Corey Efaw - Academia.edu (original) (raw)
Papers by Corey Efaw
Journal of Visualized Experiments
Journal of The Electrochemical Society, 2022
Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly ch... more Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly characterize Li metal in standard batteries due to the extremely weak scattering from Li atoms. In this work, it is demonstrated the stripping and plating of Li metal can be effectively quantified during battery cycling in appropriately designed synchrotron XRD experiments that utilize an anode-free battery configuration in which a Li-containing cathode material of LiNi0.6Mn0.2Co0.2O2 (NMC622) is paired with a bare anode current collector consisting of either Cu metal (Cu/NMC) or Mo metal (Mo/NMC). In this configuration, it is possible to probe local variations in the deposition and stripping of Li metal with sufficient spatial sensitivity to map the inhomogeneity in pouch cells and to follow these processes with sufficient time resolution to track state-of-charge-dependent variations in the rate of Li usage at a single point. For the Cu/NMC and Mo/NMC batteries, it was observed that the i...
Advanced Energy Materials, 2021
For lithium–sulfur battery commercialization, research at a pouch cell level is essential, as som... more For lithium–sulfur battery commercialization, research at a pouch cell level is essential, as some problems that can be ignored or deemed minimal at a smaller level can have a greater effect on the performance of the larger pouch cell. Herein, the failure mechanisms of Li–S pouch cells are deeply investigated via in operando pressure analysis. It is found that highly porous structures of cathodes/separators and slow electrolyte diffusion through cathodes/separators can both lead to poor initial wetting. Additionally, the Li‐metal anode dominates the thickness variation of the whole pouch cell, which is verified by in situ measured pressure variation. Consequently, a real‐time approach that combines normalized pressure with differential pressure analysis is proposed and validated to diagnose the morphology evolution of the Li‐metal anode. Moreover, applied pressure and porosity/tortuosity ratio of the cathode are both identified as independent factors that influence anode performance. In addition to stabilizing anodes, high pressure is proven to improve the cathode connectivity and avoid cathode cracking over cycling, which improves the possibility of developing cathodes with high sulfur mass loading. This work provides insights into Li–S pouch cell design (e.g., cathode and separator) and highlights pathways to improve cell capacity and cycling performance with applied and monitored pressure.
Nanoscale Corrosion Characterization of Surface Hardened Martensitic Stainless Steel: Resolving L... more Nanoscale Corrosion Characterization of Surface Hardened Martensitic Stainless Steel: Resolving Local Contributions to Bulk Materials Performance, MICHAEL HURLEY*, COREY EFAW, PAUL DAVIS, ARMEN KVRYAN, BRIELLE IBE, and NICK CARTER (Micron School of Materials Science and Engineering, Boise State University, Boise, ID; mikehurley@boisestate.edu, coreyefaw@u.boisestate.edu, pauldavis2@boisestate.edu, armenkvryan@u.boisestate.edu, brielleibe@u.boisestate.edu, nickcarter@u.boisestate.edu) Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Recent advances in characterization tools have progressed the means to resolve features that control corrosion mechanisms. However, many localized techniques do not...
ECS Meeting Abstracts, 2021
Nuclear energy has been increasingly recognized as an effective and low carbon-emission energy so... more 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 ma terials c an be utilized to correlate with sensor output. A c ommon issue with zirconium cladding i s the sothe 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, a nd at om probe tomography.
