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Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e. , the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni–Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni–Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni–Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Many phase transformations produce crystallographic variants. Under a mechanical constraint, certain variants could be selected. Although efforts have been made on resolving the selection rule, the transformation lattice deformation associated selection mechanism has not been well addressed. Thus in the present work, α variant selection of the β to α transformation in a metastable β-Ti alloy under compression was studied. Results show that the selection of the α variants is strongly affected by the imposed strain and the applied load with dependence on the local crystal perfection of the β grains. In the slightly deformed β grains, 2 Burger orientation relationship (BOR) variants forming 'cross-shaped' clusters and interrelated by a 90° rotation around the <1 1.38 2. 38 ¯ 0> α axis are selected and form in large quantities (group I variants). Such variants consume the maximum deformation work by the applied load. This energy consumption is rooted from the maximum strain contribution of the selected variants to the macroscopic deformation and the maximum shear stress from the external load resolved on their { 1 1 ¯ 2 ¯ } β < 1 ¯ 1 1 ¯ > β systems for transformation. In the heavily deformed β grains occupied by dislocation slip bands, several numbers (2–4) of BOR variants are selected but form in much less quantities (group II variants). The selection energy criterion is still obeyed by the group II variants but with restriction from the local deformation. The present work provides clear information on the interweaving of the imposed compression with the internal lattice deformation and its impact on the β to α transformation variant selection. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Phase transformation in solids always follows specific orientation relationships (ORs). The OR usually ensures a minimum lattice deformation for the structure change. However, in many cases different ORs are respected by the same transformation. The selection role and the link between the OR and the existing crystal defects needs further investigation. Thus, in this work, the α to β heating phase transformation induced by high density Electric Current Pulse (ECP) treatments in an annealed Cu–40%Zn alloy was investigated. Results show that the β phase obeys the K-S OR when formed along the α grain boundaries or in their vicinities, or the N-W OR when formed in the α grain interiors. In the former sites, the {111} α /< 1 1 ¯ 0 > α dislocation arrays were frequently observed, whereas in the latter, the {111} α /< 11 2 ¯ > α stacking faults were often found. Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111} α plane in the < 1 1 ¯ 0 > α direction, whereas under the N-W OR the maximum deformation is a shear on the {1 11 } α plane in the < 11 2 ¯ > α direction. Thus the existing {111} α /< 1 1 ¯ 0 > α dislocation arrays along the α grain boundaries and in their vicinities provide pre-strain required by the transformation via the K-S path, whereas the {111} α /< 11 2 ¯ > α stacking faults boarded by {111} α /< 11 2 ¯ > α partial dislocations offer pre-strain facilitating the transformation via the N-W path. The present results provide new information on the role of crystal defects on phase transformation strain path and the selection of transformation ORs. [ABSTRACT FROM AUTHOR]

