【作者】 杨鑫；

【导师】 奚正平； 刘咏；

【作者基本信息】 中南大学 ， 材料学， 2012， 博士

【摘要】 近年来,γ-TiAl基合金作为新一代轻质高温结构材料在全世界范围内引起了学者和科研机构广泛的关注和研究,但是较低的室温断裂韧性和高温蠕变抗性严重阻碍了γ-TiAl基合金的和实际应用。本工作以等离子旋转电极雾化(Plasma Rotating Electrode Processing,PREP)技术制备γ-TiAl基预合金粉末为原料,采用放电等离子烧结(Spark Plasma Sintering,SPS)技术可以快速制备具有细小晶粒且含有少量高温残留β相的TiAl基合金。利用光学显微镜(OM)、X射线衍射(XRD)、扫描电镜(SEM)、差热分析(DSC)、透射电镜(TEM)等分析手段系统的研究了工艺参数对获取高质量预合金粉末的影响,以及放电等离子烧结温度和成型压力对粉末烧结体显微组织、力学性能和粉末体致密化机理的影响,通过以上研究得到如下结论：1、对PREP制备的Ti-47A1-2Cr-2Nb-0.2W(at.％)预合金粉末研究结果表明,采用该技术制备的合金粉末平均粒径约为85μm,球形度高,纯度高,平均氧、氮杂质分别为550ppm和40ppm。确定了粉末直径与电极棒转速关系为：d=(12000/n)D-1/2-140.确定了粉末颗粒内部显微组织随着颗粒直径的增加由亚晶组织向等轴胞状枝晶演化规律；粉末主要由大量α2相和少量的γ相组成,随着粒径减小,γ相含量随之减小,α2/γ比例增加,且会生成一定量的β(B2)相。2、研究了SPS烧结温度和成形压力对粉末烧结体显微组织和力学性能的影响。发现,在不同成形压力下烧结温度超过1100℃时均可获得致密度高于99.6%的致密体。SPS烧结TiAl基合金过程中,成形压力不会对烧结体显微组织产生影响,成形压力增加会改变烧结初期粉末体密度和后期致密化程度。不同温度烧结试验表明合金粉末在900℃时,粉末烧结体以γ相为主,含少量α2相；当烧结温度超过1200℃时,组织内存在少量高温残留B2相,α2相、β相和γ相存在如下晶体学关系：(0001)α2//｛111｝∥｛110｝β,<1120>α2//<110>γ//<111>β；1300℃烧结样品中有形变诱导α2→γ相变以及形变孪晶产生。粉末烧结体显微组织主要取决于烧结温度,1100℃时为双态组织、1200℃时为近层片组织、1250℃为全层片组织。SPS烧结合金在1100℃具有最高的压缩断裂强度、压缩延伸率和拉伸断裂强度,分别为2367MPa.600MPa和2.25%。室温拉伸性能测试表明,合金断裂方式为脆性断裂,高温(800℃时)拉伸时强度和延伸率分别为435MPa和45%。3、系统研究了SPS制备TiAl基合金过程的致密化机理。发现,在SPS制备TiAl基合金过程中致密化可分为三个不同阶段,即初期粉末颗粒重排阶段、中期电火花放电烧结颈形成阶段和高温塑性变形致密化阶段。同一烧结温度下烧结中期小尺寸粉末颗粒相互粘结,随着烧结温度的升高,大尺寸颗粒发生大幅度塑性变形从而完成致密化。相同烧结温度下增加成形压力会增加第一阶段致密化程度和第三阶段致密化速率,通过分析确定了烧结过程的致密化方程：第一阶段：p1=(2.65±0.4)0.01Pt,(1<t<100s)；第二阶段致基本密度保持不变；第三阶段：p3=0.942exp[(2.62±0.6)×10-4t],(600<t<1100s).4、通过SPS烧结制备了TiA1-Nb(5-10at.％)复合材料。分析了不同Nb添加量和烧结温度对复合材料显微组织和室温断裂韧性的影响。研究表明,添加Nb元素粉末可以提高材料致密度,在1150℃时致密度可达99.2%。通过改变烧结温度和Nb粉添加量可以生成具有多相组织的复合材料,并有效提高复合材料的室温断裂韧性,最高室温断裂韧性为28.7MPa·m1/2。更多还原

