Fe Mn Si Cr Shape Memory Alloys Research Articles

Uncommon Cold-Rolling Faults in an Fe–Mn–Si–Cr Shape-Memory Alloy

The paper analyzes the occurrence of evenly spaced cracks on the surface of lamellar specimens of Fe-28Mn-6Si-5Cr (mass %) shape-memory alloy (SMA), during cold rolling. The specimens were hot rolled and normalized and developed cold rolling cracks with an approximate spacing of about 1.3 mm and a depth that increased with the thickness-reduction degree. At normalized specimens, X-ray diffraction patterns revealed the presence of multiple crystallographic variants of brittle α′ body-bcc martensite, which could be the cause of cold-rolling cracking. Both normalized and cold-rolled specimens were analyzed using scanning electron microscopy SEM. SEM micrographs revealed the presence of several crystallographic variants of α′-body-centered cubic (bcc) and ε hexagonal close-packed (hcp) martensite plates within a γ-face-centered cubic (fcc) austenite matrix in a normalized state. High-resolution SEM, recorded after 25% thickness reduction by cold-rolling, emphasized the ductile character of the cracks by means of an array of multiple dimples. After additional 33% cold-rolling thickness reduction, the surface of crack walls became acicular, thus revealing the fragile character of failure. It has been argued that the specimens cracked in the neutral point but preserved their integrity owing to the ductile character of γ-fcc austenite matrix.

Adapting Fe–Mn–Si–Cr shape memory alloy for laser powder bed fusion by adjusting the Mn content

AbstractShape memory alloys (SMA) are functional materials exhibiting the shape memory effect. Conventional standard manufacturing technologies for shape memory alloys include melting, casting, and forming. These often require additional machining steps to achieve the final component shape, limiting the geometric design. The production of SMAs using additive manufacturing technologies opens up new possibilities, but research has been limited, especially for iron-based SMAs. The present study investigated whether an iron-based SMA alloy powder suitable for the additive manufacturing process can be produced via atomization and subsequently 3D printed with laser powder bed fusion (PBF-LB/M). Two FeMnSi SMAs with varying manganese contents of 23.6% and 28.5% were successfully atomized and laser processed. The effect of the manganese content on the shape memory characteristics was investigated by means of dilatometry. A 5 wt% change in manganese content was found to impact the onset of the keyhole fusion mode for PBF-LB/M as well as the resulting shape memory characteristics. Reduced manganese content is shown to enhance the shape memory effect of FeMnSi.

Whole martensitic transformation process in Fe–Mn–Si–Cr shape memory alloy by improved characterization of volume resistivity

Since Fe–Mn–Si–Cr shape memory alloy (Fe-SMA) has been discovered, an improvement of the shape memory effect (SME) is challenged. Numerous attempts have been made thus far to improve SME. To effectively control such performance, it becomes imperative to comprehend the real-time behavior of the stress-induced martensitic transformation (SIMT), which is strongly related to SME, during deformation. In the past, the measurement of volume resistivity has been proposed and applied to capture the forward SIMT behavior during the tensile loading process at various strain rate. However, it is quite hard to find investigations of the reverse martensitic transformation after the loading, which is more essential for understanding SME. Therefore, a comprehensive evaluation of the whole SIMT process including the loading process at various strain rate is strongly required. In this study, an improved measurement of volume resistivity is established by introducing both the four-probe and active-dummy techniques with a development of a diameter measurement at high temperature. Then, the volume resistivity of Fe-SMA is measured during the whole deformation including loading under tension at various strain rate, unloading, and heating processes. Besides, the microstructures of Fe-SMA after unloading and heating processes are observed, respectively. Finally, the real-time whole martensitic transformation in Fe-SMA is first investigated with the novel optimization method. The negative strain rate of change in volume resistivity during whole processes can be found. More amount of austenite can be obtained at lower strain rate, resulting in a larger SME.

