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1

Enhancement of bending fatigue life in TiNi shape-memory alloy tape by nitrogen ion implantation

Takeda K., Matsui R., Tobushi H., Homma S., Levintant-Zayonts N., Kucharski S.

Archives of Mechanics

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2015

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Vol. 67, nr 4

293--310

EN

A shape-memory alloy (SMA) is expected to be applied as intelligent or smart material since it shows the functional characteristics of the shape memory effect and superelasticity. Most SMA elements, with these characteristics, perform cyclic motions. In these cases, the fatigue property of SMA is one of the most important issues in view of evaluating functional characteristics of SMA elements. The fatigue properties are complex since they depend on stress, strain, temperature and their hysteresis. If an SMA is implanted with high-energy ions, the thermomechanical properties of the material may change, resulting in long fatigue life. In the present study, the nitrogen ion implantation was applied to modify the surface of a TiNi SMA tape and the influence of implantation treatment on the bending fatigue properties was investigated.

2

Effect of cold deformation to transformation temperatures of Ni-Ti shape memory alloys

Yildiz C., Kucukyildirim B. O., Akdogan Eker A.

Archives of Materials Science and Engineering

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2014

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Vol. 66, nr 1

37--44

EN

Purpose: of this paper is to predict the effect of cold deformation to transformation temperatures of Ni–Ti SMA and clarify the relation between cold forming ratio and the parameters of thermo mechanical process. Design/methodology/approach: 25% and 50% cold formed Ni–Ti wire specimens are investigated for defining the relation between the ratio and the parameters of thermo mechanical processes of without annealing and with annealing at 450°C for 30 minutes and 600°C for 30 minutes. Moreover, Differential Scanning Calorimeter (DSC) measurements are applied to the specimens for obtaining the transformation temperatures. Findings: According to DSC measurements, start and finish temperatures of transformation of austenite, martensite and rhombohedral phases are calculated. Hence, their changes are related to deformation and annealing, so it can be implied that cold deformation process leads to increases in the temperature ranges, slow occurrences of transformation and also the shape memory property lowers. Research limitations/implications: It is observed that some specimens exhibit rhombohedral phase transformation and this affects the martensite phase transformation temperatures and results. Thus, annealing time can be increased to determine both positive and negative effect to transformation properties for further research. Moreover, the grainsize and form may also be effective, so this case should be studied. Practical implications: can be stated as suitable transformation temperatures must be paid attention to desired service temperatures of products those are made of Ni–Ti SMAs, otherwise they may exhibit unexpected shape change due to the affected transformation temperature. Originality/value: is that Ni–Ti SMAs may be exposed to cold deformation when they are manufactured for being given any product shape, so with this research it can be determined that if their transformation temperatures change or not. Besides, it can be implied that they change and so this study presents the changes, the intervals of start and finish temperatures relevant to the cold forming ratio.

3

Influence of nitrogen ion implantation on deformation and fatigue properties of TiNi shape-memory alloy wire

Takeda K., Mitsui K., Tobushi H., Levintant-Zayonts N., Kucharski S.

Archives of Mechanics

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2013

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Vol. 65, nr 5

391--405

EN

A shape memory alloy (SMA) is expected to be applied as intelligent material since it shows the unique characteristics of the shape memory effect and superelasticity. Most SMA elements, with these characteristics, perform cyclic motions. In these cases, fatigue of SMA is one of the important properties in view of evaluating functional characteristics. The fatigue properties are complex since they depend on stress, strain, temperature and time. If SMA is implanted by high energy ions, the thermomechanical properties may change, resulting in long fatigue life. In the present study, the nitrogen ion implantation was applied to modify TiNi SMA wire surface and the influence of implantation treatment on the tensile deformation and bending fatigue properties was investigated.

4

Pseudoviscoelastic behavior of TiNi shape memory alloys under stress-controlled subloop loadings

Tobushi H., Endo M., Ikawa T., Shimada D.

Archives of Mechanics

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2003

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Vol. 55, nr 5-6

519-530

EN

The superelastic behavior of TiNi shape memory alloy under various subloop loadings were investigated. The results obtained can be summarized as follows. (1) In the case of subloop loading under strain-controlled conditions, the reloading curve passes through the unloading-start point. In the case of stress-controlled conditions, the return-point memory does not appear. (2) In the case of subloop loading under stress-controlled conditions, strain increases under constant stress in the loading process and decreases in the unloading process. (3) In the case of subloop loading under stress-controlled conditions, stress decreases under constant strain in the loading process and increases in the unloading process. (4) The above-mentioned behavior concerning the return-point memory and the pseudoviscoelastic behavior similar to creep and stress relaxation, appear according to the martensitic transformation and the reverse transformation based on the variation in stress and temperature.

5

Deformation behavior of TiNi shape-memory alloy under strain- or stress-controlled conditions

Tobushi H., Okumara K., Endo M., Tanaka K.

Archives of Mechanics

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2002

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Vol. 54, nr 1

75-91

EN

The deformation properties of TiNi shape-memory alloy subjected to strain control and stress control were investigated experimentally. The results obtained are summarized as follows. (1) In the case of a full loop, the stress-strain curves under stress-controlled conditions are similar to those under strain-controlled conditions with high strain rate. The overshoot and undershoot do not appear at the start points of the stress-induced martensitic transformation in these curves. (2) In the case of subloop under stress-controlled conditions, temperature decreases and therefore the strain increases owing to the martensitic transformation at the early stage of the unloading process. At the early stage in the reloading process, temperature increases and therefore the strain decreases owing to the reverse transformation. (3) In the case of subloop under stress-controlled conditions, the starting stresses of the martensitic transformation and the reverse transformation in the loading and unloading processes coincide with the transformation stresses under strain-controlled conditions with low strain rate, respectively. (4) The deformation behaviours for a subloop under stress-controlled conditions are prescribed by the condition for progress of the martensitic transformation based on the transformation kinetics. (5) The deformation behaviors subjected to cyclic loading under stress-controlled conditions at constant temperature are also prescribed by the conditions for progress of the martensitic transformation.