Effect of Cr content on microstructure and mechanical properties of TiAl/Ni-Cr-Si-B/42CrMo brazed joints

In this study, Ni-Cr-Si-B brazing filler metals (BFMs) is employed to braze TiAl and 42CrMo alloys. The effect of Cr content (0–17 wt%) in Ni-Cr-Si-B BFMs on joint microstructure and mechanical properties are investigated. Through EDS analysis, XRD patterns, and ternary phase diagrams, the typical joint microstructure is determined: 42CrMo/ TiC+Cr₂B/Ni(s.s.) + (Ni, Ti)-B/ TiB₂+TiNi₂Al+Ti₂NiAl₃/ TiAl. When 0Cr BFM is employed, large and brittle (Ni, Fe)-B borides are present in the center of the joint. As the Cr content in the BFMs gradually increased, the precipitation of (Ni, Fe)-B boride is suppressed, B is consumed by both Cr and Ti, leading to the formation of Cr₂B and TiB₂ intermetallic compounds (IMCs). Meanwhile, Cr promotes the diffusion of C from the 42CrMo base metal (BM), resulting in the formation of TiC IMCs in Zone I. When the Cr content exceeds 10 wt%, Cr competes with Ti for C. As a result, the amount of the TiC phase decreases, and coarse Cr - (B, C) IMCs are formed. The shear test results indicate that the maximum shear strength of 244.3 MPa by controlling Cr content in the BFM at 7 wt%. When the Cr content in the BFM is less than 17 wt%, fracture occurs at the interface between the TiNi₂Al+Ti₂NiAl₃ ternary compound layer and the TiAl BM. When the Cr content in the BFM exceeds 17 wt%, the crack propagates along the large and brittle Cr- (B, C) IMCs.