THE EFFECT OF AGEING AND ANNEALING ON SHAPE MEMORY PROPERTIES OF CU-AL-NI MELT SPUN RIBBONS P. La Roca 1 , C. Sobrero 1 , J. Malarría 1 , Ph. Vermaut 2 , P. Ochin 3 , R. Portier 2 1 Instituto de Física Rosario (CONICET-UNR), Bv. 27 de Febrero 210 bis, 2000 Rosario, Argentine 2 Laboratoire de Métallurgie Structurale, Ecole Nationale Superieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 PARIS Cedex 5, France 3 Institut de Chimie et des Materiaux Paris Est, 15 rue Georges Urbain, Vitry-sur-Seine 94407, France Cu-13.5Al-5.5Ni-1Ti tapes with a grain refined microstructure of ~2 μm were obtained by melt spinning. The martensitic transformation temperatures of the precursor alloy were M s = 101°C, M f = 96°C, A s = 112°C, A f = 124°C. As result of the rapid solidification process, the transformation temperatures of this high temperature shape memory alloy shift below 0°C, and the melt spun ribbons are in the austenite phase at room temperature. Figure 1 shows the characteristic cell morphology of the austenite microstructure: The cells are clearly outlined by plates of martensite placed on their boundaries, corresponding mainly to the 18R orthorhombic structure although the 2H hexagonal structure was also observed. A high cooling rate causes a decreased order, reduced grain size, and an increased density of dislocations [1]. The concentration of quenched-in vacancies promotes more marked ageing phenomena in this melt spun alloys and the stability of the transformation temperature is an important requirement for a predictable performance. The course of the transformation was analyzed by DSC scans allowing an ageing stage for short times (30 min.) at the maximum temperature for the experiment (selected in the range 70°C-300°C) between the heating/cooling runs. A two stage evolution was clearly identified: ageing below 200°C lead to an increase of the transformation temperatures mainly due to progressive recovery of short range order (reversion of the disorder retained during the alloy solidification). For runs beyond 200°C a more pronounced increase in transformation temperatures is accompanied by a broadening and distortion of the peaks due to phase decomposition and precipitates evolution. According to these results a standard thermal treatment of 200°C x30 min was introduced as a reference state (Figure 2). Microstructural analysis of the austenite reveals that after ageing at 200°C no traces of martensite phase is observed at the cell boundaries. This suggests that the platelets maybe arise to relax thermal stresses introduced in the fabrication procedure. An evolution in the precipitates size and distribution cannot by fairly established since the former martensite plates partially mask the precipitates. Anyway, no important evolution of precipitates is expected for such ageing temperature and involved holding time [2]. To recover the high phase transformation temperatures of the master alloy, above 80°C, the ribbons must be re-homogenized by annealing at temperatures above 700°C (Figure 2) and quenched. Annealing in the range 700°C-900 °C leads to pronounced microstructural changes related with Ostwald ripening of Cu-Al-Ni-Ti precipitates: Fine precipitates are dissolved while medium and coarse precipitates grows up (Figures 4–5). X-phase aluminum reach precipitates [3] grow depleting aluminum concentration in the matrix, raising the transformation temperatures of the alloy. On the other hand, as it can be seen in Figure 6, an increase of the strain associated with the phase transition is observed, since the dissolution of fine evenly dispersed precipitates may favor moving interfaces. Annealing at temperatures as high as 900°C intensify this phenomenon and leads also to a noticeable enlarge in the grain size, from an average size of ~2 μm to ~6 μm. The striking increase in the transformation strain is ascribed mainly to this latter effect, since the accommodation of variants due incompatibilities between different oriented grains and grain boundaries constrains can be improved. Acta Microscopica, Vol. 18, Supp. C, 2009 217