Solidification Behavior And Microstructure Evolution Of High Nb Containing TiAl Alloys

?-TiAl alloys are regarded as a promising candidate in aeronautics,astronautics and automobile industry due to their superior properties such as low density,high specific strength,high specific stiffness,excellent oxidation resistance and creep strength.In recent years,a large part of scientific work has been dedicated to improve the service temperature of TiAl alloys,and the addition of Nb becomes an increasing tendency.High Nb containing TiAl alloys have attracted significant attentions because of their high service temperature,i.e.,850^oC.However,the application of these alloys is limited by problems including micro-segregation,inhomogeneous microstructure,sharp texture and severe cracking tendency.The problems mentioned above are closely related to the phase transformation behaviors and a deep understanding microstructure evolution of these alloys is of great importance.In this work,the phase transformation behaviors and microstructure evolution of a typical high Nb containing TiAl alloy,i.e.,Ti-45Al-8.5Nb-?W,B,Y?alloy was investigated.Then,based on the phase transition sequence,a novel two-step isothermal treatment?TIT?during solidification process was proposed to control the microstructure of Ti-45Al-8.5Nb-?W,B,Y?alloy and the influence of TIT on lamellar structure,homogeneity,texture and cracking was analyzed.Thermal cycling in mushy zone was performed to refine the microstructure of peritectic-solidifying Ti-48Al-2Cr-2Nb alloy and?-solidifying Ti-45Al-8.5Nb-?W,B,Y?alloy.Results show that the refinement effect is strongly dependent on solidification routes.The lamellar colony of Ti-48Al-2Cr-2Nb alloy was refined with the increasing of thermal cycles and the dendrite morphology evolves as the following process:columnar?disfigured?spheroidal.These phenomena are resulted from the remelting of dendrites during thermal cycling and the reinforced convection caused by electromagnetic stirring.As for the Ti-45Al-8.5Nb-?W,B,Y?alloy,melt cycling treatment has no significant effect on grain size because?grains produced by???transformation cannot be refined by thermal cycling treatment.The effect of solidification rates on the solidification behavior of Ti-45Al-8.5Nb-?W,B,Y?alloy was investigated.Results show that with the decrease of solidification rate,the solidification routes change from?solidification to partial peritectic solidification due to Al enrichment in residual liquid.The variation of solidification routes results in a non-uniform microstructure that fine lamellar structures form in dendrite core and coarse lamellar structures form in interdendritic region.The average lamellar colony size increases with the solidification rate decreasing.The microstructure is the most inhomogeneous at the solidification rate of 10^oC/min.The average lamellar colony size and microstructure homogeneity are controlled by the volume fraction of peritectic?phase.An in-situ observation on?to?phase transformation in Ti-45Al-8.5Nb-?W,B,Y?alloy was conducted using high-temperature laser-scanning confocal microscopy.Results show that the nucleation of?grain occurs at?grain boundaries or on borides.The morphology of the?phase is strongly dependent on the cooling rate.On fast cooling,?phase presents a Widmannst?tten morphology with a crystallographic relationship between the matrix?phase,whereas the equiaxed ones form at slow cooling developing as a spherical precipitation.EBSD was employed to investigate the evolution of texture.Results show that the volume fraction of Burgers?phase decreases with cooling rate reducing.The orientation relationship between?matrix and TiB is:{011}_???001?_B27 and<111>_??[010]_B27,and there are two groups of orientation relationship between?matrix and TiB,which cause the formation of Burgers and non-Burgers?phase,respectively.Competitive growth between these two groups of?phase occurs at fast cooling,i.e,non-Burgers?phase is inhibited and the Widmannst?tten?phase can fully develop.A novel two-step isothermal treatment during solidification process was proposed to control the microstructure of Ti-45Al-8.5Nb-?W,B,Y?alloy.The alloy exhibited a non-uniform lamellar structure after non-isothermal treatment.A homogeneous microstructure is obtained after 1^st IT,which is attributed to the elimination of peritectic?phase and the microstructural refinement by???transformation.During 2^nd IT,the spheroidizing of the Widmannatatten-?laths and the nucleation and growth of equiaxed?phase happens resulting in the formation of equiaxed lamellar colonies and reduction of the texture.After TIT,a fine and uniform lamellar structure without micro-segregations as well as reduced texture is obtained so that the cracking tendency is reduced.This new approach is particularly suitable for high quality castings of high Nb containing TiAl alloys.