MPE Research Areas

Personnel: James Morris (PI), Cai-Zhuang Wang (PI), Kai-Ming Ho (PI), Bruce Harmon (PI)

Abstract:
We are interested in material-specific mechanical and thermodynamic properties of materials. In particular, we use ab initio calculations, combined with empirical methods (such as tight-binding and EAM potentials) to examine the structure and energy of lattice distortions, including elastic deformations, phonon modes and defect structures. We relate these calculations to specific experimental observations, including high-resolution studies of defect structures, neutron scattering studies of phonon modes and pre-martensitic phenomena, and mechanical properties such as ductility.

Recent Results:
One of the fundamental questions in materials science is to understand the nucleation and growth of twin boundaries, and the competition between twinning and slip deformation modes. These issues can play an important role in the ductility of materials. We are currently studying these issues in hcp metals, where the ability to twin makes Zr and Ti very ductile even at low temperatures. Conversely, materials such as Mg and Be do not twin, and are brittle. Our large-scale atomistic simulations have shown that dislocation cores may nucleate twin boundaries. In the figure at right, a dislocation in an hcp metal has dissociated into a large twin nucleus (upper part of figure), with a small partial dislocation at the bottom. These two dislocations are connected by a stacking fault, making the arrangement difficult to move. Instead, tension along the c-axis has the effect of causing the twinned region to grow. This arrangement has not been seen in previous simulations, due to the small simulation sizes used previously. This provides a microscopic explanation for the observed fact that systems that twin under c-axis compression also twin under tension.

Grain boundaries are important in semiconductors, due to their mechanical and electrical properties. They may provide preferential sites for point defects (such as dopants), and also provide low-energy diffusion pathways. We have been able to identify low energy structures for grain boundaries, such as the one shown here, using a combination of classical, tight-binding and ab initio calculations. The resulting structure shown here is in close agreement with structures seen in high-resolution microscopy. The electronic structure-based calculations, including both the empirical tight-binding approach and the non-empirical ab initio calculations, show that the energy is very sensitive to the structure, while the classical potentials do not correctly capture this.

We have also explored precursor phenomena in martensitic transformations. Martensitic systems undergo first-order transitions which are characterized by a lattice strain. There are a number of experimental anomalies in the high-temperature parent phase, from direct TEM observations of “tweed” structure, low elastic constants, and Brillouin-zone dependence of both X-ray and neutron scattering. We have performed large-scale simulations of both the high-temperature bcc phase of Zr, as well as the transformation from bcc to hcp on cooling. These simulations allow for a full calculation of the dynamic structure factor. We have also monitored the scattering as the system transforms from the bcc to the hcp phase, and simultaneously observed the actual atomic arrangements during the transformation process, in order to learn about the nucleation and growth processes. In the figure at right, we demonstrate the final microstructure resulting from the martensitic nucleation and growth dynamics in our simulation.

Significance:
By using a combination of approaches at the atomic scale, we have been able to demonstrate how the structure and energy of defects affect observable macroscopic properties, such as the deformation behavior of materials. We have been able to predict the structure of defects, such as grain boundaries and dislocations, and their corresponding energies. We have also been able to reproduce scattering experiments where anomalies have occurred, and relate this to the underlying, non-linear dynamics associated with the transformations. The unique predictive calculations resulting from these studies provide valuable microscopic information about the deformation and transformation behaviors in materials.

Future Work:
Much work remains to be done. We are currently working on predicting the structure and energies of the “tension” twins in HCP metals, using a combination of classical and ab initio calculations. We are using our tight-binding model of Mo to calculate the structure and energies of dislocations in this BCC system. We anticipate performing similar calculations to examine dislocations in Si, again using our tight-binding model.

