Induced formation of nano precipitates and improve microstructure and mechanical properties by ultrasonic wave in the solid-liquid two-phase area of as-cast Ti48Al2Cr2Nb 2.5 C alloy

To refine the microstructure and form Ti3Al2 and C14 nano particles, and then improve strength and strain simultaneously, the Ti48Al2Cr2Nb 2.5 C alloy has been prepared under ultrasonic wave. Size of lamellar colony, lamellar spacing, phase composition, compressive properties and the related mechanisms have been studied. Results show that size of lamellar colony decreases from 40.7 to 33.0 mu m and lamellar spacing decreases from 1.5 to 0.8 mu m. Average length-diameter ratio of Ti2AlC decreases from 5.4 to 1.9 with increasing the ultrasonic treatment time from 20 to 100 s. Two kinds of nanoparticles form. One is hexagonal Ti3Al2 phase, which has incoherent relationship with gamma phase. Another is Ti(Al, Cr)2 phase with C14 structure. The Ti2AlC and alpha grain are refined by undercooling nucleation and dendrite fragmentation in solid-liquid two-phase area. The active dislocations and vacancy under ultrasonic promote the diffusion of Ti and Cr, and then induce the Ti3Al2 and Ti(Al, Cr)2 formed during solid-phase transformation. Compressive results show that the strength increases from 1901 to 2532 MPa, and the strain simultaneously increases from 27 to 34% with increasing the time from 0 to 100 s. The lattice distortion caused by the difference in thermal expansion coefficient between the Ti3Al2, Ti(Al, Cr)2 and matrix and the microstructure refined by ultrasonic jointly hinder dislocation slip to strengthen the alloy. The refinement and uniform distribution of Ti2AlC hinder crack propagation to improve toughness.