The β to α phase transformation of Ti alloys progresses in a displacive-diffusive mixed-mode. The associated transformation strain has important influence on the resultant microstructure. In this work, the microstructural features of α precipitates in a metastable β Ti alloy, Ti-7333, were thoroughly investigated. Special attention was paid to the intragranular α for the advantage of a stress-free transformation environment. Results show that the constituents of each α precipitate is not single. Two kinds of nano-sized α domains exist. One is situated on the broad face of the major α precipitate (named interface α), and the other goes through the major α (termed penetrating α). The interface α is related with the β matrix by the Burgers orientation relationship (BOR) and with the major α by a 60°/ 〈 11 2 ¯ 0 〉 α rotation. The nucleation of such α particles is induced by the largest shear strain generated by the formation of the major α. They act as stress-associated sympathetic nuclei of the neighboring α precipitates and eventually contribute to the formation of the triangular α cluster. The penetrating α does not obey the BOR with the β matrix but is related with the major α by a 60° rotation around another 〈 11 2 ¯ 0 〉 α axis. The nucleation of such α is induced by the largest normal strain generated by the formation of the main α plate. This work provides comprehensive information on the displacive characters of the β to α transformation and their impact on the microstructure of metastable β Ti alloys. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A statistical analysis is conducted on sequential {1 0 −1 2}–{1 0 −1 2} double extension twinning in a Mg AZ31 alloy obtained after two mutually perpendicular successive compressions. Indeed, primary twins form during the first compression, which produce secondary twins during the second compression. Here, “sequential double twinning” is adopted to describe this twinning mechanism, which is different from classic double twinning obtained during monotonic loading. Double extension twins, no matter sequential or classic ones, can be classified into Groups I–IV according to their misorientations with respect to the parent grain. In the present study, the majority of the sequential double extension twins have relatively high Schmid factors (SFs) and a misorientation of <0, 14, −14, 1> 60° with respect to the parent grain (Group III). However, the classical SF based analysis fails to explain why secondary twin variants in Group III are preferred over their nearest orientated neighbors in Group IV, since their SF differences are negligible. Therefore, a static double inclusion model is developed to analytically compute the elastic energy in a final configuration of sequential double twinning. The model well explains the predominance of secondary twins in Group III by a noticeable greater decrease of free energy after secondary twinning. [ABSTRACT FROM AUTHOR]

The microstructural and crystallographic features of seven-layer modulated (7M) martensite in Ni 50 Mn 30 Ga 20 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. [ABSTRACT FROM AUTHOR]

The crystal structure of modulated martensite in Mn-rich off-stoichiometric Ni 2 Mn 1.44 In 0.56 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. [ABSTRACT FROM AUTHOR]

A statistical analysis is conducted on the formation of {1 0 -1 2} extension twin variants with low Schmid factors (SFs), based on large data sets acquired by electron backscatter diffraction on a deformed Mg AZ31 alloy. Two physical conditions of the analyzed low SF twins are characterized: those solely at grain boundaries (group 1) and those in a pair of cross-boundary twins (group 2). The presence of a majority of them can be explained by strain compatibility arguments. The twins in group 1 require not only the most or more accommodation through basal slip with very low critical resolved shear stress (CRSS), but also the least or less accommodation through pyramidal slip with very high CRSS, in their neighboring grains. Those in group 2 also require the least or less accommodations through pyramidal slip and/or contraction twinning with very high CRSSs, but in their neighboring twins. However, they require the most or more accommodation through prismatic slip and/or extension twinning with low CRSSs. This is due to the texture of the material with respect to the loading geometry that renders different low CRSS accommodation systems available to the two different groups of low SF twins. The shears induced by low SF twins can be accommodated by several different deformation systems. In such cases, twin variant selection criteria depending on only one deformation system are limited in the prediction of twin variants. [ABSTRACT FROM AUTHOR]

In this paper Ti (T40) is deformed by interrupted in situ tensile tests. Twinning activation and evolution are evidenced by scanning electron microscopy/electron backscatter diffraction and analyzed by examining their strain accommodation in the frame of all the potential accommodative systems in their neighborhoods. It is revealed that the formation of twins results from the local incompatible strains produced by dislocation slips in grain boundary areas. Either high positive Schmid factor and low positive Schmid factor twins or even negative Schmid factor twins could be activated. Twins are always accompanied by certain accommodative deformation (twinning and slip) in their immediate neighborhoods. Accommodative slips and twins have different impacts on the further development of the initiating twins. Twins accommodated by twins and possessing high Schmid factors have a higher capacity to propagate and repeatedly nucleate; whereas twins accommodated by slips and possessing low Schmid factors have less power and are static. Twins can be permanent or transient depending on their Schmid factors. For those with positive Schmid factors, their strain contributions are consistent with the macroscopic strains, thus they are stable, whereas those with negative Schmid factors are unstable. The formation of such twins is due to the unbalanced local constraints induced by high Schmid factor deformation that generate the so-called back-stress. These twins make opposite contributions to the macroscopic strains. With deformation, the back-stress could be accommodated by local high Schmid factor compatible deformation. The crystal of the twinned part has to make a positive contribution in accordance with the macroscopic strains to be compatible with its surroundings, which may give rise to the activation of detwinning. [ABSTRACT FROM AUTHOR]

