Twin-related Variants Research Articles

Revisiting the Crystallography of {225}γ Martensite: How EBSD Can Help to Solve Long-Standing Controversy

Explaining the crystallography of iron alloys martensite with a {225}γ habit plane remains a challenging task within the phenomenological theory of martensite crystallography. The purpose of this study is to re-examine the martensite formed in a Fe-8Cr-1.1C alloy using EBSD, which has a better angular resolution than the conventional transmission electron diffraction techniques previously used. The results show that the single morphological plates, which hold a near {225}γ habit plane, are bivariant composites made up of two twin-related variants. It is shown that a {113}γ plane is systematically parallel to one of the three common 112α planes between the two twin-related crystals. This observation suggests that the lattice invariant strain of transformation occurs through a dislocation glide on the {113}γ ⟨110⟩γ system, rather than through twinning as is commonly accepted. Based on this assumption, the predictions of Bowles and Mackenzie’s original theory are in good agreement with the crystallographic features of {225}γ martensite. Unexpectedly, it is the high shear solution of the theory that gives the most accurate experimental predictions.

In-depth understanding of {110}-type domain walls in bismuth titanate ceramics

Bismuth titanate(BIT) ceramics have been widely studied for high temperature applications thanks to their high Curie temperature and large spontaneous polarization. The underlying domain structures play significant roles in regulating the macroscopic properties of ferroelectric materials, while there is only limited knowledge of the domain structures for BIT ceramics until now, substantially bad for the development of BIT-based new materials. In this study, the intriguing domain walls(DWs) in BIT ceramics were investigated in detail. The atomic-scale observation reveals that there primarily exist energetically favorable {110}-type 90° DWs in BIT ceramics. Combined with the theoretical analysis, these DWs are not only elaborated under multiple piezoelectric response modes, but also further corroborated by the quantitative reconstruction of spatial polarization distributions of twin-related ferroelectric variants. This work sheds insight into domain structure and piezoelectric response mechanism in BIT ceramics, and contributes to promoting their more applications in advanced electron devices.

Unconventional twin deformation of Ni-Mn-Ga 7M martensite under tension mediated by the collective lattice reorientation from a-c twin to b-c twin

Deformation of martensite twins is a prerequisite to obtain the shape memory effect in shape memory alloys, which is conventionally conceived to be realized by twinning/detwinning based on the simple shear. Here, we report the unconventional twin deformation of seven-layered modulated (7M) martensite under tension in a directionally solidified Ni50Mn30Ga20 alloy. Based on the neutron diffraction and EBSD measurements, it is evidenced that the deformation of 7M martensite in the variant colonies with unfavorable Schmid factors is achieved by the collective lattice reorientation from four twin-related variants (a-c twin) to another type of four twin-related variants (b-c twin), rather than the detwinning of existing variants. Such deformation is not caused by the simple shear, but originated from the local atomic rearrangements that convert the initial lattice to the reoriented lattice. Moreover, the collective conversion from the initial self-accommodated variants to the reoriented variants follows a specific orientation relationship, where the deformation of individual variant yields the consistent dilatational strain along the loading direction and effectively accommodates the macroscopic strain. The present study provides clear evidence on the unconventional twin deformation mediated by the collective lattice reorientation, which is expected to deepen the understanding of martensite deformation behaviors in shape memory alloys.

High pressure effect on the substructure and hardness of IF steel during martensitic transformation

In the present investigation, the microstructure and hardening of an IF steel after one thermal cycle (heating to 1050°C and holding for 30 min followed by 10°C/s cooling to room temperature) under hydrostatic pressure of 1-5 GPa were studied. Experimental results show that typical lath martensite was induced, giving rise to significant hardening from 80 to 780 HV. The lath martensite shows hierarchical packet-block-lath structure and obeys the classical K-S orientation relationship between martensite and austenite. High pressure influences the propensity and size of martensitic variants. As the pressure increases from 1 to 5 GPa: 1) single-variant blocks gradually replace those with dual-variants of same Bain group; 2) twin-related variants become predominant; 3) variants decrease their thickness from micron- to nano-scale. The hardening mechanism was analyzed assuming a linear additivity of carbon-independent contributions from (block) boundary strengthening and (forest) dislocation strengthening. High pressure was proposed as an effective method to widely tune the martensitic transformation independent of alloy element, showing potential scientific and technological importance.