Graphene— a hexagonal lattice encompassing a single layer of carbon atoms—has made great advancem... more Graphene— a hexagonal lattice encompassing a single layer of carbon atoms—has made great advancements in electronic devices, flexible electronics, and more recently as an electrically conductive bioscaffold for stem cell growth and differentiation. For this study, chemical vapor deposition(CVD) is used for graphene growth with copper as the metal catalyst. Graphene films, transferred from the metal catalyst to glass slides via chemical or electrochemical techniques, are used as bioscaffolds for the growth and differentiation of C2C12 stem cells. The ability to transfer graphene from the metal catalyst to a substrate of choice is a highly desirable and beneficial property. Traditional transfer techniques demonstrate potential limitations for tissue engineering applications as they require the use of a harmful, highly corrosive copper etchant, iron(III) chloride (Fecl3). Additional concerns are associated with the presence of metal catalyst even after long processing times. An alternative method, based on water electrolysis, electrochemical delamination, provides a transfer method with high efficiency, low cost recyclability, and minimal use of etching chemicals, and has been successfully performed. This project aims to compare the two transfer techniques to determine the optimal transfer method for graphene transfers to establish an ideal environment for C2C12 cell growth and differentiation
The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility desi... more The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility designed to simulate rapid reactor transient conditions up to 600 °C. Zirconium and its alloys are known to undergo a transition from parabolic to linear oxidation kinetics, termed breakaway. After breakaway, the oxide formed is porous and non-protective which significantly decreases its mechanical stability as a cladding material. In this work, the oxidation behavior of Zircaloy-3, Zircaloy-4, and Zr-1Nb is evaluated in isothermal and rapid transient conditions (air at ≤ 820 °C). Separate effects studies were done on plate samples in air (80% N2+O2) and oxygen (80% Ar+O2) to characterize the effect of nitrogen on the oxidation behavior during isothermal oxidation using thermogravimetric analysis. It was observed that the three alloys exhibit decreased resistance to breakaway in air. In addition, the effects of chamfering and welding on the oxidation behavior were investigated during isothermal and rapid transient oxidation. All alloys were affected by welding, however only the Fe/Sn containing alloys were affected by plastic deformation. Advanced characterization techniques (scanning Kelvin probe force microscopy and Raman spectroscopy) were performed on cross sections of oxidized specimens to further investigate the effects of oxide phase and composition on breakaway
ECS Meeting Abstracts, 2021
ECS Meeting Abstracts, 2021
CORROSION, 2015
The relative electrochemical properties of second phases compared to the surrounding matrix gives... more The relative electrochemical properties of second phases compared to the surrounding matrix gives rise to localization of corrosion on magnesium (Mg) alloys. Localized corrosion and its subsequent propagation in Mg alloys is largely driven by so-called 'micro-galvanic coupling' of microstructural constituents within the alloy microstructure. In the present work atomic force microscopy (AFM) imaging coupled with Scanning Kelvin Probe Force Microscopy (SKPFM) were used to generate surface Volta potential maps of a range of Mg alloys. In this manner, the relative Volta potential difference(s) between the respective alloy matrix phase and the microconstituent phase(s) of each sample were determined. Correlations between relative Volta potentials and phase composition were then inferred based on comparison of AFM optical and topographical images with corresponding SEM images and EDS maps of the same or similar features. Sample preparation technique, testing conditions, and proper calibration of the SKPFM were all seen to influence the Volta potentials acquired. Since the relative Volta potential difference is known to serve as an index for local corrosion, particularly under thin electrolyte layers and in chloride solutions-a review of published SKPFM data was conducted to provide a critical assessment of the surface Volta potential differences between different microconstituent phases in a variety of Mg alloys to aid in understanding and future improvement of the atmospheric corrosion of Mg alloys.
Nuclear energy and electrochemical energy storage, such as batteries, are key parts to the clean ... more Nuclear energy and electrochemical energy storage, such as batteries, are key parts to the clean energy transition of critical infrastructure. This work aims to define, monitor, and modify interfacial layers that would improve the utility of materials in harsh environments seen in nuclear and energy storage applications. First, the studying of zirconium alloys, which is used as nuclear cladding, was done to better understand the degradation mechanisms within an extreme environment. High-resolution characterization techniques were used to correlate corrosion mechanisms to equivalent circuit models from novel in-pile electrochemical impedance spectroscopy sensors. Advancement in this sensor technology could provide further insight and monitoring of the complex degradation mechanisms in a harsh nuclear core environment. A novel method was developed to spatially map Raman spectral features throughout the oxide cross-section, revealing a direct correlation between tetragonal zirconia pha...
