Growth Of Martensite Research Articles

Molecular dynamics simulations of the effects of defects on martensite nucleation

The effects of various lattice defects, such as a single edge dislocation, dislocation configurations, a low-angle grain boundary, and a high-angle grain boundary, on martensite nucleation and growth were investigated by performing molecular dynamics simulations, using EAM interatomic potentials for Ni–Al alloy. Stress induced and thermally activated martensitic transformations were studied in the cases that various defects were introduced into the simulated system. The simulation results show that the nucleation patterns were closely related to the stresses of the dislocation configurations, in the sense that the locations where stresses assist the lattice distortion of the transformation are favorable for martensite nucleation. A symmetric, tilt low-angle grain boundary is not favorable for martensite nucleation, because the stresses of the constituent dislocations cancel one another and stresses that assist the lattice distortion cannot be produced. The low-angle boundary hinders the martensite growth due to the high stability of this type of dislocation configuration. A relaxed, high-angle grain boundary (coincident site lattice) is also not favorable for martensite nucleation, because of the lack of long-range stress field.

A physically based model for the isothermal martensitic transformation in a maraging steel

Isothermal transformation from austenite to martensite in steel products during or after the production process often show residual stresses which can create unacceptable dimensional changes in the final product. In order to gain more insight in the effects influencing the isothermal transformation, the overall kinetics in a low Carbon-Nickel maraging steel is investigated. The influence of the austenitizing temperature, time and quenching rate on the transformation is measured magnetically and yields information about the transformation rate and final amount of transformation. A physically based model describing the nucleation and growth of martensite is used to explain the observed effects. The results show a very good fit of the experimental values and the model description of the transformation, within the limitations of the inhomogeneities(carbides and intermetallics, size and distribution in the material and stress state) and experimental conditions.

Comparison of interfacial structure-related mechanisms in diffusional and martensitic transformations

Some central problems in understanding the similarities of and the differences between ledgewise martensitic and ledgewise diffusional growth are examined. Martensitic growth can be described in terms of a lattic correspondence and a plane undistorted by the shear transformation. Diffusional growth can be similarly described in some cases but not in others. On the basis of the Sutton-Balluffi definitions of glissile and sessile boundaries, only misfit dislocations (on terraces or risers) or orthogonal sets of disconnections provide a truly sessile interface. When closely spaced structural ledges (now termed “structural disconnections”) are present during diffusional growth, they must have been glissile in the formation of a local equilibrium structure during the initial stages of growth. Once they are in local equilibrium and evenly spaced, however, they can only move synchronously because of their local strain interaction. Under these circumstances, extrinsic sources of growth ledges are required to move such interfaces in a diffusional manner. During martensitic growth, however, disconnections in the form of transformation dislocations can move freely in a synchronous manner. Also, on this basis, the apparent (undistorted) habit plane is generally useful in deducing the transformation mechanism during martensite formation, but is only occasionally so during diffusional growth, where only the terrace plane is generally useful.

Aspects of the surface relief definition of bainite

Criticisms of reports that ferrite laths have a sessile interfacial structure are refuted. A local lattice correspondence, achieved across coherent regions between interfacial defects, suffices to produce surface reliefs and martensitic crystallography [Prog. Mater. Sci. 42 (1997) 101]. However, formation of tent-shaped surface reliefs by monocrystalline laths/plates is inconsistent with martensitic growth.

Numerical modelling of martensitic growth in an elastoplastic material

Numerical modelling of martensitic growth in an elastoplastic material

Numerical modelling of martensitic growth in an elastoplastic material

Abstract A finite-strain-continuum thermomechanical approach is applied to the problem of temperature-induced martensitic transformation in elastoplastic materials. The key issue is to determine the transformation-induced stress and plastic strain fields and their effect on the thermodynamics and kinetics of transformation. The problems of appearance and growth of a temperature-induced rectangular martensitic unit in an austenitic matrix in both bulk and near-free-surface environments are formulated, solved and analysed. Very complex and heterogeneous stress-strain fields in the austenite and martensite phases and their non-monotonic time variation are predicted. Plastic shear strain at some points can reach 60∼ and, after an elastic growth stage, change sign and vary by 40∼. The interface velocity as a function of temperature and interface position is calculated. After the appearance of a martensitic particle, transformation work, which is the only variable part of the interface driving force, decreases by a factor of two during lengthening of the particle by 10∼. The component of the athermal interfacial friction due to dislocation forest hardening increases significantly as well, leading to interface deceleration and growth arrest. A free surface does not significantly affect the driving force until its distance from the moving interface falls below 0.75 times the particle thickness. Then the transformation work increases while the accumulated plastic strain and associated dislocation forest hardening decrease sharply. If the interface is not arrested at this point, it then accelerates to the free surface where an asymmetric plastic zone leaves ‘tent-shaped’ surface relief. The motion of a small step at the martensitic plate interface is studied as well. In contrast with elastic growth, the thermodynamic conditions for lengthening and thickening are almost equivalent for plastic growth. The results obtained allowed us to address the following fundamental problems: firstly, the morphological transition from plate to lath martensite; secondly, the relation of thermally activated kinetics at the interface level to athermal kinetics at the macrolevel; thirdly, the concentration of dislocations and plastic strain in martensite rather than in austenite despite the much higher yield stress of martensite in comparison with austenite.

