Crystal Structure Of Martensite Research Articles

Shape memory and elastocaloric properties of melt-spun NiMn-based Heusler alloys

In the present work, towards further development of NiMn-based Heusler-type elastocaloric alloys a series of melt-spun ribbons have been prepared from three multicomponent Ni-Mn-Sn-based metamagnetic shape memory alloys (MetaMSMAs) and two multicomponent Ni-Mn-Ga-based ferromagnetic shape memory alloys (FSMAs). Their thermoelastic martensitic transformation (TMT), crystal structure and thermomechanical properties have been studied in detail. Whereas FSMAs ribbons showed a conventional stress-strain behavior at TMT and conventional elastocaloric effect (eCE), MetaMSMAs ribbons, in turn, exhibited highly pronounced inverse stress-strain and inverse eCE characteristics at TMT. The abnormal thermomechanical behavior of the MetaMSMA ribbons is produced by the presence of significant internal stresses due to appropriate amount of quenched-in defects resulting from the rapid solidification of melt-spun alloys. The results of the present study can serve as a guide for the design of new eCE-efficient materials in the form of ribbon.

Terahertz-induced martensitic transformation in partially stabilized zirconia

Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the decay channel that triggers the transformation. The phonon mode required for the martensitic transformation can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with approaches based on nonlinear phononics.

Structure-property control of polycrystalline Ni-Mn-Ga by moderate Co-doping

It is well-known that the Co-doping of ferromagnetic shape memory alloys (FSMAs) is a crucial tool to control their multifunctional properties. The present work investigates the use of small quantities of Co to fine-tune the transformation, structure, microstructure, mechanical and magnetic properties of the polycrystalline Ni49.8Mn28.5Ga21.7 (at%) alloy, as the representative of the most elaborated magnetostrain-active Ni-Mn-Ga FSMAs. A series of seven alloys, undoped and doped with 0.5, 1, 1.5, 2, 3, and 4 at% Co, was fabricated using a standard induction melting technique with subsequent heat treatment. Alongside establishing the evolution of the basic characteristics as a function of the Co content in this series, an unusual effect of Co-doping on the grain microstructure was found. A Co-doping up to 1.5 at% instead of Mn significantly increased the average grain size while importantly maintaining the 10 M martensitic crystal structure. At Co concentrations of 1–1.5 at%, a microstructure with an average grain size of about 2.00 mm was formed with a twin structure, enabling the experimental observation of magnetic-field-induced twin variant rearrangement. At higher levels of Co-doping, the grain size was essentially reduced, and the crystal structure of the martensitic phase became 2 M martensite. The decreasing grain size and changing crystal structure are attributed to the progress of γ-phase precipitates. Alongside the academic aspect, the results of the present work point to the commercial advantage of fabricating 10 M Co-doped Ni-Mn-Ga actuating elements made from large grains of polycrystalline ingots obtained by a standard melting facility instead of grown single crystals.

Thermo-magneto-mechanical properties of near stoichiometric Ni2MnGa (Mn > Ga) thin films deposited by radio-frequency magnetron sputtering on Si substrate

Near stoichiometric annealed thin films (1.5 to 3 µm) of Ni2MnGa alloy and whose compositions are such that the Martensitic Transformation Temperatures TM(e/a) are in the range -60 °C to 200 °C have been deposited by radio-frequency magnetron sputtering on SiO2/Si substrate. Crystallographic structure characterization such as planar spacing d(i), Thermal Expansion Coefficient CTE and structural domain size L(i)*, has been realized by X-ray diffraction over a wide temperature range from 25 to 205 °C. The mechanical properties such as the Young's modulus (E) and hardness (HB), have been measured by ultra-nano-indentation tests carried out from ambient temperature to 175 °C. SQUID-VSM magnetometer measurements from -263 °C to 127 °C provide some magnetic characteristics: evolution of the magnetization M versus the temperature and the Curie temperature TC. Three diffraction peaks which can be attributed to the tetragonal (NM) or modulated (5 M or 7 M) martensitic structures of polycrystalline films have been identified. At ambient temperature Ta, the calculated planar spacing d(i) associated with at least two diffraction peaks present a linear decreasing with TM(e/a). As a function of the temperature and for the austenitic state only one value of the CTE has been determined. For the martensitic state, two values have been extracted and whose change is close to TM(e/a). No evident evolution has been observed at the vicinity of the Curie temperature. The Young’ modulus and particularly the hardness are very dependent on the film composition, the temperature and the state of the microstructure. At room temperature HB linearly decreases with TM(e/a). As a function of the increasing temperature, for the martensitic state, E slightly decreases whereas HB is almost constant. In the austenitic domain, E slightly increases and HB linearly increases with the temperature. A phenomenological model has been proposed. The whole of the annealed films show a ferromagnetic behavior and from the thermomagnetic curves it has been shown that four distinct states can be observed: para or ferromagnetic austenite and para or ferromagnetic martensite. A transition on the magnetization, certainly due to a change of the martensitic crystal structure, occurs at the neighborhood of TC. Mechanical (HB) and magnetic (M) properties are strongly linked and at Ta it has been shown that M is a linear decrease function of HB.

