Ricardo Romero

Elastocaloric effect associated with the martensitic transition in shape-memory alloys.

The elastocaloric effect in the vicinity of the martensitic transition of a Cu-Zn-Al single crystal has been studied by inducing the transition by strain or stress measurements. While transition trajectories show significant differences, the entropy change associated with the whole transformation (DeltaS_(t)) is coincident in both kinds of experiments since entropy production is small compared to DeltaS_(t). The values agree with estimations based on the Clausius-Clapeyron equation. The possibility of using these materials for mechanical refrigeration is also discussed.

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.

Quenched-in defects and martensitic transformation in CuAlBe shape memory alloys

Abstract From positron lifetime and calorimetric measurements, the concentration and nature of the defects introduced by quenching from different Tq temperatures, and their influence on the martensitic transformation undergone by a CuAlBe shape memory alloy are studied. The main effect of quenches is to modify the concentration of single vacancies in the system. The obtained dependence of the thermodynamic properties (transition temperatures and latent heat) of the martensitic transition upon Tq, is explained in terms of the effective relative change of Be concentration due to quenched-in vacancies. Results are compared with published data for other Cu-based shape memory alloys.

THE USE OF SHAPE-MEMORY ALLOYS FOR MECHANICAL REFRIGERATION

This letter reports on stress–strain experiments on a Cu–Zn–Al single crystal performed using a purpose-built tensile device which enables the load applied to the specimen to be controlled while elongation is continuously monitored. From the measured isothermal tensile curves, the stress-induced entropy changes are obtained at different temperatures. These data quantify the elastocaloric effect associated with the martensitic transition in shape-memory alloys. The large temperature changes estimated for this effect, suggest the possibility of using shape-memory alloys as mechanical refrigerators.

Stress- and magnetic field-induced entropy changes in Fe-doped Ni–Mn–Ga shape-memory alloys

Isothermal stress- and magnetic field-induced entropy changes in a Fe-doped Ni–Mn–Ga alloy have been measured in the limits of low applied stress and magnetic field. We have obtained that in this limit while elastocaloric is conventional, giving rise to an increase of entropy when a stress is applied, magnetocaloric effect is inverse, which means that entropy decreases by application of an applied magnetic field. This inverse effect is a consequence of the magnetostructural coupling driven by the martensitic transition.

Temperature contour maps at the strain-induced martensitic transition of a Cu–Zn–Al shape-memory single crystal

We study temperature changes at the reverse strain-induced martensitic transformation in a Cu–Zn–Al single crystal. Infrared thermal imaging reveals a markedly inhomogeneous temperature distribution. The evolution of the contour temperature maps enables information to be extracted on the kinetics of the interface motion.

Quenching effects in Cu–Al–Mn shape memory alloy

Abstract 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 L2 1 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.

Aging behavior in Cu-Al-Be shape memory alloy

This article reports positron annihilation spectroscopy and calorimetric measurements of the aging behavior in a Cu–Al–Be shape memory alloy. An excess of single vacancies is retained in the alloy as a result of a quench. All vacancies in excess disappear after long aging time, and a migration energy EM=1.0±0.1 eV for this process has been found to be larger than in other Cu-based shape memory alloys. The good correlation found for the concentration of vacancies and the shift in the martensitic transition temperature demonstrates that, in Cu–Al–Be, changes in the transition after a quench are deeply related to the excess of vacancies.

The effect of post-quench aging on stabilization of martensite in Cu–Zn–Al and Cu–Zn–Al–Ti–B shape memory alloys

Abstract The effect of post-quench aging in polycrystalline β Cu–Zn–Al and Cu–Zn–Al–Ti–B on the martensite stabilization has been studied. The samples were heat treated at 1073 K and subsequently quenched into water at 373 K. After the quench, the samples were aged in the β phase, 373 K≤ T ≤ 403 K, during 300 s ≤ t β ≤10 5 s. The stabilization tests were performed by applying compressive stresses at temperatures above the reverse transformation temperature, A f followed by martensite aging under constant strain. When unloading the sample, it recovers its original shape but the reverse transformation stress is shifted to lower values. The magnitude of the stress shift, a measure of martensite stabilization, is a function of the thermal treatment and aging time. The process continues until a saturation is reached. It was found that the stress shift saturation value depends on the temperature and the aging time in the β phase. The results show that the stabilization depends on the vacancy concentration and that dislocations can not be the dominating type of vacancy sinks in martensite.

Kinetics of the phase separation in Cu–Al–Mn alloys and the influence on martensitic transformations

We have determined the shape of the miscibility gap of the Cu–Al–Mn alloy system along the Cu3Al → Mn composition line ((Cu3Al)1 − x Mnx with 0 ⩽ x ⩽ 1) using electrical resistance measurements. Electrical resistance measurements have also been used to study the kinetics of the phase separation that takes place within the miscibility gap. The effect of such a phase separation on the magnetic properties of the system and on the martensitic transition displayed by these alloys has also been analysed. The results obtained show that the phase separation produces an enhancement of the ferromagnetic character of the system as a result of the development of ferromagnetic L21 domains. In addition, the phase separation gives rise to an increase in the transition temperature and to a reduction in the associated entropy change. Both effects are a consequence of the growth of L21 Mn-rich domains that do not transform martensitically.