Shiteng Zhao

Shiteng Zhao

Materials Scientist, Shock Physicist, Electron Microscopist

Publications

Defect reconfigration in a Ti-Al alloy via electroplasticity

Published in Nature Materials, 2020

It has been known for decades that the application of pulsed direct current can significantly enhance the formability of metals. However, the detailed mechanisms of this effect have been difficult to separate from simple Joule heating. Here, we study the electroplastic deformation of Ti-7Al, an alloy that is uniquely suited for uncoupling this behaviour because, contrary to most metals, it has inherently lower ductility at higher temperature. We find that during mechanical deformation, electropulsing enhances cross-slip, producing a wavy dislocation morphology, and enhances twinning, which is similar to what occurs during cryogenic deformation. As a consequence, dislocations are prevented from localizing into planar slip bands that would lead to the early failure of the alloy under tension. Our results demonstrate that this macroscopic electroplastic behaviour originates from defect-level microstructural reconfiguration that cannot be rationalized by simple Joule heating.

Recommended citation: Shiteng Zhao, Ruopeng Zhang, Yan Chong, Xiaoqing Li, Anas Abu-Odeh, Eric Rothchild, Daryl C. Chrzan, Mark Asta, J.W.Morris Jr., Andrew M. Minor. (2020). "Defect reconfigration in a Ti-Al alloy via electroplasticity." Nature Materials. 1(3). http://sdlszst.github.io/files/Zhao_27_Nature_Materials_Electroplasticity in Ti-Al.pdf

Short-range order and its impact on the CrCoNi medium-entropy alloy

Published in Nature, 2020

Traditional metallic alloys are mixtures of elements in which the atoms of minoriety species tend to be distributed randomly if they are below their solubility limit, or to form secondary phases if they are above it. The concept of multiple-principal-element alloys has recently expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements. This group of materilas has received much interest owing to their enhanced mechanical properties. They are usually called MEA in ternary systems and HEA in quaternary or quinary systems, alluding to theri high degree of configurational entropy. However, the question has remined as to how random these solild solutions actually are, with the influence of short-range order being suggested in computational simulations but not seen experimentally. Here we report the observation, using energy-filtered TEM of structure features attibuted to short range order in the CrCoNi MEA. Increasing amounts of such order give rise to both higher stacking fault energy and hardness. These findings suggest that the degree of local ordering at the nanmetre scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of MEAs and HEAs

Recommended citation: Ruopeng Zhang, Shiteng Zhao (contributed equally),Jun Ding, Yan Chong, Tao Jia, Colin Ophus, Mark Asta, Robert Ritchie, Andrew Minor. (2020). "Short-range order and its impact on the CrCoNi medium-entropy alloy." Nature. 581(283-287). http://sdlszst.github.io/files/Zhao_26_Nature_Short-range order and its impact on the CrCoNi medium-entropy alloy.pdf

Direct imaging of short-range order and its impact on deformation in Ti-6Al

Published in Science Advances, 2019

Chemical short range order(SRO) within a nominally single phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relatiuon to deformation using energy filtered imagin in a TEM. The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing sub-nanometer SRO enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocatio plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history.

Recommended citation: Ruopeng Zhang, Shiteng Zhao (Contributed equally),Colin Ophus, Yu Deng, Shraddha Vachhani, Burak Ozdol, Rachel Traylor, Karen Bustillo, J.W. Morris Jr., Daryl C. Chrzan, Mark Asta, Andrew Minor. (2019). "Direct imaging of short-range order and its impact on deformation in Ti-6Al." Science Adcances. 5(eaax2799). http://sdlszst.github.io/files/Zhao_24_ScienceAdvances_Direct imaging of short range order and its impact on deformation in Ti-6Al.pdf

Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys

Published in Progress in Materials Science, 2018

High-entropy alloys (HEAs), also known as multi-principal element alloys or multi-component alloys, have been the subject of numerous investigations since they were first described in 2004.We review the principal mechanical properties of these alloys with emphasis on the face-centered cubic systems, such as the CrCoNi-based alloys. Their favorable mechanical properties and ease of processing by conventional means suggest extensive utilization in many future structural applications.

