Phenomenological model of variant change during magnetization of martensite

The present study deals with the straining of ferromagnetic martensite in a magnetizing field. A phenomenological model of the movement of the interface between martensitic variants is developed for calculation of the strength of the magnetic field required for a variant change. The effect of uniaxial stress on interface movement is also examined. The results are applied to the martensite phases of NiMnGa and FePd ferromagnetic shape-memory alloys.     

Energy contributions in the martensitic transformation of shape-memory alloys

Abstract

The thermoelastic martensitic transformation in shape-memory alloys is studied thermodynamically. Calorimetric experiments on the Cu─Zn─A1 alloy system reveal that the transformation takes place with a practically negligible entropy production. The usual hysteretic subloop behaviour during partial cycling is obtained for the first time by calorimetry. An analysis of the measurements gives the quantitative behaviour of elastic and dissipative energies with the volume fraction of martensite.     

In situ stress relaxation mechanism of a superelastic NiTi shape memory alloy under hydrogen charging

On account of its good biocompatibility, superelastic Ni–Ti arc wire alloys have been successfully used in orthodontic clinics. Nevertheless, delayed fracture in the oral cavity caused by hydrogen diffusion can be observed. The in situ stress relaxation susceptibility of a Ni–Ti shape memory alloy towards hydrogen embrittlement has been examined with respect to the current densities and imposed deformations. Orthodontic wires have been relaxed at different martensite volume fractions using current densities of 5, 10 and 20 A/m² at 20 °C. The in situ relaxation stress shows that, for an imposed strain at the middle of the austenite–martensite transformation, the specimen fractures at the martensite–austenite reverse transformation. However, for an imposed strain at the beginning of the austenite–martensite plateau, the stress decreases in a similar way to the full austenite structure. Moreover, the stress plateau has been recorded at the reverse transformation for a short period. For the fully martensite structure, embrittlement occurs at a higher stress value. This behaviour is attributed to the interaction between the in situ austenite phase expansion and the diffusion of hydrogen in the different volume fractions of the martensite phase, produced at an imposed strain.     

Distortion analysis of magnetic excitation – a novel method for non-destructive evaluation of depth of surface-hardening in ferritic steels

The influence of ferritic steel samples with different case-depths on the distortion behaviour of magnetic excitation voltage has been investigated. The systematic changes in the height and position of the peak and the trough on the ((dV_E/dt) vs. V_T) profile (distortion analysis of the magnetic excitation, DAME profile) reflect the difference in the magnetization process and hence the effect of distortion of V_E in samples with different case-depths. This study shows the potential of this simple DAME method for evaluation of ferromagnetic steel components.     

Hydrogen-assisted damage in austenite/martensite dual-phase steel

For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1?1?0}_M localized slip and {1?1?2}_M twin and (2) cracking of martensite–martensite grain interfaces. The former resulted in nanovoids along the {1?1?2}_M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.     

Screw dislocation in functionally graded magnetoelectroelastic solids

A screw dislocation in a functionally graded magnetoelectroelastic material is investigated. The material properties exponentially changing along both x and y directions are considered and the mechanical–electric–magnetic coupling is discussed. Closed-form expressions for the mechanical, electric and magnetic components are derived using the general stress function method. The solutions can be applied as a fundamental result and reduced into the classic and piezoelectric cases. The study puts forth a direct way for screw dislocation analysis in inhomogeneous structures with multifield coupling.     

Influence of interface migration during annealing of martensite/austenite mixtures

Tempering of martensite in the absence of carbide precipitation leads to carbon partitioning into retained austenite. If the martensite/austenite interface is assumed to remain stationary during this process, the phase compositions reach a condition that has recently been called constrained carbon equilibrium. If iron atoms are sufficiently mobile at the interface, longer partitioning times may lead to migration of the ferrite/austenite interface. The interface may be expected to move in either direction, depending on the specific details of the phase fractions and compositions controlling the chemical potential of iron at the interface. If interface migration occurs during carbon partitioning, the situation is more complicated and conditions could exist where the interface moves first in one direction and then the other.     

Electron-phonon interactions in conducting polymers

The magnetic properties of conducting polymers are investigated within a one-dimensional periodic Anderson model in which effects stemming from electron–lattice intercations are explicitly included. The effective Hamiltonian derived via bosonization exhibits ferromagnetic ground state in the intermediate coupling regime. For these ferromagnetic polymers, the results show that phononic contributions have insignificant effect on ferromagnetism.     

The inherent tensile strength of iron

The cohesive energy of Fe as a function of structure, strain and magnetic state has been computed using the full potential linearized augmented-plane-wave method within the framework of density functional theory and the generalized gradient approximation. Calculations corresponding to uniaxial stress in the <100> direction reveal that the ideal tensile strength of bcc Fe is about 14.2GPa and is determined by instability with respect to transformation into an unstable ferromagnetic fcc structure. The low-energy fcc phase is a modulatedantiferromagnetic fcc structure that is connected to the bcc phase via a first-order magnetic transformation and does not compromise its ideal strength.     