Martensitic transformation near room temperature and hysteresis in (Ni–Co)50–Mn–Sn metamagnetic shape memory alloys

The martensitic transformation behaviors, magnetic properties, crystal structures, and entropy change (ΔS) of Ni50−xCoxMn50−ySny alloys were systematically investigated. Samples were prepared by induction melting, followed by solution heat treatment at 1273 K for 1 day and quenching. A magnetic phase diagram with a broad range of compositions was established, from which a series of alloys with varying Co concentrations was selected, such that their martensitic transformation starting temperatures (TMss) were near room temperature. The thermal and magnetic-field hystereses of these samples were systematically investigated, and a clear composition dependence was observed. The dissipation energy (Edis), which governs the transformation hysteresis, was found to be smaller for alloys with x = 5–8 than for those with x = 0–3. This behavior was found to originate from the difference in crystal structures of the martensite phases, i.e., the crystal structures of the martensite phases are 4O and 6M for x = 0–3 and x = 5–8, respectively. Moreover, the geometric compatibility between the parent and martensite phases was found to be the direct cause of the difference in dissipation energy in Ni–Co–Mn–Sn alloys.

Formation and evolution of precipitates in an additively manufactured near-α titanium base alloy

Titanium base alloys are frequently used in laser powder bed fusion manufacturing processes and enable the production of lightweight and complex components. This study describes the influence of the heat input and various post-process heat treatments on the martensite formation and its decomposition in an additively manufactured Ti-6Al-2Sn-4Zr-2Mo-Si alloy. The change of the martensite crystal structure from orthorhombic to hexagonal, caused by additional heat input, was proven by high-energy X-ray diffraction. It is shown that the heat input of the laser affects the diffusion of alloying elements such as Mo and Si. This behavior was investigated by atom probe tomography, which confirms clustering of Mo and Si at dislocations and grain boundaries, and allows for linking the heat input during the manufacturing process with the morphology of the observed clusters. Moreover, particular emphasis is laid on explaining the formation mechanism of (Ti,Zr)6Si3 silicide particles during a subsequent heat treatment.

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.

Large Elastocaloric Effect in Fe‐Doped Co–Fe–V–Ga Shape Memory Alloys

Herein, the impact of Fe substitution for Co on martensitic transformation (MT), crystal structure, elastocaloric effect (eCE), and mechanical characteristics in Co51V34Ga15 shape memory alloys (SMAs) is systematically investigated. The temperatures of MT are found to be properly adjusted by varying the Fe content in Co51−xFexV34Ga15 (x = 0, 0.3, and 0.6) alloys, shifting toward the lower values as the Fe concentration increases. Furthermore, alloying with Fe is shown to significantly improve the eCE and superelastic properties of these alloys. Under the uniaxial stress of 600 MPa, the Co50.7Fe0.3V34Ga15 alloy exhibits a significant adiabatic temperature drop of −9.5 K under adiabatic cooling conditions. Moreover, it demonstrates good stability during the cycling test with a stable temperature change of −6.6 K remaining nearly constant after 420 cycles under 500 MPa. Additionally, a maximum compressive strength of approximately 1500 MPa is achieved in the Co50.7Fe0.3V34Ga15 alloy, which makes it the promising candidate for multifunctional materials.

