Subgrain Coalescence Research Articles

Texture and microstructure development during intercritical rolling of low-carbon steels

Laboratory rolling trials have been performed to investigate the development of microstructure and crystallographic texture during and after intercritical rolling. The finishing temperature was varied over a wide range, and samples were taken just prior to the last pass, after quenching following the last pass, after air cooling and coiling, and after accelerated cooling and coiling. Cooling of the samples to the entry temperature for the last pass does not influence the texture of the sample, nor do higher cooling rates after austenitic finishing within the range of cooling rates in this study, although it may cause a refinement of the ferrite grains. Recrystallization after intercritical rolling leads to a decrease in texture intensity. In the case of recrystallization of low-carbon steels, the nucleation mechanism is strain-induced boundary migration (SIBM), which leads to unfavorable textures for deep drawing. In the case of recrystallization of interstitial-free (IF) steel after ferritic rolling, the nucleation mechanism shifts from the SIBM mechanism at high finishing temperatures to subgrain coalescence at (SGC) low finishing temperatures. The latter mechanism leads to more favorable textures for deep-drawing applications. Transformation-induced (TI) nucleation explains the occurrence of a sudden increase in ferrite grain size after high-temperature intercritical deformation of low-carbon (LC) steels.

An investigation into the cause of inhomogeneous distributions of aluminium nitrides in silicon steels

Abstract Two electrical steels containing 0.014 wt.% aluminium and 0.013–0.014 wt.% nitrogen were characterised to determine the operating recrystallisation mechanism following different heat treatment schedules. It was found that the operating recrystallisation mechanism depended on grain orientation with sub-grain coalescence and low-angle boundary movement occurring for {111}〈uvw〉-oriented grains and high-angle boundary movement occurring for {110}〈uvw〉-oriented grains in the narrow deformed bands. In addition, the number density of AlN precipitates (characterised using TEM) was larger in the {111}〈uvw〉-oriented grains compared to the {110}〈uvw〉-oriented grains. The higher number density of precipitates was due to the presence of dislocations remaining after recrystallisation.

Recrystallization Behavior in an Al–Cu–Mg–Fe–Ni Alloy with Trace Scandium and Zirconium

The Vickers hardness variation of a cold-rolled Al–Cu–Mg–Fe–Ni alloy to which trace scandium and zirconium were added had been measured during annealing. The microstructures of alloy were observed using optical microscope and transmission electron microscope (TEM). It was shown that the dispersed Al3(Sc, Zr) particles could pin dislocations, stabilize substructures and impede the movement of boundaries. Therefore, the recrystallization of the Al–Cu–Mg–Fe–Ni alloy was retarded. The mechanical properties of the alloy with Al3(Sc, Zr) particles at both ambient and elevated temperatures were improved. Subgrain coalescence was involved in the recrystallization process.

The Evolution of the Goss Texture in Silicon Steel

The Goss orientation, {l_brace}110{r_brace} , in about 3% silicon steel has been the subject of speculation from the scientific and technological points of view. The Goss texture, which is not stable with respect to plane strain deformation, rotates toward the {l_brace}111{r_brace} orientation. The Relaxed Constraints (RC) Taylor model, in which shear strains parallel to the rolling direction may occur, causes the formation of the {l_brace}111{r_brace} orientation. The {l_brace}111{r_brace} rolling component is known to lead to the Goss orientation after recrystallization. Raabe and Luecke found that the orientation intensity of the Goss component in the recrystallization texture increased with increasing orientation intensity of the {l_brace}111{r_brace} component in the rolling texture. The above results can explain the structure memory mentioned in an article by Inokuti et al. Inokuti et al. found that the nucleation of {l_brace}110{r_brace} secondary grains takes place in the vicinity of the steel surface zone. The nuclei for secondary grains were found to be the large primary recrystallized grains of near {l_brace}110{r_brace} orientation which were formed by coalescence of several subgrains. They called the nucleation of the Goss orientation structure memory because the nucleation site was thought to be the site of the original Goss orientation formed during hot rolling.more » The purpose of this work is to discuss the texture transformation from {l_brace}111{r_brace} to {l_brace}110{r_brace} based on the maximum energy release theory.« less

A comprehensive model of recrystallization for interstitial free steel

A computer model of the recrystallization process is proposed which incorporates both microstructure and crystallographic texture using a three dimensional lattice. This model is applied to the recrystallization of cold-rolled interstitial free (IF) steel, assuming oriented nucleation at the grain boundaries. The nucleation mechanism is implemented using the concept of subgrain coalescence. This model provides the quantitative information of a change in the volume fraction of the recrystallization, the grain size distribution, and the crystallographic texture.

Isokinetic analysis of nanocrystalline nickel electrodeposits upon annealing

The grain growth kinetics of nanocrystalline nickel electrodeposits was studied by transmission electron microscopy and differential scanning calorimetry at different heating rates. It was found that, upon annealing, the nanocrystals in the nickel electrodeposits appeared to grow abnormally and released about 415.7 ± 3.5 J/mol of heat. The mechanism of the abnormal grain growth was attributed to the subgrain coalescence. The method for determination of the grain growth activation energy as well as all the other kinetic parameters in the Johnson-Mehl-Avrami equation was proposed based on an isokinetic analysis. This method is applicable to general types of transformation process governed by a single activation energy under the isokinetic condition. The activation energy for the grain growth of nanocrystalline nickel electrodeposits was found to be about 131.5 kJ/mol using this method. The difference between the present method and the Kissinger and Ozawa method was addressed in terms of their physical backgrounds.

