Compound Twins Research Articles

Crystal Structure and Microstructure of Martensite in Ni-Mn-In Alloys and the Behavior of Variant Rearrangement under Uniaxial Loading

In the present work, the crystal structure and microstructure of martensite in Ni-Mn-In alloys and the variant rearrangement behavior under the external mechanical loading were investigated. Results show that the martensite has an incommensurate 6M modulated structure (I2/m (α0γ)00) with the modulation wave vector q = 0.3437c*. Microstructure of martensite is in plate shape and self-organized with four orientation variants that are alternately distributed and related in either type-I, or type-II or compound twin relation. The variant interfaces are in coincidence with their corresponding twinning plane and then should be considered coherent. Under the uniaxial compression, the loading located in the common positive Schmid factor zone of the three types of detwinning systems might be favorable to obtain the single variant state. This study is expected to offer a fundamental information of crystal structure, microstructures and variant rearrangement behaviors in Ni-Mn-In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.

Crystallographic insights into Ni–Co–Mn–In metamagnetic shape memory alloys

In the present work, the morphological and crystallographic features of the 6M modulated martensite in the Ni45Co5Mn37In13 alloy were investigated by electron backscatter diffraction and high-resolution transmission electron microscopy (HRTEM) at room temperature. The 6M modulated martensite is in plate form and organized in colonies within which the plates stretch roughly in the same direction. Each colony has four types of orientation variants that are related to three kinds of twin relations, i.e. \{ 1 \bar 2\bar 3\} _{\rm M} type I, \langle \bar 3\bar 3{{1}} \rangle _{\rm M} type II and {{\{ 103\} }}_{\rm M} compound twins. The twinning shears of type I and type II twins are the same and equal to 0.2681, being about one order of magnitude higher than that of the compound twin (0.0330). Variant interfaces are microscopically defined by their corresponding twinning plane K 1. The HRTEM investigations show that the interfaces of the type I twin are straight and coherent at atomic scale, whereas those of the type II and compound twins are `stepped'. The step height of the compound twin interfaces is much larger than that of the type II twin interfaces. In view of variant organization, there is only one oriented type I interface and one compound twin interface, but there are two oriented type II interfaces which have an angular deviation of ±5.32° with respect to the type I twin interface. The results of the present work provide comprehensive information on morphological and crystallographic features of Ni–Co–Mn–In metamagnetic shape memory alloys.

Crystal structure of modulated martensite and crystallographic correlations between martensite variants of Ni50Mn38Sn12alloy

A comprehensive study on the crystal structure, the microstructure and the crystallographic features of the martensite in an Ni50Mn38Sn12alloy has been conducted in the present work. The results show that the martensite possesses a 4O modulated structure. The martensite is organized into broad plates in the original austenite grain. The plates contain irregularly shaped colonies with two characteristic microstructural patterns: a classical lamellar pattern and a herringbone pattern. Crystallographic analyses by scanning electron microscopy/electron backscatter diffraction demonstrate that in each colony there are four orientation variants (A, B, C and D) and they form three types of twins (type I, type II and compound twin). The interfaces between corresponding variants are coincident with their twinning planeK1. The interface planes of the compound twin pairs A&D and B&C can have one or two different orientations, which leads to the two microstructural patterns. The corresponding variants in neighboring colonies within one broad plate (intra-plate colonies) possess close orientations, but the type I and the type II twin relationships are interchanged. The variants in neighboring colonies situated in adjacent plates (inter-plate colonies) are type I or type II twin related but with some angular deviations. The plate interface is defined by the {221} plane of the variant pair with largest thickness. The results of the present work provide comprehensive microstructural and crystallographic information on modulated martensite in NiMnSn alloys that is useful for the understanding of their specific functionalities and helpful for further investigation on property optimization of these materials.

Constrained hierarchical twinning in Ru-based high temperature shape memory alloys

The microstructure of high temperature shape memory alloys of the Ru-Ta and Ru-Nb family are investigated in detail. After cooling from the high temperature cubic β phase the off-stoichiometric alloys show a compound twinning in the tetragonal β′ phase which prevails at room temperature and can be observed as two levels of laminates A and C [1]. The stoichiometric alloys are characterized by a second phase transformation from β′ to monoclinic β″, which adds another type of twinning. The latter belongs to the monoclinic P2/m structure and has been created out of one of the lost symmetry elements from the tetragonal P4/mmm phase during the β′ → β″ transformation. The unusual characteristic of this new twinning, called B, is the fact that it has an intermediate size between the A and C twins that belong to the high-temperature transformation β′ → β″. This is due to a second constrained martensitic transformation.

