Research of different mechanisms in the weak/strong acoustic active zones on microstructure evolution and mechanical property of Ti48Al2Cr2Nb2.5C composites

Abstract

To reveal the coupling mechanism between melt superheat and ultrasonic action, Ti48Al2Cr2Nb2.5C alloy was prepared by arc melting with ultrasound treatment under different temperature gradients. And numerical simulation, microstructure evolution and Vickers hardness were researched in detail. Simulation results indicate that there are weak acoustic active zone (WZ) and the strong acoustic active zone (SZ) defined by acoustic pressure distribution. The acoustic pressure in SZ is over 10 MPa with an intense acoustic cavitation effect, and the acoustic streaming effect is significant in UC 350 due to the higher acoustic pressure and fluid velocity of 0.19 m/s. Experiment results reveal that the lamellar colony size increases from 39.9 to 67.3 µm and then decreases to 50.9 µm, and the aspect ratio of Ti2AlC decreases from 6.5 to 3.3 with overheating increasing in WZ. However, Al segregation is severe. In SZ, the lamellar colony size decreases from 80.3 to 43.7 µm and then increases to 56.1 µm. The aspect ratio of Ti2AlC decreases from 3.9 to 2.5. From UC 150 to UC 250 in WZ, temperature gradient increasing suppresses the ultrasonic refinement effect. With overheating increasing, the critical ultrasonic cavitation intensity decreases, and the acoustic cavitation effect increases, refining the lamellar colony and Ti2AlC. The stronger acoustic pressure and acoustic streaming effect increase the nucleation rate to refine the lamellar colony and element distribution uniformly in SZ. In WZ, Vickers hardness increases from 191.4 to 223 HV and then decreases to 211.2 HV. Vickers hardness increases from 211.3 to 227.7 HV but decreases to 208.4 HV in UC 300 and then decreases to 208.8 HV in SZ. The highest Vickers hardness in WZ and SZ are in UC 350. Lamellar colony refinement, the aspect ratio of Ti2AlC reduction and Al segregation attenuation improve the Vickers hardness.