First-principles investigations of point defect behavior and elastic properties of TiNi based alloys
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1128-U09-03
First-principles investigations of point defect behavior and elastic properties of TiNi based alloys Jian-Min Lu, Qing-Miao Hu, Rui Yang Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ABSTRACT First-principles calculations by the use of a plane-wave pseudopotential method are performed to investigate intrinsic point defect behavior in TiNi. The results show that TiNi is an antisite type intermetallic compound. The calculated interaction energies between the point defects demonstrate that Ti antisites are attractive to each other whereas Ni antisites are mutually repulsive. The attraction between Ti antisites indicates that excess Ti in TiNi may agglomerate so that a Ti-rich phase can easily precipitate. The repulsion between Ni antisites implies that the excess Ni is of certain solubility in TiNi. This result explains well the asymmetric feature of TiNi field in the binary phase diagram. In order to understand the correlation between the composition dependent elastic modulus and martensitic transformation (MT) temperature, the elastic moduli critical to MT, i.e. c′ and c44, are calculated as a function of the composition of the offstoichiometric TiNi and a series of ternary TiNi-X alloys by the use of exact muffin-tin orbital method in combination with coherent potential approximation. It turns out that, generally speaking, the early transition metal (TM) alloying elements in the periodic table increase c′ but decrease c44; the middle ones increase both c′ and c44, whereas the late ones decrease c′ but increase c44. An examination of the theoretical composition dependent elastic modulus and the experimental MT temperature shows that the MT temperature is more sensitive to the variation of c44 than to that of c′.
INTRODUCTION As one of the earliest discovered shape memory alloys, TiNi has already been used successfully in industry. The shape memory effect of this alloy is ascribed to the reversible martensitic transformation (MT) between the high temperature parent cubic B2 phase and the low temperature martensitic orthorhombic B19 or monoclinic B19' phase. However, the fundamental physics underlying the martensitic transformation are still to be clarified. One of the ambiguous issues is the precursor effect appearing just above the MT temperature as demonstrated by the central peak in the elastic neutron phonon spectrum measurement, diffuse scattering (corresponding to tiny domains) and extra diffuse peaks in X-ray diffraction, and remartensitic attenuation in ultrasonic attenuation experiments [1]. With the aid of numerical simulations, Kartha and colleagues suggested that the intrinsic chemical disorder could be one of the driving forces for pretransitional phenomena [2]. The chemical disorder in intermetallic compounds generally manifests itself by the existence of point defects, i.e. vacancies and antisite atoms. Therefore, knowledge of the point defects and their interactions is crucial for our understanding of p
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