Slipchenko V. Effect of silver and tantalum on the structure and properties of Ti-Ni-based shape memory alloys

The thesis is devoted to a research of the effect of silver and tantalum as well as technological factors on the structure and properties of Ti-Ni-based shape memory alloys. These alloying elements positively influence the casting properties of Ti-Ni-based alloys: they decrease the linear shrinkage ratio and increase the resistance to formation of casting cracks and the ability of metal to recover its shape. Martensitic transformation characteristic temperatures in such alloys tend to decrease. In this work an ambiguous effect of the cooling rate of semi-products on the structure formation and martensitic transformation parameters is analyzed. At 2°C/sec rate, dendritic structure and depleted solid solution are developed in the semi-products of Ti-Ni-based alloys, which leads to a wide temperature interval of martensitic transformation and incomplete shape recovery. At rates over 100°C/sec, a solid solution with homogeneous distribution of chemical elements, narrow interval of martensitic transformation and full shape recovery is formed. Using complex methods of analysis, the content of alloying elements that provides the ability for high temperature rolling deformation is determined, and the conditions under which it is possible to obtain high-quality semi-products in the form of ribbon are established. The results of studying the influence of contact welding on the parameters of martensitic transformation in Ti-Ni-based alloys are given which significantly enhance the potential for design of welded structures for medical devices, and chemical compositions of medical alloys are proposed.