Giant 5.8% magnetic-field-induced strain in additive manufactured Ni-Mn-Ga magnetic shape memory alloy

Laser powder bed fusion (L-PBF) additive manufacturing process was employed to manufacture polycrystalline Ni-Mn-Ga samples. The samples were heat-treated for chemical homogenization and grain growth. It is demonstrated that the chemical composition, resulting martensitic crystal structures, and phase transformation temperatures of the L-PBF-built Ni-Mn-Ga can be precisely changed in-situ by controlling the selective evaporation of Mn through adjusting the process parameters. Subsequently, repeatable and fully reversible magnetic-field-induced strain of 5.8% was measured in a single crystalline grain of an additive manufactured polycrystalline Ni-Mn-Ga sample exhibiting a 10M martensitic structure at ambient temperature. The results indicate that L-PBF can be used to manufacture Ni-Mn-Ga devices containing active parts that can be strained by an external magnetic field.

Mechanical property enhancement of the Ag–tailored Au–Cu–Al shape memory alloy via the ductile phase toughening

The Au–Cu–Al biocompatible shape memory alloys (SMAs) have attracted much attention due to the requirements of biomedical applications. However, brittleness remains a critical issue in the intermetallics of Au–Cu–Al alloys. In this study, to solve the dilemma of embrittlement, Ag was introduced into the Au–Cu–Al alloy to realize the ductile phase toughening (DPT) since the ductile disordered–fcc phase was predicted to precipitate in the brittle parent β phase based on the Ag–Au–Cu phase diagram. Microstructure observations, chemical composition analysis, crystal structure identifications, and thermal analysis revealed that the Au–28Cu–16Al–28Ag (mol.%) alloy was mainly composed of the Ag–rich α1 primary solidification phase with disordered–fcc structure and the eutectic structure of the α1 phase and the β phase with L21 structure. The ductile phase was successfully inserted into the brittle Au–Cu–Al intermetallic while the martensitic transformation temperature and crystal structures remained almost uninterrupted. In the cyclic loading–unloading tensile tests, the alloy mainly composed of the β phase failed in the early elastic region due to its embrittlement; while the alloy mainly composed of the α1 phase performed 29% in the total strain deformation. The Au–28Cu–16Al–28Ag alloy, which is composed of the β phase and the α1 phase, solve the issue of embrittlement; in addition, shape recovery was also found in this alloy during unloading. According to the microstructure observations, cyclic tensile tests, and fracture surface observations, the aforementioned improvement of strengthening and enhancement of ductility were brought from the insertion of the ductile α1 phase into the brittle β/β grain boundaries.

Crystal structure characterization of martensite of Cu–Zn–Al ternary alloy by spherical aberration corrected scanning transmission electron microscopy

The crystal structure of martensite in Cu-27at.%Zn-9.0 at.%Al alloy has been studied by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and geometrical phase analysis (GPA) to examine possible changes in atomic rearrangements during martensitic transformation of this ternary system. Observation along [100]M zone axis is suitable for examining a chemical order of the martensite, and showed that, despite the non-stoichiometry of the alloy, atomic columns containing Al atoms are imaged and distinguished from the others. On the other hand, observation along [010]M zone axis directly revealed that the parent and martensitic phases possess L21 and 18R (21‾) structures, respectively. These observations suggested that the martensite retained the local chemical order of the parent phase without shuffling before and after the transformation. GPA revealed that the interface between the two phases was coherent with tilting of the basal plane approximately 6° across the boundary, which makes otherwise large inclination small during the martensitic transformation.

Martensitic transformation, crystal structure and shape memory effect in Ni55−xMn25Ga20Cox alloys

ABSTRACTThe effect of cobalt addition instead of nickel on the crystal structure, martensitic transformation behaviour and shape memory effect were investigated. Within the analysed range of chemical composition, a single non-modulated martensite was detected at ambient temperature. Cobalt addition modified the lattice parameters and, thus, affected the tetragonality of the martensite unit cell. The hysteresis of martensitic transformation was differently affected by the type of heat treatment applied. For furnace cooled samples, the hysteresis decreased from 50°C to 30°C; in the case of water quenched samples, the hysteresis sharply increased up to 60°C. The shape memory effect, measured as the recoverable strain upon annealing after compression tests, reached a fully recoverable deformation at 10 at.-% of cobalt.This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.