Abstract: Structural modulation of martensitic phases is regarded as a prerequisite for magnetically or electrically induced reversible strains in shape-memory alloys. Controversy surrounds the crystal structure of modulated martensite. Here, we explore this critical issue through combined spatially resolved microstructural and crystallographic characterizations of a polycrystalline Ni53Mn22Ga25 alloy, by comparing the high-resolution Kikuchi patterns with those predicted according to various hypotheses—7M(IC) or nanotwin combination structure—on the modulated martensite. Detailed analysis has demonstrated that the modulated martensite plates may possess a monoclinic incommensurate superstructure, other than the nanotwinned tetragonal non-modulated structure. Such an incommensurate superstructure can generate a more favorable plate interface configuration for field-driven twinning/detwinning, capable of producing large reversible actuation strain. Moreover, the thermodynamically metastable modulated martensite may transform into the non-modulated martensite by further local lattice distortion, which would degrade the magnetic shape-memory effect. By taking into account the microstructure–property correlation, the ambiguity concerning the modulated martensitic structure could be clarified, which is essential to the understanding of the stability of modulated martensitic phases and their functionality as shape-memory materials. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

An as-quenched structural steel is tempered at 600 and 650 °C for 1 h without and with a 14-T magnetic field. The magnetic field can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. Moreover, the magnetic field can obviously retard the formation and growth of `distortion-free'' regions in the matrix, though without having any noticeable effect on the orientation distribution of the `distortion-free'' part. Investigating this subject contributes to the understanding of the way a magnetic field influences phase transformation in solid metallic materials. [Copyright &y& Elsevier]

Shape memory alloys, especially the newly developed Ni–Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni–Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants in the first linear state. It proceeds to the steady state by intensive detwinning of the these variants and by twinning of the remaining variants that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). These three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni–Mn-Ga 7M martensite at each characteristic deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds. Image 1 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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//[11 ]NM as well as (001)7M//(112)NM and [ 00]7M//[11 ]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. [Copyright &y& Elsevier]

