Tarak Bouraoui

Effect of Hydrogen on the Tensile Strength of Aged Ni–Ti Superelastic Alloy

Because of its good corrosion resistance and biocompatibility, superelastic Ni–Ti wire alloys have been successfully used in orthodontic clinics. However, delayed fracture in the oral cavity has been observed. The susceptibility of a Ni–Ti shape-memory alloy toward hydrogen embrittlement has been examined with respect to the current densities and aging in air at room temperature. Orthodontic wires have been cathodically hydrogen charged using a different current density of 5, 10, and 20 A/m2 from 2 to 24 h in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test has been 20–90 MPa higher than that without hydrogen charging. In addition, embrittlement takes place in the austenite–martensite transformation plateau. For a short period of charging, the Ni–Ti alloy conserves its superelastic behavior. However, after 24 h of aging in air at room temperature, fracture at the austenite–martensite transformation plateau takes place earlier.

Hydrogen effect on the austenite–martensite transformation of the cycled Ni-Ti alloy

Because of its biocompatibility, superelastic Ni-Ti wire alloys have been successfully used in orthodontic clinics. The susceptibility of Ni-Ti shape memory alloys toward hydrogen embrittlement has been examined with respect to the residual stress after a few number of cycles in air at room temperature. Orthodontic wires have been cycled until having an imposed deformation of 2.1%, 4%, and 7.7% between 1 and 50 cycles and then have cathodically been charged by hydrogen with a current density of 10 A/m2 for 4 h in a 0.9% NaCl aqueous solution at room temperature. Throughout cycling, a residual strain has been formed and has increased by the number of cycles and the value of the imposed deformation. After hydrogen charging, the critical stress enhances when the number of cycles is great and the value of the imposed deformation is high. In addition, an embrittlement occurs for the specimen submitted to 50 and 30 cycles with an imposed strain of 2.1% and 4%, respectively. Nevertheless, no embrittlement has been detected after 50 cycles until 7.7% of the imposed deformation. The results of this study imply that the embrittlement could be related to the discontinuity in the distribution of defects created by partial superelastic cycling.

Strain rate response of a Ni–Ti shape memory alloy after hydrogen charging

In this work, we investigate the susceptibility of Ni–Ti superelastic wires to the strain rates during tensile testing after hydrogen charging. Cathodic hydrogen charging is performed at a current density of 10 A/m² during 2–12 h in 0.9% NaCl solution and aged for 24 h at room temperature. Specimens underwent one cycle of loading-unloading reaching a stress value of 700 MPa. During loading, strain rates from 10−6 to 5 × 10−2  s−1 have been achieved. After 8 h of hydrogen charging, an embrittlement has been detected in the tensile strain rate range of 10−6  to 10−4 s−1. In contrast, no embrittlement has been detected for strain rates of 10−3 s−1 and higher. However, after 12 h of hydrogen charging and 24 h of annealing at room temperature, the embrittlement occurs in the beginning of the austenite-martensite transformation for all the studied strain rate values. These results show that for a range of critical amounts of diffused hydrogen, the embrittlement of the Ni–Ti superelastic alloy strongly depends on the strain rate during the tensile test. Moreover, it has been shown that this embrittlement occurs for low values of strain rates rather than the higher ones. This behaviour is attributed to the interaction between the diffused hydrogen and growth of the martensitic domain.

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

Because of its good corrosion resistance and biocompatibility, superelastic Ni-Ti wire alloys have been successfully used in orthodontic clinics. However, delayed fracture in the oral cavity has been observed. The susceptibility of a Ni–Ti shape memory alloy towards hydrogen embrittlement has been examined with respect to the residual stress after a few numbers of cycles and ageing in air at room temperature. Orthodontic wires have been cathodically charged by hydrogen with a current density of 10 A/m2 from 4h in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test of the as-charged specimen has been 30 MPa higher than that without hydrogen charging. However, after 1 to 50 cycles followed by hydrogen charging, the austenite-martensite plateau is decreased outstandingly compared to the monotonous tensile test of the as-charged by hydrogen specimen. Moreover, compared to the non-cycled and hydrogen charged material, the cyclic deformed and charged by hydrogen specimens present an increase of the initial stress for inducing martensite structure. This increase is about 110 MPa after 50 cycles. In addition, an embrittlement has been detected for the specimen submitted to 50 cycles and hydrogen charging for 4h. This behavior is attributed to the generated dislocations during cyclic deformation which act as trapping sites of hydrogen and a barrier for the austenite-martensite transformation.

Modeling of hydrogen effect on the superelastic behavior of Ni-Ti shape memory alloy wires

Superelastic NiTi wires are widely used in orthodontic treatments, but sometimes fracture can be observed after few months of use in buccal cavity and attributed to the degradation of NiTi mechanical properties due to hydrogen absorption. In this paper, a modeling approach is proposed in order to describe the effect of hydrogen diffusion on the transformation properties of NiTi SMAs. In order to experimentally predict such effects, cathodic hydrogen charging was performed at a current density of 10 for 6h, 24h, 48h and 72h in 0.9% NaCl aqueous solution at room temperature. Tensile tests were carried out shortly after hydrogen charging. The obtained stress-strain curves showed an increase of yield transformation stresses for forward and reverse martensitic transformations and a decrease of maximum transformation strain. Using Ficks second law, the transformation temperatures variation can be expressed as a function of the mean concentration of absorbed hydrogen and then taked into account in the SMA constitutive model developed by Chemisky et al (2011). The numerical results are compared to the experimental ones to calibrate the proposed method. Simulations showed that hydrogen diffusion induces a shifting of transfomation temperatures, a decreasing of maximum transformation strain and an increasing of yield transfomation stresses.

Shape Memory Effect Improvement and Study of the Corrosion Resistance of the Fe-8Mn-6Si-13Cr-6Ni-12Co Alloy

Fe-8Mn-6Si-13Cr-6Ni-12Co shape memory alloys are characterized by a good corrosion resistance and a modest shape memory effect. Experimental tests of the corrosion resistance of Fe-8Mn-6Si-13Cr-6Ni-12Co have been studied and compared with the Fe-32Mn-6Si alloy using weight loss, free corrosion and polarization resistance tests. The shape memory effect measurement of the Fe-8Mn-6Si-13Cr-6Ni-12Co and Fe-32Mn-6Si alloys has been also tested after 5% of deformation. The results of corrosion experimental tests show that the Fe-8Mn-6Si-13Cr-6Ni-12Co SMA is not very active in the Na2SO4 solution at pH4 at room temperature. In addition, its corrosion resistance is better than the Fe-32Mn-6Si and almost similar to the 316L stainless steel. To improve the shape memory effect of the Fe-8Mn-6Si-13Cr-6Ni-12Co, a thermomechanical treatment has been applied by 8% prestrain in tensile and followed by heat treatment at 1320K for 1 hour. The results show an improvement in the shape memory effect after 5% of deformation in tensile test.