Sequence of structural and magnetic transitions in Ni48Co2Mn39Sn11 shape memory alloy

The structural and magnetic transitions in Ni₄₈Co₂Mn₃₉Sn₁₁ shape memory alloy were systematically investigated. During cooling, the paramagnetic austenite transforms into paramagnetic martensite at T_M∼375 K, followed by a gradual transition from paramagnetic to superparamagnetic martensite around T_S∼320 K. Upon further cooling through T_P∼100 K, the superparamagnetic clusters collectively freeze into a superspin glass state as corroborated by aging, rejuvenation, and memory effects. Consequently, the unique transition sequence of paramagnetic austenite→paramagnetic martensite→superparamagnetic martensite→superspin-glass martensite is disclosed.     

Electron acoustic imaging of Mn<Subscript>50</Subscript>Ni<Subscript>28</Subscript>Ga<Subscript>22</Subscript> ferromagnetic shape memory alloy

The novel ferromagnetic shape memory alloy Mn₅₀Ni₂₈Ga₂₂ exhibits a single martensite phase with tetragonal structure at room temperature. Its martensite structure was investigated by scanning electron acoustic microscopy. Stripe twin variants exist in every grain and exhibit the configurations of the typical self-accommodation arrangement. The interfaces between twin variants are straight and clear. However, the magnetic domain walls obtained by the Bitter method coincide well with the twin variant boundaries. The first direct evidence of the coincidence between micron-scale magnetic domains and martensite variants is presented. These results will be beneficial for the development of ferromagnetic shape memory alloys and for the understanding of the correlation between magnetic domains and crystallographic twin variant domains as well. PACS  68.37.Hk; 75.70.Kw; 75.60.Ch; 73.50.Rb; 43.58.+z     

有重位点阵关系两相的复合点阵(Ⅱ)——两相空间点阵及其点阵平面的重合系数

本文用数论分析方法,给出有重位点阵关系两相空间点阵及其点阵平面重合系数求法的普遍而简便的公式。空间点阵的重合系数α₂⁽³⁾=1/|C⁽¹⁾|,点阵平面的重合系数α₂^(h)= (H⁽²⁾C⁽¹⁾/|C⁽¹⁾|, C⁽¹⁾为重位点阵基矢对应矩阵,H⁽²⁾=[h₁⁽²⁾h₂⁽²⁾h₃⁽²⁾].在C⁽¹⁾未知的情况下,空间点阵及其点阵平面的重合系数可以通过两相基矢有理对应矩阵φ及重合系数矩阵C来求,这时α₂⁽³⁾= k₁⁽²⁾k₂⁽²⁾/d³, α₂^(h)=(CH⁽²⁾,dk₁⁽²⁾)/d².求矩阵C比求矩阵C⁽¹⁾要方便得多。 关键词：     

Investigating the phase transformations in AuCuPd with computer simulation of ion images

We present the results from investigations of structural formation in gold-copper-palladium jewelry alloys at low-temperature atomic ordering. It is found that Au20.6-Cu55.2-Pd24.2 (in at %) alloy is ordered in the L1₀ structure through the formation of an intermediate metastable phase with B2 structure. This phase transformation presumably occurs via a martensite mechanism. Orientation relations between B2 and L1₀ lattices are obtained from the experimental data.     

Pairs of inversely occupied electronic states in the energy range best suited to wave generation

For a model electronic spectrum of a bcc crystal, characteristic s-surfaces are found that separate, in the quasi-momentum space, pairs of electronic states potentially active in the generation of elastic waves at the stage of growth of a martensite crystal. Numerical estimation of the fraction of active electronic states (R_eff/R) is performed for the energy range Δ = 0.2 eV near the Fermi level. The ratio R_eff/R is calculated as a function of the parameter ratio for the interaction between the nearest and the second neighbors for both bcc and fcc lattices. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 44–48, November, 2005.     

Violation of the sequence of martensite crystals formation on cooling and their shrinking on heating during B2 ↔ B19′ martensitic transformation in Ti40.7Hf9.5Ni44.8Cu5 shape-memory alloy

An in situ transmission electron microscopy study of the B2 ? B19′ martensitic transformation in Ti_40.7Hf_9.5Ni_44.8Cu₅ shape memory alloy was carried out. It was observed that the sequence of the martensite crystals shrinking on heating differed from the sequence of the martensite crystal appearance on previous cooling. This was shown that strain nanodomain formation on cooling prior to the forward martensitic transformation resulted in accumulation of the elastic energy. This led to the dependences of the elastic energy stored on cooling or released on heating on the volume fraction of the martensite phase became different. In this case, at the same volume fraction of the martensite phase, the configuration of the martensite crystals on cooling and heating was different and it was a reason for a violation of the sequence of the martensite crystal formation on cooling and its shrinking on heating.     