Precipitation is one of a few effective ways enabling to reduce the brittleness of NiFeGa magnetic shape memory alloys. However, the physical origin behind precipitation and the key factors deciding precipitation remain unknown. There is still a lack of available methods to control the precipitation of NiFeGa. To this end, the precipitation of Ni 2 FeGa is systemically studied by combining first-principles calculations and experimental examinations from the following four aspects: the stability of the matrix, the crystal structure and stability of precipitates, the key factors dominating precipitation and the strategy of tailoring precipitation. Results show that the L2 1 matrix phase possesses weak elastic and dynamic stabilities, which may be linked to the occurrence of precipitation. It is identified theoretically and experimentally that the precipitate has L1 2 rather than the previously reported FCC structure. The underlying mechanism was clarified from electron density of state and chemical bond. Valence electron concentration e/a and magnetism are found to be the dominant factors affecting the relative stability of precipitate against matrix with sensitivity coefficients of ‒175.1 and 67.4 meV, respectively. The increase of e/a prefers stabilizing the precipitate, but the enhanced magnetism favors the matrix. Conversely, the influence of lattice volume on precipitation is weak from the aspects of both sensitivity and adjustable range. Lastly, an effective strategy of tuning precipitation, i.e., by tailoring e/a and magnetism, is proposed and verified. This work is expected to lay a theoretical foundation for tailoring precipitation and further optimizing the mechanical and functional performances of ferromagnetic Heusler alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Orientation related grain growth stimuli during primary and secondary recrystallization of the metal matrix in laminated metal/non-metal composites demonstrate unique effect on orientation evolution but has rarely been investigated. In this work, the recrystallization and grain growth of Cu in a graphene nanosheets (GNSs) reinforced laminated Cu matrix composite during sintering was thoroughly investigated. The microstructure, texture and lattice strain evolution of the Cu/GNS composite was examined referenced to the Cu stack without GNSs by ex-situ and in-situ orientation characterization techniques (SEM-EBSD, neutron diffraction and synchrotron radiation) from mesoscale to macroscopic scale. The results evidenced that a strong Cube orientation was produced in the Cu/GNS composite instead of the individual non-Cube orientations in the pure Cu stack without GNSs. Detailed strain-state analysis of the Cu foils in the Cu/GNS composite revealed that the anisotropic expansion behavior of the GNS that is incompatible with that of the Cu foils imposed multiple elastic constraints to the foils during the sintering process, resulting in a biaxial isostrain state in the surface layers and a uniaxial compressive strain state in the central layer of each Cu foil. The elastic anisotropy of Cu favors the growth of the Cube oriented grains to minimize the total strain energy. This work clarified the thermal strain induced abnormal grain growth of selected orientations. The mechanism revealed can be useful for analysing abnormal grain growth in elastically strained materials and can also be applied to fabrication process for texturization or even monocrystallization. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Herein, we have revealed the phase transition path and magnetic properties of Ni–Mn–In alloys with excess Ni or Mn (Ni 24+ x Mn 12 In 12- x , Ni 24+ x Mn 12- x In 12 and Ni 24 Mn 12+ x In 12- x) by using ab initio calculations. It is demonstrated that excess Ni atoms prefer to be adjacent to each other for the Ni 24+ x Mn 12 In 12- x alloys, whereas excess Ni (Mn) atoms tend to keep away from each other for the Ni 24+ x Mn 12- x In 12 and Ni 24 Mn 12+ x In 12- x alloys. The variation of the lattice constants, which are dominated by the atomic radius, are basically the same for the three alloy systems. Excess-Ni decreases the stability of the ferromagnetic austenite (FA) as well as the 6M and NM martensites. Meanwhile, the Curie temperature (T C) is reduced but the martensitic transformation temperature (T M) is increased due to the presence of excess-Ni. In comparison, excess-Mn destabilizes the FA phase, but stabilizes the 6M and NM phases. Excess-Mn has a similar impact on the increase of T M as that of excess-Ni, but has a weak effect on T C , but increases T M. The critical composition of the martensitic transformation and martensitic transition path are determined. The highest total magnetic moment of the FA phase appears with Mn-excess compositions whilst the smallest total magnetic moment is found with Ni-excess compositions. The changes of the total magnetic moments for the 6M and NM phases demonstrate an opposite trend to that of FA phase. The above results are expected to provide information for the prediction of phase diagram and magnetic properties of Ni–Mn–In alloys. [ABSTRACT FROM AUTHOR]

