Pseudoelasticity in Fe3Ga single crystals

doi:10.1016/j.scriptamat.2005.03.032

Scripta Materialia

Volume 53, Issue 2, July 2005, Pages 253-257

https://doi.org/10.1016/j.scriptamat.2005.03.032 Get rights and content

Abstract

We first found pseudoelasticity in Fe₃Ga single crystals regardless of a thermoelastic martensitic transformation. Superpartial dislocations with Burgers vector of 1/4[1 1 1] were pulled back by nearest-neighbour and next-nearest-neighbour antiphase boundaries during unloading, resulting in the pseudoelasticity.

Introduction

Pseudoelasticity generally results from a thermoelastic martensitic transformation: a stress-assisted transformation during loading and the reverse transformation during unloading [1]. In contrast, Fe₃Al single crystals with the D0₃ structure exhibited pseudoelasticity though martensitic transformation never occurred in these crystals [2], [3], [4]. The amount of shape recovery in Fe₃Al single crystals showed a maximum at around 23.0at.%Al and the recoverable strain was approximately 5.0% [3], comparable with that of Ti–Ni alloys [1]. In Fe-rich Fe₃Al single crystals with Al contents between 22.0 at.% and 25.0 at.%, superpartial dislocations with Burgers vector (b) of 1/4〈1 1 1〉 moved individually dragging the nearest-neighbour antiphase boundary (NNAPB) during loading, though four superpartials bound by NNAPB and next-nearest-neighbour antiphase boundaries (NNNAPB) moved in a group in Al-rich Fe₃Al [3]. In Fe-rich Fe₃Al, NNAPB pulled back the superpartials during unloading, resulting in the pseudoelasticity. We call this phenomenon “APB pseudoelasticity”.

Both aluminium and gallium are IIIB-group elements. Furthermore, Fe₃Ga is known to have the D0₃ structure in the temperature range 873–923 K [5]. Thus, there is a possibility that Fe₃Ga single crystals as well as Fe₃Al demonstrate APB pseudoelasticity irrespective of the martensitic transformation. However, pseudoelasticity has not yet been reported for Fe–Ga alloys. In the present study, we discovered for the first time pseudoelasticity in Fe₃Ga single crystals. It is strongly suggested that APB pseudoelasticity appeared in these single crystals, similar to Fe₃Al.

Section snippets

Experimental procedure

A master ingot of Fe₃Ga was prepared in a plasma arc furnace. A Fe₃Ga single crystal was grown from the ingot by the floating zone method at a growth rate of 5 mm/h. The chemical composition of the crystal was determined to be Fe–24.2at.%Ga by a wet chemical analysis. After homogenisation at 1373 K for 48 h, the single crystal rod was solution treated at 1073 K for 1 h and subsequently quenched in ice brine. After quenching, the samples were annealed at 853–923 K for ordering. According to a Fe–Ga

Results and discussion

Fig. 1 shows a stress–strain curve of Fe–24.2at.%Ga single crystal annealed at 873 K for 10 h and then compressed to ε_p = 5.0%. A stress–strain curve of Fe–23.0at.%Al single crystal [3] is also shown in the figure. Fe₃Ga single crystal yields at 561 MPa and shows weak work hardening during loading, while a plastic strain of 5.0% is completely recovered during unloading. Moreover, the shape of the stress–strain curve of Fe–24.2at.%Ga single crystal is quite similar to that of Fe–23.0at.%Al crystal

Conclusions

Fe–24.2at.%Ga single crystal with an appropriate heat treatment exhibited pseudoelasticity, though a thermoelastic martensitic transformation never occurred in the crystals. NNAPB and NNNAPB may pull back the superpartials with b = 1/4[1 1 1] during unloading in the crystal, resulting in the pseudoelasticity.

Acknowledgments

This work was supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Part of this work was carried out at the Strategic Research Base “Handai Frontier Research Centre” supported by the Japanese Government’s Special Coordination Fund for Promoting Science and Technology. This work was also supported by “Priority Assistance of the Formation of Worldwide Renowned Centers of Research—The 21st Century COE Program