Recommended citation: Zezhou Li, Shiteng Zhao, Robert Ritchie, Marc Meyers. (2018). "Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys." Progress in Materials Science. 5(eaax2799). http://sdlszst.github.io/files/Zhao_19_PMS_mechanical behavior of HEA.pdf

Spall strength dependence on grain size and strain rate in tantalum

Published in We examine the effect of grain size on the dynamic failure of tantalum during laser shock and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples, thus, momocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultrafine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra and intergranular in the poly and ultrafine grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is a strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length of geometrically necessary dislocations required to form a grain boundary void is lower than that of grain interior void with the same maximum diameter, thus the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall experimental results and also predict grain boundary spearation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra- and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals , 2018

T.P.Remington, E.N. Hahn, S. Zhao (Corresponding author), R.Flanagan, J.C.E. Mertens, S. Sannagjoamrad, T.G. Langdon, C.E.Wehrenberg, B.R.Maddox, D.C.Swift, B.A.Remington, N.Chawla, M.A.Meyers. (2018). "Spall strength dependence on grain size and strain rate in tantalum." Acta Materialia. 158(313-329).

Recommended citation: Spall strength dependence on grain size and strain rate in tantalum

Spall strength dependence on grain size and strain rate in tantalum

Published in Acta Materialia, 2018

We examine the effect of grain size on the dynamic failure of tantalum during laser shock and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples, thus, momocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultrafine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra and intergranular in the poly and ultrafine grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is a strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length of geometrically necessary dislocations required to form a grain boundary void is lower than that of grain interior void with the same maximum diameter, thus the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall experimental results and also predict grain boundary spearation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra- and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals

Recommended citation: T.P.Remington, E.N. Hahn, S. Zhao (Corresponding author), R.Flanagan, J.C.E. Mertens, S. Sannagjoamrad, T.G. Langdon, C.E.Wehrenberg, B.R.Maddox, D.C.Swift, B.A.Remington, N.Chawla, M.A.Meyers. (2018). "Spall strength dependence on grain size and strain rate in tantalum." Acta Materialia. 158(313-329). http://sdlszst.github.io/files/Zhao_18_Acta Mater_Spall strength depends on grain size.pdf

Shock-induced amorphization in silicon carbide

Published in Acta Materialia, 2018

While SiC has been predicted to undergo pressure induced amorphization, the microstructural evidence of such a drastic phase change is absent as its brittleness usually prevents its successful recovery from high-pressure experiments. Here we report on the observatio of amorphous SiC recovered from laser-ablation-driven shock compression with a peak stress of about 50GPa. TEM reveals that the amorphous regions are extremely localized , forming bands as narrow as a few nanometers. In addition to these amorphous bands, planar stacking faults are observed. Large-scale non equilibrium MD simulations elucidate the process and suggest that the planar stacking faults serve as the precursors to amorphization. Our results suggest that the amorphous phase produced is a high density form, which enhances its thermodynamical stability under the high pressures combined with the shear stresses generated by the uniaxial strain state in shock compression

Recommended citation: S. Zhao, R. Flanagan, E.N. Hahn, B. Kad, B.A.Remington, C.E. Wehrenberg, R. Cauble, K. More, M.A. Meyers. (2016). "Shock-induced amorphization in silicon carbide." Acta Materialia. 158(206-213). http://sdlszst.github.io/files/Zhao_17_Acta Mater_shock induced amorphization in Silicon Carbide.pdf

Generating gradient germanium nanostructures by shock-induced amorphization and crystallization

Published in PNAS, 2017

Gradient nanostructures are attracting considerable interest due to their potential to obtain superior structural and functional properties of materials. Applying powerful laser-driven shocks (stresses of up to one-third million atmospheres, or 33 gigapascals) to germanium, we report here a complex gradient nanostructure consisting of, near the surface, nanocrystals with high density of nanotwins. Beyond there, the structure exhibits arrays of amorphous bands which are preceded by planar defects such as stacking faults generated by partial dislocations. At a lower shock stress, the surface region of the recovered target is completely amorphous. We propose that germanium undergoes amorphization above a threshold stress and that the deformation-generated heat leads to nanocrystallization. These experiments are corroborated by molecular dynamics simulations which show that supersonic partial dislocation bursts play a role in triggering the crystalline-to-amorphous transition.