【Abstract】 Currently, gamma titanium aluminides based alloys, with their low density, high specific strength and stiffness, high corrosion and oxidation resistance at high temperature, make them one of the most prospering high temperature structure materials to replace heavier nickel-based superalloys in the applications of aerospace and automotive components such as nozzles, divergent flaps, compressor blades, etc., which attracts a lot of researches’and research institutions’attention. In this paper, a novel powder metallurgy technique, namely spark plasma sintering, was successfully used to prepare refined-grain TiAl alloys with a little amount of high-temperature retained β phase particles through prealloyed TiAl-based powders produce by plasma rotating electrode processing (PREP); the process of PREP, sintering and densification mechanisms and microstructure and mechanical properties of as-SPSed bulk samples were investigated systematically. The following results had been obtained:1) Prealloyed Ti-47Al-2Cr-2Nb-0.2W powders produced by PREP were at narrow size distribution with mean particle size d50being85μm; The average contents of impurities O and N were550ppm and40ppm, respectively, and the content of O was increasing with decline of the particle size, but remained less than1000ppm, while N stayed unchanged; The sphericity started reducing when the mean particle size surpassed150μm, and the surface became rough as well. Regress analysis showed that the relation between diameter of electrode bar and rotating speed was: d=(12000/n) D-1/2-140. Further investigation indicated that inner microstructure of the particle had changed from sub-microcrystalline to equiaxed cellular crystalline as the particle sized increased; The main phase of the particle was a2with minor y phase, and the content of y declined with particle size decreasing, resulting in a2/y ratio increasing, and B2phase was observed at the same time, which was mainly because of the higher cooling rate during the PERP; Dendritic segregation appeared between dendritic arms, resulting from the difference of distribution modulus at the interface between solid-liquid during cooling process. 2) Influences of sintering temperature and pressure on microstructure and mechanical properties during SPS had been studied. During SPS, we found that relative density of SPSed TiAl bulk samples was more than99.6%when the sintering temperature was higher than1500℃; further investigation revealed that increasing the pressure could enhance the relative density of the loose density and decrease the solidification time at initial stage of SPS, but had little impact on the microstructure. The process of α2→γ phase transformation completed when sintering temperature closed to900℃, and the main phase in the bulk was y, with minor a2phase. When the sintering temperature was higher than1200℃, a small amount of B2phased were also discovered, which was caused by higher cooling rate during SPS. Microstructure of the SPSed bulk samples primarily depended on sintering temperature, with duplex phase at1100℃, near lamellar at1200℃and full lamellar at1250℃. The highest compressive fracture strength2367MPa, elongation2.25%and tensile fracture strength600MPa were achieved at1100℃sintering temperature. The highest Vickers hardness at the same temperature was516Hv. The tensile test results demonstrated that the fracture mode of the SPSed TiAl bulk was brittle mode, and superplastic behavior appeared when testing temperature was at800℃, with tensile strength435MPa and elongation45%, respectively.3) Identification of the microstructure mechanisms during SPSed TiAl densification had been studied systematically. Through sintering at three different pressures and six different sintering temperatures for5min, it was found that a SPS circle for TiAl bulk material was consist of four stages:initial pressure of machine, formation of sintering necklace, the growth of necklace and high-temperature creep stage. Formed pressure had no influence on the densification. Densification took place between900℃and1200℃via plastic deformation of the powders. TEM observation showed that the repartition of the plastic deformation was correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occurred. At1250℃, a2phase, y phase and retained P phase followed the crystalline oriental relationship as below:(0001)a2//{111}γ//{110}β,<1120>α2//<110>γ//<111>β And at above1250℃, deformation induced phase transformation and deformation twins occurred.4) A TiAl-Nb composite had been synthesized by SPS from pre-alloyed TiAl powders and elemental Nb powders at the molar ratio of90:10and95:5, and the as-prepared samples were mainly constituted of y phase, O phase, niobium solid solution(Nbss) phase and B2phase. The fracture toughness was about28.7MPa-m-1/2. The ductile phase played an important role in absorbing the fracture energy in front of the cracks and B2phase could branch the propagation of the cracks. It was discovered that sintering temperature could be decreased by adding Nb powders and relative density99.2%was obtained at1150℃. The microhardness of each phase of the composite was tested.更多还原