Achieving high strength-ductility synergy in TiBw/near α-Ti composites by ultrafine grains and nanosilicides via low-temperature severe plastic deformation

The common problem of low strength-ductility matching prevails in near-α high-temperature titanium matrix composites (TMCs). In this work, the design strategy of ultrafine grains and dispersed (Ti, Zr)6Si3 nanoprecipitates in the microstructure of TiBw/near α-Ti composites via low-temperature isothermal multidirectional forging (IMDF), is expected to break the trade-off dilemma between strength and ductility. The results show that with the decrease in the temperature of IMDF, the grain scale decreased from 0.98 μm to 0.59 μm, and the location of silicide precipitation shifted from phase boundaries and grain boundaries to α-grain boundaries and intracrystalline regions. The experiments confirm that the local segregation of Si and the temperature of thermomechanical deformation are the key factors affecting the precipitation behavior of silicides. With the decrease of the deformation temperature, the precipitation mechanism of silicides changes from a single diffusion-controlled precipitation to the coupling of two mechanisms, namely, elemental diffusion and dislocation-assisted nucleation, which facilitates the successive precipitation of nanometer-sized silicides at the grain boundaries and in the inner regions. The ultimate tensile strength (UTS) and elongation of the composites were substantially increased after IMDF at 950 and 800°C, especially the excellent performance at 800°C, where the strength reached 1320.3 MPa and the elongation was 5.8%. The room and high-temperature strengthening and failure mechanisms of the composites are analyzed and discussed, and the yield strength (YS) increments provided by various strengthening mechanisms at room temperature are quantified, aiming to provide a potential preparation strategy for the synergistic strengthening of near-α TMCs with ultrafine grains and nanoparticles.