A novel stress-relaxing approach based on forming Fe–Ni invar effect interlayer for joining C/C composites

Taking advantage of the invar effect of Fe–Ni alloy at low temperatures, a novel stress-relaxing approach based on forming Fe–Ni invar interlayer was proposed for joining carbon fiber reinforced carbon (C/C) composites in this paper. Mixed Fe + Ni62Ti38 (at %) powders were used as the filler materials. Microstructure, mechanical property, and formation mechanism of the bonded joints were investigated. The results indicate that, based on the eutectic reaction of (Fe,Ni)–C at the C/C/interlayer interface and the in-situ reaction of Ti–C in the interlayer, a (Fe,Ni)–C eutectic reaction layer was formed at the C/C side interface and the Fe–Ni invar effect interlayer reinforced by TiC particles was obtained. Owing to the low thermal expansion coefficient and good plasticity of the Fe–Ni alloy matrix at low temperatures, the high residual thermal stress of the joint was effectively reduced and the shear strength of the joint was improved. With the increase of Ni content in the Fe–Ni alloy matrix of the interlayer, the shear strength of the joint decreases gradually. When the composition of the Fe–Ni matrix was designed to be Fe–36Ni, i.e., the invar alloy with the lowest thermal expansion coefficient at 100 °C, the average shear strength of the bonded joint reached the highest value of 36 MPa. The fracture of the joint occurred in the (Fe,Ni)–C eutectic reaction layer close to the C/C composite.