Microstructural Evolution and Mechanical Properties of TC4 Titanium Alloy During Acculative Roll Bonding Process

TC4 titanium alloy is highly promising for aerospace and medical implant applications due to its low density, high strength, corrosion resistance and biocompatibility, and the ultra-fine grains of TC4 alloy by accumulative roll bonding (ARB) can efficiently improve the low temperature super-plasticity and biocompatibility for its widespread applications. However, the ARB process for TC4 alloy has been limited due to the high deformation resistance and low anti-oxidant ability. In this work, ARB was conducted for the ultra-fine grains of TC4 titanium alloy, and the effects of ARB temperatures and layer numbers on the bonding interface and microstructure were investigated as well as the deformation mechanism of themixed alpha/beta phase structure, and the influences of ARB processing on the mechanical properties were studied. The good interface bonding could be fabricated by the proper ARB temperature (near 720 degrees C), the anti-oxidation treatment and the multilayer with the high deformation, which always takes on the hardened interface with the high oxidation contents, and the interface bonding strength increases with the increase of the ARB layers and temperature through the process of the diffusion and the necking fracture. The deformation process is composed by the cooperation deformation of alpha/beta structure and the shear deformation during ARB processed TC4 alloy, during which the beta phase at the grain boundary changes from the long strips to the short bands to deform with hcp alpha phase, while the shear bands with severe local-deformation is used to adapt the severe plastic deformation. The deformed microstructure is composed of the equiaxed structure (about 300 nm spacing) and the elongated deformation structure (about 400 nm spacing), in which the equiaxed structure comes from the function of the deformation temperature, local shear deformation and the local overheat. Additionally, the inhomogeneous microstructure and properties along the thickness direction can be observed, and the high hardness can be obtained at the bonding interface, which gradually distributes homogeneous with the increase of ARB layers. The strength of ARB processed TC4 sheets increases with the increase of ARB layers, which can get to 1325 MPa after 16 ARB layers, and simultaneously the plasticity decreases to 5.4%. The ductile fracture can be observed with the low ARB layers, while the mixed structure of the quasi-cleavage and ductile fracture is obtained with the increase of ARB layers.