Toward Improving Ambient Volta Potential Measurements with SKPFM for Corrosion Studies

Scanning Kelvin probe force microscopy (SKPFM) is used in corrosion studies to quantify the relative nobility of different microstructural features present within complex metallic systems and thereby elucidate possible corrosion initiation sites. However, Volta potential differences (VPDs) measured via SKPFM in the literature for metal alloys exhibit large variability, making interpretation and application for corrosion studies difficult. We have developed an improved method for referencing SKPFM VPDs by quantifying the closely related work function of the probe relative to an inert gold standard whose modified work function is calculated via density functional theory (DFT). By measuring and tracking changes in the probe vs. gold VPD, this method compensates for some of the complex effects that cause changes in an individual probe\u27s work function. Furthermore, it provides a path toward direct, quantitative comparison of SKPFM results obtained by different researchers. Application of this method to a Cu-Ag-Ti eutectic braze of a steel sample imaged with multiple SKPFM probes of differing compositions led to enhanced repeatability both within and among probe types, as well as enabled the calculation of modified work function values for each of the microstructural constituents present

Localized Deformation in Ni-Mn-Ga Single Crystals

The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence the relationship between structure and nanoscale magnetomechanical properties, is of interest for their potential applications in actuators. Furthermore, due to its crystal structure, the behavior of Ni-Mn-Ga is anisotropic. Accordingly, nanoindentation and magnetic force microscopy were used to probe the nanoscale mechanical and magnetic properties of electropolished single crystalline 10M martensitic Ni-Mn-Ga as a function of the crystallographic c-axis (easy magnetization) direction relative to the indentation surface (i.e., c-axis in-plane versus out-of-plane). Load-displacement curves from 5–10 mN indentations on in-plane regions exhibited pop-in during loading, whereas this phenomenon was absent in out-of-plane regions. Additionally, the reduced elastic modulus measured for the c-axis out-of-plane orientation was ∼50% greater than for in-plane. Although heating above the transition temperature to the austenitic phase followed by cooling to the room temperature martensitic phase led to partial recovery of the indentation deformation, the magnitude and direction of recovery depended on the original relative orientation of the crystallographic c-axis: positive recovery for the in-plane orientation versus negative recovery (i.e., increased indent depth) for out-of-plane. Moreover, the c-axis orientation for out-of-plane regions switched to in-plane upon thermal cycling, whereas the number of twins in the in-plane regions increased. We hypothesize that dislocation plasticity contributes to the permanent deformation, while pseudoelastic twinning causes pop-in during loading and large recovery during unloading in the c-axis in-plane case. Minimization of indent strain energy accounts for the observed changes in twin orientation and number following thermal cycling

Characterization of High-temperature Polishing Techniques for Magnetic Shape-memory Alloy Ni2MnGa

Magnetic shape-memory alloys (MSMA) such as Ni2MnGa exhibit a magnetic field-induced, reversible strain through the motion of twin boundaries. Twin boundaries arise from the diffusionless transformation from the cubic austenite to the tetragonal martensite phase. Twin boundary formation changes X-ray diffraction patterns, defines the stress-strain behavior, and leads to surface reliefs which can be characterized with optical microscopy, electron microscopy, and atomic force microscopy. The accurate characterization of the surface relief requires a high quality surface finish with minimal surface roughness. Samples that are in the martensite phase at room temperature show surface topography that is indicative of the twin boundary size, angle, and position. Polishing at room temperature can remove this topography, yielding a smooth surface which is not indicative of the highly twinned microstructure of the bulk. After room temperature polishing, thermal or mechanical cycling can result in historic twins whose angles and position do not necessarily coincide with the bulk microstructure. We polished Ni2MnGa at elevated temperature in the cubic austenite phase. Upon cooling to martensite, the resulting surface relief more accurately represented the twinning structure in the bulk sample. We present the micrographic evidence supporting our theory that high-temperature polishing is necessary for surface characterization of twinned MSM alloys

