M. Morin

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Study of the order-disorder phase transitions in Cu-Al-Be shape memory alloys

Abstract We have investigated the order-disorder phase transition in Cu-Al-Be alloys close to the eutectic composition. X-ray diffraction measurements have been performed to characterize the ordered structure and to follow the evolution of the degree of long-range order with temperature. Results show that the transition is first order and takes place, on cooling, from the disordered to a D03 structure. Taking into account these results, a mean field model has been developed in order to study the superlattice formation in these alloys. Present X-ray measurements together with previous calorimetric measurements of the martensitic transition undergone by Cu-Al-Be alloys, have enabled a determination of their metastable phase diagram. It is remarkable that although the addition of a small quantity of Be has a strong effect on the martensitic transformation, it does not modify the symmetry properties of the binary Cu3Al.

Characterization of dislocations formed cyclical ß1⇌ß1′ martensitic transformations in CuZnAl alloys

Determination des vecteurs de Burgers des dislocations qui restent dans la phase β 1 apres une transformation martensitique induite de trois facons differentes. Etude sur trois alliages Cu-14,1 a 14,7% at. Zn-17,1% at. Al

Comments on the physical basis of the active materials concept

During the last decade constant improvements have been made in materials and structures design and control. But now some performance objectives cannot be achieved using classical technologies and require the use of the smart materials concept. But it is at the actuation end of the equation that smart materials and structures present the greatest challenge. It is here in particular that improved and even new materials have a leading role to play. Piezoelectrics, electrostrictives, photostrictives, magnetostrictives, electroactive polymers, shape memory materials, carbon nanotubes, rheological fluids,...all have their important contributions to make. So this paper aims to perform a brief review of the physical basis of the active materials behaviour.

Electro-thermomechanical behaviour of a Ti-45.0Ni-5.0Cu (at.%) alloy during shape memory cycling

Abstract In the present paper, electrical resistance (ER) changes are measured simultaneously with the stress-assisted two-way memory effect (SATWME) in Ti-45.0Ni-5.0Cu (at.%) wires during thermal cycling (max. 15 cycles) for several different stress levels. Interesting qualitative evolutions of the e-ER-T loops during cycling are observed as a function of the applied stress. On cooling, for stresses higher than 175 MPa, a clear deviation of the e-T curves is verified and the reversion of this anomaly is not observed during heating. After some cycles, serrations are frequently observed on the ER-T loops essentially below M f and above A f , indicating an interaction between the formation and reversion of oriented martensite variants with the defects introduced during the thermomechanical cycling. A linear relationship is observed between ER and e for the direct and reverse transformation ranges. The characteristic slope d(Δ R / R )/de is slightly dependent on the applied stress and on the number of thermal cycles.

Temperature and magnetic-field dependence of the elastic constants of Ni-Mn-Al magnetic Heusler alloys

We report on measurements of the adiabatic second-order elastic constants of the off-stoichiometric Ni54Mn23Al23 single-crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behavior of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.

Acoustic emission field during thermoelastic martensitic transformations

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.

Disorder-induced critical phenomena in magnetically glassy Cu-Al-Mn alloys

Measurements of magnetic hysteresis loops in Cu-Al-Mn alloys of different Mn content at low temperatures are presented. The loops are smooth and continuous above a certain temperature, but exhibit a magnetization discontinuity below that temperature. Scaling analysis suggest that this system displays a disorder-induced phase transition line. Measurements allow one to determine the critical exponents

Influence of the mechanical loading history on the stress assisted two way memory effect in a Ti-Ni-Cu alloy

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Influence of thermal cycling on the reversible martensitic transformation in a Cu-Al-Ni shape memory alloy

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