Fe Mn Si Cr Ni Shape Memory Alloy Research Articles

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.

Effect of the Cr Content on Shape-Memory Effect and Corrosion Resistance of FeMnSiCrNi Shape-Memory Alloys

Effect of the Cr Content on Shape-Memory Effect and Corrosion Resistance of FeMnSiCrNi Shape-Memory Alloys

Properties of FeMnSiCrNi Shape-Memory Alloys with (0.1~0.5wt%) Nb Additions

Properties of FeMnSiCrNi Shape-Memory Alloys with (0.1~0.5wt%) Nb Additions

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.

Alloy design of FeMnSiCrNi shape-memory alloys related to stacking-fault energy

The stacking-fault energy (γ sf ) of iron-based shape-memory alloys was calculated by the extended dislocation-node method. The results show that Ni and Mn increase the γ sf of the alloys with an austenite structure, while Cr and Si decrease it. An expression relating the alloying elements of Ni, Cr, Mn, and Si to the γ sf of the alloys is established. Moreover, in terms of the γ sf values of the alloys and the Schaeffler diagram, the alloy design of iron-based shape-memory alloys is carried out. It is found that the alloy having the lowest γ sf has the best shape-memory effect (SME), with a martensite transition temperature (M s ) being a little lower than ambient temperature. The corresponding composition of the alloy is located in the γ-phase zone near the phase line between the γ and γ+e phase zone and the triple point of α+γ+e in the Schaeffler diagram.

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.

The γ → ε-martensitic transformation and its reversion in the FeMnSiCrNi shape-memory alloy

The hcp martensitic transformation and its reversion in an Fe-16.86Mn-4.50Si-10.30Cr-5.29Ni alloy have been studied. The fine structure and morphologies of e-martensite were systematically investigated using transmission electron microscopy. It is found that, in the overlapped region of the stacking faults, the nucleus of e-martensite may form by shear and grow along the {111}γ plane, which is not the stacking fault plane. The nucleus may grow into a small, thin plate and may act as the basal structure unit of e-martensite. When the stress increases, the thin plates continue to grow into an e-martensite ribbon along this preferred orientation and, during the subsequent recovery annealing, the reverse transformation of these basal structure units occurs. As a result, the e-martensite plates shrink in both length and thickness.

Microstructural analysis of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: Part I—calculated description of the microstructure

The γ(f.c.c.)– ε(h.c.p.) martensitic transformation is achieved by the introduction of stacking faults on each second compact plane of the f.c.c. structure. These stacking faults are created by the motion of Shockley partial dislocations. Depending on the Burgers vector of these dislocations, the martensite does not require a macroscopic shape change (self-accommodated martensite) or a homogeneous lattice shape change (monopartial martensite). Based on the monopartial nature of the stress-induced martensite, a model describing the martensitic morphology in the Fe–Mn–Si based shape memory alloys is presented. The theoretical results are compared with some observations in a Fe–Mn–Si–Cr–Ni shape memory alloy.

Microstructural analysis of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy : Part II: Transformation reversibility

The shape memory effect exhibited by some Fe–Mn–Si based alloys is related to the γ(f.c.c.)- ε(h.c.p.) martensitic transformation. In these alloys, the shape memory effect, incomplete even for low deformation rates, drastically decreases when the deformation rate increases. The evolution of the microstructure of the martensite, and particularly the interaction of martensite plates and grain boundaries, is studied in a Fe–Mn–Si–Cr–Ni alloy to determine the origin of the loss of shape memory. The influence of the back-stress created by the dislocations located at the tip of the martensite bands is pointed out. The reduction of the shape memory is attributed to the accommodation of this back-stress when the deformation rate, i.e. the martensite band width, increases. The same kind of analysis is used to analyse the problem of superelasticity.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.