Study of the microstructure and thermomechanical properties of Mo/graphite joint brazed with Ti–Zr–Nb–Be powder filler metal

The aim of this work was to braze molybdenum and graphite with Ti–40Zr–8.5Nb–1.5Be filler metal in order to demonstrate the possibility of its application for X-ray tube target brazing, further to investigate the joint microstructure using energy-dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), and electron microscopy as well as to conduct shear and unbrazing tests. It is shown that the brazed joint consists of matrix from β-(Ti, Mo) solid solution, mixed ZrC and TiC carbide layers at the braze/graphite interface, and beryllides TiBe2 and MoBe2 located at the grain boundaries of β-(Ti, Mo). The presented data made it possible to propose a brazed joint formation mechanism and explain the concentration of beryllides at the grain boundaries during brazing, as well as the mixed carbide layer formation from the side of the graphite. The mechanical tests showed that Mo/graphite brazed joints have a shear strength of at least 28.0 ± 0.9 MPa. However, sample failure occurred through the graphite due to the graphite surface mechanical treatment and the presence of a ductile β-Ti phase in the joint. The evaluation of joint thermal properties was performed using unbrazing tests. The unbrazing temperature was 1882 °C, which was caused by formation of refractory phases during brazing. The microstructure study shows that unbrazing occurs through the β-(Ti, Mo) phase with grain boundaries and beryllides eutectic melting.