# Presentation: 312238

## Kinetics of Structural Transformations in Metal and Ceramic SystemsMicrostructure in Decomposition of Metastable  Ceramic MaterialsArmen G Khachaturyan, Rutgers University, DMR Award # 0242619

-    Motivated by the discovery of exceptional hardness in decomposing metastable ** solid  solution of Al2O3-Al2MgO4 spinel, the phase field microelasticity theory and model are developed to study the mechanisms controlling its microstructure responsible for the special property of this and similar materials.
-    The picture shows the simulated microstructure during decomposition. It demonstrates that the decomposition occurs by an unusual two-step process. The first step is a decomposition producing a labyrinth-like structure consisting of two phases of the same symmetry but different composition. It is followed by the second step that is ordering and self-assembly of ordered domains into alternating twin-related lamellae within particles of one of the phases. This dense distribution of these immobile domains  should suppresses plastic deformation and drastically increase hardness. The long-range elastic interaction plays the key role in the microstructure spatial patterning.
-    The two-step sequence of  the structural transformations obtained in this simulation was confirmed by electron microscopic observations.
-    This model has a potential to be used for design of an optimal structure and establishing critical values of the processing parameters affecting these structures.

- Phase Field prototyping of transformations in nonstoichiometric Mg-Al spinel: an isostructural decomposition followed by multivariant crystal lattice rearrangement

- Black and white represent different compositions; red and green represent different orientation domains of ordered phase.

- t*=0.2

- t*=0.6

- t*=2

- t*=4

- t*=6

- t*=9

- t*=10

- t*=15

## Kinetics of Structural Transformations in Metal and Ceramic Systems Surface Instability and Dislocation Dynamics in Heteroepitaxial Thin Films Armen G Khachaturyan, Rutgers University, DMR Award # 0242619

- Simulated evolution of surface morphology driven by epitaxy stress relaxation during annealing.

- Y.U. Wang, Y.M. Jin and A.G. Khachaturyan, “Phase Field Microelasticity Modeling of Surface Instability of Heteroepitaxial Thin Films,” *Acta Mater*., 52, 81, 2004.
- Y.U. Wang, Y.M. Jin and A.G. Khachaturyan, “Phase Field Microelasticity Modeling of Dislocation Dynamics near Free Surface and in Heteroepitaxial Thin Films,” *Acta Mater*., 51, 4209, 2003.

-    Defect-free crystalline epitaxial films are in a high demand for semiconductor and electro-optical device industry. However the existence of internal stress generated by the crystal lattice mismatch makes epitaxial films susceptible to the formation of defects, mainly surface roughening and dislocations. The computational modeling is an effective way to study the mechanisms of formation and evolution of defects. Understanding of these mechanisms is critical for a development of a new generation of high performance materials and improvement of the performance of existing ones.
-    The PFM simulations based on the solution of the exact elasticity equations show the details of the temporal evolution of the morphological instability in an epitaxial  film that eventually produces a periodical array of islands, and the generation and movement of a threading dislocation.

- The simulated motion of threading dislocation in heteroepitaxial film depositing misfit dislocation behind at film/substrate interface (numbers indicate the simulated sequence of dislocation configurations; different grayscales indicate different degrees of slip).

## Kinetics of Structural Transformations in Metal and Ceramic SystemsAdaptive Ferroelectric State Near Morphotropic Phase Boundary Armen G Khachaturyan, Rutgers University, DMR Award # 0242619

**Kinetics of Structural Transformations in Metal and Ceramic Systems****Adaptive Ferroelectric State Near Morphotropic Phase Boundary **Armen G Khachaturyan, Rutgers University, DMR Award # 0242619

- Y.M. Jin, Y.U. Wang, A.G. Khachaturyan, J.F. Li and D. Viehland, “Conformal Miniaturization of Domains with Low Domain Wall Energy: Monoclinic Ferroelectric States near Morphotropic Phase Boundaries,” *Phys. Rev. Lett*., 91, 1976011-1976014, 2003.
- Y.M. Jin, Y.U. Wang, A.G. Khachaturyan, J.F. Li and D. Viehland, “Adaptive Ferroelectric States in Systems with Low Domain Wall Energy: Tetragonal Microdomains,” *J. Appl. Phys*., 94, 3629-3640, 2003.

-    A theory is developed to explain recently observed unusual monoclinic ferroelectric states (FEm) as well as their special properties and responses to temperature, electrical field, and compositional change. These states are observed in Pb(Mg1/3Nb2/3)O3-*x*PbTiO3 and Pb(Zn1/3Nb2/3)O3-*x*PbTiO3 ferroelectric oxides near the morphotropic phase boundary between rhombohedral and tetragonal ferroelectric phases.
-    According to the theory, the FEm phases are not homogenous phases. They are rather nano-scale multi-domain adaptive states, which are formed in cases of abnormally small energies of domain walls as a result of accommodation of the misfit-generated stress and electric field. The microdomain structure can be resolved only under high resolution diffraction conditions.
-    The measured crystal lattice parameters and electric properties of the adaptive state, in fact, are microdomain-averaged values and can be calculated without any fitting parameters.  The predictions of the theory are rigidly obeyed over the entire stability range of the FEm phase.

- (a) The structure of adaptive phase resembles the stress-accommodating polydomain structure in CuAu alloy (dark-field TEM, Syutkina and Jakovleva 1967). (b) The microdomain-averaged polarization **P** of the adaptive ferroelectrics consisting of twin-related microdomains of tetragonal ferroelectric phase with polarizations **P**(1) and **P**(2).

- The fulfillment of the invariance condition (a prediction by the theory) am+cm=at+ct over the range of (c) temperature and (d) applied electric field E. (Noheda et al. 2000)

- (a)

- (b)

- (c)

- (d)