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...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 ofα2 phase in binary alloy increased with the cooling rates,which is caused by moreαphases directly transforming toα2 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 200-600μm of as⁃cast to 18μm of the alloy cooled at 4.9×10^5 K/s,which is caused by the undercooling induced from rapid solidification.Lamellar spacing was decreased from 4.5μm of as⁃cast to 1.1μm by rapid solidification.With the increase of cooling rate,the content ofα2 phase increased andγphase 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 ofα2 phase.展开更多
Solute diffusion controlled solidification model was used to simulate the initial stage cellular to dendrite transition of Ti44Al alloys during directional solidification at different velocities. The simulation result...Solute diffusion controlled solidification model was used to simulate the initial stage cellular to dendrite transition of Ti44Al alloys during directional solidification at different velocities. The simulation results show that during this process, a mixed structure composed of cells and dendrites was observed, where secondary dendrites are absent at facing surface with parallel closely spaced dendrites, which agrees with the previous experimental observation. The dendrite spacings are larger than cellular spacings at a given rate, and the columnar grain spacing sharply increases to a maximum as solidification advance to coexistence zone. In addition, simulation also revealed that decreasing the numbers of the seed causes the trend of unstable dendrite transition to increase. Finally, the main influence factors affecting cell/dendrite transition were analyzed, which could be the change of growth rates resulting in slight fluctuations of liquid composition occurred at growth front. The simulation results are in reasonable agreement with the results of previous theoretical models and experimental observation at low cooling rates.展开更多
基金National Natural Science Foundation of China(Grant No.51825401)the China Postdoctoral Science Foundation(Grant No.2019TQ0077).
文摘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 ofα2 phase in binary alloy increased with the cooling rates,which is caused by moreαphases directly transforming toα2 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 200-600μm of as⁃cast to 18μm of the alloy cooled at 4.9×10^5 K/s,which is caused by the undercooling induced from rapid solidification.Lamellar spacing was decreased from 4.5μm of as⁃cast to 1.1μm by rapid solidification.With the increase of cooling rate,the content ofα2 phase increased andγphase 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 ofα2 phase.
基金National Natural Science Foundation of China (50434030)
文摘Solute diffusion controlled solidification model was used to simulate the initial stage cellular to dendrite transition of Ti44Al alloys during directional solidification at different velocities. The simulation results show that during this process, a mixed structure composed of cells and dendrites was observed, where secondary dendrites are absent at facing surface with parallel closely spaced dendrites, which agrees with the previous experimental observation. The dendrite spacings are larger than cellular spacings at a given rate, and the columnar grain spacing sharply increases to a maximum as solidification advance to coexistence zone. In addition, simulation also revealed that decreasing the numbers of the seed causes the trend of unstable dendrite transition to increase. Finally, the main influence factors affecting cell/dendrite transition were analyzed, which could be the change of growth rates resulting in slight fluctuations of liquid composition occurred at growth front. The simulation results are in reasonable agreement with the results of previous theoretical models and experimental observation at low cooling rates.
