Fe Mn Si-based Shape Memory Alloys Research Articles

Microstructural mechanism underlying the stress recovery behavior of a Fe–Mn–Si shape memory alloy

Recovery stress governs the prestress strengthening effect of Fe–Mn–Si-based shape memory alloys in civil engineering applications. Although the effects of grain size, texture, and precipitates on recovery stresses have been extensively studied, the generation of recovery stress has not been thoroughly discussed regarding martensite growth behavior, especially considering the interaction of martensite variants. This study investigates the stress recovery and ε martensite growth behavior of a Fe–30Mn–6Si–5Cr (wt.%) alloy subjected to varying levels of deformation. The Fe–30Mn–6Si–5Cr (wt.%) alloy exhibits a yield strength of approximately 450 MPa, with an ultimate elongation of 44 % and a tensile strength of 950 MPa observed in uniaxial incremental cyclic tensile tests. Additionally, the results reveal an initial increase in recovery stress with pre-strain, peaking at 7 %, followed by a subsequent decrease with further deformation. With increasing pre-strain, the primary ε martensites initially grow parallelly, followed by the appearance of secondary martensites with a different orientation, intersecting multiple parallel primary martensites along with the increasing density of dislocations and stacking faults. A microstructural mechanism is proposed to elucidate the stress recovery behavior, encompassing the impact of primary martensites and secondary martensites interaction, and the influence of dislocations and stacking faults. This work offers microstructural scenarios that help to understand the generation of recovery stress and provides guidance for regulating recovery stress in Fe–Mn–Si-based shape memory alloys for civil engineering applications requiring prestress strengthening.

Fabrication of FeMnSi-based shape memory alloy components with graded-microstructures by laser powder bed fusion

This work demonstrates the tailoring of the microstructure of two FeMnSi-based shape memory alloys during laser powder bed fusion. Significant variations in the microstructure of the parts can be induced by modifying the scanning speed or the scanning strategy during the process. Specifically, the volume phase fractions, texture, and grain size and morphology change with the applied processing parameters as the thermal history experienced by the material is modified. Additionally, the generally undesired Mn evaporation during laser melting can be deliberately exploited to manipulate the phase composition in the final microstructure. Taking advantage of this, graded-microstructure samples with tailored amounts of the two phases (bcc-δ and fcc-γ) and distinct microstructure at different locations can be fabricated. A good combination of pronounced shape memory effect and ultrahigh strength can be achieved in this way. The findings of this study offer valuable insights that can drive future research and advancements in additive manufacturing, opening exciting opportunities for the fabrication of multifunctional and high performance materials.

Influence of thermomechanical treatment on the shape memory effect and pseudoelasticity behavior of conventional and additive manufactured Fe–Mn–Si–Cr–Ni-(V,C) shape memory alloys

This study evaluated the influence of heat treatment and thermomechanical training on the microstructural evolution and mechanical characteristics of conventional and additive-manufactured FeMnSi-based shape memory alloys. The conventional samples were produced by casting and rolling. The additive-manufactured samples were manufactured using the laser powder bed fusion (L-PBF) technique. Both specimens were subjected to the same heat treatment and thermomechanical training. The heat treatment involved solution annealing at 1050 °C for 2 h and aging at 750 °C for 6 h, and the thermomechanical training concluded with a 4% elongation at ambient temperature followed by annealing at 250 °C for 15 min. This training cycle was repeated four times for each sample after heat treatment. The heat treatment improved the pseudoelasticity and shape memory effect of the samples. Although training further enhanced the pseudoelasticity, it also reduced the shape memory effect. Thermomechanical training led to the formation of a large number of stacking faults, which facilitated the inverse phase transformation of martensite (ε) to austenite (γ) during unloading, resulting in improved pseudoelasticity. The heat-treated additive-manufactured samples showed the highest total recovery strain owing to the pseudoelasticity and shape memory effect. This characteristic could be due to the smaller grain size and higher volume fraction of precipitates. The precipitates and grain refinement improved the conditions for partial dislocation motion by increasing the back stresses on the martensite tip.