Microstructure evolution and thermal stability of equiatomic CoCrFeNi films on (0001) α-Al2O3

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K.The fcc-phase persists while the original <111> texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[11¯0]//α-Al2O3(0001)[11¯00]) and OR2 (M(111)[11¯0]//α-Al2O3(0001)[112¯0])and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature.Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures.

In-situ observation of nucleation, growth and interaction of deformation-induced [formula omitted] martensite in metastable Ti–10V–2Fe–3Al

Microstructure evolution, dominated by the formation of deformation-induced α″ martensite during the 3-point bending of a metastable β Ti–10V–2Fe–3Al alloy containing 5% α, was observed in-situ using forward-scatter electron imaging and electron backscattering diffraction. α″ plates nucleated heterogeneously from {580}α″ habit planes at α-β interfaces and triggered the nucleation of additional α″ plates at the advancing interface during growth and upon impingement with high-angle boundaries. Further thickening of the impinged α″ plates led to a build-up of strain energy which induced 13031‾0α'' twinning within obstacle-forming α″ plates. Until now, this twinning system was observed in Nb and Ta alloyed Ti shape memory alloys only. This in-situ study is also the first to document the activation of 13031‾0α'' twinning by α″ impingement in a metastable β matrix. The first forming variants were {011}α″ compound twin-related variant pairs, each forming from up to two individual habit planes. These variant pairs were consistently predicted by determining the grain-average stress with finite element analysis and computing the available work from the transformation strain of the α″ variants. At higher indenter loads, grains that were unfavourably oriented to martensite formation accommodated the local stress by forming aggregates of different α″ variants, obeying either {111}α″ type I or 211α'' type II twin relationships. Scanning transmission electron microscopy revealed α″ substructures comprising columnar domains oriented perpendicular to the habit plane along with the absence of lattice invariant deformation. The growth of α″ plates involved ledge formation at α″-β interfaces.

Internal structure evolution of L12 variants in aged Fe-Ga alloys

Abstract Recently, ferromagnetic Fe-Ga composites prepared by aging the metastable body-centered-cubic (BCC) precursor within the ordered face-centered-cubic (FCC) L12 phase regime have been found to yield novel properties, such as highly thermal-stable magnetization, stress-insensitive magnetic permeability, and sign-changed-magnetostriction. Accurate control of the macroscopic properties requires a thorough understanding of the microstructure evolution during the aging process. Here we performed an aging-time dependent study on a Fe73Ga27 alloy to investigate the transformation process and the associated internal structure evolution. The differential scanning calorimetry (DSC) measurements reveal that the BCC to L12 transformation is diffusion-controlled. The detailed transmission electron microscope (TEM) investigations reveal that this transformation has also displacive feature, exhibiting shear-induced twin-related L12 variants with multilayer internal structure prior to approaching phase equilibria. The twin boundary and sublayer interface are found to contain {111}-L12 stacking faults. Especially, the stacking faults within individual L12 variants disappear after long-term aging. These findings suggest that this transformation is not only driven by the large free energy difference between the metastable BCC and the equilibrium L12 phases but also driven by reducing the stacking fault energy, which may help to understand its slow transformation kinetics and to discover novel properties through engineering the microstructure of Fe–Ga alloys.

Effect of tensile deformation on variant selection in {225}γ plate martensite and {557}γ lath martensite

The effect of tensile deformation on variant selection has been studied in {225}γ plate martensite and {557}γ lath martensite formed in high- and low-carbon steels, respectively. Different thermomechanical treatments are performed to analyse the effect of applied tensile stress, in the elastic and plastic range and the effect of prior plastic tensile deformation on variant selection. The variant selection is studied by computing the work of the deformation associated with the transformation in the applied stress field. Our analysis indicates that the deformation that needs to be considered to model the selection is based on lattice strains combinations. For {225}γ plate martensite, the average deformation resulting from the combination of the lattice strains of twin-related variants is the relevant deformation gradient for the work computation. On the contrary, in lath martensite, the combination of the lattice strains of low-misoriented variants appears to better model the selection phenomenon.

