MECHANICAL ENGINEERING & MATERIALS SCIENCE
Scott X. Mao, PhD
538D Benedum Hall | 3700 O’Hara Street | Pittsburgh, PA 15261
William Kepler Whiteford Professor
P: 412-624-9602 C: 412-624-4846 sxm2@pitt.edu
Revealing the real-time atomic-scale structural evolution is central to understanding and controlling the mechanical degradation of high-performance engineering and energy materials. However, it has been an outstanding challenge to explore those processes due to technical difficulties. Here, we developed a novel in-situ
nanomechanical/electrochemical testing setup inside transmission electron microscope (TEM), which provides an unprecedented in-situ atomistically-resolved approach for discovering the previously unknown mechanisms in nanosized engineering and energy materials.
Formation of Monatomic Metallic Glasses
Deformation-Induced Stacking Fault Tetrahedra
Twin-Size Dependent Deformation and Failure
How to make a monatomic metallic glass by vitrification has been a longstanding challenge for materials scientists. Here we find an experimental method to approach the vitrification of monatomic metallic liquids by achieving an ultrahigh quenching rate of 1014 K/s. Using this method, liquid bodycentered cubic metals (e.g. pure tantalum and vanadium) are successfully converted into monatomic metallic glasses, offering unique possibilities for studying the structureproperty relationships of glasses. We further show the great controllable process of reversible vitrification–crystallization. The ultrahigh cooling rate also makes it possible to explore the fast kinetics and structural behavior of supercooled metallic liquids. (Nature, (2014) 512, 177-180)
Stacking fault tetrahedral (SFT), the 3D crystalline defects, are often observed in quenched or irradiated face-centred cubic metals and alloys. All of the stacking fault tetrahedra experimentally observed till date are supposed to originate from vacancies. We discovered that surface-nucleated dislocations can strongly interact inside the confined volume of Au nanowires, leading to a new type of dislocation-originated SFT, in distinct to the widely believed vacancyoriginated SFT. This discovery sheds new light onto the size effect on the mechanical behavior of small-volume materials and advances our fundamental knowledge of the 3D volume defects. (J.W. Wang, et al. Nature Communications (2013) 4, 2340).
Although nanoscale twinning is an effective mean to enhance the strength of metals, twin-size effect on the deformation and failure of nanotwinned metals remains largely unexplored, especially at the minimum twin size. Here, a new type of size effect in nanotwinned Au nanowires (NWs) is presented. As twin size reaches the angstrom-scale, Au NWs exhibit a remarkable ductile-to-brittle transition that is governed by the heterogeneousto-homogeneous dislocation nucleation transition. Quantitative measurements show that approaching such a twin size limit gives rise to the ultra-high strength in Au NWs,close to the ideal strength limit of perfect Au. (J.W. Wang, et al. Nature Communications (2013) 4, 1742).
Atomic-Scale Lithiation Process of Si Anodes Understanding the atomic-scale structural evolution during the electrochemical reactions in solid-state electrodes is critically important to the development of high-performance Li-ion batteries. Here, we show the first atomic-scale lithiation process of both crystal-Si (c-Si) and amorphous-Si (a-Si). The lithiation of c-Si is controlled by the atomic-scale ledge mechanism, resulting in the crystallographic orientation
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dependence of lithiation-induced swelling; while the lithiation of a-Si is mediated by an unexpected twophase mechanism, in contrast to the widely believed single-phase mechanism. These discoveries elucidate the atomistic origin of morphological change and degradation in lithiated electrodes. (Nature Nanotechnologies (2012) 7, 749-756; Nano Letters (2013) 13, 709-715; Science (2010), 330, 1515-1520). DEPARTMENT OF MECHANICAL ENGINEERING AND MATERIALS SCIENCE
