Fe Mn Al Ni Shape Memory Alloys Research Articles

Abnormal Grain Growth and Pseudoelasticity of Industrially Processed Fe–Mn–Al–Ni Shape Memory Alloy Joined by Metal Inert Gas Welding

The effect of metal inert gas welding on the microstructure, abnormal grain growth and the pseudoelastic properties of industrially processed Fe–Mn–Al–Ni shape memory alloy sheets were studied. Square-butt welds were manufactured using similar filler material. The influence of alternating mean arc linear energy on the microstructure of the individual zones is shown. A correlation between the process parameters, the associated heat input, the grain morphology and the α/γ-ratio could be deduced. As the mean arc linear energy increases, the α/γ-ratio in the fusion zone and the heat-affected zone increases. To evaluate the influence of the welding process on abnormal grain growth, a post-weld cyclic heat-treatment was carried out. Although no loss of major alloying elements in the fusion zone was observed after welding, metal inert gas welding has a significant effect on grain growth upon cyclic heat treatment. While abnormal grain growth occurred in the base material, a polycrystalline microstructure characterized by significantly smaller grain sizes was visible in the former fusion zone. Incremental strain tests revealed severe plastic deformation near the grain boundaries in the former fusion zone. However, the grain boundaries of the bamboo-like grown grains in the base material turned out to be more critical for structural failure.

Effects of V addition on microstructure and pseudoelastic response in Fe–Mn–Al–Ni alloys

Avoiding quench cracks and controlling the formation process of NiAl precipitates are essential to regulate the microstructure and pseudoelastic properties of Fe–Mn–Al–Ni shape memory alloys (SMAs). In this study, we investigated the effect of 1.5 at.%V addition on the quench sensitivity of Fe–Mn–Al–Ni SMAs and the precipitation behavior of NiAl precipitates in air-cooled Fe–Mn–Al–Ni–V SMAs. It is found that the V addition significantly inhibits the precipitation process of non-transforming γ-phase. Polycrystalline sample of bamboo structure was prepared by abnormal grain growth and tested by incremental strain cycling tensile test at room temperature. The polycrystalline Fe–Mn–Al–Ni–V SMAs obtain fully reversible thermoelastic martensitic transformation without quenching, which also prevents the formation of quenching cracks. Single crystal samples were further prepared by directional recrystallization. High number-density and relatively large-size coherent β-NiAl precipitates were obtained to strengthen the matrix in the single crystal without aging, resulting in large stress hysteresis and irrecoverable strain. After aging Fe–Mn–Al–Ni–V single crystal for 3 h at 200 °C, both the stress hysteresis and irrecoverable strain decreased, thereby achieving good pseudoelasticity in Fe–Mn–Al–Ni–V SMAs.

Structural and superelastic properties of Fe–Mn–Al–Ni shape memory alloy sheets produced on industrial process routes by hot rolling

In the present study the structural and functional properties of Fe–Mn–Al–Ni shape memory alloy sheets produced on an industrial process route focusing on hot rolling were investigated. The as-processed condition is characterized by a high fraction of the non-transforming γ-phase, which ensures good workability, but is associated with poor superelasticity. The alloy shows good structural properties with a yield strength of about 600 MPa, which is well above the usual transformation stress related to the martensitic phase transformation for the investigated alloy composition. After solution annealing, a microstructure showing no preferred orientation being characterized by distinctly larger grains is present. The results obtained reveal that the previous thermo-mechanical processing had no impact on the subsequent texture, however, provided a sufficient amount of driving force for abnormal grain growth. Imposed by a cyclic heat treatment, oligocrystalline structures with grain sizes above 10 mm can be achieved in the industrially processed material, which show superelastic properties similar to material processed in small batches in the laboratory.

A Review Article on FeMnAlNi Shape Memory Alloy

Shape memory alloys (SMAs) are the materials which remember their original shape once after the deformation has occurred. In recent days, researchers started working on Fe-based shape memory alloys as NiTi shape memory alloys has few drawbacks. Febased shape memory alloys show better advantages over NiTi SMAs. FeMnAlNi SMA has advantage of wide range of operating temperature and low stress dependence. This review article provides information on work carried out on FeMnAlNi SMA which will help the researchers to carry further research work on the alloy for various applications.

