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

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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.