Mn Si Cr Ni Shape Research Articles

Unveiling the microstructure evolution and mechanical properties in a gas tungsten arc-welded Fe–Mn–Si–Cr–Ni shape memory alloy

Fe–Mn–Si–Cr–Ni shape memory alloys (SMAs) are unique low-cost materials with shape memory properties that grant them the ability to be used in both functional and structural applications. Such SMAs are especially sought in the construction sector for the creation of new components and/or the reinforcement of damaged ones. In this study, a Fe–17Mn–5Si–10Cr–4Ni–1(V, C) wt% SMA was gas tungsten arc welded, with the objective to investigate the microstructure and mechanical performance changes occurring after welding. A comprehensive assessment of processing, microstructure and properties relationships was established combining microscopy (optical and electron), synchrotron X-ray diffraction, microhardness mapping and tensile testing including cycling assessment of the joint’s functional performance. It is shown that the present SMA has good weldability, with the joints reaching nearly 883 MPa at fracture strain of 23.6 ± 2.1%. Alongside this, several microstructure differences were encountered between the as-received and as-welded condition, including the formation of ferrite and Fe5Ni3Si2 P213 cubic precipitates amidst the fusion zone in the latter region.Graphical abstract

Preparation and properties of Fe–Mn–Si–Cr–Ni shape memory alloy

Preparation and properties of Fe–Mn–Si–Cr–Ni shape memory alloy

Effects of Ti on microstructure and properties of Fe–Mn–Si–Cr–Ni shape memory alloy

The effects of Ti on the microstructure and properties of Fe–20Mn–6Si–8Cr–5Ni shape memory alloy prepared by powder metallurgy were investigated. The results showed that trace Ti elements could improve the alloy's compressive yield strength and fracture strength. With the increase of Ti, a precipitated phase gradually appeared in the alloy, which adversely affected the mechanical properties of the alloy. The addition of Ti increased the lattice microstrain of the alloy, which provided the driving force for the martensitic phase transformation and facilitated the shape memory properties of the alloy. There was a relationship between the Ti content and the corrosion resistance of the alloy, with a small amount of Ti producing dispersion strengthening to improve the corrosion resistance of the alloy, and an excess of Ti causing composition segregation to make the alloy susceptible to corrosion. Ultimately, 0.5 wt% Ti was found to have good overall mechanical properties with a compressive yield strength of 1567 MPa and fracture strength of 2140MPa, its superelastic strain was 2.8% and 3.1% (at 10% and 15% pre-strain), respectively.

On the significance of C and Co on shape memory performance of Fe–Mn–Si–Cr–Ni shape memory alloy

Fe–Mn–Si bearing shape memory alloys are used as structural and functional materials, and have the potential for application in industrial and architectural fields. Fe–Mn–Si–Cr–Ni based shape memory alloys were selected to elucidate the impact of C and Si content on shape memory effect, where Si was replaced by Co. In this regard, Fe–15Mn–5Si–8Cr–5Ni, Fe–15Mn–5Si–8Cr–5Ni-0.1C, and Fe–15Mn–3Si–8Cr–5Ni-0.1C–2Co alloys were designed to simultaneously elucidate the shape memory effect (SME) and mechanical properties in relation to the microstructure. The study indicated that the addition of 0.1 wt% C significantly improved the SME of Fe–Mn–Si–Cr–Ni alloy, and the shape recovery rate of Fe–15Mn–5Si–8Cr–5Ni-0.1C alloy aged at 750 °C for 4 h was maximum at 55.45%. The content of thermally-induced martensite was reduced, which led to decrease in the probability of α′-martensite formation, and promoted inverse transformation of stress-induced martensite and enhanced the shape memory effect. In addition, the strength of the parent phase was enhanced by segregation of elements during aging. Furthermore, the plastic deformation involving slip during stress-induced martensitic transformation was reduced. The reduced content of α′-martensite promoted reversal of stress-induced martensite. On the other hand, Co increased the stacking fault energy and reduced nucleation of stress-induced martensite, such that the number density of stacking faults decreased the shape memory effect of the experimental steel.

