Temporal Evolution Of Microstructure Research Articles

Temporal evolution of microstructure and its influence on micromechanical and tribological properties of Stellite-6 cladding under aging treatment

Temporal evolution of microstructure and its influence on micromechanical and tribological properties of Stellite-6 cladding under aging treatment

Microstructural evolution and kinetics of phase separation in binary polymer blends under electric fields

Temporal evolution of microstructure and kinetics of phase separation under electric fields (E-fields) were numerically investigated for binary polymer blends at both off-critical and critical compositions, where the morphology, structure factor, characteristic size, and anisotropy parameter were adopted. The results show that the E-field can significantly influence morphological development and kinetic process of phase separation, and the influence rule at the off-critical composition is distinctly different from the rule at the critical composition. With the enhancement of E-field strength, the growth exponent in the x direction (perpendicular to the E-field) decreases and the growth exponent in the y direction (parallel to the E-field) increases. Moreover, as initial composition raises, anisotropy parameter firstly increases and then decreases, and reaches to a maximum at the critical composition. The simulation results were also compared with existing experimental results, and exhibit several different aspects. This study reveals the influence rule of E-fields on phase separation process of binary polymer blends. On the other hand, it can provide a scheme to predict and design anisotropic microstructures by E-fields, and further guide to produce polymeric composite materials with superior mechanical and physical properties.

Strain-rate-sensitive mechanical response, twinning, and texture features of NiCoCrFe high-entropy alloy: Experiments, multi-level crystal plasticity and artificial neural networks modeling

The present work adopts an extended multi-level crystal plasticity finite element method (CPFEM) framework coupled with an improved artificial neural network (ANN) algorithm to predict the quasi-static and dynamic uniaxial tensile mechanical response, twinning and texture characteristic in NiCoCrFe high-entropy alloy. Firstly, a Split-Hopkinson tensile bar setup is utilized to achieve high strain rates, and the initial microstructure and texture data used in CPFEM is characterized by electron backscattered diffraction. The experimental results show the effect of twins on strain hardening is more obvious with the increase of strain rates. Next, a developed dislocation-density based hardening law taking into account rate and temperature sensitive property is firstly extended by calculating the slip/twin activation stresses through thermal activation mechanism, Hall-Petch resistance, forest dislocation drag and slip-twin interactions. The simulations reveal some underlying defect mechanisms using the temporal evolution of microstructure and macro response over a wide range of strain rates. Then, an advantageous ANN model combined with the Genetic algorithm (GA) optimized is applied to exploit potential law of massive experimental and CPFEM data and serve engineering applications efficiently. The verification results capture well the features of strain rate sensitivity with CPFEM and experimental data sources. Finally, the predictive ability of two models is further demonstrated by validating the uniaxial tension of NiCoCrFe HEA. It is clear that the improved idea (an ANN model through GA optimization) is more efficient than the physical mechanism based CPFEM model.

Evolution of Microstructure and Carbon Distribution During Heat Treatments of a Dual-Phase Steel: Modeling and Atom-Probe Tomography Experiments

The temporal evolution of microstructures and carbon distributions in a Fe-0.323C-1.231Mn-0.849Si (mol pct) dual-phase steel during heat treatments are simulated using a two-dimensional cellular automaton model. The model involves austenite nucleation, phase transformations controlled by ferrite (α)/austenite (γ) interface mobility and the local carbon concentration, and long-range carbon diffusion. It is also coupled with a solute drag model to account for the effect of substitutional elements on the interface migration. The results show that after holding at 800 °C for 300 seconds the transformed γ-volume fraction is lower than the paraequilibrium prediction. During subsequent cooling at 6 °C s−1, the γ → α transformation takes place after a stagnant stage; the carbon concentrations in both the α- and γ-phases increase and become non-uniform. When cooled below 450 °C, the γ-volume fraction is nearly unchanged. A small amount of carbon enriched martensite, transformed from the remaining γ-phase, exists in the room temperature microstructure. The simulated microstructures and carbon concentrations in martensite compare reasonably well with the experimental micrographs and atom-probe tomographic measurements. During tempering at 400 °C, martensite decomposes and the carbon concentration in the α-matrix increases. The simulation results are used to understand the mechanisms of yield strength variations after different heat treatments.

Dislocation mechanism based size-dependent crystal plasticity modeling and simulation of gradient nano-grained copper

Overcoming the trade-off between strength and ductility in metallic materials is a grand challenge. Recently, materials with a gradient nano-grained (GNG) surface layer adhering to a ductile coarse-grained (CG) substrate have been proposed to overcome this long-standing dilemma. Constitutive modeling and simulation are crucial to understand the deformation mechanisms controlling the strength and ductility in GNG/CG materials, and to enable theory to guide microstructure optimization for upscaling. Here, we develop a dislocation mechanism based size-dependent crystal plasticity model, where multiple dislocation evolution mechanisms are considered. Furthermore, damage evolution and mechanically driven grain growth during the deformation of GNG/CG materials are incorporated into the constitutive model to study the role of microstructure gradient in the overall plastic response. The developed size-dependent constitutive model was implemented within a finite-strain crystal plasticity finite element framework, and used to predict the tensile mechanical behavior of GNG/CG copper, including yield stress, strain-hardening and ductility with a highly simplified geometrical representation of the microstructure. The simulations reveal some of the underlying deformation mechanisms controlling ductility and strengthening in terms of the spatial distribution and temporal evolution of microstructure and damage. The model was also used to demonstrate optimization of strength and ductility of GNG/CG copper. By manipulating the thickness of the GNG layer and the grain size of the CG substrate, the strength increase is associated with a loss of ductility showing the same linear inverse relationship observed experimentally for GNG/CG copper, which indicates the improvement over the typical nonlinear trade-off between strength and ductility.

Temporal evolution of microstructure and rheology of sheared two-dimensional foams

We measure the shear rheology of two-dimensional liquid foams in a Couette device, and correlate the variation of the shear stress with the evolution of the foam microstructure. For a monodisperse foam, inception of shearing results in a rapid rise of the shear stress, which then saturates into a quasi-steady state of fluctuations about a time-independent mean value. The dominant frequency of the oscillation correlates closely with the time scale of one row of bubbles sliding past the next row. The mean stress decreases with increasing bubble size. In bidisperse foams, the shear stress exhibits two regimes of transient behavior. If the shear rate and the large-to-small bubble size ratio are both below certain thresholds, the stress behaves similarly to that in a monodisperse foam. Above these thresholds, however, the instantaneous shear stress oscillates around a mean value that declines gradually over several minutes. The foam eventually approaches a quasi-steady state similar to that of a monodisperse foam. This gradual decrease in shear stress can be attributed to the size-based segregation of bubbles. Finally, we propose a mixing rule that relates the effective viscosity of a bidisperse foam in the quasi-steady state to those of monodisperse foams made of the two constituent bubble sizes.

A multiscale coupled Monte Carlo model to characterize microstructure evolution during hot rolling of Mo-TRIP steel

A multiscale modelling framework has been proposed to characterize microstructure evolution during hot strip rolling of transformation-induced plasticity (TRIP) steel. The modelling methodology encompasses a continuum dislocation density evolution model coupled with a lumped parameter heat transfer model which has been seamlessly integrated with a mesoscale Monte Carlo (MC) simulation technique. The dislocation density model computes the evolution of dislocation density and subsequently constitutive flow stress behaviour has been predicted and successfully validated with the published data. A lumped-parameter transient heat transfer model has been developed to calculate the average strip temperature in the time domain. The heat transfer model incorporates the effect of plastic work for different strain rates in the energy conservation formulation. A coupled initial value problem solver has been developed to integrate the system of stiff ordinary differential equations in the time domain to predict dislocation density and temperature profiles simultaneously. The temporal evolution of microstructure during hot rolling of TRIP steel is simulated by the MC method incorporating thermal and dislocation density data from the continuum models. Simulated microstructural maps, kinetics of recrystallization and grain size evolution have been generated in a 200×200 lattice system at different strain rates and temperatures. The simulation code has been implemented in a high-performance grid computing network. The predicted temporal evolution of grain size, recrystallized fractions and flow stress have been validated with the published literature and found to be in good agreement, confirming the predictive capability of the integrated model.

The temporal evolution of microstructures during structural transition of D022 and L12 involved with transient phases

A microscopic phase-field method was employed to study the evolution in microstructure of Ni–Al–V alloys during the ordering process. The FCC→L12/D022 phase transition proceeds with an intermediate transient L10/L10 (M=1) phase at the critical Al level where the two phases co-exist. Additionally, L12 may act as a transient phase of D022 at compositions not in favor of D022, and vice versa. The rearrangement of atoms during the structural transition undergoes three classical stages: a respective disorder stage with atoms distributing uniformly; a second transient phase with atoms aligning in either L10/L10 (M=1) configuration with slight atom fluctuation, or in L12-Ni3Al/D022-Ni3V configuration, with atoms highly aligned, and lastly; a third equilibrium L12-Ni3Al/D022-Ni3V phase. Further studies on the atomic occupancy probability demonstrated that L10 and L10 (M=1) are low-ordered phases with atomic occupancy probability fluctuation within a narrow range; conversely, L12 and D022 are ordered phases showing significant atomic occupancy probability fluctuations when ordering as well as when act as a transient phase.

