Study On The Microstructure And Properties Of Ti-46.5Al-4Nb-1.5V-0.5Cr Alloy By Cold Crucible Directional Solidification

TiAl-based alloy is an intermetallic compound and it has been recognized as the most promising lightweight structural materials at high temperature,because of its excellent performance,such as low density,high melting point,high specific strength,high temperature strength and good oxidation resistance.However,it has disadvantages,mainly including poor plasticity at room temperature and poor processing ability,leading to its restriction in industrial application.In orded to improve the properties of TiAl based alloys,?-phase is introduced into the microstructure to improve the high-temperature deformation ability of the alloy by adding ?-phase stabilizing elements.In this paper,Ti-46.5Al-4Nb-1.5V-0.5Cr(at.%)alloy was prepared by cold crucible directional solidification techneque in orded to investigate the influence of different process parameters on the solidification structure of the alloy.The room temperature mechanical property of the alloy was analyzed by room temperature compression and fracture toughness test.In addition,the deformation mechanism of directional solidified TiAl alloy was studied by high temperature compression deformation experiments,which can provide a reference for hot processing technology.In this paper,Ti-46.5Al-4Nb-1.5V-0.5Cr alloy billets were prepared by changing parameter of the power and pulling rate.The results showed that when the pulling rate was 1.0 mm/min,the power was 45 KW or 50 kW,the directional solidification effect was poor.When the power was too low to make the internal temperature of the sample melten pool was inhomogeneous,forming a "W" shaped solid-liquid interface,leading to the poor directional solidification effect.With power increasing,the superheat of the melt increased and electromagnetic stirring enhanced.It was benificial to directional solidification.Thus the power increased to 55 kW,the alloy has a excellent directional solidification orientation.When the power was 50 kW,the pulling rate was 0.6 mm/min or 0.8 mm/min,the solid-liquid interface was concave interface,and the heat transfer between the solid-liquid interface was stable,and the mushy zone was composed of dendritic dendrites with definite orientation,resulting in excellent directional solidification effect of the alloy.The straight or nearly flat solid-liquid interface was beneficial to obtain directional solidification sample with good directional effect and columnar crystal parallel to the direction of pulling.When the pulling rate was 1.0mm/min,the longitudinal temperature gradient would increase with the power increasing from 45 kW to 55 kW,which makes the phenomena of supercooling weakened,leading to inhibiting ? phase solidification.In addition,the lamellar spacing refined with the nucleation rate increasing.The alloy with the process parameters of 55 kW and 1.0 mm/min had a mushy zone composed of dendrites.The primary dendrite arm was perpendicular to the secondary dendrite arm,showing the solidification characteristics of ? phase and the directional effect was excellent.When the pulling rate was constant,the compressive strength and fracture toughness of the alloy were higher than that of the original ignot with the power increasing.Because the directional solidification technique reduced the casting defects.When the power was 50 kW,pulling rate increased from 0.6 mm/min to 0.8 mm/min,which makes tendency of composition supercooling aggravating and leads to promoting ? phase solidification.And they had the mushy zone composed of dendrites,the angle between the primary dendritic arm and the secondary dendritic arm is 90° or 60°,showing the ? phase solidification and ? phase solidification.With the increasing of the pulling rate,the proportion of large angle lamellae increased in the alloy structure,leading to the compressive strength increasing and fracture toughness of the alloy decreasing.The alloy sample(55 kW,1.0 mm/min)had excellent directional effect and primary phase was ?-phase.Its room temperature compressive strength was 1249 MPa,and fracture toughness was 26.1 MPa · m1/2.The alloy was subjected to high temperature compression deformation test at 1050~1150?,0.001~0.1S-1 by Gleeble-1500 D thermal simulation test machine.The true strain was 0.69.The flow stress was closely sensitive to the deformation temperature and strain rate.The flow stress would increase with decreasing the temperature and increasing the strain rate.According to the flow curves and the hyperbolic-sine equation,the deformation activation energy of directionally solidified TiAl alloy was 589.26 kJ/mol.During the compressive hot-deformation of Ti Al-based alloy,the ?-phase was elongated and perpendicular to the compression direction and the lamellar colonies featured bent and buckled lamellar,and the orientation was perpendicular to the compression direction.Dynamic recrystallization occurred in the lamellar structure as well as lamellar colonies interface,resulting in forming ? equiaxed grains.The ?-phase fractured,broke and spheroidized under a high strain rate.The existence of ?-phase can coordinate the deformation of ?2/? grains and delay germination of crack.In a word,?-phase was beneficial to deformation of TiAl alloy.