Fe Mn Si-based Shape Memory Alloys Research Articles

Recovery stress formation in a restrained Fe–Mn–Si-based shape memory alloy used for prestressing or mechanical joining

The formation of recovery stresses in a restrained Fe–Mn–Si–Cr–Ni–VC shape memory alloy used for mechanical coupling and prestressing has been investigated with special emphasis on non-ideal restraints that might reduce the recovery stress. The evolution of recovery stresses in the presence of different initial gaps and elastic restraints was investigated experimentally. The results show that the alloy can still produce substantial recovery stresses even under non-ideal restraining conditions. In a second part, the experimental results are analyzed using techniques from constitutive modelling to investigate the evolution of the recovery stress in a more rigorous way. For this purpose the measured strain is decomposed into its elastic, thermal expansion, and transformation parts. With these strain components, the development of the recovery stress during heating and cooling can be described for all different constraint conditions in a unified way. The analysis also shows why optimizing a material for high recovery stress is different from optimizing it for a large recovery strain.

Role of carbon in improving the shape memory effect of Fe–Mn–Si–Cr–Ni alloys by thermo-mechanical treatments

To clarify the role of carbon in improving the shape memory effect of Fe–Mn–Si-based shape memory alloys by thermomechanical treatments, we investigated the effect of optimum thermomechanical treatments on shape memory effect and microstructures of Fe–14Mn–5Si–8Cr–4Ni and Fe–14Mn–5Si–8Cr–4Ni–0.12C alloys. The Cr23C6 particles in optimum thermomechanical-treated Fe–14Mn–5S–8Cr–4Ni–0.12C more effectively prevented collisions between stress-induced ϵ martensite bands than the residual α′ martensite in optimum thermomechanical-treated Fe–14Mn–5Si–8Cr–4Ni. This result is attributed to the thinner width of stress-induced ϵ martensite bands in optimum thermomechanical-treated Fe–14Mn–5S–8Cr–4Ni–0.12C compared to optimum thermomechanical-treated Fe–14Mn–5Si–8Cr–4Ni. In addition, the Cr23C6 particles formed at more sites and provided more obstacles as compared with the residual α′ martensite. Accordingly, the recovery strain of Fe–14Mn–5Si–8Cr–4Ni–0.12C was higher than that of Fe–14Mn–5Si–8Cr–4Ni. It is concluded that carbon addition is beneficial to further improving the shape memory effect of Fe–Mn–Si-based shape memory alloys by thermomechanical treatments.

Shape memory effect in Fe–Mn–Ni–Si–C alloys with low Mn contents

An attempt was made to develop a new Fe–Mn–Si–based shape memory alloy from a Fe–17Mn–6Si–0.3C (mass%) shape memory alloy, which was previously reported to show a superior shape memory effect without any costly training treatment, by lowering its Mn content. The shape memory effect and the phase transformation behavior were investigated for the as-solution treated Fe–(17–2 x)Mn–6Si–0.3C– xNi ( x = 0, 1, 2, 3, 4) polycrystalline alloys. The shape recovery strain exceeded 2% in the alloys with x = 0–2, which is sufficient for an industrially applicable shape memory effect; however, it suddenly decreased in the alloys between x = 2 and 3 although the significant shape recovery strain still exceeded 1%. In the alloys with x = 3 and 4, X-ray diffraction analysis and transmission electron microscope observation revealed the existence of α′ martensite, which forms at the intersection of the ɛ martensite plates and suppresses the crystallographic reversibility of the γ austenite to ɛ martensitic transformation.

Surface treatment and corrosion behaviour of Fe–32Mn–6Si shape memory alloy

Abstract Fe–Mn–Si-based shape memory alloys are characterized by a good shape-memory effect, but with a modest corrosion resistance. The goal of this study is to try out a suitable surface treatment in order to improve the corrosion resistance of the Fe–32Mn–6Si alloy without impairing its shape-memory properties. The implemented solution is a zinc-electrodeposited coating achieved by chromium conversion treatment. The results show, thanks to electrochemical and thermomechanical measurements, and corrosion tests performed in salt spray chamber, that zinc-electrodeposited coating followed by chromium conversion layer can be considered as a good anticorrosion solution of the Fe–32Mn–6Si alloy.

