M. Sade

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

Abstract Experiments have been performed in Cuue5f8Znue5f8Al 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.

The effect of temperature on pseudoelastic cycling of CuZnAl single crystals

Fatigue, due to pseudoelastic transformations, has been studied by several researchers during the last few years. A transition between an ordered bcc phase ({beta}) and an 18R structure is found in single crystals of Cu-Zn-Al of 3/a = 1.48. It consists in the formation of martensite (M), when a stress is applied on the {beta} phase, and in the retransformation to the original structure after unloading, giving rise to pseudoelastic behavior. Repetition of this process induces changes in the material, which finally leads to fracture. Understanding the evolution of materials during cycling should be the first step in controlling fatigue in these materials. Research shows that temperature, during fatigue experiments, plays an important role in material behavior. For example, the number of cycles to reach fracture is clearly higher at liquid nitrogen temperature if compared with fatigue at room temperature. In order to explain this, the evolution of the material during cycling has to be considered. Several aspects should be analyzed: (a) the evolution of the stress-deformation curves ({sigma}-{var_epsilon}); (b) the introduction of bulk defects; and (c) the formation of surface defects. Previous work shows that surface defects, with an intrusion-extrusion morphology, form after cycling at room temperature. These defects,morexa0» which accommodate in bands, parallel to the habit plane of the transformation (habit plane defects), join to form microcracks, which are believed to be the main cause for fracture. On the other hand, the intrusion-extrusion defects are absent at liquid nitrogen temperatures, giving rise to a longer life of the material. So far, no information is available on the effect at intermediate temperatures on the evolution of {sigma}-{var_epsilon} curves as well as on bulk defects. The purpose of this work is to report a further advance in these studies for temperatures ranging from {minus}196 C to 50 C.«xa0less

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

Abstract 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.

Conceptual design of actuator applications with Cu–Zn–Al single crystals

Abstract A systematic approach to the conceptual design of actuator applications with single-crystals of Cu–Zn–Al shape memory alloys (SMA) is presented. The actuator considered here, a device capable of doing work in response to temperature changes, is based on a single-crystal nucleus of a Cu–Zn–Al alloy coupled to a conventional spring that represents the load to be displaced. The different steps and factors involved in the conceptual design are described. In this stage, a full transformation cycle approach was adopted. In this way the effects of different parameters (cycle number, friction, temperature range, load level) on the actuator behavior can be studied.

Stage for in situ mechanical loading experiments in a scanning electron microscope (Philips 515) with a small chamber

Martensitic transitions involve changes in structure which can be induced thermally as well as under applied mechanical stresses. The thermomechanical characteristics of these displacive transitions are strongly affected by defects which may be introduced in the material by thermomechanical treatments as well as by the transformation itself. To better study the effects of surface defects on these phase transitions high resolution equipment is needed to follow the movement of interfaces. In this work a developed stage is presented. It allows to perform in situ mechanical tests under well controlled temperature conditions and to thermally cycle a sample under controlled stress or strain level in a scanning electron microscope (Philips 515) with small chamber. The test temperature can be easily chosen in the range from −30 to 80u2009°C. The stage is suitable to be used in bigger chambers of similar equipment. Thermal cycles are obtained using a Peltier thermobattery. The load and deformation are monitored by a l...

Numerical simulations of the pseudoelastic effect in CuZnAl shape-memory single crystals considering two successive martensitic transitions

Shape memory alloys (SMAs) with double martensitic transitions are potential candidates for highly effective, nonstandard mechanical damping systems. This paper presents a numerical model that can be used to simulate pseudoelasticity in systems with two successive martensitic transformations, such as adequately oriented CuZnAl, CuAlNi and CuAlBe single crystals. The model is based on stress versus strain data obtained from the tensile test of a CuZnAl single crystal and is able to simulate the mechanical damping of SMAs with two successive martensitic transitions. The numerical model has been implemented as an algorithm and used to assess the mechanical damping capacity of a system based on CuZnAl SMA single crystals, considering the complete β-18R–6R cycle. A numerical model with a single degree of freedom is used and the behavior of the SMA-based damper is analyzed both under free and forced oscillation conditions. The results obtained indicate that the alloy studied is a very effective mechanical damper.

Cryostat for fatigue tests down to liquid‐nitrogen temperatures

A cryostat was constructed and installed in a MTS 810 servohydraulic testing machine for studying fatigue properties associated with pseudoelastic cycling of shape memory alloys down to liquid‐nitrogen temperatures. Special attention was focused on eliminating temperature fluctuations during the fatigue experiments. In order to perform long time fatigue tests, an automatic controlled system which supplies the cryostat with liquid nitrogen was developed and described in detail.

Diffusive Phenomena and the Austenite/Martensite Relative Stability in Cu-Based Shape-Memory Alloys

The main characteristic of martensitic phase transitions is the coordinate movement of the atoms which takes place athermally, without the contribution of diffusion during its occurrence. However, the impacts of diffusive phenomena on the relative stability between the phases involved and, consequently, on the associated transformation temperatures and functional properties can be significant. This is particularly evident in the case of Cu-based shape-memory alloys where atomic diffusion in both austenite and martensite metastable phases might occur even at room-temperature levels, giving rise to a variety of intensively studied phenomena. In the present study, the progresses made in the understanding of three selected diffusion-related effects of importance in Cu–Zn–Al and Cu–Al–Be alloys are reviewed. They are the after-quench retained disorder in the austenitic structure and its subsequent reordering, the stabilization of the martensite, and the effect of applied stress on the austenitic order. It is shown how the experimental results obtained from tests performed on single crystal material can be rationalized under the shed of a model developed to evaluate the variation of the relative stability between the phases in terms of atom pairs interchanges.

The Relevant Role of Dislocations in the Martensitic Transformations in Cu–Al–Ni Single Crystals

The interaction between dislocations and martensitic transformations in Cu–Al–Ni alloys is shortly reviewed. Results from many researchers are critically analyzed towards a clear interpretation of the relevant role played by dislocations on the properties of shape memory alloys in Cu-based alloys. Both thermally and stress-induced transformations are considered and focus is paid on two types of transitions, the β→β′ and the formation of a mixture of martensites: β→β′xa0+xa0γ′. After cycling in the range where both martensites are formed, the twinned γ′ phase is inhibited and cycling evolves into the formation of only β′. A model which considers the difference in energy of each γ′ twin variant due to the introduced dislocations quantitatively explains the inhibition of γ′ in both thermally and stress-induced cycling. The type of dislocations which are mainly introduced, mixed with Burgers vector belonging to the basal plane of the β′ martensite, enables also to explain the unmodified mechanical behavior during β→β′ cycling. The reported behavior shows interesting advantages of Cu–Al–Ni single crystals if mechanical properties are comparatively considered with those in other Cu-based alloys.