Subgrain Growth Research Articles

A generalized model for radiation-induced amorphization and crystallization of U 3Si and U 3Si 2 and recrystallization of UO 2

A rate-theory model of radiation-induced amorphization and crystallization of U 3Si during ion irradiation has been generalized to include U 3Si 2 and UO 2. The generalized model has been applied to ion-irradiation and in-reactor experiments on U 3Si and U 3Si 2 and provides an interpretation for the amorphization curve (dose required to amorphize the material as a function of temperature), for the ion-radiation-induced nanoscale polycrystallization of these materials at temperatures above the critical temperature for amorphization, as well as for the role of the small crystallites in retarding amorphization. An alternative mechanism for the evolution of recrystallization nuclei is described for a model of irradiation-induced recrystallization of UO 2 wherein the stored energy in the UO 2 is concentrated in a network of sinklike nuclei that diminish with dose due to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular dislocation structures that evolve relatively early in the irradiation period. The complicated kinetics involved in the formation of a cellular dislocation network are approximated by the formation and growth of subgrains due to the interaction of shock waves produced by fission-induced damage to the UO 2.

A re-evaluation of the mechanism of SIBM

The process for creating a new recrystallized grain by the bulging out of an existing grain boundary within the deformed structure was recognized by Beck and Sperry and described as strain induced boundary migration, often now as SIBM. For many years SIBM seemed to be accorded less significance than the subgrain growth models for nucleation of recrystallization. However, the importance of SIBM is increasingly recognized, for example in the case of cube texture development in aluminum alloys. There is increasing evidence that high angle boundaries in deformed metals are not only residues of the prior grain structure but may also be generated by the deformation itself. These also participate in the nucleation of recrystallization on annealing and must do so by some form of SIBM mechanism.

Statistical evaluation of subgrain size growth in Hg 1 − xCd xTe (MCT)

Technological applications of single crystal MCT with x = 0.2 for infrared detectors require improved electrical parameters which are closely related to the presence of defects (dislocations, subgrains, precipitates). The authors have already established that edge dislocations were arranged forming mostly tilt-type sub-boundaries (1). In this paper, subgrain (SG) growth on thermal annealing (30–60 days at 650 °C) was studied by X-ray topography. Results were evaluated statistically, and SG size was found to be Log-normal distributed. Some remarks on the slight growth in SG size are given.

Discontinuous subgrain growth in deformed and annealed {110} 〈001〉 aluminium single crystals

Single-phase crystals of Al-0.05%Si of the Goss orientation {110} 〈001〉 have been deformed in channel die plane strain compression and found to be stable up to true strains of 3.0. The deformation microstructure contains neither long range orientation gradients nor heterogeneities and the crystals are resistant to recrystallization in the range 250–350°C, although rapid recrystallization can be induced if the crystal surfaces are lightly abraded. During annealing it is found that whilst the general subgrain structure coarsens gradually, a few subgrains grow rapidly to diameters of over 100 μm. Such discontinuous subgrain growth, which is topologically similar to primary recrystallization and abnormal grain growth has not previously been reported. It is shown that the rapidly growing subgrains are associated with slightly larger misorientations than the average subgrains. The origin of discontinuous subgrain growth is analyzed in terms of the orientation dependence of low angle boundary energies and mobilities.

Change in Boundary Misorientation with Subgrain Growth in a Microduplex Stainless Steel

Change in Boundary Misorientation with Subgrain Growth in a Microduplex Stainless Steel

Subgrain growth and misorientation of the α matrix in an ( α + γ) microduplex stainless steel

The change of the ( α + γ) microduplex structure during annealing in an Fe-26%Cr-7%Ni alloy has been investigated with emphasis on the change in the misorientation of the α matrix subgrain boundaries with subgrain growth in a large range of subgrain size. A long time holding up to 360 ks at 1273 K caused significant structure coarsening, and the growth of the α matrix subgrains and γ particles obeyed the third power law. On the other hand, the average misorientation of the α subgrain boundaries changed insignificantly, even though the α subgrain size markedly increased from 2.6 to 15.6 μm. This insignificance in the misorientation resulted from the fact that the variation of the local lattice rotation in the α matrix is not monotonic but periodic, and no long range lattice curvature exists.