Correlation between crystallographic and microstructural features and low hysteresis behavior in Ni50.0Mn35.25In14.75 melt-spun ribbons

In this work, crystallographic, microstructural and magnetocaloric investigations were performed on textured Ni50Mn35.25In14.75 melt-spun ribbons with low thermal (6 K) and magnetic-field induced hysteresis (−0.73 J kg−1 at 2 T) and moderate maximum magnetic entropy change ΔSMmax (11 J kg−1 K−1 at 5 T) at room temperature (302 K). The austenite in the ribbons crystallizes into a L21 structure, whereas martensite has a monoclinic incommensurate 6 M modulated structure as determined with the superspace theory. By means of electron backscatter diffraction technique, the morphological and crystallographic features of microstructure were systematically characterized. Ribbons possess a fine microstructure with an average grain size (initial austenite phase) of around 10 μm, whereas the 6 M martensite has a self-accommodated microstructure with 4 kinds of twin-related martensite variants. During inverse martensitic transformation, the austenite prefers to nucleate at the grain boundaries of initial austenite. By means of cofactor conditions and crystallographic orientation analyses, the good geometrical compatibility between austenite and martensite was confirmed. Based on the crystal structure and microstructure information obtained, the reason of the low thermal hysteresis was discussed.

Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels

We investigated the effects of C concentration variation from 0.028 to 0.058 wt pct on microstructure of the coarse grained heat-affected zone (CGHAZ) of low heat input girth welded Ti-Nb microalloyed steels by using electron microscope and atom probe tomography. It is found that the CGHAZ microstructure exhibits a systematic response to C variation. Increased C raises the temperature for precipitation of NbC. This leads to coarser (Ti, Nb)N-Nb(C, N) but finer delayed strain-induced NbC in the high-C steel than in the low-C steel. Fine strain-induced NbC are ineffective in preventing austenite grain coarsening in CGHAZ due to their fast dissolution upon heating. For a given inter-particle spacing originally determined by (Ti, Nb)N particles, increased epitaxial growth of Nb(C, N) on pre-existing (Ti, Nb)N in the high-C steel results in a smaller austenite grain size of 34 µm in the CGHAZ of the high-C steel than that of 52 µm in the low-C steel. Increased C promotes a microstructure consisting of bainitic lath structure with C Cottrell atmospheres at dislocation debris and martensitic layers of 30 to 100 nm in thickness at inter-lath boundaries in the CGHAZ. Increased C promotes configuration of crystallographic variants belonging to different Bain groups in the neighbors, preferentially twin-related variant pairs within a bainite packet.

Orientation of austenite reverted from martensite in Fe-2Mn-1.5Si-0.3C alloy

The orientation of austenite (γ) reverted from lath martensite has been studied using electron backscatter diffraction (EBSD) in Fe-2%Mn-1.5%Si-0.3%C (mass%) alloy. Two morphologies of γ, acicular and globular, are formed during reversion. Nucleation sites for acicular γ are lath, block and sub-block boundaries, while those for globular γ are prior γ grain boundaries, as well as inside packets. Both acicular and globular γ hold a near Kurdjumov-Sachs (K-S) orientation relationship with at least one of its surrounding tempered martensite blocks. Most acicular γ have almost identical orientations with the prior γ, while only a part of acicular γ have a specific twin-related variant with the prior γ. Furthermore, grain boundary globular γ has almost identical orientations with the prior γ into which it does not grow. The formation of acicular and grain-boundary globular γ was attributed to strong variant selection during γ nucleation at lath, block and sub-block boundaries. On the other hand, orientation relationship analyses among intragranular globular γ, tempered martensite and cementite suggest that the nucleation of γ at cementite and tempered martensite interfaces causes weakening of the variant selection for the formation of intragranular globular γ.

Crystallographic features of the martensitic transformation and their impact on variant organization in the intermetallic compound Ni50Mn38Sb12 studied by SEM/EBSD.

