The Study Of Phase Transformation And Electronic Structure Of Ni-Ti(Ta) Alloys

The NiTi shape memory alloys (SMA), as an implanted material, are currently a topic of notable interest in the medicine field because of its unique shape memeroy effect and superelasticity. The superelasticity is mostly used as implanted materials. Aging treatment is an effective method to improve the shape memory effect and superelasticity of Ni-rich Ni-Ti alloy. However, the deeper investigations on superelasticity are lacked. So far, the problem that the high nickel content of NiTi could cause toxicity is disputed, more researchs are needed to do. Recently, the NiTi alloys have been used in the interventional operation. With low X-ray visibility, the detection of the very small implantable devices or tools that are made of NiTi alloys can be very difficulut. To improve the visibility is demanded.Aiming at some aspects metioned above, the effects of aging on microstrcture and phase transfonnation and their mechanism in NiTi alloy were investigated. The microstructure and phase transformation were also studied in the Ni-Ti-Ta alloys, which were added element Ta with high X-ray visibility and excellent biocompatibility. To understand the mechanism of superelasticity, tha ab initio calculations were carried out on R-phase structure, point defects, and mechanism of the effects of temperature and stress on the transformation in the NiTi SMA and Ta occupation in the Ni-Ti-Ta alloys. The primary conclusions are listed as follows:Aging will result in a three-stage transformation in the Ti-50.6at.%Ni alloy. The Ti₃Ni₄ precipitation induces the inhomogeneity of the matrix, both in term of composition and internal stress fields. It is attributed to the occurrence of multi-stage transformation. The Ti₃Ni₄ precipitates do not affect the B2?R transformation temperatures, but strongly affect the R ?B19' transformation temperatures with increasing aging time. The martensite morphology is heavily affect by the Ti₃Ni₄ precipitates. The internal defect is (001)m compound twin after short time aging; <011>m type â…¡ twin is gradually observed with increasing aging time.The microstructures in the casted Ni-Ti-Ta alloy are composed of primary NiTi dendrite and net-like eutectic. The net-like eutectic is made up of NiTi matrix and β-Ta phase. The (Ti,Ta)₂Ni particles inhomogeneously exit in the eutectic regions. The Ta atoms in Ni-Ti-Ta alloys are proposed to replace Ti atoms, instead of Ni atoms. With the Ta addition increased, the amount of β -Ta phase and the volume of unit cell increase. The substructures of Ni-Ti-Ta aresmilar to those of NiTi alloy. The main substructure in NisoTijsTas alloy is (01 l)M type IItwining.The transformation sequence of Ni-Ti-Ta alloy is one-stage B2**B19\ Ta addition depresses the transformtion temperatures of Ni-Ti alloy. If Ta addition is constant, the transformation temperatures decrease with the Ni/Ti ratio increases. The transformation temperatures of ternary Ni-Ti-Ta alloy decrease more quickly than those of binary Ni-Ti alloy under the same number of thermal cycles. It is attributed to the presence of more second-phase particles which induce more dislocations and stress fields. The transformation temperature increases with incrasing aging temperature below 550 °C. However, the transformation temperature decreases with increasing aging temperatuere above 550 °C.Predeformation induces martensitic stabilization. This is because deformation introduces dislocations, which result in increasement of the energy barrier. The 4% deformation might induce plastic strain for the ï¿¡ -Ta. The Ni-Ti-Ta alloys exhibit a wide transformation hysteresis compared to Ni-Ti binary alloys. The most shape memory recovery strain of 5.6% is obtained after deformation 8% in NisoTitfTas alloy among the three kinds of Ni-Ti-Ta alloys.The ab initio calculations were carried out to study the R-phase structure. The calculated results indicate that the P3 structure changes into P31m , while the P3 structure is unchanged after geometry optimization. According to the analysis on the earlier ab initio calcalated results and the comparison with the ground-state energies, densities of states (DOS), bond-lengthes between P31m and optimized P3, it is suggested the P31m is a correct ground-state crystal structure of the R-phase. The calculated X-ray diffraction pattern and electron diffraction patterns are consistent with experimental results.The mechanism about the effects of temperature and stress on the martensitic transformation is explained in terms of electronic structure. Decreasing temperature and increasing deformtion can make the DOS at Fermi energy higher, which decrease the stabilization and induce martensitic transformation.The theoretical calculations indicate that the formation energy of Ti vacancy is slightly lower than that of Ni vacancy in NiTi alloy. It is easier to form Ti vacancy than to form Ni vacancy. The Ni vacancy, Fe substituting Ni and Al substituting Ti can induce a small gap between valence and conduction bands in their band structures, which result in decreasing conductibility and inducing R-phase transformation. The Ti vacancy, Ta substituting Ti, Cu substituting Ni and Al substituting Ni can't induce a small gap between valence and conduction bands in their band structures. Thus it can't induce R-phase transformation. This indicates thatthe R-phase transformation is relative to the gap of the band structure. By comparing the formation energy of Ta replacement in Ti and Ni, it is suggested that the Ti atom in Ni-Ti-Ta alloy is occupied by Ta atom. This is consistent with the experimental result.