Recrystallization Interface Research Articles

Recrystallisation and Trace-Element Mobility in Zircons: Implications for U-Pb Dating

Complex microstructures of zircon recrystallisation have been discovered in igneous and metamorphic zircons from the Jack Hills Metasedimentary Belt in Western Australia, and the granitic injection complex of Harris in north-west Scotland. Cathodoluminescence (CL), electron backscatter diffraction (EBSD) and wavelength dispersive spectroscopy (WDS) analysis reveal that recrystallisation in the studied zircons is often characterised by (1) translation, bending and fading of the oscillatory zoning; (2) structural recovery of the zircon; (3) an enrichment in Hf and depletion in Y in recrystallised zircon; and (4) the formation of a recrystallisation interface with a minor (<2°) misorientation. A new, composite recrystallisation model in which trace element and dislocation migration occurs shortly after crystallisation during magmatic cooling and pipe diffusion along dislocation arrays formed by the accumulation of dislocations allows enhanced diffusion to enrich Hf and leach U, Pb and Y. After the recrystallisation event, subsequent metamictization of primary zircon (typically with oscillatory zoning) creates zones that can structurally recover through a diffusion-reaction re-equilibration mechanism, resulting in further trace-element mobility. These mechanisms can create complex microstructures in zircons, suggesting that a detailed understanding of the crystallisation and recrystallisation history of a zircon may be required to accurately interpret its U-Pb ages.

Modelling discontinuous dynamic recrystallization using a quantitative multi-order-parameter phase-field method

A multi-order-parameter phase-field model was built by coupling a phase-field model with a physically-based statistical nucleation model to predict the microstructure evolution and flow stress responses of discontinuous dynamic recrystallization in 304L stainless steel. Individual growth kinetics simulations of recrystallized nucleus showed that the critical nuclei size was determined by balance between the local stored energy difference and grain boundary energy. This was different from the widely-used semi-analytical Roberts-Alhblom model of nucleation criterion and showed good agreement with Bailey-Hirsch model. The migration rate of the recrystallization interface did not follow the monotone change but strongly depended on the deformation conditions. The overall simulations of dynamic recrystallization agreed well with experimental observation. The characteristic features such as effect of deformation conditions on the peak stress, critical strains and grain size were quantitatively captured by the model. The sensitivity of grain boundary mobility to both temperature and strain rate was found from simulation. If the initial grain size decreased to a critically small value, the enhanced work hardening effect due to grain refinement maybe results in the dramatic increase of nucleation density, and hence finer steady-state grain size. The transition from single peak flow behaviors to multiple peak implies the change of dominating recrystallization behavior from nucleation to interface migration.

Grain reorientation during zone annealing of MA6000

Abstract Reorientation towards a 〈110〉 fibre texture by the submicrometre unrecrystallised grains of the mechanically alloyed nickel based superalloy MA6000 has been observed behind the secondary recrystallisation interface during the zone annealing process. The driving force is considered to be the potential reduction in grain boundary energy, and possible mechanisms are those of grain rotation or the preferential growth of suitably oriented annealing twins. From orientation analyses by microbeam electron diffraction it is demonstrated that the reorientation arises from the latter process.MST/3615

Growth of secondary recrystallised grains during zone annealing of oxide dispersion strengthened alloys

Electron backscattered pattern analysis has been usedto identify the changes in microtexture of a zone annealed rod of mechanically alloyed nickel MA 6000 between the initial and steady state growth of the coarse longitudinal grains. As the secondary recrystallisation interface travels further down the specimen, the <110> fibre texture becomes increasingly ‘tight’, and the grain size and the proportion of ‘special’ boundaries both increase. A development of the Gladman model of secondary recrystallisation is presented which accountsfor this behaviour.MST/3037

Growth of secondary recrystallised grains during zone annealing of oxide dispersion strengthened alloys

Electron backscattered pattern analysis has been usedto identify the changes in microtexture of a zone annealed rod of mechanically alloyed nickel MA 6000 between the initial and steady state growth of the coarse longitudinal grains. As the secondary recrystallisation interface travels further down the specimen, the <110> fibre texture becomes increasingly ‘tight’, and the grain size and the proportion of ‘special’ boundaries both increase. A development of the Gladman model of secondary recrystallisation is presented which accountsfor this behaviour.MST/3037

Effect of process variables on the structure of directionally recrystallized MA-6000

The effect of zone speed during directional recrystallization of MA-6000 in two conditions of extrusion has been investigated. Electron back-scattered patterns have been used to compare the microtextures developed and to characterize the longitudinal grain boundaries formed. In both extrusions a [110] texture developed; in addition, small (<10μm) highly misoriented grains were detected, their frequency of occurrence decreasing with increasing zone traverse rate. A high density of small annealing twins was also observed after low traverse rates. It is suggested that both features are formed at the secondary recrystallization interface but fail to grow with the rest of the grain. Except at the minimum zoning rate, the high temperature extrusion material failed to directionally recrystallize but developed equiaxed grains. It is concluded that under these conditions, the rate of traverse exceeds the maximum growth rate of the grains. No microstructural differences were detected between the materials after differing extrusion conditions, and the origin of the difference in boundary mobility is considered to arise from solute pinning.

