Calculation of Defect Properties of NiTi and FeTi
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CALCULATION OF DEFECT PROPERTIES OF NiTi AND FeTi 2 RUSSELL T. LUTTON 1 , MICHAEL J. SABOCHICKit and NGHI Q. LAM
'Air Force Institute of Technology, Department of Engineering Physics, Wright-Patterson Air Force Base, OH 45433-6583 2 Argonne National Laboratory, Materials Science Division, Argonne, IL 60439
ABSTRACT The energies and configurations of interstitials and vacancies in the B2 ordered compounds NiTi and FeTi were calculated using atomistic simulation. The stable configuration of a vacancy after the removal of an Ni atom was a vacant Ni site; similarly, the removal of an Fe atom in FeTi resulted in a vacant Fe site. Removal of a Ti atom in both compounds, however, resulted in a vacant Ni or Fe site and an adjacent antisite defect. The effective vacancy formation energies in NiTi and FeTi were calculated to be 1.48 and 1.07 eV, respectively. Interstitials in NiTi formed split configurations consisting of a Ni-Ni dumbbell oriented in the direction with one or two adjacent antisite defects. The Fe interstitial in FeTi had a similar configuration, except the dumbbell contained Fe atoms. The Ti interstitial in FeTi formed an Fe-Fe dumbbell.
INTRODUCTION A recent computer simulation study of the crystalline-to-amorphous (C-A) transition of CumTin compounds under electron irradiation indicated that point defects were a critical factor in the transition [1,2]. In an accompanying study of point defects in CuTi and CuTi 2 , it was found that the stable interstitial configurations consist of Cu atoms in a split interstitial, surrounded by one or more antisite defects [3]. From the observation that these configurations were rather complicated, it was presumed that interstitials in the compounds were less mobile than their pure metal counterparts. This could explain how point defect concentrations can increase sufficiently during electron irradiation to induce amorphization [3]. The purpose of the present work was to calculate the point-defect properties of NiTi and FeTi. These properties are compared with those calculated for the CumTin compounds, and the implications regarding amorphization are discussed. In computer simulation studies of the amorphization of NiTi and FeTi, it has found that pointdefect introduction is necessary to trigger the C-A transition [4,5]. The findings for FeTi were similar to those for the CunTin compounds; the chemically-disordered state had a lower energy than the amorphous state [4]. In NiTi, however, chemical disorder alone was capable of storing enough energy to favor the transition, although, as in the other compounds, the transition was not observed without point defect introduction [5].
METHOD The computational methods employed in the present work were molecular statics, molecular dynamics and Monte Carlo, using a modified version of the code DYNAMO [6]. The simulations used a fixed cubic volume containing N = 1024 atoms and having periodic boundaries.
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Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
208
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