ECS Meeting Abstracts, 2020
Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its hig... more Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its high theoretical capacity and low electrochemical potential. However, the cascading heterogeneity from the formation of the solid-electrolyte interphase (SEI) up to the non-uniform current distribution leads to both poor efficiency via rapid consumption of active materials as well as safety issues from treacherous dendrite growth. An SEI that is ionically conductive while electrically resistive, viscoelastic, electrochemically stable, and possesses good mechanical properties is paramount for hindering these failure mechanisms. The discovery of novel approaches to producing an ideal SEI is a crucial piece to solving the lithium metal anode puzzle. One route of controlling the SEI is through interface engineering by producing a stable artificial SEI. In this work, a mixed artificial SEI was produced with aim toward improving ionic conductivity while retaining electrochemical stability and mec...
ECS Transactions
An improved relative scaling of Volta potential differences (VPD) acquired from scanning Kelvin p... more An improved relative scaling of Volta potential differences (VPD) acquired from scanning Kelvin probe force microscopy (SKPFM) was developed by quantifying the probe work function. In corrosion studies, SKPFM has been used to identify local nobility of complex metallic systems and provide theoretical corrosion initiation sites. However, large variability in measured VPD values for metallic phases has led to controversy in their interpretation. Tracking changes of the probe work function has been shown to decrease the variability seen in SKPFM results. To quantify the work function of SKPFM probes, the measured VPD of an inert gold standard was compared to the work function theoretically calculated by density functional theory (DFT) first-principles. For proof of concept, a stainless steel sample joined by a Cu-Ag-Ti brazing material was characterized by SKPFM with three different types of probes.
Materials
Historically, high carbon steels have been used in mechanical applications because their high sur... more Historically, high carbon steels have been used in mechanical applications because their high surface hardness contributes to excellent wear performance. However, in aggressive environments, current bearing steels exhibit insufficient corrosion resistance. Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Magnetic force microscopy showed differences in magnetic domains between the matrix and carbides, while scanning Kelvin probe force microscopy (SKPFM) revealed a 90–200 mV Volta potential difference between the two phases. Corrosion progression was monitored on the nanoscale via SKPFM and in situ atomic force microscopy (AFM), revealing different corrosion modes among heat treatments that pred...
Nuclear Materials and Energy
Journal of Applied Physics
The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence t... more 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 caxis: 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. I.
Journal of Visualized Experiments
Journal of The Electrochemical Society, 2022
Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly ch... more Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly characterize Li metal in standard batteries due to the extremely weak scattering from Li atoms. In this work, it is demonstrated the stripping and plating of Li metal can be effectively quantified during battery cycling in appropriately designed synchrotron XRD experiments that utilize an anode-free battery configuration in which a Li-containing cathode material of LiNi0.6Mn0.2Co0.2O2 (NMC622) is paired with a bare anode current collector consisting of either Cu metal (Cu/NMC) or Mo metal (Mo/NMC). In this configuration, it is possible to probe local variations in the deposition and stripping of Li metal with sufficient spatial sensitivity to map the inhomogeneity in pouch cells and to follow these processes with sufficient time resolution to track state-of-charge-dependent variations in the rate of Li usage at a single point. For the Cu/NMC and Mo/NMC batteries, it was observed that the i...
Advanced Energy Materials, 2021
For lithium–sulfur battery commercialization, research at a pouch cell level is essential, as som... more For lithium–sulfur battery commercialization, research at a pouch cell level is essential, as some problems that can be ignored or deemed minimal at a smaller level can have a greater effect on the performance of the larger pouch cell. Herein, the failure mechanisms of Li–S pouch cells are deeply investigated via in operando pressure analysis. It is found that highly porous structures of cathodes/separators and slow electrolyte diffusion through cathodes/separators can both lead to poor initial wetting. Additionally, the Li‐metal anode dominates the thickness variation of the whole pouch cell, which is verified by in situ measured pressure variation. Consequently, a real‐time approach that combines normalized pressure with differential pressure analysis is proposed and validated to diagnose the morphology evolution of the Li‐metal anode. Moreover, applied pressure and porosity/tortuosity ratio of the cathode are both identified as independent factors that influence anode performance. In addition to stabilizing anodes, high pressure is proven to improve the cathode connectivity and avoid cathode cracking over cycling, which improves the possibility of developing cathodes with high sulfur mass loading. This work provides insights into Li–S pouch cell design (e.g., cathode and separator) and highlights pathways to improve cell capacity and cycling performance with applied and monitored pressure.
