Investigation Of Properties And Metal Injection Moulding Of Sn Micro-alloyed High Nb Containing TiAl Alloy

TiAl alloys exhibit unique properties,including low density,high specific strength,excellent oxidation resistance and creep resistance.The combination of those properties makes TiAl attractive for high-temperature applications.The alloys can be processed by powder metallurgy(PM),especially metal injection moulding(MIM),which is a cost-effective method that can fabricate small components with complex geometry on large scale.In addition,MIM is able to form near-net shaped parts with homogeneous and refined microstructures.However,the alloy powders are difficult to achieve full densification due to their low sintering activity.In this work,Sn micro-alloyed high Nb containing TiAl alloy has been produced by pressureless sintering with pre-alloyed high Nb-TiAl alloy powder and Sn powder as the sintering aid.The effects of Sn addition on the sintering densification,mechanical properties,high temperature compression deformation behavior and high temperature oxidation behavior of sintered alloys have been investigated based on theoretical calculation and experimentation.MIM technology of TiAl alloy is also studied in this paper.The results are shown as follows.Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y-xSn(TiAl-xSn)alloys with a high relative density can be obtained by pressureless sintering with various Sn additions(0～5at.%),at the sintering temperature ranging from 1430℃ to 1540℃.The results show that a small addition of Sn significantly reduces the densification temperature,and increases the relative density of sintered parts.Therefore,the small addition of Sn could effectively refine the microstructure and improve mechanical properties.The TiAl-lSn alloy sintered at 1510℃ for 2h shows a relative density of 98%with no β phase segregation.The specimens exhibit fine and uniform full lamellar microstructure,and the α2/γ lamellar colonies size are about 60-80μm.The lamellar spacing is increased with more than 1at.%Sn addition.Excessive’ addition of Sn(>5at.%)leads to the formation of Sn segregation around the grain boundaries.The hardness of Sn micro-alloyed TiAl alloy increased with Sn addition and the sintered TiAl-1Sn alloys have been found to possess superior room-temperature mechanical properties,with a Rockwell hardness of 72.2HRA,a compressive strength of 2938MPa,a yield strength of 680MPa,and a compression ratio of 29%,which is obviously higher than those of TiAl alloys.Sn mainly dissolves into α2 phase and few stays in y phase.The volume fraction of α2 phase obviously decreases in the alloy with Sn addition.During the high temperature compression deformation,the yield strength of the TiAl-xSn(x=0,0.5,1,1.5)alloys rises,whereas the ductility declines with the deformed temperature increasing and the strain rate decreasing.The ductile-brittle transition temperature(DBTT)is decreased by Sn addition.DBTT is 900℃ for TiAl-OSn alloy and 700 ℃ for TiAl-lSn alloy.Under the deformed condition of 800℃/0.002s-1,the deformation twinning is the main deformation mechanism for TiAl-OSn alloy and both deformation twinning and dislocations slipping are observed in TiAl-lSn alloy.The interrelations of peak flow stress,strain rate and deformed temperature can be described by Arrhenius equation modified by the hyperbolic sine function,which indicates that the hot compression deformation is controlled by thermal activation.Under the deformed condition of 700～1000℃/0.2～0.002s-1,the activation energy of high temperature deformation is 311kJ/mol for TiAl-OSn alloy and 429kJ/mol for TiAl-lSn alloy.The TiAl-xSn(x=0.5,1,1.5)alloys possess superior oxidation resistance compared with Sn-free alloy.At 900℃ for 100h in static air,the mass gains of the TiAl-xSn(x=0.5,1,1.5)alloys are less than that of TiAl-OSn alloy.Under the same condition,their oxide scales and structure formed on the surface of the alloys are obviously different from each other.For the TiAl-xSn(x=0.5,1,1.5)alloys,the sequence of the compact thin oxide layer from the surface to the substrate is an Al2O3 layer with Ti02 particles/TiO2 layer/Nb(Sn)-rich transition layer with a minor amount of TiN/Ti3Al layer/substrate.By comparison,the oxide scale of the TiAl-OSn is thick.The sequence of the oxide layer from the surface to the substrate is TiO2 layer/Al2O3 layer/mixed loose TiO2+Al2O3 layer/Nb-rich transition layer/substrate.Sn is mainly distributed in the transition layer,which results in effectively impeding the diffusion of Ti and O ions.Based on the first principles theories,Sn-doped γ-TiAl alloy systems have been studied with the concentration of dopant atoms selected as 1/54.The lattic parameters,the average formation energy,axial ratio,elastic modulus,and overlap population of these systems are calculated by CASTEP.The stability of Sn-dopedγ-TiAl systems is slightly lower than the pure y-TiAl system.Sn has higher possibility to occupy the positions of A1 atoms.The analysis of axial ratio and elastic modulus shows that doping of Sn will improve the ductility of y-TiAl alloy.As for the overlap population of the Sn-doped γ-TiAl compared with the pureγ-TiAl,it can be found that doping with Sn can weaken the anisotropy of the covalent bonds in γ-TiAl systems,which contributes partly to the ductility improvement of γ-TiAl alloys.A new catalytic debinding binder for MIM has been developed in this paper.It consists of 85wt.%POM(Polyoxymethylene),5wt.%HDPE(High Density Polyethylene),2wt.%EVA(Ethylene Vinyl Acetate Copolymer),3wt.%CW(Carnauba Wax)and 5wt.%SA(Stearic Acid).A critical powder loading of 65vol%is achieved for this feedstock.An ideal rheology is achieved by feedstock and the fluidity exponent n and the flow activation energy E are 0.521 and 28.18kJ/mol respectively,at 180℃ for a shear rate of 1412s-1.The appropriate debinding parameters are 115℃ for 6h,with an HNO3 feed rate of 1.3g/min.Owing to the low debinding temperature and short time,the impurity contents slightly increases.Effects of sintering temperature and holding time on the mechanical properties,microstructure and density have been investigated.The relative densities of alloys increase with the rise of sintering temperature and holding time,while excessive temperature and holding time result in microstructure with coarse grains,which reduces the mechanical properties.TiAl-lSn alloy sintered at 1510℃for 2h shows a tensile strength of 402MPa,which is lower than those of the as-cast materials.The elongation of 1.16%is close to those of the as-cast materials.The alloy has a compressive strength of 2930MPa and a compressibility of 34%,which are higher than those of the as-cast materials.