Study On Ti-43Al-9V-0.3Y Powders Densified By Spark Plasma Sintering And Its Microstructure And Mechanical Properties

Due to its relatively low density,high specific strength,modulus and excellent high-temperature creep resistance and oxidation resistance,TiAl alloy is considered to be a light-weight high-temperature structural material with great application potential in the aerospace and automotive fields,however,the poor plasticity at room temperature（RT）and poor hot working performance of TiAl alloys limit the application.Compared with traditional TiAl alloys,β-solidifing TiAl alloys,which solidify via body-centered cubicβphase region,allow for the formation of refined microstructures with balanced properties,has garnered interest.In addition,adding an appropriate amount of yttrium（Y）element to the TiAl alloy can significantly reduce the grain size and the lamellar spacing,and improve the strength and plasticity at room temperature,as well as high temperature deformation and oxidation resistance.Therefore,in this paper,the gas atomized Ti-43Al-9V-0.3Y powder was used as the raw material,and fine-grainedβ-solidifing TiAl alloy was densified by Spark Plasma Sintering（SPS）technology.The microstructure and phase structure of Ti-43Al-9V-0.3Y powders with different sizes were systematically studied in detail.Formation mechanism of SPS sintering neck and Prior Particle Boundaries（PPBs）of Ti-43Al-9V-0.3Y alloy was analyzed.The microstructure and mechanical properties of Ti-43Al-9V-0.3Y alloy prepared by SPS and the existing form and distribution of Y and its effect on microstructure and mechanical properties were investigated.First,through the characterization of the atomized Ti-43Al-9V-0.3Y powders with different sizes,it was found that there are three kinds of solidified microstructure according to the size of the powders:noncharacteristic planar crystal structure,cellular crystal structure and dendritic structure.The powders with a diameter of less than 15μm are composed of martensiticα′phase and part of the matrix residualβ0 phase,while the powders with a diameter of 20-50μm are composed ofβ0 single phase,and there is no trace of martensitic transformation.The microstructure of powders with diameter greater than 100μm consists ofβ0 phase,martensiticα′phase andα/α₂ phase.The content ofαphase increases with the increase of powder size,andγphase grains are precipitated in someαgrains.The martensitic phase is mainly distributed in the interdendritic region.The Y-rich precipitates in the large-size dendritic powders are mainly composed of a large number of nano-sized YAl₂ distributed in the interdendritic region,accompanied by a small amount of yttria.The Y-rich phase in the small and medium-sized powders has a low degree of segregation,but fine YAl ₂ and Y₂O₃ particles can still be observed in the microstructure.The study about SPSed sample sintered at 900-1000℃found that the microstructure is mainly composed of recrystallizedγandβ0 phase,sintered neck structures composed mainly ofα₂ phase were formed at the overlap of the original powder,and as the sintering temperature increased from 900℃to 1000℃,the sintering neck size grew up.The reason for the formation of sintering neck is that the oxygen enrichment in the sintering neck region caused by the oxidation of the original powder surface reduces theαtransition temperature;When the sintering temperature reaches 1100℃,the completely densified sintered body formed,but PPBs structure is still widely distributed in the sintered body,PPBs structure is mainly composed ofα₂/γlamellar colonies andγgrains.The PPBs structure is evolved from the sintering neck formed at low temperature,and its formation mechanism is directly related to the enrichment of oxygen element.By increasing the sintering temperature and lengthening the holding time,PPBs can be effectively eliminated and a uniform duplex microstructure can be obtained,which is achieved by promoting the diffusion of oxygen elements and improving theα-phase formation ability of the matrix.The microstructure and mechanical properties of SPS sintered Ti-43Al-9V-0.3Y alloy were investigated.It was found that when sintered at 1100℃to 1150℃（holding for 5 min）,the microstructure of Ti-43Al-9V-0.3Y alloy is a uniform Duplex microstructure consisting ofα₂/γlamellar colonies,β0 andγphase,accompanied by very fine Y-rich precipitates distributed uniformly in the matrix.There are two kinds of ultrafine Y-rich precipitates in the microstructure,namely YAl₂ and Y₂O₃.The orientation relationship between Y₂O₃ andγ-TiAl matrix is as follows:[011]Y₂O₃∥[001]γand（6-33）Y₂O₃∥（-2-20）γ,and the orientation relationship between YAl₂ andγ-TiAl matrix can be described as follows:[4-61]YAl₂∥[123]γ,（320）YAl₂∥（-1-11）γ,the two Y-rich particles can pin grain boundaries,hinder grain growth,and hinder dislocation movement during plastic deformation.Therefore,the alloy with duplex microsturcture has excellent tensile properties at room temperature.The highest yield stress,ultimate stress and plastic elongation are 920.2 MPa,1113.1 MPa and 1.44%,res pectively.When the sintering temperature is higher than 1200℃,the microstructure of the alloy evolves into a Nearly Lamellar（NL）structure composed of largeα₂/γlamellar colonies and a small amount ofβ0 andγgrains distributed around them,some Y-rich precipitates grew and enriched,and showed significant oxidation aggregation locally.Compared with the duplex alloy,the tensile strength and ductility of the high-temperature sintered alloy decrease significantly at room temperature due to the coarsening of lamellar colonies,the decrease ofγphase grains and the segregation of Y-rich phase.The high temperature test results show that the NL alloy sintered at 1200℃exhibits the highest ultimate tensile strength at various tensile temperatures,which are:700℃-881.57 MPa,750℃-743.63 MPa and 800℃-603.58 MPa,respectively.However,the high temperature plastic deformation capacity of the alloy sintered at 1200℃is much lower than that of the alloy with duplex microstructure.At 800℃,the small-size ofα₂/γlamellar colonies in the duplex microstructure can coordinate the plastic deformation ofγphase,while the larger sizeα₂/γlamellar colonies in the NL microstructure are difficult to coordinate the deformation.Finally,the microstructural evolution and mechanical properties of high yttrium content TiAl alloy prepared by SPS were investigated.It was found that by increasing the temperature,the duplex microstructure,NL microstructure and Fully Lamellar（FL）microstructure could be obtaine d,respectively.The duplex microstructure sintered at 1100℃had the highest compressive strength and ductility at room temperature.In the duplex and NL microstructure,YAl₂precipitates are dispersed in the matrix in the form of fine particles,while in the FL microstructure,a network structure consisting of hard and brittle striplike YAl ₂precipitates is formed,resulting in the division of the alloy microstructure and significantly decrease the mechanical properties.