Solid-state refrigeration based on the caloric effects has been conceived to be a high-efficient and environmental-friendly alternative to replace the vapor-compression refrigeration technique. The implementation of solid-state refrigeration requires that the refrigerants should possess not only remarkable caloric effect but also wide working temperature region. In this work, we demonstrate that various caloric effects can be achieved successively in a multiferroic Ni 50 Mn 35 In 15 meta-magnetic shape memory alloy prepared by directional solidification, including inverse magnetocaloric effect around inverse martensitic transformation, conventional magnetocaloric effect around Curie transition and elastocaloric effect above Curie transition. Among them, the elastocaloric effect is particular striking, where a giant adiabatic temperature variation up to –19.7 K is achieved on removing a moderate stress of 350 MPa due to the elimination of negative magnetic contribution, with the specific adiabatic temperature change of 56 K GPa–1. Furthermore, through the combination of these successive caloric effects, a broad refrigeration temperature region covering the temperature range from 270 K to 380 K can be achieved. It is demonstrated that the combination of various caloric effects could be a promising way to extend the working temperature range of solid state refrigeration. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni 50 Mn 30 Ga 20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e. , type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10} 7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e. , {1 –2 –10} 7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1} A //{1 –2 10} 7 M and <1 0 –1> A //<10 10 1> 7 M , in contrast to that of self-accommodated case, i.e. , {1 0 1} A //{1 –2 –10} 7 M and <1 0 –1> A //<–10 –10 1> 7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Shape memory alloys exploit the crystallographically reversible martensitic transformation to achieve the significant shape memory effect. However, the crystallography of reverse transformation from deformed martensite to austenite that specifies the strain recovery remains less understood. In this work, based on the detailed microstructural and crystallographic examinations on the deformed seven-layered modulated (7M) martensite in a Ni 50 Mn 30 Ga 20 alloy through uniaxial tension, it is revealed that the reverse transformation from deformed martensite to austenite may not conform to the transformation crystallography of self-accommodated martensite, but involve more crystallographic routes and higher lattice discontinuity to resume the crystallographic orientation of austenite, owing to the activation of new twinning systems in the deformed martensite. Comprehensive knowledge on the reverse transformation of deformed martensite is of practical importance for understanding the intrinsic characteristics of shape memory behavior and theoretical interest for insights into the crystallographic reversibility of martensitic transformation. [Display omitted] [ABSTRACT FROM AUTHOR]

TiNiSi-Type MnCoSi alloys have attracted much attention as a potential magnetostrictive material, owing to the magnetic-field-induced metamagnetic transition. In the present work, the textured bulk MnCoSi alloys with [100]o preferred orientation were prepared by a multi-stage thermal treatment. Giant magnetostriction was achieved as much as −5268 ppm in the [100]o oriented stoichiometric MnCoSi alloys at 300 K and under a magnetic field of 3 T. The substitution of Si with Ge element results in a linear magnetostrictive behavior in the [100]o oriented MnCoSi 0.84 Ge 0.16 alloys, which means that large magnetostriction can be obtained under a low magnetic field. The magnetostriction for the MnCoSi 0.84 Ge 0.16 alloys reaches up to −327 ppm and −330 ppm under a magnetic field of 0.5 T at 280 K and 300 K, respectively. In a magnetic field of 1 T, the magnetostriction of the MnCoSi 0.84 Ge 0.16 alloys is −923 ppm and −821 ppm at 280 K and 300 K, respectively. Through the systematic investigation of the crystal structure, microstructure, and magnetic properties of the MnCoSiGe alloys, the achievement of a linear magnetostrictive performance is ascribed to the enhancement of magnetization and decreasing of the width of martensite variants. Our work greatly enhances the low-field magnetostrictive performance of MnCoSi-based alloys and makes them of interest in potential applications. [Display omitted] [ABSTRACT FROM AUTHOR]

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 Ni 50 Mn 30 Ga 20 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. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The effects of strong magnetic fields on the columnar-to-equiaxed transition (CET) have been investigated experimentally. Six alloys have been directionally solidified at low growth speeds (1–10μms−1) under magnetic fields up to 10T. Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET. The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically. Numerical results reveal that the value of the thermoelectric magnetic force increases as the magnetic field intensity and the temperature gradient increase. A torque is created on cells/dendrites and equiaxed grains. This torque breaks cells/dendrites and drives the rotation of equiaxed grains. The rotation of equiaxed grains in the mushy zone will further destroy cells/dendrites. Thus, with the increase of the magnetic field intensity and the temperature gradient, the volume fraction of equiaxed grains in front of columnar dendrites increases. When the magnetic field intensity and the temperature gradient reach a critical value, the growth of columnar dendrites is blocked and the CET then occurs. The present work may initiate a new method of inducing the CET via an applied strong magnetic field during directional solidification. [Copyright &y& Elsevier]

