| 其他摘要 | Owing to their excellent shape memory effect, superelasticity, corrosion resistance and biocompatibility, TiNi-based alloys have become one of the most important shape memory alloys, which are widely used in aerospace components, machines, meters, automobile and biomedical engineering, et al. However, as their applications are gradually extended, TiNi alloys are confronted with increasingly stringent requirements for their performance. To understand the underlying mechanism of shape memory effect and mechanical properties of TiNi alloys is a prerequisite to further alloy designing. First-principles calculations can predict the physical and chemical properties of materials, not relying on any experimental or experiential data, which makes it a real prediction. The applications of first-principles calculations to the designing of TiNi alloys will physically facilitate the comprehension of behavior of these alloys.
In this dissertation, two first-principles methods, the plane-wave pseudopotential method (PW-PP) and the exact muffin-tin orbitals within coherent potential approximation (EMTO-CPA), are employed to investigate the effects of point defects and alloying elements on martensitic transformation of TiNi alloys.
The point defect behavior of TiNi binary alloys is studied. A method based on statistical mechanics and first-principles calculations is used to provide the point defect concentrations. It indicates that TiNi alloys are of antisite-type intermetallics. In Ti-rich TiNi alloys, excess Ti atoms occupy the Ni sublattice, which form antisite defects; only very small amount of vacancies exist. On the other hand, in Ni-rich TiNi alloys, Ni antisites on Ti sublattice are dominant with vacancies being few. There are a large amount of vacancies in Ti-rich vacancies at elevated temperature. The interactions of point defects are also calculated. The nearest-neighboring Ni antisite and Ti antisite are attractive to each other, whereas the second-nearest neighboring Ni antisites repel each other and Ti antisites mutually attract, which may result in Ti-rich domains in stoichiometric TiNi. The strong repulsive interaction between the Ni antisites and the attractive interaction between the Ti antisites also explain the high solubility of excess Ni and low solubility of excess Ti in TiNi observed in experiments.
The effects of composition on properties of TiNi are investigated. The results show that, in Ni-rich compositions, the shear modulus c44 of non-basal shear (001) [1 0] increases with the increasing Ni content, while the shear modulus c' of basal shear (1110) [1 0] decreases. The increase of c44 is a factor that would depress the martensitic transformation start temperature Ms, while the decrease of c' is the opposite. But the increasing rate of c44 surpasses the decreasing rate of c'. The combined effect of both moduli is lowering the Ms with the increase of Ni content. The anisotropy A (c44/c') increases with increasing Ni content, which means that the coupling of c44 and c' is weakened, and it becomes more difficult for TiNi to perform monoclinic distortion. But the value of A is not large enough to prohibit this distortion, so the product of martensitic transformation of TiNi alloys is monoclinic phase B19'. The G/B ratio increases when the Ni content increases, which lowers the ductility of TiNi alloys. But the values of G/B are generally low, so TiNi alloys are ductile.
The heat of formation of 12 TiNiX ternary alloys as a function of alloy compositions is calculated by using the EMTO-CPA implementation, which is used to determine the site occupation of X from phase stability point of view. It shows that, alloying atoms Al, Sc, Zr have strong preference to occupy the Ti sublattice; Cr, Mn, Fe, Co prefer to the Ni sublattice; V, Cu, Au, Pd, Pt do not have preference to occupy any particular sublattice, determined by the compositions of TiNiX alloys. Also, the bonding charge density of X with its surrounding atoms is obtained from CASTEP calculations, which tells the general trend that, an alloying atom, which forms a stronger bond with surrounding atoms when on Ti sublattice than that when on Ni sublattice, prefers to occupy Ti sublattice; and vice versa.
The effects of alloying element X on martensitic transformation of the TiNiX alloy, from the elastic stability point of view, is analyzed by using the moduli c44 and c' of B2 parent phases of 12 TiNiX ternary alloys, which are obtained from EMTO-CPA calculations. The main results are: (1) In Ni-rich Ti50-xNi50Alx, Al increases both the shear moduli c44 and c', which makes the both shears more difficult and thus causes the transformation temperature to drop down. In Ni-rich Ti50-xNi50Scx, Sc causes c44 to slightly increase but c' to dramatically increase, which provides a combined effect of making shear harder, thus lowering the transformation temperature. (2) In Ni-rich Ti50-xNi50Vx, V almost does not change c44, but depress c' a lot, which reduces the resistance to shear and thus will raise the transformation temperature. In Ni-rich Ti50-xNi50Zrx, c44 decreases slowly and then does so quickly, and c' increases steadily, which are combined to make the transformation temperature decrease, then followed by increase. (3) In Ti-rich Ti50Ni50-xXx (X = V, Cr, Mn, Fe, Co), X mainly keeps c44 unchanged or slightly increases it, but increases c' a lot, which causes the transformation temperature to drop. (4) In Ti-rich Ti50Ni50-xXx (X = Pd, Pt, Au), three elements Pd, Pt and Au always decrease c', but perform varying effects on c44: Pd first raises c44, and then depresses it; Pt and Au first raise c44, and then keep it almost constant. The correlated effect of both moduli is to make the shear difficult at the beginning and then make it easy, which causes the transformation temperature drop first and then increase. (5) In Ti-rich Ti50Ni50-xCux, Cu increases c44 but meanwhile depress c', which results in no obvious effect of Cu on transformation temperature. It comes to a conclusion, from above, that the addition of alloying elements affects the shear moduli c44 and c' of TiNi alloy, both of which influence the martensitic transformation temperature together. The impact of c44 and c' is different, which is as following: if the addition of alloying elements changes c44 a little, then only when it changes c' dramatically can it change transformation temperature; if the addition of alloying elements changes c' a little, then the transformation temperature is generally determined by c44. |
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