Microstructural Investigation of a FeMnAlNi Shape Memory Alloy Processed by Tungsten Inert Gas Wire and Arc Additive Manufacturing

In the present study, tungsten inert gas wire and arc additive manufacturing was used to process an iron-based FeMnAlNi shape memory alloy. By a layer-by-layer method, a wall structure with a length of 60 mm and a height of 40 mm was generated. Bidirectional welding ensured grain growth parallel to the building direction. To maintain a nearly constant temperature–time path upon cooling, the structure was fully cooled after each weld to room temperature (298 K). With this approach, an anisotropic microstructure with a grain length of up to 8 mm (major axis) could be established. The grain morphology and formed phases were investigated by optical microscopy and scanning electron microscopy. The images revealed a difference in the orientation with respect to the building direction of the primarily formed γ grains along the grain boundaries and the secondarily formed γ grains in the heat-affected zones. Subgrains in the α matrix were observed also by scanning electron microscopy. With X-ray diffraction, the preferred orientation of the α grains with respect to the building direction was found to be near ⟨100⟩. Overall, an anisotropic polycrystalline material with a columnar texture could be produced, with a preferred grain orientation promising high values of transformation strains.

Effect of Crystallographic Orientation and Grain Boundaries on Martensitic Transformation and Superelastic Response of Oligocrystalline Fe\u2013Mn\u2013Al\u2013Ni Shape Memory Alloys

In situ tensile tests employing digital image correlation were conducted to study the martensitic transformation of oligocrystalline Fe–Mn–Al–Ni shape memory alloys in depth. The influence of different grain orientations, i.e., near-〈001〉 and near-〈101〉, as well as the influence of different grain boundary misorientations are in focus of the present work. The results reveal that the reversibility of the martensite strongly depends on the type of martensitic evolving, i.e., twinned or detwinned. Furthermore, it is shown that grain boundaries lead to stress concentrations and, thus, to formation of unfavored martensite variants. Moreover, some martensite plates seem to penetrate the grain boundaries resulting in a high degree of irreversibility in this area. However, after a stable microstructural configuration is established in direct vicinity of the grain boundary, the transformation begins inside the neighboring grains eventually leading to a sequential transformation of all grains involved.

Nanoscale twinning in Fe–Mn–Al–Ni martensite: a backscatter Kikuchi diffraction study

Iron-based Fe-Mn-Al-Ni shape-memory alloys are of rather low materials cost and show remarkable pseudoelastic properties. To further understand the martensitic transformation giving rise to the pseudoelastic properties, different Fe-Mn-Al-Ni alloys have been heat treated at 1473 K and quenched in ice water. The martensite, which is formed from a body-centred cubic austenite, is commonly described as face-centered cubic (f.c.c.), even though there are also more complex, polytypical descriptions of martensite. The presently studied backscatter Kikuchi diffraction (BKD) patterns have been evaluated, showing a structure more complex than simple f.c.c. This structure can be described by nanoscale twins, diffracting simultaneously in the exciting volume. The twinned structure shows a tetragonal distortion, not uncommon for martensite in spite of the lack of interstitial elements. These features are evaluated by comparing the measured BKD patterns with dynamically simulated ones.

Functionally graded structures realized based on Fe–Mn–Al–Ni shape memory alloys

In the present study a novel approach to obtain functionally graded properties in an Fe–Mn–Al–Ni shape memory alloy is introduced. A single crystalline sample showing superelastic properties was aged in a partially martensitic state. As a result, the area aged in austenite is characterized by a changed superelastic hysteresis with different critical stresses for forward and backward transformation, whereas the properties in the area aged in martensite remained almost unchanged. Transmission electron microscopy studies revealed that the mean average size of nanometric β precipitates, which strongly influence the transformation temperatures and eventually the transformation stresses, is about 9 nm in the area aged in martensite and about 12 nm in the area aged in austenite. Based on these results it seems to be feasible to tailor Fe–Mn–Al–Ni components in a way allowing them to show locally different functional properties.

On the Influence of Microstructure on the Corrosion Behavior of Fe–Mn–Al–Ni Shape Memory Alloy in 5.0 wt% NaCl Solution

Herein the corrosion behavior of Fe–Mn–Al–Ni shape memory alloys characterized by different microstructures in a 5.0 wt% NaCl solution is investigated. Open circuit potential and potentiodynamic polarisation tests are conducted. Generally, Fe–Mn–Al–Ni shows corrosion properties being similar to pure iron. However, the polarization curves of single crystals indicate the presence of an unstable passive system. The corrosion damage is analyzed by means of optical microscopy and electron microscopy. It is revealed that polycrystalline samples consisting of two different phases induced by heat treatment, i.e., α‐phase and γ‐phase or β‐Mn phase and γ‐phase, are suffering a selective corrosion attack of the α‐phase and β‐Mn–phase, respectively. On the contrary, the single crystal seems to form a protective layer on the surface. Upon presence of stress‐induced martensite, a selective corrosion attack of γ′–martensite is observed.

