Materials Science Forum Vols. 715-716

Abstract: During plastic deformation, a metallic structure is deformed inhomogeneously near hard inclusions. Hence both the materials strengthening and recovery and thus softening depends on the local position. There are thus high gradients of point defects, such as vacancies and interstitials, of the dislocation density and hence of the strain energy. Those gradients govern the diffusional flow, whose pile-up influences the climbing of edge dislocations, i.e. recovery and materials softening, respectively.

Abstract: Grain growth in nanocrystalline Ni has been simulated by molecular dynamics. The simulations show the creation of a high density of twin boundaries during the growth as well as the formation of vacancies consistent with recent experimental observations. The growth follows parabolic kinetics with the diameter increasing with the square root of time consistent with behavior of conventional scale metals but in disagreement with prior simulation results.

Abstract: Oxide dispersion strengthened ferritic steels (so-called nanostructured ferritic alloys, NFAs), which are candidate structural materials in next generation nuclear power plant, have attracted much attention during recent years. In this work, iron oxide as oxygen carrier and titanium, yttrium hydrides were together mechanically milled with Fe-14Cr-3W gas-atomized powder. The thermal stability and recrystallization behaviour of the as-milled ferritic powder were studied by means of metallography, SEM, TEM and microhardness test. After ball milling for 48h, complete solid solution of bcc-Fe was formed in the as-milled powder. The thermal analysis results show that dispersed oxides with an average diameter of 5nm precipitate from the supersaturated matrix at about 850 °C. During annealing at temperatures from 800 to 1000 °C, a large number of equiaxed grains as fine as few hundreds of microns were found embedding in the matrix; the recrystallized grains stay quite stable and show minor dependence on annealing temperature and time. After being heated to 1200 °C for extended time, abnormal grain growth took place and resulted in bimodal grained structure. The effect of secondary particles on the thermal stability and recrystallization behavior of the ferritic steel was also discussed.

Abstract: In the present work, for the description of grain coarsening, a probabilistic and a deterministic 2D cellular automaton simulation setup were developed. The results of the simulation have been validated by solution annealing experiments of austenitic stainless steel 304L (Fe-18Cr-8Ni) at different temperatures and times. Both cellular automata models show an excellent correlation between the experimental determined data and grain growth kinetics based upon considerations of temperature and second phase particles. Additionally, a two parameter approach of the probabilistic model was implemented, resulting in determining the grain sizes limiting normal and abnormal grains and accurate description of grain growth.

Abstract: The low carbon steels were smelted with special oxide introduction technique and the HAZ properties has been studied with thermal simulation. The optical microscope, SEM and TEM were used to analyze the composition, size and distribution of the inclusions, and the mechanical properties after thermal simulation were also investigated. The influence of oxide inclusions on the austenite grain size was also studied. The results show that after the smelting the inclusion is complex, in the core is Ti oxides about 1-3 micron and around it is MnS. When the reheat temperature is below 1000, the size of austenite grain is the same for experimental steel and base steel. However, when the reheat temperature is over than 1100, the size of austenite grains in experimental steel is one third of that in base steels. After thermal simulation, with the t_8/5 increasing the toughness of HAZ decreased. The austnite grain size also increased. The microstructure is composed of intergranular ferrite and intragranular acicular ferrite. Therefore by introducing the fine oxide inclusion to the steel the austenite grain was refined and during the phase transformation the acicular ferrite formed at inclusions at first. These two factors are the main causes to improve the toughness of heat affected zone for steels produced by oxide metallurgy technique.

Abstract: The standard Monte Carlo (MC) Potts model is modified regarding the mobility of grain boundaries and their junctions allowing the simulation of a size effect observed in nanocrystalline grain growth. In large simulation studies different properties are measured. For initially very small grains the early growth regime corresponds to a separate coarsening state, which is characterised by an average growth law and a self-similar grain size distribution that both show strong deviations from the parabolic normal grain growth behaviour. The simulation results can be described by a theoretical model based on a statistical mean-field theory for nanocrystalline grain growth.

Abstract: Reduced activation ferritic-martensitic steels are considered for future applications in fusion power plants. The thermal stability of two steels (ODS-Eurofer and Eurofer-97) was evaluated in cold-rolled samples annealed below 800°C. These grades have similar chemical compositions except for the presence of nanosized Y₂O₃ particles in ODS-Eurofer steel. This nanosized dispersion is very effective to prevent recrystallization. On the other hand, full recrystallization occurs in Eurofer-97 steel when annealed above 700°C. The low volume fraction of recrystallized grains (< 0.1) in ODS-Eurofer steel annealed at 800°C can be explained by nucleation at prior grain boundaries and around large M₂₃C₆ particles. Further growth of these nuclei is impeded by Zener drag and concurrent recovery. Static recovery is the main softening mechanism in the ODS-steel below 800°C.

Abstract: A phase field model to study the microstructural evolution of a polycrystalline dual-phase material with conserved phase fraction has been implemented, and 2D simulations have been performed. For 2D simulations, the model predicts the cubic growth well-known for diffusion-controlled systems. Some interphase boundaries are found to show a persistent non-constant curvature, which seems to be a feature of multi-phase materials. Finally, it is briefly outlined how this model is to be applied to investigate microstructural evolution in duplex steel.

Abstract: A reversion of the strain produces a modification of the static recrystallization kinetics. Initially, the reversion increases the recrystallization time, that reaches a maximum at a certain strain, and decreases again for increasing reverse strains. This transient on recrystallization kinetics develops over a strain interval similar to that of the microstructural and stress-strain transients. At strains beyond the transient, the reversion can be regarded as a shift on the strain axis. However, at the authors knowledge there is no formulation able to describe the material behaviour during the transient. The present work introduces an equivalent strain concept based on the substructural dissolution/build-up processes taking place as a result of the strain reversal. This formulation allows including the effect of the strain path on recrystallization models.

Abstract: The static recrystallisation behaviour of cold rolled and annealed TWinning Induced Plasticity (TWIP) steels is important for its industrial production. The recrystallisation kinetics have been determined for an Fe-Mn-C-Si-Al TWIP steel using hardness measurements and microstructure analysis: it has been shown that recrystallisation progresses rapidly with increased annealing temperature. Recrystallisation was faster at higher cold reductions, and a smaller final grain size was observed at lower annealing temperatures. This indicates that the mechanism is nucleation dominated at lower temperatures; grain growth at higher temperatures appears similar for all reductions. The recrystallisation results in a crystallographic texture where the main components of the cold rolling texture are preserved in the final texture after annealing, although some randomisation was observed.