Zn Al Single Crystals Research Articles

Large pseudoelastic strain in double β-18R–6R martensitic transformation with reduced 6R martensite stabilization in CuZAl alloys

In Cu–Zn–Al single crystals, two sequential martensitic transformations can be induced by mechanical stress, depending on the chemical composition and the crystallographic orientation of load application. The first transition occurs between a metastable austenitic phase named β and a martensitic phase named 18R. As the applied load increases, a martensite to martensite transition occurs and the 18R transforms into the 6R phase. A large pesudoelastic tensile strain of up to 18% can be obtained. The mechanical hysteresis associated with the 18R–6R transformation is high, which makes it a potential candidate for mechanical damping applications. However, during pseudoelastic cycling the 6R phase is simultaneously submitted to plastic deformation and martensite stabilization. Mechanical behavior can be improved by introducing γ phase nanoprecipitates, which reduces plastic deformation and dynamic martensite stabilization. The effects of different mean precipitate sizes on 18R–6R mechanical cycling behavior are studied in this work. Results show that martensite stabilization kinetics decreases to a large extent as mean precipitate size increases. This leads to a rather reproducible mechanical behavior at least for 50 cycles. We propose a mechanism based on the interaction between precipitates and out of equilibrium vacancies to explain the great reduction in dynamic stabilization of 6R martensite.

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.

Comment on “Effects of heat-flux features on the differential scanning calorimetry curve of a thermoelastic martensitic transformation” by Benke et al. [Mater. Sci. Eng. A 481–482 (2008) 522

The martensitic transformation of a Cu–Zn–Al single crystal was measured in a differential scanning calorimeter at different heating rates. The curves were analyzed in order to detect the initial and final temperatures of the transition. The results were analyzed using both, the method suggested by Benke et al. and the conventional method of extrapolating the temperatures to zero rate. They were also compared with those determined in the same calorimetric sample using the electrical resistance measurement. It is concluded that the transition happens in a range higher than that detected by calorimetry and that the inflexion point is not a useful point for a proper temperature determination.

Microstructural changes in β–Cu–Zn–Al due to SHI irradiation

The effects of irradiation with swift heavy ions on L2 1 long range ordered β phase Cu–Zn–Al single crystals with surface orientation close to (0 0 1) β were investigated. The resulting change in structure and microstructure were characterized by transmission electron microscopy. A phase transformation to a close-packed phase, loops and cavities were observed and characterized after the irradiation process. The origin and the stability of the microstructural changes due to irradiation are discussed.

In-situ observations of a martensitic transformation in a Cu–Zn–Al single crystal driven by stress or strain

We have studied the stress–strain hysteresis loops during the martensitic transition of a Cu–Zn–Al single crystal. The transition was mechanically induced either under stress-driven or strain-driven conditions. The comparison between the two mechanisms shows significant differences in the transformation path. In situ optical observations in strain-driven experiments reveal retransformation which results in a re-entrant behaviour. No re-entrancy is found in stress-driven experiments. The hysteresis is larger in stress-driven experiments.

Micromechanical analysis of precipitate effects on shape memory alloys behaviour

In this paper, we study the effect of elastic precipitates in a shape memory (SMA) alloy matrix. In the first part, we give the constitutive equations for a pure shape memory alloy single-crystal based on a micro-mechanical approach. Then, we derive a model that takes into account the presence of elastic precipitates in a single-crystal SMA matrix using the Mori–Tanaka scale transition technique. In the third part we give the associated polycristal behaviour based on a self-consistent approach. Finally, we discuss the results of the numerical computation for a Cu–Zn–Al single crystal with elastic Zr precipitates.