Recrystallization kinetics within adiabatic shear bands

Small recrystallized grains (0.1-0.2 μm diameter) are observed to form in adiabatic shear bands of shock-prestrained copper. However, the mechanism for recrystallization under the high strain, high-strain-rate conditions within shear bands is somewhat unclear. The kinetics of two classical mechanisms for recrystallization, high angle boundary migration and subgrain coalescence, are compared with the time-temperature profile determined for these adiabatic shear bands. It was found that the kinetics of the existing models are inadequate to explain the observed grain sizes, with the kinetics being several orders of magnitude slower than the deformation, time and/or the cooling time of the shear bands. Tests conducted at liquid nitrogen temperature also demonstrated that temperature did not play a major role in dynamic recrystallization under these circumstances. A dynamic recrystallization model is suggested in which mechanically-driven subgrain rotations assist the mechanism for recrystallization. This approach may enable dynamic recrystallization to proceed at very high strain rates, with only limited thermal assistance.

Towards Three-Dimensional Modelling of Subgrain Coalescence

Hu (1962, 1963) suggested that the nucleation process during recrystallisation can arise from subgrain coalescence. In the present paper, a new modelling approach has been presented for subgrain rotations. It is suggested that the required mass transfer due to compatibility criteria, can be modelled as geometric friction forces. By employing this new concept, it has been shown that three-dimensional models can be derived, based on classical mechanics. The derived model has shown-that the small and regular subgrains are the most likely candidates for rotation. Subgrain coalescence is strongly temperature dependent since diffusion of vacancies are required for the rotation.

Prediction of the Onset of Static Recrystallization after Hot Deformation.

In order to determine the kinetics of recrystallization in plain carbon steels, double-hit compression tests have been performed using a computerized Gleeble machine. Based on the nucleation mechanism of subgrain coalescence and the measured softening data, a kinetic model was developed to predict the start times for the static recrystallization occuring after hot deformation. Both the model calculations and experimental observations indicate that the onset of recrystallization is decelerated with increasing reheating temperatue but speeded up as the deformation temperature is increased. The former case can be attributed to the larger initial austenite grain size produced at higher reheating temperatures, which in turn results in larger subgrains and reduces the rate of coalescence. The latter case is due to the higher boundary diffusivity and mobility attained at higher deformation temperatures, which accelerates the process of subgrain coalescence. The accelerating effects of strain and strain rate can also be similarly explained by means of the higher dislocation density and finer subgrains generated during the higher strain and strain rate deformation. The good agreement obtained between model predictions and experimental results finally supports the view point that the coalescence of subgrains could be the rate controlling mechanism for the nucleation process of static recrystallization in austenite.

Recrystallization of molybdenum wire doped with lanthanum oxide

Recrystallization mechanism of La 2O 3-doped molybdenum wire was investigated. Samples of 1 mm in diameter were annealed for 1 h at various temperatures. Substructures were observed by transmission electron microscopy. Subgrain structures including dislocations of high density and small particles were stable up to about 1650 °C. Upon annealing at 1800 °C large grains with high aspect ratio along the working direction partly developed, though subgrains composed of dislocations simultaneously remained. Additionally, dislocations in the coarsened grain were not perfectly removed due to the pinning effect of second particles. Upon annealing over 2000 °C, on the other hand, elongated grain structures developed in the whole area. It is concluded that the grain coarsening was attributed to the primary recrystallization (subgrain coalescence) involving a rearrangement of dislocations rather than to the secondary recrystallization involving a nucleation process in primary grains.

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 ‡C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 ‡C and 1227 ‡C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 ‡C, while an arrest of grain growth was observed at a peak temperature of 1200‡C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy.

Computer Simulation of 2-Dimensional Subgrain Coalescence

Computer Simulation of 2-Dimensional Subgrain Coalescence

Irradiation-induced grain growth in gold and copper: In situ HVEM studies at 75-300 K

When thin polycrystalline films of Au, Cu and various other materials are subjected to energetic ion irradiation, the average grain size increases even at cryogenic temperatures. As is the case with many ion beam processes, this phenomenon of ion irradiation induced grain growth exhibits only a very mild temperature dependence. This contribution is based on in situ experiments, performed at the HVEM-Tandem User Facility at Argonne National Laboratory. This Facility interfaces a 2 MV Tandem ion accelerator and a 0.6 MV ion implanter to a 1.2 MV AEI high voltage electron microscope, which allows a wide variety of in situ ion beam experiments to be performed with simultaneous irradiation and electron microscopy or diffraction. A series of in situ ion and/or electron irradiation experiments is being performed at the HVEM-Tandem Facility at Argonne which have shown clearly for fine grained Au films that two mechanisms for growth are operative for the ion beam case: grain boundary migration as in normal thermal grain growth and grain coalescence which is similar in appearance to recrystallization by subgrain coalescence. Especially in the case of Au for which ion-induced growth is relatively rapid, such in situ experiments also demonstrate the importance of dislocation activity which is a consequence of the collision cascade damage associated with ion irradiation. Existing theories for irradiation-induced grain growth assume that growth occurs by boundary migration and that only point defects generated at grain boundaries are responsible for the growth phenomenon.