Anomalous cyclic oxidation behaviour of a Fe–Mn–Si–Cr–Ni shape memory alloy

Fe–Mn–Si–Cr–Ni alloys are known by their shape memory properties, however, oxidation studies are still incipient. This study evaluated an Fe-17Mn-5Si-10Cr-4Ni-VC alloy on cyclic oxidation tests at 800, 900 and 1000°C. Mass variation was evaluated and oxide layers were examined by SEM, EDS and XRD. An anomalous mass variation was observed: after initial spallation, the material restarted mass gain. The formed Mn-depleted zone changed the austenite microstructure to ferrite on the metal/oxide interface and better oxide anchoring was promoted through the roughness generated. The result makes this alloy an interesting candidate for applications where cyclic oxidation is an important issue.

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.

Characterization of phases in an Fe–Mn–Si–Cr–Ni shape memory alloy processed by different thermomechanical methods

The presence of second phases in FeMnSi-based systems has been widely discussed in the literature, and also in our previous works. In this manuscript, various specimens of an Fe–15Mn–5Si–9Cr–5Ni wt.% (nominal composition) shape memory alloy were processed by different thermomechanical treatments in order to identify the phases present in each condition. TEM-EDS results indicate that the chemical composition of the second phase we found in all cases is close to 19Mn–xSi–12Cr–5Ni at. %, with x varying between 5 and 11, and Fe balance. This is compatible with a Fe5Ni3Si2 type intermetallic phase. It forms in a temperature range between 600 and 900°C, independently of the thermomechanical processing and the coexisting phases. Rietveld refinement of X-ray patterns was done using Maud software, and the phase was found to be isostructural to the pi-phase Cr3Ni5Si2 with space group P213 and lattice parameter equal to 0.6227(7) nm. Phase quantification, crystallite size and accumulated microstrain were also computed as a convenient set of variables, necessary for improving refinement quality. Relevant microstructural information, i.e. stacking fault and twinning probabilities, along with dislocation density, were calculated for austenite, the matrix phase, contributing to the reliability of the determination of the crystallographic characteristics of the second phase.

Effect of copper addition on the microstructure and shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys

The effect of Cu addition on the microstructure and shape recovery behaviour of Fe–Mn–Si–Cr–Ni shape memory alloys has been studied. Microstructural characterization revealed that Cu is soluble in these alloys at least up to 3wt% in the austenite (γ) phase and acts as a strong γ stabilizer. The γ–ε martensite transformation start temperature (MS) and the reverse ε–γ transformation start temperature (AS) were observed to decrease with Cu addition, although at different rates. Copper addition promotes the formation of incoherent Fe5Ni3Si2 type intermetallic π-phase precipitate during ageing and its amount increases with the amount of Cu in the alloy. Most importantly, Cu results in the deterioration of shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys, by increasing stacking fault energy (SFE), by decreasing the resistance to plastic deformation and by lowering the MS temperature of the γ phase.

A Novel Training‐Free Cast FeMnSiCrNi Shape Memory Alloy Based on Formation of Martensite in a Domain‐Specific Manner

AbstractThe proposed key to producing a good shape memory effect in FeMnSi alloys is to reduce or even prevent the collisions between martensite bands. A method for realizing this is to make stress‐induced martensite bands form in a domain‐specific manner. We developed a novel training‐free cast Fe18Mn5.5Si9.5Cr4Ni alloy with residual lathy delta ferrite based on this idea. The recovery strain reached 6.4% only after annealing the cast Fe18Mn5.5Si9.5Cr4Ni alloy. Microstructure observations indicated that the lathy delta ferrite made the stress‐induced martensite form in a domain‐specific manner by first subdividing grains into smaller domains. We hypothesize that through adjusting alloy compositions, solidification parameters, and heat treatment technique, the shape recovery of cast FeMnSi alloys can be further improved. Such a finding will provide a novel method for developing training‐free FeMnSi shape memory alloys.

Wear characteristics of a new type austenitic stainless steel

In order to solve the problem of the poor wear resistance in conventional austenitic stainless steels, a new type austenitic stainless steel was designed based on Fe–Mn–Si–Cr–Ni shape memory alloys in this article. Studies on its wear resistance and wear mechanism have been carried out by comparison with that of AISI 321 stainless steel using friction wear tests, X-ray diffraction, scanning electron microscope. Results showed that the wear resistance of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was better than that of AISI 321 stainless steel both in dry and oily friction conditions owing to the occurrence of the stress-induced γ → e martensitic phase transformation during friction process. This article also compared the corrosion performance of the two stainless steels by testing the corrosion rate. Results showed that the corrosion rate of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was notably lower in NaOH solution and higher in NaCl solution than that of AISI 321 stainless steel.

