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Martensitic nucleation under normal circumstances is a heterogeneous process, and the heterogeneity can be explained by the potency distribution of defects that provide the nucleation sites, including both preexisting and autocatalytically generated defects. The potency distribu- tion of the preexisting defects has been shown earlier by small-particle studies to follow an exponential function, whereas that of the autocatalytic defects is found in the present work to obey a Gaussian function. A major distinction between these two distributions is that the number density of thepreexisting defects increases monotonically with decreasing defect potency, while the number density of theautocatalytic defects is distributed about a mode, giving a saturation level of its cumulative distribution. As a result of these potency distributions, the nucleating sites of both origins exhibit distributions in their activation energies for nucleation, and a distributed- activation kinetic model is now proposed to take these variations into account for martensitic transformations. The features of this model are tested with considerable success in experiments on an Fe-32.3Ni alloy, leading to calculations of the entire course of martensitic transformation curves (including the athermal, anisothermal, and isothermal contributions), the changing shapes of time-temperature-transformation (TTT) diagrams for a range of Fe-Ni compositions, and the grain-size dependence of “bursting” behavior.

An earlier theory for the nonequilibrium transformation of austenite in Fe-C alloys to partially supersaturated plates of ferrite is extended to include a recent solute trapping model by Aziz. The previous model yielded a relationship between interface velocity and supersaturation for a specified transformation temperature. However, a unique growth velocity was determined from a velocity maximization criterion that could not be justified. By including the Aziz solute trapping function as a third interface response function (in addition to functions describing the diffusion field velocity and interface mobility), it has been possible to obtain a more physically based unique solution to the growth velocity for a specified temperature. The new calculations on an Fe-C alloy indicate a smoothly increasing supersaturation during both nucleation and growth, as the transformation temperature is reduced. Both the nucleation and growth processes tend to become completely diffusionless only below the martensite start temperature.

Using a dislocation model of interfacial structure, kinetic theories of dislocation motion are adapted to predict the mobility of martensitic interfaces. Defining generalized driving forces and activation parameters, analytical models are developed which describe the kinetics of motion controlled by various types of obstacle interactions. The behaviors of martensitic interfaces and slip dislocations in identical microstructures are compared. For a lattice-invariant shear by slip, the martensitic interface behaves similarly to a collection of glide dislocations. The interface/obstacle interaction is much weaker if the martensite is internally twinned, giving a higher relative mobility.

The mobility of theβ 1-γ 1 ′ martensitic interface in thermoelasticβ-Cu-Al-Ni alloys has been investigated using stress-assisted single interface transformation over the temperature range 180 to 410 K. The imposed interfacial velocities varied between 10-6 and 10-2 ms-1. The kinetic behavior is found to be consistent with thermally-activated interfacial motion, although an anomalous temperature dependence is observed below 210 K. This low temperature anomaly is attributed to the effect of the experimentally verified elastic softening. At higher temperatures, measured activation energies of 1.5 × 10-20 to 3.5 × 10-20 J and activation volumes of 103 to 104 atomic volumes are rationalized in terms of the rate-controlling interaction of the moving interface with fine-scale lattice displacements, the tweed structure, observed by transmission electron microscopy. Coarse particles ofγ- and 2H-phase, detected by the TEM, are found to give rise to noncontact particle/interface interactions which are too long-range to be surmounted by thermal activation. Consequently, they produce athermal friction stresses opposing interface motion.

True-stress (σ), true-strain (e) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs e) of materials in both conditions have a sigmoidal shape at temperatures above Msσ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below Msσ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below Msσ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above Msσ. Comparison of σ-e andf-e curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above Msσ, but there is also a dynamic “transformation softening” contribution which becomes dominant below Msσ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above Msσ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below Msσ.

The early stages of aging/tempering of ferrous martensites have been studied over a wide range of carbon contents by numerous experimental techniques. A kinetic framework is presented which allows a correlation of many diverse observations. Three general regimes of structural change termed relaxation, aging, and tempering can be further subdivided, thus identifying two stages of aging (A1 and A2) which precede the conventional stages of tempering (T1-T3). The first stage of aging involves clustering of carbon atoms in c-oriented octahedral sites, while the second stage is associated with a modulated tweed microstructure containing high-carbon regions which may be interpreted as either advanced carbon clusters or coherent transition carbides. Alloy compositions are proposed to better correlate future complementary experiments required to determine these unusual structures more precisely.

