Study On The Mechanism Of ????? Phase-transformation And The Law Of Strength And Tougheness Of ?-CEZ Titanium Alloy

?-CEZ alloy(The nominal composition in weight percent is Ti-5Al-4Mo-4Zr-2Sn-2Cr-1Fe),which was a kind of near?titanium alloy,was developed for aeroengine compressor disks,springs,fastens,et al in 1990s by CNECMA company and CEZUS company of Franch.This alloy can get very high strength after solution and ageing treatment because of high content of?stabilizer elements Mo,Cr and Fe.Most of researchs on?-CEZ alloy have concentrated on its structural stability,high temperature deformation microstructures,phase transformation kinetics,hot-working characters and structure/mechanical properties relationships.However,little researches have been reported on?-CEZ alloy about?phase,secondary?phase,related transformations,microstructures and properties.Therefore,in this paper,Mechanisms of?????phase transformations of?-CEZ alloy were analyzed on the basis of observing microstructures of?phase,?phase and?phase by OM,SEM and high resolution transmission electron microscopy(HRTEM).The phase transformation dynamics models of continuous heating and isothermal aging of?-CEZ alloy were established by changing heating temperature and holding time.Furthermore,On the basis of phase transformation researches,the structures and properties of different heat-treated?-CEZ alloy were compared and analyzed.The main research contents and conclusions are as follows:(1)Recrystallization rule of?-CEZ alloy was studied.It was found that with the increasing of solid solution temperature below transus temperature,the rod-like primary?phase reduced and evolved into spherical,and the dissolution rate of primary?phase decreased.The kinetics expression of grain size and temperature was(?)=3.27×10⁶×exp)(-93.5/8.314T),and the expression of grain size and time was(?)=61.56t^0.38.In addition,by comparing the air-cooled structures after solution treated at different temperatures,it was found that the higher the solution temperature,the lower the stability of residual?phase.(2)The aging process of?-CEZ alloy can be divided into three parts by comparing the hardness after ageing at different temperatures.The hardness of the alloy was the lowest when the alloy was aged below 300?for 2 hours because no secondary?phases were precipitated.When the alloy was aged between 350?and 550?,very fine nano-sized needle-like secondary?phases were precipitated,so the hardness of the alloy increased rapidly.While increasing ageing temperature to above 600?,?phase grown up,especially the?_WGB and?_WM grown dramaticly,so the hardness of?-CEZ alloy decreased again.(3)The precipitation law of secondary?phase in?-CEZ alloy during heating process of ageing was studied by DSC.It was found that the starting and ending temperatures of precipitating secondary?phase increased as heating rate increased.The precipitation law of?phase can be divided into two parts by temperature.Under 500?,due to the auxiliary effect of?phase,fine secondary?phase can be precipitated rapidly during the heating process.Over 600?,larger size?_WGB and?_WM were precipitated at grain boundaries and in grains respectively.Furthermore,between 700?and 800?,the precipitation rate of secondary alpha phase is similar(with the similar Avrami index n),however,secondary?phases grow rapidly when the temperature reaches 850?(reaction rate factor k increased significantly).(4)The type,size,shape and microstructure of?phase in?-CEZ alloy were investageted by HRTEM.The fine mini-spheroid?precipitates,as small as 1nm-2 nm,would form from?matrix phase while the?-CEZ alloy was quenched in water,following solution treated at 920? for 2 hours.The ideal structure of ? phase would be expected to{111}?planes shift 0.5d₂₂₂ along<111>direction.The structural relationship between?phase atoms and?phase atoms in a cell is as follows::(1/3)<(?)2>?(?)<((2/3)(1/3)(1/2))>?,(2/3)<(?)2>?(?)<((1/3)(2/3)(1/2))>?;The corresponding relationship between?phase and?phase after shear is as follows:<111>??<0001>?,<1(?)0>??<11(?)0>?.(5)The relationship between?phase and?phase in?-CEZ alloy was comprehensively analyzed.It was found that isothermal?with a long axis about 10nm would be formed,which would help secondary?phase precipitated in adjacent area of?matrix.The transformation process of???phase in?-CEZ alloy can be summarized as follows:???+athermal???+isothermal???+isothermal?+secondary?.(6)By increasing the solution temperature of?-CEZ alloy from 830?to 870?,it was found that the content of primary?phase decreased and the?grain boundary appeared,so the room temperature strength and plasticity of the alloy decreased.After ageing at 650?for 6 hours,a large number of fine needle-like secondary?phases precipitated,which made the strength increase significantly and the plasticity decrease slightly.Furthermore,after ageing process,the strength increased and the plasticity decreased with the increase of solution temperature.By analyzing the plane fracture toughness,it was found that the fracture toughness increases slightly with the increase of solution temperature.However,the K_IC of?-CEZ alloy with equiaxed?phase was ranging from 30 to 45 MPa m^1/2.In contrast,lamellar primary?phase can cause crack deflection and significantly improve the fracture toughness of the?-CEZ alloy.(7)By comparing the microstructures and properties of?-CEZ alloy with water cooling,oil cooling and air cooling after solution treatment,it was found that the strength of the alloy increases slightly as the cooling rate decreases,the equiaxed primary?phase grows up to rod and long strip shape,and the content of?phase increases relatively.After aging,needle-like secondary?phase precipitated from metastable?phase,and the strength of the alloy increases rapidly.With the decrease of the cooling rate of original solution,the strength decreased and the plasticity increased.(8)Microstructures and properties of?-CEZ alloy after aged from 550?to 650?,followed solution treated at 850?were analyzed.It was found that the primary?phase transformed to rod-like and content increased with the aging temperature increasing from550?to 650?,meanwhile the size of needle-like secondary alpha phase increased obviously.Therefore,the strength of the?-CEZ alloy decreased,the plasticity and fracture toughness increased.