Microstructural evolution of the martensitic cast steel GX12CrMoVNbN9‐1 during long‐term annealing and creep

The microstructure of four specimens of the martensitic cast steel GX12CrMoVNbN9‐1 crept to fracture at 873 K for 220‐33027 h was quantified by electron microscopy with regard to large M23C6‐particles, small V‐ and Nb‐containing particles, dislocations, pores, and oxidation layers. The laws of time‐dependent coarsening of particles and strain‐controlled growth of subgrains are consistent with those previously established for X20(22)CrMoV12‐1. Using these data the microstructural model of deformation developed for X20(22)CrMoV12‐1 can explain the creep behaviour of GX12CrMoVNbN9‐1. The minimum in creep rate is related to the coarsening of particles and subgrains. At low stresses (< 140 MPa) the fraction of small slowly coarsening V‐ and Nb‐containing particles leads to improved creep resistance. The material resists against pore formation; fracture occurs by necking.

Microstructure and deformation rate during long‐term cyclic creep of the martensitic steel X22CrMoV12‐1

The microstructure of three specimens of the martensitic steel X22CrMoV12‐1 which had been subjected to long‐term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = −1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20(22)CrMoV12‐1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth.

Metallurgical reactions in two industrially strip-cast aluminum-manganese alloys

Precipitation, phase transformation, subgrain growth, and recrystallization that occur during heat treatment of two strip-cast, cold-rolled, high manganese aluminum alloys have been studied mainly by transmission electron microscopy (TEM). The alloys differ in silicon content. The isothermal heat treatments have been performed in a salt bath at temperatures between 330 °C and 530 °C for times up to 1000 hours. Size distributions for each type of secondary particle have been determined. After short annealing times, small quasicrystals precipitated and subsequently transformed to α phase. The densities of these precipitates controlled dislocation movement and regulated subgrain sizes. Prolonged heating resulted in peritectoid reactions to Al6Mn or Al12Mn. Recrystallization, which is associated with the formation of Al12Mn, is advanced by increasing the silicon content; the nucleation and growth of Al12Mn occurs only at the expense of other phases that stabilize the subgrain network.

On grain and subgrain rotations in two dimensions

Computer simulations of subgrain growth by coalescence in two dimensions have been carried out. The model governing the dynamics allows the subgrains to rotate in order to reduce the sub-boundary energies. The purpose of the model is to study this mechanism separately; thus, the sub-boundaries are not allowed to migrate out of their initial positions. Hence, a coarsening of the subgrain structure occurs due to coalescence only. Results from several simulations are discussed. It was found that the mean subgrain size increased as an exponential function of time. The effect of the initial distribution of orientations and angles of misorientation has also been studied. It was found that the width of the distribution of orientations is important for the evolution of the mean subgrain size. The model and, consequently, the simulations concern subgrain rotations leading to coalescence. Based on these results, the general case of grain rotations in two dimensions has been discussed. It has been suggested that grain rotations depend on the grain boundary energy as a function of the misorientation.

Subgrain growth in heavily deformed aluminium—experimental investigation and modelling treatment

A comprehensive study of the annealing behaviour of two variants of commercial purity aluminium alloys has been carried out. The primary objectives of this work have been to improve the understanding of the recovery processes in itself as well as to study the effect of recovery on the annealing behaviour of heavily deformed aluminium. The experimental part has focused, firstly on describing the as deformed structure with emphasis on sub-boundary misorientations and long range lattice curvature, secondly on following subgrain growth and characterization of the softening reaction. The modelling part treats subgrain growth as a reaction controlled by sub-boundary migration. The significance of sub-boundary flexibility in this context is emphasized, an aspect which requires lateral drift of ledges as an important element in the boundary migration mechanism. A subgrain growth model based on thermally activated migration of ledges has been developed and applied to the experimental results.

