Study of residual stresses, microstructure, and hardness in FeB and Fe2B ultra-hard layers

Abstract

Boriding is a thermochemical diffusion-based process of achieving ultra-hard surface on metals. Two distinct crystalline phases, i.e. tetragonal Fe2B and orthorhombic FeB can exist in the surface layer penetrated by boron ions. In our contribution, we have studied the microstructure, the hardness, and the spatial distributions of both phase composition and residual stresses (RS) in samples exhibiting either single-phase Fe2B or duplex Fe2B-cum-FeB character. The indispensable knowledge of the elastic constants used in the stresses calculations from the measured strains by X-ray diffraction were gained from the refined lattice parameters of both iron borides employing density functional theory implemented in CASTEP software by Materials Studio. In the studied case, there is only minor occurrence of preferred orientation in the Fe2B phase and the evaluated RS have compressive character gradually decreasing from its maximum value on the very surface.