Effect of thermal cycling on the R-phase and martensitic transformations in a Ti-rich NiTi alloy
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I.
INTRODUCTION
NITI-BASED alloys are among the shape memory alloys most technologically used, and, therefore, are one of the most studied. Up to now, many investigations on the effect of thermal cycling on transformation temperatures have been conducted; above all, they have focused on equiatomic NiTi[1,2,3] or Ni-rich alloys.[4,5,6] Very few studies are related to the thermomechanical behavior of a Ti-rich NiTi.[7,8,9] It is well established that the transformation temperatures could be modified by the concentration of Ni and/or the initial annealing state. The purpose of the present work is to study the premartensitic (R phase) and the martensitic transition occurring in a Ti-49.6 pct Ni alloy which was initially solution treated. The investigation was carried out using electrical resistivity, internal friction, and elastic modulus measurements. We examined with great attention the effect of thermal cycling on the transition sequences on heating and on cooling runs. In the present text, we shall refer to the various phases as follows: A, the high-temperature phase, also called austenite; R, the phase induced by a rhombohedral distortion of the austenite; and M the lowtemperature phase, also called martensite. II.
EXPERIMENTAL PROCEDURE
The alloy composition was determined by chemical analysis to be Ti-49.6 at. pct Ni. The samples were cut in flat strips with typical dimensions of 5 3 30 3 0.2 mm. They were solution treated in a vacuum furnace (P , 1026 torr) for 10 hours at 800 7C, then furnace cooled (approximately 4 7C/min). Electrical resistance was measured as a function of temperature using a conventional four-probe method. Elastic modulus and internal friction measurements were performed on a vibrating reed device. The apparatus has been described elsewhere.[10] Briefly, in the dynamic regime, the flexural modulus (F) is determined by measuring the natural
V. PELOSIN, Professor Assistant, and A. RIVIERE, Professor, are with the Laboratory of Mechanic and Physic of Materials, ENSMA/LMPM, Site du Futuroscope, 86960 Futuroscope Cedex, France. Manuscript submitted June 9, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
frequency of the reed. It is recalled that F is related to the resonant frequency (f) by the formula f 5 0.16 t/L2=F/r where t, L, and r are, respectively, the thickness, the length, and the density of the sample. In our case, the free oscillations of the specimen were found to be 273 Hz at room temperature (20 7C) and for the first thermal cycle. The internal friction is calculated from the free decay of the amplitude vibration. Temperature cycles (N) were performed between room temperature (or 0 7C, for resistivity experiments) and 150 7C. Heating and cooling rates were generally 2 7C/min. The incidence of the temperature rate was studied between 0.1 7C/min and 2 7C/min for N . 30 (with a stabilized sample structure). III.
RESULTS
A. Resistivity The resistivity measurements enabled a preliminary study of phase transformations in the Ti-rich (Ti-49.6 pct Ni) alloy. The sample was submi
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