Variant organization and mechanical detwinning of modulated martensite in Ni–Mn–In metamagnetic shape-memory alloys

Based upon microstructural and crystallographic characterizations, the variant organization and mechanical detwinning of 6M modulated martensite in Ni–Mn–In alloys have been studied. In the stress-free circumstance, each martensite colony is composed of four distinct orientation variants that are related to one another in three twin relations. Adjacent variants with type-I twin relation possess straight interfaces, whereas those with type-II or compound twin relation have stepped interfaces at atomic scale. Schmid factor (SF) calculations show that under uniaxial compression condition, the loading orientation zones with high SFs for the type-I and type-II detwinning systems are close to each other, being at about 90° away from those of the compound detwinning system. The detwinning resistances of the different types of twins are considered to be associated with their twinning shears and twin boundary structures. Type-I twin possesses much larger detwinning resistance than that of type-II twin, though they have the same amount of twinning shear (0.2392). Compound twin has the smallest detwinning resistance, which is benefited from its tiny twinning shear (0.0277) and stepped twin interface. Under external loading, martensite variants react locally within colonies through detwinning if the loading orientation is favorable. It is possible to obtain single variant state in some colonies when the loading orientation is located in the common positive SF zone of the three detwinning systems. This study is expected to provide fundamental information on local variant reorientation of 6M Ni–Mn–In martensite during deformation.

Crystal structure and crystallographic characteristics of martensite in Ni50Mn38Sb12 alloys

For Ni–Mn–Sb ferromagnetic multifunctional alloys, the crystal structures of martensite variants and the orientation relationships between them are decisive factors for their magnetic field-induced behaviours and are hence of importance. Such information has rarely been reported in the literature. In the present work, the crystal structure, microstructure and orientation relationships of Ni–Mn–Sb martensite were thoroughly investigated by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Through XRD analyses, the crystal structure of the martensite, including the crystal system, the space group, the lattice parameters and the atomic coordinates, was fully resolved. The structure is orthorhombic and can be represented with a 4O superstructure. EBSD analyses show that the Ni–Mn–Sb martensite has a lamellar form. One martensite lamella corresponds to one orientation variant. The lamellae are organized in long plate-shaped colonies. Within each colony, four distinct orientation variants (A, B, C and D) appear repeatedly and extend in roughly the same direction. The four variants are twin related to one another, with variants A and C (or variants B and D) forming a type I twin, variants A and B (or C and D) a type II twin, and variants A and D (or B and C) a compound twin. The complete twinning elements for each twin relation were thus fully determined. The interfaces between the variants were identified to be their corresponding twinning planes. All these results provide fundamental information for Ni–Mn–Sb alloys that is useful for interpreting their magnetic and mechanical characteristics.

Transformation twinning and deformation twinning of NiTi shape memory alloy

Transformation twinning and deformation twinning of NiTi shape memory alloy are investigated on the basis of a nominal chemical composition of Ni50Ti50 and Ni50.9Ti49.1 (atomic percent), respectively. In the case of Ni50Ti50 alloy, {111¯} type Ⅰ twin and 〈011〉 type Ⅱ twin can be observed in the NiTi matrix, but it seems that {111¯} type Ⅰ twin is dominant. In the case of as-rolled Ni50Ti50 alloy subjected to heating for 2h at 850°C and subsequent quenching into liquid nitrogen, in particular, the primary twin and the secondary twin can be observed. The occurrence of the secondary twin plays a critical role in guaranteeing the compatibility of martensitic transformation. In the Ni50.9Ti49.1 samples aged at 600°C, the occurrence of martensitic twins is attributed to the local inhomogeneous composition in the microstructures and the dislocation networks at the interface between the incoherent Ni4Ti3 precipitates and the B2 matrix. The (001) martensitic compound twins appear in the Ni50.9Ti49.1 sample subjected to local canning compression at room temperature and subsequent annealing at 600°C. The phenomenon is attributed to the fact that the lattice distortion along with the dislocation defects frequently occurs in the coarse-grained NiTi sample derived from crystallization of the amorphous phase and thus contributes to the occurrence of the (001) martensitic compound twins. A high density of dislocations is distributed in the B2 austenite matrix and {114} B2 austenitic compound twin can be observed in the Ni50.9Ti49.1 sample subjected to local canning compression at 300°C, which indicates that dislocation slip and deformation twinning play an important role in local canning compression of Ni50.9Ti49.1 alloy at 300°C.

