Seminar & Colloquium
[세미나: 7월 8일(월), 오전 11시] Prof. Seok-Woo Lee, University of Connecticut
Title
Superelasticity of ThCr2Si2-Structured Intermetallic Compounds: Making and Breaking Bonds in the Solid State
Speaker
Prof. Seok-Woo Lee, Associate Professor, Department of Materials Science and Engineering, University of Connecticut
* Education
- Sep. 2006 - Jun. 2011 Ph.D. in Materials Science and Engineering, Stanford University, CA, USA
o Advisor: Prof. William D. Nix
o Thesis title: The Plasticity of Metals at the Sub-Micrometer Scale and Dislocation Dynamics in a Thin Film
- Sep. 2004 - Aug. 2006 M.S. in Materials Science and Engineering, Korea University, Seoul, South Korea
o Advisor: Prof. Jae-Chul Lee
o Thesis title: Crystallization induced plasticity in bulk amorphous alloys
- Mar. 1997 - Feb. 2004 B.S. Materials Science and Engineering, Seoul National University Seoul, South Korea
* Professional Experience
- Aug. 2020 - present Associate Professor, Materials Science and Engineering, University of Connecticut
- Aug. 2014 - Aug. 2020 Assistant Professor, Materials Science and Engineering, University of Connecticut
- Oct. 2011 - July. 2014 Kavli Nanoscience Institute Postdoc Fellow, Applied Physics and Materials Science, California Institute of Technology
- Sep. 2006 - Sep. 2011 Research Assistant, Materials Science and Engineering, Stanford University
| Date | Monday, July 8th , 2024
| Time | 11:00 ~
| Venue | 33동 125호(WCU 다목적실)
[Abstract]
Elastic strain limit, the measure of the maximum elastic deformability, of most crystalline solids is less than one percent because plastic deformation or fracture usually occurs at very small strain. To obtain a large elastic strain limit, a crystalline material needs to undergo a reversible structural transition.
In this presentation, a new class of superelastic materials, ThCr2Si2-structured intermetallic compounds (CaFe2As2, (CaK)Fe4As4, LaRu2P2, SrNi2P2), which undergo a unique reversible structural transition, lattice collapse-expansion, will be introduced. Under uni-axial compression along c-axis, Si-Si type bonds are formed, leading to the lattice collapse with more than 10% decrease in length. Once the applied stress is relaxed, thermal vibration breaks Si-Si type bonds, and the lattice expands back to the original length. This making and breaking bond process can induce the giant elastic strain limit up to 17%. If the temperature is low enough (below 40K), Si-Si type bonds can be maintained even without applied stress, leading to cryogenic shape memory effect. It is also possible to induce the thermal actuation if the residual compressive stress is developed by adding nanoscale precipitation. Some ThCr2Si2-structured intermetallic compounds are high temperature superconductor, and their superconductivity can be switched on and off through lattice collapse-expansion process. SrNi2P2 is special because it exhibits superelasticity under both compression and tension because only the fraction of P atoms in SrNi2P2 are bonded at the stress-free state. C-axis compression forms a chemical bond between unbonded P atoms (2/3 of P atoms) and causes lattice collapse while c-axis tension breaks a chemical bond between bonded P atoms (1/3 of P atoms) and causes lattice expansion. As a result, SrNi2P2 exhibits tension-compression asymmetry in mechanical response, and this asymmetry leads to the elastocaloric effect comparable with conventional shape memory alloys such as Nitinol.
In addition, we recently discovered that CaFe2As2 exhibits a unique hysteresis behavior in the load-depth curve for a-axis nanoindentation, which does not cause lattice collapse-expansion. Transmission electron microscopy revealed that many new grain boundaries are created through multiple instances of atomic layer buckling and the nucleated dislocations are piled-up near these grain boundaries. These piled-up dislocations cause a reversed plastic flow due to the back stress (the Bauschinger effect), leading to a large hysteresis loop in the nanoindentation load-depth curve. Density Functional Theory calculation confirmed that CaFe2As2 has an anisotropic layered structure, where atomic layer buckling and dislocation nucleation can occur easily.
Note that ThCr2Si2-structure is known as the most populous of all structural types. The theory predicts the presence of over 2500 compounds, and many of them could exhibit the same lattice collapse-expansion process. Therefore, a combined study of machine learning, single crystal growth, and mechanical characterization could lead to a discovery of a large group of new superelastic materials.
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