The interaction of the twinnedβ -1-γ 1 ′ , martensitic interface with various experimentally observed obstacle particles is analyzed using a specific dislocation model for the interface. The strain interaction of particles with simple shears and tetragonal distortions and their modulus interactions are treated as functions of particle crystallographic orientation, particle position with respect to interfacial intersection, and particle size. Differences from previous predictions of a simple general interface model arise primarily from differences in the assumed interfacial trajectory relative to the particles. The finite-particle calculations indicate that the point-particle approximation is valid for a particle radius less than one-tenth the interfacial twin period. Overall agreement with the experimentally measured interfacial mobility behavior is greatly improved over the previous simple model prediction. The measured athermal component of the driving force for interfacial motion is consistent with the strain and modulus interaction with 2H-phase particles. The activation-energy /driving-force relations obtained from the thermally activated component are reasonably represented by the strain interaction with the fine-scale atomic displacements of the tweed structure.

Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamic conditions, aimed at elucidating the fundamental mechanisms of shear localization underlying both adiabatic shear localization and fracture processes. Experiments were also devised to study the effect of hydrostatic pressure and austenitizing temperature on the critical strain to localization. Experimental evidence strongly suggests that strain localization in the steels investigated is driven by microvoid softening controlled by nucleation at 100 nm scale particles. This is supported by the observed pressure dependence of the instability strain, enhanced resistance to shear instability with particle dissolution, and direct observation of microvoids at these particles in deformed material. For the steels investigated with approximately equivalent strength levels, a direct correlation between the crack extension force and shear instability is demonstrated. Consequently, both fracture toughness and shear localization are dependent on the size, type, and distribution of second phase particles.

Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites.

A regular solution model is fit to experimental equilibrium temperatures(T 0) and pressures(P 0) for diffusionlessδ→α transformations in Pu-Ga and Pu-Al alloys, in order to define the chemical free energy change ΔG forδ→ α transformation. Analysis of reported isothermalδ→ αtransformation rate data in terms of nucleation-controlled martensitic kinetics gives a nucleation activation energyQ which is a nonlinear function of ΔG. The activation volumeV * defined byδQ/δΔG is of the order of 10 to 40 atomic volumes, suggestive of rate control by an interfacial Peierls barrier. The grain size dependence of the transformation-start temperature at a fixed cooling rate of 2.08 × 10-2 Ks-1 is measured in a Pu-1.7 at. pct Ga alloy, revealing an inhibition of transformation at fine grain sizes. The overall kinetic behavior is characteristic of a martensitic mechanism.

Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.

Low-temperature plastic flow in TRIP steels has been found to be controlled by stress-assisted isothermal martensitic transformation. For these conditions, the thermodynamics and kinetic theory of martensitic transformations leads directly to constitutive relations predicting the dependence of flow stress on temperature, strain, strain-rate, and stress-state, consistent with the observed behavior of TRIP steels. Guidelines are obtained for the control of temperature sensitivity, σ -ɛ curve shape, and stress-state effects to achieve novel mechanical properties.

Microstructural changes which accompany the first stage of tempering have been studied by transmission electron microscopy (TEM) and electrical resistometry in two Fe-Ni-C alloys that form platelike martensite. The e-carbide transition phase in these alloys adopts a platelike shape with a habit plane near {012=α. Electron diffraction data indicate that the carbide may be partially ordered, resulting in orthorhombic symmetry, and therefore, this phase is designated as e′- carbide. The carbide particles contain a fine internal substructure which appears to represent an internal accommodation deformation (faulting) on the carbide basal plane. Detailed analysis of the kinematics of carbide precipitation suggests that the observed habit plane and accommodation deformation permit an invariant-plane strain transformation which minimizes elastic strain energy. The apparent selection of only a limited number of possible orientation variants is explained in terms of the symmetry of the parent martensitic phase, which is known to undergo spinodal decomposition prior to the nucleation of the transition carbide. The martensitic substructure is not found to exert any significant influence on this overall precipitation behavior.

The growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault plane are examined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all martensitic transformations. Embryo thickening by the nucleation of discrete loops of transformation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest” dislocations intersected by embryos growing in the fault plane. Lattice softening phenomena may lower the critical chemical driving force for nucleation, but are not essential for martensitic nucleation by the proposed faulting mechanism.

The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedMs temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered.

A collaborative study of the aging of virgin Fe-Ni-C martensites has combined the techniques of transmission electron microscopy (TEM), atom-probe field-ion microscopy (APFIM), and electrical resistometry. Aging at room temperature leads to the rapid development of a finescale structural modulation along 〈203 〉 lattice directions. Atom-probe analysis of Fe-15Ni-lC martensite reveals the formation of carbon-rich regions whose carbon concentration increases with time and approaches 11 at. pct C on prolonged aging. The early stage kinetics of this process are composition-dependent and are consistent with carbon-diffusion control. The morphological features of the aging reaction are explained by elastic strain-energy considerations. In accordance with previous thermodynamic models, it is concluded that virgin Fe-C martensites are unstable and that phase separation occurs by a spinodal mechanism. The martensitic substructure does not appear to exert any substantial influence on this decomposition behavior.

The plastic flow behavior of Hadfield manganese steel in uniaxial tension and compression is shown to be greatly influenced by transformation plasticity phenomena. Changes in the stress-strain (σ−e) curves with temperature correlate with the observed extent of deformation twinning, consistent with a softening effect of twinning as a deformation mechanism and a hardening effect of the twinned microstructure. The combined effects give upward curvature to the σ−e curve over extensive ranges of plastic strain. A higher strain hardening in compression compared with tension appears to be consistent with the observed texture development. The composition dependence of stacking fault energy computed using a thermodynamic model suggests that the Hadfield composition is optimum for a maximum rate of deformation twinning. Comparisons of the Hadfield steel with a Co-33Ni alloy exhibiting similar twinning kinetics, and an Fe-21Ni-lC alloy deforming by slip indicate no unusual strain hardening at low strains where deformation is controlled by slip, but an unusual amount of structural hardening associated with the twin formation in the Hadfield steel. A possible mechanism of anomalous twin hardening is discussed in terms of modified twinning behavior (pseudotwinning) in nonrandom solid solutions.

ESR 4340 steel forgings tempered to a hardness of HRC 55 exhibit a severe loss of tensile ductility in the short transverse direction which is strain-rate and humidity dependent. The anisotropy is also reflected in blunt-notch Charpy impact energy, but is absent in the sharp-crack fracture toughness. Brittle behavior is associated with regions of smooth intergranular fracture which are aligned with microstructural banding. Scanning Auger microprobe analysis indicates some intergranular segregation of phosphorus and sulfur in these regions. The anisotropic embrittlement is attributed to an interaction of nonequilibrium segregation on solidification with local equilibrium segregation at grain boundaries during austenitizing. This produces defective regions of enhanced intergranular impurity segregation which are oriented during forging. The regions are prone to brittle fracture under impact conditions and abnormal sensitivity to environmental attack during low strain-rate deformation. A relatively sparse distribution of these defects (∼10cm−3) accounts for the discrepancy between smooth bar and blunt-notch testsvs sharp-crack tests. Isotropie properties are restored by homogenization treatment. For application of these steels at extreme hardness levels, homogenization treatment is essential.

Comparison of the S-N curves of warm-extruded TRIP steels, heat treated to give two stabilities with respect to deformation-induced martensitic transformation during testing, reveals a beneficial effect of the transformation on fatigue behavior under stress control conditions. The proportionality of fatigue strength to ultimate tensile strength is maintained to higher strength levels relative to other steels. For a stress ratio of R = 0.1, a fatigue strength at 10 to the seventh power cycles of 180 ksi (1,240 MPa) is obtained. (Author)