Development of microstructures of high grain aspect ratio during zone annealing of oxide dispersion strengthened superalloys

Two extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. A <110> texture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towards <110> has been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lower temperature. Quenching experiments have shown that nucleation of secondary recrystallisation occurs at temperatures higher than that at which the recrystallisation interface grows at the zoning speed employed. It is suggested that the microstructure develops via the thermally activated unpinning of interfaces which have mobility advantages.MST/1948

Development of microstructures of high grain aspect ratio during zone annealing of oxide dispersion strengthened superalloys

AbstractTwo extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. A texture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towards has been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lowe...

Secondary recrystallisation of two oxide dispersion strengthened ferritic superalloys: MA 956 and MA 957

The response to isothermal recrystallisation and zone recrystallisation of rods of MA 956 and MA 957 has been compared. Isothermal recrystallisation produced equiaxed millimetre sized grains in MA 957, whereas MA 956 had grains of high aspect ratio. This is attributed to an anisotropic distribution of oxide particles in MA 956. In both alloys the initial strong 〈110〉 fibre texture is transformed into textures lying on the 〈001〉–〈111〉 zone axis. A discrete zone recrystallisation interface could not be formed in MA 956 under the range of conditions investigated. This may be accounted for by an anisotropic boundary pinning force. This results in small transverse velocities and also a different zone recrystallisation texture in MA 956 from that in MA 957.MST/1860

Secondary recrystallisation of two oxide dispersion strengthened ferritic superalloys: MA 956 and MA 957

The response to isothermal recrystallisation and zone recrystallisation of rods of MA 956 and MA 957 has been compared. Isothermal recrystallisation produced equiaxed millimetre sized grains in MA 957, whereas MA 956 had grains of high aspect ratio. This is attributed to an anisotropic distribution of oxide particles in MA 956. In both alloys the initial strong 〈110〉 fibre texture is transformed into textures lying on the 〈001〉–〈111〉 zone axis. A discrete zone recrystallisation interface could not be formed in MA 956 under the range of conditions investigated. This may be accounted for by an anisotropic boundary pinning force. This results in small transverse velocities and also a different zone recrystallisation texture in MA 956 from that in MA 957.MST/1860

Grain boundary segregation in an oxide-dispersion-strengthened ferritic steel

Oxide-dispersion-strengthened (ODS) ferritic steels are being developed in the nuclear industry for advanced fast and fusion reactor systems [1, 2]. These materials offer improved elevatedtemperature strength characteristics compared with the currently favoured 12% Cr martensitic alloys [3], together with high swelling resistance under irradiation [4]. Attention is presently focused on the MA 957 superalloy [nominal composition Fe-14Cr1Ti-0.3Mo-0.27Y203 (wt%)], which contains a fine yttria dispersion in a fully ferritic 14% Cr matrix, and is manufactured by the powder metallurgy route of mechanical alloying, viz. ball-milling, compacting and extrusion [5]. Significant enhancement of creep ductility is possible in ODS materials for matrix grain structures of high aspect ratio [6]. Such directional grain coarsening is conventionally achieved by a process of zone annealling, which essentially results in secondary recrystallization [7]. The secondary recrystallization response of extruded bars of MA 957 has recently been characterized by Evens et al. [8]. In these studies a significant curvature of the recrystallization interface was noted in longitudinal sections of partially zone-annealled samples, the interface becoming more steeply inclined to the normal with increasing zoning rate. The curvature of the interface is shown in Fig. la and is not thought to arise from variations in temperature across the section of the bar. This is confirmed by holding the specimen stationary in the temperature gradient for 30 min; this would result in a planar interface if a diametral temperature gradient exists, but the interface remains curved, as shown in Fig. lb. A similar curved interface has been observed by Marsh and Martin [9] in MA 6000 (a yttriastrengthened nickel-based superalloy), which was ascribed to strain gradients in the original extrusion, leading to differences in texture and hence grain boundary mobility. Evens et al. [8] alternatively suggest that in MA 957 ferritic superalloy the phenomenon is a consequence of gradients in solute segregation which influence grain boundary mobility. Therefore, in order to provide more information on this topic, an accurate analysis of grain boundary microchemistry was carried out on MA 957 using a field-emission gun scanning transmission electron microscope (FEGSTEM).

Micromechanisms of texture development during zone annealing of MA 6000 extrusions

AbstractBars of MA 6000 have been extruded at a ratio of 2·75:1 at a temperature of 1100°C, subjected to a zone recrystallisation treatment in a calibrated gradient furnace, and the (secondary) recrystallisation interface studied. By selected area channelling, a 〈110〉 texture has been identified, which is attributed to the ability of 〈110〉 tilt boundaries to form low energy interfaces of a close packed nature. A curved secondary recrystallisation interface is observed and this is shown to result indirectly from the strain gradients during extrusion. A microbeam electron diffraction technique has enabled the orientations of the submicrometre sized equiaxed grains in the hot worked orientations to be studied. It has been shown that, as the temperature is increased in the less severely deformed regions of the samples, rotation of the grains to lower energy orientations (〈110〉) occurs.MST/1211