Thermomechanical behavior of Fe–Mn–Si–Cr–Ni shape memory alloys modified with samarium

The deformation and training behavior of Fe–14Mn–3Si–10Cr–5Ni (wt.%) shape memory alloys containing samarium addition has been studied in the iron-based shape memory alloys. It is noticed that thermomechanical treatment (training) has significant influence on proof stress, critical stress and shape memory behavior of the alloys. The improvement in shape memory behavior can be attributed to the decrease in the proof stress and critical stress which facilitates the formation of ɛ (hcp martensite). It is also observed that alloy 2 containing samarium undergoes less softening as compared to alloy 1 with training which inhibits the formation of ά (bcc martensite) and thus enhances the shape memory behavior. The excessive thermomechanical treatment with increase in the training cycle has led to the formation of ά (bcc martensite) along with ɛ (hcp martensite) in the alloy 1 which appeared to have decline in the shape memory effect. This has been demonstrated by the examination of microstructure and identification of ά (bcc martensite) martensite in the alloy 1 as compared to alloy 2.

Improvement of shape memory effect in an Fe–Mn–Si–Cr–Ni alloy fabricated by equal channel angular pressing

The effects of equal channel angular pressing (ECAP) and subsequent annealing on the microstructures, martensite transformation and shape memory effect (SME) in an Fe–Mn–Si–Cr–Ni alloy were investigated. The results showed that the fined-grained Fe–Mn–Si–Cr–Ni shape memory alloy with the size of several microns was obtained by ECAP. The SME was improved to 95% when bent 4.33% at 200 K.

Quantification of the amount of ɛ martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy by means of electron backscatter diffraction

The determination of the volume fraction of ɛ martensite is of crucial importance to study the shape memory behaviour of an iron-based shape memory alloy (SMA). Several techniques have been used in literature trying to obtain a reliable measurement of this volume fraction. The results of these attempts were, however, not satisfying. In the present work, the amount of ɛ martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy was measured by means of electron backscatter diffraction (EBSD) before deformation, after deformation, and after subsequent annealing. Despite the specific nature of the austenite-to-ɛ-martensite transformation, which affected the image quality parameter, it was proven, through the high confidence index, that a correct indexing of the diffraction pattern of ɛ martensite is possible with this technique. The evolution of the ɛ martensite volume fraction could readily be followed with a reasonable accuracy, through the entire processing. These results showed that EBSD has a high potential to quantify the ɛ martensite fraction in an iron-based SMA.

Effects of homogenization time on the both martensitic transformations and mechanical properties of Fe–Mn–Si–Cr–Ni shape memory alloy

In this work, we have investigated some physical and mechanical properties of martensitic transformation in Fe–12.5%Mn–5.5%Si–9%Cr–3.5%Ni (wt) shape memory alloy. Compressive deformation at room temperature, which applied to ɛ(hcp) phase within austenite phase cause the formation of α′(bcc) martensite crystals at intersection of prior ɛ martensite plates and also new formed ɛ martensite plates were observed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction methods were applied for revealing microstructural and crystallographic features of martensitic transformations in this alloy. Also effect of prior homogenization time on the martensitic transformations and their mechanical features were stress–strain curves.

Analysis of wear resistance and its mechanism in an Fe–Mn–Si–Cr–Ni shape memory alloy

The wear resistance and its mechanism in an Fe–17Mn–5Si-10Cr–4Ni shape memory alloy have been studied by means of friction wear tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results show that the wear resistance of Fe–17Mn–5Si–10Cr–4Ni alloy in oily friction is remarkably higher than that in dry friction. Compared with stainless steel, the wear resistance of Fe–17Mn–5Si–10Cr–4Ni alloy in dry friction is lower, but higher remarkably in oily friction. From XRD analysis and SEM observations, it is found that there are large amounts of ɛ martensite in the worn surface of Fe–17Mn–5Si–10Cr–4Ni alloy after oily friction, but no ɛ martensite in the worn surface of the alloy after dry friction. From these results, it is concluded that the mechanism for high wear resistance of Fe–Mn–Si shape memory alloy in oily friction is stress-induced γ → ɛ martensitic transformation.