Nanoscale Corrosion Characterization of Surface Hardened Martensitic Stainless Steel: Resolving L... more Nanoscale Corrosion Characterization of Surface Hardened Martensitic Stainless Steel: Resolving Local Contributions to Bulk Materials Performance, MICHAEL HURLEY*, COREY EFAW, PAUL DAVIS, ARMEN KVRYAN, BRIELLE IBE, and NICK CARTER (Micron School of Materials Science and Engineering, Boise State University, Boise, ID; mikehurley@boisestate.edu, coreyefaw@u.boisestate.edu, pauldavis2@boisestate.edu, armenkvryan@u.boisestate.edu, brielleibe@u.boisestate.edu, nickcarter@u.boisestate.edu) Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Recent advances in characterization tools have progressed the means to resolve features that control corrosion mechanisms. However, many localized techniques do not...
ECS Meeting Abstracts, 2021
Nuclear energy has been increasingly recognized as an effective and low carbon-emission energy so... more 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 ma terials c an be utilized to correlate with sensor output. A c ommon issue with zirconium cladding i s the sothe 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, a nd at om probe tomography.
Graphene— a hexagonal lattice encompassing a single layer of carbon atoms—has made great advancem... more Graphene— a hexagonal lattice encompassing a single layer of carbon atoms—has made great advancements in electronic devices, flexible electronics, and more recently as an electrically conductive bioscaffold for stem cell growth and differentiation. For this study, chemical vapor deposition(CVD) is used for graphene growth with copper as the metal catalyst. Graphene films, transferred from the metal catalyst to glass slides via chemical or electrochemical techniques, are used as bioscaffolds for the growth and differentiation of C2C12 stem cells. The ability to transfer graphene from the metal catalyst to a substrate of choice is a highly desirable and beneficial property. Traditional transfer techniques demonstrate potential limitations for tissue engineering applications as they require the use of a harmful, highly corrosive copper etchant, iron(III) chloride (Fecl3). Additional concerns are associated with the presence of metal catalyst even after long processing times. An alternative method, based on water electrolysis, electrochemical delamination, provides a transfer method with high efficiency, low cost recyclability, and minimal use of etching chemicals, and has been successfully performed. This project aims to compare the two transfer techniques to determine the optimal transfer method for graphene transfers to establish an ideal environment for C2C12 cell growth and differentiation
The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility desi... more The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility designed to simulate rapid reactor transient conditions up to 600 °C. Zirconium and its alloys are known to undergo a transition from parabolic to linear oxidation kinetics, termed breakaway. After breakaway, the oxide formed is porous and non-protective which significantly decreases its mechanical stability as a cladding material. In this work, the oxidation behavior of Zircaloy-3, Zircaloy-4, and Zr-1Nb is evaluated in isothermal and rapid transient conditions (air at ≤ 820 °C). Separate effects studies were done on plate samples in air (80% N2+O2) and oxygen (80% Ar+O2) to characterize the effect of nitrogen on the oxidation behavior during isothermal oxidation using thermogravimetric analysis. It was observed that the three alloys exhibit decreased resistance to breakaway in air. In addition, the effects of chamfering and welding on the oxidation behavior were investigated during isothermal and rapid transient oxidation. All alloys were affected by welding, however only the Fe/Sn containing alloys were affected by plastic deformation. Advanced characterization techniques (scanning Kelvin probe force microscopy and Raman spectroscopy) were performed on cross sections of oxidized specimens to further investigate the effects of oxide phase and composition on breakaway
ECS Meeting Abstracts, 2021
ECS Meeting Abstracts, 2021
CORROSION, 2015
The relative electrochemical properties of second phases compared to the surrounding matrix gives... more The relative electrochemical properties of second phases compared to the surrounding matrix gives rise to localization of corrosion on magnesium (Mg) alloys. Localized corrosion and its subsequent propagation in Mg alloys is largely driven by so-called 'micro-galvanic coupling' of microstructural constituents within the alloy microstructure. In the present work atomic force microscopy (AFM) imaging coupled with Scanning Kelvin Probe Force Microscopy (SKPFM) were used to generate surface Volta potential maps of a range of Mg alloys. In this manner, the relative Volta potential difference(s) between the respective alloy matrix phase and the microconstituent phase(s) of each sample were determined. Correlations between relative Volta potentials and phase composition were then inferred based on comparison of AFM optical and topographical images with corresponding SEM images and EDS maps of the same or similar features. Sample preparation technique, testing conditions, and proper calibration of the SKPFM were all seen to influence the Volta potentials acquired. Since the relative Volta potential difference is known to serve as an index for local corrosion, particularly under thin electrolyte layers and in chloride solutions-a review of published SKPFM data was conducted to provide a critical assessment of the surface Volta potential differences between different microconstituent phases in a variety of Mg alloys to aid in understanding and future improvement of the atmospheric corrosion of Mg alloys.
