The two way shape memory effect (TWSME) was induced in Cu–Al–Ni single crystals by a training method which consisted of applying tensile or compressive stresses at room temperature and keeping the sample under stress in the martensitic phase at the same temperature for a certain time. After quenching from the β stability region the studied alloys exhibit a simultaneous transformation to β′(18R) and γ′ (2H) martensites. Subsequent ageing in the β phase favours the formation of the γ′ phase. An important improvement of the TWSME has been obtained both in the tension and compression modes in the samples aged at 470 K before training, coincident with a transformation predominantly to the γ′ martensitic phase. In trained samples, a subsequent heating for short times at 520 K produces a stable and maximum TWSME of 4% always in elongation sense, both for the samples trained in tension and compression mode.

Simultaneous measurements of acoustic emission and thermal emission have been made during the martensitic transformation of a Cu-Zn-Al alloy after quenching from temperatures Tq between 300 and HOOK. Just after the quench a strong dependence of the thermal and acoustic data on Tq has been observed. This dependence is almost suppressed when the alloy is aged (ageing time t≈50 ks) at room temperature (ageing temperature Ta≈295 K). The results show that the transformation temperature and enthalpy are sensitive to the state of the β-phase after the quench and that the transformation entropy is independent of Tq The acoustic emission is very sensitive to the excess of quenched-in free vacancies in the system. Permanent effects due to γ-type precipitates have also been detected from the energy dissipated by irreversible processes.

The study of martensitic transformations in shape memory alloys by means of techniques such as unconventional scanning calorimetry, acoustic emission and optical microscopy has made it possible to establish the existing links between the hysteresis cycle and the internal state of a given material We have studied the changes in the hysteresis cycle, concerning its shape, width and relevant temperatures, which are produced as a consequence of different thermal treatments and repetitive thermal cycling, for a Cu-Zn-Al single crystal. The effect of thermomechanical treatments in Cu-Zn-Al samples with different surface orientation is also studied. Relevant changes in a mixed hysteresis cycle (Cu-Al-Ni polycrystal with two martensite phases) are observed in annealing treatments. On the other hand, observation of the interfaces movement in reduced thermal cycling establishes the temperature scale and the geometrical conditions to obtain a repetitive/recoverable hysteresis cycle

1.i) The temperatures for forward transformation are always lower than standard ones in the domain of t★ studied. This cannot be explained by a change in the composition of the β phase and therefore implies a relative stabilisation of β phase versus martensite.2.ii) The temperatures for reverse transformation are lower than the standard ones for growing times t★ ⩽ 20 s and higher for t★ > / 40 s. A change of behaviour in this time interval is also present in acoustic emission results. It could be atributed to a change in the coherence of the precipitates.3.iii) The cycle of hysteresis is always wider than the standard one. This is associated with an increase of friction.

The use of stabilized stress induced martensite to obtain two way memory effect has been reported as easier than traditional methods of training. A SEM study of various cycles of transformation-retransformation, at different heating-cooling rates, on Cu-Al-Zn single crystals showing thermoelastic martensitic transformations with stabilized stress induced martensite is presented.

Optical observations, simultaneous with the detection of acoustic emission during the martensitic transformation of a 68.5Cu-14.9Zu-16.6Al (at%) show that: (a) the recorded acoustic emission associated with a martensite microplate is reproducible, and for each event the activity and the echoes last about 1 ms; (b) the intensity of acoustic emission is more important in the reverse transformation; (c) the acoustic activity is related to accelerations in the movement of the martensite microplates. The results for the observed microplates indicate the following. (1) During the process of forward/reverse transformation two kinds of acoustic emission are detected, only one of them being associated with the appearance/disappearance of visible martensite domains in the optical microscope (dimensions about 1 jim), while the other, equivalent to the acoustic emission of the β phase, and with comparable intensity to that produced by a microplate, is not related with any observable microplate, and is probably connected to lesser changes in the surface martensite. (2) The acoustic emission peaks can be related to the existence of pinnings, created by defects, in the martensite domains. (3) There is no proportionality between the acoustic emission and the mass of the material. (4) The thermal hysteresis can be associated with the delay in the reverse transformation caused by pinning of the martensite.

A systematic study of the thermal energy released in the martensitic transformation undergone by a monocrystalline alloy of composition 74.41 Cu-18.18 Zn-7.41 Al (wt%) has been made. The values for Δ H in the β → M and M → β transformations have been determined, as a function of the heat treatment and the number of cycles. The experimental results establish that the transformation enthalpy is reproducible when the initial conditions are well determined. For different samples, in thermal cycles 1 to 10, Δ H is approximately constant, and by rapid cycling, the enthalpy of the 100th cycle is slightly smaller. Thermal cycling makes the effects of heat treatment less important and smooths the picture of the transformation dynamic. A qualitative study of several features of the acoustic emission produced during the transformation, as its signal/noise ratio and waveform, has also been carried out.

Fatigue properties associated with the β-18R martensitic transformation are analyzed in CuZnAl single crystals. The fatigue induced changes are studied by optical, scanning and electron microscopy. It is found that in the interior of the crystals dislocation defect arrays are created which act as local obstacles for the martensitic transformation and lead to the formation of extrusions and holes at the surface, which with increasing numbers of cycles join to form continuous cracks of about 1 μm width. Crack propagation seems to be little affected by composition or orientation.

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