Recommended citation: Shiteng Zhao, Bimal Kad, Christopher Wehrenberg, Bruce Remington, Eric Hahn, Karren More, Marc Meyers. (2017). "Generating gradient germanium nanostructures by shock-induced amorphization and crystallization." PNAS. 114(9791-9796). http://sdlszst.github.io/files/Zhao_15_PNAS_laser_nanogradient_Ge.pdf

Directional amorphization of boron carbide subjected to laser shock compression

Published in PNAS, 2016

Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laserdriven shock compression, unprecedented high strain rates can be achieved, here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45-50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B4C.

Recommended citation: Shiteng Zhao, Bimal Kad, Bruce Remington, Jerry LaSalvia, Christopher Wehrenberg, Kristopher Behler, Marc Meyers. (2016). "Directional amorphization of boron carbide subjected to laser shock compression." PNAS. 113(12088-12093). http://sdlszst.github.io/files/Zhao_11_PNAS_Directional amorphization of boron carbide subjected to laser shock compression_PNAS_final.pdf

Amorphization and nanocrystallization of silicon under shock compression

Published in High power pulsed laser driven shock compression and recovery experiments on (001) silicon unveiled remarable structural changes above a pressure threshold. Two distinct amorphous regions were identified:(a) a bulk amorphous layer close to the surface and (b) amorphous bands initially alligned with {111} slip plabnes. Further increase of the laser energy leads to the recrystallization of amorphous silicon into nanocrystals with high concentration of nanotwins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. MD simulation corroborates the amorphization, showing that it is initated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory., 2015

S. Zhao, E.N. Hahn, B. Kad, B.A.Remington, C.E. Wehrenberg, E.M.Bringa, M.A. Meyers. (2015). "Shock-induced amorphization in silicon carbide." Acta Materialia. 103(519-533).

Recommended citation: Amorphization and nanocrystallization of silicon under shock compression

Amorphization and nanocrystallization of silicon under shock compression

Published in Acta Materialia, 2015

High power pulsed laser driven shock compression and recovery experiments on (001) silicon unveiled remarable structural changes above a pressure threshold. Two distinct amorphous regions were identified:(a) a bulk amorphous layer close to the surface and (b) amorphous bands initially alligned with {111} slip plabnes. Further increase of the laser energy leads to the recrystallization of amorphous silicon into nanocrystals with high concentration of nanotwins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. MD simulation corroborates the amorphization, showing that it is initated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory.

Recommended citation: S. Zhao, E.N. Hahn, B. Kad, B.A.Remington, C.E. Wehrenberg, E.M.Bringa, M.A. Meyers. (2015). "Shock-induced amorphization in silicon carbide." Acta Materialia. 103(519-533). http://sdlszst.github.io/files/Zhao_07_Acta_shock amorphization of silicon.pdf

Influence of Severe Plastic Deformation on the Dynamic Strain Aging of Ultrafine Grained Al-Mg alloys

Published in Acta Materialia, 2014

This paper is about the dynamic strain aging of materials.Especially the influence of severe plastic deformation on DSA of ultrafine grained Al-Mg alloys. We use confined channel die pressing at room temperature to fabricate alloys with submicrometer grain sizes. Subsequently, strain rate jump tests were conducted to measure the strain rate sensitivity of the materials. It was shown that the critical strain for initiation of serrated flow increased considerably with increasing strain and Mg content contray to the behavior of the coarse grained and non-deformed counterparts.In addition, the instantaneous stress response and the instantaneous strain rate sensitivity during rate jumps were always positive and increased monotonically with pre-strain. The steady state strain rate sensitivity was negative and decreased firstly with progressing pre-strain up to certain point and then increase again. This mechanical behavior of Al-Mg alloys is discussed on the basis of recently developed DSA models by relating the microstructure evolution of Al-Mg alloys during SPD to the influence of SPD on DSA.

Recommended citation: Shiteng Zhao, Chenlu Meng, Fengxin Mao, Weiping Hu, Guenter Gottstein. (2014). "Influence of Severe Plastic Deformation on the Dynamic Strain Aging of Ultrafine Grained Al-Mg alloys." Acta Materalia. 76(54-67). http://sdlszst.github.io/files/Zhao_01_Acta_Influence of SPD on DSA.pdf