Man Yoo, Oak Ridge National Laboratory (Deformation properties of HCP metals)
Sid Yip and Ju Li, Massachusetts Institute of Technology (Dislocations in HCP and diamond-structured materials)
Ye Yiying, Wuhan University, China (Ab initio calculations of material properties)

Last update: February 19, 2008
Home \| Research \| People \| Awards \| MPC \| Contact \| Ames Laboratory \| IPRT \| Iowa State University

Ab Initio Studies of Defect Structures and Phase Transformations Personnel: James Morris (PI), Cai-Zhuang Wang (PI), Kai-Ming Ho (PI), Bruce Harmon (PI) Abstract: We are interested in material-specific mechanical and thermodynamic properties of materials. In particular, we use ab initio calculations, combined with empirical methods (such as tight-binding and EAM potentials) to examine the structure and energy of lattice distortions, including elastic deformations, phonon modes and defect structures. We relate these calculations to specific experimental observations, including high-resolution studies of defect structures, neutron scattering studies of phonon modes and pre-martensitic phenomena, and mechanical properties such as ductility. Recent Results: One of the fundamental questions in materials science is to understand the nucleation and growth of twin boundaries, and the competition between twinning and slip deformation modes. These issues can play an important role in the ductility of materials. We are currently studying these issues in hcp metals, where the ability to twin makes Zr and Ti very ductile even at low temperatures. Conversely, materials such as Mg and Be do not twin, and are brittle. Our large-scale atomistic simulations have shown that dislocation cores may nucleate twin boundaries. In the figure at right, a dislocation in an hcp metal has dissociated into a large twin nucleus (upper part of figure), with a small partial dislocation at the bottom. These two dislocations are connected by a stacking fault, making the arrangement difficult to move. Instead, tension along the c-axis has the effect of causing the twinned region to grow. This arrangement has not been seen in previous simulations, due to the small simulation sizes used previously. This provides a microscopic explanation for the observed fact that systems that twin under c-axis compression also twin under tension. Grain boundaries are important in semiconductors, due to their mechanical and electrical properties. They may provide preferential sites for point defects (such as dopants), and also provide low-energy diffusion pathways. We have been able to identify low energy structures for grain boundaries, such as the one shown here, using a combination of classical, tight-binding and ab initio calculations. The resulting structure shown here is in close agreement with structures seen in high-resolution microscopy. The electronic structure-based calculations, including both the empirical tight-binding approach and the non-empirical ab initio calculations, show that the energy is very sensitive to the structure, while the classical potentials do not correctly capture this. We have also explored precursor phenomena in martensitic transformations. Martensitic systems undergo first-order transitions which are characterized by a lattice strain. There are a number of experimental anomalies in the high-temperature parent phase, from direct TEM observations of “tweed” structure, low elastic constants, and Brillouin-zone dependence of both X-ray and neutron scattering. We have performed large-scale simulations of both the high-temperature bcc phase of Zr, as well as the transformation from bcc to hcp on cooling. These simulations allow for a full calculation of the dynamic structure factor. We have also monitored the scattering as the system transforms from the bcc to the hcp phase, and simultaneously observed the actual atomic arrangements during the transformation process, in order to learn about the nucleation and growth processes. In the figure at right, we demonstrate the final microstructure resulting from the martensitic nucleation and growth dynamics in our simulation. Significance: By using a combination of approaches at the atomic scale, we have been able to demonstrate how the structure and energy of defects affect observable macroscopic properties, such as the deformation behavior of materials. We have been able to predict the structure of defects, such as grain boundaries and dislocations, and their corresponding energies. We have also been able to reproduce scattering experiments where anomalies have occurred, and relate this to the underlying, non-linear dynamics associated with the transformations. The unique predictive calculations resulting from these studies provide valuable microscopic information about the deformation and transformation behaviors in materials. Future Work: Much work remains to be done. We are currently working on predicting the structure and energies of the “tension” twins in HCP metals, using a combination of classical and ab initio calculations. We are using our tight-binding model of Mo to calculate the structure and energies of dislocations in this BCC system. We anticipate performing similar calculations to examine dislocations in Si, again using our tight-binding model. Interactions: Man Yoo, Oak Ridge National Laboratory (Deformation properties of HCP metals) Sid Yip and Ju Li, Massachusetts Institute of Technology (Dislocations in HCP and diamond-structured materials) Ye Yiying, Wuhan University, China (Ab initio calculations of material properties) Return to Exploratory Research page.

	Phone: +1 (515)294-4446 \| Fax +1(515)294-8727 \| Security & Privacy Notice \| Disclaimer Ames Laboratory is a member of the Institute for Physical Research and Technology, a consortium of research and technology-transfer centers at Iowa State University. Webmaster: bcarsten@ameslab.gov Copyright©2006 Last update: February 19, 2008