Crystal Structure of Martensite and Orientation Relationships During Thermoelastic Martensitic Transformations in Ni-Mn-Based Alloys

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Novel insight into the formation of α″-martensite and ω-phase with cluster structure in metastable Ti-Mo alloys

On the basis of the “-Mo-Ti-Mo-” linear unit along the specific <111>β, <110>β, and <100>β directions, the cluster structures of α″-martensite and ω-phase were constructed in metastable Ti-Mo alloys to examine phase stability, elastic property, and crystal structure evolution by first-principles calculations combined with experimental analyses. With the increase in Mo content, the orthorhombicity and shuffle magnitude of {110}β plane along <110>β direction decreased, leading to change in the crystal structure of martensite from hexagonal close-packed to orthorhombic structure; the displacive collapse degree of {112}β plane along <111>β direction decreased, indicating that the crystal structure of ω-phase transited from hexagonal to trigonal structure. The softening effect of tetragonal shear elastic constant (C′) and Young's modulus (E100) was favorable for the shuffle and shear components of α″-martensite, respectively, whereas that of shear modulus (G111) was beneficial to the collapse component of ω-phase. The competition among C′, E100, and G111 affected the phase transformation following the sequence of hexagonal close-packed α′-martensite, orthorhombic α″-martensite, hexagonal ω-phase, and trigonal ω-phase in metastable titanium alloys.

The effect of the processing parameters on the martensitic transformation of Cu-Al-Mn shape memory alloy

The influence of processing, heat-treatment and aging of Cu-(8–9) wt% Al-(7–10) wt% Mn alloy on the kinetics and temperatures of martensitic transformation was investigated by calorimetric measurements. Cu-Al-Mn alloy was prepared by continuous casting, melt-spinner and by melting in the electric arc furnace. Alloys were further heat-treated at 900 °C for 30 min and quenched in water, as well as aged at 300 °C for 1 h. Differential Scanning Calorimetry (DSC) was performed at 3 heating/cooling cycles from −50 to 250 °C. Non-isothermal measurements were determined at five different heating/cooling rates: 5, 10, 15, 20 and 25 °C/min. Activation energy was obtained according to Ozawa and Kissinger kinetic models. Microstructure analysis of investigated systems was performed by scanning electron microscopy (SEM). Results indicate the most intensive formation of martensitic structure in the as-cast state during continuous casting, where the partially formation of needle-like and V-shape martensite was observed. After solution treatment and quenching as well as aging, completely martensitic phase occurred in continuously cast alloy. XRD analysis detected Cu2AlMn, Cu3Al and Al4Cu9 phases in quenched specimens of continuously cast Cu-Al-Mn alloy and ribbon. The highest impact of the solution treatment and aging on the shifting of the martensitic temperatures was observed for Cu-Al-Mn ribbons, while in continuously cast Cu-Al-Mn alloy heat treatment and aging induced formation of different martensitic crystal structures. Kinetic investigations showed increasing start martensitic temperatures, Ms, and wider temperature interval of martensitic transformation with higher cooling rate. The highest values of activation energy of martensitic transformation was obtained for the continuously cast Cu-Al-Mn alloy.

Martensite crystal structure in Ru-based high temperature shape memory alloys

High temperature shape memory alloys RuNb and RuTa were investigated by X-ray diffraction in order to determine the structure of the low temperature β” martensite. Powder X-ray diffraction and Rietveld analysis confirm the monoclinic symmetry and allow a precise determination of the atom positions. The low temperature microstructure is the result of a martensitic transformation from a tetragonal β′ martensite. The analysis of how it is controlled by the space groups of the two phases and by their relative orientation is performed by transmission electron microscopy. A three level twinned microstructure is recognized in the β” phase. The investigation at the smallest scale twins reveals the presence of wavy contrast boundaries which are typical for translation interfaces. High resolution transmission electron microscopy images, accompanied by phase contrast imaging analysis, have been used to determine the translation vectors associated to the interfaces. These displacements are the translations of the tetragonal β′ phase which are lost during the transformation. The combination of X-ray diffraction and transmission electron microscopy investigation allows the suggestion of a global distortion mechanism that explains the atomic displacements in the β” form.

Using Kerr Microscopy for Direct Observation of Magnetic Domains in Ni–Mn–Ga Magnetic Shape Memory Alloy

Giant magnetic-field-induced strain up to 12% in Ni–Mn–Ga single crystal is due to magnetically-induced reorientation of the twinned martensitic low symmetry crystal structure. An important interplay is arising from the interaction between ferroelastic domains (twin domains) and magnetic domains. Here, we studied the magnetic domains in an Ni50Mn28Ga22 single crystal exhibiting 6% strain by using a magneto-optical indicator film and for the first time also directly by the Kerr magneto-optical microscopy. To compare the magnetization behavior of bulk and surface, we measured the magnetization loops by the Kerr effect (MOKE) and by the vibrating sample magnetometry. We demonstrate that using a particular arrangement of the Kerr microscopy enables visualizing the magnetic domains structure and its evolution in the magnetic field.

