【作者】 张俊红；

【导师】 黄伯云；

【作者基本信息】 中南大学 ， 材料学， 2003， 博士

【摘要】 TiAl基合金因其具有高的比强度、比刚度，以及较好的高温抗蠕变、抗氧化性能等优点，引起了国内外学者的广泛关注。但这类材料具有本质脆性，一般难以加工成形，阻碍了其实用化。在超塑性状态下成形是目前解决TiAl基合金成形最为有效的方法之一。为此，本文开展了TiAl基合金的超塑性研究工作。首先，采用快速包套锻热机械处理和非晶晶化法两种工艺制备了细晶TiAl基合金，为超塑性变形提供条件；然后深入、系统地研究了TiAl基合金锻态变形组织、锻态双态组织的拉伸超塑性性能和粉末冶金态组织的压缩超塑性性能，包括力学性能和显微组织演变；分析了TiAl基合金的超塑性变形机理；并对超塑性变形过程中的孔洞行为与断裂机理进行了研究。 快速包套锻热机械处理和非晶晶化法分别是制备细晶TiAl基合金的有效IM（铸锭冶金）和PM（粉末冶金）方法。采用终锻温度为950℃的三步快速包套锻热机械处理工艺，得到了晶粒尺寸约为0.6μm的亚微米级TiAl基合金。对于铸造TiAl基合金而言，晶粒细化主要表现为层片细化，通过研究层片结构在热压力下的塑性变形行为和热处理时组织的演变，分析了层片的细化机理。非晶晶化法是一种新颖的非平衡技术，该技术采用机械合金化法制备Ti-50 at.％Al非晶粉末，经预压成形，然后在40MPa，1000℃，1h条件下热压，得到了晶粒尺寸为0.15μm的微细晶TiAl基合金。在机械合金化过程中，Ti-Al元素粉末的结构演变过程为：Ti+Al→Ti（Al）过饱和固溶体（h.c.p）→非晶相。在本研究所采用的成分范围内，Ti-Al系具有大而负的混合热，Ti，Al两元素间存在不均衡扩散，符合形成非晶的SSAR（固态非晶化反应）机制。Ti-50 at.％Al非晶粉末在660-750℃之间发生晶化反应，其晶化过程为：非晶→α相（h.c.P）→γ，其中，α相是由无序非晶相向有序γ相转变的介稳相。 采用上述工艺制备的细晶TiAl基合金获得了良好的拉伸超塑性。其中，锻态变形组织在1050℃，8×10^-5S^-1条件下得到了566％的延伸率；锻态双态组织在1050℃，1×10^-4S^-1条件下，得到了350％的延伸率。尤其令人关注的是，锻态变形组织表现出了优良的低温超塑性，它在800℃即出现超塑性，其延伸率达到313％；在900℃，5×10^-4S^-1，常压空气中博士学位论文摘要得到了413%的最大延伸率;并且，在lx10一35一‘的较高应变速率和常压空气中，还得到了350%的延伸率。这个结果对于TIAI基合金超塑性成形技术的工业化应用具有重要意义。在压缩状态下，T认1基合金也表现出超塑性。 采用恒应变速率和应变速率递增法研究了TIAI基合金超塑性变形的力学性能:延伸率、应力一应变曲线、应变速率敏感性因子和变形激活能。不同组织形态TIAI基合金的超塑性力学性能不同，它们受变形条件、显微组织、晶粒尺寸、预变形、晶界结构等因素的影响。重点研究了锻态变形组织的力学性能，发现超塑性变形时，其流变应力、应变速率、变形温度和晶粒尺寸间存在特殊的依赖关系，并且这种依赖关系随温度区间而转变。在大量实验数据的基础上，通过计算表征各变形量间依赖关系的参数，包括:应变速率敏感性因子m值、表观激活能QaPP值、晶粒尺寸因子p值，经实验拟合首次得到了TIAI基合金锻态变形组织超塑性变形的经验状态方程。此方程为分析TIAI基合金的超塑性变形机理提供了依据。在锻态变形组织中，超塑性流变曲线中存在强烈的加工硬化现象，组织中由预变形引起的高密度位错是引起这种现象的主要原因。 采用光学显微镜（0A）、扫描电镜（SEM）、透射电镜（T EM）等分析手段对TIAI基合金超塑性变形过程中组织演变研究的结果表明，经过超塑性变形，三种组态TIAI基合金具有基本相同的组织特征，包括（l）晶粒仍然保持等轴晶形状;（2）有动态再结晶发生;（3）有位错运动。其中，经过超塑性变形的TIAI基合金中的位错密度较小，主要分布在晶界和三角晶界附近，晶内位错在晶界处容易被晶界“吸收”而产生强烈的攀移和相消现象。晶界、三角晶界、晶界上的QZ相及杂质相是位错的“源”。在铸态双态合金中，经超塑性变形后，组织中还有亚晶存在。 提出了TIAI基合金的超塑性变形机理。在TIAI基合金中，超塑性流变是以晶粒群沿剪切带共同滑动的形式进行的。由晶界位错的滑移和攀移运动共同协调的晶粒群协同滑动是TIAI基合金的超塑性变形机理，其中，晶界位错的攀移运动是变形的回复机制，晶界位错的滑移运动既是变形机制又是变形的回复机制。这个机理对晶界滑动的微观过程作出了合理的解释。采用该机理本文对T认1基合金超塑性本构方程进行了理论推导，推导出的方程与实验得到的方程具有相同的形式，说明—11—博士学位论文摘要本研究给出的超塑性变形机理具有相当程度的合理性。另外，在TIAI基合金中，动态再结晶和亚晶对超塑性变形亦有贡献。 研究了超塑性TIAI基合金中的孔洞行为，提出了一种新的孔洞形核机制—在晶粒群协调滑移带（CGBS）交割处形核。在TIAI基合金中，超塑性变形时，孔洞形核于晶/相界、三叉晶界和晶粒群协调滑移带（CGBS）交割处。张应力作用下，过饱和空位向晶界汇流，更多还原

