The isothermal relaxation process near room temperature in b Cu-Zn-Al and Cu-Zn-Al-Ni industrial alloys which follows a step quenching or an up quenching thermal treatment has been studied by isothermal calorimetric technique. The transition temperature Ms and its evolution have been obtained by differential scanning calorimetry and by detecting the acoustic emission. The dissipated energy of the sample measured during the relaxation process present a complex behaviour characterized by a continuous slowing down. On the other hand the evolution of Ms at the beginning of the phenomenon obeys a different law. We show that all measured evolutions are much more slow n the case of Cu-Zn-Al-Ni alloy, comparing to Cu-Zin-Al alloy. The presence of nickel, which slows diffusion processes can explain this behaviour influenced by quenching rate and annealing time. We interpret the relaxaton processes and the evolutions of Ms in term of atomic reordering (B2 and L21). These phenomena are so slow in Cu-Zn-Al-Ni alloys near room temperature that the equilibrium state of order cannot be reached

The isothermal relaxation process in a beta -Cu-Zn-Al single crystal which follows quenches from different Tq temperatures has been studied by means of an isothermal calorimetric technique at three Ta temperatures. The relaxation has been attributed mainly to a reordering process which takes place through a vacancy mechanism. The dissipated energy during the process has been measured for the different Ta and Tq temperatures. The authors have found that the relaxation process is not of a single exponential type and that, in all cases, the recorded curves giving the dissipated thermal power, W(t), against time show long tails. The kinetics of the process have then been characterized by means of a relaxation time tau , defined as the time at which the product tW(t) versus time shows a maximum. Assuming an Arrhenius law dependence of tau on Ta and Tq, they have obtained effective activation energies of 0.76+or-0.03 eV and 0.43+or-0.03 eV for vacancy migration and formation respectively.

Neutron diffraction experiments have been carried out in order to investigate the ordered structures of the high temperature phase of two different Cu-Zn-Al alloys. It has been confirmed that the alloys exhibit an L21 superstructure below a critical temperature T(c2); a B2 superstructure between T(c2) and T(c1), and a disordered BCC Structure above T(c1).The evolution of the degree of order has been investigated by measuring the change with temperature of the intensity of the 111 and 200 reflections. Results indicate that both transitions are second order. The critical temperatures have been determined as well as the critical exponent-beta. Within the experimental accuracy, it has been found that both transitions belong to the universality class of the three dimensional Ising model.

We report measurements of the attenuation of longitudinal elastic waves, carried out using a broad-band immersion pulse-echo technique. The ultrasonic attenuation has been measured as a funcion of temperature and frequency during the martensitic transformation of monocrystalline and polycrystalline Cu-Zn-A1 alloys. It has been shown that the most likely mechanism producing ultrasonic attenuation is the scattering of the uItrasonic waves by regions with different acoustic impedances.

Measurements of the enthalpy (DELTAH) and entropy (DELTAS) changes associated with the martensitic transformation of Cu-Al-Be single crystals of different compositions, were performed by differential scanning calorimetry. In addition, the elastic constants of the body-centered-cubic parent phase single crystals were measured as a function of temperature, using the ultrasonic pulse-echo frequency varying - phase detection method, and the value of the elastic anisotropy (A=C44/C') was computed at the martensitic transformation temperature. The results are in good agreement with a theoretical expression obtained according to models previously proposed, which relates DELTAS to the elastic anisotropy of the parent phase at the martensitic transformation temperature.

Ultrasonic attenuation has been measured using a broad-band immersion pulse-echo technique on Cu-Zn-Al alloys. The use of two different polishing conditions of the surface of the sample has enabled the effect of surface distortion associated with the martensitic transformation undergone by these alloys to be quantified. The behaviour of the ultrasonic attenuation coefficient as a function of both frequency and temperature is presented. The results are interpreted in terms of the scattering of the ultrasonic beam caused by the co-existence within the material of regions with different acoustic impedances.