The effect of carbon content on the c/a ratio of as-quenched martensite in Fe-C alloys

The as-quenched martensite crystal structure is widely accepted as body-centered tetragonal. The classical Honda & Nishiyama model, c/a = 1 + 0.045wt%C, was obtained based only on the experimental results for steels containing more than 0.6wt% of carbon. This model was used to predict that steels containing less than 0.6wt% of carbon would follow the same relation. However, it was reported by Sherby that the steel with less than 0.6wt% of carbon is body-centered cubic, and c/a ratio equals to 1. This debate is caused by highly overlapped martensite (002) and (200) peaks when the carbon content of steel is less than 0.6wt%. In this paper, the martensite lattice parameters of the as-quenched steels with different carbon contents were measured using high-resolution X-ray diffractometer. Rietveld refinement was used to deconvolute overlapped martensite peaks. For the steel with less than 0.6wt% of carbon, the structure is proved to be body-centered tetragonal. It was found that the relationship between c/a ratio and carbon content follows as c/a = 1 + 0.031wt%C.

Controlling DC permeability in cast steels

Annealing (at multiple cooling rates) and quenching (with tempering) was performed on specimens of cast steel of varying composition. The aim was to devise a method for selecting the steel with the highest permeability, from any given range of steels, and then increasing the permeability by heat treatment. Metallographic samples were imaged using optical microscopy to show the effect of the applied heat treatments on the microstructure. Commonly cast steels can have DC permeability altered by the careful selection of a heat treatment. Increases of up to 381% were achieved by annealing using a cooling rate of 6.0°C/min. Annealing was found to cause the carbon present in the steel to migrate from grain boundaries and from within ferrite crystals into adjacent pearlite crystals. The migration of the carbon resulted in less carbon at grain boundaries and within ferrite crystals reducing the number of pinning sites between magnetic domains. This gives rise to a higher permeability. Quenching then tempering was found to cause the formation of small ferrite crystals with the carbon content of the steel predominately held in the martensitic crystal structures. The results show that with any given range of steel compositions the highest baseline DC permeability will be found with the steel that has the highest iron content and the lowest carbon content. For the samples tested in this paper a cooling rate of 4.5°C/min resulted in the relative permeability of the sample with the highest baseline permeability, AS4, increasing from 783 to 1479 at 0.5T. This paper shows how heat treatments commonly applied to hypoeutectoid cast steels, to improve their mechanical performance, can be used to also enhance electromagnetic properties of these alloys. The use of cast steels allows the creation of DC components for electrical machines not possible by the widely used method of stacking of electrical grade sheet steels.

The Mechanism of Orientation Dependence of Cyclic Stability of Superelesticity in NiFeGaCo Single Crystals Under Compression

Using single crystals of the Ni49Fe18Ga27Co6 (at.%) alloy, oriented along [001]- and [123]-directions, cyclic stability of superelasticity is investigated in isothermal loading/unloading cycles at T = Af +(12–15) K (100 cycles) under compressive stress as a function of given strain per cycle, presence of disperse γ-phase particles measuring 5–10 μm, austenitic (B2 or L21) and stress-induced martensitic crystal structure (14M or L10). It is shown that single-phase L21-crystals demonstrate high cyclic stability during L21–14M-transitions with narrow hysteresises Δσ < 50 MPa in the absence of detwinning of the martensite. During the development of L21–14M stress-induced transformation, the reversible energy ΔGrev for these crystals exceeds the dissipated energy ΔGirr, and ΔGrev/ΔGirr = 1.7–1.8. A significant degradation of superelasticity is observed in [123]-oriented crystals during the development of L21–14M–L10-transformations followed by detwinning of the L10-martensite crystals and heterophase (B2+γ) single crystals, irrespective of their orientation during the B2–L10-transition. In the latter case, martensitic transformations are characterized by a wide stress hysteresis Δσ ≥ 80 MPa and the dissipated energy exceeds the reversible energy ΔGrev/ΔGirr = 0.5. The empirical criterion, relying on the analysis of the reversible-to-irreversible energy ratio, ΔGrev/ΔGirr, during stressinduced martensitic transformations, can be used to predict the cyclic stability of superelasticity in NiFeGaCo alloys subjected to different types of heat treatment.