The Burns Research Group is always working on a wide variety of environmental cracking problems. Current research topics, along with student and sponsorship information, can be found in the descriptions below:
1. Effect of Galvanically Induced Corrosion Damage on the Fatigue Crack Formation Behavior of Al 7050-T7451
Researcher(s): Carly Cocke Sponsor(s): Office of Naval Research (ONR) Project Description: Galvanic corrosion damage in aircrafts is produced when steel fasteners are coupled with aluminum substructure in atmospheric conditions. Interaction of the galvanic corrosion damage with aircraft operational loading poses a challenge on the structural integrity and fatigue behavior of the airframe components. The objectives of this project are to quantify the humid air crack formation life and microstructure scale crack progression from the galvanic corrosion morphologies, and to identify features that govern crack formation process. Corrosion damage is induced under electrochemical conditions representative of a galvanic crevice in atmospheric conditions with and without corrosion inhibitors. 3D characterization of the corrosion damage is achieved using white light profilometry, microscopy and x-ray tomography. Crack formation life (Ni) to ≈10 μm and local microstructure-scale crack growth rates (da/dN) are determined in Al 7050-T7451 exposed in humid air using fractography of load-induced crack surface marker-bands. Data obtained from the characterization of underlying microstructure, 3D corrosion morphology feature, crack formation and crack growth rate will be used to evaluate safe life and damage tolerant engineering models, and to identify the influence of corrosion inhibitors on fatigue initiation behavior.
2. Influence of γ’ Precipitate Morphology on Crack Tip-Damage Interactions in Monel K-500
Researcher(s): Zach Harris Sponsor(s): DoD Corrosion Office – Technical Corrosion Collaboration Project Description: Nickel-based superalloys exhibit exceptional mechanical properties and excellent resistance to chemical degradation. However, despite the alloy system’s inherent resistance to chemical attack, they have proven to be susceptible to hydrogen environment-assisted cracking (HEAC) when stressed in aggressive environments that electrochemically produce hydrogen. Though the influence of hydrogen embrittlement on mechanical properties has been well studied, the contribution of microstructural variables with regards to the proposed mechanisms are still unclear. One feature that has yet to be thoroughly evaluated in the literature is the influence of γ’ precipitate morphology on local deformation structures near a crack tip. Furthermore, the effect of changing the precipitate morphology on the accuracy of current micromechanical models used for predicting HEAC kinetics is not known. Thus, a systematic study of the influence of precipitate morphology on HEAC of Ni-based superalloys is needed to not only address identified knowledge gaps in the scientific literature, but to also evaluate the effect of microstructural variability on HEAC modeling. This study will seek to address this need through a three phase approach. First, a metallurgical study will be completed to evaluate the γ’ precipitate morphology as a function of applied heat treatment. These results will also be used to better understand the general slip behavior and slip-grain boundary interactions as a function of the precipitate morphology. Second, the influence of the precipitate morphology on HEAC will be evaluated using fracture mechanics testing on single-edge notched specimens at varying hydrogen concentrations. And third, the influence of the precipitate morphology on the crack-wake damage structures will be evaluated using state-of-the-art characterization techniques.
3. The Evolution of Grain Boundary Precipitates in Al-Mg alloys at Mildly Elevated Temperatures
Researcher(s): Raewyn Haines Sponsor(s): Office of Naval Research (ONR) Project Description: Al-Mg alloys are widely used in the marine industry due to the fact that they are light-weight, strong, and resistive to corrosion. However, these alloys are susceptible to IGSCC when a β phase precipitates on the grain boundaries during thermal exposure and dissolves during subsequent immersion in NaCl solutions. The amount of β on the grain boundaries is quantified by the Degree of Sensitization (DoS), measured by a standard nitric acid mass loss test; it has been proven that the rate of IGSCC increases with increasing DoS. Current research studies the rates of IGSCC caused by the dissolution of these precipitate particles with respect to variations in material orientation, composition, and temper. However, investigation of each of these material properties is performed under the assumption that the DoS is a sufficient measurement of sensitization, and that a single numerical DoS measurement accurately represents the same sensitization conditions across all ranges of material, regardless of differences in material orientation, composition, and temper. It is hypothesized that the numerical DoS is insufficient, in some cases, to quantify the magnitude and extent of sensitization; instead it is proposed that the microscopic morphology of β particles on the grain boundaries is a more precise means of predicting rates of IGSCC. This research aims to provide fractographic images of the volume and distribution of β particles on the grain boundaries, in order to enable a more accurate, standardized measurement of sensitization, and thus more accurate predictions of IGSCC rates and component failure.
4. Mechanistic Studies of Intergranular Corrosion and Stress Corrosion Cracking Under Atmospheric Exposure Conditions
Researcher(s): Patrick Steiner Sponsor(s): Office of Naval Research (ONR) Project Description: One of the frontiers of corrosion science is localized corrosion and stress corrosion cracking under atmospheric conditions. This project takes previous fatigue stress corrosion cracking experiments performed on Al-MG 5XXX series alloys (primarily AA5083 and AA5456) under full immersion conditions, and seeks to expand it to various atmospheric environments such as misting, thin electrolyte films, and targeted wicking into the crack tip. In particular it will determine to what degree the mechanisms of intergranular stress corrosion cracking previously developed for full immersion conditions can be extended to atmospheric conditions. Crack growth kinetics will be measured using direct current potential drop, which allows for micron scale crack extension measurements.
5. Mitigation of Stress Corrosion Cracking and Intergranular Corrosion in AA5xxx Alloys with Smart Functional Coatings
Researcher(s): Matthew McMahon Sponsor(s): Office of Naval Research (ONR) Project Description: AA5xxx alloys are currently utilized as lightweight, high strength alloys to replace heavier steels in the new Littoral Combat Ship. These alloys are strengthened in part by Mg saturation, which upon exposure to service conditions of 50 degrees Celsius or more over months at a time, catalyzes the sensitization of this metal through beta-phase precipitation at grain boundaries. Under tension, this dissolvable secondary phase will increase the incidence of stress corrosion cracking, which currently threatens the future of these novel light alloys for naval use. With Zn and Mg functional coatings, this interdisciplinary project seeks to mitigate these deleterious effects through enhancing coating capabilities in terms of electrochemical potential throwing power, which allows for the protection of exposed alloy surfaces across coating defects. Testing is done on highly sensitized AA5456 and AA5083 alloys to prove effective protection abilities in candidate coatings at the most high-risk alloy states in full saline immersion, with salinities across a range of strengths and salt types to consider various environmental scenarios that these ships may encounter
6. Effect of High Altitude (Low Temperature and Water Vapor Pressure) Environments on Fatigue Crack Propagation Rates in Aerospace Aluminum Alloys
Researcher(s): Luke Brown Sponsor(s): DoD Corrosion Office – Technical Corrosion Collaboration Project Description: The objective of this research is to examine the deleterious effect of high purity water vapor on fatigue crack growth rates (da/dN) in aluminum alloys (AA7075-T651 and AA2199-T86). Growth rates declined with decreasing water vapor pressure over a wide range of ΔK-values. This behavior demonstrated consistency with current environmental theories where crack growth is limited by molecular flow or H-diffusion in the crack tip process zone. A novel minimum in growth rates was observed at intermediate water vapor pressures and low ΔK. This mechanism suggests testing protocol-dependent environmental fatigue behavior, where variations in crack wake fracture morphology and crack opening displacement change the molecular flow of water vapor to the crack tip. This investigation will better inform protocols in selecting environment appropriate crack growth rates for linear elastic fracture mechanics (LEFM) modeling.
7. The Effect of High Altitude Environments on the Dislocation Structure Evolution during Fatigue Cracking of Legacy and Next Generation Aerospace Aluminum Alloys
Researcher(s): Adam Thompson (collaboration with Helge Heinrich) Sponsor(s): Internal Project Description: This work will characterize and analyze the effect of low temperature and low water vapor pressure environments on dislocation structure developed at the crack tip during fatigue loading of 7075. A combination of standard and high resolution electron backscatter diffraction (EBSD) analysis, as well as FIB/TEM analysis will be used. The standard EBSD analysis will give general deformation data over a relatively large area (mm scale). High resolution EBSD, analyzing small shifts in EBSD patters, allows for more detailed observation of dislocation densities, but over a smaller area (100s of microns). FIB liftouts for TEM analysis show the dislocation structures in great detail, but over a very limited area (~10 microns). By combining all three techniques, the general dislocation structure may be more fully understood than would otherwise be by any technique applied individually. This data will be used to understand the mechanism leading to the observed discrepancies in fatigue behavior between tests at similar water vapor pressures but different temperatures.
8. Metal-Rich Primers to Mitigate Multiple Forms of Corrosion Including Environmental Cracking of High Performance Alloys
Researcher(s): Kristyn Ardrey Sponsor(s): DoD Corrosion Office – Technical Corrosion Collaboration Project Description: Metal-Rich Primers (MRPs) are being becoming increasingly popular as environmentally friendly means of protecting DOD assets from the aggressive environments in which they operate. However there is insufficient understanding and a lack of a DOD testing protocols to ascertain whether a given metal rich primer mitigates corrosion for a variety of DOD alloys. This work will investigate the chemical and electrochemical mechanisms of inhibition of corrosion and environmental cracking for metal primers in the presence of various relevant environmental conditions, alloy compositions, and mechanical loading states. Critically, the mechanism of damage will change with these variables, as such the means of inhibition must be tuned to the specific application. For example, suppression of stress corrosion cracking requires protection of depassivated metal and mitigation of corrosion of grain boundary phases that are unique to each high performance DOD alloy. The metal rich primer must be tuned to function to achieve these goals. Accelerated testing that validates the likelihood of success in field deployment is needed. Field and accelerated lab assessment/evaluation of these systems will enable the development of a mil-spec procedure to rapidly and effectively test different metal rich primers, alloy substrates, and environments. Improved life cycle costs will be enabled by both practical and scientific understanding of such environmental friendly chromate primer replacement options.