Microstructure evolution and improved mechanical strength of the C/C-Mo joint by adjusting the joining temperature in a wide temperature range

In order to reduce the residual thermal stress to obtain the C/C-Mo joints with high mechanical properties, the composition and distribution of the phases were optimized by adjusting the joining temperature in a wide range. As the joining temperature increases from 1180 degrees C to 1380 degrees C, the joint strength first increases and then decreases, which reaches a maximum (47.84 MPa) at 1330 degrees C, 42.30% higher than the joint brazed at the common utilizing temperature of the BNi-5 filler (1180 degrees C). The interlayer of the joint is divided into two regions: Zone 1 with Ni-rich MoNiSi and a little Ni solid solution (Ni-ss), and Zone 2 with Mo-rich MoNiSi and high content of Ni-ss. As the temperature increases, the thickness of Zone 2 increases while that of Zone 1 reduces due to the phase transition from Ni-rich MoNiSi to Mo-rich MoNiSi. This microstructure evolution introduces more Ni-ss in the joint, improving the joint strength by reducing residual thermal stress via plastic deformation. Furthermore, a Ni-based interfacial layer including complex compounds with smaller sizes in the joints brazed at higher temperature leads to the stronger bonding between C/C composites and braze. The straight fracture path along the interface changed to the tortuous path in the braze seam, leading to more energy consumption, which results in the improved joint strength. The excessive temperature (1380 degrees C) would lead to the big sizes of the brittle MoNiSi phase, resulting in the formation of pores and cracks in the joints and poor joint strength.

Automated summary

By adjusting the joining temperature, the composition and distribution of phases in C/C-Mo joints were optimized to achieve maximum strength, leading to improved joint strength. The gradual phase transition in the different regions introduced more Ni-ss, reducing residual thermal stress and enhancing joint strength.