Effects of Rapid Cooling Rate on Microstructure Formation and Microhardness of Binary Ti-44Al Alloy

In order to refine microstructure grains and improve mechanical properties of TiAl alloys, Ti44Al (at.%) alloy was rapidly solidified by melt spinning under different cooling rates. Microstructure and microhardness of the alloy before and after rapid solidification were investigated. XRD results show that the ratio of α2 phase in binary alloy increased with the cooling rates, which is caused by more α phases directly transforming to α2 phases. Grain morphology changed from long dendrite to the mixture of equiaxed and dendrite to equiaxed with the increase of cooling rates. The grain size was refined from 200-600 μ m of as-cast to 18 μ m of the alloy cooled at 4.9×105K/s, which is caused by the undercooling induced from rapid solidification. Lamellar spacing was decreased from 4.5 μ m of as-cast to 1.1 μ m by rapid solidification. With the increase of cooling rate, the content of α2 phase increased and γ phase decreased gradually. Rapid solidification can reduce the segregation of elements. The microhardness was improved from 247 HV to 556 HV, which results from grain refinement strengthening, reduction of lamellar spacing, and more content of α2 phase.