Holding-temperature effects on thermally and stress induced martensitic transformations in an FeMnSiCr SMA

Abstract Dynamic mechanical analysis (DMA) involves applying a mechanical deformation under dynamic regime, with controlled amplitude and frequency, either isothermally or during temperature variation. These experiments, performed with strain sweep or temperature scan enable to monitor internal friction variation or to identify the presence of martensitic transformation that governs the mechanism of shape memory alloys (SMAs), respectively. These features of DMA device can be successfully handled in order to apply isothermal mechanical treatments with various frequencies and amplitudes. In the present paper, the experimental alloy was a Fe-Mn-Si-Cr SMA, obtained by ingot metallurgy, which was hot rolled, and water quenched to retain martensitic phases. The samples were subjected to solution treatments at 1050°C for five different periods of time: 2, 4, 6, 8 and 10 hrs. Heat treatment effects were evaluated by differential scanning calorimetry (DSC) before being mechanically treated by DMA strain sweeps with three different frequencies; 1, 5 and 10 Hz. A precipitation phenomenon was identified by DSC after 8 hrs. solution treatment time, which can be responsible for the decrease of internal friction average values. Storage modulus increased, during DMA strain sweeps, due to work hardening, but experienced a saturation tendency at a strain amplitude of 0.022 % for which a maximum internal friction value of 0.045 has been associated. Microstructural changes were evaluated by scanning electron microscopy.

Effect of Severe Plastic Deformation on the Structure and Magnetic Behaviour of an Fe-Mn-Si Shape Memory Alloy

This work focuses on the relation between severe plastic deformation process via HSHPT and magnetic properties of an Fe–Mn–Si-Cr shape memory alloy. High speed high pressure torsion (HSHPT) was applied on cast state of alloy. Microstructure evolution of severe plastic deformed iron based alloy subjected to different deformation degree were investigated. The microstructure and phase compositions of alloy were characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. The magnetic properties are discussed on the basis of the severe plastic deformation process and the underlying martensitic transformation. Thermomagnetic curves, between 150 K and 390 K and magnetic hysteresis loops at 300 K temperatures were measured. The thermo-resistivity measurements were done by standard four-probe. The magnetic properties are interpreted and correlated with microstructure.

Structural effects of high-temperature plastic deformation process on martensite plate morphology in a Fe-Mn-Si-Cr SMA

An Fe–Mn–Si–Cr shape memory alloy (SMA) was induction melted, in a cold crucible furnace, cast into cylindrical moulds and swaged by means of two hot working processes. From repeatedly hot forged and hot rolled billets, different specimens were prepared with longitudinal and transversal orientations with respect to main plastic deformation direction. After corresponding metallographic preparation, structural analyses comprising optical (OM), scanning electron (SEM) and atomic force (AFM) microscopy observations were performed with the aim to emphasise the structural effects, of the two above mentioned plastic deformation processes on martensite plate morphology, size and distribution. The paper reports the results recorded by OM, SEM and AFM and corroborates martensite plate surface relief with high–temperature plastic deformation process. The results show that martensite plates were shorter and deeper in hot forged specimens as compared to hot rolled ones, an effect which is atypical for Fe–Mn–Si–Cr SMAs.

Hardness-gradient reversion in FeMnSiCr shape memory alloy modules produced by high-speed high pressure torsion

High pressure torsion (HPT) processing technology, consisting in the obtainment of (ultra)fine bulk metallic structure during 2–3 complete rotations of the superior anvil at low speed (~10-1 rpm) under high applied pressure (~ GPa) applied on the lower anvil, has been modified as to allow the application of elevated number of rotation numbers (~102 rpm). By high-speed high pressure torsion (HS-HPT), coned-disk spring shape modules were processed from an as cast Fe-28Mn-6Si-5Cr (mass %) shape memory alloy (SMA). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies revealed that the modules became nanostructured as an effect of HS-HPT processing. After processing, a hardness gradient was obtained along the truncated cone generator, increasing from inner to outer areas, due to different deformation degrees in these zones. After complete flattening, the measurements revealed that the hardness gradient maintained its value but reversed its variation sense.

Effects of hot working procedure on surface relief characteristic in an Fe–Mn–Si–Cr shape memory alloy

The paper reports the evolution of surface relief and shape memory behavior in an Fe–28Mn–6Si–5Cr (mass%) shape memory alloy (SMA) as a function of the applied hot working procedure. The objective of this paper is to discuss the relationship between surface relief characteristic, structural changes and the type of hot working processing. Particular aspects concerning the relief of martensite plates were reported after the alloy was subjected to two types of hot working procedures, namely hot rolling and hot forging at 1273 K. The observations were performed by means of optical (OM) and atomic force (AFM) microscopy, on bi‐dimensional and three‐dimensional micrographs, corroborated with statistical evaluations. Aiming to reveal the presence of solid phase transitions enabled by hot working, differential scanning calorimetry (DSC) measurements were performed between room temperature and 673 K.

