Corrosion behavior of Ni + -Ion Irradiated NiTi Alloys
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CORROSION BEHAVIOR OF Ni -ION
IRRADIATED NiTi ALLOYS
R. WANG AND J. L. BRIMHALL Pacific Northwest Laboratory,
P.O. Box 999,
Richland,
Washington
99352.
ABSTRACT + Corrosion behavior of Ni -irradiated NiTi alloys was studied in chloride solutions, together with unirradiated Ion NiTi material with different surface conditions. irradiation with either 2.5 or 5 MeV Ni ions transformed the NiTi surface into an amorphous layer up to 1.5 micrometers thick. Studies of corrosion potential vs. time and polarization behavior indicated a small enhancement of the passivation for the Ni -irradiated NiTi over the The unirradiated NiTi with a unirradiated NiTi. mechanically polished, coarse surface was susceptible to pitting and crevice corrosion attack in 1 N HCI solution. The homogeneous amorphous structure in the irradiated alloy retarded this type of localized corrosion.
INTRODUCTION Aqueous corrosion behavior of metal and alloy surfaces can be altered Depending on the type and the energy of by high energy ion-implantation. the ions, the surfaces can be chemically and/or structurally modified. Extensive studies have been made of surface alloying with ion-implantation. Typically, improved behavior in both general and pitting corrosion has been observed, such as in Fe and Fe-based alloys implanted with ions of passive This is true especially if the structure of metals, e.g., Cr, Ta and Ti. the metal surface was modified into an amorphous phase [1]. On the other hand, implantation of inert gas ions into pure Fe without the effect of change in the corrosion behavior [2]. surface alloying resulted in little In the latter case, degradation of the corrosion behavior was sometimes So far there is little observed due to ion-induced defect structures [3]. reported work on the effects of amorphization of a crystalline, single-phase alloy by ion-implantation without chemical change of the surface. This paper concerns the study of the effects of ion induced amorphization on the corrosion behavior of a single-phase NiTi alloy. The fine-grained NiTi structure can be transformed into an amorphous phase by Therefore, irradiation with one of its own constituents, i.e., Ni ions [4]. it is possible to study the effects of this structural modification with minimal effects from the compositional variation.
MATERIALS The as-cast NiTi alloy, supplied by Raychem Corporation, had a nominal The microstructure was single-phase NiTi with composition of 50% Ni-50% Ti. Specimen discs of 3 mm in diameter and a few isolated particles of Ti 2 Ni. 0.1 mm thick were extracted from the cast alloy for ion irradiation and The specimens were polished to a smooth finish, corrosion measurements. then surfaces representing three different conditions were prepared: electropolished in perchloric/alcohol solution, abraded on 600 grit paper A sputter deposited alloy was also prepared paper. and abraded on 240 grit using the Raychem material as the sputtering target.
Mat. Res.
Soc. Symp. Proc. Vol.
27 (1984) 0
Elsevier Science Publishing Co.,
Inc.
730
ION I
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