Valence electron structure of cementite phase and its interface and the tempering phenomenon

Tempering is an important phenomenon in ferrous alloys. Most steels, especially alloying steels, are used after quenching and tempering. To design the composition of quenching and tempering steels, control the tempering process more effectively, improve the properties after tempering and realize the potentials of steels, the essence of tempering process and the properties of tempering products have to be understood. In this paper, the phase structure factors and interface conjunction factors of common alloying elements in cementite and its interface are calculated out. The relationships between these valence electron structure parameters and (i) the phenomena of martensite decomposition, (ii) transformation, gathering and growth of the carbides and (iii) the mechanical properties of tempering products are bult up. The nature of the effect of alloying elements on tempering process and properties of tempering products is uncovered on the level of valence electron structure. One new theoretical foundation for alloy design is provided.

First stages of martensitic growth studied by TEM in a Cu-Al-Ni single crystal and associated mechanical spectroscopy instabilities

The martensitic transition of a Cu-Al-Ni single crystal has been studied by isothermal mechanical spectroscopy and in situ transmission electron microscopy (TEM) observations. A damping instability (with time) has been found at low frequency at the beginning of the transition. At the same time, TEM study has evidenced the presence of thin martensite plates which appear just at the beginning of the (massive) martensite formation and growth. The martensite plate growth rate is quite low and non continuous. While high internal friction in the transition domain is usually associated with the mobility between martensite/austenite interfaces, the present internal friction anomaly has been ascribed to the unstable motions of the first martensite plates.

201 楕円体マルテンサイト相の成長を考慮した微視領域におけるTRIP鋼の変形挙動の数値シミュレーション(計算力学)

201 楕円体マルテンサイト相の成長を考慮した微視領域におけるTRIP鋼の変形挙動の数値シミュレーション(計算力学)

Observation on Plastic Accommodation of Shape Strain in Martensitic Transformation in Fe-Ni-C Shape Memory Alloys

Thermally induced and stress-induced martensitic transformations in Fe-31Ni-0.3C and Fe-31Ni-0.4C (mass%) alloys have been studied using atomic force microscopy (AFM) to clarify whether or not plastic accommodation occurs during the growth of martensite. It was found that plastic deformation in austenite always accompanies thermally induced martensites for both the as-solution treated sample and ausformed sample. However, for stress-induced martensites, no plastic deformation occurs in austenite. A brief discussion is given on such difference in accommodation of the shape strain of martensite.

Some remarks on the nucleation and growth of martensite

Abstract It is generally agreed that the difficult step in the formation of isothermal as well as athermal martensite is the nucleation since the growth occurs at a high speed. Previously the difference between M s and M g , the temperature below which martensite can grow if it is already nucleated, was determined for an Fe-1.62%C alloy. The difference was surprisingly small indicating that the difference in driving force for nucleation and growth is small. This experimental result is here compared with results on both athermal and isothermal martensite. A discussion about the critical step for the formation of lath and plate martensite formed by rapid continuous cooling is also given.

Morphology transitions of deformation-induced thin-plate martensite in Fe–Ni–C alloys

The morphology and crystallography of deformation-induced martensites formed during isothermal tensile tests in Fe–30Ni–0.34C and Fe–25Ni–0.66C alloys were investigated by means of optical, transmission electron and scanning electron microscopy. Transitions in morphology from thin plate to coupled plate, to lenticular coupled-plate martensite and from thin plate to lenticular to compact martensite have been observed with increasing deformation. Stress favours the growth of martensite when concurrent plastic strain allows accommodation of macroscopic transformation strains and the change of the Bain strain accommodation mechanism. Mobile dislocations and emission dislocations are directly related to the change of the Bain strain accommodation mechanism from twinning to slip.