EBSD characterization of crystallographic orientations and twin interfaces of modulated martensite in epitaxial NiMnGa thin film

The crystal structure and the global texture of 7M modulated martensite in epitaxial Ni50Mn30Ga20 thin films were investigated by X-ray diffraction (XRD), and the local crystallographic orientations correlated with microstructural features were revealed by electron backscatter diffraction (EBSD). The microstructure of 7M martensite can be classified into two distinct constituents. One refers to long straight strips with relatively homogeneous contrast, running parallel to one edge direction of the substrate. The other refers to shorter and bent plates with relatively high relative contrast, being oriented with the plate length direction roughly in 45° with respect to the substrate edges. Each 7M martensite plate is designated as a single orientation variant, and four orientation variants that are twin-related one another constitute one variant group. With the local crystallographic orientations of martensite plates and the orientations of inter-plate interface traces, the Type I, Type II and compound twin interfaces in the low and high relative contrast zones were determined.

Evolution of Intergranular Stresses in a Martensitic and an Austenitic NiTi Wire During Loading–Unloading Tensile Deformation

In situ synchrotron X-ray diffraction testing was carried out on a martensitic and an austenitic NiTi wire to study the evolution of internal stresses and the stress-induced martensite (SIM) phase transformation during room temperature tensile deformation. From the point of lattice strain evolution, it is concluded that (1) for the martensitic NiTi wire, detwinning of the [011]B19′ type II twins and the {010}B19′ compound twins is responsible for internal strains formed at the early stage of deformation. (2) The measured diffraction moduli of individual martensite families show large elastic anisotropy and strong influences of texture. (3) For the austenitic NiTi wire, internal residual stresses were produced due to transformation-induced plasticity, which is more likely to occur in austenite families that have higher elastic moduli than their associated martensite families. (4) Plastic deformation was observed in the SIM at higher stresses, which largely decreased the lower plateau stresses.

Effects of Co and Al addition on martensitic transformation and microstructure in ZrCu-based shape memory alloys

The effect of ternary alloying element Al and quaternary alloying element Co on the martensitic transformation of ZrCu-based shape memory alloy was investigated. The results show that the addition of Al and Co in ZrCu alloy decreases both the martensitic transformation temperature and the martensitic transformation temperature hysteresis. Transmission electron microscope (TEM) observations reveal that the Cm martensite structure is the preferential formation phase. The intervariant structures in ZrCuAlCo alloy are (021) type I twins, while the dominant substructures inside the martensite variant are the (001) compound twins. With the increase of Co content, tensile fracture strength and strain are improved obviously.

Generalized stacking fault energies in the basal plane of triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

Molecular dynamics and molecular statics simulations were used to investigate dislocation glide in the basal plane (i.e. the – plane, where , and define the edge vectors of the unit cell) of -triamino--trinitrobenzene (TATB) using generalized stacking fault energy surfaces (-surfaces). Triclinic symmetry and molecular packing arrangement result in two glide plane types for the same glide plane normal. The unstable stacking fault energies () are less than at K and atm, indicating easy dislocation glide. Glide in the and – directions is preferred compared to the direction at atm. A stable stacking fault is predicted in the – direction at atm and 5 GPa. A compound twin with energy is predicted to be stable in the basal plane. Unequal values of about the stable stacking fault in the and – traces indicate an asymmetric barrier to dislocation glide. High pressure ( GPa) results in an increase in the values. The extremely small barriers to twinning and dislocation glide might be sources for observed second-harmonic generation in the nominally centrosymmetric crystal.

Effects of aging on the shape memory behavior of Ni-rich Ni50.3Ti29.7Hf20 single crystals