Sub-grain boundaries in Cd xHg 1− xTe and CdTe

Sub-grain boundary structure in Cd x Hg 1− x Te and CdTe has been investigated and is found to result primarily from slip and polygonisation. Sub-grain coalescence occurs during Cd x Hg 1− x Te high-temperature annealing, and using this process boundary-free material can be obtained.

Decomposition of deformed ferrite in a duplex stainless steel

The decomposition of deformed ferrite in a duplex stainless steel (U44L) has been examined using transmission electron microscopy. During annealing, austenite precipitation was extremely rapid and pinned the developing ferritic subgrain structure. Hence, the ferrite recrystallized via a continuous mechanism. Deformation of the ferrite, and to a lesser extent the austenite during growth/coarsening of the austenite, led to a ‘secondary recovery’ process. Subgrain coalescence, within the ‘primary’ ferrite subgrain structure and in the austenite particles, was observed after long term anneals: this process took place by the dissolution of the sub-boundaries formed during the secondary recovery process. In addition to the formation of austenite, M23C6, σ, and martensite were also observed.MST/311

Decomposition of deformed ferrite in a duplex stainless steel

The decomposition of deformed ferrite in a duplex stainless steel (U44L) has been examined using transmission electron microscopy. During annealing, austenite precipitation was extremely rapid and pinned the developing ferritic subgrain structure. Hence, the ferrite recrystallized via a continuous mechanism. Deformation of the ferrite, and to a lesser extent the austenite during growth/coarsening of the austenite, led to a ‘secondary recovery’ process. Subgrain coalescence, within the ‘primary’ ferrite subgrain structure and in the austenite particles, was observed after long term anneals: this process took place by the dissolution of the sub-boundaries formed during the secondary recovery process. In addition to the formation of austenite, M23C6, σ, and martensite were also observed.MST/311

Dislocation substructures formed during the flow stress recovery of high purity aluminum

Flow stress recovery experiments were performed on polycrystalline high purity aluminum following room temperature tensile prestrains of 5%, 10%, 15%, 20% and 25%. Isothermal recovery temperatures of 120, 160 and 200°C were used for recovery times up to 300 h. It was found that work hardening at room temperature is accompanied by the development of dislocation tangles that form the boundaries of a dislocation cell structure. These tangles were found to develop along {100},{110} and {111} planes. The cells formed decrease in size with increasing strain up to 15%, thereafter remaining constant at an average size of about 1.8 μm. The cell structure converts to a sharply defined subgrain structure during recovery as the tangles are rearranged into networks. The network planes are the same as those on which the original tangles were formed. During recovery following tensile prestrains less than about 15%, the subgrain size tends to grow by two mechanisms: (1) subgrain coalescence and (2) subboundary coalescence. The flow stress related to formation of dislocation substructures has been found to be inversely proportional to either the cell or the subgrain size for prestrains less than 15%, where the dislocation density in, and the misorientation angle of, the cell or subgrain boundaries was found to be nearly constant. In this region, conversion from cell to subgrain by recovery annealing results in a 24% loss of flow stress, independent of the prestrain used. Above about 15% tensile prestrain, the flow stress continues to rise without a significant decrease in the cell or subgrain size but with an increase in the density of dislocations in the cell walls and the average misorientation angle of the subboundaries. Furthermore, the subgrains formed after strains greater than 15% are more thermally stable, i.e. they do not grow appreciably during recovery annealing for 300 h at a temperature of 160°C.

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.

Kinetics of sub-grain coalescence—A reconsideration of the theory

The theory for the kinetics of sub-grain coalescence originally developed by Li [ J. appl. Phys. 33, 2958 (1962)] has been modified; the original theory significantly underestimates the observed times for coalescence in aluminium at temperatures below 350°C. The first modification considers the climb mobility of dislocation loops rather than that of isolated edge dislocations. For mobility limited by bulk diffusion the result is similar to that given by Li, but mobility limited by pipe diffusion is possible for dislocation loops and is found to give much higher mobilities at temperatures less than 350°C. An additional modification is the relaxation of Li's assumption of a uniform dislocation array now known to be incorrect. A simple computer simulation of coalescence by independent climb of a set of dislocation loops, is shown to give non-uniform dislocation distributions very similar to those observed. In combination with the pipe diffusion model for climb mobility, the simulation matches the reported kinetics of coalescence. Finally, the model for coalescence developed here, involving interaction of different dislocation loops, is shown to account for the known heterogeneity of sites for coalescence in aluminium. It is predicted that coalescence requires two adjacent high-angle boundaries to act as dislocation sinks.

Dynamic subgrain coalescence during low-temperature large plastic strains

Dynamic subgrain coalescence during low-temperature large plastic strains