Effect of off-stoichiometry on the mobility of point-like defects and damping in binary Cu–Al martensites S. Kustov a,b, * , S. Golyandin b , K. Sapozhnikov b , J. Pons a , E. Cesari a , J. Van Humbeeck c a Universitat de les Illes Balears, Cra Valldemossa, km. 7.5, 07122 Palma de Mallorca, Spain b A.F. Ioffe Physico-technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia c Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium Received 21 March 2005; received in revised form 23 June 2005; accepted 30 December 2005 Available online 3 March 2006 Abstract An attempt is undertaken to define a factor, apart from temperature, which allows for the mobility of quenched-in defects in a Cu–Al system of martensitic alloys. Experiments were performed with single-and polycrystalline alloys with nominal Al content ranging from 10 to 13.5 wt.%. An acoustic technique, operating for temperatures ranging from ambient down to 10 K, was applied to detect the existence of mobile quenched-in defects. No mobility of quenched-in defects was detected for either single-or polycrystalline alloys with Al con- tents close to and below the Cu 3 Al stoichiometric one (Cu–12.4 wt.% Al), corresponding to perfect DO 3 ordering. For the alloy Cu–13 wt.% Al, mobility of quenched-in defects was detected starting from temperature of around 70 K. It is suggested that the off-stoichiom- etry of the alloys is accommodated through anti-site defect formation on the Al-deficient side and through vacancy formation on the Al- rich side of the phase diagram. The composition dependence of damping of Cu–Al martensites exhibits a sharp maximum around the stoichiometric composition Cu 3 Al (Cu–12.4 wt.% Al), which is presumably formed by variations of ordered domain size and point defect concentration on both sides of the stoichiometry. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Shape memory alloys; Ordering; Vacancies; Diffusion; Acoustic methods 1. Introduction The presence of mobile point-like defects plays a crucial role in the properties of Cu-based martensitic alloys, pro- moting diffusion-related phenomena. Diffusion determines the stability of the martensitic transformation through the martensite stabilization effect [1–4] and precipitation of equilibrium phases [5], damping properties of martens- ites through pinning and, hence, immobilization of inter- faces and internal defects of martensitic variants [6–9]. All these phenomena are especially undesirable for shape- memory alloys, since diffusion-related effects affect the sta- bility of their functional properties. In martensites, often produced by quenching from high temperatures, mobile point-like defects are supposed to be quenched-in vacancies [4,10–12]. However, martensites are usually formed from ordered structures, and the ordering process, as a rule, cannot be suppressed even by quenching. The mechanism of point defect formation in ordered struc- tures is substantially different from that in disordered alloys: the concentration and type of point defects (vacancies or anti-site defects) depend on the deviation from stoi- chiometry [13]. As a most general rule, vacancy diffusion- related phenomena in metals are considered to be irrelevant for low homologous temperatures T [ 0.3T m , where T m is the melting temperature. Apart from deviations from the stoichiometry, martensitic structures with different order- ing and stacking sequence can be formed depending on minor changes in composition. As far as the authors are aware, the possible effects of all these structural character- istics on the formation and mobility of quenched-in defects have not been studied. Cu–Al martensites are convenient materials for studying both the type/presence of point-like 1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.12.035 * Corresponding author. Tel.: +34 971 17 1375; fax: +34 971 17 3426. E-mail address: sergey.kustov@uib.es (S. Kustov). www.actamat-journals.com Acta Materialia 54 (2006) 2075–2085