Microstructure evolution and strength-ductility synergy mechanisms in the gradient transition zone between Ti-5.5Al-3.7Sn-3.6Zr-0.5Mo-0.4Nb-1.0Ta-0.4Si and Ti-22Al-25Nb alloys

With the development of advanced aircraft engines towards high thrust-weight ratio, traditional titanium alloys with single homogeneous structure and mechanical property can no longer meet the service requirements of aero-engine compressor discs. Laser deposition technology was adopted in this study to prepare the Ti-5.5Al-3.7Sn-3.6Zr-0.5Mo-0.4Nb-1.0Ta-0.4Si/Ti-22Al-25Nb dual alloys with a composition interval of 20 wt% at the connection interface. Then the microstructure evolution law, phase precipitation path, and strengthening and plasticizing mechanisms of the gradient structure were studied. Results show that as the content of Ti-22Al-25Nb alloy increases, the phase composition gradually transforms from basketweave structure dominated by coarse α/α₂ lamellae at the Ti-5.5Al-3.7Sn-3.6Zr-0.5Mo-0.4Nb-1.0Ta-0.4Si (Ti60) rich end to a multi-phase coexistence structure dominated by dispersed O phase at the Ti-22Al-25Nb rich end. Based on the thermodynamic calculations and EBSD analysis, the liquid solidification and solid-state phase transformation paths in the gradient structure were explored, and the secondary O phase was clarified mainly precipitating through the α₂ + β/B2 → O peritectic reaction. The dual-alloy component presents excellent strength-plasticity matching with room-temperature tensile strength reaching 1038 MPa and elongation reaching 9.5%. The strengthening mechanism mainly comes from the refinement of lamellar α/α 2 and the pinning effect of dispersed O on dislocation slip. The improvement of plasticity mainly depends on inducing GND accumulation at the soft/hard phase boundary of multi-phase heterostructure to construct long-range back stress and front stress fields, achieving dynamic redistribution and homogenization of loads between different phases. Moreover, the introduction of plastic strain component along the c-axis direction by pyramidal slip can further improve the deformation coordination.