Precipitation processes in near-equiatomic TiNi shape memory alloys
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I. INTRODUCTION
TiNi alloys of near-equiatomic composition are one of the most technologically important shape memory alloys. Recently, several investigations 1-4 have been made in order to improve the memory properties of such alloys, i.e., the shape memory effect, pseudoelasticity, 1'2'3 and the reversible shape memory effect. 4 Some of the improved methods 2'4 have basically the same feature: making use of precipitation in Ni-rich Ti-Ni alloys. Others involve thermomechanical treatment. 1.3Therefore, it is necessary to clarify the aging behavior for the further improvement of memory properties in the TiNi alloy system. Phase separation and the phase diagram in the Ti-Ni system have been studied by many investigators) -13 Some of them 5-s reported that a eutectoid reaction (TiNi ~ TiNi2 + TiNi3) took place about 650 ~ Others 9-13 argued against this proposal and contrary results were reported following diffusion couple experiments. 12:3Wasilewski et al. 11suggested that a peritectoid reaction [TiNi + TiNi3 ~ Ti42Ni58 (Ti~Ni3)] occurred at 625 --- 20 ~ in Ni-rich alloys. In addition, it is well known that fine precipitates form in aged or slowly cooled Ni-rich TiNi alloys. 2'4'7'8'14-16 The purpose of the present paper is thus to clarify the nature of phase separation in near-equiatomic TiNi using conventional metallographic techniques.
II.
EXPERIMENTAL PROCEDURE
Ti-50 pct and 52 pct Ni (at. pct) alloys were prepared from 99.7 pct sponge Ti and 99.9 pct electrolytic Ni by arc melting in an argon atmosphere. The ingots were remelted several times for homogenization. The compositions of the alloys were determined by chemical analysis to be Ti-50.08 and 51.96 at. pct Ni, and the oxygen contents were 280 and 310 ppm, respectively. Plate specimens for optical and scanning electron microscope (SEM) studies were cut from slabs 4 mm thick, which were hot-forged and rolled from the initial ingots. The specimens were annealed at 1000 ~ for 1 hour in evacuated quartz capsules and then quenched into ice water without breaking the capsules, and then aged M. NISHIDA, formerly with the University of lllinois, is with the Department of Metallurgy, Kumamoto University, Kumamoto, Japan. C. M. WAYMAN is with the Department of Metallurgy and Mining Engineering and Materials Research Laboratory at the University of Illinois, Urbana, IL 61801. T. HONMA is with the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan. Manuscript submitted May 21, 1985. METALLURGICALTRANSACTIONS A
under several conditions. Following this, they were lightly mechanically polished and then electropolished in an electrolyte consisting of 5 pct H 2 5 0 4 and 95 pct methanol (by volume) and finally etched in a solution consisting of I0 pct HF, 40 pct HNO3, and 50 pct H20 (by volume). Disk specimens 0.25 mm thick for transmission electron microscopy (TEM) studies were sliced from rods 3 mm in diameter (which were hot forged and drawn from ingots) and then heat-treated in same manner as for the optical microscopy specimens
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