Relational Science: A Synthesis

A synthesis of the two primary theory structures in Robert Rosen’s relational complexity, (1) relational entailment mapping based on category theory as described by Rosen and Louie, and (2) relational holism based on modeling relations, as described by Kineman, provides an integral foundation for relational complexity theory as a natural science and analytical method. Previous incompatibilities between these theory structures are resolved by re-interpreting Aristotle’s four causes, identifying final and formal causes as relations with context. Category theory is applied to introduce contextual entailment algebra needed to complete the synthesis. The modeling relation is represented as a recursive four-cause hierarchy, which is a unit of both whole and part analysis (a ‘holon’) that relates realized and contextual domains of nature as complementary inverse entailments between structure and function. Context is a non-localized domain of distributed potentials (models) for existence, as contrasted with the realized domain of localized interactive and measurable events. Synthesis is achieved by giving modeling relations an algebraic form in category theory and by expanding relational analysis to include contextual entailments. The revised form of analysis is applied and demonstrated to examine Rosen’s M-R diagram, showing that structure–function relations imply adaptive interaction with the environment, and that contextual relations imply three forms of the M-R entailment corresponding with the generally known three forms of life; Archaea, Bacteria, and Eukaryota, which can be represented by their holon diagrams. The result of this synthesis is a consistent foundation for relational science that should have important implications in many disciplines.     

Semiquantitative analysis of surface relief due to martensite formation in Fe-Mn-Si-based shape memory alloys by atomic force microscopy

Semiquantitative analysis of the surface relief caused by martensite formation in Fe-Mn-Si-based shape memory alloys has been performed by atomic force microscopy. It is found, for the thermally induced martensite transformation, that all three possible variants of martensite with the same {111} habit plane appear while, for the stress-induced martensite transformation, only one martensite variant is likely to form. For the former case, martensite plates with various variants are formed on the same habit plane in such a way that the shape strains of those plates are self-accommodated macroscopically, but each martensite plate itself is a single variant and not a multivariant plate as has been reported in the literature. For the latter case, it is especially emphasized that all the martensite plates formed in the well 'trained' sample have the same variant, which is one of the most important conditions for exhibiting a good shape memory effect.     

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon undergoes a B2 ? B19′ transformation upon cooling and heating. When the Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)₂Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)₂Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)₂Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.     

Exact solution of the one-dimensional Potts bridge model with competing interactions

Abstract

The exact solution of the q-state Potts bridge model with competing ferromagnetic and antiferromagnetic interactions is obtained and corresponding regular and critical behaviour studied.     

Creep strengthening by lath boundaries in 9Cr ferritic heat-resistant steel

The interactions between dislocations and lath boundaries in Grade 91 steel were observed by an in situ transmission electron microscopy tensile test at 973 K. Dislocations glided slowly and bowed out in a martensite lath interior. The ends of the dislocation were connected to the lath boundaries. In a tempered specimen, the pinning stress caused by the lath boundary was estimated to be >70 MPa with a lath width of 0.4 μm. In crept specimens, lath coarsening reduced the pinning effect.     

Hippocampal differentiation without recognition: an fMRI analysis of the contextual cueing task

A central role of the hippocampus is to consolidate conscious forms of learning and memory, while performance on implicit tasks appears to depend upon other structures. Recently, considerable debate has emerged about whether hippocampal-dependent tasks necessarily entail task awareness. In the contextual cueing task, repetition facilitation is implicit, but impaired in patients with amnesia. Whether the hippocampus alone or other MTL structures are required is unclear. Event-related functional magnetic resonance imaging revealed hippocampal activity that differentiates novel from repeated arrays. This pattern of results was observed without recognition of the repeating arrays. This finding provides support for the claim that the hippocampus is involved in processes outside the domain of conscious learning and memory.     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

Critical factors governing the thermal stability of austenite in an ultra-fined grained Medium-Mn steel

The time-evolution of retained austenite at 650 °C is shown to be a fair indicator of the factors governing the austenite stability. The grain size contribution to austenite stability is evidenced and a martensite start temperature (M_s) law applicable to medium-Mn steels and including both the chemical and size effects is determined.     

Ab initio study of Heusler alloys Co2XY (X = Cr,Mn, Fe; Y = Al,Ga) under high pressure

The structural, electronic, and magnetic properties of ferromagnetic Heusler alloys Co₂XY (X?=?Cr, Mn, Fe; Y?=?Al, Ga) have been investigated by means of the all-electron full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange and correlation potential. The main focus of this study is to elaborate the changes brought about in the electronic and the magnetic properties by applied pressure. The calculated total spin magnetic moments of all the compounds are found to be in good agreement with experiments. Out of these compounds, Co₂CrAl is found to be perfectly half-metallic (HM) and the other compounds are found to be nearly HM. Thus the HM to metal transition was observed at 75?GPa pressure for Co₂CrAl and the nearly half-metal to half-metal transition was observed at 40?GPa and 18?GPa for Co₂CrGa and Co₂MnAl, respectively, while no transition is observed in other compounds under investigation. It can be clearly seen that pressure affects the minority spin states rather than the majority states, leading to a slight reconstruction of the minority spin states with a shift in the Fermi level driving the above-mentioned transition.