Matthias Militzer

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.

Austenite grain growth kinetics in Al-killed plain carbon steels

Austenite grain growth kinetics have been investigated in three Al-killed plain carbon steels. Experimental results have been validated using the statistical grain growth model by Abbruzzese and Lücke, which takes pinning by second-phase particles into account. It is shown that the pinning force is a function of the pre-heat-treatment schedule. Extrapolation to the conditions of a hot-strip mill indicates that grain growth occurs without pinning during conventional processing. Analytical relations are proposed to simulate austenite grain growth for Al-killed plain carbon steels for any thermal path in a hot-strip mill.

Laser-ultrasonic monitoring of phase transformations in steels

The ferrite-austenite transformations in A36 and in IF steels were monitored using laser-ultrasonics. Sudden variations of ultrasonic attenuation were observed at transformation temperatures. Dilatometry and standard metallographic observations support an interpretation of these attenuation variations in terms of nucleation and growth of the new phase. Laser-ultrasonics is a new technique to monitor microstructural changes that take place during phase transformations. This information is obtained in real-time and may greatly facilitate laboratory studies on phase transformations. Using ultrasonic scattering models, and with reliable values of the austenite elastic constants as a function of temperature, laser-ultrasonics could provide a quantitative evaluation of grain sizes during phase transformations.

Phase-Field Modeling for Intercritical Annealing of a Dual-Phase Steel

A phase-field model has been developed to describe microstructure evolution during intercritical annealing of a commercial DP600 dual-phase steel. The simulations emphasize the interaction between ferrite recrystallization and austenite formation from a cold-rolled pearlite/ferrite microstructure at high heating rates. The austenite-ferrite transformations are assumed to occur under conditions where only carbon partitions between the phases by long-range diffusion. A solute drag model has been integrated with the phase-field model to describe the effect of substitutional alloying elements on the migration of the ferrite/austenite interface. Experimental results including recrystallization and transformation kinetics as well as austenite morphology have been successfully described by carefully adjusting both the austenite nucleation scenario and the interface mobilities.

Study of the interaction of solutes with Σ5 (013) tilt grain boundaries in iron using density-functional theory

Substitutional alloying elements significantly affect the recrystallization and austenite-ferrite phase transformation rates in steels. The atomistic mechanisms of their interaction with the interfaces are still largely unexplored. Using density functional theory, we determine the segregation energies between commonly used alloying elements and the Σ5 (013) tilt grain boundary in bcc iron. We find a strong solute-grain boundary interaction for Nb, Mo, and Ti that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundary, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk.

Abnormal Grain Growth in Electrochemically Deposited Cu Films

Cu interconnects are essential in advanced integrated circuits to minimize the RC delay. In manufacturing these devices, Cu is deposited electrochemically using a plating bath containing organic additives. The as-deposited nanocrystalline Cu films undergo self-annealing at room temperature to form a micronsized grain structure by abnormal grain growth. Systematic experimental studies of self-annealing kinetics on model Cu films deposited on a Au substrate suggest that the rate of grain size evolution depends primarily on the initial grain size of the asdeposited film. A model for the observed abnormal grain growth process is proposed. Assuming that desorption of the organic additives leads to mobile grain boundaries, the onset of abnormal grain growth is attributed to a sufficiently low additive concentration such that a full coverage of all grain boundaries cannot be maintained. The incubation time of abnormal growth is then a logarithmic function of the initial grain size. The probability to find a growing grain is proportional to the number of grains per unit volume. This assumption is seen to be in good agreement with the experimental observations for subsequent abnormal grain growth rates. The limitations of the proposed model and the challenges to obtain further insight into the complex microstructure mechanisms during self-annealing are delineated.

3D phase field modelling of recrystallization in a low-carbon steel

Intercritical annealing is a critical processing step to manufacture dual-phase (DP) steels. As part of modelling the microstructure evolution in an intercritical-annealing cycle, a 3D multi-phase field model has been employed to simulate recrystallization during heating of a low-carbon steel that is used to produce commercial DP600 grade. The cold-rolled microstructure obtained from metallographic observations is used as the initial structure in the model. The nucleation conditions and the effective interface mobility are employed as adjustable parameters to fit the experimentally measured kinetics of isothermal recrystallization and then applied to non-isothermal recrystallization. The model predictions are in good agreement with experimental data for recrystallization during continuous heating. The model provides realistic recrystallized microstructures as initial conditions for modelling the subsequent formation and decomposition of austenite.

Phase field simulation of austenite grain growth in the HAZ of microalloyed linepipe steel

Abstract Phase field modelling is used to simulate austenite grain growth in the heat affected zone (HAZ) of an X80 linepipe steel. The HAZ experiences a very steep temperature gradient during welding which restricts grain growth. In addition to this phenomenon known as thermal pinning, austenite grain growth is affected by pinning due to precipitates and their potential dissolution. Grain growth has first been simulated for bulk samples subjected to rapid heating conditions to replicate thermal cycles at various positions in the HAZ. Effective grain boundary mobilities are introduced that are consistent with strong pinning at lower temperatures and weak pinning at higher temperatures. These two temperature regimes are separated by the estimated dissolution temperature of fine NbC precipitates. These mobility relationships are then used to predict austenite grain growth in the HAZ using typical time–temperature profiles.

Phase field modelling of austenite formation from ultrafine ferrite–carbide aggregates in Fe–C

Abstract In this paper, austenite formation in an Fe – C system was simulated with a phase field approach. The model deals with a detailed description of morphological changes during austenite formation from ultrafine ferrite – cementite aggregates isothermally annealed in the intercritical region to form dual phase microstructures. Long-range diffusion of carbon is explicitly considered. The model is capable of resolving carbide particle sizes of about 100 nm to simulate the morphological complexity during austenite formation. Simulations were carried out in two- and three-dimensions. It was observed that morphological aspects of austenite formation depend significantly on spacing and distribution of cementite particles that provide suitable nucleation sites for austenite. This dependency can primarily be attributed to overlapping diffusion fields and curvature effects.

Modeling self-annealing kinetics in electroplated Cu thin films

Electroplated Cu films exhibit a microstructure evolution at room temperature, called self-annealing. The kinetics of this recrystallization process is strongly influenced by plating conditions in the form of Cu film thickness, current density, and additive content in the electrolyte. Existing models have been used and improved to describe the kinetics of self-annealing. This gives the possibility to evaluate the influence of processing parameters and predict self-annealing behavior of electroplated Cu thin films.