Phase-field simulations of magnetic field-induced strain in Ni2MnGa ferromagnetic shape memory alloy

The magnetization and magnetic field-induced strain behavior of the ferromagnetic shape memory alloy, Ni₂MnGa, under constant compressive stress were studied using the phase-field method. Based on the evolving magnetic domain and martensitic structures, we analyzed the cycling effect, magnetization hysteresis, strain recoveries, and coupling between the domain wall and martensite twin boundaries. We compared the switching behavior of single variant and multivariant martensite structures. We observed three types of magnetic field-induced strain mechanisms, depending on the magnitude of the applied compressive stress. The study revealed that the martensite microstructure of the magnetic shape memory alloy plays an important role in magnetization and strain evolution during loading and unloading of an external magnetic field under different stress conditions. The results are compared with existing experimental observations.     

Annealing effect on phase transformation in nano structured Ni–Mn–Ga ferromagnetic shape memory alloy

Nano structured Ni_52.6Mn_23.7Ga_24.3 alloy was prepared using the ball milling technique. High martensitic transition temperatures are observed in the range between 336 and 367 K. The X-ray diffraction profile revealed that annealed Ni–Mn–Ga powder at 1073 K displays mixture phases of austenite and martensite. Annealing at 1173 K induces phase transformation from mixture phase to Heusler L2₁ structure, which confirms the high-temperature shape memory effect. On the contrary, the milled sample shows no evidence of shape memory effect. Furthermore, annealing at higher temperature (1273 K) shows the accumulation of oxidation, which leads to the loss of shape memory effect. The grain size increases with increasing annealing temperature and causes deterioration in the soft magnetic properties.     

NiTi记忆合金中R相和马氏体相中孪晶界面的群论分析

利用母相群对低温相群陪集分解的理论,确定了NiTi形状记忆合金中R相存在四种变体,变体之间以{110}_p和{100}_p作为孪晶界面,四种变体可构成三种自适应群。马氏体相中存在三种变体,变体间的孪晶界面为{110}_p,三种变体可以构成四种呈三角形分布的自适应群。所得结果与实验工作基本一致。 关键词：     

Implementation of the FCHC lattice gas model on the Connection Machine

The 4-dimensional FCHC lattice gas model has been implemented on a Connection Machine CM-2 with 16K processors. Symmetries are used to reduce the collision table to a size that fits into local memory. This method avoids the degradation of the Reynolds coefficientR _*, but at the price of increased computing time. Bit shuffling between parallel lattices is introduced to reduce the discrepancy between measured viscosities and those predicted from the Boltzmann approximation. Thereby a model with a negative shear viscosity is obtained: a fluid having a uniform initial velocity is unstable and organized nonuniform motions develop. Because of the buildup of very strong correlations between the parallel lattices, the discrepancy with the Boltzmann values decreases only very slowly with the number of parallel lattices.     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.     

Transformation plasticity and shape memory effects in TiNi alloy after low-temperature treatment

The effect of low-temperature annealing on the structure, kinetics of martensitic transformations, and functional properties of an equiatomic TiNi shape memory alloy is studied. Low-temperature annealing of the TiNi alloy is shown to decrease the temperature of the end of the forward martensite transformation M _f and the temperature of the onset of the reverse transformation A _s , which increases the transformation temperature range. As a result, the shape memory effect is improved due to a decrease in the irreversible strain. These phenomena are assumed to be caused by the hardening of the TiNi alloy induced by low-temperature annealing.     

