Study On Non-equilibrium Solidification Characteristics And Microstructure Of Ti-Al Binary Alloy

Sub-rapid solidification process is not limited by process equipment and product size,easy to industrialize,and has many advantages of rapid solidification,which has good application prospects.TiAl alloy belongs to a typical peritectic system.The change of cooling rate during solidification can make its solidified structure present various morphologies.Sub-rapid solidification technology can maximize the performance potential of alloy materials.In this paper,TiAl alloy specimens with different cooling rates were obtained by casting TiAl alloy plates with different thickness.The effects of cooling rates on the solidification structure,peritectic point and Cp point,the solid phase transition temperature of TiAl binary alloy under sub-rapid solidification were studied.Firstly,the relationship between cooling rate and wall thickness was determined by numerical simulation and experiment.Then,the influence of cooling rate on solidification behavior,law of formation and evolution of solidification structure of TiAl alloy under sub-rapid solidification was studied.The following main research results were obtained:1)Solidification behavior of TiAl alloy under sub-rapid solidificationThe cooling rates of Ti47Al alloys whose wall thicknesses respectively are 1mm,2mm,4mm,6mm and 8mm are 1081K/s,490K/s,260K/s,170K/s and 120K/s,respectively.The relationship between the cooling rate and the wall thickness obtained by the fitting process is:y=1074.81x-1.05With the increase of cooling rate,the lattice constants of the ? phase and the a2 phase decrease.The diffraction peaks of(111)? and(002)?2 face shift to a higher angle,and the orthogonal distortion(c/a)increases.The peritectic reaction temperatures of Ti47Al alloys whose cooling rates respectively are 1081K/s,260K/s and 170K/s are 1483?,1472? and 1480?,and the solid phase transition(???+?)temperatures are 1301?,1304.8? and 1307? respectively.The relationship between the difference between the solid phase transition temperature of Ti47Al alloy under sub-rapid solidification and the solid phase transition temperature under equilibrium solidification and the cooling rate is as follows:y=-50.33e-x/79.69+50.35Under sub-rapid solidification,the cooling rate increases,and the peritectic reaction point and the C? point move toward the high A1 direction.Compared with Al content of the peritectic point and the C? point under equilibrium solidification,the relationship between difference between Al content of peritectic point and the C? point under the sub-rapidally solidified conditions and under equilibrium solidification conditions and cooling rate respectively are:y=ln(0.987+0.00123x);y=0.045x0.2852)Solidification structure of TiAl alloy under sub-rapid solidificationUnder the condition of sub-rapid solidification,the solidification structure of TiAl alloy with the solidification path of beta phase is dominated by coarse lamellar structure,while other structures are few.With the increase of Al content or the decrease of cooling rate,the number of coarse lamellar structure will decrease.The solidification structure of sub-peritectic TiAl alloy is full lamellar structure with fine lamellar layer and the size of dendrite will decrease with the increase of Al content.The solidification structure of hyperperitectic TiAl alloy is equiaxed dendrite with full lamellar structure.With increase of the cooling rate,the content of ? phase decreases and the content of ?2 and B2 increases.Under sub-rapid solidification conditions,the cooling rate is not enough to destroy the relationship between ?2 phase and ? phase.That is:(0001)?2//(111)? will not change,and the secondary dendrite arm spacing decreases with the increase of Al content or cooling rate,and the relationship between secondary dendrite arm spacing and cooling rate is as follows:?=186.36v-0.36Under sub-rapid solidification conditions,the composition of TiAl alloys with three solidification paths is inhomogeneous.At the same time,the composition inhomogeneity increases with the increase of cooling rate at the grain boundary,and the Al content increases with the increase of Al content at the grain boundary.