Advances in Science and Technology Vol. 78

Abstract: Triple-shape memory effect (SME), i.e., to recover the original shape through one intermediate shape upon heating, has been demonstrated as an intrinsic feature of thermo-responsive shape memory polymers (SMPs) after being uniformly programmed, but seemingly has yet been achieved in shape memory alloys (SMAs). In this paper, we study two programming approaches, in which the deformation is uniform throughout the whole sample length without involving any permanent change in material properties at all, to realize the triple-SME in NiTi SMAs. We show that the triple-SME can be tailored to meet the temperature/strain requirements. With this technique, now we are able to achieve step-by-step motion control in SMAs.

Abstract: In this paper we firstly propose and study a microscopic model of twin boundary motion in the Heusler Ni-Mn-X (X= Ga, In, Sb, Sn) alloys on real tetragonal lattice using the first principles and Monte Carlo simulations. The two variants of the low temperature martensite which divided by twin boundary are considered. The Heisenberg model for magnetic subsystem and the Blume-Emery-Griffiths (BEG) one for structural subsystem with magnetostructural interaction between these subsystems are used. The influence of external magnetic field and anisotropy on the twin boundary motion is studied. It is shown that proposed model gives the picture of twin boundary motion as in experiments.

Abstract: Stability of the crystal structure is determined by the competition between attractive and repulsive interatomic forces. Using many-body exponential potentials it can be shown that the bcc structure corresponding to austenitic phases is more stable for low values of the q-parameter characterising the attractive forces for a fixed value of the p-parameter describing the repulsive forces. The structural stability can be changed with the acting pressure that may alter the martensitic transformations from the bcc-austenite to a close-packed structure. The effect of pressure is examined in a generic model employing many-body potentials and the results are compared with ab initio calculations for zirconium representing a monoatomic material with displacive phase transformation.

Abstract: The phase diagram for MnBi was investigated in high fields up to 18 T at temperatures ranging from 300 to 730 K. We used the differential thermal analysis (DTA), in order to examine the equilibrium phase change of ferromagnetic MnBi by applying high magnetic fields. In particular, the first-order magnetic phase transition to the paramagnetic phase at the decomposition temperature Tt ~ 628 K for ferromagnetic MnBi was evaluated in fields up to 26 T. It was found that Tt increases with increasing magnetic fields at the rate of 2 K/T in low fields up to 18 T, and clearly deviates from the linear increase above 20 T. From a viewpoint of application, it is important that the decomposition of MnBi can be controlled by a magnetic field. As a result, Tt on the liquid phase line changes the amount of Mn content from 10 to 16.5at.% at 26 T, and the heat-treatment at 26 T improves the volume fraction of MnBi. Further, it is quite interesting to directly synthesize ferromagnetic MnBi from the liquid phase without the paramagnetic phase transformation.

Abstract: Composition dependence of the compatibility condition at junction plane (JP) (interface between habit plane variants (HV)) was evaluated by geometrically non-linear theory of martensite in Ti-Nb-Al shape memory alloys that have β (cubic) to α” (C-orthorhombic) martensitic transformation. The kinematic compatibility (KC) condition requires non-zero rotation of HV to form compatible JP; the angle of this rotation is termed θ. This means that the invariant habit plane (HP) and the compatible JP are not formed simultaneously. It turned out that twelve types of θ exist depending on the pair of HV. Composition dependence of each type of θ was systematically investigated. The previous results of transmission electron microscopy observations were also discussed in terms of the present results.

Abstract: Foams and other highly porous metallic materials with cellular structures are known to have many interesting combinations of physical and mechanical properties. That makes these systems very attractive for both structural and functional applications. Cellular metals can be produced by several methods including liquid infiltration of leachable space holders. In this contribution, results on metal foams of Cu based shape memory alloys (SMAs) processed by molten metal infiltration of SiO2 particles are presented. By using this route, highly homogeneous CuZnAl SMA foams with a spherical open-cell morphologies have been manufactured and tested. Morphological, thermo-mechanical and cycling results are reported.

Abstract: The rational design of shape memory alloy (SMA) actuators requires reliable data on the fatigue strength of the material under cyclic thermal activation (functional fatigue). Test results on SMAs under functional fatigue are scarce in the technical literature and the few data available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by another SMA element, their stress state is far from constant in operation. The mismatch between actual working conditions and laboratory arrangements leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Four test procedures are planned, covering most of the typical situations occurring in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress-strain variation. The paper describes the experimental apparatus specifically designed to implement the four loading conditions and presents fatigue results obtained from commercial NiTi wires tested under three of the four protocols.

Abstract: The transformation behavior of shape memory alloys is simulated for complex loadings of stress, strain and temperature. Calculations are made by using the “Accommodation Model” which is a constitutive model for shape memory alloys considering the accommodation behavior of the transformation strain. Calculated results are presented for the transformation behavior in the axial and shear stress state. These results are compared with those obtained by the experiment where tube specimens of the Ti-Ni shape memory alloy are subjected to the axial and torsion loading. The proposed constitutive model can predict the transformation behavior including the plastic strain effect of polycrystalline Ti-Ni shape memory alloys under non-proportional multiaxial loading condition.

Abstract: The paper deals with the analytical modelling of a shape memory alloy Negator spring. Negator springs are spiral springs made of strip metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbour. This configuration allows a constant force mechanical response and very long strokes, limited mainly from the total length of the spring. The authors investigate the behaviour of the spring made of a shape memory alloy (SMA). The intrinsic characteristic of SMA is to have two different elastic moduli at different temperatures. This difference can be exploited in order to have a net actuation force for the entire very long stroke, overcoming the two major drawbacks of the SMA actuators, short strokes and output force which varies linearly during the travel.