Michael D. Sangid

Energy barriers associated with slip-twin interactions

The energetics of slip–coherent twin boundary (CTB) interactions are established under tensile deformation in face centered cubic (fcc) copper with molecular dynamics simulations, exploring the entire stereographic triangle. The CTBs serve as effective barriers in some crystal orientations more than others, consistent with experimental observations. The resulting dislocation structures upon slip–twin reactions are identified in terms of Burgers vector analysis. Visualization of the dislocation transmission, lock formation, dislocation incorporation to twin boundaries, dislocation multiplication at the matrix–twin interface and twin translation, growth, and contraction behaviors cover the most significant reactions that can physically occur providing a deeper understanding of the mechanical behavior of fcc alloys in the presence of twin boundaries. The results make a distinction between deformation and annealing twins interacting with incident dislocations and point to the considerable role both types of twins can play in strengthening of fcc metals.

Laser-Driven Micro Transfer Placement of Prefabricated Microstructures

Microassembly of prefabricated structures and devices is emerging as key process technology for realizing heterogeneous integration and high-performance flexible and stretchable electronics. Here, we report on a laser-driven micro transfer placement process that exploits, instead of ablation, the mismatch in thermomechanical response at the interface of a transferable microstructure and a transfer tool to a laser pulse to drive the release of the microstructure from the transfer tool and its travel to a receiving substrate. The resulting facile pick-and-place process is demonstrated with the assembling of 3-D microstructures and the placement of GaN light-emitting diodes onto silicon and glass substrates. High-speed photography is used to provide experimental evidence of thermomechanically driven release. Experiments are used to measure the laser flux incident on the interface. These, when used in numerical and analytical models, suggest that temperatures reached during the process are enough to produce strain energy release rates to drive delamination of the microstructure from the transfer tool.

Analyzing the effects of powder and post-processing on porosity and properties of electron beam melted Ti-6Al-4V

ABSTRACT Metal additive manufacturing techniques such as the powder-bed systems are developing as a novel method for producing complex components. This study uses synchrotron-based X-ray microtomography to investigate porosity in electron beam melted Ti-6Al-4V in the as-built and post-processed state for two different powders. The presence of gas porosity in the starting powder was shown to correlate to porosity in the as-built components. This porosity was observed to shrink after a hot isostatic press treatment, but grow following a subsequent heat treatment. Crystal plasticity simulations were used to observe the effects of various observed pore sizes on mechanical behavior under loading. GRAPHICAL ABSTRACT IMPACT STATEMENT This work combined µSXCT and crystal plasticity simulations to characterize porosity through each processing stage of EBM Ti-6Al-4V. Gas porosity was detected in the as-HIPed state, and growth was observed after subsequent heat treatment.

MicroStamping for Improved Speckle Patterns to Enable Digital Image Correlation

Surface strain measurements using image correlation require a pattern to be applied to the surface of the object being measured. Lithography, the most widely used method for repeatable patterning is expensive, requiring dedicated technical staff and significant infrastructure. Lithography is time consuming, often requiring several days for each patterning application, which limits throughput. An innovative method has been developed and tested whereby repeatable patterns for image correlation are applied without dedicated technical staff or special infrastructure and can be completed in a few minutes rather than days. This new method is more amenable to application of patterns to complex surface geometries and larger surface areas. The new micro stamping method allows for higher contrast patterning materials, which improves the accuracy of strain measurements using image correlation.

Bayesian uncertainty quantification and propagation for validation of a microstructure sensitive model for prediction of fatigue crack initiation

A microstructure and deformation mechanism based fatigue crack initiation and life prediction model, which links microstructure variability of a polycrystalline material to the scatter in fatigue life, is validated using an uncertainty quantification and propagation framework. First, global sensitivity analysis (GSA) is used to identify the set of most influential parameters in the fatigue life prediction model. Following GSA, the posterior distributions of all influential parameters are calculated using a Bayesian inference framework, which is built based on a Markov chain Monte Carlo (MCMC) algorithm. The quantified uncertainties thus obtained, are propagated through the model using Monte Carlo sampling technique to make robust predictions of fatigue life. The model is validated by comparing the predictions to experimental fatigue life data.

Study of Structure and Deformation Pathways in Ti-7Al Using Atomistic Simulations, Experiments, and Characterization

Ti-7Al is a good model material for mimicking the α phase response of near-α and α+β phases of many widely used titanium-based engineering alloys, including Ti-6Al-4V. In this study, three model structures of Ti-7Al are investigated using atomistic simulations by varying the Ti and Al atom positions within the crystalline lattice. These atomic arrangements are based on transmission electron microscopy observations of short-range order. The elastic constants of the three model structures considered are calculated using molecular dynamics simulations. Resonant ultrasound spectroscopy experiments are conducted to obtain the elastic constants at room temperature and a good agreement is found between the simulation and experimental results, providing confidence that the model structures are reasonable. Additionally, energy barriers for crystalline slip are established for these structures by means of calculating the γ-surfaces for different slip systems. Finally, the positions of Al atoms in regards to solid solution strengthening are studied using density functional theory simulations, which demonstrate a higher energy barrier for slip when the Al solute atom is closer to (or at) the fault plane. These results provide quantitative insights into the deformation mechanisms of this alloy.

An experimental and computational study of size-dependent contact-angle of dewetted metal nanodroplets below its melting temperature

Decorating 1D nanostructures (e.g., wires and tubes) with metal nanoparticles serves as a hierarchical approach to integrate the functionalities of metal oxides, semiconductors, and metals. This paper examines a simple and low-temperature approach to self-assembling gold nanoparticles (Au-np)—a common catalytic material—onto silicon nanowires (SiNWs). A conformal ultra-thin film (i.e., <15 nm thick) is deposited onto SiNWs and thermally dewetted, forming nanoparticles in the 6–70 nm range. Two parameters of its morphology are dependent upon dewetting conditions: particle size and particle contact angle. Using transmission electron microscopy imaging, it is found that annealing temperature profile has a strong effect on the particle size. Additionally, the contact angle is found to be dependent on particle size and temperature even below the eutectic temperature of the Au-Si alloy. Molecular dynamics simulations were performed to investigate potential explanations for such experimental observation. In this...

Investigation of fatigue crack incubation and growth in cast MAR-M247 subjected to low cycle fatigue at room temperature

MC carbide particles (with Hafnium and/or Tantalum as constituent metallic element, M) were observed to crack extensively in a cast polycrystalline nickel-base superalloy, MAR-M247, when subjected to low-cycle fatigue loading at room temperature. High resolution secondary electron images taken on the surface of a double edge notch test specimen revealed that approximately half the carbide particles cracked in the highly-strained notch section of the specimen. These images further illustrated that the average surface area of cracked particles was approximately three times that of the uncracked particles. Additional analysis illustrated that the cracks within a large number of particles aligned nearly perpendicular to the loading direction. However, high aspect ratio particles (with aspect ratio

Validation of Microstructure-Based Materials Modeling

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