Degradation Of Shape Memory Effect Research Articles

Phase field study on the microscopic mechanism of the cyclic degradation of shape memory effect in nano-polycrystalline NiTi shape memory alloys

A three-dimensional non-isothermal and crystal plasticity based phase field model was proposed to simulate the cyclic deformation of nano-polycrystalline NiTi shape memory alloys (SMAs) reflecting their one-way shape memory effect (OWSME) and stress-assisted two-way effect (SATWSME), and then the microscopic mechanism of the cyclic degradation of shape memory effect (SME) was systematically revealed. The proposed model was numerically implemented in a two-dimensional plane strain nano-polycrystalline NiTi SMA system, where each grain was divided into a grain-interior (GI) phase and grain-boundary (GB) phase, and the GI phase was set to be embedded in the GB phase. For the GI phase, a crystal plasticity based phase field model was newly established by considering four inelastic deformation mechanisms, i.e., martensitic transformation (MT), martensitic reorientation (MR), austenitic plasticity caused by dislocation slipping and martensitic plasticity caused by dislocation slipping and deformation twinning, simultaneously; for the GB phase, a visco-plastic model was used to describe its plastic deformation. The simulated results show that: during the cyclic deformation of nano-polycrystalline NiTi SMAs involving the OWSME or SATWSME, the local internal stress caused by the MR or MT can lead to the accumulation of plastic deformation, which further increases the fraction of the martensite variants with favorable orientations by inducing a specific internal stress field after cooling and then results in the accumulation of residual martensite phase after heating; the accumulation of the irrecoverable strain mainly from the martensitic twinning and the decrease of recoverable strain cause the cyclic degradation of OWSME, which is further dependent on the loading rate and loading level; the SATWSME of the SMAs can exhibit a cyclic strengthening during the cyclic loading with a relatively low constant stress, while a cyclic degradation with a relatively high stress, which results from the competition between the transformation strain and the irrecoverable strain mainly caused by the austenitic plasticity that increases gradually during the cyclic deformation involving the SATWSME. These new findings are very helpful to clarify the physical nature of the cyclic degradation of SME and then alleviate the functional fatigue of NiTi SMAs by modulating the material design.

A Thermo-Mechanical Constitutive Model of Glassy Shape Memory Polymers

Glassy shape memory polymer materials are applied successfully in biomedical fields due to their large recovery deformation, excellent biocompatibility and unique biodegradability. To predict the thermo-mechanical behavior of glassy shape memory polymers in biomedical devices accurately, a reasonably three-dimensional thermo-mechanical constitutive model must be established firstly. A one-dimensional linear-elastic constitutive model proposed by Tobushi et. al. (1997) was extended to capture the loading level dependent degradation of shape memory effect by introducing new nonlinear evolution equations with threshold values. Comparisons between experiments and simulations were carried to validate the extended model. Simulation results agree with experiments well, especially for the high loading levels.

On the degradation of shape memory effect in trace Ti-added Cu–Zn–Al alloy

Present study concerns the effect of trace addition of Ti on the factors governing the microstructural evolution in Cu–Zn–Al shape memory alloy during different thermal treatments. It has been revealed that trace addition of Ti has significantly reduced the transformation temperatures. This in turn, has reduced the susceptibility to microstructural degradation induced by the ageing effect. It has been shown that Ti forms fine precipitate at the β-phase temperature, which do not contribute to the processes related to the degradation of shape memory effect, rather aids in the nucleation of favorable martensite variants. Moreover, addition of Ti has led to a finer grain size of the alloy due to the pinning effect of the precipitate along with the reduced solute drag.

The Influence of Heat Treatment on Shape Memory Effect

The investigation results of the influence of various quenching treatments (direct quenching, up-quenching and step-quenching) on microstructure, crystal structure, phase transformation temperatures, mechanical properties and shape memory effect in Cu-25.38Zn-3.3Al and Cu-18.05Zn-5.35Al are presented in this work. The behavior of quenched specimens during aging at temperatures in the range 200 to 500°C for times up to 30 hours is also described. These results enable to determine optimal quenching media from the point of relevant properties - firstly shape memory effect. Besides, aging at 200 to 500°C for different times caused the degradation of shape memory effect and change of phase transformation temperatures. During aging neither coarsening of α-precipitates nor formation of γ-precipitates were observed. The ultimate operating temperatures of devices or parts, which could be made of these alloys have been defined.