Recovery revisited

Recovery of mechanical properties during annealing of deformed metals has been modelled based on a microstructural representation comprising two elements, (i) the cell/subgrain structure (size δ) and (ii) the dislocation density (ρ) within the subgrains. These two microstructural elements are treated as independent internal state variables and the recovery of flow stress obtained by adding the time dependent contributions due to subgrain growth [σ ∝ 1/δ( t)] and dislocation network growth ▪. The growth of a dislocation network has been treated in terms of thermally activated glide, thermally activated cross-slip, climb and solute drag as rate-controlling mechanisms. Subgrain growth has been analysed in a manner analogous to normal grain growth, with climb of the boundary dislocations being the rate controlling mechanism. The model has successfully been applied in the interpretations of recovery observations in iron, aluminium and AlMg alloys. It follows from the theoretical treatment as well as from the analysis of experimental data that the characteristic logarithmic time dependence of low temperature recovery is the result of a reaction controlled either by thermally activated glide of jogged screw dislocations or by solute drag. It has been demonstrated that a mechanism based on thermally activated cross-slip does not apply in this context.

Interactions Between Subgrain Boundaries and Carbides in P91 Steel Characterized by Fully Automatic Image Analysis / Charakterisierung der Wechselwirkungen zwischen Subkorngrenzen und Carbiden im Stahl P91 durch vollautomatische Bildanalyse

In the present study a method suitable for evaluation of the particle ― boundary distance using fully automatic image analysis has been described. The method has been demonstrated in the case of three different microstructural states of the tempered martensite ferritic steel P91. The following results were obtained: In the state before creep (as-received) 70% of the M 23 C 6 carbides are situated near the subgrain boundaries. The distance from the subgrain boundary where statistically relevant population of carbides can be found is less than 0.1 μm. The influence of creep at 600° C, 130 MPa on the microstructure of the as-received state is relatively weak what is reflected by a very limited subgrain growth and by almost the same statistical distribution of the carbide-subgrain boundary distances

EBSP: A powerful tool in studying recrystallization of metals

The Electron Back Scattering Pattern Technique (EBSP), for measurements of crystal orientations is now a well established technique for microtexlure detennination. The application of the technique to material problems has increased quite substantially the last years, both in basic research and in more industry related problems. When studying the recrystallization process in detail a relatively large area of a specimen has to be investigated to obtain good statistical results. With a resolution of the order of 0.5-1 μm and a short processing time the EBSPtechnique has shown to be advantageous compared to TEM on one side and X-ray and neutron diffraction on the other. By the EBSP-technique we are able to follow the local texture evolution during transformation and are at the same time able to analyse changes in the untransfonncd matrix (i.e. subgrain growth etc.).It is well accepted that the formation of recrystallization nuclei is a highly heterogeneous process.

Effect of Austenitizing Temperature on Microstructure and Mechanical Properties of 12% Cr Steel.

The effect of austenitizing temperature were investigated on the microstructure and mechanical properties of 12%Cr steel. Low-temperture austenitizing below 1000°C induced the carbide coarsening during subsequent tempering at 750°C for 1 hr due to the nucleation effect of undissolved M23C6. The large and spheroidized carbides enhanced the subgrain growth. On the other hand, the complete dissolution of M23C6 above 1000°C caused the fine carbide formation on lath boundaries, which retarded the subgrain growth during tempering. Furthermore, the dissolution of Nb(C, N) above 1100°C enhanced the tempering resistance through increasing the stability of lath morphology and reducing the growth rate of M23C6. The increase in strength with increasing austenitizing temperature was attributed to the fine carbide distribution and the high dislocation density. Further, as the austenitizing temperature increased, the impact energy markedly reduced, due to the large proir austenite grain size and the high strength.

