Development of Cr_2Nb Intermetallic Compounds with High Fracture Toughness as Ultra High Temperature Structural Materials

为了制备出能在1473K高温下使用且具有足够室温断裂韧性的Cr_2Nb金属间化合物，通过合金设计、显微组织控制和先进工艺对制备的合金的力学性能进行了研究。得到了以下结果：1)利用x射线和透射电镜观察了Cr_2Nb Laves合金的组织。研究了其高温压缩变形行为，并讨论了其与激活位错结构的关系。因此，变形开始于1473K，并通过激活1/2 <011>位错的Schockley部分或动态再结晶来完成。2)澄清了Cr-Nb-V、Cr-Nb-Mo和Cr-Nb-W三元合金体系在1623K时的等温相图。结果表明，C15 Cr_2Nb Laves相与bcc固溶体直接平衡，不形成任何三元中间化合物。研究了V、Mo、W、Ti等三元合金元素对C15 Cr_2Nb合金行为、晶格性能和相稳定性的影响。结果表明，V和W的溶解度均大于10at%， W的溶解度约为5at%， Ti向Cr_2Ti形成连续的固溶体范围。通过炼金术测定，V占据Cr的子晶格，Mo的子晶格，Ti的子晶格，W的子晶格。TEM观察表明，V的加入增强了C15组织的稳定性，而Mo、W和Ti的加入降低了C15组织的稳定性。4)在较宽的实验温度范围内，通过压缩试验研究了Cr_2Nb和bcc固溶体三元合金的力学性能。由C15相和富cr - bcc固溶体组成的合金在低温下具有较高的断裂韧性，而由C15相和富nb - bcc固溶体组成的合金在高温下具有较高的断裂强度。

In this study, to develop the Cr_2Nb intermetallics which can be used at 1473K and the enough fracture toughness at ambient temperature, the mechanical properties of the alloys prepared by alloy design, microstructural control and advanced process were investigated. The following results were obtained.1) Structures of Cr_2Nb Laves alloy were observed by x'-ray and (HR) TEM.High temperature compressive deformation behavior was investigated, and discussed in correlation with the activated dislocation structures. Consequently, the deformation begins at 1473K,and accomplished by an activation of the Schockley partials of 1/2 <011> dislocations, or the dynamic recrystallization.2) The isothermal phase diagrams of Cr-Nb-V,Cr-Nb-Mo and Cr-Nb-W ternary alloy systems at 1623K were clarified. It was shown that the C15 Cr_2Nb Laves phases are directly equilibrated with bcc solid solution without forming any ternary intermediate compounds.The effect of ternary alloying elements such as V,Mo, W and Ti on the alloying behavior, the lattice properties and the phase stability of the C15 Cr_2Nb was investigated. It was shown that the solubility of V and W are more than 10at%, that of W is about 5at%, and Ti forms the continuous solid solution range toward to Cr_2Ti. The ALCHEMI determined that V occupies the sublattice of Cr, Mo, Ti and W do the sublattices of Nb. Also, TEM observation on the microstructure of the alloys with ternary elements suggested that V enhance the stability of C15 structure but Mo, W and Ti reduce the stability of the C15 structure.4) The mechanical properties of the ternary alloys consisting of the Cr_2Nb and the bcc solid solution were investigated by compression test in a wide range of test temperature. High fracture toughness at low temperature was observed on the alloys consisting of the C15 phase and Cr-rich bcc solid solution, while high strength at high temperature was observed on the alloys consisting of the C15 phase and Nb-rich bcc solid solution.