L Journal of Alloys and Compounds 310 (2000) 300–305 www.elsevier.com / locate / jallcom Effect of thermal cycling through the martensitic transition on the internal friction and Young’s modulus of a Ni Ti alloy 50.8 49.2 a, a a b b a * B. Coluzzi , A. Biscarini , R. Campanella , G. Mazzolai , L. Trotta , F.M. Mazzolai a Istituto Nazionale per la Fisica della Materia Condensata, Perugia, Italy b University of Perugia, Department of Physics, Perugia, Italy Abstract The internal friction (IF) and Young’s modulus (E ) of a Ni Ti is affected by thermal cycling. With increasing the number n of 50.8 49.2 thermal cycles, the IF peak P (P ) occurring at the austenite / martensite transition temperature decreases to disappear almost AM MA completely. Meanwhile the associated E( T ) minimum at the beginning ( n , 250) deepens and then becomes progressively shallower and 3 3 wider (3 3 10 # n , 18 3 10 ). The strong sensitivity of P (P ) to thermal cycling and to impurity (hydrogen) contents suggests that AM MA this peak is predominantly associated with stress-assisted collective motions of twin boundaries located inside the martensite platelets embedded within the austenite phase, rather than with the martensitic transition itself. However, as the high temperature tail of P starts AM at temperatures appreciably higher than the martensite start-temperature M , a premartensitic contribution to damping is also present. The s 3 widening of the E( T ) minimum indicates that, for n $ 3 3 10 , the direct transformation is to some extent hindered by the dislocation network introduced by thermal cycling. A not thermally activated IF peak P , which is believed to be due to stress-assisted motions of TWM (001) compound twin boundaries in the homogeneous martensite state, grows with increasing n.  2000 Elsevier Science S.A. All rights reserved. Keywords: Internal friction; Young’s modulus; NiTi alloy; Martensitic transition; Thermal cycling 1. Introduction solution-treated Ni rich alloy. Furthermore, it was hoped that a systematic investigation might lead to a better For technological applications one of the most important characterisation and understanding of the time independent properties of shape memory alloys is their stability with part of the IF associated with the transition [5,6]. respect to thermal cycling. To test this property some research has been carried out in the NiTi alloy system over the past few years. Depending on the alloy composition, 2. Experimental the stress loading conditions and the thermo-mechanical history of the material, more or less marked changes have A specimen of nominal composition 50.8 at.% Ni been observed in the behaviour of the martensitic transition prepared by CNR-TEMPE in the shape of a bar of with increasing the number n of thermal cycles [1–4]. It dimensions 44.4 34.7 30.8 mm was used in the present appears that in solution-treated (annealed at 1200 K then experiments. The coefficient of mechanical energy dissipa- 21 water-quenched) alloys large densities of dislocations and tion Q was deduced either from the decay of free dislocation tangles are introduced in the high temperature oscillations of the specimen excited in flexure in its free– B2 phase by repeated transformation cycles [1]. These free fundamental and overtone modes, or from the width of dislocations are considered to be the physical source of the the resonance curve under conditions of forced vibrations. observed changes in the characteristic temperatures of the The Young’s modulus was derived from the resonance martensitic transition. frequency, which ranged between about 1.3 and 7.1 kHz. The present work has been undertaken to investigate the A semi-automatic set-up was used which has been de- effect of dislocations introduced by thermal cycling on the scribed elsewhere [7]. The measurements were carried out 27 internal friction (IF) and the Young’s modulus ( E ) of the at vibration amplitudes of the order of 10 and at 22 cooling / heating rates of about 1.5 310 K/s. Prior to the *Corresponding author. measurements, the specimen was permanently deformed at 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(00)00968-3