Finite element simulation of martensitic transition based on thermo-mechanical model

Martensitic phase transition and martensite phase have an important effect on the physical and deformation properties of a material. In this paper, martensitic phase transition in elastic materials is modeled by using thermo-mechanical phenomenological theory. Based on the model proposed, stress-induced martensitic transition in Cu–Zn–Al shape memory alloy is studied by using finite element method. Stress–strain responses of Cu–Zn–Al single crystals under different applied stress rate are simulated by thermo-mechanically coupled calculation. Pseudoelastic deformation, hysteresis of transformation and hardening in stress–strain response are predicted in calculation results. Stress–strain curves determined from numerical simulation have a good agreement with those observed in experiments.

Vacancy influence on the stabilization of martensitic Cu–Zn–Al single crystals

On a well-ordered β phase Cu–Zn–Al single crystal, the vacancy influence on martensitic stabilization has been studied by compression tests. Systematic studies on the influence of stabilization time, temperature, and the influence of the vacancy concentration on the stabilization process show that the characteristic time for stabilization and vacancy migration are several orders of magnitude different. It is found that only a small fraction of the initial vacancy concentration contributes to the process and that the remnant vacancies are unable to promote further stabilization. A model is proposed witch can reproduce the essential features of isothermal martensite stabilization.

Finite-strain micromechanical model of stress-induced martensitic transformations in shape memory alloys

A micromechanical model of stress-induced martensitic transformation in single crystals of shape memory alloys is developed. This model is a finite-strain counterpart to the approach presented recently in the small-strain setting [S. Stupkiewicz, H. Petryk, J. Mech. Phys. Solids 50 (2002) 2303–2331]. The stress-induced transformation is assumed to proceed by the formation and growth of parallel martensite plates within the austenite matrix. Propagation of phase transformation fronts is governed by a rate-independent thermodynamic criterion with a threshold value for the thermodynamic driving force, including in this way the intrinsic dissipation due to phase transition. This criterion selects the initial microstructure at the onset of transformation and governs the evolution of the laminated microstructure at the macroscopic level. A multiplicative decomposition of the deformation gradient into elastic and transformation parts is assumed, with full account for the elastic anisotropy of the phases. The pseudoelastic behavior of Cu–Zn–Al single crystal in tension and compression is studied as an application of the model.

Serrated yielding in long-range ordered 18R Cu–Zn–Al single crystals

The jerky flow of long-range ordered 18R Cu–Zn–Al single crystals is studied on compressive deformation tests between 293 and 523 K. The results are analysed in terms of the Portevin–Le Chatelier effect, and the apparent activation energy for this and other LRO alloys is discussed in relation to a dislocation interaction and diffusion model.

Stability and stabilization of 2H martensite in Cu–Zn–Al single crystals

The stabilization of the 2H martensitic phase in Cu–Zn–Al single crystals with an electron concentration e/a = 1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic β phase by the direct β → 2H or the indirect β → 18R → 2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted σT1. It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300 K, is only obtained if an ageing is performed at a stress above σT1. Additionally, the stresses of the transformation to another martensitic phase, called 18R2, were found to be constant when the value of σT1 is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.

The influence of small impurity contents on the diffusion controlled stabilization of Cu–Zn–Al martensitic single crystals

The stabilization of the 18 R stress induced martensite in Cu–Zn–Al single crystals has been measured as a function of ageing time in differently prepared crystals. It is concluded that only small impurity contents of a concentration similar to that of the quenched-in vacancies can affect notably the stabilization kinetics.

Hysteretic behaviour of a Cu–Zn–Al single crystal during superelastic shear deformation

The present work investigates the hysteretic behaviour of Cu–Zn–Al single crystal submitted to superelastic shear tests. Major symmetric loops were performed within the shear strain limit of the stress-induced martensitic transformation. Minor loops inside the major loops allowed to analyse more closely the hysteretic behaviour of the Cu–Zn–Al single crystal. Experimental results obey most of the phenomenological observations previously established for polycrystalline Ni–Ti shape memory alloys.