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The Fe₃Ga base alloy is known to exhibit pseudoelasticity and the solute hardened Fe₃GaB holds considerable promise as well. The present work aims at developing a theoretical model to establish the critical twinning stress in Fe₃Ga with varying boron concentration. The theoretical model is based on the atomistic–micromechanical approach where we utilize density functional theory and Peierls Nabarro formalism at an atomic scale and the Eshelbian anisotropic elasticity at microscale. Using local strain measurements at the grain scale, we also show the experimental evidence of effect of boron in elevating the twinning stress in Fe₃Ga. The work calculates the interaction energies associated with the presence of boron in octahedral, tetrahedral and xenohedral sites in the D0₃ lattice and the transition from octahedral to xenohedral sites upon twinning. The model distinguishes the elevation of twinning stress depending on the interstitial site and the transition between the sites.
Modeling of pseudoelasticity via reversible slip in Fe<inf>3</inf>Ga
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The purpose of this paper is to propose a mesoscale model incorporating back stresses responsible for cyclic reversible dislocation motion. It has been reported that single crystals of Fe3Ga with the cubic D03 structure exhibit pseudoelasticity of several percent strain and these recoverable strains are comparable to NiTi alloys [21]. The reversible slip in Fe3Ga and Fe3Al can occur over a broader range of temperature [21–24].
The Fe₃Ga alloy with the cubic D0₃ lattice possesses considerable recoverable strain due to the slip reversibility. Pseudoelasticity via reversible slip in Fe₃Ga is studied with atomistic simulations. An extended Peierls–Nabarro model incorporating the Generalized Stacking Fault Energy (GSFE) is established to determine Peierls stress in D0₃ and L1₂ Fe₃Ga. The back stress and frictional stress are predicted during loading and unloading process. These stress magnitudes govern the reversible slip in Fe₃Ga. The results show that the reversible slip observed experimentally in D0₃ Fe₃Ga is induced by its larger back stress compared to its frictional stress. In contrast, the reversible slip cannot appear in L1₂ since its back stress is not large enough to pull back superpartials, and thus the existence of L1₂ will suppress the pseudoelasticity of D0₃ Fe₃Ga and results in decreasing the strain recovery. The present study has explored the theoretical foundations of this phenomenon arising from high back stresses responsible for cyclic reversible dislocation motion.
Anomalous growth of antiphase domains in Ti<inf>3</inf>Al
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Anomalous antiphase domain (APD) growth was observed in Ti₃Al after prolonged annealing, and its mechanism was investigated by detailed transmission electron microscopy observation. Significant numbers of dislocations were observed around anomalously coarsened APDs, and APD boundaries (APDBs) were terminated with dislocations having non-c-axis components of Burgers vectors (b) parallel to the phase-shift vector R of the APDBs. This implies that the anomalous APD coarsening occurs via APDB elimination by motion of superpartial dislocations whose bs can cancel the R of APDBs.
Multimode pseudoelasticity in Fe-23.8 at% Ga single crystals with D0<inf>3</inf> structure
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Fe3Ga is a Fe-based intermetallic compound with the D03 superstructure of which single-phase field exists around 873–923 K [4]. We first observed that Fe3Ga single crystals with the D03 structure demonstrated giant pseudoelasticity irrespective of martensitic transformation [5,6]. In the D03 structure (Fig. 1(a)), a 〈111〉 superdislocation is generally dissociated into four 1/4〈111〉 superpartials bound by the nearest-neighbour (NN) and next-nearest-neighbour (NNN) antiphase boundaries (APB) as shown in Fig. 1(b) [7].
Pseudoelastic behaviour of D0₃-ordered Fe–23.8 at% Ga single crystals subjected to different heat treatments was examined focusing on the deformation mode. There existed three kinds of deformation modes in the crystals depending on the heat treatment: stress-induced martensitic transformation, dislocation glide and twinning which had a potential to show large pseudoelasticity. Numerous martensites with the 14M structure were stress-induced during loading and disappeared during unloading resulting in perfect pseudoelasticity with small stress–strain hysteresis. On the other hand, 1/4[111] superpartial dislocations dragging the antiphase boundaries (APB) moved backward during unloading due to the APB tension, which also led to large strain recovery. Twinning and untwinning of 2.2^T-type pseudo-twins accompanying both a serrated flow and a clicking sound were also responsible for the pseudoelasticity. It was also found that the pseudoelastic behaviour associated with the three deformation modes was strongly affected by stress sense and loading axis.
Effect of the ordering process on pseudoelasticity in Fe<inf>3</inf>Ga single crystals
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The pseudoelastic behaviour of Fe₃Ga single crystals with the D0₃ structure was investigated focusing on the annealing conditions and Ga content. In these D0₃–ordered Fe₃Ga single crystals, uncoupled and paired 1/4[1 1 1] superpartial dislocations moved, dragging the nearest-neighbour and next-nearest-neighbour anti-phase boundaries (APBs), respectively. These APBs pulled back the superpartials during unloading, resulting in pseudoelasticity, though a martensitic transformation never occurred in the crystals. The amount of strain recovery increased with increasing Ga content up to 25.4 at.% when the crystals were annealed at 853–893 K within 10 h. In particular, nearly perfect pseudoelasticity with a recoverable strain of 5% was obtained at 24.2 and 25.4 at.% Ga after an appropriate heat treatment. Isothermal annealing in the D0₃ single phase region was effective in improving pseudoelastic properties. Moreover, the metastable D0₃ phase also exhibited pseudoelasticity, even if the (α + L1₂) or (D0₃ + L1₂) dual phase was thermally stable in the equilibrium state.
Effect of heat treatment on pseudoelasticity in Fe-25.0 at.% Al single crystals
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% was favourable for the appearance of perfect pseudoelasticity. More recently, we also found that D03-ordered Fe3Ga single crystals showed giant pseudoelasticity arising from the to-and-fro motion of 1/4〈111〉 superpartials [11]. According to the Fe–Al phase diagram shown in Fig. 1, the transformation pathway of Fe–25.0 at.
The pseudoelastic behaviour of Fe–25.0 at.% Al single crystals annealed in the D0₃, (α + D0₃), (α + B2) or B2 phase region was examined focusing on the dislocation and ordered domain structures. When large pseudoelasticity appeared in the crystals, 1/4[111] superpartial dislocations moved individually dragging the nearest-neighbour antiphase boundaries (NNAPB). During unloading, the superpartials were pulled back by the surface tension of the NNAPB resulting in pseudoelasticity. Large pseudoelasticity took place only in the D0₃-ordered crystals, while the crystals annealed in the (α + D0₃), (α + B2) or B2 phase region exhibited small strain recovery. The surface tension of the NNAPB in the D0₃ phase was higher than that in the B2 one resulting in the larger recoverable strain in the D0₃-ordered crystals. Since a specific type of ordered domain boundaries in the D0₃ phase played an important role in the pseudoelasticity, a coarsening of the ordered domains caused a decrease in strain recovery.

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Pseudoelasticity in Fe3Ga single crystals

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgments

Acta Mater

J Alloys Compd

Scripta Mater

Shape memory materials

Pseudoelasticity in Fe₃Ga single crystals