Early stages of recrystallization in nickel

This paper reports on an investigation into the early stages of recrystallization in nickel from the standpoint of the misorientation between newly formed recrystallized grains and the surrounding deformed microstructure. For the case of recrystallization “nuclei”(i.e., l-μm diameter) tended to be either not low-gS CSLs or less close to a low-gS CSL than for the “nucleus” case. The interpretation of these data is that the very earliest stage of recrystallization is facilitated at a boundary which is more mobile than average, often a characteristic of a low-gS CSL. After nucleation of recrystallization has been established, the important criterion for the recrystallization interface is a large capacity for dislocation absorption, which is associated with general boundaries rather than CSLs. The number of boundaries bordering the new grains and their geometrical characteristics were consistent with a model which relates sides per grain to grain boundary energy.

Amorphization of germanium, gallium phosphide, and gallium arsenide by laser quenching from the melt

The direct formation of amorphous layers on (111) Ge, GaP, and GaAs crystals is demonstrated to occur when these are transiently melted with 2.5-ns UV laser pulses below particular radiation energy density thresholds. Above these thresholds, the melt resolidification rate is reduced and in each case the (111) crystals exhibit a transition to a regime of profuse twin defect nucleation. Also for each material, laser melting of (001) crystal leads to defect-free recrystallization when amorphization does not occur. All amorphous layers exhibit a cellular thickness modulation which may be due to a fundamental instability in the recrystallization interface.

Recrystallization of a nickel-base superalloy: Kinetics and microstructural development

The primary recrystallization of a commercial nickel-base superalloy, Nimonic 115, has been investigated. The migration kinetics of the recrystallization interface have been examined by light optical microscopy and hardness testing; the reasons for a high apparent value of activation energy so obtained are discussed. The grain refinement resulting from recrystallization has been studied by transmission electron microscopy. A mechanism, based on enhanced recovery near to the recrystallization interface leading to repeated nucleation of recrystallization, is proposed.

Modulated structures and metastable dopant concentrations in silicon annealed with Q-switched laser pulses

Radiation pulses from Q-switched lasers can be used to transiently melt and modify the near surface regions of solids. Indeed the annealing is of such short duration that new, extremely rapid solidification phenomena are now amenable to study. Most work has probed the basic crystal growth behavior of elemental Si. This process is often strongly influenced by the presence of high concentrations of impurities: either growth instabilities or metastable, supersaturated solid solution formation may occur. At the highest resolidification rates (≳10 m/s) the recrystallization interface can break down with the formation of an amorphous solid phase. This paper describes the various processes which can occur and identifies limiting behavior in high speed crystal growth.

Molecular Dynamics of a Laser-Annealing Experiment

A study of a laser-annealing system using a new molecular dynamics method modified to faithfully simulate typical experimental conditions is presented. Following melting, the recrystallization interface is layer structured in the melt. Rapid recrystallization, impurity segregation and constitutional undercooling, and the dynamics of the processes are demonstrated.

The application of convergent-beam electron diffraction to the detection of small symmetry changes accompanying phase transformations II. Recrystallization of superalloys

Abstract This paper gives an example of the application of convergent-beam electron diffraction (CBED) to the detection of subtle changes in the ordering of precipitate phases which may accompany the passage of a recrystallization interface. This alteration in the ordered state of the γ phase in one nickel-base superalloy is shown not to involve any change in composition but rather the redistribution of the atomic species with respect to the atomic sites in the precipitate phase, leading to a higher degree of ordering than is normally associated with γ′. A tetragonal distortion is induced by this ordering, and it is shown that this may also influence the subsequent precipitation of cooling γ′ in the matrix. A mechanistic interpretation, based on the reactions occurring in the moving grain boundary which forms the recrystallization interface, is presented.

The recrystallization of nickel-base superalloys

The effects of recrystallization on the γ′ distribution in four nickel-base superalloys of varying γ′ volume fraction (Nimonics PE16, 80A and 115, and Udimet 720) have been studied by transmission electron microscopy. These effects are explained in terms of high solubility and diffusivity in the recrystallization interface, and it is suggested that high diffusivity assumes greater importance as the amount of solute dissolved in the boundary increases. Some attention is given to the nucleation of recrystallization. It is shown that in one of the alloys (Udimet 720), nucleation at grain boundaries involves subgrain coalescence. Subsequent growth of the nucleus occurs by strain-induced boundary migration.

Theoretical considerations in the zone melting of organic substances: Analysis of the diffusion gradient

Further evidence is gathered in the theory of the zone melting of organic compounds under ideal equilibrium conditions by a kinetic study of the diffusion gradient in the melt zone. A mathematical analysis is developed for the concentration distribution of impurities in the moving molten zone, which yields an expression for the concentration gradient at the liquid-solid recrystallisation interface. These expressions describe the transport process and a measure of the rate of segregation of impurity at the recrystallisation interface. In addition, these equations have been programmed on a computer and concentration profiles for two zone velocities have been graphed. It has been found that the concentration gradient is independent of the length of the zone under ideal equilibrium conditions for pure diffusion.