EFFECTS OF TEMPERATURE GRADIENT ON LAMEL- LAR ORIENTATIONS OF DIRECTIONAL SOLIDIFIED TiAl-BASED ALLOY

Directional solidification experiments under heating temperatures of 1580℃and 1650℃were performed on Ti-47Al-2Cr-2Nb alloy in order to obtain the evolution of lamellar grains.Prom microstructural analysis in the mushy zone of directional solidified ingots,βphase was firstly solidified, thenαphase was formed through peritectic reaction;αphase depended on the pre-existedβphase on which it nucleated,and only one of the 12αorientation variants was selected during solid stateβ→αtransformations.As the average temperature gradient of the mushy zone was increased from 40 K/cm to 160 K/cm,the solidification interface morphologies were changed from columnar dendrite to celluar dendrite.Eliminating the influence of cutting plane to theγ-lamella orientation,it was shown that the columnar lamellar grains with an angle of approximately 74°to growth direction gradually overgrew the ones with the angle of nearly 45°to the growth direction at the withdrawal rate of 1 mm/min and temperature gradient of 40 K/cm.Increasing temperature gradient to 160 K/cm,the grains with the angle of about 74°progressively rejected other with the angle of nearly 90°to the growth direction.Calculation of theβdendrite preferred growth orientation indicated thatβdendrites tend to grow along the 110_βorientation at the present solidification experiments.Increasing temperature gradient,the preferred growth tendency ofβdendrite became more drastically,other orientations,such as the 001_βdirectional orientation,could rapidly be replaced.