Simulation and experimental characterization of VC precipitation and recovery stress formation in an FeMnSi-based shape memory alloy

In this study, TC-PRISMA was employed to simulate the precipitation (number density and radius) of an FeMnSi-based shape memory alloy (FeMnSi-SMA), with a chemical composition of Fe–17Mn–5Si–10Cr–4Ni-1(V, C) (wt%), during one-step and two-step aging processes. In addition, transmission electron microscopy (TEM) was used to study the microstructures of the specimens, particularly the VC precipitates, under several selected two-step aging conditions to validate the simulation results. Furthermore, thermomechanical characterization was conducted to investigate the recovery stress of the FeMnSi-SMA under the selected two-step aging conditions. It was found that, compared to only one-step aging, the first aging at a lower temperature (600 °C) followed by a second aging at a higher temperature (670 °C) can take advantage of both high nucleation rate and high growth rate. This achieves a similar precipitation state in a significantly shorter time (approximately 1/6 of the total duration of the one with one-step aging). The recovery stress value (509 MPa) corresponding to the two-step aging condition (first step: 600 °C for 20 h; second step: 670 °C for 6 h) shows a similar stress value range (514 MPa) corresponding to the one-step aging condition (600 °C for 144 h); however, the fracture strain is 40% higher. The number density and VC radii simulated by TC-PRISMA are generally consistent with the TEM observations.

Effect of low-temperature precipitates on microstructure and pseudoelasticity of an Fe–Mn–Si-based shape memory alloy

Fe–Mn–Si-based shape memory alloys (Fe-SMAs) have attracted much research attention due to their potential applications for vibration mitigation, energy dissipation, and re-centering in the construction sector. Because of the crucial impact of precipitation on the pseudoelasticity (PE) behavior of Fe-SMAs, the equilibrium phase diagram of an Fe–17Mn–5Si–10Cr–4Ni–1(V C) (wt%) SMA was used in this study to identify a low-temperature precipitate and study its effect on the microstructure and PE of the alloy after a low-temperature aging process. Transmission electron microscopy (TEM) studies revealed that aging at 485 °C for 6 h after aging at 750 °C for 6 h led to the precipitation of fresh, parallelogram-shaped, (Cr–V–C)-rich precipitates along with elliptical-shaped, V-rich precipitates in the austenite grains of the recrystallized samples. Numerous parallel stacking faults (SFs) were formed due to the presence of the precipitates within the austenite grains. It is postulated that such an arrangement of SFs can further improve the PE by reducing the activation energy for the nucleation of ɛ-martensite laths and inhibiting them from colliding with each other and consequent formation of α'-martensite, resulting in a residual strain reduction to 2.7% after 4.0% tensile straining. • The equilibrium phase diagram of an Fe–17Mn–5Si–10Cr–4Ni–1(V C) (wt%) shape memory alloy is studied to identify a low-temperature precipitate. • The effect of the low-temperature precipitate on the microstructure and pseudoelasticity of the alloy is investigated. • Primary aging at 750 °C followed by aging at 485 °C leads to the formation of fresh, parallelogram-shaped, (Cr–V–C)-rich precipitates along with elliptical-shaped, V-rich precipitates. • It is postulated that the parallel alignment of stacking faults can improve the pseudoelasticity by reducing the probability of the collision of ε-martensite laths with each other and consequent α’-martensite formation.

