Multiscale microstructural consideration of enhanced shear strength in TiAl intermetallic/K4169 alloy composite joints prepared by vacuum brazing with (Ti, Zr)-Ni-based amorphous filler metal

Abstract

TiAl intermetallic could be used to replace Ni-based alloy in assemblies to generate excellent specific strength. A (Ti,Zr)-Ni-based amorphous filler metal Ti21.25Zr25Ni25Cu18.75 (at.%) was designed using a cluster-plus-glue-atom model to successfully vacuum braze K4169 and TiAl bimetallic assemblies. At various brazing temperatures and holding time, the quantitative relationships between lattice distortion, grain boundary, dislocation density, and hardness, elastic modulus, shear strength of the joints were investigated. Meanwhile, the fracture mechanism of the joints was revealed. The brazed seam mainly consisted of solid diffusion reaction layers (Zones I and III) and filler metal residue zone (Zone II). When the brazing temperature increased to 1030 °C, grain refinement occurred in the brazed seam. Zone I was primarily composed of (Ni)ss[0-11]+TiNi[011]/(Cr,Fe,Ni)ss[0-11]/(Ti,Zr)Ni[0-1-1]+(Cr,Fe,Ni)ss[0-11]. The (Ti,Zr)(Ni,Cu)[001] and (Ti,Zr)(Ni,Cu)[101] intermetallic compound-based solid solutions were formed in Zone II. And the lattice distortion of (Ti,Zr)(Ni,Cu)[101] and (Ti,Zr)(Ni,Cu)[001] was 32.05% and 14.82%, respectively. As a result, the proportion of low angle grain boundaries (LAGBs) and deformed grains in Zone II rose to 38.6% and 38.7%. In Zones I and III, the proportion of LAGBs reduced to 8% and 3.4%, respectively. As the holding time increased, the long-range diffusion of Al in Zone II caused the (Ti,Zr)(Ni,Cu)[001] with cubic structure to transform into (Ti,Zr)(Ni,Cu,Al)[00-1] with hexagonal crystal system structure, where the lattice distortion was 4.42% and 10.49% for a and c. At 1030 °C/10 min, the average geometrically necessary dislocation densities (GNDs) in Zones I, II and III were 9.87 × 1014 m−2, 8.55 × 1014 m−2 and 11.4 × 1014 m−2, respectively. Therefore, the shear strength of joints reached 322 MPa due to the lattice distortion, dislocation strengthening and fine grain strengthening. Meanwhile, the plastic and brittle hard phases were generated in Zone II and displayed a mechanical interlocking structure that contributed to the performance of the joint. Both (Ti,Zr)(Ni,Cu)[001] and (Ti,Zr)(Ni,Cu)[101] in Zone II formed along different low-index cleavage planes during transgranular fracture. The cracks initiated in this region extended to the interface between Zones I and II and exhibited bimodal grain characteristics.