Two-way Memory Strain Research Articles

Quantitative analysis of the microstructure in NiTi wires by Synchrotron X-ray diffraction: An approach to the cause of the Two Way Shape Memory Effect

Abstract To understand the complex behaviour of the NiTi shape memory alloys, the use of advanced characterization techniques as Synchrotron X-ray diffraction, becomes more and more necessary. In this paper, this technique was performed in heat-treated, thermally cycled at zero stress and two-way memory trained NiTi wires to evaluate through microstrain and retained martensite which is the predominant micro-mechanism that cause the Two Way Shape Memory Effect. The microstructural effects in the structure of the Austenite by thermal and training methods were quantified separately for each NiTi sample, allowing us to calculate how much of the accumulated microstrain in the Austenite phase is due to the increment in density of dislocations and how much is consequence of the decrease in crystallite size. To achieve this, two different methodologies were used. The mean microstrain and shift index were calculated by analysing the whole diffractogram with the Rietveld method and the Double-Voigt Approach. The individual peak shift and the peak broadening of the Austenite phase were evaluated by the modified Williamson-Hall plot. The results show that as higher is the increase in microstrain by thermal and training procedures, higher is the two-way memory strain of the NiTi sample. The micromechanical results pointed out that there is a certain formation of retained martensite during two-way memory training, but is not the main cause of the two-way shape memory effect. What is more, it seems that an early formation of retained martensite during training may decrease the generation of two-way shape memory effect. Pre-training thermal procedures that minimize the formation of retained martensite and maximize the increase in microstrain are fundamental to improve the generation of the NiTi two-way memory strain by subsequent training.

Influence of Vibration on Thermomechanical Property and Transformation Behavior of TiNi Shape Memory Alloy

The strain recovery character and transformation behavior of a TiNi alloy are investigated by means of mechanical testing and DSC measurement before and after vibration. The DSC results show that the second reverse transformation peak temperature of TiNi alloys after vibration under constraint increases slightly during the subsequent heating, and the transformation heat decreases, while the transformation heat of the first reverse transformation increases. After constrained vibration, the first decline of the two-stage recovery strain obtained during the second heating process is obviously smaller than the one without vibration, and the total recoverable strain also decreases. For the prestrained martensitic TiNi fiber after a heating-cooling cycle, the two-way memory strain increases about 1.2% after vibration. All these phenomena are considered to be closely related to the vibratory stress relief during vibration.

TWO-WAY SHAPE MEMORY EFFICIENCY OF THE Ti-Ni 54.4 Wt. (%) ENHANCED BY THERMAL TREATMENTS

This paper experimentally analyzes the influence of repeated thermal cycling at zero stress on the two-way training efficiency for developing the two-way shape memory effect in TiNi shape memory alloys. Four different sets of TiNi wire are used in the study: two sets have been heat-treated to ensure a transformation path with no R-phase, whereas the other two sets have had the same heat-treatment which is then followed by repeated thermal cycling at zero stress to stabilize their phase transformation behavior. Subsequently, thermal cycling under constant stress two-way training is performed on each sample set. The study then analyzes the training efficiency and the recoverable two-way memory strain for each sample set. The results suggest that to obtain the best two-way material performance, that is, to obtain a substantial two way memory strain together with an increase in the efficiency of the training, the TiNi wire should be thermally cycled at zero stress prior to training until the phase transformation is stabilized.

New Understanding of the Influence of the Pre-Training Phase Transformation Behaviour on the TWSME in NiTi SMA Wires

Changes in the phase transformation behaviour in two-way shape memory NiTi wires were quantified by X-ray diffraction as a function of temperature. The results were compared with those obtained from the electrical resistivity measurements and applied loading method. The weight fraction diagrams enable us to observe that the higher is the level of the R-phase in the pre-training path of the NiTi sample, the lower will be the two-way memory strain obtained. Substantial values of two-way memory strain were found to be associated with pre-training paths characterized by simultaneous transformation of austenite to R-phase plus martensite. Finally, a comparison of these three experimental techniques led to a new interpretation of the electrical resistivity curves for obtaining the transformation temperatures of two-way shape memory NiTi wires. This may help to clarify the relationship between the shape of the electrical resistivity curve and the two-way memory strain that occurs in the NiTi sample.

