Study On Microstructure And Mechanical Properties Of Novel Tininb Shape Memory Alloys With Inhomogeneous Composition

In recent years,with the requirements of miniaturization,weight reduction on devices and harsh conditions for their usage,shape memory alloys are required to exhibit high superelastic stress in a wide temperature range and at low temperatures.However,the existing shape memory alloys cannot meet the requirements.Our group has found that the Ti_x Ni_yNb_z alloy exhibits high superelastic stress in a wide temperature range?-196?to 60??,which is 3 times wider than that of commercial NiTi shape memory alloy.However,the microstructure and mechanical properties of this alloy have not been systematically studied.In this paper,the microstructure of Ti_xN_iyNb_z alloy in ingot,plate,warm drawn wire,cold drawn wire,and the wires after aging have been studied by using scanning electron microscopy,transmission electron microscopy,and in-situ high-energy X-ray diffraction technique.Meanwhile,we also investigated the microstructure and mechanical properties of other 14 TiNiNb alloys,which are newly designed and prepared.Firstly,we have observed the microstructure of Ti_xNi_yNb_z alloys by different status.The results show that the ingots are mainly composed of two components with Nb-rich and Nb-lean areas.This microstructure has good stability during aging at 435?and515?.The warm drawn wires are mainly composed of marbling-like structure with different Nb contents,and the width of the marbling band becomes narrower as decreasing of the diameters;The average grain size of cold drawn wires is about 20 nm after aged at 435?for 20 min.In addition,Ti_x+1.5Ni_y+0.5Nb_z-2-2 alloys have been designed and prepared.The microstructure of these alloy ingots and warm drawn wires are studied.It is found that there is no Nb-rich phase,and only dual compoments with different Nb contents.The cold drawn wires after aging at 405?for 20 min exhibit high superelastic stress from-196?to 80?,much wider than that of the shape memory alloy reported in the literature.The microstructure of 13 species of TiNiNb alloys with different Nb contents is also explored in this dissertation.We found that with the reduction of Nb content in the ingots,the dual components with different Nb contents become weaker and weaker,and with the increase of Nb contents,more and more white Nb-rich phase are formed.