Revisiting Cu-based shape memory alloys: Recent developments and new perspectives

Cu-based shape memory alloys belong to one important class of functional alloys, presenting shape memory effect and superelasticity due to their reversible martensitic transformation. Although they have been extensively studied since the middle of the last century, there are still many challenges to be solved. In the last decades, these alloys were extensively studied regarding new compositions, processing routes, phase transformation, mechanical and functional properties. Aspects of the thermoelastic phase transformation have been described using thermodynamic and thermo-mechanical studies, while the role of metallurgical features (such as grain size and morphology, ordering, precipitates and second phases) have been described mainly by phenomenological approach. In this sense this review discusses the advances in the general fundamentals of Cu-based shape memory alloys, the recent developments in processing routes, compositions, and applications in the last years. Graphical abstract

Effect of reverse thermoelastic phase transformation on stress of onset of structural deformation in titanium nickelide

The experimental study results of the dependence in the stress value of the onset of the structural deformation on the degree of the reverse phase transformation occurring after the complete direct martensitic transformation in titanium nickelide are described. It is established that these results are qualitatively and quantitatively correctly described in the framework of the model according to which the maximum value of the intensity of the intrinsic inelastic deformation of the martensitic part of the representative volume of the shape-memory alloy is used as a parameter of isotropic hardening.

Structural Transitions in a Co2NiGa Ferromagnetic Shape Memory Alloy

The development of ferromagnetic shape memory for practical applications needs to overpass brittleness issues, in addition to the control of the magnetoelastic domains. The Co-Ni-Ga system can provide adequate structural particularities to increase the ductility. This paper reports on structural observations of the martensitic transformation in a Co2NiGa alloy, in the as-cast and in plastically deformed state. Characterization has been performed before and after the heat treatment, using in-situ X-ray diffraction, optical and electron microscopy, as well as DSC measurements performed on heating and cooling of the samples. The observations show a β + γ two-phase structure that can be further influenced by quenching. The structural contribution on the deformation capacity of the alloys is analyzed, based on the changes in the pattern of transformation. Severe plastic deformation by cold rolling leads to the disappearance of the thermoelastic phase transformation.

Development of Texture-Controlled Bulky Actuator/Sensor Materials by Combining Rapid-Solidified Fiber/Ribbon Elements and Spark Plasma Sintering/Joining(SPSJ)

ABSTRACTThe authors have showed that rapid-solidified (RS) melt-spun fiber/ribbon/foil type samples have very often strongly textured fine columnar grains, and those can have very high performance of the metallic actuator/sensor properties. However, as for the applications of these sample materials in smart material systems, the developed rapid-solidified samples are thought to be inevitably too small force to move the machines and structures in the engineering field. In this paper, we propose one novel material processing technique that can produce the bulk type solid-state actuator/sensor materials by combing the rapid-solidified fiber/ribbon and short time spark plasma sintering/joining (SPS) method. The produced samples, both 1) the disk-type sample from stacked layers of RS-ribbons and 2)the sintered compact from ball-milled RS-fibers of shape memory Ti50Ni40Cu10 (NITINOL) alloy, showed clear thermoelastic phase transformation by DSC evaluation as well as shape recovery. The same approach was also tried for making bulky magnetostrictive FeGa(Galfenol) alloy systems from stacked FeGa ribbons. Laminated FeGa ribbons maintained large magnetostriction of about 100–200ppm under increasing stressed condition as much as in the case of the strongly textured ribbon sample.

A Synchrotron X-Ray Study of Texture Induced by Application of Magnetic Fields During Phase-Transformations in Shape-Memory Ni-Mn-Ga

Ni-Mn-Ga alloys can exhibit a thermoelastic phase transformation near room temperature, which is associated with the shape-memory effect (i.e., temperature-induced strain recovery after twinning) or superelasticity (i.e., strain recovery after a stress-induced phase transformation). This work uses a synchrotron X-ray transmission technique to investigate texture induced by applying magnetic fields during the phase transformation in polycrystalline Ni2MnGa. Synchrotron X-ray radiation is ideally suited for such investigations since the measurements are representative of twinning in the bulk, in contrast with measurements from conventional X-ray sources that represent surface measurements affected by surface relaxation. Magnetic texturing of polycrystalline Ni2MnGa, by cooling through the phase-transformation in the presence of a magnetic field, has potential to lead to polycrystalline materials with more compatible field- induced strains and hence increased twin boundary mobility upon application of a magnetic and/or stress field.