Thermodynamics of martensite formation in Fe–Mn–Al–Ni shape memory alloys

Fe−Mn−Al−Ni based shape memory alloys are regarded suitable for large scale applications due to relatively low materials costs and a good cold workability. The austenitic state consists of an A2 structured matrix with coherent B2 precipitates which are required to ensure thermoelasticity, supposedly due to a strengthening effect on the matrix. The martensitic state is based on the A1 structure. For a series of alloys, the martensite start temperatures were determined to be below ambient temperature, whereas for the same alloy compositions T0 temperatures of about 1000 °C are predicted for the transformation of the A2 matrix into A1 by means of CalPhaD-based calculations. It is shown by thermodynamic considerations that the undercooling of about 1000 K is caused by the resistance of the B2 precipitates (~NiAl with some content of Fe and Mn) to undergo the transformation to the L10 structure enforced by the martensitic transformation of the matrix.

Abnormal grain growth in Fe–Mn–Al–Ni shape memory alloy with higher Al content

The effect of Al content on the abnormal grain growth of Fe–Mn–Al–Ni shape memory alloy is investigated in comparison with the previously reported Fe–Mn–Al–Ni–Ti alloy. Microstructural observations revealed that compared with 1.5Ti, 17Al (adding 2 at.% Al to the conventional Fe–Mn–Al–Ni) showed the potential to grow into a large single crystal. The obtained results indicate that the size control of the second phase, which can be achieved not only by introducing Ti, but rather universally by stabilizing BCC phase, and the resultant subgrain structure are vital for abnormal grain growth.

Induction Butt Welding Followed by Abnormal Grain Growth: A Promising Route for Joining of Fe–Mn–Al–Ni Tubes

The present study focuses on induction butt welding of Fe–Mn–Al–Ni tubes. By comparing different processing routes, characterized by different temperatures and forces during welding, it was possible to find adequate process parameters for realization of defect-free joints. Moreover, it was feasible to fully reset the microstructure prevailing in the heat-affected zone by a subsequent cyclic heat treatment promoting abnormal grain growth. Tensile testing up to a maximum strain of 6% revealed excellent pseudoelastic properties of the final microstructural condition. The present study shows for the first time that welding with superimposed pressure is well suited for joining of Fe–Mn–Al–Ni shape memory alloys. Furthermore, it is revealed that abnormal grain growth induced by a cyclic heat treatment can be applied independently of the geometry of the component.

Some structural effects related to the abnormal grain growth in FeMnAlNi shape memory alloys

Abstract Oligocrystalline Fe43.5Mn34Al15Ni7.5 alloys, with an average grain size larger than their thickness/ diameter, experience superelasticity between 223 … 423K which recommends them for large scale potential applications in seismic-protection industries. Besides oligocrystallinity which is necessary in order to avoid the presence of triple junctions between grains, the body centered cubic (bcc) austenite matrix has to be reinforced by the coherent precipitation of ordered β-NiAl bcc, in such a way as to enable the reversible stress induced formation of γ’ face centered cubic (fcc) martensite. The present paper describes the processing procedure of three alloys with nominal chemical composition Fe43.5Mn34Al15±1.5Ni7.5∓1.5 After hot rolling, annealing, cold rolling, cyclic heat treatment around solvus temperature (enhancing γ-fcc phase decomposition and the occurrence of a discontinuous phenomenon called abnormal grain growth) and solution treatment with final water quenching (meant to avoid γ-fcc formation and to enable coherent precipitation), chemical composition effects have been analyzed by optical microscopy (OM). In each of the processing steps, the evolution of average grain size was monitored and correlated with chemical composition effects. A limited oligocrystalline structure was obtained at Fe43.5Mn34Al16.5Ni6 after cyclic heat treatment.

Large Dimension and Low-Cost Fe-SMA Rods

The effects on sample size on abnormal grain growth in inexpensive FeMnAlNi shape memory alloy (SMAs) wires and rod were investigated. It was shown that repeated heat treatments between single phase (bcc) and two phase (bcc + fcc) region resulted in abnormal grains with bamboo structure both in small sized wires and large diameter rods. Microstructural features were investigated using electron backscatter diffraction (EBSD) and it was found that 0.5 mm wires possess strong [011] texture whereas, large diameter rods have random texture after grain growth heat treatments.

