【作者】 黄丽芳；

【导师】 张凯锋；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 γ-TiAl基合金具有低密度、高熔点、良好的高温强度以及出色的抗氧化、抗蠕变和抗疲劳性能,成为最有发展前景的航空材料之一。但是TiAl基合金塑性差、难加工的缺点限制了它的广泛应用。晶粒尺寸是影响材料性能的重要因素,以传统的铸造和热变形加工方法一般难以获得细小均匀的组织。脉冲电流辅助烧结由于其升降温速度快、烧结时间短、组织结构可控、致密度高等优点被用于制备TiAl基合金。本文利用脉冲电流辅助烧结技术,对γ-TiAl预制合金粉进行烧结。在烧结过程中,设定了不同的烧结温度、升温速度、烧结时间等工艺参数,研究其对合金微观组织的影响。对烧结后不同组织的试样进行了高温拉伸实验,得出了不同组织对超塑性能的影响,实现了低温超塑性。在超塑性拉伸实验的基础上,对等轴近γ组织和全片层组织进行了叶片的超塑挤压模拟尝试。脉冲电流辅助烧结:当γ-TiAl粉末升温速率分别以4.5、3.8、3、2.5、2℃/s加热到1250℃,烧结5min的条件下,分别得到了等轴细晶近γ组织、细晶双态组织、近片层组织、细全片层组织、粗全片层组织。且晶粒细小,等轴近γ组织晶粒最小为4μm。高温拉伸实验:对烧结后不同组织的试样进行高温拉伸测试。结果表明等轴细晶组织高温拉伸性能相对最优,在950℃~1000℃,应变速率为1.040×10-4s-1~2.083×10-4s-1时获得的延伸率超过240%,最大延伸率达到437%,实现了低温超塑性。通过观察在拉伸材料上发现了孔洞,材料内的孔洞对材料的延性有一定的帮助,拉伸后期成为断裂的原因,通过断口分析,高温拉伸断口基本上为延性断裂。叶片模拟挤压:以高温拉伸实验为指导,设计了细晶等轴近γ组织和全片层组织的高温超塑挤压试验。结果表明,等轴近γ组织在1250℃,压头速度为0.05mm/min中成形质量良好,性能得到改善。而全片层组织在1250℃,0.02mm/min压头速度下,发生了动态再结晶,形成了组织均一的等轴近γ组织,各向异性差异小,材料性能得到改善。更多还原

【Abstract】 γ-TiAl based alloys are promising candidates for elevated temperature structural materials due to their superior characteristics, such as low density, high young’s modulus, excellent strength and oxidation resistance at elevated temperature. However, they suffer from room temperature brittleness and poor workability, hindering their applications. The grain size has a very important effect on the properties of material. However,it is difficult to prepare TiAl alloy with fine and homogeneous microstructure by traditional casting and mechanical working processes. Pulse Current Auxiliary Sintering(PCAS), because of its quick heating rate, short sintering time, controllable microstructure, high material density, is videly used to produce TiAl alloys with refined microstructure.In this paper, TiAl based alloys from the prealloyed powder of were prepared by pulse current auxiliary sintering. In the synthesis process, influences of different parameters, including heating-up rate, sintering temperature and duration time, on obtained microstructures in sintered products were studied. Effect of microstructure on properties at elevated temperature was tested. High tensil properties were obtained at relatively low temperature. Based on the tensil properties, superplastic extrusion of blade was conducted.Pulse Current Auxiliary Sintering: When powders were synthesized in the oven at 1250℃for 5min with heating rates of 4.5, 3.8, 3, 2.5, 2℃, near gamma, duplex, nearly lamellar, fine fully lamellar and coarse fully lamellar microstructures were obtained, correspondingly. Fine grain size was abtained. The grain size of nearγmicrostructure has reached 4μm。Tensile testing at elevated temperature: Tensile tests of different microstructure were conducted in temperature range of 950℃~1000℃. The results showed that one with equi-axed fine grains possessed more superior elevated temperature tensile properties. When the strain rate range was 1.040×10-4s-1~2.083×10-4s-1, the obtained elongation was over 240%. The maximum elongation got to 437%. High superplastic behavior at relatively low temperatures realized. Cavities were observed in the sample after tensil tests. It is analyzed that cavities wre helpful to elongation at first. But they lead to the fracture of alloy eventually. The morphologic of the cross section showed that a majority of high temperature tensile fracture was ductile.Imitated extrusion of blade: Based on the high temperature superplasic properties, with nearγmicrostructure and fully lamellar microstructure, models of blade were extrued at 1250 ~1350℃. The results show that when the sample with nearγ microstructures was extruded at 1250℃with the extrusion rate 0.05mm/min, good quality and improved mechanical properties were obtained. Whenγ-TiAl alloys with fully lamellar microstructures was extruded at 1250℃with the extrusion rate 0.02mm/min, dynamic recrystallization happened. The microstructure transferred to equiaxial and uniformγ-TiAl with nearγmicrostructures. The properties of the material were improved.更多还原