Bimodal eutectic titanium alloys: Microstructure evolution, mechanical behavior and strengthening mechanism

We report a novel microstructure evolution and corresponding mechanical behavior of bimodal eutectic (Ti63.5Fe26.5Co10)(82)Nb12.2Al5.8 alloys processed by semi-solid sintering (SSS) of the as-milled alloy powders with various glass contents resulted from various milling times. Results show that the as-milled alloy powders have more homogeneous element distribution and higher content of glassy phase with increased milling time. Correspondingly, although the SSSed bulk alloys possess the same constituted phases of the bcc beta-Ti, bcc B2 Ti (Fe, Co) and fcc Ti-2(Co, Fe), their eutectic structures containing bcc beta-Ti and bcc B2 Ti(Fe, Co) evolve from irregular eutectic, to partial coarse eutectic, to fine cellular eutectic matrix, and finally to typical nano and ultrafine lamellar eutectic matrix. Interestingly, it is the first time to report that the lamellar eutectic matrix has an bimodal structure consisting of interleaving nano- and ultrafine-grained B2 Ti(Fe, Co) and bcc beta-Ti lamellae. Corresponding to the evolution of eutectic structure, the SSSed bulk alloys exhibit a gradual increase in yield strength and plastic strain. Especially, the SSSed bimodal eutectic alloy has ultra-high yield strength of 2050 MPa and large plasticity of 19.7%, superior to those of equivalent counterparts. Theoretically, strengthening mechanism of the SSSed bimodal eutectic alloy can be mainly rationalized as ordering strengthening of B2 superstructured Ti(Fe, Co) and coherency strengthening between bcc beta-Ti and bcc B2 Ti (Fe, Co) lamellae inside the lamellar eutectic matrix.