Electrochemical Impedance Spectroscopy for Materials Research

Electrochemistry is chemical reaction that involves charge transfer across a solid/liquid interface. Electrochemistry is all around us in the form of batteries, corrosion, and biological cell communication. Understanding and predicting electrochemical interactions is necessary for reducing the costs of corrosion and advancing energy storage technology. A method to investigate interfacial reactions is Electrochemical Impedance Spectroscopy (EIS). A small amplitude alternating potential is applied to the sample over a range of frequencies and the response is measured. An electrochemical interface can be modeled as an electric circuit and EIS provides a method to resolve the electrical components such as capacitors, resistors, or inductors. Using this powerful but accessible technique, reactions on the surface of materials can be characterized. EIS is a pivotal research technique used at Boise State to predict corrosion rates, test semiconductor device performance, deposit DNA origami nanostructures, and characterize fuel cell and battery performance

Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed

3D Li-Ion Batteries Through Advanced Manufacturing

Increasing demands in storage systems for enhanced energy and power are driving innovative research in the design of better material systems and devices. Li-ion batteries are the leading technology and major power source for portable electronics; however, they fail to meet the demands for sustainable energy applications, such as electric vehicles and storage for renewable energy. 3D batteries have potential to maximize energy and power densities through nanoarchitecture, reducing ionic and electronic diffusion lengths. We focused on developing an advanced manufacturing process that is capable of fabricating 3D Li-ion electrodes to maximize volumetric energy and power densities while maintaining mechanical strength. Electrode materials were characterized via scanning electron microscopy and X-ray diffractometry. With the implementation of new technologies for this project, we learned about the support and processes required to enable effective R&D. This includes the contributions and requirements of work controls, procurement support, safety evaluation, laboratory training requirements, and project management

First-Principles Surface Interaction Studies of Aluminum-Copper and Aluminum-Copper-Magnesium Secondary Phases in Aluminum Alloys

First-principles density functional theory-based calculations were performed to study θ-phase Al2Cu, S-phase Al2CuMg surface stability, as well as their interactions with water molecules and chloride (Cl−) ions. These secondary phases are commonly found in aluminum-based alloys and are initiation points for localized corrosion. Density functional theory (DFT)-based simulations provide insight into the origins of localized (pitting) corrosion processes of aluminum-based alloys. For both phases studied, Cl− ions cause atomic distortions on the surface layers. The nature of the distortions could be a factor to weaken the interlayer bonds in the Al2Cu and Al2CuMg secondary phases, facilitating the corrosion process. Electronic structure calculations revealed not only electron charge transfer from Cl− ions to alloy surface but also electron sharing, suggesting ionic and covalent bonding features, respectively. The S-phase Al2CuMg structure has a more active surface than the θ-phase Al2Cu. We also found a higher tendency of formation of new species, such as Al3+, Al(OH)2+, HCl, AlCl2+, Al(OH)Cl+, and Cl2 on the S-phase Al2CuMg surface. Surface chemical reactions and resultant species present contribute to establishment of local surface chemistry that influences the corrosion behavior of aluminum alloys

Determination of Zirconium Oxide Chemistry Through Complementary Characterization Techniques

Nuclear energy has been increasingly recognized as an effective and low carbon-emission energy source. Nuclear reactors are susceptible to adverse effects, which can lead to potentially severe consequences, though they are low in probability. To ensure safety and improved monitoring of reactors, there have been increasing interests in developing sensors to monitor key parameters relating to the status within a reactor. In order to improve sensor accuracy, high-resolution characterization of cladding materials can be utilized to correlate with sensor output. A common issue with zirconium cladding is the so-called breakaway phenomenon , a critical factor seen as the transition from an initially passive zirconia to an active material. Existing research presents many factors that contribute to the breakaway mechanism, ultimately resulting in difficulty to predict its activation and propagation. As part of the effort to develop sensors, an improved understanding of pre- and post-breakaway zirconium alloys (Zr, Zr-2.65Nb, Zry-3, and Zry-4) is accomplished with Raman spectroscopy, scanning Kelvin probe force microscopy, and atom probe tomography