F.C. Lovey

On the origin of the intrinsic thermoelasticity associated with a single-interface transformation in Cu-Zn-Al shape-memory alloys

Abstract Thermoelasticity is one of the important characteristics of the martensitic transformation in shape-memory alloys, that is the need for continuous undercooling (overheating) in order to force the transformation (reversion) to proceed. In this work the thermoelastic behaviour of a single-interface L21-to-18R transformation has been studied. It was found that the slope of curves of interface position against temperature varied from 0.05 to 0.004 K μm−1 depending on the thermal treatment to which the sample had been subjected. The smallest hysteresis width in a carefully polished material was found to be about 0.06 K. A simple model suggests that the thermoelasticity is ultimately related to the interaction of the martensite with pre-existent dislocations in the matrix.

Pseudoelastic fatigue of NiTi wires: frequency and size effects on damping capacity

The damping properties associated with hysteretic behavior of the pseudoelastic stress–strain (σ–e) curves of NiTi shape memory alloy (SMA) wires were studied. Damping was characterized for wires of 2.46 and 0.5 mm diameter using samples of 120 mm in length. The effect of the frequency and size of the wire on the σ–e curves were studied in the 3 × 10 − 5–3 Hz range, with 8% maximal strain. Damping associated parameters, such as hysteresis width, dissipated energy and specific damping capacity (SDC), defined as the ratio between the hysteretic energy and the maximum strain work over a complete pseudoelastic cycle, show maximum values at a specific frequency for each size diameter. These findings were explained in terms of the temperature effects associated to the heat of transformation. Results show that NiTi wire of 0.5 mm diameter has the highest SDC when cycling around 0.1 Hz while wire of 2.46 mm diameter has the highest SDC at 0.01 Hz. At 1 Hz, the SDC for 0.5 mm diameter wire is around twice that of 2.46 mm diameter wire.

Plasticity in β phase Cu-Zn-Al alloys

Abstract The β Cu-Zn-Al alloys undergo pseudo-elastic martensitic transformations or plastic deformations under an applied stress depending on the temperature and alloy composition. It is of particular interest to study the interactions between both deformation modes. In this paper a detailed study of the plastic deformation in the β phase of Cu-Zn-Al alloys is carried out around room temperature. The stress-strain behaviour shows that the addition of Al increases the critical resolved shear stress for plastic deformation compared with that of binary Cu-Zn. Optical and electron microscopy observations show that the slip direction is [111] and the glide planes are (112) in the twinning sense or a combination of {123}-type planes when the (112) is no longer operative. In no case is slip observed on the (101) or on the (211) antitwinning plane. The effect of the L21 long-range ordering on the dislocation characteristics is also investigated. In particular, it is found that the superlattice dislocations split...

Martensitic transformation in Cu-Mn-Al alloys

The Cu-Mn-Al alloys exhibit, in a certain range of composition, an A2 (BCC) phase which is stable at high temperatures (around 1,050 K). Two ordering reactions take place on lowering the temperature A2 {yields} B2 and B2 {yields} L2{sub 1}, with a distribution of antiphase domains. Spinodal decomposition can be found in the as quenched specimen for some compositions, as shown by Sato et al. In addition a martensitic transformation can also occur, with an Ms strongly dependent on the composition. In this work the dependence of the Ms on composition is investigated, for alloy compositions along the Cu{sub 3} Al-Cu{sub 3}Mn{sub 2} line of the ternary phase diagram. The results, together with other data taken from the literature, are analyzed in terms of a model, based on the pair interchange energies, developed by Ahlers.

Pseudoelastic fatigue of CuZnAl single crystals: the effect of concomitant diffusional processes

Abstract Experiments have been performed in CuZnAl single crystals in order to further understand the evolution of material properties after continuous pseudoelastic cycling. In a fatigue experiment of this type, permanent and recoverable effects are observed for temperatures above 273 K. Experiments in the stress–strain–time space have been designed in order to separate both contributions. The occurrence of recoverable changes can be related to the ordering of the beta-phase and to the stabilization of the martensite, both depending in turn on the atomic diffusion phenomena in these alloys above 273 K. The kinetics of these processes show a considerable increase of the associated time constant after the pseudoelastic cycling procedure. The relation of these changes with the microstructural evolution of the material is analyzed and a discussion is offered on the role that each mechanism, either diffusive or due to microstructural changes, plays on fatigue. The relevance of defining a reference state in order to determine the consequences of pseudoelastic cycling is shown. A model which considers the stabilization of martensite and the beta recovery has been used to reproduce the stress deformation behavior after cycling. An enhancement of stabilization kinetics during fatigue has allowed us to obtain a closer fit with the experimental results.

