Divyanshu Gupta. Wed. Feb 20, 9:00AM 1:00PM JHE 326H

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1 Divyanshu Gupta Twin boundaries in GaP nanowires: Electronic structure and optical properties Formation of multiple twin boundaries during the growth of semiconductor nanowires is very common. However, the effects of such planar defects on the electronic and optical properties of nanowires are not very well understood. We use a combination of ab initio simulation and experimental techniques to study these effects. Twin boundaries in GaP are shown to act as an atomically-narrow plane of wurtzite phase with a type-i band alignment homostructure. Presence of twin boundaries leads to the introduction of shallow trap states observed in photoluminescence studies. These effects should have a profound impact on the efficiency of nanowire-based devices.

2 Chao Zheng Structural Dynamics in Hybrid Halide Perovskites: Bulk Rashba Splitting, Spin Texture, and Carrier Localization Hybrid halide perovskites, with (CH3NH3)PbI3 being a prominent member, has attracted enormous interest as a solar cell absorber material. In this paper, we present results for an effective relativistic band structure of (CH3NH3)PbI3 with the focus on the dispersion of electronic states near the band edges of (CH3NH3)PbI3 affected by thermal structural fluctuations. The dynamic Rashba splitting diminishes down to 4 mev as the molecular dynamics progress to longer times (beyond 2 ps) the valence band becomes disordered and loses the Bloch character that undermines the argument about an indirect band gap present in the dynamic structure of (CH3NH3)PbI3 at room temperature.

3 David Jeon High Temperature Oxidation Behavior of Ferritic Stainless Steels for Hot End Components of Automotive Exhaust Systems A comparative study of the high temperature oxidation behavior of AISI 409, AISI 439, AISI 436 and a multilayer steel (MLS) is being investigated as it pertains to the hot end components of automotive exhaust gas systems. The multilayer steel is comprised of chromium-enriched surfaces layers formed on an interstitial free steel substrate by a pack cementation process. Each material was exposed in two simulated hot gas combustion environments including: (i) flowing air and (ii) flowing air with 12% water vapor for durations of 100 h and 500 h. The high temperature oxidation behavior of the materials was compared using mass gain measurements, X-ray diffraction (XRD) of the oxide scales to determine the type of oxide formed along with Scanning Electron Microscopy (SEM) cross-sectional imaging coupled with Energy- Dispersive X-Ray Spectroscopy (EDS) to document the mode and extent of attack whilst exploring links between oxidation susceptibility and microstructure. Initial findings indicate that AISI 436 stainless steel hast the best high temperature performance among the benchmark materials followed by AISI 439 and 409 respectively solely based on mass gain results and oxide scale thickness. The lower chromium content of 409 and the absence of molybdenum in both 439 and 409 contribute to their weaker high temperature oxidation performance. The multilayer steel performs as well as the 436 stainless steel when comparing surface oxide scales, but poorly when comparing mass gain. The multilayer steel s exposed IF substrate forms a thick, iron-based oxide which contributes to the high mass gain. The iron based oxide easily descales, exposing the substrate to further oxidize. To account for this, the depth of substrate loss was measured f to determine the extent of oxidation. Further testing and analysis must be conducted to determine the multilayer steel s capability to replace current ferritic stainless steels used in automotive exhaust systems.

4 Connie Pelligra The Influence of Transformation Induced Plasticity on Damage Development in QP1500 Recent advances in the optimization of heat treatment procedures to stabilize retained austenite at ambient temperatures has led to an industrial break-through in attaining advanced high strength steels with exceptional combinations of strength and ductility. For example, transformation-induced plasticity (TRIP) strengthening is known to increase elongation and delay necking. Considerable research has been invested in creating these steels; however, there is little known about the extent to which transformation-induced plasticity in 3G steels can suppress damage. The quantification of damage as a function of TRIP using strain as the intermediate variable would have a significant impact in all forming operations using 3G steels. The relationship of strain to damage of a Fe-0.2C-3.4Mn- 1.6Si QP1500 steel has been determined through in-situ SEM tensile tests coupled with x-ray computed microtomography (XCT), while the strain to TRIP relationship is determined via XRD. This has enabled a study of the micro-mechanisms of microstructural damage in steels exhibiting substantial TRIP effects during deformation.

5 Mohammad Ghoncheh Investigations on Hot Tearing and High-temperature Constitutive Behavior of Dual-phase Advanced High-Strength Steel Dual-phase (DP) advanced high-strength steels (AHSSs) are widely used in automotive industries due to their advantageous e.g., bake hardening effect, excellent combinations of strength, ductility, and strength-hardening capacity. However, solidification defects during casting of DP AHSS slabs make these ferrous alloys expensive. Hot tearing, a crack that forms in solidifying alloys at high solid fraction, is a very complicated defect because of its interplay between long-range stresses, solidification shrinkage, and microstructure. Understanding the mechanisms of hot tear nucleation and propagation would be helpful for reducing product failure resulting from this defect. Toward this end, synergy between high-temperature tensile properties achieved via Gleeble instrument and in-situ microstructural observation using laser confocal microscopy has been employed. Laser confocal microscope allows us to observe melting path of DP AHSS as well as solid-state phase transformation, while Gleeble instrument as a programmable machine provides us thermomechanical schedule in which samples can be pulled up even within the mushy zone. Interplay among instruments results and fractographs will lead to formulate high-temperature constitutive behavior of DP AHSS and develop a new mechanism of hot tearing susceptibility.