A novel joining of Cf/C composites using AlCoCrFeNi2.1 high-entropy brazing filler alloys

A novel brazing of C-f/C composites was achieved using AlCoCrFeNi2.1 high-entropy alloys (HEAs) as the interlayer. The microstructure and shear strength of the joints brazed under different processing parameters was evaluated. The typical structure of the C-f/C composite joints could be described as C-f/C / zone 2/ zone 1/ zone 2/ C-f/C. The zone 1 was a continuous metallic layer which was mainly composed of the BCC and M7C3 coupled with some FCC and gamma' phases. The zone 2 was actually a mixture of discrete graphite and filling phases. The filling phases was dominated by the FCC with embedded gamma', while a small amount of the BCC and M7C3 was also incorporated. The formation of M7C3 and transformation of the FCC to BCC took place in the solid interlayer in initial heating, and simultaneously some porous graphite appeared at the substrate/interlayer interface. Upon cooling, the liquid in the middle of the braze seam was solidified to generate the zone 1, while the liquid pouring into the porous structure beside the interlayer gave rise to the zone 2 in the joint. Elevating the brazing temperature could increase the average shear strength of the joints because it accelerated the liquid penetration filling the porosities in the zone 2 and also produced a hybrid structure in the joints. In this work, the highest shear strength of the joints achieved 21.9 MPa when brazed at 1440 degrees C for 10 min. The hybrid structure observed showed particular role in mitigating the thermal residual stresses, providing a unique idea in joining carbonbased materials as well as extending the application of C-f/C composites.