【作者】 彭超群；

【导师】 黄伯云；

【作者基本信息】 中南大学 ， 材料科学与工程， 2001， 博士

【摘要】 通过可行性分析、模拟试验和感应热处理试验，采用光学显微镜、扫描电镜、透射电镜、Ｘ射线衍射仪和室温压缩试验等手段，系统研究了快速加热循环热处理对TiAl基合金显微组织和室温力学性能的影响，得到以下主要结论。 1)循环热处理正交试验(L₁₆（4⁵）)表明：根据不同热处理条件（加热速度υ_h=25～100℃/s，保温温度θ=1310～1340℃，保温时间t=2～5min，冷却速度υ_c=20～160℃/s，循环次数n=1～7），可获得4种不同类型的显微组织，即典型层片组织，细小层片、较粗层片及块状相混合组织，细小近层片组织，块状组织；在上述试验参数范围内，当加热速度低于25℃/s，且保温湿度低于1310℃或循环次数小于3时，TiAl基合金仅发生层片粗化或少量形核。由极差分析可知，加热速度、保温温度、保温时间、冷却速度和循环次数对硬度值的影响按从大到小排列的顺序为：υ_h；t，θ，υ_c，n，其中加热速度对硬度的影响最大。 2)加热速度单因素优化试验表明：以100～3200℃/s（θ=1330℃，t=2min，υ_c=50～80℃/s，n=5）进行快速热处理可以使晶粒尺寸细化至50μm以下。 3)循环热处理可以在热加工基础上进一步细化TiAl基合金显微组织；对二次锻原始组织，随着循环次数增加，层片组织不断细化，当循环次数为7次时，全层片组织的晶团平均尺寸可从50μm左右细化至20μm以下。 4)利用感应加热设备对TiAl基合金进行循环热处理，可以获得晶团尺寸约为50μm、层片间距约为12×10^-2μm的TiAl基合金细小全层片组织。 5)感应加热快速热处理可以大幅度提高TiAl基合金的硬度，最大硬度可达412HV₅；经1000℃，12～48h时效处理后，硬度仍可保持在350HV₅以上，且当时效时间达到12h以后基本保持不变。 6)感应加热循环热处理可以较大幅度提高TiAl基合金的压缩力学性能，屈服强度、最大流变应力和压缩率的最大值分别可达806.6MPa，1740MPa和19.4％；最佳综合力学性能在1000℃时效至24h时获得，其值为屈服强度745.1MPa、最大流变应力1672.2MPa和压缩率19.4％。 7)经感应热处理和时效后的室温压缩断裂仍为较典型的解理断裂，且裂纹扩展以穿晶断裂为主。 8)在快速加热、短时保温循环热处理过程中，细小全局片组织以晶界形核为主，并可发生相界形核；新层片晶团的长大可用台阶机制描述。更多还原

【Abstract】 Through feasibility analyses, simulated tests and induction heat-treatments, the effects of rapid heating cyclic heat treatments on microstructures and mechanical properties of a TiAI-based alloy (Ti-33Al-3Cr, mass fraction , %) were systematically studied by means of optical microscopy. scanning electron microscopy. transmission electron microscopy. X-ray diffractometry and compression tests, and the following main conclusions are drawn.1)The L16(45) orthogonal cyclic heat treatment tests show that, according to different heat treatment conditions(heating rate vh=25?100 /s. holding temperature 0=1 310?I 340 , holding time t=2? mm, cooling rate vp20?160 /s. cycling number n1 ?), four kinds of microstructures can be obtained, i.e. typical lamellar structure. mixed microstructure of fine lamellae, relatively coarse lamellae and massive phase, fine nearly-lamellar microstructure, and massive microstructure. In the selected ranges of the above parameters, when the heating rate is lower than 25 /s and the holding temperature is lower than 1 310 or the cycling number is smaller than 3, only lamellar coarsening and formation of some new nuclei occur. The hardness (HV5) can be increased by rapid heating cyclic heat treatment, and the error range analysis shows that the effects of heating rate. holding temperature, holding time, cooling rate and cycling number on hardness, from large to small, are as follows: vh; t, 6, v~ and n, among which the effect of heating rate is by far the most evident.2)The optimized single factor test of heating rate shows that, rapidly heated at a rate of 100~? 200 /s ( ~=l 330 , t=2 mm, v~50-?80 C/s, n=:5). the hardness of the TiAl-based alloy can be increased by 30?0 HV5 compared to that of the as-cast alloy and the microstructure can be refined to be less than 50 I~ m.3)The inicrostructure of the thermo-mechanicallly treated TiAI alloy can be further refined by rapid heating cyclic heat treatment. For the two-step microstructure, with increasing cycling number, the lamellar structure is continuously refined. When the cycling number reaches 7, the colony size can be refined from about 50 i~ m to be lessIIthan 20 ii m.4)The microstructure of the hAl-based alloy with a colony size of about 50 I.~ m arida lamellar spacing of about 12X 1W2 j.A m can be obtained. The HV5 hardness can besubstantially increased and the highest hardness reaches 412 HV5. After aging at 1 000 C for 12?8 h, the hardness remains above 350 HV5, and when the aging time is longer than 12 h, the hardness almost keeps unchanged.5)The compression mechanical properties of the hAl-based alloy can also besubstantially enhanced by rapid heating cyclic induction heat treatment. The maximumvalues of yield strength. largest flow stress and compression ratio respectively reach806.6MPa, 1 740 MPa and 19.4%. The optimized mechanical properties are obtained byaging at 1 COOt for 24 h, and the corresponding values are respectively yield strength745.1MPa, largest flow stress 1 740 MPa and compression ratio 19.4%.6)The compression fracture of TiAl-based alloy at room temperature after cyclic induction heat treatment and aging is still typical cleavage and the crack propagations occur transgranularly.7)In the process of rapid heating and short-time holding cyclic heat treatment, the nucleations of fine lamellar microstructure mainly occur along the grain boundaries and also at the phase interfaces. The growth of new lamellar colonies can be described by the ledge mechanism.更多还原