Materials Science Forum Vols. 715-716

Abstract: Microstructural changes due to isothermal heat treatment in the bent regions of cold folded nickel alloy (Ni200), mild steel, and Titanium (Ti6-4) sheet samples, followed by the EBSD (electron back-scattered diffraction) technique are reported. Large strain gradients that vary from being tensile to compressive exist in the bent regions and have been used to study their influence on recovery and recystallisation processes. Isothermal heat treatment at 600°C in the Ni200 and steel sheets results in recrystallisation process to start in regions with the highest strains and progresses towards the centre of folded regions. Grain growth was observed to be dependent on both the sign of the strain and its level. Generally larger grain sizes were associated with tensile strains in both Ni200 and steel. Abnormal grain growth occurred after prolonged heat treatment in the Ni200 sample in regions with low levels of tensile strains found in the middle of the sheet thickness. Although complete recrystallisation was not observed in the steel sample significantly large grains formed in regions with low strain levels. Only recovery processes were observed in the Ti6-4 samples. Some twinning was also observed in Ti6-4 that was confined in the regions with compressive strains. Crystallographic textural changes were observed during this study and will be reported in future publications.

Abstract: The microstructure of a material determines its mechanical properties. Since microstructure can be tailored by thermo-mechanical processing of the metal, it is important to understand how the microstructure evolves under thermo-mechanical processing. We have constructed a phase field formalism to study recrystallization and grain growth in polycrystalline material. A unique feature of our model is that the Euler Angles (φ₁,φ,φ₂), obtained from Electron Back Scattered Diffraction (EBSD) data of a polycrystalline sample can be taken as an input to our model. In our model, the grain orientations at discrete grid points are represented by a non-conserved vector field, namely a quaternion. The free energy used for the evolution of the local orientations contains bulk energy for various preferred grain types and grain boundary energy. The grain orientations evolve in time following a Langevin dynamics. So far we have established that the rate of grain growth follows the usual L ~ t^1/2 scaling law when the grain boundary energy is independent of the misorientation angle between neighboring grains. Work on other aspects of this model is in progress.

Abstract: Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products. The present work focuses on computation of incubation time t_inc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting t_inc equal to interpass time. Simulations allow for prediction of the onset of austenite static recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Abstract: The densification and grain growth during sintering of alumina-zirconia (Al₂O₃ZrO₂) ceramics were simulated using a modified phase field method, which considered simultaneously a density field, a composition filed and orientation fields. The results indicate that the model can capture the main microstructure features in the different stages of sintering. A higher relative green density leads to a higher final density and a larger final grain size in the sintered ceramics. A higher volume fraction of the ZrO₂ phase results in a lower relative density and a smaller final grain size.

Abstract: Stress relaxation was studied in a series of low carbon, high Mn microalloyed steels containing 0.012, 0.06 and 0.1 wt% Nb. The stress-relaxation curves were modeled using a physically-based model that takes into account the time evolution of precipitation, recovery and recrystallization as well as their interactions. The results confirm that high Mn-high Nb design can offer distinct advantage over the low-Mn design for the application of near net shape processing.

Abstract: Three dimensional (3D) phase field modelling is used to simulate austenite grain growth in X80 linepipe steel for thermal paths that are typical in the heat affected zone (HAZ). In the HAZ austenite grain growth is affected by pinning due to precipitates and their potential dissolution. Effective grain boundary mobilities are introduced that are consistent with strong pinning at lower temperatures and weak pinning at higher temperatures separated by the estimated dissolution temperature range of fine NbC precipitates. These mobility relationships are then used to describe austenite grain growth in bulk samples subjected to rapid heating and cooling conditions to replicate thermal cycles at various positions in the HAZ.

Abstract: Size effects observed in nanocrystalline grain growth are modeled by attributing each type of grain boundary junction an own specific energy and finite mobility. By considering grain growth as a dissipative process that is driven by the reduction of the Gibbs free interface and junction energy a general grain evolution equation is derived that separates into nine types of possible growth kinetics. The corresponding self-similar grain size distributions are derived and compared with results from modified Monte Carlo Potts model simulations taking into account size effects in triple and quadruple junction limited grain growth.

Abstract: Microstructural design of a new generation of 9%Cr steels for fossil power plants is considered. It was shown that microstructural stability of 9%Cr steels impairs their creep resistance. Two types of restoration processes can occur in the heat resistance steels under creep conditions: (i) normal grain growth and (ii) dynamic recovery. The first process associates with the migration of high-angle boundaries (HAGB) of blocks of tempered martensite lath structure (TMLS). However, their migration is negligible even during creep deformation. Boundaries of packets and prior austenite boundaries (PAB) are effectively pinned by precipitations of M₂₃C₆ and Laves phase Fe₂(W,N). The second process consists of transformation of lath boundaries to subboundaries and their subsequent migration (subgrain coarsening) under creep. Under aging the migration of low-angle boundaries (LAGB) is retarded by uniformly distributed nanoscale M(C,N) dispersoids and particles of M₂₃C₆ precipitated on these boundaries under tempering. Under creep the dissolution of M₂₃C₆ carbides located along LAGBs and coagulation of uniformly distributed M(C,N) carbonitrides facilitates LAGB migration. It was shown that the normal grain growth is not important for deterioration of creep strength. Conversion of the lath boundaries into subgrain boundaries strongly decreases creep rate. In contrast, continuous subgrain coarsening is the main process restricting the ability of the 9%Cr steel for long-range service under creep conditions. Tertiary creep is attained due to the occurrence of subgrain coarsening.

Abstract: A novel set-up developed to continuously observe and measure stress driven grain boundary migration is presented. A commercially available tensile/compression SEM unit was utilized for in-situ observations of mechanically loaded samples at elevated temperatures up to 850°C by recording orientation contrast images of bicrystal surfaces. Two sample holders for application of a shear stress to the boundary in bicrystals of different geometry were designed and fabricated. The results of first measurements are presented.

Abstract: The deformed microstructure and recrystallization behavior of copper samples processed using equal channel angular extrusion (ECAE) have been investigated. The heavily deformed microstructure was found to be non-uniform through the sample thickness and to vary in a manner consistent with the non-uniform distribution of strain imposed by processing. The through-thickness heterogeneity of the deformed microstructure resulted in a different extent of recrystallization in different layers during annealing. Recrystallized grains were also observed in samples that were not annealed, but stored at room temperature, which indicates that the deformed microstructure of ECAE-processed pure copper is unstable even at room temperature. In each sample, recrystallization was found to initiate in regions containing predominantly large misorientations.