Effect Of B And C On Solidification Segregation And Hot Ductility Of U720Li Alloy

Ni-base U720Li is a ?'-Ni₃?Al,Ti?precipitation-hardened wrought superalloy that is widely used for turbine disks in the temperature range of 650⁷30 ? owing to the excellent creep,fatigue,oxidation and corrosion resistances at elevated temperatures.However,the home made bars of U720Li alloy with a diameter fit for aero engine disk forging have not been worked out till now.The high Al,Ti contents?AI+Ti?7.5%wt.%?leads to serious solidification segregation and deteriorates the hot ductility,which makes it very difficult for the U720Li alloy to be processed through the conventional cast and wrought?C&W?route.C and B are two important strengthening elements in U720Li alloy,which significantly influence the segregation,as-cast microstructure and hot workability.Therefore,the effect of B and C on the solidification segregation,homogenization and hot ductility of U720Li alloy were investigated in the present thesis.The M₃B₂ boride and Ni₅Zr phase are always precipitated in front of the eutectic??+?'?in the interdendritic region of as-cast U720Li alloy,which indicates that B and Zr are segregated at the solid/liquid interface during solidification.The Incipient melting temperatures of the low-melting point phases Ni5Zr,M₃B₂ and ?-Ni3Ti are measured,and a three-steps homogenization treatment has been established accordingly,which eliminates the low-melting point phases in as-cast microstructures,reduces the segregation degree of elements and improves the hot ductility of U720Li alloy efficiently.The mechanism for??+?'?precipitation has been investigated by observing the nucleation and growth of the two-phases microstructure in the liquid pools.The alternate ?/?' structure were always observed to be initiated at the edge of the liquid pools and grow into the liquids.At the center of the liquid pool,there were honeycomb-like y phase and lath-like ? phase instead of??+?'?.And hence it is believed that the ?' is nucleated via a peritectic reaction of?L + ????' at the edge of liquid pool,and the peritectic reaction naturally precedes over the eutectic reaction of?? +?'?,because the energy for the ?' nucleation from the y matrix via peritectic reaction is much lower than that from the liquid pools,which is verified by the fact that no??+?'?eutectic was observed at the center of the liquid pool when cooled at a rate as high as 100 ?/s.The y and y' in??+?'?tend to grow into a lamellar structure because it is difficult for them to nucleate directly from the liquids,and hence,the y'finally grow into larger particle.B greatly retards the y matrix solidification which is verified by the fact that it obviously increases the area fraction of liquid pools but markedly reduces that of the segregated region enriched in Ti around the liquid pools.Based on this phenomenon,B is assumed to form a layer at the solid/liquid interface and inhibits the attachment of atoms on or jumping off the solid front,and hence lowers the y matrix solidification rate.The retarding of y matrix solidification results in deeper undercooling and more eutectic forming element Ti left in the residual liquids,which in turn promotes the eutectic??+?'?precipitation.The increasing eutectic??+?'?precipitation consumes more Ti and Zr atoms,and hence reduces the ?-Ni3Ti and Zr-rich phase formation.The effect of B on the hot ductility of U720Li alloy has been investigated.The results indicate that the eutectic??+?'?is the main source of crack initiation in as-cast alloy.B addition promotes the eutectic??+?'?precipitation at the grain boundaries,which significantly favors the crack propagation of as-cast U720Li alloy and deteriorates its hot ductility.However,after the homogenization treatment the tensile fracture mode changes from intergranular to transgranular and the hot ductility is increased substantially with increasing B content.The effect of C on the solidification segregation and hot ductility of U720Li alloy has been examined.It has been found that C addition increases the blocky MC carbide formation considerably,which consumes more Ti,B,Mo and Zr and hence reduces the precipitation of eutectic??+?'?,? phase,borides and Zr-rich phase significantly.Appropriate amount of C can significantly reduce cracking at the eutectic??+?'?/matrix interface in as-cast alloy and the crack initiation at the grain boundaries of the as-homogenized alloy,and hence improves the hot ductilities of as-cast and as-homogenized U720Li alloy.However,too much C results in the formation of much larger carbides,which promotes crack initiation along the carbide/matrix interface and worsens the hot ductility.