The mechanical and magnetic properties of Ni-Mn-Sb intermetallic compounds are closely related to the martensitic transformation and martensite variant organization. However, studies of these issues are very limited. Thus, a thorough crystallographic investigation of the martensitic transformation orientation relationship (OR), the transformation deformation and their impact on the variant organization of an Ni50Mn38Sb12 alloy using scanning electron microscopy/electron backscatter diffraction (SEM/EBSD) was conducted in this work. It is shown that the martensite variants are hierarchically organized into plates, each possessing four distinct twin-related variants, and the plates into plate colonies, each containing four distinct plates delimited by compatible and incompatible plate interfaces. Such a characteristic organization is produced by the martensitic transformation. It is revealed that the transformation obeys the Pitsch relation ({0[Formula: see text]}A // {2[Formula: see text]}M and 〈0[Formula: see text]1〉A // 〈[Formula: see text]2〉M; the subscripts A and M refer to austenite and martensite, respectively). The type I twinning plane K1 of the intra-plate variants and the compatible plate interface plane correspond to the respective orientation relationship planes {0[Formula: see text]}A and {0[Formula: see text]}A of austenite. The three {0[Formula: see text]}A planes possessed by each pair of compatible plates, one corresponding to the compatible plate interface and the other two to the variants in the two plates, are interrelated by 60° and belong to a single 〈11[Formula: see text]〉A axis zone. The {0[Formula: see text]}A planes representing the two pairs of compatible plates in each plate colony belong to two 〈11[Formula: see text]〉A axis zones having one {0[Formula: see text]}A plane in common. This common plane defines the compatible plate interfaces of the two pairs of plates. The transformation strains to form the variants in the compatible plates are compatible and demonstrate an edge-to-edge character. Thus, such plates should nucleate and grow simultaneously. On the other hand, the strains to form the variants in the incompatible plates are incompatible, so they nucleate and grow separately until they meet during the transformation. The results of the present work provide comprehensive information on the martensitic transformation of Ni-Mn-Sb intermetallic compounds and its impact on martensite variant organization.

Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)

Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation.

{225}\u03b3 habit planes in martensitic steels: from the PTMC to a continuous model

Fine twinned microstructures with {225}γ habit planes are commonly observed in martensitic steels. The present study shows that an equibalanced combination of twin-related variants associated to the Kurdjumov-Sachs orientation relationship is equivalent to the Bowles and Mackenzie’s version of the PTMC for this specific {225}γ case. The distortion associated to the Kurdjumov-Sachs orientation relationship results from a continuous modeling of the FCC-BCC transformation. Thus, for the first time, an atomic path can be associated to the PTMC.

Crystal Structure, Microstructure and Martensitic Transformation Path in Ni-Mn-In Alloys

In the present work, the crystal structure, microstructure and martensitic transformation path in Ni-Mn-In alloys were systematically studied. Results show that the austenite has a highly ordered cubic L21 structure. The martensite phase possesses a 6M incommensurate monoclinic modulated structure. The microstructure of martensite is in plate shape and self-organized in colonies. The maximum of 6 distinct martensite colonies and 24 kinds of variants in one parent grain are observed. Both of K-S and Pitsch orientation relations are found to be appropriate to describe the lattice correspondence between the parent and product phase. However, the transformation path related to Pitsch relation should be the real one that governs the transformation process in Ni-Mn-In alloys. With the determined martensitic transformation path, the formation mechanism of the microstructure of martensite phase is revealed. The 6 distinct martensite colonies are respectively generated by the six (110) planes of the cubic austenite phase during martensitic transformation. Each (110) plane transforms into four twin-related variants by changing the directions of the transformation plane and direction.

Microstructural evolution associated with martensitic transformation in Ni-Mn-Ga alloy

Based on the spatially resolved electron backscatter diffraction technique, the microstructural evolution accompanying the martensitic transformation (austenite to 7M martensite) and the intermartensitic transformation (7M martensite to NM martensite) was studied on a polycrystalline Ni53Mn22Ga25 alloy. Results show that the 7M martensite plate groups transformed from initial austenite have a diamond-shape with four twin-related variants. The 7M to NM intermartensitic transformation was accompanied by the thickening of martensite plates. With the experimental results, the characteristics of microstructural evolution during the phase transformations were further analyzed.

