【作者】 何涛；

【导师】 冯勇；

【作者基本信息】 西北工业大学 ， 材料学， 2019， 博士

【摘要】 β-CEZ钛合金（Ti-5Al-4Mo-4Zr-2Sn-2Cr-1Fe）是20世纪90年代由法国的CNECMA和CEZUS公司研制的近β钛合金。该合金的β稳定元素含量高,在固溶时效处理后具有很高的强度,主要用于制造压气机盘、弹簧和紧固件。此前,研究人员对β-CEZ钛合金的热加工特点进行了相对系统的研究,对其α和α″相变、组织和性能都有了一定了解。但是,对该合金的亚稳ω相变研究较少,对其热处理过程中相和组织的演变关系尚不明确,对相变后组织和性能的研究也不深入。因此,本文利用OM、SEM和HRTEM等技术观察了β-CEZ钛合金在固溶时效热处理时的β相、ω相和α相的组织结构特点,分析了β-CEZ钛合金的β→ω→α相变机理;借助DSC和XRD等手段获得了固溶态β-CEZ钛合金相变与加热温度和保温时间的关系,建立了固溶态β-CEZ钛合金连续升温和等温时效的相变动力学模型;此外,在相变研究的基础上,对比分析了不同热处理后β-CEZ钛合金的组织和强韧性。主要的研究内容和结论如下:（1）将β-CEZ钛合金加热到不同温度固溶处理,研究其静态再结晶规律。通过在830℃以上的两相区固溶,发现随着加热温度升高,初生α相的比例降低,尺寸减小,趋于球状,且α相的溶解速度下降;通过分析相变点上下的β晶粒尺寸随加热温度和保温时间的变化关系,获得了β-CEZ钛合金的晶粒尺寸和加热温度的动力学关系式（?）=3.27×10⁶×0)（-93.5/8.314）以及晶粒尺寸和加热时间的动力学关系式（?）=61.56^0.38。另外,通过对比不同温度固溶后β-CEZ钛合金的硬度和空冷组织,发现固溶温度越高,残余β相的稳定性越低,析出次生α相的速度越快。（2）通过对比β-CEZ钛合金不同温度时效2h后的硬度变化规律,将时效过程分为三部分。在300℃以下,没有发生明显相变,合金硬度与固溶态相当,约300HV;在350℃至550℃之间时效,析出非常细小的纳米级针状次生α相,合金硬度迅速升高到约500HV;在600℃以上时效,晶界附近析出较大尺寸的α_WGB,α_WM也长大非常显著,合金硬度又逐渐降低。（3）对比分析不同加热速率和不同时效温度的DSC曲线,研究时效加热时β-CEZ钛合金中次生α相的析出规律。发现提高升温速率,β-CEZ钛合金中相变开始和结束温度都升高,且相变可分为低温和高温两部分。在500℃以下,由于ω相的辅助作用,升温过程就可以快速析出弥散细小的次生α相;在600℃以上,在晶界附近和晶内分别析出尺寸较大的α_WGB和α_WM,且在700℃至800℃,次生α相析出速度相近（Avrami指数n相当）,但当温度达到850℃,高温下α相快速长大,导致反应速率因子k增大。（4）采用高分辨透射电子显微镜（HRTEM）分析了β-CEZ钛合金中ω相的类型、尺寸、形状和微观结构。确认β-CEZ钛合金在水冷固溶处理后,β基体的{111}β面沿着<111>β方向坍塌析出颗粒状的无热ω相（尺寸约1nm～2nm）;在一个晶胞内的β相原子和ω相原子的结构关系为:（1/3）<（?）2>β（?）<（（2/3）（1/3）（1/2））>ω,（2/3）<（?）2>β（?）<（（1/3）（2/3）（1/2））>ω;切变以后β相和ω相的对应关系为<111>β∥<0001>ω,<1（?）0>β∥<11（?）0>ω。（5）综合分析β-CEZ钛合金中ω相和α相的关系,结合其他近β钛合金中ω相的研究结果,发现在低温时效时,β-CEZ钛合金中会析出长轴约10nm的椭球状等温ω相,在等温ω相的邻近位置会析出次生α相。可将β-CEZ钛合金中β→ω→α相的转变过程概括为:β→β+无热ω→β+热ω→β+热ω+次生α。（6）通过调整固溶加热温度,研究β-CEZ钛合金在830℃至870℃固溶及时效以后的强韧性。发现随着固溶温度的升高,初生α相含量减少,开始出现β晶界,合金的室温强度和塑性都降低。时效以后,合金在固溶组织的基础上析出大量尺寸细小的针状次生α相,使β-CEZ钛合金的强度比固溶后显著升高,且时效以后的强度随着原固溶温度的升高而升高。通过分析平面断裂韧性,发现随着固溶温度的升高,β-CEZ钛合金的组织粗化,断裂韧性略有升高。但初始组织为等轴状的合金的K_IC较低。相比而言,片层状的初生α相可以引起裂纹的偏转,显著提高β-CEZ钛合金的断裂韧性。（7）通过对比分析固溶以后水冷、油冷和空冷三种冷却方式对β-CEZ钛合金组织和性能的影响,发现随着固溶以后的冷却速度降低,等轴状初生α相向棒状和长条状长大,α相占比相对增多,所以合金的强度略微升高。在时效以后,亚稳β相中析出弥散细小的针状次生α相,合金的强度迅速升高,且随着原固溶冷速的降低,合金强度降低,塑性升高。（8）通过对比在850℃固溶,在550℃至650℃不同温度时效的β-CEZ钛合金的组织和性能,发现随着时效温度升高,初生α相由等轴状向棒状转化,含量升高,针状次生α相尺寸明显长大,合金的强度降低,塑性升高,断裂韧性升高。更多还原

【Abstract】 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.更多还原