Ll. Mañosa

Magnetic-Field-Induced Effects in Martensitic Heusler-Based Magnetic Shape Memory Alloys

Publisher Summary The discovery of the ferromagnetic (FM) Heusler alloy Cu 2 MnAl, in the beginning of the twentieth century, made considerable impact in the field of magnetism. Although Cu 2 MnAl contained no FM element, it had a very high Curie temperature in excess of 600 K. Features related to the magnetostructural interplay in Heusler alloys are observed in the phonon and magnetization properties. This chapter discusses Heusler-based magnetic shape memory alloys. The complex behavior displayed by these materials is mainly a consequence of the strong coupling between magnetism and structure, which is driven by the martensitic transition. Magnetic shape memory properties are the best known functionalities shown by this class of materials. They refer to the ability of these alloys to show strong response in shape, strain, and dimensions to applied magnetic fields. The magnetic field triggers the changes of shape caused by either inducing the structural transition or rearranging the martensite variants. The first observation of the magnetic-field-controlled shape memory effect was made in Ni 2 MnGa. It is suggested that the mechanism giving rise to the large magnetostriction in Ni 2 MnGa consisted of a twin-related variant reorientation through field-induced twin-boundary motion.

Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically and the latter require large electric fields that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We find barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our findings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

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.

Neutron diffraction study of long-range atomic order in Cu-Zn-Al shape memory alloys

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 Tc2; a B2 superstructure between Tc2 and Tc1, and a disordered BCC structure above Tc1. 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.

Selective spin-state and metal-insulator transitions in GdBaCo 2 O 5:5

Abstract By means of ultra-high resolution synchrotron diffraction and calorimetry measurements we have studied the metal–insulator transition in GdBaCo 2 O 5.5 . The appearance of the metallic state is attributed to a sudden excitation of some electrons in the octahedra ( t 2 g 6 state) into the Co e g band (final t 2 g 4 e g 2 state). In contrast, the t 2 g 5 e g 1 state in the pyramids does not change at the transition. Calorimetry measurements show that the insulator-to-metal transition is first order and the entropy change estimated from the latent heat corroborates the pictured scheme.

Effect of γ precipitates on the martensitic transformation of β CuZnAl studied by calorimetry

Abstract 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. All the aforementioned effects are permanent. Therefore, it is possible to modify the martensitic transformation of shape memory alloys in a reproducible and controllable manner.

Systematic study of the martensitic transformation in a Cu-Zn-Al alloy. Reproducibility of the thermal energy results and cycling effects

Abstract 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.

Acoustic emission in martensitic transformations

Abstract 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 Greens 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) 〈1 1 0〉 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.

Inverse barocaloric effects in ferroelectric BaTiO3 ceramics

We use calorimetry to identify pressure-driven isothermal entropy changes in ceramic samples of the prototypical ferroelectric BaTiO3. Near the structural phase transitions at ∼400 K (cubic-tetragonal) and ∼280 K (tetragonal-orthorhombic), the inverse barocaloric response differs in sign and magnitude from the corresponding conventional electrocaloric response. The differences in sign arise due to the decrease in unit-cell volume on heating through the transitions, whereas the differences in magnitude arise due to the large volumetric thermal expansion on either side of the transitions.

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.