Abstract: The effect of a high magnetic field on the morphology and preferred orientation relationship of the Al–Al3Ni fiber eutectic has been investigated during directional solidification. Results indicate that an application of a high magnetic field has decreased the eutectic spacing and caused the appearance of the band-like structure. Moreover, it has been found that a high magnetic field has changed the preferred orientation relationship of the Al–Al3Ni eutectic. Indeed, the application of a 12T high magnetic field has caused the preferred orientation relationship of the Al{331}//Al3Ni{001}; Al [110]//Al3Ni[010] in the case of no magnetic field to transfer into the Al{001}//Al3Ni{102}; Al[102]// Al3Ni[010]. Further, in order to investigate the nucleation and growing mechanism of the Al–Al3Ni eutectic, the growth and morphology relationships between the Al–Al3Ni eutectic and the primary Al and Al3Ni phases have been investigated during volume and directional solidification, respectively. It has been found that the growth and orientation of the primary Al3Ni phase and the heating flow extraction direction have determined the growth of the Al–Al3Ni eutectic. This implies that the Al–Al3Ni eutectic growth may be capable of being controlled by adjusting the alignment of the Al3Ni eutectic phase and the heating flow extraction direction. The above experimental results have been discussed from the effect of a high magnetic field on the diffusion process and the epitaxial growth of the eutectic. [Copyright &y& Elsevier]

Abstract: The effects of a strong magnetic field on Al–Al2Cu and Pb–Sn lamellar eutectics during directional solidification have been investigated experimentally. The results show that the application of a strong magnetic field caused “tilting and oscillatory” morphological instabilities and deformation of the eutectic lamellae. Moreover, it was found that the Al–Al2Cu eutectic grain became aligned under a strong magnetic field and that with an increase in the magnetic field intensity this alignment was gradually enhanced. Further, the stresses caused by the magnetization force and the thermoelectric magnetic force during directional solidification under a strong magnetic field were analyzed and it was found that they are likely responsible for the “tilting and oscillatory” morphological instabilities and deformation. This is experimental evidence that the stresses imposed on a solid are capable of inducing the morphological instabilities of lamellar eutectics. The magnetic crystalline anisotropy of the Al2Cu phases and the growth relationship between the primary Al2Cu phase and the eutectic phases was investigated and it was found that the Al2Cu phase had a remarkable magnetic crystalline anisotropy which determined the growth of the Al–Al2Cu eutectic grain. Thus, alignment of the Al–Al2Cu eutectic grain under a strong magnetic field may be attributed to the magnetic crystalline anisotropy of the Al2Cu phase. Based on the growth behaviour of Al–Al2Cu and Pb–Sn lamellar eutectics under a strong magnetic field, an alignment model of lamellar eutectics during directional solidification under a strong magnetic field is proposed. [Copyright &y& Elsevier]

Abstract: The influence of an axial high magnetic field (up to 12T) on the stability and morphology of the liquid–solid interface of a binary alloy has been investigated experimentally during the directional solidification of the Al–0.85wt.% Cu and Zn–2.0wt.% Cu alloys. Experimental results indicate that the high magnetic field caused the breakdown of a planar interface into cellular undulations and the formation of an irregular shape. Specifically, for the Zn–2.0wt.% Cu peritectic alloy, a wavy band-like structure appears under a high magnetic field. Moreover, the high magnetic field promoted the enrichment of the solute Cu element in the diffusion boundary layer. A theory about the magnetization and solute build-up in the diffusion boundary layer under a high magnetic field for a binary alloy has been proposed. This magnetization and solute build-up could be partly responsible for the breakdown of the planar interface and the formation of the band-like structure in a peritectic alloy. Moreover, the stresses in the solid near the interface under a high magnetic field were analyzed, measured and simulated numerically. It is suggested that they are responsible to the interface irregularity, and are also capable of inducing the interface instability. [Copyright &y& Elsevier]