Microstructure And Properties Of Rapidly Solidified Tial Based Alloys

Binary Ti Al alloys(Ti-44 Al,Ti-48Al) and high Nb containing Ti Al alloys(Ti44Al6Nb1.0Cr, Ti44Al6Nb1.0Cr2.0V) ribbons were produced by chill-block melt-spinning method. The thesis studied the phase compositions, microstructure and mechanical properties of these Ti Al based alloys prepared by traditional casting and rapid solidification. The effect of Al content on microstructure of binary Ti Al alloys at the same cooling rate was investigated. The influence mechanism by alloying elements of Nb, Cr and V on the microstructure and mechanical properties of rapidly solidified alloys were analyzed.Ti-44 Al and Ti-48 Al are consisted of γ phase and α2 phase under the methods of traditional casting and rapidly solidified, the content of α2 phase increases with the cooling rate. The primary structure of binary Ti-(44, 48)Al(at.%) is lamellar structure in conventionally cast condition. The grain size is about hundreds of microns. With increasing of cooling rate, the microstructure changes into equiaxed grains and the grain size is reduced to tens of microns gradually under the condition of rapidly solidified. The elements content of rapidly solidified alloys is more homogeneous. The microhardness of rapid solidification alloys is bigger than that of traditional casting alloys. Especially the microhardness of rapidly solidified Ti-44 Al alloy is improved at least 100%.Conventionally casting Ti44Al6Nb1.0Cr(2.0V) alloys are consisted of γ phase, α2 phase and B2 phase.There are only γ phase and α2 phase in rapidly solidified alloys. The content of B2 phase was too little to be detected by XRD. The grain size has decreases with the increasing of cooling rate. The segregation emerges in grains under the method of rapid solidification. The segregation of high Nb containing Ti Al alloys is improved effectively. Similarly the microhardness of rapidly solidified Ti44Al6Nb1.0Cr and Ti44Al6Nb1.0Cr2.0V alloys are increased too.The microstructure of rapidly solidified Ti-44 Al alloy was in compared with Ti-48 Al alloy’s under the same cooling rate of 4.9×105K/s. The grain size is even smaller. The high content of Al makes the γ phase form easily. α→γ has decreased the grain size.Addition of Nb, Cr and V to Ti Al based alloys leads differences. For one thing, the high melting elements of Nb, Cr and V enhanced the melting point of alloys. The nucleation rate increases with the increasing of degree of supercooling; For another, the diffusion reaction is hindered in solidification by adding low diffusivity elements of Nb, Cr and V. The grain size decreases by the slowly growth rate. Refined crystalline strengthening and solution strengthening enhances the microhardness under rapid solidification.