Microstructure And Properties Of TiNi Alloy Prepared By Mechanical Alloying

The TiNi shape memory alloy which has been studied since 1970 wins some of the industry’s favor for its excellent performance. At present nearly equiatomic TiNi shape memory alloy is the largest amount of applications, because it has excellent shape memory effect and super elasticity, and high strength, non magnetic, corrosion resistance, wear resistance, good biocompatibility, high damping and resistance mutation characteristics, which has been widely used in many fields as a new type of functional material.TiNi alloy products prepared by traditional method has composition segregation phenomenon. TiNi alloy prepared by mechanical alloying(MA) preparation technology, is a kind of amorphous or microcrystalline structure materials.MA can improve the brittle of intermetallic compounds. In this paper, the main properties and micro structure of TiNi powder products prepared by MA were studied. MA products were studied from the view of thermodynamics and dynamics.The phase, morphology and elemental composition of products with different milling time were tested by XRD, SEM, and EDS. The results show that: microstructure of Ti-Ni powder during ball milling changes obviously with the increase of ball milling time, which gradually forms substitution solid solution, begins to appear amorphous after ball milling 19h, amorphous and Ni solid solution phase after 27h, the intermetal compounds of TiNi, Ni3Ti, Ti3Ni4 phase with solid phase reaction after high energy ball milling 30h. In the process of ball milling, powder composition tends to be uniform, a laminated composite particles.Thermodynamic calculation of TiNi system in MA process was carried out using Miedema semi empirical model, results show that the phases during MA process is the solid solution followed by amorphous and intermetallic compounds, which is more consistent with experimental results. In addition, a large number of defects produced during mechanical alloying process also provide convenient conditions for atom diffusion. The grain refinement, a lot of dislocation and various defects during the ball milling process induce the solid solubility of Ti in Ni increased significantly, which is one of the main reasons causing the alloying.DSC testing and annealing treatment on the alloy powders ball milled for 30 hours were carried out, results of DSC show that the crystallization of amorphous alloy and the continuous phase transformation took place during the heating process, and the glass transition temperature and phase transition temperature would reduce significantly with increase of the ball milling time. XRD results showed that the main products in annealing under the temperature of 1000 ℃ were Ni3Ti, Ti2Ni, TiNi2, TiNi and TiC, the main products in the annealing at the temperature of 650 ℃ are similar to those of under the temperature. But the TiNi is not the main phase in annealing at the above temperature, the content of TiNi annealed at 650 ℃ was significantly lower.The samples were sintered by using the conventional vacuum sintering and plasma sintering in vacuum, the results show that:the pure TiNi phase with shape memory effect was not obtained in two kinds of sintering method, but the higher the temperature of sintering, the relative proportion of TiNi is higher, at the same time, the specimen density increased. The specimen sintered by plasma method is with high dense, high content of TiNi phase, fine grains in micrometer scale, hardness up to 900HV which is higher 2～5 times than that of TiNi alloy prepared by conventional melting method. Test the mechanical properties of the plasma sintering of samples, There are certain shape memory effect in the specimen sintered by plasma method according to mechanical test results.