Superelastic response in 〈1 2 2〉-oriented single crystals of FeMnAlNi shape memory alloy in tension and compression

The superelastic response of the Fe43.5Mn34Al15Ni7.5 single crystalline shape memory alloy, oriented along the 〈122〉 direction was investigated. The quenched single crystals demonstrated superelastic strains of 4.0% in tension and 2.0% in compression. Ageing at 200 °C for 3 h led to the suppression of superelasticity in compression. An asymmetry in the tension and compression properties, probably associated with the ability of the martensite crystals to detwin under stress, was detected. It is likely that the detwinning of γ′-martensite in compression leads to an increase in the mechanical hysteresis Δσ and the stresses σcr required for the onset of strain associated with the stress-induced martensitic transformation in quenched crystals, suppressing superelasticity in the aged crystals. Tension without the detwinning of γ′-martensite result in a weak influence of coherent B2 (bcc/NiAl) precipitates on the mechanical hysteresis Δσ.

Stress- and Magnetic Field-Induced Martensitic Transformation at Cryogenic Temperatures in Fe–Mn–Al–Ni Shape Memory Alloys

Stress-induced and magnetic-field-induced martensitic transformation behaviors at low temperatures were investigated for Fe–Mn–Al–Ni alloys. The magnetic-field-induced reverse martensitic transformation was directly observed by in situ optical microscopy. Magnetization measurements under pulsed magnetic fields up to 50 T were carried out at temperatures between 4.2 and 125 K on a single-crystal sample; full magnetic-field-induced reverse martensitic transformation was confirmed at all tested temperatures. Compression tests from 10 to 100 K were conducted on a single-crystal sample; full shape recovery was obtained at all tested temperatures. It was found that the temperature dependence of both the critical stress and critical magnetic field is small and that the transformation hysteresis is less sensitive to temperature even at cryogenic temperatures. The temperature dependence of entropy change during martensitic transformation up to 100 K was then derived using the Clausius–Clapeyron relation with critical stresses and magnetic fields.

Microstructural design considerations in Fe-Mn-Al-Ni shape memory alloy wires: Effects of natural aging

In this study, FeMnAlNi shape memory alloys are demonstrated to experience room temperature (RT) aging, which manifests itself as a gradual increase in critical transformation stress with time when the sample is kept in a stress-free condition. This effect is sufficient to create superelasticity in the solution treated condition, which does not normally show superelasticity soon after heat treatment. This phenomenon is due to the nucleation and coarsening of nanoprecipitates at room temperature despite the high melting point of the alloy. RT aging also influenced precipitation hardened FeMnAlNi samples by causing an increase in stress hysteresis and hardening rate.

Electron beam welding of Fe–Mn–Al–Ni shape memory alloy: Microstructure evolution and shape memory response

The present study reports on the impact of abnormal grain growth (AGG) on the microstructural evolution following electron beam (EB) welding of Fe–Mn–Al–Ni shape memory alloy (SMA). Polycrystalline sheet-like material was EB-welded and a cyclic heat treatment, studied in previous work, was conducted for inducing AGG and a bamboo-like microstructure, respectively. Optical and electron microscopy were carried out to characterize the prevailing microstructure upon cyclic heat treatment. For characterization of the functional properties following AGG, a load increase test was conducted. The current results clearly show that good shape memory response can be obtained in Fe–Mn–Al–Ni SMA upon EB welding and subsequent post-heat treatment. These results further substantiate the potential use of conventional processing routes for Fe–Mn–Al–Ni SMA.

Relationship between martensitic reversibility and different nano-phases in a FeMnAlNi shape memory alloy

In order to suppress the crack formation, water or air cooling after solution treatment is employed to introduce γ phase in a Fe43.61Mn34.74Al13.38Ni8.27 alloy. In addition to the γ phase, nano-sized B2 and β-Mn phases with an average diameter of 7.8nm are precipitated in α phase for water cooled samples, while nano-sized β-Mn phase of 11.5nm is mainly found in the α phase for air cooled samples. This nano-sized coherent B2 phase is beneficial to improving the martensitic reversibility, while the nano-sized incoherent β-Mn phase has no positive effect on achieving the good martensitic reversibility. Furthermore, the nano-sized coherent B2 phase is more effective to avoid the formation of hairpin-shaped dislocations at γ′/α interface than the nano-sized incoherent β-Mn phase. The above two reasons are the main causes of that the reversibility of stress-induced γ′ martensite in the water cooled samples is better than that in the air cooled samples. As a result, the water cooled samples exhibit strong shape memory effect while the air cooled samples have weak one. For the purpose of obtaining the good shape memory effect in FeMnAlNi alloys, the nano-sized coherent B2 phase is expected to be precipitated.

Cyclic degradation in bamboo-like Fe–Mn–Al–Ni shape memory alloys — The role of grain orientation

In the present study the cyclic deformation behavior within differently oriented grains in Fe–34.8Mn–13.5Al–7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor.