Modeling the effect of primary and secondary twinning on texture evolution during severe plastic deformation of a twinning-induced plasticity steel

Modeling the effect of deformation twinning and the ensuing twin-twin- and slip-twin-induced hardening is a long-standing problem in computational mechanical metallurgy of materials that deform by both slip and twinning. In this work, we address this effect using the twin volume transfer method, which obviates the need of any cumbersome criterion for twin variant selection. Additionally, this method is capable of capturing, at the same time, secondary or double twinning, which is particularly important for modeling in large strain regimes. We validate our modeling methodology by simulating the behavior of an Fe-23Mn-1.5Al-0.3C twinning-induced plasticity (TWIP) steel under large strain conditions, experimentally achieved in this work through equal-channel angular pressing (ECAP) for up to two passes in a 90&deg; die following route BC at 300 &deg;C. Each possible twin variant, whether nucleating inside the parent grain or inside a potential primary twin variant was predefined in the initial list of orientations as possible grain of the polycrystal with zero initial volume fraction. A novelty of our approach is to take into account the loss of coherency of the twins with their parent matrix under large strains, obstructing progressively their further growth. This effect has been captured by attenuating growth rates of twins as a function of their rotation away from their perfect twin orientation, dubbed here as &ldquo;disorientation&rdquo; with respect to the mother grain&rsquo;s lattice. The simulated textures and the hardening under tensile strain showed very good agreement with experimental characterization and mechanical testing results. Furthermore, upper-bound Taylor deformation was found to be operational for the TWIP steel deformation when all the above ingredients of twinning are captured, indicating that self-consistent schemes can be bypassed. <br /

An Elastic Phase Field Model for Thermal Oxidation of Metals: Application to Zirconia

A multi-phase field model was developed for non-selective oxidation of metals which captures both the oxidation kinetics and stress generation. Phase field formulation involved a non-conserved phase field variable as the marker for the metallic substrate, oxide scale, and a fluid phase containing oxygen, and a conserved phase field variable representing the concentration of oxygen. The evolution equations of the phase field variables were coupled to the mechanical equilibrium equations to investigate the evolution of stress generation in both the oxide scale and the underlying metal. The governing equations were solved in a finite element framework. This phase field model predicts the oxygen composition depth and stress profiles in the oxide layer and at the metal-oxide interface. The model was proven successful in predicting the observed evolution of oxide thickness and growth stresses for Zircaloy-4 oxidized at 900 °C. The results of phase field simulations showed that the generation of stresses upon oxidation tends to slow down the oxidation kinetics, and this substantially improved the model predictability of experimental data

High-temperature oxidation behaviour of base material and laser-weld specimens of a thin FeCrAl-RE foil at around 900°C

High Temperature Corrosion and Protection of Materials 6 Les Embiez (France) 16-21 mai 2004 ed. P. Steinmetz, I.G. Wright, G. Meier et al... Trans Tech PublicationsInternational audienceIsothermal oxidation of laser welded FeCrAl-RE samples containing specific fractions of seams in a bead-on-plate "configuration" has been performed at around 900°C and studied using TGA, SEM, TEM and EPMA techniques. An important reduction in the alumina-growth rate on the fusion zone occurs at 900°C, thereby, suppressing the discontinuous increase in mass gain commonly observed during the high temperature oxidation of alumina-forming alloys. This phenomenon is mainly related to the concomitant dramatic chromium carbide precipitation at the fusion zone/scale interface and possible earlier injection of the rare earth elements into the scale. The former, which is linked to the laser melting-induced high free carbon, contributes to the increase in effectiveness of the diffusion barrier provided by the thermally growing scale. The latter is correlated with the initial high Ce+La enrichment at the fusion zone surfaces and is manifested by the elimination of detrimental platelet transformation during the initial stages of oxidation

Phase transformation and growth of alumina on a thin FeCrAl-RE foil at around 900°C

High Temperature Corrosion and Protection of Materials 6 Les Embiez (France) 16-21 mai 2004 ed. P. Steinmetz, I.G. Wright, G. Meier et al... Trans Tech PublicationsInternational audienceWe performed isothermal oxidation tests on a 95ﾵ m thin commercial FeCrAl foil at around 900ﾰC in ambient and synthetic air in order to examine the effects of microstructural and morphological evolution of metastable aluminas on the overall growth kinetics by mean of TGA, AET, FEG-SEM, TEM, EPMA and GIXRD techniques. The origin of platelet-like morphology formation at an early stage of oxidation is examined and discussed. Further heating produces phase transformation and significant structural changes of this morphology. Formation of a corundumalumina-rich compact layer from an outer metastable layer by a coupling phenomenon between grain growth and the sintering of the platelets was identified as responsible of the discontinuous increase in growth rate at around 900ﾰC widely reported in the literatur

A Phase Field Model for Stress Induced Martensitic Phase Transformation in Zirconia

In this work, we present a phase field model to understand how applied stresses can trigger the tetragonal to monoclinic (T→M) phase transformation in zirconium oxide at temperatures higher than Ms (martensitic transformation start temperature during cooling). We consider the effect of stress on T→M transformation by applying the stresses explicitly on the computational domain by adding them as boundary conditions in the mechanical equilibrium equations. Different loadings effect on stress induced T→M phase transformation was studied and it was shown that regardless of stress loading direction, monoclinic twinning plane would be (100)m. Results also showed that the external stress increased the production of those variants whose transformation strains were aligned with the applied stress direction

Phase Field Modeling of Tetragonal to Monoclinic Phase Transformation at Zirconium Oxide

Tetragonal to monoclinic phase transformation at the zirconium oxide layer is an important factor in changing the oxidation kinetics and also promoting breakaway oxidation in Zircaloy fuel rod claddings. This transformation affects the stress state at the oxide layer close to the metal/oxide interface. The amplitude of these internal stresses can significantly change the kinetics of oxidation. In this work, we presented a phase field model to investigate the tetragonal to monoclinic phase transformation in zirconium oxide layer. All necessary driving forces including bulk free energy, interfacial energy and elastic energy were taken into account, and the symmetry reduction between parent and products has been assigned to different non-conserved order parameters. Transformation stages from nucleation, growth and coarsening of variants were simulated. Development of internal stresses due to this transformation, which is the source of oxidation breakaway, was studied

Shape Memory Effect and Pseudoelasticity Behavior in Tetragonal Zirconia Polycrystals: A Phase Field Study

Martensitic tetragonal-to-monoclinic transformation in zirconia is a double-edged sword , enabling transformation toughening or shape memory effects in favorable cases, but also cracks and phase degradation in undesirable scenarios. In stressed polycrystals, the transformation can burst from grain to grain, enabling stress field shielding and toughening in an autocatalysis fashion. This transformation strain can be recovered by an adequate thermal cycle at low temperatures (when monoclinic is stable) to provide a shape memory effect, or by unloading at higher temperatures (when tetragonal is stable) to provide pseudoelasticity. We capture the details of these processes by mining the associated microstructural evolutions through the phase field method. The model is both stress and temperature dependent, and incorporates inhomogeneous and anisotropic elasticity. Results of simulations show an ability to capture the effects of both forward (T → M) and reverse (M → T) transformation under certain boundary conditions

Phase Field Modeling of Stress-Induced Tetragonal-to-Monoclinic Transformation in Zirconia and its Effect on Transformation Toughening

This paper proposes a two-dimensional elastic phase field model for capturing the effect of external stress on the tetragonal-to-monoclinic (T → M) phase transformation in zirconia. The model was able to predict the sensitivity of the monoclinic microstructural formation and evolution to the external loading conditions. The effect of stress on the T → M phase transformation was captured by explicitly applying stresses on the computational domain by entering them in the mechanical equilibrium equations as boundary conditions. Simulation results showed that, regardless of the stress loading direction, the monoclinic twinning plane always corresponded to {1 0 0} m. Results of simulations showed that external stress favors the production of monoclinic variants which exhibit transformation strains aligned with the applied stress direction. When applied to the transformation toughening phenomenon in zirconia, the model was able to elucidate the mechanisms of phase transformation ahead of a crack tip, including the generation of a compressive stress field responsible for the retardation of further crack growth. This work presents the first model capable of demonstrating the process of transformation toughening and crack closure in zirconia

Effect of Variant Strain Accommodation on the Three-Dimensional Microstructure Formation during Martensitic Transformation: Application to Zirconia

This paper computationally investigates the effect of martensitic variant strain accommodation on the formation of microstructural and topological patterning in zirconia. We used the phase-field technique to capture the temporal and spatial evolution of embryonic formation of the monoclinic phase in tetragonal single crystals. The three-dimensional simulations were able to capture the formation of all the possible monoclinic variants. We used the multivariant single embryo as an initial condition to mitigate the lack of nucleation criteria at the mesoscale. Without a priori constraint, the model can select the transformation path and final microstructure. The phase-field model was benchmarked against experimental studies on surface uplift formation in zirconia reported by Deville et al. (Acta Mater 2004;52:5697, Acta Mater 2004;52:5709). The simulations showed the excellent capabilities of the model in predicting the formation of a surface relief induced by the tetragonal to monoclinic martensitic transformation