The Development of the Amorphous Phase in NiTi During Heavy Ion or Electron Bombardment
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THE DEVELOPMENT OF THE AMORPHOUS PHASE IN NiTi DURING HEAVY ION OR ELECTRON BOMBARDMENT J. L. BRIMHALL,* H. E. KISSINGER,* and A. R. PELTON** *Pacific Northwest Laboratory, Richland, WA 99352; **Ames Laboratory, Ames, IA 50011
ABSTRACT A supralinear dose dependence for the amorphous transformation was observed in NiTi during bombardment with 2.5 MeV Ni+ ions. These results are consistent with a mechanism that requires cascade overlap to obtain a critical defect density for the amorphous transformation. Direct amorphization in the cascades was not resolvable. The temperature dependence of the minimum dose required for complete amorphous transformation had the same form as that observed for amorphization of silicon. Amorphization caused by electron bombardment required a higher dose than by ion bombardment. Different degrees of homogeneity of the damage state between ions and electrons can explain the dose dependence on particle type. INTRODUCTION This paper reports on a study of the mechanisms by which an intermetallic compound, NiTi, transforms to an amorphous phase during ion irradiation. Although there have been extensive studies on the mechanisms of amorphous transformation in silicon, [1] little attention has been devoted to other materials. In particular, the importance of displacement cascades and whether amorphous zones are formed directly in the cascades was investigated in this work. The NiTi alloy was irradiated with 2.5 MeV Ni + ions at ambient temperature and the amorphous transformation followed by TEM observations. In addition, electron irradiation was carried out over a limited dose range and results compared with those from ion bombardment. The results are discussed in terms of current mechanisms for amorphization. Experimental Procedure The NiTi alloy was made by arc melting and had a nominal composition of 50% Ti - 50% Ni. Discs, 3 mm wide and 0.25 mm thick, were punched from rolled sheet and polished to a smooth finish. All discs were annealed at 0 900 C for 24 hrs prior to ion bombardment. The microstructure was predominantly NiTi phase with isolated particles of Ti 2 Ni. The were bombardedwith 2.5 MeV Ni+ ions to doses ranging from 4 x 10o 3 /cm to 1 2 5 x 10 /cm , i.e., 0.020 to 3 displacements per atom (dpa) according to calculated damage rates [2]. The maximum damage ocpurred at a depth of -0.6 ýim. The 3on current was normally 0.16 iamp/cm but currents as high as 1.6 iamp/cm were used. The same alloy was el tron irradiated in the HVEM at Lawrence Berkeley Laboratory to doses >10 e/cm so as to produce damage states equivalent toldpa [3]. Standard microscopic techniques were used to record the microstructural changes. Selective electro thinning was used to observe the surface region and a region -0.3 pm from the surface. The influence of the deposited Ni ion was negligible in these regions. The amorphous volume was
Mat. Res. Soc. Symp. Proc. Vol. 27 (1984)DElsevier Science Publishing Co., Inc.
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determined by estimating a radius for the irregular amorphous regions and assuming a three
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