Study On The Mechanism Of β→ω→α Phase-transformation And The Law Of Strength And Tougheness Of β-CEZ Titanium Alloy

β-CEZ alloy（The nominal composition in weight percent is Ti-5Al-4Mo-4Zr-2Sn-2Cr-1Fe）,which was a kind of nearβtitanium 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 ofβstabilizer elements Mo,Cr and Fe.Most of researchs onβ-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 onβ-CEZ alloy aboutωphase,secondaryαphase,related transformations,microstructures and properties.Therefore,in this paper,Mechanisms ofβ→ω→αphase transformations ofβ-CEZ alloy were analyzed on the basis of observing microstructures ofβphase,ωphase andαphase by OM,SEM and high resolution transmission electron microscopy（HRTEM）.The phase transformation dynamics models of continuous heating and isothermal aging ofβ-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-treatedβ-CEZ alloy were compared and analyzed.The main research contents and conclusions are as follows:（1）Recrystallization rule ofβ-CEZ alloy was studied.It was found that with the increasing of solid solution temperature below transus temperature,the rod-like primaryαphase reduced and evolved into spherical,and the dissolution rate of primaryαphase 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 residualβphase.（2）The aging process ofβ-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 secondaryαphases were precipitated.When the alloy was aged between 350℃and 550℃,very fine nano-sized needle-like secondaryαphases were precipitated,so the hardness of the alloy increased rapidly.While increasing ageing temperature to above 600℃,αphase grown up,especially theα_WGB andα_WM grown dramaticly,so the hardness ofβ-CEZ alloy decreased again.（3）The precipitation law of secondaryαphase inβ-CEZ alloy during heating process of ageing was studied by DSC.It was found that the starting and ending temperatures of precipitating secondaryαphase increased as heating rate increased.The precipitation law ofαphase can be divided into two parts by temperature.Under 500℃,due to the auxiliary effect ofωphase,fine secondaryαphase can be precipitated rapidly during the heating process.Over 600℃,larger sizeα_WGB andα_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,secondaryαphases grow rapidly when the temperature reaches 850℃（reaction rate factor k increased significantly）.（4）The type,size,shape and microstructure ofωphase inβ-CEZ alloy were investageted by HRTEM.The fine mini-spheroidωprecipitates,as small as 1nm-2 nm,would form fromβmatrix phase while theβ-CEZ alloy was quenched in water,following solution treated at 920℃ for 2 hours.The ideal structure of ω phase would be expected to{111}βplanes shift 0.5d₂₂₂ along<111>direction.The structural relationship betweenβphase atoms andωphase atoms in a cell is as follows::（1/3）<（?）2>β（?）<（（2/3）（1/3）（1/2））>ω,（2/3）<（?）2>β（?）<（（1/3）（2/3）（1/2））>ω;The corresponding relationship betweenβphase andαphase after shear is as follows:<111>β∥<0001>ω,<1（?）0>β∥<11（?）0>ω.（5）The relationship betweenωphase andαphase inβ-CEZ alloy was comprehensively analyzed.It was found that isothermalωwith a long axis about 10nm would be formed,which would help secondaryαphase precipitated in adjacent area ofβmatrix.The transformation process ofβ→αphase inβ-CEZ alloy can be summarized as follows:β→β+athermalω→β+isothermalω→β+isothermalω+secondaryα.（6）By increasing the solution temperature ofβ-CEZ alloy from 830℃to 870℃,it was found that the content of primaryαphase decreased and theβgrain 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 secondaryαphases 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 ofβ-CEZ alloy with equiaxedαphase was ranging from 30 to 45 MPa m^1/2.In contrast,lamellar primaryαphase can cause crack deflection and significantly improve the fracture toughness of theβ-CEZ alloy.（7）By comparing the microstructures and properties ofβ-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 primaryαphase grows up to rod and long strip shape,and the content ofαphase increases relatively.After aging,needle-like secondaryαphase precipitated from metastableβphase,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 ofβ-CEZ alloy after aged from 550℃to 650℃,followed solution treated at 850℃were analyzed.It was found that the primaryαphase 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 theβ-CEZ alloy decreased,the plasticity and fracture toughness increased.