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.
The crystal structure of modulated martensite in Mn-rich off-stoichiometric Ni2Mn1.44In0.56 alloy was determined with high-resolution powder neutron diffraction and synchrotron X-ray diffraction in the frame of (3 + 1)-dimensional superspace theory. The average crystal structure and the modulation wave vector were firstly derived by analyzing the reflection separations induced by the martensitic transformation on the basis of the transformation orientation inheritance. This treatment could be applied to predetermine the modulated structures of materials with displacive structural transformation. The crystal structure of modulated martensite was finally refined by the Rietveld method. Results show that the martensite possesses an incommensurate 6M modulated structure of superspace group I2/m(α0γ)00, with lattice parameters a = 4.3919(4) Å, b = 5.6202(1) Å, c = 4.3315(7) Å, and β = 93.044(1)°, and the modulation wave vector q = 0.343(7) c*. The detailed site occupations for extra-Mn atoms with respect to the stoichiometric case were investigated by ab initio calculations. The extra-Mn atoms have a preference to be uniformly dispersed. A threefold layered superstructure in the 3-dimensional space was proposed to approximately describe the incommensurate modulated structure. This 6M superstructure model is considered to be representative for off-stoichiometric Ni–(Co)–Mn–In modulated martensite with martensitic transformation around room temperature. The present study is expected to offer an important basis for reliable crystallographic and microstructural characterizations on Ni–Mn–In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.
The growth characteristics of eutectic Si in unmodified and Sr-modified Al–12.7%Si alloys were investigated by microstructure-correlated crystallographic analyses. For the unmodified alloys, the formation of repeated single-orientation twin variants enables rapid growth of eutectic Si according to the twin plane re-entrant edge (TPRE) mechanism. Microscopically, Si crystals are plate-like elongated in one 〈1 1 0〉 direction that is not in accordance with the 〈1 1 2〉 growth assumed by the TPRE model. The 〈1 1 0〉 growth direction is realized by paired 〈1 1 2〉 zigzag growth on parallel twinning planes, leading to alternate disappearance and creation of 141° re-entrants. As each twinning plane is associated with three re-entrants, Si crystals may extend in three co-planar 〈1 1 0〉 directions and cause the formation of equilateral plates. With the formation of α-Al around eutectic Si, the number of re-entrants is reduced. The planar isotropic growth of eutectic Si becomes anisotropic, leading to the formation of long plates. The reduction of the number of re-entrants also accounts for the width and thickness changes over the length of Si plates. This complex growth mode results in Si crystals exposing only their low-energy {1 1 1} planes to the melt. For the Sr-modified alloys, substantial changes appear in the eutectic Si morphology, attributable to the restricted TPRE growth and the impurity induced twinning (IIT) growth. The former enhances lateral growth by forming new twins with parallel twinning planes, while the latter leads to isotropic growth by forming differently oriented twins.
The microstructural and crystallographic features of seven-layer modulated (7M) martensite in Ni50Mn30Ga20 thin films grown on MgO(0 0 1) substrate were revealed by electron backscatter diffraction (EBSD) and secondary electron imaging (SEI). Locally, each group of martensite plates consisted of four orientation variants that are twin-related to one another. The Type-I and Type-II twin relationships were most prevalent in individual plate groups with low or high relative SEI contrast, respectively. A general procedure was developed to quantitatively assess the accommodation capacities of twin variant pairs constrained by the rigid substrate. It is understood that the configuration of either Type-I twin variant pairs in low relative contrast zones or Type-II twin variant pairs in high relative contrast zones can effectively accommodate the shear deformation in the film normal direction – a deformation that tends to “peel” the film surface off the substrate. As a consequence, it brings about the preferential formation of Type-I and Type-II twins during the martensitic transformation. This finding is of significance as it highlights the role of external constraint on microstructure control toward property modification.
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. (0 0 1)7M//(1 1 2)NM and [1 0 0]7M//[1 1 ]NM as well as (0 0 1)7M//(1 1 2)NM and [ 0 0]7M//[1 1 ]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.
For Ni–Mn–Ga alloys, giant magnetic-field-induced strains may be achieved in a modulated martensitic state, offering attractive chances for academic and practical exploration. However, the metastability of modulated martensite imposes a severe constraint on the capacity of these alloys as promising materials for sensors and actuators. In the present work, we conduct both experimental examinations and ab initio calculations to seek potential remedies of this critical problem through composition tuning. Results show that, for Group II alloys having modulated martensite at reasonable temperatures, the increase in Ni addition results in an enhanced tendency to the formation of non-modulated (NM) martensite, whereas the proper Mn addition leads to the stabilization of seven-layered modulated (7M) martensite, which serves as the structural ground state of martensite. By correlating the microstructural evolutions with the two-stage phase transformation (i.e. austenite → 7M martensite → NM martensite), it is demonstrated that the 7M martensite possesses lower energy barriers in terms of the lattice distortion of parent austenite and the interfacial energy of martensitic variants, which plays a vital role in bridging the austenite to NM martensite transformation. This result is expected to provide useful information for the design of these new functional materials.