Stresses related to the shape memory effect in Fe–Mn–Si-based shape memory alloys

The shape memory behaviour of two Fe–Mn–Si-based alloys has been investigated. Two compositions were studied, i. e. Fe29Mn7Si and Fe29Mn7Si5Cr (numbers indicate mass%). Characterizations of the martensitic transformation and the different structural constituents were performed using optical microscopy and X-ray diffraction methods. Transformation temperatures were determined by thermodilatometry on undeformed and deformed samples. The shape recovery was quantified by means of bending tests and dilatometry experiments on compressed samples. A procedure was designed to measure the recovery stresses caused by the dimensional changes of the sample due to the shape memory effect. The recovery stress is defined as the stress that is generated when the recovery of deformation is impeded under constraint. The mechanical results are discussed on the basis of the underlying transformation and deformation processes.

Shape recovery characteristics of biaxially prestrained Fe–Mn–Si-based shape memory alloy

Fe–Mn–Si-based shape memory alloy has already been used practically for steel pipe joints. In most of the applications including the steel pipe joints, it is possible to estimate the reduction of diameter from the experimental data of the shape recovery after uniaxial stretching of the alloy materials. However, studies on shape recovery effects after biaxial stretching are important for the extensive applications of the alloy. In this study, we investigated the shape recovery strain after uniaxial and biaxial stretching and the microstructures of the alloy in order to see the effects of uniaxial and biaxial prestrain on the stress-induced martensitic transformation. Amounts of shape recovery strain in the biaxially prestrained specimens are smaller than those in the uniaxially prestrained specimens. Transmission electron microscopy revealed that reverse transformations of stress-induced martensitic ɛ-phase are prevented by slip bands formed at the same time in the biaxially prestrained specimens, but not in the uniaxially prestrained specimens. The technological data and interpretations presented in this study should be useful in forming design guidelines for promoting the extensive applications of Fe–Mn–Si-based shape memory alloy.

The effect of pre-rolling Fe–Mn–Si-based shape memory alloys: Mechanical properties and transmission electron microcopy examination

Fe–Mn–Si-based alloys have been promising systems for shape memory applications for a long time. However, the need of a complicated training process in order to get the desired properties avoided their practical use. Recently, several new Fe–Mn–Si-based alloys, with interesting properties, containing different types of precipitates have been developed. In these new systems, the shape memory improvement is obtained either by the sole introduction of precipitates, or by the combination of a simple thermomechanical treatment followed by a subsequent precipitation. In this work, we investigate the effect of a thermomechanical treatment performed at a temperature of 870 K on the shape memory properties of an Fe–28Mn–6Si–5Cr (wt.%). We have found that a simple treatment, without the necessity of introducing precipitates, is enough to obtain good shape memory properties. Transmission electron microscopy shows that a large density of stacking faults is produced during the treatment. From these observations we deduce that this large stacking fault density is clearly related to the observed properties.

Effect of Alloying Additions on the SFE, Neel Temperature and Shape Memory Effect in Fe-Mn-Si-based Alloys

A range of Fe–Mn–Si-based shape memory alloys has been investigated to examine the interplay of composition, stacking fault probability (SFP) and Neél temperature on the shape memory effect (SME). It has been found that the SFP (inversely proportional to stacking fault energy) showed little correlation to the SME for the range of alloy compositions examined. Further, the Neél temperature was not found to exhibit a significant effect on the SME. The addition of interstitial elements, however, was found to markedly decrease the SME.

Strengthening of a FeMnSi-based shape memory alloy by dispersion of χ-phase domains

The FeMnSi-based shape memory alloy (SMA), governed by the γ ⇔ ϵ phase transformation, is shown to be strengthened remarkably by the introduction of a non-deformable phase via beating and heating treatments, improving the shape memory effect (SME) against an opposing stress as high as 450 MPa. Dispersion of the non-deformable phase is also found to increase pseudo-elasticity. The non-deformable phase contributing to the strengthening is identified as having a bcc-based structure of I 3m, i.e. α-Mn-type χ-phase, by analyzing the zero- (ZOLZ) and first-order Laue zones (FOLZ) in microdiffraction patterns. This phase is found to be non-deformable, at least up to 1.2 GPa, by semi-in situ compression tests made on thin foil specimens, which were prepared for observation with a high-voltage electron microscope (HVEM). The origin of the strengthening and pseudo-elasticity of the FeMnSi-based SMA is discussed, with reference to dispersion–hardening, under the chemical driving force acting on the Shockley partial dislocations, which convey the γ ⇔ ϵ phase transformation.

Austenite stability in Fe–Mn–Si-based shape memory alloys

The stability of austenite in a number of Fe–Mn–Si-based shape memory alloys has been investigated. It was found that a grain boundary precipitate of BCC structure is formed over a wide range of alloy compositions and heat treatment temperatures. This grain boundary phase has been identified as the chi ( χ) phase. Although up to 3 vol.% of the grain boundary precipitate was generated by isothermal aging in the range 500–800 °C, it was found not to markedly affect the mechanical properties or the shape memory effect. Nano-indentation indicated that the hardness and strength of the parent and precipitate phase are very similar, as are their compositions.