Grain boundary engineering in a thermo-mechanically processed Nb-stabilized austenitic stainless steel

Three different thermo-mechanical strategies—annealing, strain recrystallization and strain annealing—were applied to a Nb-stabilized 304H austenitic stainless steel in order to study their effects on grain boundary character distribution (GBCD). An Electron Backscatter Diffraction (EBSD) analysis revealed specific combinations of cold reduction-temperature-time that favor annealing twinning. A uniform increase in microstructural size and special boundaries (particularly for Σ3, Σ9 and Σ27 boundaries) was achieved under strain annealing conditions (low cold reductions) and long times at high temperatures (≥ 990°C). These conditions provide a high fraction of special boundaries (about 80%), which replace the random grain boundary network and thus optimize the GBCD. The profuse presence of Σ3n boundaries is attributed to the geometric interaction of twin-related variants during grain boundary migration. In addition to all this, precipitation takes place at the temperature range where optimum GBCD is achieved. The significance of precipitation in the different strategies was also tackled.

Conjugated precipitation of twin-related α and Ti2Cu phases in a Ti–25V–3Cu alloy

The formation of conjugated precipitates in a ternary Ti–25V–3Cu (wt.%) (Ti–24.4V–2.3Cu (at.%)) alloy subjected to isothermal aging at 500°C was investigated in this study. X-ray diffraction confirms that the equilibrium precipitates in the alloy are hexagonal α phase and body-centered-tetragonal (bct) Ti2Cu intermetallic compound. These precipitates are composed of two geometrically symmetrical components, which are verified by transmission electron microscopy (TEM) to be twin-related variants of α and Ti2Cu phases. Atomic-level characterizations incorporating high-resolution TEM and atom probe tomography were used to investigate the early-stage nucleation process of the precipitates. The results reveal that precipitation occurs heterogeneously on plate-like (or disc-like) metastable Cu-enriched GP zones that preferentially inhabit {110}β planes. Crystallographic orientation analyses suggest that the Ti2Cu phase is related to the matrix via a definite orientation relationship of (01¯3)Ti2Cu‖(011)β and [100]Ti2Cu‖[100]β, while the growth of the α phase is governed by the Potter’s OR such that (11¯01)α‖(011)β and [112¯0]α‖[11¯1]β. (01¯3)Ti2Cu and (11¯01)α are the twin planes of the respective phases. In spite of highly distinct crystal structures, the conjugated α and Ti2Cu phases attain a nearly perfect lattice correspondence along certain crystallographic planes, which results in a low interfacial energy configuration and favours the co-development of these phases.

Crystallographic insights into the intermartensitic transformation in Ni–Mn–Ga alloys

Ferromagnetic shape memory alloys such as Ni–Mn–Ga usually undergo intermartensitic transformation upon cooling or heating. Here, an attempt is made to explore the microstructural features and orientation relationships associated with the intermartensitic transformation from seven-layered modulated (7M) martensite to non-modulated (NM) martensite in a polycrystalline Ni53Mn22Ga25 alloy. Based on electron backscatter diffraction analysis, it is demonstrated that the intermartensitic transformation proceeds in an in-plate manner (through atomic reshuffling and lattice distortion) with specific orientation relationships between the two martensitic phases, i.e. (001)7M//(112)NM and [100]7M//[111¯]NM as well as (001)7M//(112)NM and [1¯00]7M//[111¯]NM, accompanied by the thickening of martensite plates and the surface relief effect. Consequently, one 7M variant (plate) evolves into one NM plate consisting of two twin-related fine variants, and four differently oriented 7M variants in one variant group result in a total of eight NM variants.

Crystallographic Orientation of the ε → α′ Martensitic (Athermal) Transformation in a FeMnAlSi Steel

The presence of athermal e- and α-martensite (α′) in the as-cast structure of a Fe-0.08C-1.95Si-15.1Mn-1.4Al-0.017N alloy has been revealed by electron backscattered diffraction analysis. The alloy exhibited two athermal martensitic transformations described by γ → α′ and γ → e → α′. The Shoji–Nishiyama orientation relationship was observed between γ-austenite and e-martensite, while α-martensite nucleated from γ-austenite exhibited a Kurdjumov–Sachs orientation relationship. Six crystallographic variants of α-martensite consisting of three twin-related variant pairs were observed in e-bands. A planar parallelism of {0001}e || {110}α′ and a directional relation of \( \left\langle {1\bar{1} 1} \right\rangle \alpha ' \) lying within 1 deg of \( \left\langle {\bar{1} 2\bar{1} 0} \right\rangle \varepsilon \) existed for these variants.