Engineering Science Paper

Since the discovery of the shape memory effect,NiTi shape memory alloy is considered to be the most important shape memory alloys for its salient superelasticity and shape memory effect Which are displayed in martensitic transformations,This material transforms martensitically from the parent B2 phase to a monoclinic martensitic phase at the transformation temperature of 333 K.This material has been used in the aerospace industry, for electronics and mechanical engineering.In the last decade NiTi-based alloys have found widespread applications in medicine also.In the case of TiNi the knowledge of the nature of its biocompatibility is very important because Ti is a good implant material but Ni is well known as a hazard material. However, biocompatibility is the surface related property.The properties of materials are determined by their atomic and electronic structures, and the quantum chemistry provided a viable method in computing the behavior of electrons and atomic nuclei under arbitrary circumstances. The first principles Density Function Theory is always the most powerful tool for calculating electronic structure and characteristics in the field of Condensed State physics.The theoretical basis of CASTEP is the density functional theory (DFT) in the local density approximation (LDA) or gradient-corrected LDA version, as developed by Perdew and Wang (GGA). The DFT description of electron gas interactions is known to be sufficiently accurate in most cases, and it remains the only practical way of analyzing periodic systems.In the paper, a first-principles plane-wave method, a total energy CASTEP code is utilized to systemically investigate the geometrical and electronic structure of NiTi .First we have studied the bulk properties of the NiTi alloy,such as lattice parameters, formation energy , elastic constant ,electronic structures, the results have good agreement with experimental and othercalculating results. Through computating energy at different crystal lattice distance ,Obtains when the crystal constant is 0.314nm, the energy is lowest.The alloy formation energy and cohesive energy reflect the material stability and unifies strong or weak between the alloy atom.Through the computation,the formation energy and cohesive energy of NiTi alloy is -0.335eV/atom and -5.003eV/atom respectively.The origin of unique B19′phase is a result of the coupling between C44 and C’,and C44 controls B19′transformation temperature,the harder C44, the lower the transformation temperature to is B19′.Furthermore, it can be seen that the anisotropy factors A of NiTi are far smaller than those of the usual martensitic alloys,It is argued that the higher A is a sufficient condition for the B2 to B19 martensitic transformation. the paper also conforms to the above viewpoint through the elastic constant computation。Niti’Fermi levels fall on a dip of a DOS curve. As well known, a dip is generally considered to divide a DOS into a bonding state and an antibonding state,thus when a Fermi level falls on the dip, the corresponding structure may be regarded as a saveenergy system, as compared with one whose Fermi level does not fall on the dip. Thus, this kind of structure is more stable;Secondly,we have studied the geometry structures and electronic structures of NiTi (100) and(110)surface, Predicted adsorption oxygen on the NiTi alloy (100) surface. As for the surface geometry structures, there are no reconstruction in the cleaned NiTi (100)surface, but there are different relaxation in the surface layers. the interlayer distance between the surface and subsurface layers,δ12= -12.76% for Ni/NiTi andδ12= -10.31%for Ti/NiTi, whereas the second interlayer distance expanded byδ23=5.19%、δ23=5.21% in both cases.As for the cleaned NiTi(110) surface,outermost surface layer shows a large rippled relaxation in which Ni atoms contract into the bulk by 0.122? and Ti atoms expand to the vacuum region by 0.198?, the interlayer distancebetween the surface and subsurface layers,δ12=-7.35%,whereas the second interlayer distance,δ23=5.5%。the ripple,γ1=15.38%、γ2=-13.98%。The calculation of the surface electronic show that Ti-terminated surface is more reactive than Ni-terminated surface on (100) surface and the NiTi (110) surface is inert. The energy and electronic structure calculation indicated the structure with single oxygen layered at the hollow position is more stable than that at top position and at bridge position on the Ti-terminated surface.The structure with double oxygen layered at the hollow and top position is more stable than the structure with double oxygen layered at bridge position.when the position stand in the middle of bridge position and hollow position and approache (100) surface by the parallel way, O2 will decompose after adsorption,the oxygen atoms near hollow position move downwards to the subsurface,and the oxygen atoms near bridge position move to the top position, an TiO2 film could form with an Ti-O bond length and an angle of bonds which is closed to those of TiO2 .