A growth manner of butterfly martensite

The growth of butterfly martensite in an Fe-Ni-V-C alloy was investigated using an optical microscope and transmission electron microscope through observing its morphology. The present butterfly martensite is dislocation-type, with a few fine twins. One wing of a butterfly martensite is layered more heavily than the other and the concave part is layered more obviously than other regions. Most butterfly martensites have a lath plate outside of and next to one wing. The outside martensite plates grow first, and then two wings of butterfly martensite. The smooth parts of a butterfly martensite grow earlier than the layered regions. A wing of a butterfly martensite grows like a group of lath martensites.

Reconstruction of Martensitic Nucleation Process from Morphological Attributes

Within the framework of the concepts of heterogeneous nucleation and wave growth of martensite the refinement of a technique of account of wave normals N W to expected habits planes are carried out. The technique allows to connect orientations of N W with definite areas of nucleation of crystals in an elastic field of defects. The specific results are presented for system Cu-Zn. As separate rectilinear dislocations with lines [1 1 1] and dislocation loops basic elements of which are pieces of these lines are considered here as probable confers of nucleation. It is shown, that the spectrum of N W can be coordinated with experimentally observable space distribution of normals N a in a vicinity of a pole [2 11 12]. The similar coordination opens additional opportunities to reconstruct a stage of martensite nucleation from morphological attributes and to transform a distribution of habits in the informative parameter.

Dislocation model for the autocatalytic growth of martensite

A two-dimensional simulation for the morphology of martensitic transformation based on a theoretical dislocation model was performed. The growth morphology of the martensite relates to many factors, such as the supercooling, the interface energy, the shear strain, the normal strain and the hydrostatic pressure. In the present study, the simulation results were compared with the different types of martensitic morphology in Fe-Ni-C alloys.

A mean-field theory of athermal martensite growth

Pattern formation in acicular martensite growth has recently been modelled. Numerical simulations of the growth process reveal that the martensite grain size distribution is governed by an unstable fixed point of a renormalization group. This fixed point corresponds to a limit of the model in which the martensite grains nucleate simultaneously and grow with a common finite velocity. We present a mean-field theory of the martensite transformation in this limit and obtain size distributions which agree well with the published simulation data.

A comprehensive dynamical study of nucleation and growth in a one- dimensional shear martensitic transition

We have constructed a complete hydrodynamic theory of nucleation and growth in a one-dimensional (1-D) version of an elastic shear martensitic transformation with open boundary conditions, where we have accounted for interfacial energies with strain-gradient contributions. We have studied the critical martensitic nuclei for this problem: Interestingly, the bulk critical nuclei aretwinned struc-tures, although we have determined that the dominant route for the formation of martensite is throughsurface nucleation. We have analytically solved for the surface nuclei and evaluated exact nucleation rates showing the strong preference for surface nucleation. We have also examined the growth of martensite: There are two possible martensitic growth fronts,viz., dynamical twinning and so-called two-kink solutions. These transformation fronts are separated by adynamical phase transition. We analytically derive this phase diagram and determine expressions for the speeds of the martensitic growth fronts.

Comment on "Kinematic scaling and crossover to scale invariance in martensite growth"

A Comment on the Letter by Madan Rao, Surajit Sengupta, and H. K. Sahu, Phys. Rev. Lett. 75, 2164 (1995). The authors of the Letter offer a Reply.Received 5 October 1995DOI:https://doi.org/10.1103/PhysRevLett.76.3234©1996 American Physical Society

Kinematic theory for scale-invariant patterns in acicular martensitas

We present a kinematic theory which explains the emergence of scale-invariant patterns in acicular martensites which occur, for example, in FeC and FeNi alloys. Scale-invariant structures emerge naturally as a consequence of competition between the average tip-velocity, ν, and the rate of nucleation, I, of the martensite grains. Martensite growth is analyzed in terms of fixed points of a well-defined renormalization group. It is shown that the stationary probability distribution of the martensite grain size P( l), is governed by two fixed points - an unstable (noncritical) fixed point at ν/ I = 0, characterized by a Gamma distribution and a stable (critical) fixed point at ν/ I = ∞, characterized by a Lévy distribution, P( l) ∼ l − α . A universal crossover function describes the SPD at intermediate values of ν/ I. The analysis may also be used to compute the tip-velocity from optical micrograph pictures of martensite grains.

Carbon Diffusion and Kinetics During the Lath Martensite Formation

Carbon Diffusion and Kinetics During the Lath Martensite Formation