Factors influencing shape memory effect and phase transformation behaviour of Fe–Mn–Si based shape memory alloys

The objective of this research work was to investigate the factors influencing the shape memory effect and phase transformation behaviour of three Fe–Mn–Si based shape memory alloys: Fe-28Mn-6Si, Fe-13Mn–5Si-10Cr-6Ni and Fe-20Mn-6Si-7Cr-1Cu. The research results show that the shape memory capacity of Fe–Mn–Si based shape memory alloys varies with annealing temperature, and this effect can be explained in terms of the effect of annealing on γ↔e transformation. The nature and concentration of defects in austenite are strongly affected by annealing conditions. A high annealing temperature results in a low density of stacking faults, leading to a low nucleation rate during stress induced γ→e transformation. The growth of e martensite plates is favoured rather than the formation of new e martensite plates. Coarse martensite plates produce high local transformation strains which can be accommodated by local slip deformation, leading to a reduction in the reversibility of the martensitic transformation and to a degradation of the shape memory effect. Annealing at low temperatures (≤673 K) for reasonable times does not eliminate complex defects (dislocation jogs, kinks and vacancy clusters) created by hot and cold working strains. These defects can retard the movement and rearrangement of Shockley partial dislocations, i.e. suppress γ→e transformation, also leading to a degradation of shape memory effect. Annealing at about 873 K was found to be optimal to form the dislocation structures which are favourable for stress induced martensitic transformation, thus resulting in the best shape memory behaviour. Transmission electron microscopy observations supported the concept that the regular overlapping of stacking faults can result in the formation of bulk e martensite plates. Stacking faults were also found to exist in e martensite plates, and it is inferred that these faults can act as embryos for e→γ reverse transformation.

Effect of L1o → Ni5Al3 Reordering on Properties of Martensitic Ni-Al Alloys

L1o→Ni 5 Al 3 reordering and related properties in powder Ni-Al alloys consisting 64-65 at.%Ni have been examined by X-ray diffraction and dilatometric and damping capacity measurements. The N1 5 A1 3 formation occurs in the L1o matrix by simple reordering of atoms with a continuous increase of the c/a ratio. As a result, degradation of shape memory effect and decrease of damping capacity are observed after short-time annealing at 200-300°C.

Evaluation of cyclic deformation and degradation of shape memory effect in TiNi alloy : Journal of JSNDI, vol. 42, no. 7 (July 1993) p. 383 Furuya, Yasubumi; Kato, Hisashi; Mitsuzuka, Kenichi; Young Chul Park

Evaluation of cyclic deformation and degradation of shape memory effect in TiNi alloy : Journal of JSNDI, vol. 42, no. 7 (July 1993) p. 383 Furuya, Yasubumi; Kato, Hisashi; Mitsuzuka, Kenichi; Young Chul Park

Thermal cyclic deformation and degradation of shape memory effect in Ti-Ni alloy: Furuya, Y., Obata, M. and Matsumoto, M. Proc. Conf. Intermetallic Compounds—Structure and Mechanical Properties, JIMIS-6, Sendai, Japan, June 1991, pp 685–689

Thermal cyclic deformation and degradation of shape memory effect in Ti-Ni alloy: Furuya, Y., Obata, M. and Matsumoto, M. Proc. Conf. Intermetallic Compounds—Structure and Mechanical Properties, JIMIS-6, Sendai, Japan, June 1991, pp 685–689

THERMAL CYCLIC DEFORMATION AND DEGRADATION OF SHAPE MEMORY EFFECT IN Ti-Ni ALLOY

Abstract The most basic single axial cyclic motion (i.e. fatigue) tests by the alternative heating up and cooling down of shape memory Ti-Ni50.2at%wires were performed. Heating up was done by direct current method or temperature controllable water-bath and the characteristics on the cyclic motion and its degradation in shape memory effect (SME) are discussed. The degradation of SME can be divided into two factors; the cyclic deformation by thermoelastic transformation ΔD and the gradual inelasatic elongation U of Ti-Ni wire. The stress dependency of each SME degradation factor, ΔD or ΔU, and the variation of transformation temperature with thermal cycles are discussed. Lastly, nondestructive estimation of total thermal fatigue life of wire is tried from the view point of the initial changing rate of these degradation parameters.

Ti-Ni形状記憶合金ワイヤーの繰返し変形動作特性と劣化

Recently, the robot actuator worked by the driving recovery-force in thermoelastic martensitic transformation of shape memory alloys (SMA) is under development. In general, such a SMA actuator necessitates a number of cyclic repeated motions, so that the investigation of the gradual decrease of recovery force with repeated motion cycles as well as the prevention of such a degradation of shape memory effect (SME) are very important for the actual use as a robot actuator. However, such research and discussions about the degradation of SME are very few up to the present. Therefore, in the present study, the most basic, single axial SMA actuator of Ni-Ti wire is made, and the cyclic motion (i.e. fatigue) test by the alternative, heating up and cooling down of SMA Ni-Ti wire is performed. Heating up of wire is done by a direct current method, and cooling down by fan in air. The characteristics on the cyclic motion and its degradation of SME is discussed. As a result, the degradation in SME can be devided in two factors; the cyclic deformation by martensitic-trasformation and the gradual elongation of Ni-Ti wire. From the investigation in the changes of these two factors with repeated motion cycles, it is found that the latter factor contributes to the degradation to a much greater extent than the former under the present fatigue test condition of Ti-Ni wire. The efficiency of training before the use of SMA material is confirmed as one of the useful measures to prevent the degradation of SME in engineering applications.