Differential scanning calorimetry study of deformed Cu–Zn–Al martensite

The effect of dislocations, produced by plastic deformation of 18R Cu–Zn–Al single crystals, on the thermally induced martensitic transformation has been analyzed by calorimetric measurements. The changes of the transformation temperatures, the heat of transformation, and the two way shape memory effect are discussed in terms of the deformation level.

Vacancies and stabilisation in martensitic Cu–Zn–Al single crystals

The stabilisation of the 18R stress induced martensite in Cu–Zn–Al single crystals has been measured as a function of the ageing time and the previous thermal treatment in the high temperature β phase. It is found that the initial and final stages of the stabilisation present a different behavior, and only the latter is related to the quenched-in vacancy concentration. It is confirmed that the martensite does not reach its thermodynamical equilibrium state, although the vacancies remain mobile. It is suggested that the evolution of the martensite can be rationalised by the combined action of mobile vacancies and precursors of the γ equilibrium phase.

Determination of the cubic–orthorhombic transformation strain in Cu–Zn–Al alloys

The length change associated with the compression induced β-18R transformation for Cu–Zn–Al single crystals (SC) is measured. The results of deformation based on the invariant plane strain model and the phenomenological theory are in very good agreement with the experimental values. The invariant plane strain components, the dilatational and the shear component, are evaluated. The resulting relative volume change going from martensite to β is ξ=−0.0016±0.0002, and the shear component is s=0.1985±0.0004.

Plastic deformation under compression of Cu–Zn–Al martensitic single crystals

In Cu–Zn–Al single crystals, a bcc to 18R martensitic transition can be induced either termally or under applied stresses. In the latter case, a martensite single crystal is obtained. In this work, the plastic deformation behavior of the 18R martensitic phase for two different compositions ( e/ a=1.43 and e/ a=1.48, M s≈263 K) is analyzed in order to find out possible differences in both the mechanical behavior and the microstructure. Optical microscopy and transmission electron microscopy were used to study surface and bulk defects. The observed primary slip system was the same for all the samples, independently of the composition and orientation: (011)[100] β, which corresponds to slip in the basal plane of the martensite. In addition, for compression axes near [100] β, a secondary slip system appeared. The corresponding surface traces were far larger than those corresponding to the primary slip system. The measured resolved shear stresses to activate the primary slip system were on average 15% lower for compositions with e/ a=1.43, compared to the samples with e/ a=1.48. The results obtained for the yield stress of the martensite lend support to the role of plasticity of this phase on the nucleation and growth of the surface and bulk defects generated after pseudo-elastic cycling.

Effect of temperature on superdislocation dissociation in Cu–Zn–Al 18R single crystals

Dislocation structure in 18R Cu–Zn–Al single crystals deformed at temperature above room temperature is studied by transmission electron microscopy. It is shown that the antiphase boundary joining the superpartial dislocations increases its width during annealing.

Microstructural evolution in the pseudoelastic cycling of Cu–Zn–Al single crystals: behavior at a transition stage

Cu–Zn–Al single crystals have been pseudoelastically cycled through the bcc-18R martensitic transformation. The test temperatures varied from low temperatures (liquid nitrogen) to above room temperature. The microstructure evolution during cycling has been analyzed by optical and transmission electron microscopy. A description of defects formed after cycling both in the bulk and on the surface is presented. A transition in the type of formed defects is found corresponding to an increase in the test temperature. Dislocation tangles parallel to the basal plane of the martensite, with trapped martensite inside, give way to dislocation arrays parallel to the habit plane of the transformation, as the test temperature increases. A correlation between this transition behavior and the mechanical evolution was found and the mechanisms of generation of bulk and surface defects are discussed. A correlation between dislocation arrays parallel to the habit plane and intrusion–extrusion defects on the surface is also presented. Both type of defects appear at temperatures higher than 173 K. The effect of point defects introduced during cycling and their change in mobility with test temperature are considered.

Pseudoelastic fatigue of CuZnAl single crystals: the effect of concomitant diffusional processes

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.