The shape memory properties of solutionized and aged Ni-rich Ni50.3Ti29.7Hf20 single crystals were investigated along the [001], [011] and [111] orientations in compression. The effects of crystal orientation and aging temperature on the transformation strain, thermal hysteresis and Clausius–Clapeyron (CC) relation were determined. Aging at 550°C for 3h introduced coherent 10–20nm precipitates in the matrix, which substantially improved the shape memory and mechanical properties of the Ni50.3Ti29.7Hf20 crystals. [001]-oriented single crystals showed high dimensional stability under stress levels as high as 1500MPa in both the solutionized and aged conditions, but with transformation strains of <2%. Thermal treatments can be used to tailor the transformation temperatures over a wide range with the martensite start temperature varying from −25°C in the solutionized case to 123°C by aging at 650°C for 3h. Compared to the solutionized condition, thermal hysteresis was reduced after aging at 550°C/3h, but increased with aging at 650°C. Perfect superelasticity with recoverable strain of >4% was observed for solutionized and 550°C/3h aged single crystals along the [011] and [111] orientations, and general superelastic behavior was observed over a wide temperature range. In contrast, aged [001]-oriented single crystals have a very high CC slope, in the range of 30–40MPa°C−1, which results in a lack of superelasticity. Theoretical transformation strains were calculated by using the energy minimization method and lattice deformation theory. The calculated transformation strains were higher than the experimentally observed strains since the calculated strains could not capture the formation of martensite plates with (001) compound twins.

Size Effects on the Martensitic Phase Transformation in Ti54.5Ni45.5 Alloy Thin Films

Ti54.5Ni45.5 alloy nonorganic thin film was prepared by direct current (DC) magnetron sputtering system and subsequent rapid annealing treatment. Electrical resistivity(ER), X-ray diffraction (XRD) and transmission electron microscope (TEM) were carried out to investigate martensitic phase transformation behaviour and microstructure. The results demonstrated that the grains of the thin film can be refined through increasing heating rate, the smaller grain size is, the lower martensitic phase transition temperature. The martensitic phase transformation and its reversible phase transition were observed as the grain size was 50nm, which was smaller than the critical dimension (60nm) of recent theoretical predictions. In addition, the martensitic variant mainly showed the single-pair morphologies with (001) compound twinning.

Preferential Morphology of Self-accommodation Microstructure in Ti-Ni-Pd Shape Memory Alloy

The formation frequency of junction planes (JPs) and orientation relationships across the JPs in self-accommodation microstructure in Ti-25Ni-25Pd was investigated by TEM and SEM-EBSD. Each martensite plates followed the invariant plane condition and identified as habit plane variants (HPV). The self-accommodation microstructure consisted of four kinds of HPV and contained three kinds of JPs. The formation frequency of each JP was 14% for {011}B19 compound twin, 36% for <111>B19type I twin and 50% for <211>B19type II twin. The formation frequency was explained by the magnitude of the incompatibility at the JPs.

Twin relationships between nanotwins inside A–C type variant pair in Ni–Mn–Ga alloy

The twin relationships of the A–C type non-modulated martensitic variant pair in Ni54Mn25Ga21 ferromagnetic shape memory alloy were investigated in detail through experimental observations and theoretical calculations. Variants A and C are microtwins connected by intervariant interface, while the lamellae in each variant are nanotwins separated by twin boundaries. The nanotwins with different thicknesses in each variant have two types: the major lamella and the minor lamella. The orientation relationships between the lamellae across an intervariant interface vary with the volume fraction of the minor lamella. The twin relationships between the nanotwins were identified experimentally and theoretically. Specifically, the lamellae in a variant have a {112} compound twin relationship, the major lamellae (or minor lamellae) are twin related with a twinning plane of high indices, but the major and minor lamellae across the A–C interface are not twin related. The mobility of various interfaces, such as the twin boundary in a variant and intervariant interface is discussed, based on the identified twin relationships.

Microstructural characterization and shape memory characteristics of the Ni50.3Ti34.7Hf15 shape memory alloy

The effect of precipitation on the microstructure and shape memory characteristics of the Ni50.3Ti34.7Hf15 shape memory alloy has been investigated via transmission electron microscopy, differential scanning calorimetry and load-biased thermal cycling tests in tension. A one-stage martensitic transformation from B2 austenite to B19′ martensite was observed in all the aged samples but the transformation temperatures followed a more complicated trend depending on specific aging conditions. The transformation temperatures decreased below room temperature when the precipitate size, and thus the interparticle distance, was below ∼20nm, as occurred after short aging times at low temperatures. On the other hand, the transformation temperatures can be increased over a wide temperature range by increasing the precipitate size and volume fraction through aging for long durations or at higher temperatures. The alloy demonstrated excellent dimensional stability under stress levels as high as 300MPa as a consequence of precipitation hardening, with a maximum fully recoverable strain of 3.3% after aging at 450°C for 10h. The transformation thermal hysteresis also decreased in the aged samples due to reduced defect generation in the precipitation-strengthened samples. The precipitate crystal structure was identified as the H-phase that was recently reported in Ni-rich NiTiHf and NiTiZr alloys, and not the Ni4Ti3-type structure as reported in a few earlier studies. The H-phase present in the Ni-rich NiTiHf alloy does not change the twinning relations in the B19′ martensite phase compared to (Ti+Hf)-rich NiTiHf alloys, being a mixture of (001) compound twins and (011) Type I twins. On the other hand, the precipitates have a significant effect on martensite morphology and load-biased thermal cycling response, both of which can be manipulated by controlling the precipitate size.