Thermo-mechanical processing and the shape memory effect in an Fe–Mn–Si-based shape memory alloy

The effect of cold rolling and annealing on the shape memory effect (SME) in an Fe–Mn–Si-based alloy has been studied. It has been found that the SME in these alloys can be significantly increased by the appropriate thermo-mechanical processing (TMP). The optimum conditions were found to be 15% cold rolling followed by annealing at 800 °C for 15 min. This produced a total strain recovery of 4.5%. TEM showed that this processing schedule produces a microstructure of evenly spaced, and well defined stacking faults throughout the parent phase. It is shown for the first time that samples processed in this way produce a larger fraction of martensite compared to samples in the as-austenitized condition. It is concluded that the stacking faults induced by TMP act as nucleation sites for martensite formation during deformation. The SME is improved primarily as a result of the increased amount of martensite that is formed in this condition.

Vibration mitigation by the reversible fcc/hcp martensitic transformation during cyclic tension–compression loading of an Fe–Mn–Si-based shape memory alloy

The present work concerns the damping behavior of an Fe–28Mn–6Si–5Cr–0.5NbC (mass%) shape memory alloy determined by low cycle fatigue tests, and the corresponding deformation mechanism under cyclic tension–compression loading. The specific damping capacity increases with increasing strain amplitude and reaches saturation at 80%, above the strain amplitude of 0.4%. Quantitative X-ray diffraction analyses and microstructural observations using atomic force microscopy revealed that a significant amount of the tensile stress-induced e martensite is reversely transformed into the austenite by subsequent compression; in other words, the stress-induced ‘reverse’ martensitic transformation takes place in the alloy.

Development of Prestressed Concrete Using Fe–Mn–Si-Based Shape Memory Alloys Containing NbC

This article reports the mechanical properties of concrete prestressed by the Fe–Mn–Si-based shape memory alloys containing NbC that exhibit an excellent shape memory effect without the so-called ‘training’ treatment. A thermomechanically treated Fe–28Mn–6Si–5Cr–0.53Nb– 0.06C (mass%) alloy was used for this purpose. Four square bars of the alloy were embedded in mortar, and heated above their reverse martensitic transformation start temperature after hardening of the mortar matrix. Three-point bending tests were performed for the mechanical property characterization. It was found that prestressing by the shape memory alloys increased the bending strength and cracking stress of the mortar.

一定応力下におけるFeMnSi基形状記憶合金の形状回復特性

FeMnSi-based shape memory alloy (SMA) is expected to be utilized for pipe installations and large structural members. Since the shape recovery action in these applications must evolve under a stress, it is important to grasp the shape recovery behavior under a controlled stress. In this study, the shape recovery behavior of FeMnSi-based SMA under a constant stress is investigated in detail, and the interpretation based on the mechanism of stress-induced martensitic transformation, which controls the shape recovery action of FeMnSi based SMA, is conducted. The stress applied to oppose the reverse transformation decreases the degree of shape recovery and increases shape recovery temperature. On the other hands the stress applied to assist the reverse transformation increases the degree of shape recovery and decreases recovery temperature. The effect of the type and amount of prestrain on the shape recovery force is also studied. The degree of shape recovery and shape recovery force shown for FeMnSi-based SMA should be useful in designing pipe joints as well as structural members with knowledge of the results and the interpretation presented in this study.

FeMnSi基形状記憶合金薄板の二軸引張変形に対する形状回復特性

FeMnSi-based shape memory alloy (SMA) has already been used practically as steel pipe joints. In most cases of the applications, it is possible to estimate the reduction of diameter from the present data measured on the shape recovery for a uniaxial stretch of FeMnSi-based SMA. However, to promote the extensive practical application of FeMnSi-based SMA, investigations of shape recovery characteristics for biaxial stretch are significant. In this study, shape recovery characteristics for uniaxial tensile stretch, biaxial tensile stretch, and the stretch of the transition field from simple tension to biaxial tension are investigated. In addition, microstructures of FeMnSi-based SMA are observed, and the mechanism of stress-induced martensitic transformation which controls shape recovery actions is interpreted. Shape recovery strain after biaxial tensile stretch is markedly smaller than that of simple tensile stretch. The relationship of shape recovery characteristics to orientations of the ε-phase is clarified from microstructural observations. The technical data and interpretation presented in this study should be useful in forming design guidelines for promoting the extensive application of FeMnSi-based SMA.