Nuclear energy and electrochemical energy storage, such as batteries, are key parts to the clean ... more Nuclear energy and electrochemical energy storage, such as batteries, are key parts to the clean energy transition of critical infrastructure. This work aims to define, monitor, and modify interfacial layers that would improve the utility of materials in harsh environments seen in nuclear and energy storage applications. First, the studying of zirconium alloys, which is used as nuclear cladding, was done to better understand the degradation mechanisms within an extreme environment. High-resolution characterization techniques were used to correlate corrosion mechanisms to equivalent circuit models from novel in-pile electrochemical impedance spectroscopy sensors. Advancement in this sensor technology could provide further insight and monitoring of the complex degradation mechanisms in a harsh nuclear core environment. A novel method was developed to spatially map Raman spectral features throughout the oxide cross-section, revealing a direct correlation between tetragonal zirconia pha...
ECS Meeting Abstracts, 2020
Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its hig... more Lithium metal is often deemed the “Holy Grail” of rechargeable battery technology, due to its high theoretical capacity and low electrochemical potential. However, the cascading heterogeneity from the formation of the solid-electrolyte interphase (SEI) up to the non-uniform current distribution leads to both poor efficiency via rapid consumption of active materials as well as safety issues from treacherous dendrite growth. An SEI that is ionically conductive while electrically resistive, viscoelastic, electrochemically stable, and possesses good mechanical properties is paramount for hindering these failure mechanisms. The discovery of novel approaches to producing an ideal SEI is a crucial piece to solving the lithium metal anode puzzle. One route of controlling the SEI is through interface engineering by producing a stable artificial SEI. In this work, a mixed artificial SEI was produced with aim toward improving ionic conductivity while retaining electrochemical stability and mec...
ECS Transactions
An improved relative scaling of Volta potential differences (VPD) acquired from scanning Kelvin p... more An improved relative scaling of Volta potential differences (VPD) acquired from scanning Kelvin probe force microscopy (SKPFM) was developed by quantifying the probe work function. In corrosion studies, SKPFM has been used to identify local nobility of complex metallic systems and provide theoretical corrosion initiation sites. However, large variability in measured VPD values for metallic phases has led to controversy in their interpretation. Tracking changes of the probe work function has been shown to decrease the variability seen in SKPFM results. To quantify the work function of SKPFM probes, the measured VPD of an inert gold standard was compared to the work function theoretically calculated by density functional theory (DFT) first-principles. For proof of concept, a stainless steel sample joined by a Cu-Ag-Ti brazing material was characterized by SKPFM with three different types of probes.
Materials
Historically, high carbon steels have been used in mechanical applications because their high sur... more Historically, high carbon steels have been used in mechanical applications because their high surface hardness contributes to excellent wear performance. However, in aggressive environments, current bearing steels exhibit insufficient corrosion resistance. Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Magnetic force microscopy showed differences in magnetic domains between the matrix and carbides, while scanning Kelvin probe force microscopy (SKPFM) revealed a 90–200 mV Volta potential difference between the two phases. Corrosion progression was monitored on the nanoscale via SKPFM and in situ atomic force microscopy (AFM), revealing different corrosion modes among heat treatments that pred...
Nuclear Materials and Energy
Journal of Applied Physics
The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence t... more 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 caxis: 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. I.