Microstructure And Mechanical Properties Of TiAl Alloys Solidified With Electric Current

TiAl alloys have excellent properties such as low density,high specific strength and modulus,good creep resistance and oxidation at high temperatures.They have been considered as one of the promising candidate materials in application of military,stationary and rotating components of propulsion system in aircraft engine industries,and also shown great potential in high temperature components in automobile,such as turbocharger and exhaust valve.However,the low ductility and poor fracture toughness at room temperature have become an urgent problem need to be solved which limits the engineering applications and further development of TiAl alloys.The electric current processing is used as a novel technique to act on the solidification process of TiAl alloys.Solidification with current could refine the microstructure and avoid the contamination of refining impurities.On this basis,changing current type and parameters could modify the solidification behavior,optimize microstructure and improve mechanical properties.Direct electric current or pulse electric current was applied on the solidification process of Ti-48Al-2Cr-2Nb alloy in the study.The influences of current parameters on growth direction,the width and the continuity of the columnar crystal,and the interface morphology,lamellar space,lamellar growth orientation,element segregation have been studied.The mechanism of microstructure formation under current effect has been analyzed.The mechanical properties of the treated alloys have been tested,and the relationship between current parameter,microstructure and mechanical properties has been established.The magnetic field and distribution of Lorentz force in the TiAl alloy melt under current effect have been simulated by Ansys software.Results show that magnetic field and Lorentz force in the TiAl alloy melt under direct electric current decreases from outside to inside,and the magnetic flux density or Lorentz force reaches the smallest value at the axis of the rod melt.The variation amplitude of the magnetic induction intensity along the radial direction is greater than that of along the axial direction.The magnetic field or Lorentz force in the melt increases with the increase of the DC current density.The magnetic induction intensity or Lorentz force is concentrated in the skin layer of the melt,and decreases from the sides of the skin layer to the center of the skin layer.The skin effect is more significant with higher current density or pulse frequency of the pulse current,and therefore,the magnetic induction intensity and Lorentz force in the skin layer are much larger.The distributions of magnetic induction intensity and Lorentz force along the axial direction in the skin layer were uniform.Application of DC current in the directional solidification process of Ti-4822 alloy shows that the Joule heat induced in the solidification process can compensate the heat transfer.The axial temperature gradient in the TiAl alloy melt increases with increment of current density,which leads to increase in growth continuity of columnar grains,decrease in the deflection angle of grain growth and decrease in the concave degree of the solidification interface.On the other hand,Joule heat will result in the dendrites remelt and break down,and increase the nucleation rate and supercooling degree,therefore,refine the width of columnar grains.The direct current promoted the composition homogenization of the melt and the microstructure of the alloy,and reduced the segregation degree of the solute element.Decrease in the deflection angle of crystal grain growth,increase in growth continuity of columnar grains,refinement of columnar grains and lamellar spacing,increase in lamellar with small angle,decrease in the segregation of alloying elements,they are all account for the improvement of the mechanical properties of the alloys.The yield strength,tensile strength and fracture toughness of Ti-4822 alloy first increase then decrease with the increment of the current density.The high temperature compressive yield strength and ultimate strength of the Ti-4822 alloy reach their maximum when the current density is 64 m A/mm².Compared with that without current,the maximum of compressive yield strength and ultimate strength increases about 131.8% and 25.3% to 1050 MPa and 1310 MPa,respectively.And the maximum of tensile strength and yield strength at room temperature are 563 MPa and 526 MPa,respectively.The maximum of the fracture toughness is 23.25 MPa·m^1/2.And the elongation reaches its maximum of 0.90% when the current density is 96 m A/mm².Application of pulse current in the directional solidification process of Ti-4822 alloy shows that the Joule heat induced in the solidification process compensates the radial heat dissipation and increases the axial temperature gradient.Increasing the density and frequency of pulse current will lead to increase in growth continuity of columnar grains,decrease in the deflection angle of crystal grain growth,decrease in the concave degree of the solidification interface and decrease in the segregation of the solute element.The magnetic pressure results from the pulse current cause the increase in the supercooling degree,and increase the nucleation rate,therefore,the spacing of primary dendrites decrease.The solid-liquid interface will destabilize for the higher perturbation during the growth of melt when increasing the density and frequency of pulse current.And therefore,the effect of the pulse current is more obvious.The variation of supercooling degree and temperature gradient caused by the Joule heat may result in the change of the lamellar spacing of TiAl alloys.The yield strength,tensile strength and fracture toughness of Ti-4822 alloy first increase then decrease with the increment of the pulse current density.The high temperature compressive yield strength and ultimate strength of the Ti-4822 alloy reach the maximum when the current density is 64 m A/mm².Compared with that without pulse current,the maximum of compressive yield strength and ultimate strength increases about 128% and 34.2% to 1020 MPa and 1396 MPa,respectively.And the maximum of tensile yield strength at room temperature are 557 MPa,respectively.The maximum of the fracture toughness is 25 MPa·m^1/2.And the tensile strength and elongation reaches the maximum of 595 MPa and 0.97% when the current density is 96 m A/mm².The yield strength,tensile strength and fracture toughness of Ti-4822 alloy increase with the increment of the pulse current frequency when the pulse current density keep constant.The high temperature compressive yield strength and fracture strength reaches the maximum of 1255 MPa and 1370 MPa when the pulse current frequency is 200 Hz.Compared with that without pulse current frequency,they improve about 180.6% and 31.7%,respectively.And the yield tensile strength and maximum tensile strength at room temperature are 568 MPa and 605 MPa.The elongation and the fracture toughness are 0.93% and 26.5 MPa·m^1/2 respectively.