【Abstract】 TiAl-based alloys have high specific strength, specific stiffness, excellent oxidation and creep resistance, which have been paid broad attentions to for years. However, the major factors limiting structural applications in this material are their ambient temperature brittleness and poor workability. It is said that superplastic forming is a novel process for fabricating titanium aluminides. So, a research on the superplasticity of the alloy is done in the paper. First, a fine-grained TiAl-based alloy is obtained by a quickly canned-forging thermo-mechanical processing or by crystallization of mechanically alloyed amorphous Ti-50at%Al. Then the tensile superplasticity of the IM (ingot metallurgy) TiAl alloy with deformed microstructure or with duplex microstructure obtained by the quickly canned-forging thermo-mechanical processing and the compress superplasticity of the PM (powder metallurgy) TiAl alloy obtained by crystallization of amorphous Ti-50at%Al powders are investigated systemically and deeply. The superplastic mechanism of the alloy is discussed. The cavitation behavior and the fracture mechanism under superplastic deformation are analyzed too.Quickly canned-forging thermo-mechanical processing is an efficient and economical IM technology. By this technology, a sub-microstructure titanium aluminide with mean grain size of about 0.6μm is obtained after 3-step forging with the final forging temperature of 950℃. For a cast TiAl-based alloy, lamellar globularization is the mechanism of grain refining. By analyzing the deformation behaviors and the microstructure evolution of lamellas during forging and the following heat treatment, the lamellar globularization mechanism of the alloy is discussed. The technology of crystallization of mechanically alloyed amorphous Ti-50at%Al powders is a novel process. By this technology, a titanium aluminide with mean grain size of about 0.15*m is obtained under 40Mpa, 1000℃,lh hot-pressing. During mechanical alloying, the amorphous phase formation process can be described by: Ti+Al→Ti (Al) supersaturated solid solution (h.c.p)→morphous phase. In our research, the Ti-Al system has a large and negative heat of mixing, the?IV ?ABSTRACTdiffusivities of the two components are great different, so, the formation of the amorphous phase obeys SSRA (solid state amorphisation reaction) mechanism. Crystallization of the amorphous phase occurs at 660-750, the process can be described by: Amorphous phase disordered a phase (.c.p) ordered y phase, during the process, the disordered a phase is a metastable phase.The fine-grained TiAl-based alloys obtained by above technologies exhibit good tensile superplastic characters. An elongation of 566% is obtained at lOSO’C^SxlO’V1 in the TiAl-based alloy with deformed microstructure, and an elongation of 350% is obtained at 1050,lxloV in the alloy with duplex microstructure. It worth especially noting that the alloy with deformed microstructure exhibits excellent low temperature superplasticity. The alloy shows superplastic characters at very low temperature- 800 (the elongation is 313%). A maximal elongation of 413% is obtained at 900,5xlO~V in air. Even at a higher strain rate of 1x1 0’V1 and in air, the elongation also reaches 350%. In addition, under compress condition, the alloy also exhibits superplastic characters.Mechanically properties of the superplastic deformation are investigated by constant strain rate tests and incremental strain rate tests. These properties include: elongation, curves of flow stress-true strain, strain rate sensitivity (m), and apparent activation energy. Results show that these mechanically properties are affect by deforming conditions, microstructure, grain size, pre-deformation, and grain boundary structure. In the research, a large amount of work is done in TiAl-based alloy with deformed microstructure. A phenomenon of strain hardening resulting from pre-deformation is observed in the alloy. And the phenomenological relationship of superplastic flow is investigated in the alloy. It is found更多还原