Hall effect in a martensitic transformation in Ni-Co-Mn-In Heusler alloys

The Hall effect, transverse magnetoresistance, and magnetization of Ni₄₈Co₂Mn₃₅In₁₅ Heusler alloys have been studied at T = 77–300 K in magnetic fields up to 15 kOe. It has been shown that a martensitic transformation is accompanied by a change in the sign of the constant of the ordinary Hall effect, which means a strong change in the electronic spectrum in the martensitic transformation, while the anomalous Hall effect (AHE) constant is positive in both the austenite and martensite phases. In both phases, there are no correlations between the AHE constant and the square of the resistivity, which are characteristic of the side jump mechanism in the AHE theory. In the near vicinity of the martensitic transformation, the field dependences of the Hall resistance are complex and nonmonotonic, indicating a change in the relative concentrations of the austenite and martensite phases in strong fields.     

Formation of self-accommodation complexes of martensite crystals during transformations of distortion type in alloys with the shape memory effect

This work discusses the formation of self-accommodation complexes of martensite crystals during transformations of distortion type and experimentally observed shape memory effects in unordered solid solutions. A method for the analysis of orientation relationships between the lattices of austenite and marten-site in terms of the possibility of formation of self-accommodation complexes is given.     

Development of NiMnGa-based ferromagnetic shape memory alloy by rapid solidification route

The ferromagnetic shape memory alloy with nominal composition of Ni_52.5Mn_24.5Ga₂₃(at%) was developed by the melt-spinning technique. The as-spun ribbon showed dominant L2₁ austenitic (cubic) structure with splitting of primary peak in the X-ray diffractogram indicating existence of a martensitic feature. The quenched-in martensitic plates were revealed from Transmission electron microscopy (TEM). Increase of magnetisation at low-temperature rise indicates martensite to austenite transformation and its reverse with a drop in magnetisation during cooling cycle. The martensite to austenite transformation can be made spontaneous at higher magnetic field.     

TEM characterization of a nitrogen implanted austenitic stainless steel

Pure austenitic stainless-steel samples (18% Cr, 10% Ni) were implanted at room temperature with nitrogen ions at an energy of 40 keV with fluences from 10¹⁷ to 6X10¹⁷ ions cm^-2. Microstructures obtained after implantation were studied by transmission electron microscopy and selected-area diffraction. The observations show the formation of ε martensite (hexagonal), of α' martensite (tetragonal) and the appearance of nitrides (Fe, Cr, Ni)₂N_1-x hexagonal or orthorhombic.     

Pairs of Electronic States with Population Inversion in the Energy Interval Optimum for Wave Generation

The characteristic s-surfaces in quasi-momentum space separating pairs of electronic states that can generate elastic waves at the stage of growth of a martensite crystal are determined for a model electronic spectrum of crystals with BCC lattices. The relative number of active electronic states (R_eff/R) is numerically estimated for the energy interval Δ = 0.2 eV counted from the Fermi energy level. The dependence of R_eff/R on the ratio of the parameters of interaction between the first and second neighbors is calculated for the BCC and FCC lattices. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 18–22, July, 2005.     

Microstructure and magnetic properties of Ni-rich Ni54Mn25.7Ga20.3 ferromagnetic shape memory alloy thin film

A Ni₅₄Mn_25.7Ga_20.3 ferromagnetic shape memory alloy thin film has been fabricated by using the RF magnetron-sputtering technique. The structure and magnetic properties of the film were systematically investigated. The results show that the film is in ferromagnetic martensite state at room temperature with the Curie temperature (T_c) of about 370 K. The saturation magnetization (M_s) of the film reaches 45 emu/g at 300 K, which is about 80% as large as that of Ni–Mn–Ga bulk material. The magnetization hysteresis loops significantly depend on temperatures. The residual magnetization (M_r) and the coercive force (H_c) increase with decreasing temperatures. The grains homogeneously distribute in the film. The microstructure of the film consists of martensite plates. The interface between the martensite variants is clear and straight, indicating a good mobility.     

Thermal neutron scattering from a hydrogen-metal system in terms of a general multi-sublattice jump diffusion model—I: Theory

A Multi-Sublattice Jump Diffusion Model (MSJD) for hydrogen diffusion through interstitial-site lattices is presented. The MSJD approach may, in principle, be considered as an extension of the Rowe et al.[1] model. Jump diffusion to any neighbours with different jump times which may be asymmetric in space is discussed. On the basis of the model a new method of calculating the diffusion tensor is advanced. The quasielastic, double differential cross section for thermal neutron scattering is obtained in terms of the MSJD model. The model can be used for systems in which interstitial jump diffusion of impurity particles occurs. In Part II the theoretical results are compared with those for quasielastic neutron scattering from the αNbH_x system.