Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr

This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness K-IC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe-3(C,B) phase are determined. Samples of the M2B phase are tasted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 +/- 1 GPa), E (373 +/- 6) GPa and K-IC (3.5 +/- 0.7 MPa root m) at 4-5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 +/- 0.7 GPa, E = 473 +/- 9 of and K-IC = 3.26 +/- 0.8 MPa root m at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3-3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. (C) 2018 Elsevier Ltd. All rights reserved.