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

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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.

Section: Introductionmentioning

confidence: 99%

Effect of Crystallographic Orientation and Grain Boundaries on Martensitic Transformation and Superelastic Response of Oligocrystalline Fe–Mn–Al–Ni Shape Memory Alloys

Bauer

Vollmer

Niendorf

2021

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“…12 were aged at 200 °C for 3 h [57], 24 h [59] and 10 h [89]. Whereas, for Ni 50.8 Ti with an ageing time of 1.5 h at 550 °C, the superelastic functionality is limited to a temperature range of about 0-90 °C [97]. The Clausius-Clapeyron slope ranges from 7 to 10 MPa/°C.…”

Section: Discussionmentioning

confidence: 99%

“…It is known that this alloy system is extremely sensitive to the characteristics of the precipitates [89] which influence the stress levels of the forward/reverse transformation. Additionally it was recently shown that the growth of precipitates is impeded in FCC martensite compared to that in BCC austenite [97]. Thus, the initial microstructure prior to ageing is expected to affect the precipitate volume fraction and size in the local regions of the sample.…”

Section: Variability In Femnalnimentioning

confidence: 99%

Unraveling Frequency Effects in Shape Memory Alloys: NiTi and FeMnAlNi

Sidharth

Mohammed

Abuzaid

et al. 2021

With the presence of internal interfaces such as the austenite-martensite interface and the internal twin boundaries in the martensite, shape memory alloys (SMAs) can be employed in passive/active damping applications. Due to the latent heat of transformation, a temperature rise/drop during a load/unload cycle is expected to dynamically couple with the mechanical response of the SMA and influence the stress levels of forward/reverse transformation and thus the hysteretic area (i.e. the dissipated energy). Additionally, the temperature change per cycle is a function of loading frequency due to momentary heat transfer effects. To this end, for the first time, we demonstrate a rate insensitive shape memory alloy system, Fe 43.5 Mn 34 Al 15 Ni 7.5 which also exhibits near-zero temperature dependent stress-strain response. Contrastingly, we show that Ni 50.8 Ti, which is widely used commercially, is highly rate sensitive. With straightforward in situ experiments, complemented with thermomechanical modelling, we pinpoint the key material parameter which dictates frequency sensitivity. The corresponding results are then discussed in the light of different mechanisms contributing to the damping capacity of SMAs.

“…However, the main drawback for an increased industrial use of Ni-Ti alloys is their high cost resulting from comparatively expensive alloying elements, high dependency of functional properties on compositional accuracy, challenging machinability [5], and weldability [6]. Therefore, low-cost iron-based SMAs, like Fe-Mn-Si-X (X = Cr, Cr-Ni, Cr-Ni-VC) [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and Fe-Mn-Al-Ni-Z (Z = Ti, Cr) [23][24][25][26][27][28][29][30][31][32], have attracted pronounced attention in recent years. Particularly, the latter show promising functional properties.…”

Section: Introductionmentioning

confidence: 99%

“…Particularly, the latter show promising functional properties. Superelastic strains up to 10% [25,27] and a low-temperature dependence of the critical stress needed for martensitic transformation [27,31] make the alloy promising for applications as damping elements. The thermo-elastic character of the phase transformation is based on the formation of the aluminum-and nickel-rich, nano-sized b-precipitates (B2, bcc), which are coherent and ordered to the disordered a matrix (A2, bcc) [27,33].…”

Section: Introductionmentioning

confidence: 99%

Electron Beam Welding of Hot-Rolled Fe–Mn–Al–Ni Shape Memory Alloy Sheets

Bauer

Wiegand

Wicke

et al. 2023

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The present study focuses on the weldability of hot-rolled Fe–Mn–Al–Ni shape memory alloy sheets by vacuum electron beam welding. Tailored process-specific welding parameters, such as preheating with electron beam or beam oscillation during welding, allowed defect-free joining with very thin weld seams and heat-affected zones. By applying a post-weld cyclic heat treatment, abnormal grain growth can be promoted across the weld seams. However, regardless of the selected welding parameters, some specimens are characterized by the formation of smaller grains within the former fusion zone. In situ incremental strain tests reveal that the former fusion zone has only a minor influence on the functional properties and is not responsible for structural failure. Thus, electron beam welding is a promising welding technology for joining Fe–Mn–Al–Ni shape memory alloys.