Thermodynamic modeling and computational predictions of NbC precipitation in Fe–Mn–Si-based shape memory alloys by the classical nucleation and growth theories

NbC precipitation in Fe–Mn–Si-based alloys is an effective method to improve the shape memory effect. In this study, the precipitation behavior was investigated using a thermodynamical model to understand the mechanism and optimize the precipitates for a better performance of Fe–Mn–Si-based shape memory alloys. The influence of alloying elements can be considered in the model by introducing interaction parameters. The precipitate size distribution, mean size, precipitate volume faction, and number density of three typical Fe–Mn–Si-based alloys with different NbC addition amounts were calculated. The results indicated that the mean size could be decreased significantly as the NbC addition increased from 0.5% to 1.0%, while the precipitate volume fraction and number density showed obvious increments. The Fe–28Mn–6Si–5Cr alloys exhibited smaller mean sizes and higher number densities than the Fe–14Mn–6Si–9Cr–5Ni and Fe–21Mn–6Si–9Cr–5Ni alloys. It was also found that the precipitate size distribution showed no evident change as the aging time increased from 0.5 h to 2 h except for the Fe–28Mn–6Si–5Cr–0.5NbC alloy in which the precipitates began to coarsen after about 1.25 h.

Enhanced pseudoelasticity of an Fe–Mn–Si-based shape memory alloy by applying microstructural engineering through recrystallization and precipitation

The aim of this work is to provide a novel understanding of pseudoelasticity mechanisms in an FeMnSi-based shape memory alloy and to utilize the identified parameters to control and enhance the mechanical behavior of the alloy. The alloy was processed by employing caliber rolling to an equivalent strain of 0.25 at room temperature. Various heat treatments from 530 to 1000 °C were applied to study the microstructural evolution and pseudoelasticity behavior during short-term post-deformation annealing (PDA) and aging. A minimum residual strain of 2.85% was achieved after 4% loading in tension by annealing the cold-worked sample at 925 °C for 50 min followed by aging at 750 °C for 6 h; this is the lowest ever reported residual strain for this alloy. Moreover, the absorbed energy increased from 17 to 22 J/cm3, indicating a 30% enhancement compared with the as-received aged sample. These improvements in pseudoelasticity and absorbed energy make this alloy more suitable for seismic damping application by providing more recentering after energy dissipation. The improvements are mainly attributed to grain refinement, which stimulates a uniform distribution of precipitates inside the austenite grains after PDA and aging. Additionally, grain refinement modifies the morphology and size of precipitates, resulting in an increased number of stacking faults and a high volume fraction of ε-martensite, and diminishes the probability of the intersection of ε-martensite laths with each other and subsequent α′-martensite formation.

Enhanced Pseudoelasticity of an Femnsi-Based Shape Memory Alloy by Applying Microstructural Engineering Through Recrystallization and Precipitation

Enhanced Pseudoelasticity of an Femnsi-Based Shape Memory Alloy by Applying Microstructural Engineering Through Recrystallization and Precipitation

Formation of metastable bcc-δ phase and its transformation to fcc-γ in laser powder bed fusion of Fe–Mn–Si shape memory alloy

The phase transformation behavior from metastable bcc-δ to fcc-γ in Fe–Mn–Si based shape memory alloy processed by laser powder bed fusion was investigated. Primary δ phase formed in this alloy due to the rapid cooling and fast solidification. Bcc-δ to fcc-γ phase transformation occurred when the volumetric energy density was high due to the process-inherent heat treatment effect. Transmission electron microscope studies confirmed Kurdjumov-Sachs orientation relationship between bcc-δ and fcc-γ phases. The analytical results revealed that the phase transformation occurred by a combined displacive-diffusional mechanisms.

Effect of Phase Changes on the Axial Modulus of an FeMnSi-Shape Memory Alloy.

The axial modulus ESMA(κ) of FeMnSi-based shape memory alloys (FeMnSi-SMAs) is a parameter introduced in this study to characterize the relationship between stress and strain behavior at the early stage of tensile loading. ESMA(κ) can be used to correctly estimate and model the interaction forces between FeMnSi-SMAs and other materials. Unlike the conventional Young’s modulus, which is usually given at room temperature, the ESMA(κ) is evaluated at different temperatures and strongly depends on phase transformation and plastic deformation. This study investigated the evolution of ESMA(κ) during and after pre-straining as well as in the course of the activation processes. The effect of different factors (e.g., phase transformation and plastic deformation) on the magnitude of ESMA(κ) is discussed. The result shows that the ESMA(κ) can differ significantly during activation and thus needs to be modified when interaction forces between FeMnSi-SMAs and other substrates materials (e.g., concrete) must be modeled and evaluated.

Influence of thermal treatment conditions on recovery stress formation in an FeMnSi-SMA

Abstract In the current study, a broad experimental program was designed and conducted to investigate the effect of aging conditions (i.e., aging temperature and time) on the thermomechanical properties (0.1% yield stress, pseudoelasticity, and recovery stress) of an FeMnSi-based shape memory alloy (FeMnSi-SMA) with a chemical composition of Fe–17Mn–5Si–10Cr–4Ni-1(V, C) (wt.%). In addition, transmission electron microscopy (TEM) was used to study the specimens' microstructures under several selected aging conditions. It was found that the aging conditions significantly affected the formation and distribution of precipitates and stacking faults in the aged specimens. At lower temperatures (e.g., 600 and 660 °C), numerous fine precipitates were randomly dispersed both in the austenite matrix and on/near stacking faults, resulting in higher yield stress and recovery stress. At elevated temperatures (e.g., 774 °C), the majority of the precipitates formed on/near stacking faults, leading to a lower yield stress and recovery stress. Besides, as the aging time increased, the pseudoelasticity increased at all aging temperatures.

Grain orientation dependence of the forward and reverse fcc ↔ hcp transformation in FeMnSi-based shape memory alloys studied by in situ neutron diffraction

The grain orientation dependence of the deformation-induced forward fcc→hcp and reverse hcp→fcc martensite transformation of a FeMnSi-based shape memory alloy was studied by in situ neutron diffraction during cyclic loading. A deformation-induced fcc→hcp transformation is observed during tensile straining to +2%. The hcp martensite phase that forms under tension partially reverts to fcc austenite upon subsequent compression from +2% → −2% for the {220}, {331} and {111} grain families aligned with respect to the loading direction but not for the {200} grain family. The martensite formation and the reversion of the individual grains can be explained by considering grain orientation dependent Schmid factors of the {111}<112> slip system underlying the fcc to hcp transformation. While for post-yield elastically compliant grains the Schmid factor of the leading partial dislocation is larger than that of the trailing partial dislocation, the opposite is true for post-yield elastically stiff grains. The former grains show a phase reversion, i.e. hcp→fcc upon compression, the latter grains do not transform back to fcc. EBSD characterization confirms the phase reversion for a <541> orientated grain by the disappearance of hcp bands. Martensite bands, which have not reverted to austenite during compression, showed a thickening. The thickening of existing bands during compression is associated with the activation of a second slip system.

Fabrication of Ferrite-Coated Magnetic Fe–Mn–Si–Cr–Ni Alloy Utilizing Selective Oxidation of Mn Element

The development of magnetic properties extends application fields of Fe–Mn–Si-based shape memory alloys (SMAs) into the field of electromagnetic sensors and actuators. In the previous studies, magnetic Fe–Mn–Si-based shape memory ribbons have been prepared by the melt-spinning technique. However, the melt-spinning technique could only obtain 2-D thin materials. In this paper, a ferrite-coated Fe–18.81Mn–5.61Si–9.31Cr–5.36Ni bulk shape memory material, exhibiting both magnetic and shape memory functions, was fabricated by means of the oxidation at 1173 K for different times. The ferritic layer was depleted in the Mn element due to the selective oxidation of the Mn element, and its growth behavior followed the parabolic law. Furthermore, we prepared not only ferrite/austenite/ferrite sandwich sheets but also matrix at ferrite core–shell wires in the case of ferrite-coated Fe–18.81Mn–5.61Si–9.31Cr–5.36Ni alloy. The shape recovery ratio of sandwich sheets decreased to 74.1% from 78.3% of the ferrite-free state while their saturation magnetization increased to 7.8 emu/g. The shape recovery ratio of core–shell wires decreased to 54.8% from 78.3% of the ferrite-free state with increasing the saturation magnetization to 36.2 emu/g. For both sandwich sheets and core–shell wires, there is a tradeoff relationship between shape memory effect (SME) and saturation magnetization. Accordingly, it is necessary to strike a balance between their shape memory and magnetic properties according to engineering applications.

On the origin of the improvement of shape memory effect by precipitating VC in Fe–Mn–Si-based shape memory alloys

We studied the role of VC precipitation in improving the shape memory effect (SME) of the as-solution treated Fe–Mn–Si-based shape memory alloys by examining the microstructures developed during aging and deformation using transmission electron microscopy and electron channeling contrast imaging. Our results suggest that VC particles are not the only product of aging. Upon aging at 650 °C, the precipitation of VC particles is accompanied by the formation of profuse dislocations (2.26 ± 0.098 × 1013 m−2). In this case, the SME is not improved compared to the as-solution treated reference state. Upon aging at high temperatures (700–900 °C), a number of stacking faults are formed accompanying the VC precipitation and the SME is significantly improved, where the recovery ratios reach almost twice that of the as-solution treated state (<50%). For these high-temperature aged states, in situ straining experiments reveal that the stacking faults rather than the VC particles play an important role in the stress-induced martensitic transformation, leading to the formation of very thin (<3 nm) martensite plates with a single crystallographic variant within each grain. These martensite plates are in contrast to the very thick (from tens to hundreds of nanometers) and multi-variant martensite plates that prevail in the as-solution treated state. By comparing the characteristics of the martensite plates between the as-solution treated and the high-temperature aged states, the reasons for the improvement of SME by precipitating VC were discussed.

Recovery stress formation in FeMnSi based shape memory alloys: Impact of precipitates, texture and grain size

FeMnSi based shape memory alloys are considered for engineering applications such as prestressing of concrete or coupling devices. For this, a high recovery stress is desired which forms when the FeMnSi alloy is first elongated and then heated above the austenite transformation temperature under mechanical constraints. In this work, we study the impact of three different microstructural properties on the recovery stress of Fe17Mn5Si10Cr4Ni1(V,C) shape memory alloys: Precipitates, texture and grain size. Precipitates allowed varying the recovery stress in a broad range between 255MPa and 564MPa. A recovery stress of 564MPa was achieved by a combination of a high materials yield strength σ0.1 of 536MPa and a shape recovery of 14%. The [001] grain orientation with a low Schmid factor has a beneficial impact on the recovery stress due to a higher yield strength when compared with the [011] orientation. A decrease of the grain size caused a Hall-Petch strengthening but had no influence on the recovery stress. However the pseudo-elasticity increased for a decrease of grain size.

Characterization of the deformation and phase transformation behavior of VC-free and VC-containing FeMnSi-based shape memory alloys by in situ neutron diffraction

The stress-induced fcc-austenite to hcp-martensite transformation in the iron based shape memory alloy (SMA) Fe-17Mn-5Si-10Cr-4Ni with and without VC precipitates is investigated by in-situ neutron diffraction measurements upon uniaxial loading and unloading. Based on experimentally derived elastic moduli the critical resolved shear stress (CRSS) for the fcc to hcp phase transformation was calculated. VC precipitates promote the martensite transformation by shifting the CRSS from 152MPa to 85MPa. A nearly perfect plastic behavior is found for the (220) grains with a high Schmid factor of 0.47. While (220), (111) and (200) oriented grains exhibit a phase transformation, (311) grains plastically deform solely by slip. During plastic deformation a load redistribution from soft behaving (220) grains to hard behaving (200) orientated grains takes place. The presence of VC precipitates leads to a broadening of the stress interval at which a martensite transformation is induced. This is explained by spatially heterogeneously distributed martensite transformation temperatures which are caused by VC precipitates. The microstructural reason for pseudo-elasticity is found to be a combination of back transformation from hcp to fcc and a reversible motion of Shockley partial dislocations.

Creep and stress relaxation of a FeMnSi-based shape memory alloy at low temperatures

The creep and stress relaxation behavior of a Fe–17Mn–5Si–10Cr–4Ni–1(V,C) (wt%) shape memory alloy at low homologous temperatures (−45°C<T<50°C) was systematically studied in stress and strain controlled tensile tests. At constant stresses at −45°C, the alloy exhibits pronounced creep up to 0.6% at 600MPa after only 30min holding time. If the strain is kept constant, a pronounced stress relaxation of up to 10% of the initial stress was observed. The final creep strains, the creep rates at a constant stress as well as the stress relaxation at a constant strain increase with decreasing temperature. In addition, the change of the recovery stress as a function of time in a restrained sample after 4% elongation and heating to different constant temperatures was monitored. It was observed that the increase of the final recovery stress is more pronounced when the holding temperature is increased. This behavior was explained with the time and temperature dependent formation of stress induced ε-martensite from the parent γ-austenite phase during mechanical loading according to the model of Kajiwara et al. as well as with the increased number of stacking faults at lower temperatures, which serve as nucleation sites for the ε-martensite formation.

Relationship among grain size, annealing twins and shape memory effect in Fe–Mn–Si based shape memory alloys

In order to clarify the relationship among grain size, annealing twins and the shape memory effect in Fe–Mn–Si based shape memory alloys, the Fe–21.63Mn–5.60Si–9.32Cr–5.38Ni (weight %) alloy with a grain size ranging from 48.9 μm–253.6 μm was obtained by adjusting the heating temperature or heating time after 20% cold-rolling. The densities of grain boundaries and annealing twins increase with a decrease in grain size, whereas the volume fraction and width of stress-induced ε martensite after 9% deformation at Ms + 10 K decrease. This result indicates that grain refinement raises the constraint effects of grain boundaries and annealing twins upon martensitic transformation. In this case, the ability to suppress the plastic deformation and facilitate the stress-induced ε martensite transformation deteriorates after grain refinement owing to the enhancement of the constraint effects. It is demonstrated by the result that the difference at Ms + 10 K between the critical stress for plastic yielding and that for inducing martensitic transformation is smaller for the specimen with a grain size of 48.9 μm than for the specimen with a grain size of 253.6 μm. Therefore, the shape memory effect declined by decreasing the grain size.

Effect of titanium addition on shape memory effect and recovery stress of training-free cast Fe–Mn–Si–Cr–Ni shape memory alloys

The shape memory effect and recovery stress of cast Fe–17.2Mn–5.28Si–9.8Cr–4.57Ni (18Mn) and Fe–17.5Mn–5.29Si–9.68Cr–4.2Ni–0.09Ti (18Mn–Ti) alloys have been investigated by optical microscopy, scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and resistivity–temperature curves. The cast 18Mn and 18Mn–Ti alloys solidified as the ferritic mode for which liquid phase fully transforms into primary δ ferrite. The role of titanium is to indirectly refine the austenite through refining the primary δ ferrite. In this case, the austenitic grains of the cast 18Mn alloy were much bigger than that of the cast 18Mn–Ti alloy, although the two alloys underwent δ→γ phase transformation. Grain refinement suppresses the stress-induced ε martensitic transformation, and thus the shape memory effect of the cast 18Mn–Ti alloy is worse than that of the cast 18Mn alloy. On the contrary, the maximum recovery stress and the recovery stress at room temperature are higher for the cast 18Mn–Ti alloy annealed at 1073K for 30min than for the cast 18Mn alloy annealed at 973K for 30min, because grain refinement suppresses the relaxation of recovery stress caused by the plastic deformation and the stress-induced ε martensitic transformation during cooling process. It is difficult to obtain the training-free cast Fe–Mn–Si based shape memory alloys with excellent shape memory effect and high recovery stress only by grain refinement.

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.