Biomedical subjects

Mikko Haataja

Publications and source records attributed to Mikko Haataja.

7 recordsLinked to original sources

PubMed2006

Phase-field simulations of velocity selection in rapidly solidified binary alloys.

Time-dependent simulations of two-dimensional isothermal Ni-Cu dendrites are simulated using a phase-field model solved with a finite-difference adaptive mesh refinement technique. Dendrite tip velocity selection is examined and found to exhibit a transition between two markedly different regimes as undercooling is increased. At low undercooling, the dendrite tip growth rate is consistent with the kinetics of the classical Stefan problem, where the interface is assume to be in local equilibrium. At high undercooling, the growth velocity selected approaches a linear dependence on melt undercooling, consistent with the continuous growth kinetics of Aziz and with a one-dimensional steady-state phase-field asymptotic analysis of Ahmad [Phys. Rev. E 58, 3436 (1998)]. Our simulations are also consistent with other previously observed behaviors of dendritic growth as undercooling is increased. These include the transition of dendritic morphology to absolute stability and nonequilibrium solute partitioning. Our results show that phase-field models of solidification, which inherently contain a nonzero interface width, can be used to study the dynamics of complex solidification phenomena involving both equilibrium and nonequilibrium interface growth kinetics.

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PubMed2006

Phase-field crystals with elastic interactions.

We report on a novel extension of the recently introduced phase-field crystal (PFC) method [Elder, Phys. Rev. Lett. 88, 245701 (2002)10.1103/PhysRevLett.88.245701], which incorporates elastic interactions as well as crystal plasticity and diffusive dynamics. In our model, elastic interactions are mediated through wave modes that propagate on time scales many orders of magnitude slower than atomic vibrations but still much faster than diffusive time scales. This allows us to preserve the quintessential advantage of the PFC model: the ability to simulate atomic-scale interactions and dynamics on time scales many orders of magnitude longer than characteristic vibrational time scales. We demonstrate the two different modes of propagation in our model and show that simulations of grain growth and elastoplastic deformation are consistent with the microstructural properties of nanocrystals.

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PubMed2004

Crossover scaling of wavelength selection in directional solidification of binary alloys.

We simulate cellular and dendritic growth in directional solidification in dilute binary alloys using a phase-field model solved with adaptive-mesh refinement. The spacing of primary branches is examined for a wide range of thermal gradients and alloy compositions and is found to undergo a maximum as a function of pulling velocity, in agreement with experimental observations. We demonstrate that wavelength selection is unambiguously described by a nontrivial crossover scaling function from the emergence of cellular growth to the onset of dendritic fingers. This result is further validated using published experimental data, which obeys the same scaling function.

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PubMed2004

Apparent hysteresis in a driven system with self-organized drag.

The motion of extended defects in materials is often resisted by interaction with diffusing impurities. The defect undergoes a transition from slow to fast migration, or vice versa. This transition is commonly hysteretic, with the defect jumping back and forth between these kinetic states. We explore such hysteresis within a kinetic Monte Carlo simulation. After identifying the slow variables, we construct an effective potential that quantitatively describes the stable and metastable states of the system (a bifurcation diagram) and the kinetics of the transitions between these. This description provides a means to determine whether hysteresis will be observed in a particular simulation and the detailed nature of this hysteresis (switching times).

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PubMed2003

Seaweed to dendrite transition in directional solidification.

We simulate directional solidification using a phase-field model solved with adaptive mesh refinement. For small surface tension anisotropy directed at 45 degrees relative to the pulling direction we observe a crossover from a seaweed to a dendritic morphology as the thermal gradient is lowered, consistent with recent experimental findings. We show that the morphology of crystal structures can be unambiguously characterized through the local interface velocity distribution. We derive semiempirically an estimate for the crossover from seaweed to dendrite as a function of thermal gradient and pulling speed.

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PubMed2002

Morphological instability and additive-induced stabilization in electrodeposition.

Experiments show that electrodeposited (ED) films exhibit rough surfaces unless the electrochemical bath contains small quantities of molecular "additive" species. We develop a model for ED with additives which shows the suppression of the morphological instability by preferential additive accumulation near surface protrusions due to complex formation and additive codeposition, and subsequent growth site blocking. Linearly stable growth can be achieved over a wide range of deposition flux at sufficiently large additive bulk concentration. We predict the growth conditions necessary for level surfaces, in good agreement with experiments.

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PubMed2002

Modeling elasticity in crystal growth.

A new model of crystal growth is presented that describes the phenomena on atomic length and diffusive time scales. The former incorporates elastic and plastic deformation in a natural manner, and the latter enables access to time scales much larger than conventional atomic methods. The model is shown to be consistent with the predictions of Read and Shockley for grain boundary energy, and Matthews and Blakeslee for misfit dislocations in epitaxial growth.

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