Composition design principles for Fe–Mn–Si–Cr–Ni based alloys with better shape memory effect and higher recovery stress

To investigate the factors affecting the recovery stress in Fe–Mn–Si–Cr–Ni shape memory alloy, the transformation behavior under constraints was studied through in situ electrical resistivity analysis. The recovery stress equations during constrained heating and cooling have been established. Composition design principles for Fe–Mn–Si–Cr–Ni shape memory alloys with better shape memory effect and higher recovery stress at room temperature have been proposed and they are different from ones which produce only the best shape memory effect. To test the principles, Fe–14.8Mn–4.78Si–7.95Cr–4.16Ni–0.18C alloy was designed according to the principles. The result shows that the recovery stress of the designed alloy is 360 MPa, 135 MPa higher than that of the alloy designed according to the principles for only obtaining the best shape memory effect under no constraints. Decreasing the amount of the plastic deformation and the ɛ martensite transformation can remarkably increase the recovery stress.

Effect of carbon content on mechanism of thermal–mechanical cycling in Fe–Mn–Si–Cr–Ni–C alloys

The effects of carbon addition on the optimum annealing temperature and the microstructures in Fe–Mn–Si–Cr–Ni–C alloys were investigated. The results show that the addition of carbon raises the optimum middle annealing temperature of thermal–mechanical cycling in Fe–Mn–Si–Cr–Ni shape memory alloys. When the carbon content is lower than 0.12 wt.%, the optimum annealing temperature is about 600 °C, and the mechanism of improving shape memory effect (SME) through the thermal–mechanical cycling is the increase of crystal defect. When the carbon content is 0.18 wt.%, the optimum temperature is raised to 1150 °C, and the mechanism of improving SME is the presence of Cr 23C 6 at grain boundaries, not the increase of crystal defect.

Structural refinement and strengthening of an Fe–Mn–Si–Cr–Ni shape memory alloy by high-speed rolling

The shape recovery under different opposing stress conditions and the various microstructures obtained have been examined following high-speed rolling. An iron-based shape memory alloy that can hardly be rolled at a high strain rate is shown to be capable of being rolled down to 50% of its original thickness by single pass. The shape recovery under the opposing stress applied during reverse transformation is found to increase notably as a result of structural refinement induced by high-speed rolling. In these tests, the specimens were twinned or transformed into hcp and bcc nanophases by the high-speed rolling performed at strain rates as high as ∼10 4 s −1. The current study emphasizes the contribution of the resultant structural refinement to the strengthening of the shape memory alloy.

Atomic force microscope study of stress-induced martensite formation and its reverse transformation in a thermomechanically treated Fe–Mn–Si–Cr–Ni alloy

Consecutive observations of the stress-induced martensite formation and its reversion by atomic force microscopy have been carried out for the fcc/hcp transformation in the thermomechanically treated sample of an Fe–Mn–Si–Cr–Ni shape memory alloy. It is found that thin martensite plates of 0.1–0.2 μm thickness, which are the same martensite variant on the same habit plane, are formed one after another at the immediate neighbor of the existing martensite plate. These martensite plates make a group of several plates within the width of 1–2 μm. This formation mode of martensite is compared with those martensite plates observed by high resolution microscopy and optical microscopy and it is concluded that the basic mode of the stress-induced transformation is that each martensite plate is induced to relieve the shape strain of the existing martensite plate for all the observed magnifications ranging from several hundreds to several millions. The first martensite plate formation is presumed to occur at the pre-existing stacking fault in austenite. In the reverse transformation on heating, it is likely that each martensite plate is reverse-transformed one after another by reverse movement of the Shockley partial dislocations residing at the tip of the plate. This seems to be true for every range of the observable magnification from namometres to microns. Such a reverse transformation mode ensures a good shape memory effect in Fe–Mn–Si-based shape memory alloys.

The influence of deformation on the microstructure and transformation temperatures of Fe–Mn–Si–Cr–Ni shape memory alloys

Abstract The characteristics of the martensitic transformation in a Fe–Mn–Si–Cr–Ni alloy and in a similar alloy containing also Co were investigated in terms of the amount and type of deformation: cold rolling, with 19 and 33% reduction of area, and wire drawing, with 20% reduction of area in the last pass. The temperatures of start and finish of the reverse transformations were measured by dilatometry. The constitution and microstructure of the alloys were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy and magnetic force microscopy. The cold forming processes applied to the alloys induced the formation of e and the α ′ martensites, even when the start temperature of thermal martensite was well below room temperature. An increase of up to 500 K in the characteristic temperatures of the reverse transformation, when compared to thermal martensite, was also observed.