Effect of annealing atmosphere on the structure and luminescence of Sn-implanted SiO2 layers

Sn nanoclusters are synthesized in 180 nm SiO2 layers after ion implantation and heat treatment. Annealings in N2 ambient at high temperatures (T⩾700°C) lead to the formation of Sn nanoclusters of different sizes in metallic and in oxidized phases. High-resolution transmission electron microscopy (TEM) analyses revealed that the formed larger nanoparticles are composed by a Sn metallic core and a SnOx shell. The corresponding blue-violet photoluminescence (PL) presents low intensity. However, for heat treatments in vacuum, the PL intensity is increased by a factor of 5 and the TEM data show a homogeneous size distribution of Sn nanoclusters. The low intensity of PL for the N2 annealed samples is associated with Sn oxidation.

Magnetic properties of Cu-Mn-Al alloys with shape memory effect

The magnetization of Cu-Mn-Al alloys with a shape memory effect in the composition line Cu{sub 3} Al-Cu{sub 3}Mn{sub 2} was measured from 5 K to room temperature and at fixed temperatures up to (5 {times} 10{sup 7})/(4 {times} {pi}) A/m magnetic fields. It was found that from room temperature to about 90 K (depending upon composition) the samples behaved as superparamagnets. This behavior is described in terms of the magnetic cluster size, the cluster concentration and the interaction between clusters for different compositions and temperatures by fitting the results with the Brillouin function proposed by Beck. Evidence was also found that below this temperature, mictomagnetism (a mixture of ferromagnetism and antiferromagnetism) is present in these alloys. Careful measurement of the magnetization during the martensitic transformation, for a sample with 11.9 at.% Mn, was done in order to estimate the magnetic energy change between {beta} and martensite phases. No noticeable effect on the Ms martensitic transformation temperature, for applied magnetics fields up to (5 {times} 10{sup 7})/(4 {times} {pi}) A/m, was found.

Hysteresis evolution in the martensitic transformation cycling in β-Cu-Zn-Al samples with γ-phase precipitates

The characteristics of the martensitic transformations in Cu-Zn-Al alloys strongly depend on the presence of {gamma}-type precipitates embedded in the transforming matrix. It has been shown that the hysteresis of a single-variant transformation in a sample containing a fine distribution of small {gamma}-type precipitates (7--10 nm average size) evolves from a relatively high value in the first cycle to an asymptotic value after several cycles. In this work the authors will show the results of the hysteresis evolution, which can be described over 3 regimes according to the precipitate size. These regimes will be related to microscopic observations of defects created around the precipitates.

Experimental and crystallographic evidence for the growth of two equivalent β-variants from one single martensite plate in a Cu-Zn-Al single crystal

Abstract The shape-memory effect is associated with a martensitic transformation in many materials. It consists in the ability of the material to recover its original shape after being deformed through or in the martensitic phase. This shape recovery occurs because the same parent phase is obtained after the retransformation. However, it is shown in this work that, under very special circumstances, two differently oriented variants of the parent phase can be obtained from a martensite single variant. This breakdown of the shape-memory effect was found after transforming a single crystal into a unique plate of martensite by single interface growth. During the retransformation two variants of the parent phase appeared which were related to each other by the martensite symmetry operations (symmetry group C2h). The experimental results are consistent with those obtained by applying the Wechsler—Lieberman—Read (WLR) phenomenological theory.

The hysteresis in the martensitic transformations due to the interaction with precipitates in Cu-Zn-Al alloys

A systematic survey on the effects of two-phase microstructures on the martensitic transformation has been presented mainly based on experimental results from iron-base alloys. In a similar fashion, the characteristics of the martensitic transformations in Cu-Zn-Al alloys depend strongly on the presence of {gamma}-type precipitates embedded in the transforming matrix. It has been observed in global transformations that the Ms temperature decreases and the hysteresis becomes wider with the precipitate size. In addition, changes of the hysteresis loops with the transformation cycles have also been reported. In this work, the dependence of the hysteresis with the precipitate size and the number of transformation cycles in stress-induced single-variant transformation is experimentally studied. The asymptotic value of the hysteresis width is analyzed in terms of a simple model based on the mechanisms of plastic deformation.