Texture evolution and fcc/hcp transformation in Fe–Mn–Si–Cr alloys by tensile deformation

In this study, evolution characteristics of the texture and microstructure of Fe–Mn–Si–Cr shape memory alloys (SMAs) upon tensile deformation were investigated. Electron backscatter diffraction analysis revealed that the tensile deformation in the rolling direction (RD) of sample sheets resulted in the evolution of microstructure containing stress-induced ɛ martensite and texture components belonging to 〈1 1 1〉 and 〈1 0 0〉//RD fibers. Quantitative measurement of ɛ martensite formation in individual austenite grains at the strain levels of 4% and 5% revealed that among the 〈1 0 0〉, 〈1 0 1〉, and 〈1 1 1〉 grains which are oriented along the tensile direction, 〈1 0 1〉 oriented grain showed the highest ɛ martensite formation rate. In the recrystallized state, the volume fraction of 〈1 0 1〉 texture component in 20% tensile deformed samples was slightly higher in the transverse direction (TD) than in the RD. Because of that preferred orientation recovery strain was slightly higher in the TD sample than in the RD sample.

Innovation in producing crane rail fishplate using Fe–Mn–Si–Cr based shape memory alloy

An Fe–Mn–Si–Cr shape memory alloy is applied to make crane rail fishplates. The shape recovery of the reverse martensite transformation from hcp to fcc is utilised to connect finite lengths of rail. Rails connected by normal steel fishplates gradually become separated at the joint and can be damaged by broken flakes in the gap and dents during heavy duty operation of the crane. To eliminate such problems, a sufficient and controlled compressive stress is required at the joint to resist the stress responsible for creating the gap. A quantitative estimate of the load required to separate the joint has been made to calibrate the compressive stress yielded by the reverse martensite transformation of fishplates. An innovative joining technique that ensures adequate and controlled compressive stress at the joined parts has been developed using a Fe–Mn–Si–Cr shape memory alloy together with sophisticated materials design and installation techniques.

Characteristics of Fe–Mn–Si–Cr shape memory alloys in centrifugal casting

Recent application of Fe–28Mn–6Si–5Cr (mass%) alloy as pipe joints in civil engineering constructions has required efficient production of these joints. Centrifugal cast materials have shown better shape recoveries than rolled materials. Crystallographic investigation showed this shape memory effect enhancement was due to a characteristic 〈0 0 1〉 columnar structure extended along the radial direction of joints. Theoretical estimation of shape recovery strain revealed the ideal microstructure made by centrifugal casting and the limitations of shape recovery strain as well.

Elastic Energy Analysis of Carbide and Nitride-Type Precipitates in an Fe–Mn–Si–Cr Shape Memory Alloy

The application of the microscopic theory of elasticity in a discrete lattice model is made on the transitional metal carbide and nitride precipitates formed in the Fe–Mn–Si–Cr shape memory alloy in conjunction with the improvement of shape memory effect (strain) and the improvement of strength. Two distinguishable methods of analysis have been established using the microscopic theory of elasticity in a discrete lattice model: One is the establishment of the description of precipitate and misfit dislocations in Fourier space. The second is the rigorous estimation of interaction energies among precipitate and misfit dislocations. The results could successfully describe the shape of the precipitate observed in the experimental investigation. It was also concluded that the elastic strain energy increases with the lattice parameter of the precipitate. Among the transition carbides and nitrides under investigations, VN, which revealed the minimum value of the elastic energy, is manifested to be the most favored one for the precipitation enhanced Fe–Mn–Si–Cr shape memory alloy. Homogeneously precipitated VN containing materials could show large deformability, higher strength by precipitation hardening and the higher shape recovery strain due to the nucleation sites of the precipitates in its reverse phase transformation.

Improvement of Oxidation Resistance of an Fe–Mn–Si–Cr Shape Memory Alloy by Annealing under Vacuum

Electron microprobe microanalysis (EPMA), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) were used for charactering the surface composition of an Fe–Mn–Si–Cr shape memory alloy. The effect of annealing under vacuum on the surface composition of the alloy was studied. It was found that the amount of manganese decreased, while the amount of chromium increased in a surface layer by annealing. The thickness of the surface layer is of the order of ten micrometers, which is comparable to that obtained by conventional plating or coating. As a result, the surface of the Fe–Mn–Si shape memory alloy annealed under a vacuum exhibited better oxidation resistant than that of the as-lapped alloy.

