[en] The martensitic transition in shape memory alloys has an evident first-order character. Nevertheless, pretransitional effects are observed. The situation is illustrated with experimental results (neutron diffraction and ultrasonic measurements) in different shape-memory materials such as Cu-based systems. Ni-Al alloys and the ferromagnetic Ni₂MnGa Heusler compound. In all cases it is shown that pretransitional behaviour is associated with low restoring forces for specific vibrational modes related to the path which lead the system towards the martensitic phase. Within the framework of the Landau theory, the existence of a first-order transition is explained by the anharmonic coupling of the incipiently unstable excitations to a secondary field. (orig.)

[en] As opposed to the existence of a single L2₁ parent phase in Ni₂MnGa at room temperature, off-stoichiometric Ni₂MnAl alloys are found in a mixed B2 (antiferromagnetic) and L2₁ (ferromagnetic) phase due to the significantly lowered B2-L2₁ ordering temperature for this compound. Despite the mixed nature of the parent phase of Ni-Mn-Al, we show that it is possible to extract information on field-induced strains occurring in the martensitic phase of Ni-Mn-Al alloys from magnetization studies

[en] An acoustic emission (AE) study of the martensitic and premartensitic transitions in nearly stoichiometric Ni-Mn-Ga alloy is reported. By comparing the AE with calorimetric results, it is shown that AE is generated at both transitions. The AE intensity at the premartensitic transition is extremely weak, in accordance with its weakly first-order character. The amplitude distribution of the AE signals recorded during the martensitic transformation is a power law, with an exponent α = 2.2 ± 0.1. This value is compared with recent results obtained in other shape-memory alloys. (orig.)

[en] We present a phenomenological model for the premartensitic phase transition in ferromagnetic Ni-Mn-Ga alloys. The model contemplates the effects of an applied mechanical stress on the premartensitic transition. It is shown that the premartensitic transition is driven by a magnetoelastic coupling. The first order character of this transition is enhanced by the application of a mechanical stress. Experimental data are also presented which support the predictions of the proposed model. (orig.)

[en] Using experimental data from ultrasonic and neutron scattering measurements for different Cu-based bcc alloys undergoing a martensitic transformation, we show that: (i) the relationship between the elastic constant C' and the zone boundary frequency ω_ZB of the associated phonon branch, both evaluated at the transition point T_M, conforms to the dependence predicted by a Landau model, recently proposed for martensitic transformations and (ii) the elastic anisotropy A at T_M has, within the experimental accuracy, a constant value; this last finding being not explained by the existing models. Some suggestions are envisaged to account for this fact. (orig.)

[en] In this paper the effect of quenching from different temperatures (T_q) in a Cu-Al-Mn alloy is studied. This alloy system, which displays an L2₁ ordered structure, transforms martensitically at an intermediate temperature T_M, and undergoes a spin freezing process at a lower temperature T_f. Positron annihilation measurements have shown that after the quench, an excess of vacancies is retained in the system, depending on T_q. In addition, both T_M and T_f have been found to be sensitive to T_q. This has been attributed to frozen-in disorder induced by the quench. Experimental results have been interpreted in terms of the growth of magnetic clusters, quenched-in vacancies and atomic disorder. (orig.)

[en] We have investigated the influence of quenches that modify the degree of atomic order on the martensitic transformation of Cu-Al-Be alloys by calorimetric, ultrasonic and neutron scattering techniques. The changes in the thermodynamic quantities and elastic constants induced by changes in the degree of order have been compared to reported results for Cu-Zn-Al alloys. Cu-Al-Be has been found to behave differently than Cu-Zn-Al. Some hypothesis accounting for these differences are envisaged. (orig.)

[en] We have studied the kinetics of thermoelastic martensitic transformations (MT), both experimentally and using spin models. Measurements of acoustic emission and thermal pulses in Cu-Zn-Al single crystals, during thermally and stress induced MT, show that the transformation takes place as a sequence of avalanches, whose sizes and durations distribute according to power laws in more than one decade. Most features observed are qualitatively reproduced by spin models that incorporate domain interactions and frozen disorder. A systematic simulation of these models at T = 0 (no fluctuations allowed) has shown that there is a critical concentration of disorder for which avalanche sizes and durations distribute following power-laws, which appear to be independent of details of the models other than their dimensionality. We discuss the connection between the experimental results and the simulations. (orig.)