condition.