Coupling Effects of Melt Treatment and Ultrasonic Treatment on Solidifying Microstructure and Mechanical Performance of Ti44Al6Nb1Cr Alloy

The coupling effects of melt treatment and ultrasonic treatment on the solidifying microstructure and mechanical performance of Ti44Al6Nb1Cr alloy are investigated. During melt treatment, a low superheat degree is beneficial for microstructure refinement, with the lamellar colony size decreasing from 512 to 243 μm, while a low cooling rate leads to the microstructure coarsening as the lamellar colony size enlarges from 458 to 615 μm. After coupling with ultrasonic treatment, under moderate superheat degree and cooling rate, the original coarse lamellar colony size is significantly refined to 56 and 38 μm, the compressive strength is improved by 60.71 and 47.89 pct, and the compressive strain is enlarged by 80.19 and 112.33 pct, respectively. It is found that the ultrasonic refining efficiency is dominated by the melt temperature, and there is an optimum temperature range near the crystallization temperature: a too-high temperature leads to the remelting of crystal nuclei, impairing the refining efficiency, whereas a too-low temperature results in high viscosity, hindering the ultrasonic effects. Under ultrasonic treatment, the melt supercooling is increased, leading to an extended constitutional supercooling region, which will enlarge the crystal nucleation; the solute enrichment is enhanced, forming a quasi-steady state with a higher solution concentration gradient, which improves the crystal growth velocity.