Epitaxially grown thin films with nominal composition Ni50Mn30Ga20 and thickness 1.5 μm were prepared on MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. The surface layer microstructures of the as-deposited thin films consist of non-modulated (NM) martensite plates with tetragonal structure at ambient temperature, which can be classified into the low and high relative contrast zones of clustered plates (i.e. plate colonies) with parallel or near-parallel inter-plate interface traces in secondary electron images. Orientation analyses by electron backscatter diffraction revealed that individual NM plates are composed of alternately distributed thicker and thinner lamellar variants with (1 1 2)Tetr compound twin relationship and coherent interlamellar interfaces. In each plate colony, there are four distinct plates in terms of the crystallographic orientation of the thicker lamellar variants and therefore, in total, eight orientation variants. For the low relative contrast zones, both thicker and thinner lamellar variants in adjacent plates are distributed symmetrically across their inter-plate interfaces (along the substrate edges). At the atomic level, there are no unbalanced interfacial misfits and height misfits, resulting in long and straight inter-plate interfaces with homogeneous contrast. However, in the high relative contrast zones, the thicker and thinner lamellar variants in adjacent plates are oriented asymmetrically across their inter-plate interfaces (at ∼45° to the substrate edges). Significant atomic misfits appear in the vicinity of the inter-plate interfaces and in the film normal direction. The former result in bending of the inter-plate interfaces, and the latter give rise to the high relative contrast between adjacent plates.
For Ni–Mn–Ga ferromagnetic shape memory alloys, a large magnetic-field-induced strain could be reached through the reorientation of martensitic variants in the martensite state. Owing to the collective and displacive nature of the austenite to martensite transformation, a certain orientation relationship (OR) between the parent and the product phase is required to minimize the transformation strain and the strain energy generated, which brings about self-accommodating groups of martensitic variants with specific orientation correlations. In this work, the microstructural and crystallographic characteristics of martensitic variants in a polycrystalline Ni50Mn30Ga20 alloy were investigated by electron backscatter diffraction analysis. With accurate orientation measurement on inherited martensitic variants, the local orientations of parent austenite grains were predicted using four classical OR for the martensitic transformation. Furthermore, a specific OR, namely the Pitsch relation with (1 0 1)A//(1 )7M and [1 0 ]A//[ 1]7M, was unambiguously determined by considering the magnitude of discontinuity between the lattices of the product and parent phases and the structural modulation of the incommensurate 7M modulated martensite. The present procedure to determine the OR, without recourse to the presence of retained austenite, is in general applicable to a variety of materials with modulated superstructure for insight into their martensitic transformation processes.
For Ni–Mn–Ga ferromagnetic shape memory alloys, the characteristic features of modulated martensite (including the number/shape of constituent variants, the inter-variant orientation relationship and the geometrical distribution of variant interfaces) determine the attainability of the shape memory effect. In the present work, a comprehensive microstructural and crystallographic investigation has been conducted on a bulk polycrystalline Ni50Mn28Ga22 alloy. As a first attempt, the orientation measurements by electron backscatter diffraction (EBSD) – using the precise information on the commensurate 5M modulated monoclinic superstructure (instead of the conventionally simplified non-modulated tetragonal structure) – were successfully performed to identify the crystallographic orientations on an individual basis. Consequently, the morphology of modulated martensite, the orientation relationships between adjacent variants and the characters of twin interfaces were unambiguously determined. With the thus-obtained full-featured image on the configuration of martensitic variants, the possibility of microstructural modification by proper mechanical “training” was further discussed. This new effort makes it feasible to explore the crystallographic/microstructural correlations in modulated martensite with high statistical reliability, which in turn provides useful guidance for optimizing the microstructure and shape memory performance.
A systematic investigation of microstructure and crystallography of the multiply twinned structure at the nanometer scale in a Ni53Mn25Ga22 ferromagnetic shape memory alloy (FSMA) was performed. Nanoscale internal twins inside the micrometer-scale martensitic lamellae were observed. In each lamella, two nanotwin variants with a compound twinning relationship exist, and the twinning elements are . Two types of lamellar interfaces, i.e., interpenetrated interlamellar interface and stepped intralamellar interface, were revealed. The orientation relationships between the nanotwins connected by these two types of interfaces were unambiguously determined, and it was found that the respective orientation relationship between the neighboring nanotwins depends strongly on the adjacency condition at the interfaces. The possible formation mechanisms of the interpenetrated interlamellar interface and stepped intralamellar interface are discussed. These results are useful for property optimization by microstructure control in the FSMAs.
A detailed study of martensitic transformation crystallography and microstructural characteristics in the Ni53Mn25Ga22 ferromagnetic shape memory alloy (FSMA) was performed by both experimental observation and theoretical calculation. It is revealed that there are two microscopically twin-related martensitic variants with a misorientation of ∼82° around the 〈1 1 0〉M axis in each initial austenite grain. The twin interface plane was determined to be {0.399 0.383 0.833}M (1.79° away from {1 1 2}M). The ratio of the amounts of the two variants inherited from one single austenite grain is about 1.70. The prevalent orientation relationship between austenite and martensite was found to be Kurdjumov–Sachs (K–S) relationship with (1 1 1)A//(1 0 1)M, . A successful explanation of the crystallographic features during martensitic transformation will shed light on the development of FSMAs with optimal performance.
In situ high-energy X-ray diffraction experiments reveal the influence of various microstresses and grain orientations on the memory effect in polycrystalline Ni2MnGa ferromagnetic shape-memory alloys during phase transformation (see figure). The “memory” of grain orientation and intergranular stress is evident; however, the microscale “memory” is degraded due to the existence of the intragranular stress.
Lattice strain distributions mapped by high energy X-ray diffraction (see Figure) in an electrodeposited iron coating with nanocrystalline grains show high nonlinearity, providing unambiguous evidence for the existence of a strong stress interaction along the growth/deposition direction in a layered material.