We present results from both, calorimetric and dilatometric studies of the isothermal ordering process taking place in a Cu-Zn-Al shape memory alloy after quenches from Tq temperatures ranging from 350 K to 1200 K The dissipated energy and the length variations of the system are obtained during the process. The change of these quantities in the whole process have been compared with the difference DELTAM(s) between M(s), measured after the relaxation and M(s) measured just after the quench. We obtain that these three quantities present, as a function of T(q), the same qualitative behaviour. These changes are then associated with changes of the L2(1) ordering after the quench in the system. The relaxational process does not follow a single exponential decay. Instead, a continuous slowing down is observed. A relaxation time tau has been defined to characterize the relaxation rate. We show that T depends on both the annealing and the quenching (T(q) < 800 K) temperatures through an Arrhenius law.

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 influence of quenching rate on the martensitic transformation of CuZnAl alloys is analysed using several experimental techniques. For samples subjected to rapid quenching, during cooling and after an ageing process, growth of a network of thin martensite needles at temperatures largely higher than the usual temperature of transformation is observed. The experimental results are interpreted by assuming that needle-like platès are associated to internal stresses retained during quench; the stresses are estimated from the shift in the transformation temperature. Nous avons étudié l'influence de la vitesse de trempe sur la transformation martensitique des alliages CuZnAl à l'aide de plusieurs techniques experimentalles. Pour des échantillos ayant subi une trempe rapide, au refroidissement après viellissement, nous avons observé la croissance de fines aiguilles de martensite à des temperatures largement superieures à la temperature normale de transformation. Les résultats experimentaux sont interpretés en supposant que la présence de ces aiguilles à haute temperatures est liée aux contraintes internes retenues lors de la trempe; nous avons estimé celles-ci à partir du décalement de la temperature de transformation.

This work deals with the study of the acoustic emission generated during martensitic transformations. We propose a source model containing both a shear and a volume change mechanism. The dynamic Green's function formalism for a continuous elastic media enables, in the far field approximation, one to obtain the radiation (pattern and kinematics) characteristics of the source. The experiments corroborate that the acoustic radiation pattern in such transformations corresponds to a predominant shear mechanism in the (110) 〈110〉 system and a volume change. The kinematics of growth for martensite plates has been experimentally obtained by making use of the Doppler effect for acoustic emission waves allowing us to obtain the velocity, depth and growth length of transformation steps.

The acoustic emission generated during a thermoelastic martensitic transformation, undergone by a trained Cu-Zn-Al alloy, has been simultaneously detected on four faces of a quasi-cubic sample with well-determined crystallographic orientations. The results, interpreted using the dynamic Green’s function formalism for elastic continuum media, provide the first experimental evidence that the acoustic source for thermoelastic martensitic transformations can be described in terms of a shear and a volume change effect.

We have studied, at different temperatures Ta, the aging behaviour of the shear elastic constant C′ = 12 (C11 - C12) of a CuZn Al SMA after quenches from various temperatures Tq (< TcDO3). Results indicate two different time evolution regimes. In the early stages C′ shows a complex behaviour which is strongly dependent on Ta and Tq whereas final stages of the relaxation process follow a simple exponential decay independent of Tq and nearly independent of Ta.

Ultrasonic attenuation during the martensitic transformation of Cu-Zn-Al alloys has been measured by means of a broad-band pulse-echo technique. A different behaviour has been found in the attenuation against temperature for single crystals and polycrystalline samples; these differences have been explained by taking account of the scattering mechanisms. It has also been observed that surface effects play a very important role in determining the overall attenuation; they produce an ultrasonic peak when the attenuation in the bulk of the material evolves differently from the losses at the surface. Finally, the results obtained have shown that, in single crystals, the ultrasonic attenuation is not proportional to the transformed fraction of the sample.

The authors have analysed the acoustic emission signals generated during the thermoelastic martensitic transformation of a Cu-Zn-Al alloy using a broad-band detection system. The analysis has been restricted to rise times which contain information related with the dynamics of the transformation mechanism. The results show the existence of two well separated rise-time signal populations. The average values of each class are 0.2+or-0.1 mu m and 2.2+or-0.5 mu m. The presence of signals with a characteristic 'fine structure' superimposed on the mean long rise-time wave has been attributed to a jerky character of the martensitic growth process.

The thermoelastic martensitic transformation in shape-memory alloys is studied thermodynamically. Calorimetric experiments on the Cu─Zn─A1 alloy system reveal that the transformation takes place with a practically negligible entropy production. The usual hysteretic subloop behaviour during partial cycling is obtained for the first time by calorimetry. An analysis of the measurements gives the quantitative behaviour of elastic and dissipative energies with the volume fraction of martensite.