Petr Šittner

Experimental study on the thermoelastic martensitic transformation in shape memory alloy polycrystal induced by combined external forces

Combined tension and torsion experiments with thin wall specimens of Cu-Al-Zn-Mn polycrystalline shape memory alloy (SMA) were performed at temperatureT =Af + 25 K. The general stress-strain behaviors due to the thermoelastic martensitic transformation, induced by a combination of external forces of axial load and torque, were studied. It is shown that the progress of martensitic transformation (MT) at general stress conditions can be well considered as triggered and controlled by the supplied mechanical work (a kind of equivalent stress) in the first approximation. Pseudoelastic strains in proportional as well as nonproportional combined tension-torsion loadings were found fully reversible, provided that uniaxial strains were reversible. The axial strain can be controlled by the change of torque andvice versa due to the coupling among tension and torsion under stress, not only in forward transformation, but also in reverse transformation on unloading. The pseudoelastic strains of SMA polycrystal are path dependent but well reproducible along the same stress path. The evolution of macroscopic strain response of SMA polycrystal, subjected to the nonproportional pseudoelastic loading cycles with imposed stress path, was systematically investigated. The results bring qualitatively new information about the progress of the MT in SMA polycrystal, subjected to the general variations of external stress.

Anisotropy of martensitic transformations in modeling of shape memory alloy polycrystals

Based on the knowledge of the anisotropy associated with the martensitic transformations obtained from tension/compression experiments with oriented CuAlNi single crystals, a simple constant stress averaging approach is employed to model the SMA polycrystal deformation behaviors. Only elastic and inelastic strains due to the martensitic transformation, variant reorientations in the martensite phase and martensite to martensite transformations in thermomechanical loads are considered. The model starts from theoretical calculation of the stress-temperature transformation conditions and their orientation dependence from basic crystallographic and material attributes of the martensitic transformations. Results of the simulations of the NiTi, NiAl, and Cu-based SMA polycrystals in stress–strain tests are shown. It follows that SMA polycrystals, even with randomly oriented grains, typically exhibit tension/compression asymmetry of the shape of the pseudoelastic σ−e curves in transformation strain, transformation stress, hysteresis widths, character of the pseudoelastic flow and in the slope of temperature dependence of the transformation stresses. It is concluded that some macroscopic features of the SMA polycrystal behaviors originate directly from the crystallography of the undergoing MTs. The model shows clearly the crystallographic origin of these phenomena by providing a link from the crystallographic and material attributes of martensitic transformations towards the macroscopic σ−e−T behaviors of SMA polycrystals.

Recovery stress generation in shape memory Ti50Ni45Cu5 thin wires

Abstract The recovery stresses evolving in constrained Ti 50 Ni 45 Cu 5 (at.%) shape memory wires were investigated in thermomechanical experiments performed in combination with electric resistance measurements. The hysteretic stress–temperature responses of the wires in constrained thermal cycles were analyzed by comparing the experimental results with simulated responses using a phenomenological algorithm developed for prediction of uniaxial thermomechanical SMA behaviors. The effects of individual SMA material parameters, constraint parameters, test boundary conditions and thermomechanical history on the evolution of recovery stresses in SMA wires are predicted.

On the anisotropy of martensitic transformations in Cu-based alloys

Abstract Tension/compression thermomechanical experiments were performed on oriented Cu–Al–Ni shape memory alloy single crystals. Response of the single crystals in thermomechanical loads was strongly temperature dependent and anisotropic, i.e. dependent on the orientation of the load axis and sense of the load. The anisotropy of the response is closely related to an anisotropy of the transformation strains due to two basic martensitic transitions β1→β′1 and β1→γ′1 in Cu-based alloys. Calculated transformation strains due to these transitions were compared with the results of compression experiments. The comparison explains well the observed anisotropy of the response of the crystals in thermomechanical loads, in particular, the experimentally observed anisotropy in transformation strains, stresses, hysteresis widths, slopes of transformation lines in stress-temperature diagrams and martensite phases formed under various stress-temperature conditions.

Thermomechanical behavior of shape memory alloy under complex loading conditions

Abstract The thermo-mechanical behavior of polycrystalline shape memory alloy (SMA) under multi-axial loading with varying temperature conditions has been studied by experiments. Recently the research has been extended theoretically and a mechanical model of polycrystalline SMA and the corresponding mesoscopic constitutive equations have been developed. The model presented in this paper is constructed on the basis of the crystal plasticity and the deformation mechanism of SMA. The variants in the crystal grains and the orientations of crystal grains in the polycrystal are considered in the proposed model; the constitutive equations are derived on the basis of the proposed model. The volume fraction of the martensite variants in the transformation process and the influence of the stress state on the transformation process are also considered. Some calculated results obtained by the constitutive equations are presented and compared with the experimental results. It is found that the deformation behavior of SMA under complex loading conditions can be well reproduced by the calculation of the constitutive equations.

