Shape Memory Steel Research Articles

Electrochemical characterization of a Fe-based shape memory alloy in an alkaline medium and the behaviour in aggressive conditions

A Fe-17Mn-6Si-19Cr-4Ni-1(V,C) shape memory steel (SMS) was characterised electrochemically in its unstrained and pre-strained conditions. The work focused on analysing the passive films generated in alkaline conditions, and on the behaviour of those passive samples in various Cl−/OH− ratios. The passive films were developed by cyclic voltammetry and their characterisation was performed by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS); these tests were also carried out in 304 L stainless steel specimens for comparison purposes. The results indicated that the film characteristics (thickness and composition) were similar in all the samples, although higher corrosion resistance was observed in the 304 L due to the higher Cr content. In addition, the behaviour of the passive samples in aggressive conditions was assessed by potentiodynamic measurements. The results stated that the shape memory steels were more sensitive than the stainless steel to the aggressive conditions, especially when the alloy was pre-strained. It was also observed that the SMS exhibited a characteristic corrosion morphology localized at the grain boundaries.

Dissimilar friction welding of NiTi shape memory alloy and steel reinforcing bars for seismic performance

Integration of NiTi superelastic bars with steel reinforcing bars is a promising path for increasing the seismic performance of reinforced concrete structures. Welding of NiTi alloy to steel is however not straightforward with standard welding techniques. Friction welding is investigated for performing this dissimilar junction using both regular friction (forced controlled) and milling (position-controlled) machines. When adequate shortenings and forging forces are selected, the welds can withstand the tensile strength of the steel rebars. Failure actually occurs in the heat-affected zone of the steel bars. The intermetallic (IM) layer at the welded interface should be kept thin and cracks should be limited in the steel layer next to the interface in order to avoid early brittle interfacial cracking.

Microstructural evaluation on shape recovery in stainless Fe-Mn-Si-Cr-Ni-Co SMA processed by wire drawing

The effect of the microstructure on the shape recovery in stainless Fe-8Mn-5Si-13Cr-6Ni-12Co shape memory steel (SSMS) was evaluated using tensile tests and reversion temperature of 600oC for a pre-strain of 4%. The tests were performed for a solution treated and annealed conditions at different temperatures after wire drawing of 57% area reduction. The best total shape recovery (TSR) was 83% for a sample deformed and annealed at 850oC. It was concluded that the elastic (or, superelastic) shape recovery (ESR) is high when the austenitic matrix strength is high surpassing the value of shape recovery due to memory effect (SR) and once the austenitic matrix becomes softer, the contribution of SR increases and that of ESR decreases.

Shape Recovery in Stainless FeMnSiCrNi(-Co) SMA Processed by ECAE

Stainless shape memory steel presents reasonable shape recovery but lower than the traditional NiTi shape memory alloys (SMA). However, recent results have shown that the shape recovery could be improved by decreasing the austenitic grain size. The present work describes the influence of the austenitic grain size on the shape recovery in stainless shape memory steel deformed by equal channel angular extrusion (ECAE) using a die intersection angle of 120o. Two alloys, FeMnSiCrNi and FeMnSiCrNiCo, were deformed by 1 ECAE pass and then they were compared in the deformed state; deformed and annealed in different temperatures for 1 h, resulting different grain sizes. Both alloys were evaluated by compression tests and the results shows an increase in total shape recovery related to grain size decrease. The best total shape recovery was 73% after a pre-strain of 4% for FeMnSiCrNi alloy.

Abrasive Wear of Fe-Mn-Si-Cr-Ni Shape Memory Stainless Steel: Preliminary Results

This study was developed to understand the influence of chemical composition and austenitic grain size on the wear resistance in stainless shape memory steel. A two-body abrasive wear device was used to understand the wear mechanism involved. They were tested pins with the following chemical composition: Fe-10.3Mn-5.3Si-9.9Cr-4.9Ni-0.006C and Fe-14.2Mn-5.3Si-8.8Cr-4.6Ni-0.008C after being austenitized at 900 and 1050 °C, followed by water quenching. The surface characterization was performed by optical microscopy and scanning electron microscopy, and the roughness profile evaluation was also conducted. The weight loss was measured after conducting the wear testing, and the wear rates were estimated. The results demonstrated that the alloy with less manganese and higher chromium content has the best wear resistance (between 17.5 and 18.9%). With an increase of the austenitic grain size there was a small reduction on the wear resistance (between 3.0 and 4.1%). The chemical composition demonstrated to have higher influence on the wear behavior than the austenitic grain size.

Plasticité de transformation d'un acier à mémoire de forme Fe_18Mn_8Cr_5Ni_5Si

Transformation plasticity of a Fe18Mn8Cr5Ni5Si shape memory steel. This paper presents an experimental study of the transformation plasticity related to the γ→ε martensitic transformation in a Fe-18Mn-8Cr-5Ni-5Si shape memory steel. Under a weak bending stress the as-quenched austenitic samples showed a transformation plasticity by cooling at 170K with a plasticity coefficient h≈2. During heating, this transformation plasticity, related to the memory effect, is reversible, partially or completely, depending on whether the initial stress was maintained or removed at 295K during heating. Nous présentons une étude expérimentale de la plasticité de transformation accompagnant la transformation martensitique γ→ε dans un acier à mémoire de forme Fe-18Mn-8Cr-5Ni-5Si. Les résultats que nous avons obtenus montrent que des échantillons austénitiques, soumis à des faibles contraintes de flexion, présentent une plasticité de transformation au refroidissement avec un coefficient de plasticité h≈2. Nous montrons que la plasticité de transformation obtenue est réversible par effet mémoire de forme, en partie ou en totalité, suivant que la contrainte est maintenue ou relâchée.

Repair and Strengthening of Concrete Structures Through Application of Corrective Posttensioning Forces with Shape Memory Alloys

Shape memory alloys recover deformations induced at lower temperatures after being heated above a transformation temperature; restraint of this shape recovery generates relatively large stresses. These stresses are used to transfer corrective forces to structural systems for strengthening and repair. For this purpose, shape memory rods are pre-elongated, anchored to the structure, and subjected to electrical resistance heating to transfer corrective forces to the structure. The project used iron-based shape memory alloys of relatively low cost; the alloy composition was selected to yield relatively high and stable levels of restrained-shape recovery stresses. Laboratory tests verified the ability of pre-elongated rods anchored onto damaged structural systems to restore structural integrity through application of corrective forces. Subsequent damaging effects could also be overcome by electrical resistance reheating of rods. A reinforced concrete bridge structure with beams lacking sufficient shear strength at longitudinal bar cutoff locations was selected for field demonstration of the technology. A design methodology was developed and verified through laboratory tests simulating conditions of the selected bridge structure. Subsequently, a detailed design was developed and the approach was implemented under field conditions. Application of corrective (posttensioning) forces to structural systems using shape memory steel provides an efficient, rapid, and convenient approach for repair and strengthening of damaged or deficient structures. The relatively large recoverable strains of shape memory alloys help reduce losses of posttensioning forces. The approach suits application to diverse structures for corrective and strengthening effects.

Effect of Amount of Deformation on the Martensitic Transformation and Shape Memory Effect in Fe-Mn-Si based Shape Memory Steel

The effect of amount of deformation on the martensitic transformation and shape memory effect in a Fe-16Mn-5Si-9Cr-4Ni alloy is studied. It is found that both E- and α'-martensite are induced by deformation. With increasing amount of deformation, γ→E→α' transformation is greatly enhanced. The highest shape memory strain 3.8% is reached when the amount of deformation is about 5%