R-phase influence on different two-way shape memory training methods in NiTi shape memory alloys

This paper studies how the R-phase developed by thermal cycling at zero stress influences two different training procedures for developing the two-way shape memory effect (TWSME) in NiTi shape memory alloys (SMAs). The two training procedures are thermal cycling under constant load and isothermal tensile deformation under martensite state. This paper studies how R-phase developed by thermal cycling at zero stress influences these training procedures and their training parameters, the two-way memory strain obtained and the transformation temperatures. The results show first, that the presence of the R-phase did not prevent substantial two-way memory strain from being obtained in any instance of training and thermal treatment; and second, that thermal cycling at zero stress prior to training has a considerable impact depending on the training method used. Finally, to obtain the best results (that is, a substantial two-way memory strain together with a minimum of final plastic strain and easy TWSME activation temperatures), the NiTi wire should be thermally cycled at zero stress prior to training until the R-phase is totally developed and trained by thermal cycling under constant load.

Appearance of Two-Way Strain in Shape Memory Effect of Ti–Ni–Nb Alloy —Influence of Applied Strain on Two-Way Strain—

A promising field for applications of shape memory alloys (SMAs) in the near future is the micro-actuator technology. Especially, two-way shape memory effect (TWSME) is the most suitable to apply in actuators, because a pre-determined response can be obtained very easily by thermal changes against shape memory elements.In this paper, the TWSME in Ti–Ni–Nb alloy was investigated quantitatively by applying various levels of pre-deformation. The deformation in a complete martensite phase was applied by a thermo-mechanical treatment in order to obtain the two-way memory strain. The experimental results indicated that the deformation mechanism in a martensite phase was just the martensite reorientation accompanied by the dislocation slip. The dislocation due to the slip deformation is the origin of the internal stress field that is necessary to generate the two-way memory strain. However, excessive introduction of the dislocation decreases the two-way memory strain. The maximum two-way memory strain observed in this experiment was 2.1% at an applied strain of 18%. In addition, pre-deformation increases the temperature of reverse transformation, but decreases the temperature of martensitic transformation. These experimental results can be explained by using the series-parallel combined model that has been suggested in our previous work.

「スマートマテリアル」 Ｔｉ‐Ｎｉ‐Ｎｂ合金の形状記憶効果における二方向ひずみの発現について〜二方向ひずみに及ぼす付加ひずみの影響〜

A promising field for application of shape memory alloys (SMAs) in the near future is in micro-actuator technology. Two-way shape memory effect (TWSME) is particularly suitable for application in actuators, since pre-determined responses can be obtained very easily by thermal changes to shape memory elements. In this paper, the TWSME, Ti-Ni-Nb alloy, was investigated quantitatively by applying various levels of pre-deformation. Deformation in a complete martensite phase was applied as a thermomechanical treatment in order to obtain the two-way memory strain. The experimental results indicated that the deformation mechanism in martensite phase was martensite orientation accompanied by dislocation slip. The dislocation due to the slip deformation is the origin of the internal stress field that is necessary to generate the two-way memory strain. However, excessive introduction of dislocation tends to decreases the two-way memory strain. The maximum two-way memory strain observed in this experiment was 2.1% at an applied strain of 18%. In addition, pre-deformation increases the temperature of reverse transformation, but decreases the temperature of martensitic transformation. These experimental results can be explained by using the series-parallel combined model suggested in our previous study.

Two-Way Shape Memory Effect Induced by Tension Training in Cu-13.4Al-4.0Ni (mass%) Alloy Single Crystals

The two-way shape memory effect in CuAlNi shape memory alloy single crystals was studied. The effect was induced by cyclic tensile deformations of superelasticity and shape memory effect, and the efficiency of both methods was examined. Compared with the similar traininig process in the single crystals of CuZnAl alloys, the CuAlNi alloy showed much smaller changes in stress-strain behaviour and in the resulted two-way memory strain. The superelasticity training had no influence on both the stress-strain behaviour and the dilatation change. By shape memory training, the two-way memory strain increased to saturate at 0.90% after 10 cycles. Associated with this change, particular martensite variants increased its fraction and became “reproductive”. However, they were not the variants that were induced during the training, and further increase in the two-way memory strain was impossible by the formation of the observed variants. It was concluded that the present training methods were not effective for the two-way memory effect in CuAlNi shape memory alloy.