Microstructure and Tensile Behavior of Ti-Rich Ti-Ni-Cu Melt-Spun Ribbon

Microstructure and mechanical properties of Ti51.5Ni25Cu23.5 ribbon fabricated by melt spinning were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile tests. Some B19 martensite crystalline with (011) compound twin was embedded in the mainly amorphous ribbon, while the ribbon annealed at 450°C for 1 h is at fully martensitic state. Annealing process alter the preferential orientation from (022)-B19 to (111)-B19. Tensile fracture stresses of as-spun ribbon and the annealed ribbon are 1257 MPa and 250 MPa, respectively. The tensile fracture morphology of as-spun ribbon shows typical vein fringe while that of the annealed ribbon reveals fine but depth-inhomogeneous dimples. After tensile deformation, the annealed ribbon exhibits typical martensitic detwinning behavior accompanying with the strain contrast.

Crystallization of amorphous NiTi shape memory alloy fabricated by severe plastic deformation

Based on the local canning compression, severe plastic deformation (SPD) is able to lead to the almost complete amorphous nickel-titanium shape memory alloy (NiTi SMA), in which a small amount of retained nanocrystalline phase is embedded in the amorphous matrix. Crystallization of amorphous NiTi alloy annealed at 573, 723 and 873 K was investigated, respectively. The crystallization kinetics of the amorphous NiTi alloy can be mathematically described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. NiTi SMA with a complete nanocrystalline phase is obtained in the case of annealing at 573 K and 723 K, where martensite phase transformation is suppressed due to the constraint of the grain boundaries. Crystallization of amorphous NiTi alloy at 873 K leads to the coarse-grained NiTi sample, where (001) martensite compound twin is observed at room temperature. It can be found that the martensitic twins preferentially nucleate at the grain boundary and they grow up towards the two different grains. SPD based on the local canning compression and subsequent annealing provides a new approach to obtain the nanocrystalline NiTi SMA.

Identification of Crystal Structure and Crystallographic Features of NiMnGa Thin Films by Combination of X-Ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD)

In this work, NiMnGa thin film composed of non-modulated martensite (NM) and seven-layered modulated martensite (7M) was produced. The crystal structure and lattice constants were determined by X-ray diffractometer (XRD). The preferred crystallographic orientation of martensite was determined using the four-circle XRD. SEM/EBSD was employed to verify the crystal structure of the martensite and to reveal its crystallographic features correlated with the microstructure. According to the XRD patterns, the crystal structure of NM and 7M was determined as tetragonal and monoclinic crystal structure, respectively. Pole figures measured by four-circle diffractometer revealed that the NM martensite possesses (004)NM and (220)NM preferred plane texture close to the substrate surface, whereas the 7M martensite has (2 0 20)7M, (2 0 )7M and (040)7M preferred plane texture close to the substrate surface. SEM/EBSD analysis shows that the surface layer of the film is mainly composed of NM martensite that is organized in variant groups. In each variant group, all the martensite plates consist of paired lamellar (112)NM compound twins and there are eight orientation variants in each variant group.

Structural transformations in NiTi shape memory alloy nanowires

Martensitic phase transformation in bulk Nickle-Titanium (NiTi)—the most widely used shape memory alloy—has been extensively studied in the past. However, the structures and properties of nanostructured NiTi remain poorly understood. Here, we perform molecular dynamics simulations to study structural transformations in NiTi nanowires. We find that the tendency to reduce the surface energy in NiTi nanowires can lead to a new phase transformation mechanism from the austenitic B2 to the martensitic B19 phase. We further show that the NiTi nanowires exhibit the pseudoelastic effects during thermo-mechanical cycling of loading and unloading via the B2 and B19 transformations. Our simulations also reveal the unique formation of compound twins, which are expected to dominate the patterning of the nanostructured NiTi alloys at high loads. This work provides the novel mechanistic insights into the martensitic phase transformations in nanostructured shape memory alloy systems.