Stress-induced martensitic transformation in Cu–Al–Zn–Mn polycrystal investigated by two in-situ neutron diffraction techniques

Dedicated to Prof. P. Lukac, Charles University, Prague on the occasion of his 65. Birthday Abstract In-situ neutron diffraction studies of internal strains, stresses and phase fractions in pseudoelastic CuAlZnMn shape memory alloy evolving during two tensile load cycles at constant temperature are reported. The results are discussed with the aim of demonstrating the applicability of this experimental technique for the shape memory alloy research. Particularly, an experimental information on the mechanisms by which individual grains of the transforming polycrystal share the macroscopic stress - load partition - as well as quantitative information on the residual stress frozen in the austenite phase after the first cycle are discussed.

Grain-resolved analysis of localized deformation in nickel-titanium wire under tensile load.

Bend it, shape it, remember it Shape-memory alloys have the useful property of returning to their original shape after being greatly deformed. This process depends on the collective behavior of many small mineral grains in the metal. Using three-dimensional x-ray diffraction, Sedmák et al. tracked over 15,000 grains in a nickel-titanium shape-memory alloy as it moved through this transformation, thus linking microscopic changes to the bulk deformation. Science, this issue p. 559 Synchrotron x-ray diffraction of 15,000 grains allows imaging of internal shear bands in NiTi alloy during deformation. The stress-induced martensitic transformation in tensioned nickel-titanium shape-memory alloys proceeds by propagation of macroscopic fronts of localized deformation. We used three-dimensional synchrotron x-ray diffraction to image at micrometer-scale resolution the grain-resolved elastic strains and stresses in austenite around one such front in a prestrained nickel-titanium wire. We found that the local stresses in austenite grains are modified ahead of the nose cone–shaped buried interface where the martensitic transformation begins. Elevated shear stresses at the cone interface explain why the martensitic transformation proceeds in a localized manner. We established the crossover from stresses in individual grains to a continuum macroscopic internal stress field in the wire and rationalized the experimentally observed internal stress field and the topology of the macroscopic front by means of finite element simulations of the localized deformation.

Martensitic transformations in [001] CuAlZnMn single crystals

Thermoelastic martensitic transformations exhibited by [001] oriented CuAlZnMn shape memory alloy single crystals were investigated by tension/compression thermomechanical experiments. Thermal cycling tests at constant applied stress, mechanical cycling tests at constant temperature as well as complex thermomechanical tests were performed to determine the transformation conditions in a wide range of applied stresses and temperatures. The martensitic phase induced typically under tensile stress (β1-18R monoclinic structure) was different from that induced by compressive stress (γ1-2H orthorhombic structure) and the related tension/compression asymmetries were investigated in detail. Particularly, the wide thermal as well as stress hysteresis due to the β1→γ1 transformation involving large compressive strains is a peculiar feature of the CuAlZnMn alloy and probably an origin of the wide hysteresis effect exhibited by the polycrystalline CuAlZnMn shape memory alloy. A mechanism based on the non-equivalent crystallographic transformation paths of the forward and reverse martensitic transformation was proposed to explain this phenomenon.

Stress induced martensitic transformations in tension/torsion of CuAlNi single crystal tube

Abstract A CuAlNi single crystal tube was loaded in tension while keeping the torsional displacements free. The tube extended and simultaneously twisted during the stress induced martensitic transformation. A cooperative activity of four habit plane/shear direction transformation systems appearing at the same time in different quadrants of the tube wall is considered to rationalize the unusual mechanical behavior.

Anisotropy of transformation characteristics of Cu-base shape memory alloys

Shape memory alloy single crystals undergoing stress induced thermoelastic martensitic transformations exhibit strong anisotropy of deformation characteristics when loaded along various crystallographic axes. In the present work, CuAlNi and CuAlZnMn single crystals of various orientations were investigated by thermomechanical tension/compression tests. In particular, the orientation dependence of transformation strains and stress-temperature critical conditions for martensitic transformation were studied. Results have been summarized in stress-temperature phase diagrams which provide sufficient basis for micromechanical modeling of Cu-based polycrystals. Experimentally measured transformation strains were found to be in reasonable agreement with predictions based on the phenomenological theory of martensite crystallography.