Effect of the Residual Deformation on the Mechanical Behavior of the Ni-Ti Alloy Charged by Hydrogen

Gamaoun, Fehmi; Skhiri, Imen; Bouraoui, Tarak

doi:10.4028/www.scientific.net/amr.324.181

Cited by 3 publications

(3 citation statements)

References 10 publications

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“…This observation is explained by the presence of hydrogen that blocks the motion of the Ni and the Ti. Consequently, they retard the occurrence of the martensite transformation and contribute to its hindrance [4,27]. Moreover, it is observed that during the plateau, the fluctuation has been heightened.…”

Section: Resultsmentioning

confidence: 99%

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

Letaief

Hassine

Gamaoun

2016

Shap. Mem. Superelasticity

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The relaxation behavior under tensile loading of a superelastic NiTi alloy was investigated after hydrogen charging with respect to aging from one to 77 days in air at room temperature. The specimens were immersed for 3 h in a 0.9 % NaCl aqueous solution and then relaxed with an imposed strain of 4.8 %-which results in half of the martensite transformation-for different strain rates of 10-4 , 10-3 , and 5 9 10-3 s-1. For the non-charged specimens, the relaxed stress at the beginning exhibited a temporary dependence on the strain rates and then reached the same equilibrium stress after 2.5 h. After hydrogen charging, this equilibrium stress did not vary for the ascharged specimen. Nevertheless, the greater the aging period is the greater the equilibrium stress is. This behavior can be attributed to the diffusion of hydrogen into the entire specimen, which hinders the relaxation mechanism of the martensite bands.

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Section: Resultsmentioning

confidence: 99%

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

Letaief

Hassine

Gamaoun

2016

Shap. Mem. Superelasticity

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“…However, we notice that the hardening-like slope of the phase-transformation pseudo-plateau grows slightly and a minor reduction of the plateau is detected. According to our previous results [5][6][7], this could be attributed to the hindrance of the austenite transformation by the absorbed hydrogen. In addition, this result indicates that after 3 h of immersion, the amount of diffused hydrogen is not enough to cause an increase in the critical stress for introducing martensite or to generate an embrittlement during the austenite transformation.…”

Section: Methodsmentioning

confidence: 99%

Effect of Hydrogen on the Stress Relaxation of Aged NiTi Shape Memory Alloys

2016

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The susceptibility of the NiTi shape memory alloy to relaxation after the hydrogen charging in an aqueous solution has been investigated with respect to ageing during one to six days in air at room temperature. The orthodontic wires have been prepared by immersing in a 0.9% NaCl solution for 3 h, under applied current density of 10 A/m 2 and then relaxed with an imposed deformation in a fully austenite state of structure and in a state with 1/3 and 2/3 of the martensite volume fraction. Through the stress relaxation, the hydrogen-charged specimen has shown a significant decrease of the stress, compared to the non-immersed alloy, when the imposed deformation was located in the plateau of the austenite-martensite transformation. It was also found that a longer ageing period is important and the properties of the wires with longer stress relaxation are similar to the those of non-charged wires. Nevertheless, no difference has been detected between the as-received and the as-charged specimens when the imposed deformation was located in the elastic deformation region of the fully austenitic structure. This behavior is attributed to the effect of the gradient of absorbed hydrogen, existing between the surface and the center axis of the studied wires, which facilitates the mobility of the martensite bands during the stress relaxation.

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“…This behaviour was attributed to the accumulation of hydrogen atoms, preferentially within regions that contain continual deficiencies, because of being trapped by retained martensite plates and dislocations, forming an obstacle which would cause fracture when the martensite domains were forced to pass across these boundaries [ 11 ]. In several studies [ 16 , 17 ], permanent defects could be obtained by cyclic deformation. In [ 18 ], the authors indicated that the pseudo-elastic hysteresis loops would decrease as the number of cycles increased and the strain hardening slope grew.…”

Section: Introductionmentioning

confidence: 99%

Strain Rate Effect upon Mechanical Behaviour of Hydrogen-Charged Cycled NiTi Shape Memory Alloy

Gamaoun

2021

Materials

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The rate dependence of thermo-mechanical responses of superelastic NiTi with different imposed strain rates after cycling from 1 to 50 cycles under applied 10−5s−1, 10−4s−1 and 10−3s−1 strain rates, immersion for 3 h and ageing has been investigated. The loaded and unloaded as-received NiTi alloy under an imposed strain of 7.1% have shown an increase in the residual deformation at zero stress with an increase in strain rates. It has been found that after 13 cycles and hydrogen charging, the amount of absorbed hydrogen (291 mass ppm) was sufficient to cause the embrittlement of the tensile loaded NiTi alloy with 10−5s−1. However, no premature fracture has been detected for the imposed strain rates of 10−4s−1 and 10−3s−1. Nevertheless, after 18 cycles and immersion for 3 h, the fracture has occurred in the plateau of the austenite martensite transformation during loading with 10−4s−1. Despite the higher quantity of absorbed hydrogen, the loaded specimen with a higher imposed strain rate of 10−3s−1 has kept its superelasticity behaviour, even after 20 cycles. We attribute such a behaviour to the interaction between the travelling distance during the growth of the martensitic domains while introducing the martensite phase and the amount of diffused hydrogen.

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Effect of the Residual Deformation on the Mechanical Behavior of the Ni-Ti Alloy Charged by Hydrogen

Cited by 3 publications

References 10 publications

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

Effect of Hydrogen on the Stress Relaxation of Aged NiTi Shape Memory Alloys

Strain Rate Effect upon Mechanical Behaviour of Hydrogen-Charged Cycled NiTi Shape Memory Alloy

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