Crystallography and elastic energy analysis of VN precipitates in Fe–Mn–Si–Cr shape memory alloys

High-resolution electron microscopy investigations are carried out to describe the morphology and crystallography of VN precipitates which are formed in an Fe–28Mn–6Si–5Cr (mass%) shape memory alloy. It is revealed that the shape change from a cube with {1 0 0} interfaces to an octahedral shape with {1 1 1} interfaces occurs on aging in the precipitate. In order to identify the equilibrium shape of the VN precipitate, elastic strain energy of the precipitate has been estimated on the basis of microscopic theory of elasticity. It is found that a coherent precipitate (∼4 nm in edge width) in a cube shape with {1 0 0} surfaces can be formed, at an early stage of precipitation, with no misfit dislocations existing at the interface of the precipitate. It is also shown that the octahedral-shaped precipitate (∼15 nm in edge width) has a minimum elastic energy, among the cube-shaped, sphere-shaped and octahedron-shaped precipitates, only when the misfit dislocations are introduced at the interfaces. The elastic interaction energy between the misfit dislocations and the precipitate–misfit dislocations is estimated for the first time using the Fourier transformed microscopic theory of elasticity.

Characterization of Fe–Mn–Si–Cr shape memory alloys containing VN precipitates

Abstract An Fe–28Mn–6Si–5Cr shape memory alloy containing VN coherent precipitates has been developed so that mechanical properties and shape recovery are high enough to be applied to pipe joining of steel pipes. The proof stress over 400 MPa and 3.4% of recovery strain are obtained on aging, where VN compounds are formed homogeneously in the parent Fe–28Mn–6Si–5Cr material. Electron microscopic observation has indicated that high-density formation of coherent VN precipitates can enhance the proof stress of the alloy. A large amount of shape recovery strain has been obtained from the shape memory alloy containing high densities of VN compounds that exhibit large size.

Effect of phase transformation on cavitation erosion resistance of some ferrous alloys

Cavitation erosion resistance of two Fe–Mn–Si–Cr shape memory alloys and five Cr–Ni–Mn stainless steels has been investigated at speed 45 m s −1 using a rotating disc rig. Based on the localized surface elasticity H e, the effect of phase transformation on cavitation erosion resistance has been discussed. The results show that the effect of phase transformation on the resistance of the tested alloys is significantly influenced by transformation characteristics. The resistance of two stress-induced hexagonal closed-packed martensite Fe–Mn–Si–Cr shape memory alloys is much better than that of three strain-induced body-centered cubic martensite Cr–Ni–Mn stainless steels. The role of phase transformation in the cavitation erosion resistance of the tested alloys is associated with the localized surface elasticity and energy absorption of phase transformation. The predominant factor characterizing the role of phase transformation in the cavitation erosion resistance of the tested alloys is the localized surface elasticity. As the value of H e is close, the resistance can be obviously improved owing to energy absorption of phase transformation.

Cavitation erosion of Fe–Mn–Si–Cr shape memory alloys

Cavitation erosion tests of three Fe–Mn–Si–Cr shape memory alloys were carried out at speed 34 and 45 m/s using a rotating disc rig, and their cavitation damage has been investigated by comparison with a referring 13Cr–5Ni–Mo stainless steel used for hydraulic turbine vanes. The research results proved that the cavitation erosion of the Fe–Mn–Si–Cr shape memory alloys is a failure of low cycle fatigue and fracture propagates along grain boundaries. After 48 h cavitation erosion the cumulative mass losses of the studied alloys at speed 45 m/s are more than theirs at speed 34 m/s; however, the effect of velocity on cavitation damage of the Fe–Mn–Si–Cr alloys is much lower than that of 13Cr–5Ni–Mo stainless steel. The cumulative mass loss of the 13Cr–5Ni–Mo stainless steel are 26.3 mg at speed 45 m/s and 3.2 mg at speed 34 m/s, and the mass losses of the Fe–Mn–Si–Cr alloys are within the range of 3.6–7.3 mg at speed 45 m/s and 2.0–4.1 mg at speed 34 m/s. The surface elasticity of the Fe–Mn–Si–Cr shape memory alloys is better than that of the 13Cr–5Ni–Mo stainless steel, and the effect of surface elasticity on cavitation damage increases with velocity. The excellent surface elasticity of the cavitation-induced hexagonal closed-packed (h.c.p.) martensite plays a key role in contribution of phase transformation to the cavitation erosion resistance of the Fe–Mn–Si–Cr shape memory alloys. The cavitation damage of the studied alloys at speed 45 m/s mainly depends on their surface elasticity, and the variation of 48 h cumulative mass loss (Δ m) as a function of the elastic depth ( h e) can be expressed as Δ m=2.695+[1371.94/(4( h e−46.83) 2+12.751)] with a correlation factor of 0.99345.