Effective elastic properties of polycrystals based on phase-field description

A combination of microelasticity, phase-field description of grain structures, and first-principles calculations is proposed to predict the effective elastic properties of polycrystals. As an example, using the single crystal elastic constants from first-principles calculations and a polycrystalline microstructure from a phase-field simulation as inputs, the effective elastic moduli of polycrystalline magnesium are obtained as a function of temperature and compared with available experimental measurements. The texture effect on the effective elastic moduli is also examined. The proposed integrated model will make it possible to model not only the temporal evolution of microstructures but also the temporal evolution of properties using the phase-field method.

Initiation of microtopography in re‐vegetated cutover peatlands: evolution of plant species composition

AbstractQuestionsHow has plant species composition changed following initiation of microstructures in re‐vegetated cutover peatlands? How many years are required for plant species composition of re‐vegetated cutover peatlands to resemble natural boreal bogs?LocationNewly formedSphagnumcarpets on restored, cutover peatlands (inCanada) or re‐vegetated spontaneously after site abandonment (inEstonia) and on undisturbed natural bogs nearby.MethodsPlant frequencies (point intercept method) and abundances (vegetation quadrats) were assessed along linear transects. At each assessment point, moss surface height was measured relative to a local reference point (lowest point on a given transect) to associate frequencies or abundances to a position in the gradient of microtopography.PCAs (separately forCanada andEstonia) were conducted to follow evolution of plant species frequency in the gradient of microtopography in re‐vegetated sites and similarity with those of natural peatlands. InCanada, regressions were also performed to estimate relationships between moss surface height and vascular plant cover (ericaceous shrubs andCyperaceae) as well as time required for vascular plant cover to become similar to that of natural bogs.ResultsSpecies composition was still dissimilar to microstructures of natural bogs 10 yr post‐restoration and 70 yr post‐abandonment; however, some trends were observed in re‐vegetated peatlands. The greatest differences were for ericaceous species (two‐ to three‐fold less abundant in re‐vegetated peatlands), dominantCyperaceae, and relative proportions ofSphagnum. In addition, hummock formation was closely related to dense (>50%) ericaceous cover.ConclusionsAll species tolerant to abiotic conditions prevailing in re‐vegetated sites contributed to initiation of microtopography, although some species were found in atypical positions within the gradient of microtopography. Random events and establishment priority seemed initially to be more important in temporal evolution of microstructures than plant interactions. However, ecological restoration could effectively reduced time needed for species occurrences to approach those in natural peatlands, relative to time required for recovery of spontaneously re‐vegetated peat extraction sites.

Diffusion during phase separation

Nearest neighbor interactions have been incorporated into novel equations that describe diffusion processes during phase separation and ordering. None of the elements of the model are new, but the gestalt is overdue. Nearest neighbor interactions are included in the free energy of a binary alloy, then the chemical potentials of the two species are derived from this free energy. The diffusion fluxes are driven by the chemical activities, which are given by the chemical potentials. This results in fourth-order difference equations that describe the motion of atoms during phase separation or ordering. Calculations using these equations describe the expected evolution of microstructures, which have often been depicted only with cartoons. In the unstable spinodal region, perturbations grow exponentially with time until the compositions reach the phase boundary compositions, and then the structures coarsen slowly. In the meta-stable region between the spinodal and the phase boundary, composition fluctuations that are large enough generate phase separation by a nucleation process. In the single-phase region outside the two-phase field, there is critical slowing down. Far away from the critical region, the equations become standard diffusion equations, where the diffusion process is driven only by entropy. Numerical solutions describe the temporal evolution of microstructures during spinodal decomposition, precipitation, and ordering. The propagation of instability fronts in the spinodal region have been observed, and their velocities have been determined. More complex interactions between atomic sites are expected to change only the details but not the essential character of the behavior observed in this model. Based on universality, it is expected that method can be applied to many situations where there is an interaction between the volume elements.

Readily regenerable reduced microstructure representations

Many of the physical properties of materials are critically dependent on their microstructure. In recent years, there has been increasing interest in using computer simulations based on phase-field models for the spatial and temporal evolution of microstructures. Although such simulations are computationally expensive, the generated set of microstructures can be stored in a repository and used for further analysis in materials design. However, such an approach requires a substantial amount of storage, for example, approximately 1 Terabyte for a single binary alloy. In this paper, we develop fast data compression and regeneration schemes for two-dimensional microstructures that can reduce storage requirements without compromising the accuracy of computed values, such as stress fields used in analysis. Our main contribution is the development and evaluation of a sparse skeletal representation scheme which outperforms traditional compression schemes. Our results indicate that our scheme can reduce microstructure data size by more than two orders of magnitude while achieving better accuracies for the computed stress fields and order parameters.