Origin of ductility of intermetallic compounds CoZr and Co39Ni11Zr50 at low temperatures

CoZr是为数不多的多晶型金属间化合物，在室温下具有极好的延展性。用Ni代替少量的Co可以提高合金的延展性。虽然对这类材料进行了许多研究，但其高延性的来源仍不清楚。二次滑移系统的易滑移和马氏体相变是高延展性的有利原因。为了解决这一技术上的重要问题，将首次在低温（300 K - 4 K）下进行机械测试。一方面，拉伸试验用于确定屈服应力、加工硬化、断裂应力和断裂应变的温度依赖关系。基于应力松弛试验的热活化分析将用于确定活化参数作为应力和温度的函数，从而得出变形机制的结论。另一方面，在室温和液氮温度下进行高压扭转实验，测量形成的局部剪切织构作为剪切应变的函数。织构积累了激活滑移系统的信息，这些信息的贡献可以通过与合适的多晶模型模拟的比较得到。此外，位错分析将由一位经验丰富的中国合作伙伴用透射电子显微镜进行，以获得位错的类型、排列和密度的信息，但不包括它们对塑性应变的贡献。还将特别注意（Co,Ni）Zr中的马氏体相变对塑性的影响（也可能是CoZr在低温下的应力诱导）。本文的研究是基于对延展性稀土金属间化合物YCu和YAg的大量研究经验。他们应该把金属间化合物延展性的原因放在更广泛的基础上，以便将来更有针对性地寻找用于结构应用的延展性金属间化合物。（Co,Ni）Zr具有较好的抗氧化性，是一种合适的模型材料。

CoZr belongs to the few intermetallic compounds, which in polycrystalline form are extremely ductile at room temperature. The ductility can be increased by substituting a small amount of Co by Ni. Although there have been made many investigations on this material class, the origin of the high ductility is still not clear. Reasons favored for the high ductility are easy slip on secondary slip systems and martensitic transformation. To solve this technologically important problem, for the first time mechanical tests will be performed at low temperatures (300 K – 4 K). On the one hand tensile tests are used to determine the temperature dependence of the yield stress, work-hardening, fracture stress and fracture strain. Thermal activation analysis based on stress relaxation tests will be used to determine the activation parameters as a function of stress and temperature allowing conclusions on the deformation mechanisms. On the other hand high pressure torsion experiments will be performed at room and liquid nitrogen temperature and the local shear texture formed will be measured as a function of shear strain. Textures accummulate the information on the activated slip systems, the contribution of which can be obtained from comparison with suitable polycrystal model simulations. In addition, dislocation analyses with transmission electron microscopy will be done by an experienced chinese partner to get information on type, arrangement and density of the dislocations, but not on their contribution to plastic strain. Special attention will be also paid to the influence of the martensitic transformation in (Co,Ni)Zr on the ductility (possibly also stress-induced in CoZr at low temperatures). The investigations are based on much experience gained on the ductile rare earth intermetallic compounds YCu and YAg. They should put the reasons for ductility of intermetallic compounds on a broader basis giving rise in the future to a more targeted search of ductile intermetallic compounds for structural applications. Because of the better oxidation resistance, (Co,Ni)Zr is a suitable model material.