The pseudoelastic behavior of Fe–Mn–Si-based shape memory alloys containing Nb andC

The pseudoelastic behavior of an Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%)alloy with respect to the thermomechanical treatment of pre-warm-rolling andsubsequent ageing is investigated. A partial pseudoelasticity appears betweenAs andMd,where As is the reverse martensitic transformation start temperature andMd is the temperature above which the martensitic transformation cannot be induced bystress. It can be reasonably interpreted as transformation pseudoelasticity, like forthermoelastic shape memory alloys such as Ti–Ni-based alloys and Cu-based alloys. Thethermomechanical treatment is effective in enhancing the pseudoelasticity by increasing theMd temperature. It is also found that the alloy exhibits a partial pseudoelasticity belowAs when the sample is deformed into a two-phase state: fcc and hcp. A conceivablereason for the driving force of the reverse transformation (from hcp to fcc) onunloading is the back-stress experienced by Shockley partial dislocations residing atthe tip of the martensite plates, which originates from NbC precipitates. Perfectpseudoelasticity is also obtained when the specimen is subjected to cyclic loading.

Martensitic surface relief in an Fe–Mn–Si-based alloy strained by bending

Abstract Samples of Fe–Mn–Si-based shape memory alloy were strained by bending. Surface relief from deformation and subsequent recovery exhibited tension–compression asymmetry. Tensile regions showed well-defined plate-shaped relief, whilst compressive regions exhibited more surface rumpling and compact relief. Irreversible plastic deformation is more pronounced in the compression region, contributing to reduced shape memory.

Improvement of shape memory properties of NbC containing Fe–Mn–Si based shape memory alloys by simple thermomechanical treatments

Abstract A systematic study of effect of pre-deformation before aging on shape memory properties in Fe–Mn–Si based alloys containing Nb and C has been performed and it is found that the room temperature (RT) deformation as well as deformation at high temperature in the austenitic range can greatly improve shape memory effect (SME). The results on two representative NbC containing alloys, i.e., Fe–28Mn–6Si–5Cr–0.53Nb–0.06C and Fe–15Mn–5Si–9Cr–5Ni–0.53Nb–0.06C (mass%) are as follows: (1) about 90% shape recovery for 4% initial strain and 250–320 MPa shape recovery stress are obtained for pre-rolling by 10–30% at 870 K and (2) in the case of pre-rolling at room temperature, 75% shape recovery for 4% initial strain is obtained with 10% rolled samples and 240–320 MPa shape recovery stress is attained for 10–30% rolling. The improved shape recoveries are fairly close to those of the trained conventional shape memory alloys (SMAs) without NbC and their shape recovery stresses are nearly twice those of the trained ones.

TEM observation of the internal structures in NbC containing Fe–Mn–Si-based shape memory alloys subjected to pre-deformation above room temperature

Abstract Shape recoveries comparable to those of the trained conventional alloys are obtained in Fe–Mn–Si-based alloys containing Nb and C by pre-rolling at 870 K and subsequent short ageing at 1070 K to produce NbC precipitates. In addition, when the amount of thickness reduction by pre-rolling is between 14 and 30%, these newly developed alloys attain recovery stresses high enough to make them useful for practical applications such as pipe joints. In this work, the microstructures resulting from the pre-rolling treatment at 870 K and subsequent ageing are investigated by means of transmission electron microscopy (TEM) in Fe–28Mn–6Si–5Cr–0.53Nb–0.06C and Fe–15Mn–5Si–9Cr–5Ni–0.53Nb–0.06C (mass%) alloys. Observations of the as-rolled microstructure indicate that the pre-rolling treatment produces a high density of stacking faults in the fcc matrix. A fine distribution of semi-coherent, nanometric scale NbC precipitates is obtained after ageing the pre-rolled samples. The amount of pre-rolling affects the size and distribution of precipitates and most of the precipitates are seen to be in close relation with stacking faults in the aged condition. The stress-induced hcp martensite is produced as lamella structure, probably due to the abundance of sites for its nucleation i. e., stacking faults and precipitates, and the width of martensite plates is extremely small. These thin martensite plates are highly recoverable, which is the indispensable condition for obtaining a large shape memory effect.

Further Improvement in the Shape-Memory Properties of FeMnSi-Based Shape-Memory Alloys Containing NbC Precipitates

Further Improvement in the Shape-Memory Properties of FeMnSi-Based Shape-Memory Alloys Containing NbC Precipitates