【作者】 高昕；

【导师】 蔡伟；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2007， 博士

【摘要】 本文采用光学显微观察、透射电子显微观察、示差扫描量热分析、X射线衍射和压缩实验等手段,系统研究了Ru基高温形状记忆合金的马氏体相变和应变恢复特性,阐明了马氏体微观组织在压缩变形中的演化规律及其与应变恢复特性之间的内在联系,研究了Fe元素对Ru-Nb合金相变和力学性能的影响规律,揭示了掺Fe改善断裂韧性的微观机制。研究结果表明合金成分对Ru基合金的相变有显著影响,Ru₅₀Ta₅₀、Ru₅₃Nb₄₇和Ru₅₀Nb₅₀合金在冷却时发生β→β′→β″两步马氏体相变,室温形成具有单斜结构的β″马氏体;而Ru₅₄Ta₄₆、Ru₄₅Ta₅₅和Ru₄₅Nb₅₅合金马氏体冷却时只发生β→β′一步马氏体相变,室温形成具有正方结构的β′马氏体;在Ru_50-xNb₅₀Fe_x（X=3.5、7、14）合金中观察到了β→β′→β″两步马氏体相变,马氏体相变温度随Fe含量的增加而降低。室温压缩变形使Ru基合金马氏体逆转变温度升高,相变滞后增宽。透射电镜观察证实,Ru基高温记忆合金室温β′和β″马氏体变体呈三角状或楔状等自协作组态,变体间界面平直,马氏体亚结构为（101）I型孪晶。室温压缩变形对马氏体组织形态有显著影响。形变量较小时,马氏体变体因择优取向而粗化;随形变量的增大,变体间呈挤碰和交叉形貌,变体间界面共格性遭到破坏,界面变得紊乱,变体内部出现位错;随形变量的进一步增大,马氏体变体失去自协作组态,大都形成单一变体,变体内部位错密度增加且形成缠结组态。Ru基记忆合金的马氏体类型不同,应变恢复特性差异很大。β′马氏体在室温压缩变形后加热,伴随β′→β转变呈现一步应变恢复;而β″马氏体变形后加热,伴随β″→β′→β两步逆相变发生两步应变恢复,对应于β′→β转变产生的可逆应变大于对应于β″→β′转变的可逆应变。掺Fe改善了Ru-Nb合金的形状记忆效应,Ru₅₀Nb₅₀的最大可完全恢复应变为2%,而Ru₄₃Nb₅₀Fe₇的最大可完全恢复应变达2.5%。适量Fe元素的加入降低了Ru-Nb合金的室温压缩强度,提高了室温塑性,掺Fe改变了Ru-Nb合金的室温压缩断裂方式,未掺Fe的Ru-Nb主要呈脆性沿晶断裂,而掺Ru-Nb-Fe合金室温呈穿晶断裂。上述断裂方式的改变是塑性增大的主要原因。更多还原

【Abstract】 The martensitic transformation and strain recovery characteristics have been systematically investigated by optical microscopy, TEM, DSC, XRD, and compression test. The effect of compression deformation on martensite microstructure and internal relations between martensite microstructure and strain recovery characteristics has been illuminated. The effect of adding Fe to martensitic transformation and mechanical properties has been studied. Micromechanism of adding Fe to improve fracture ductility at room temperature of Ru based alloys has been explained.The experimental results show that a two-step phase transformation occurs when cooling Ru₅₀Ta₅₀, Ru₅₃Nb₄₇, and Ru₅₀Nb₅₀ alloys from high temperature to room temperature, formingβ″martensite of monoclinic structure. While one step phase transformation occurs during the cooling procedure of Ru₅₄Ta₄₆, Ru₄₅Ta₅₅, and Ru₄₅Nb₅₅ alloy from high temperature to room temperature,β″martensite of tetragonal structure. For Ru_50-xNb₅₀Fe_x（X=3.5, 7, 14） alloy, two-step phase transformation occurs when cooling these alloys from high temperature to room temperature. Keeping the content of Nb, transformation temperatures drop down with reducing of Fe content. Deformation of Ru based alloys makes that martenstic inverse transformation temperatures increase and phase transformation hysteresises become wider.The study of TEM research shows that thermalβ″andβ′martensite exist at room temperature in Ru based high temperature shape memory alloys. The martensite variants ofβ″andβ′martensites exhibit triangle-like or wedge-like self-accommodating morphology, with alternating regular bands inside; this kind of variants boundary is straight. The twinning relationship between the substructural bands is （101） Type I mode. Compression deformation at room temperature has great influence on martensite microstructure. The main deformation mechanism involved varies as the following sequence: When the specimen deforms, the reorientation and coalescence ofβ′andβ″martensite variants take place. While deformation increases the self-accommodated morphology begins to be broken, injection of the foreign variant into the substructural bands can be found, dislocations can be found in martensite varients. With further deformation, the self-accommodated morphology has been completely broken, uniform variant of martensite is formed, the density of dislocations in martensite variant is increasing and forming winding dislocation.Martensite type has great influence on strain recovery characteristics. After compression deformation and heating,β′martensite shows only one step shape recovery withβ′→βinverse martensitic transformation happening. When deforming at room temperature and heatingβ″martensite, two step shape recovery occurs withβ″→β′andβ′→βtwo-step inverse martensitic transformation happening, and recovery strain ofβ′→βinverse martensitic transformation is good than that ofβ″→β′martensitic inverse phase transformation.The addition of Fe can improve the shape memory effect, the maximal complete recovery strain of Ru₅₀Nb₅₀ is 2%, while the maximal complete recovery strain of Ru₄₃Nb₅₀Fe₇ is 2.5%.The addition of Fe decreases the compression strength and improves the ductility at room temperature of Ru-Nb alloy. The addition of Fe changes the failure mode of Ru-Nb alloy, failure mode of Ru-Nb alloys is brittle intergranulal fracture. failure mode of Ru-Nb-Fe alloys is ductile intragranular fracture. The change of failure mode is the main reason of ductility increment.更多还原