Effects of interdiffusion on shear response of semi-coherent {111} interfaces in Ni/Cu

Oct 1, 2022

Abstract:

Intermixing of chemical species as a result of interdiffusion during the manufacturing of metallic nanolaminates leads to diffuse interface structures which have distinct properties compared to corresponding atomically sharp interfaces. The effect of interdiffusion-induced changes to the interface structure on interface shear strength is complex due to the presence of solute atoms and a reduced misfit dislocation density. The shear responses of {111} Cu/Ni nanolaminate interfaces with varying levels of interdiffusion are studied using atomistic methods to elucidate the effect of interface structure changes on shear deformation mechanisms. Models with diffuse interfaces exhibit improved interface shear strength relative to the atomically sharp case; however, shear strength does not increase monotonically with solute concentration. The distribution of maximum changes in energy per misfit node and non-uniform misfit node displacements, filtered using microrotation vector analysis, suggest heterogeneous interface resistance to sliding. No strong correlation is found between solute concentration near misfit node centroids and misfit node displacements, indicating the importance of the longer-range misfit dislocation structure. Increased activation of misfit dislocation glide is associated with larger solute concentrations as a result of increased misfit node displacements. Analysis of change in energy during the shear deformation process, however, reveals that interface sliding is dominated by the misfit node behavior. These findings highlight the importance of modeling realistic diffuse interface structures and emphasize the competing effects of solute concentration and interface misfit dislocation density.

Coarse-grained atomistic modeling of dislocations and generalized crystal plasticity

Apr 6, 2022

Abstract:

Recent developments in generalized continuum modeling methods ranging from coarse-grained atomistics to micromorphic theory offer potential to make more intimate physical contact with dislocation field problems framed at length scales on the order of microns. We explore a range of discrete dynamical and continuum mechanics approaches to crystal plasticity that are relevant to modeling behavior of populations of dislocations. Predictive atomistic and coarse-grained atomistic models are limited in terms of length and time scales that can be accessed; examples of the latter are discussed in terms of interactions of multiple dislocations in heterogeneous systems. Generalized continuum models alleviate restrictions to a significant extent in modeling larger scales of dislocation configurations and reactions, and are useful to consider effects of dislocation configuration on strength at characteristic length scales of sub-micron and above; these models require a combination of bottomup models and top-down experimental information to inform parameters and model form. The concurrent atomistic-continuum (CAC) method is extended to model complex multicomponent alloy systems using an average atom approach. Examples of CAC are presented, along with potential to assist in informing parameters of a recently developed micropolar crystal plasticity model based on a set of sub-micron dislocation field problems. Prospects for further developments are discussed.

Lattice dislocation induced misfit dislocation evolution in semi-coherent {111} bimetal interfaces

Jul 1, 2021

Abstract:

Characterization of misfit dislocation evolution in bimetal interfaces is critical for understanding plasticity in nanolaminates as local misfit dislocation structures affect dislocation/interface interactions. This work utilizes the Concurrent Atomistic- Continuum method to probe the evolution of misfit structures at semi-coherent Ni/Cu and Cu/Ag interfaces impinged by dislocation pileups generated via nanoindentation. A continuum microrotation metric is computed and used to visualize the evolution of the interface misfit dislocation pattern. The stress state from approaching dislocations induces mixed contraction and expansion of misfit dislocation structures. A lower misfit dislocation density coincides with greater localized deformation for atoms near misfit nodes for Ni/Cu. The increased misfit dislocation density for Cu/Ag alternatively distributes the deformation over a larger percentage of atoms at the interface. Interface sliding is found to facilitate deformation extending into the bulk lattices centered on misfit nodes. The depth of penetration of those fields is greater for Ni/Cu than for Cu/Ag.

Silane functionalization effects on dispersion of alumina nanoparticles in hybrid carbon fiber composites

Aug 10, 2018

Abstract:

Hybrid carbon fiber reinforced polymer composites are a new breed of materials currently being explored and characterized for next-generation aerospace applications. Through the introduction of secondary reinforcements, such as alumina nanoparticles, hybrid properties including improved mechanical properties—fracture toughness, for example—and stress-sensing capabilities can be achieved. However, problems with manufacturing can arise resulting from the inherent variability of the manufacturing techniques along with the tendency for the nanoparticles to agglomerate. Photoluminescence spectroscopy is used to investigate the effects of adjustments to manufacturing processes and silane functionalization on particle dispersion and sample consistency between samples of the same type. This work finds that application of surface treatments on the nanoparticles improved their dispersion, with the reactive treatment providing for the most consistency among samples. Improvements to dispersion and increased consistency resulting from specific changes in manufacturing processes were shown numerically. Findings provide a manufacturing recommendation to achieve optimum dispersion and mechanical properties of the composite.

Piezospectroscopic evaluation and damage identification for thermal barrier coatings subjected to simulated engine environments

Aug 25, 2017

Abstract:

The application of high temperature ceramic coatings has enabled aircraft and power generation turbines to run at higher inlet temperatures for greater efficiency. Their use extends the lifetime of the superalloy blades that bear thermal gradients and mechanical loads during operation. In this work, ex-situ photo-luminescence spectroscopy was conducted to investigate the stresses within the thermally grown oxide of a thermal barrier coated tubular sample following complex realistic conditions, such as induced thermal gradients, and long duration aging. The resulting high spatial resolution stress contour maps highlight the development of the thermally grown oxide in response to the complex conditions. The outcomes highlight both the role of the aging process and the oxide growth’s influence on the stress profile which varies spatially across the specimen. The results further provide early detection of micro-damaged zones in the oxide layer nondestructively. Improving the understanding of the coating system’s response to loading conditions will allow for more accurate system modeling and early detection and monitoring of damage zones, which is critical for improving efficiency and longevity of aircraft and power generation turbines.

Quantifying Alumina Nanoparticle Dispersion in Hybrid Carbon Fiber Composites Using Photoluminescent Spectroscopy

Feb 1, 2017

Abstract:

Composites modified with nanoparticles are of interest to many researchers due to the large surface-area-to-volume ratio of nano-scale fillers. One challenge with nanoscale materials that has received significant attention is the dispersion of nanoparticles in a matrix material. A random distribution of particles often ensures good material properties, especially as it relates to the thermal and mechanical performance of composites. Typical methods to quantify particle dispersion in a matrix material include optical, scanning electron, and transmission electron microscopy. These utilize images and a variety of analysis methods to describe particle dispersion. This work describes how photoluminescent spectroscopy can serve as an additional technique capable of quickly and comprehensively quantifying particle dispersion of photoluminescent particles in a hybrid composite. High resolution 2D photoluminescent maps were conducted on the front and back surfaces of a hybrid carbon fiber reinforced polymer containing varying contents of alumina nanoparticles. The photoluminescent maps were analyzed for the intensity of the alumina R1 fluorescence peak, and therefore yielded alumina particle dispersion based on changes in intensity from the embedded nanoparticles. A method for quantifying particle sedimentation is also proposed that compares the photoluminescent data of the front and back surfaces of each hybrid composite and assigns a single numerical value to the degree of sedimentation in each specimen. The methods described in this work have the potential to aid in the manufacturing processes of hybrid composites by providing on-site quality control options, capable of quickly and noninvasively providing feedback on nanoparticle dispersion and sedimentation.

Characterization of Hybrid Carbon Fiber Composites using Photoluminescence Spectroscopy

Jan 5, 2017

Abstract:

Hybrid carbon fiber reinforced polymers (HCFRPs) are a new breed of material that are currently being explored and characterized for next generation aerospace applications. Through the introduction of secondary reinforcements, such as alumina nanoparticles, it is possible to achieve improved mechanical behavior and enable structural sensing to create unique hybrid properties. The photoluminescent properties of the alumina inclusions allow for the application of local stress measurements through piezospectroscopy (PS) in addi- tion to dispersion characterization. Measuring the shift in emission wavenumber at several points across the face of a sample allows for determination of the local stress through the application of the PS relationship. Measuring local intensity differences across the face of the sample, alternatively, allows for the determination of relative local particle concentra- tion for dispersion characterization. Through investigation of an HCFRP sample loaded with 10 wt% of alumina nanoparticles, it was found that stress was greater in regions with high relative particle concentrations upon mechanical loading. Further investigation also found evidence of particle-matrix debonding, characterized by a lower particle stress re- sponse to increasing composite strain at higher loads. In order to address both of these issues silane coupling agents are utilized to adjust particle behavior. It is found that the use of these treatments results in improved particle dispersion and reduced sedimentation. A reactive and non-reactive surface treatment were compared and it was found that the reactive treatment was more effective at improving dispersion for the weight percentage investigated. The outcomes of this work demonstrate the potential of utilizing the photo- luminescent sensing capability of these reinforcing particulates to tailor the design of the hybrid carbon fiber composites.

Characterizing Mechanical Properties of Hybrid Alumina Carbon Fiber Composites with Piezospectroscopy

Jan 1, 2016

Abstract: Carbon fiber composites have become popular in aerospace structures and applications due to their light weight, high strength, and high performance. Recently, scientists have begun investigating hybrid composites that include fibers and particulate fillers, since they allow for advanced tailoring of mechanical properties, such as improved fatigue life. This project investigated a hybrid carbon fiber reinforced polymer (HCFRP) that includes carbon fiber and additional alumina nanoparticle fillers, which act as embedded nano stress sensors. Utilizing the piezospectroscopic effect, the photo-luminescent (PL) spectral signal of the embedded nanoparticles has been monitored as it changes with stress, enabling non-contact stress detection of the material. The HCRFPs stress-sensitive properties have been investigated in-situ using a laser source and a tensile mechanical testing system. Hybrid composites with varying mass contents of alumina nanoparticles have been studied in order to determine the e↵ect of particle content on the overall stress sensing properties of the material. Additionally, high resolution photoluminescent maps were collected from the surfaces of each specimen in order to determine the particulate dispersion of specimens with varying alumina content. The dispersion maps also served as a method of quantifying particulate sedimentation, and can aid in the improvement of the manufacturing process. The results showed that the emitted photoluminescent spectrum can indeed be captured from the embedded alumina nanoparticles, and exhibits a systematic trend in photoluminescent peak shift with respect to stress, up to a certain critical stress. Therefore, the non-contact stress sensing results shown in this work have strong implications for the development of multi-functional hybrid composites to support structural health monitoring and nondestructive evaluation (NDE) of aerospace structures.