Gary R. Purdy

Modeling recrystallization of microalloyed austenite: effect of coupling recovery, precipitation and recrystallization

Abstract In this contribution, existing models for precipitation, recovery and recrystallization have been coupled, with their interdependencies explained, to describe the microstructural evolution in a supersaturated alloy after hot deformation. Microalloyed austenite has been used as an example system and the time evolution of the precipitate diameter and the recrystallization and softening fractions are compared with the available experimental data. The model predictions are in excellent quantitative agreement with the experimental observations. Particular attention is paid to the occurrence of ‘plateaus’ or ‘humps’ in the softening and recrystallization fraction plots. In both cases, the incorporation of recovery is an essential ingredient for a quantitative description of the microstructural evolution in the hot-worked structure.

O-lattice analyses of interfacial misfit. I. General considerations

Abstract Misfits between lattice points of intersecting crystals and between corresponding lattice planes are analysed using O-lattice constructions referred to both direct and reciprocal spaces. O-lattice planes are described by the same expression as the ‘Moire planes’ formed by the mismatch of correlated planes, which facilitates the study of the O lattice from diffraction patterns. The normalized primary O-lattice planes (those related to the closest packed atomic planes) with the smallest interplanar spacing are considered to be planes of minimum mismatch, and hence to represent plausible low-energy interfaces. An interface of arbitrary orientation is assumed to be stepped microscopically along the primary O-lattice planes. The solution for the O-lattice cells is derived. Based on this cell structure, possible configurations of the misfit dislocations in a primary O-lattice plane are obtained; this provides a basic description of interfacial structure.

Observations of Thin Crystals of TiNi in Premartensitic States

Thin films of TiNi have been examined in the temperature range 0°–200°C, using transmission electron microscopy and selected‐area diffraction. The CsCl diffraction patterns are accompanied by strong localized diffuse scattering, which becomes more pronounced as the temperature is reduced, approaching that of a phase transformation. A series of experiments designed to determine the directions of maximum diffusely scattered intensity suggest that these are 〈210〉 and 〈321〉, and that scattering is largely confined to reciprocal lattice directions subtending small angles with the incident electron beam. Since the phase transformation is thought to be of the second kind, and diffusionless, the results are interpreted in terms of large‐amplitude, short‐wavelength atomic displacements, reflecting incipient mechanical instability. Several diffraction patterns and dark‐field photomicrographs are presented to indicate the scale of the domain structure of the transformation product.

A study of the influence of Mn and Ni on the kinetics of the proeutectoid ferrite reaction in steels

Abstract The kinetic transition between partitioned and unpartitioned growth of proeutectoid ferrite has been studied for high-purity Fe–C–Mn and Fe–C–Ni alloys, and for temperatures just above the eutectoid. These results (and certain results of previous investigations) are compared with computed paraequilibrium and equilibrium ternary phase diagrams, and it is shown that the transition occurs well within the paraequilibrium two-phase regions, but significantly outside the limit predicted by the local equilibrium analysis of the ternary precipitation reaction. These observations are interpreted in terms of solute drag theory. It is inferred that both Mn and Ni exert a drag on the moving ferrite/austenite interfaces, and that this drag force is due to substitutional solute diffusion within the moving interface. The equilibrium binding energies of each of the substitutional solutes to the boundary are expected to be of order RT .

O-lattice analyses of interfacial misfit. II: Systems containing invariant lines

Abstract Mismatch in interfaces containing an invariant line is analysed on the basis of the O-lattice concept and the properties of the invariant line strain. An optimum orientation relationship is proposed to satisfy the requirement for the existence of the O lines. The corresponding habit plane will contain the O lines, and the mismatch in the habit plane will be accommodated completely by a single set of dislocations. The concept of an effective invariant line is introduced. An example is given for a Zr-2.5 wt% Nb alloy, showing the method of search for the optimum orientation relationship. The corresponding interfacial structure in the habit plane is also provided.

A model for the competition of precipitation and recrystallization in deformed austenite

Abstract The hot-deformation of a microalloyed steel sets the stage for the processes of static-recrystallization and strain-induced precipitation. A physically-based model was developed to describe the interaction of these two processes. The precipitates were assumed to form, exclusively, on dislocations. Dynamic effects as well as static recovery were ignored. Given the alloy composition, deformation temperature and dislocation density, the model is able to predict the recrystallized fraction as a function of time. The model may be used to construct recrystallization–time–temperature (RTT) maps as well as deformation–temperature (DT) maps. The predictions of the model are in excellent qualitative agreement with experimental observations.

Particle coarsening in binary and multicomponent alloys

The theory of particle coarsening (Ostwald ripening) for multicomponent alloys was reviewed recently by Umantsev and Olson. In addition, they presented their own formulation for systems with a diagonal diffusivity matrix and they discussed how coarsening is affected by non-ideal behavior of the solid solution surrounding the precipitate particles. The present communication extends their work to include off diagonal terms in the diffusivity and shows that coarsening is independent of the solid solution thermodynamics followed by the continuous phase. This independence was recognized before, because changes in the diffusivity due to non-ideal behavior are balanced by changes in the concentration differences, due to surface curvature, that drive diffusion.

Overview no. 38: On the nature of the bainite transformation in steels

Abstract An overview is given of the similarities and differences among the members of the series of products, Widmanstatten ferrite, upper bainite, lower bainite, lath martensite. It is argued that this should be regarded as a continuous series even though there are significant distinctions between neighbouring products. It is proposed that important changes occur in the properties of the α γ interface with decreasing temperature and result in changes from one product to the next.

Autocatalytic nucleation and elastic stabilization of linear arrays of plate-shaped precipitates

Abstract Autocatalytic nucleation of linear arrays of elastically interacting plate-shaped precipitates (each causing tetragonal distortion) is considered. It is shown that the strain fields of large plate like precipitates can interact with those of smaller ones in a repeated nucleation process, such that successive new plates are formed in the stress and diffusion fields of existing ones. The resulting elastically-locked arrays are stabilised against growth and/or coarsening. It is shown that the elastic interaction energy may play a decisive role in the nucleation stage of θ′ in Al 3 wt.% Cu. The θ′ microstructure, in relatively early stages of development, is often inhomogeneous and consists mainly of linear (parallel inclined and cross-like) stacks, which are energetically favoured.

On the stability of arrays of precipitates

Abstract The problem of the stability of an array of elastically interacting plate-shaped precipitates is considered. Algebraic conditions for stability against coarsening are developed, and the results of detailed numerical tests of the stability against displacement and of the stability against coarsening, of a particularly simple and symmetric array are reported.