Microstructure and Mechanical Properties of Titanium–Zirconium–Molybdenum and Ti2AlNb Joint Diffusion Bonded with and without a Ni Interlayer

Abstract

Diffusion bonding of titanium–zirconium–molybdenum (TZM) and Ti2AlNb with and without a Ni interlayer is studied, and microstructure, residual stress, and mechanical properties of the joints are investigated. The results show that cracks appear in the TZM substrate near the joint during the direct diffusion‐bonding process, owing to detrimental residual stress from coefficient of thermal expansion (CTE) mismatch of the bonded couples. Herein, the maximum shear strength is only 240 MPa, corresponding to the joint bonded at 950 °C for 60 min at 20 MPa. When a 60 μm‐thick Ni interlayer is added, sound diffusion‐bonded joints without cracks are obtained, and the maximum shear strength reaches 340 MPa. The typical microstructure of the TZM/Ni/Ti2AlNb joint is TZM/NiMo + Ni3Mo/Ni4Mo/residual Ni/TiNi3/AlNi2Ti + Ti3Ni4 + (Ti, Ni, Nb) phase/Ti2Ni + B2(Ni)/Ti2AlNb. The residual Ni interlayer in the joint absorbs residual stress by plastic deformation, which is beneficial for the improvement of the joint shear strength. The residual stress in the joint is reduced from 347 to 78 MPa by increasing the thickness of the Ni interlayer from 0 to 120 μm. Fracturing in the joint occurs in the Ti2Ni phase because the Ti–Ni compounds have the highest microhardness values.