Grain orientation dependence of the forward and reverse fcc ↔ hcp transformation in FeMnSi-based shape memory alloys studied by in situ neutron diffraction

Highlights

• In situ cyclic deformation experiments with in FeMnSi alloy during neutron diffraction.

• Deformation induced fcc.→hcp and its reversion hcp→fcc in FeMnSi was studied.

• Grain orientation determines the amount of martensite formation and its reversion.

• Two fundamentally different transformation behaviors are identified.

• For strains <2% one primary slip plane is active during tension and compression.

Abstract

The grain orientation dependence of the deformation-induced forward fcc→hcp and reverse hcp→fcc martensite transformation of a FeMnSi-based shape memory alloy was studied by in situ neutron diffraction during cyclic loading. A deformation-induced fcc→hcp transformation is observed during tensile straining to +2%. The hcp martensite phase that forms under tension partially reverts to fcc austenite upon subsequent compression from +2% → −2% for the {220}, {331} and {111} grain families aligned with respect to the loading direction but not for the {200} grain family. The martensite formation and the reversion of the individual grains can be explained by considering grain orientation dependent Schmid factors of the {111}<112> slip system underlying the fcc to hcp transformation. While for post-yield elastically compliant grains the Schmid factor of the leading partial dislocation is larger than that of the trailing partial dislocation, the opposite is true for post-yield elastically stiff grains. The former grains show a phase reversion, i.e. hcp→fcc upon compression, the latter grains do not transform back to fcc. EBSD characterization confirms the phase reversion for a <541> orientated grain by the disappearance of hcp bands. Martensite bands, which have not reverted to austenite during compression, showed a thickening. The thickening of existing bands during compression is associated with the activation of a second slip system.

Introduction

The shape memory effect (SME) in FeMnSi-based alloys was first discovered by Sato in the 1980 ies [1,2]. The SME in FeMnSi-based alloys is based on the deformation-induced transformation from the parent face-centered cubic (fcc) phase into the hexagonal-closed packed (hcp) phase and its reversion upon heating. The mechanism of the fcc→hcp transformation is similar to deformation twinning in the fcc crystal structure: the fcc lattice can be transformed into a hcp lattice by the formation of partial dislocations separated by stacking faults on every second {111} layer while a deformation twin is formed by stacking faults on every subsequent {111} layer. If a tensile load is changed into compressive, the phase transformation can be reverted (hcp→fcc) or a twin can be de-twinned. The fcc→hcp transformation is usually observed in alloys with stacking fault energies <20 mJ/m² [[3], [4], [5]], whereas deformation twinning is observed for higher stacking fault energies between 20 and 40 mJ/m² [6,7].

Compared to the most widely studied shape memory alloy (SMA) NiTi, FeMnSi-based SMAs have the advantage of low material and manufacturing costs. The combination of low costs and the unique thermo-mechanical behavior triggered extensive research of the alloy system for the last four decades. Significant efforts have been made to increase the SME by optimizing e.g. the chemical composition [1,8], introducing thermo-mechanical treatments [9,10] and the formation of precipitates [11]. By using carbide- or nitride-based precipitates an almost perfect SME in polycrystalline FeMnSi-based shape memory alloys was achieved for strains up to around 4% without the need of training [11]. In the recent years, FeMnSi-based SMAs were considered for various applications such as pre-stressing of concrete structures for civil engineering [[12], [13], [14]], clamping and coupling [15,16], and also seismic damping [17,18]. In all these applications, the FeMnSi-based SMA might be subject to cyclic loading and is thus prone to fatigue failure. It has been shown that in comparison with steels, FeMnSi-based SMA exhibit excellent low cycle fatigue properties which is attributed to the reversible fcc ↔ hcp transformation [19]. The low cycle fatigue properties of FeMnSi-based alloys were improved recently by modifying the alloy composition so that a high degree of reversibility fcc ↔ hcp and a high yield strength were achieved at the same time [[20], [21], [22]]. Another way to improve the low cycle fatigue properties would be to modify and select the texture of the material in a favorable way. For this, the impact of grain orientation on the deformation induced forward and reverse fcc↔hcp transformation must be known. A detailed understanding of the forward fcc→hcp and reverse hcp→fcc phase transformation and its grain orientation dependence is thus of particular interest, but while detwinning in fcc crystals has been frequently studied [23,24], the phase reversion for the fcc→hcp is not well understood up to now. The deformation-induced phase reversion hcp→fcc was evidenced by atomic force microscopy by the appearance and disappearance of the surface relief [25]. However, in the latter study the impact of the grain orientation on the phase reversion was not studied. There is a significant impact of the grain orientation on the forward phase transformation fcc→hcp [26] and thus it can be expected that the grain orientation also has an impact on the phase reversion hcp→fcc. Recently, Lee et al. investigated twinning-detwinning in gold nanowires and explained this phenomenon with the grain orientation dependence of the Schmid factor (SF) for the leading partial dislocation and the trailing partial dislocation under tension and compression [27]. The effect of grain orientation with respect to the loading direction has been shown to affect the SF of the leading partial dislocation and the trailing partial dislocation and the associated TRIP [7,28] and TWIP mechanisms [29]. While the tension-compression asymmetry for the fcc-hcp transformation is already studied in detail for sole tension or compression experiments [30], the grain orientation dependent behavior of the fcc-hcp transformation in FeMnSi-based SMAs during a tension-compression cycle has not been studied so far.

Neutron diffraction is a unique method to study the deformation mechanism of a statistically large number of grains due to the high penetration depths of neutrons in metals and the associated large interaction volume. In the past, the fcc→hcp transformation in FeMnSi-based SMAs during static loading was studied by in situ neutron diffraction [26,[31], [32], [33]]. However, in none of these studies, the deformation-induced phase reversion hcp→fcc nor any cyclic loading including tension and compression was studied. Recently, a neutron diffraction study during a tension-compression cycle of a TWIP steel gave insights into the grain orientation dependence of twinning and detwinning [24]. It was found that the grain orientation has an impact on whether detwinning takes place.

In the present study the impact of grain orientation on the deformation-induced forward (fcc→hcp) and reverse (hcp→fcc) phase transformation behavior in FeMnSi-based shape memory alloys is studied in detail by employing in situ neutron diffraction during a tension-compression cycle. To complement the in situ neutron diffraction tests, the electron backscatter diffraction (EBSD) was undertaken at the unloaded state after reaching the maximum loads (both tension and compression). The presented EBSD study allows assessing the deformation behavior, the activation of martensite variants and the reversion of the transformation upon the tension-compression cycle for individual grain orientations.