Fundamental Aspect of Texture Formation in Low Carbon Steel.

The mechanism of the texture formation in low carbon steels is not fully understood as yed. To clarify this, a series of systematic investigations has been made. Summarizing these results and comparing these with those of other investigations, possible mechanisms of texture formation are discussed. In polycrystalline iron, deformation and recrystallization in grain boundary regions play the most important role in the texture formation. In this respect, results obtained on single and bi-crystals do not provide much informations. In polycrystals, crystal rotation occurs during rolling along different paths from that observed in single crystals, affected by grain boundary constraints significantly. Approaches to the stable end orientation seems to be closely related with the development of the stable dislocation substructures. The development of the rolling texture is therefore strongly affected by the metallurgical factors that influence the evolution of the dislocation substructures. {111} recrystallized grains seem to be formed from the grain boundary regions of the same orientation through the subgrain growth mechanism.

The effect of recovery annealing after small plastic deformations on the yield strength of polycrystalline aluminium

Investigations of the yield strenght of polycrystalline aluminium have been performed, looking at the effect of recovery annealing after small plastic deformations. The kinetics of the yield strength changes were determined as a function of temperature and time. Results were discussed in relation to electron microscopy observations of changes in the microstructure. It has been found that changes in the yield strength in fine-grained aluminium after small plastic deformations and recovery annealing are controlled by the mechanisms on grain boundaries whereas in coarse-grained aluminium the most important effects are associated with mechanisms in the grain interior. In fine-grained aluminium these mechanisms are: the annihilation of dislocations in grain boundaries which leads to a decrease of the total dislocation density, and the change of the grain boundary characteristics enhanced by this annihilation. In coarse-grained polycrystal the basic mechanisms are rearrangement of tangled dislocations and dislocation walls into subboundaries and growth of subgrains.

Modelling high temperature creep of academic and industrial materials using the composite model

The composite model of plastic deformation of materials developing a subgrain structure provides a framework for coupling the deformation of the soft subgrain interior and the hard subgrain boundaries through internal stresses. By specifying the kinetic laws for deformation of soft and hard regions according to the dominant dislocation obstacles, the model has been applied to three materials: pure aluminium, the Ni-based alloy NiCr22Co12Mo (Inconel 617) and the ferritic 12% Cr steel X 20 CrMoV 12 1. The formulation of the kinetic laws for the soft region was based on thermally activated glide through the dislocation structure in the case of aluminium and on solute and particle hardening in the case of the alloys. The kinetic laws for the hard region were similar in all cases, differing mainly in the amount of particle hardening. It is shown that the model is capable of modelling the transient creep of pure aluminium between two different steady states, accounting for strain hardening due to the formation of the subgrain structure during the primary creep of NiCr22Co12Mo and estimating the degree of softening which can be expected as a result of subgrain growth and particle coarsening during the long-term creep of the 12% Cr steel.

Effect of precipitates on subgrain growth during continuous recrystallisation in Al–4Cu

AbstractIt was found that on annealing supersaturated and cold rolled Al–4Cu at 280°C subgrains were initially formed as a result of the rearrangement of dislocations. Concurrent with the formation of subgrains having well defined boundaries, θ particles were formed, situated mostly on the subgrain boundaries, and the growth of subgrains was therefore severely hindered by the particles, leading to continuous recrystallisation for annealing times up to 10 000 min at 280°C. The pinning force due to particles located on the subgrain boundaries was much greater than that due to randomly dispersed particles. Subgrain growth was found to be arrested for particle parameters f/r of greater than 0·25 μm−1, where f is the particle volume fraction and r is the mean particle radius.MST/1572

Effect of precipitates on subgrain growth during continuous recrystallisation in Al–4Cu

Effect of precipitates on subgrain growth during continuous recrystallisation in Al–4Cu