Microstructure evolution and strength-toughness synergy mechanism in as-cast Ti-7Mo-4Al-3Nb-2Cr-2Zr-xTa alloy

Traditional metal strengthening and toughening methods are difficult to break through the inverted law of strength and plasticity of titanium alloys. To improve strength and toughness simultaneously, Ti-7Mo-4Al-3Nb-2Cr-2Zr-xTa alloys were prepared by reducing the size of α phase and increasing the density of dislocation. The microstructure was observed and the synergistic mechanism of strength and toughness was studied. Results show that the size of β grain decreases from 2.1 to 0.4 mm with the increase of Ta. There are two kinds of α phase, one is αp phase and another is αs phase. The aspect ratio of αp and αs phase both decreases with the Ta content increasing from 0 to 0.5 wt%, which is from 9.7 to 6.1 and 8.1 to 7.1. With the continuous increase of Ta content, the aspect ratio increases instead. The density of dislocation reaches the maximum at 0.5 wt%, which is 1.6 × 1015 m−2. The large atomic radius of Ta hinders the growth of β grains, thus limiting the transition from β phase to α phase. The increase in dislocation density is caused by the limited transition from β phase to α phase and the lattice distortion after adding Ta. When the Ta content is 0.5 wt%, the strength and toughness reach the highest simultaneously, which are 1283 MPa and 71 MPa·m1/2. The precipitation strengthening caused by the refined α phase and the dislocation strengthening caused by increased dislocation density is the decisive factors in improving strength and toughness simultaneously.