Julie Tucker

As the use of computers in education increases, adaptive learning platforms are becoming more common. However, these adaptive systems are typically designed to support acquisition of declarative knowledge and/or procedural fluency but rarely address conceptual learning. In this work, we developed the Crystallography Adaptive Learning Module (CALM) for materials science to provide students a tool for individualized conceptual learning. We used a randomized quasi‐experimental design comparing two instructional designs with different levels of computer‐provided direction and student agency. Undergraduate students were randomly assigned to one of two different instructional designs; one design had students complete an individualized, adaptive path using the CALM (N = 80), and the other gave students the freedom to explore CALM's learning resources but with limited guidance (N = 85). Within these two designs, we also investigated students among different cumulative grade point average (GPA) groups. While there was no statistically significant difference in the measure of conceptual understanding between instructional designs or among the groups with the same GPA, there is evidence to suggest the CALM improves conceptual understanding of students in the middle GPA group. Students using CALM also showed increased participation with the interactive learning videos compared to the other design. The number of videos watched in each instructional condition aligns with overall academic performance as the low GPA group received the most assigned supplements but watched the least videos by choice. This study provides insight for technology developers on how to develop educational adaptive technology systems that provide a proper level of student agency to promote conceptual understanding in challenging STEM topics.

In this article, the corrosion resistance of a cobalt–chromium‐based laser cladding reinforced with different microparticles: boron nitride, graphene oxide, and graphite, added for increased tribological performance, is explored. Samples are fabricated by premixing cobalt–chromium powder with microparticle additions and cladding onto 316L stainless steel base metal. The corrosion behavior is measured in industrially relevant applications: 1 m acetic acid and 3.5 wt% NaCl, using open‐circuit potential, electrochemical impedance spectroscopy, linear polarization resistance, and cyclic polarization. Laser ablation–inductively coupled plasma–mass spectrometry is used to analyze the distribution of the chemical elements throughout the coatings. The reference cladding's corrosion resistance is outstanding in both electrolytes, with a corrosion rate (CR) of ≤0.19 μm year⁻¹ and no pitting tendencies. With the addition of microparticles, the claddings maintain their remarkable pitting resistance, but show an increase in CR up to 0.98 μm year⁻¹ due to the nonuniform distribution of the microparticles into the matrix.

Additive manufacturing (AM) tools are capable of applying overlay austenitic stainless steel (SS) claddings to carbon steel components. The benefits of this approach over arc welding include a smaller heat-affected zone, residual stress reduction, and material savings. In particular, wire-directed energy deposition is a suitable technique because of its low material cost and high rate of production compared to other AM methods. However, metallurgical variations in composition, phase fraction, and microsegregation can potentially influence the corrosion behavior of such claddings. In this work, 309L SS is clad on carbon steel substrates and electrochemical methods are used to measure their general and pitting corrosion resistance in simulated marine environments (3.5 wt% NaCl solutions). Two-layer claddings are fabricated with four laser powers to understand the effects of bulk chemical composition, austenite/δ-ferrite phase fractions, and individual phase compositions on corrosion behavior. The two-layer claddings are compared to a single-layer cladding, wrought 304 SS, and the carbon steel substrate for a comprehensive assessment of corrosion performance. The two-layer claddings are remarkably resistant to general corrosion in the 3.5 wt% NaCl environment because of their high Cr content (21.6 wt% to 23.3 wt% Cr). The single-layer cladding exhibits localized corrosion at unmixed Fe-rich peninsulas that originate at the dissimilar metal boundary and protrude into the first cladding layer. All two-layer claddings possess higher pitting corrosion resistance than wrought 304 SS, demonstrating their effectiveness as a corrosion-resistant barrier. The pitting corrosion resistance is superior for claddings made with lower laser powers, due to low dilution and greater δ-ferrite contents.

Despite their exceptional mechanical and corrosion properties, duplex stainless steels (DSS) have not found widespread use in high-temperature applications due to concerns over thermal aging and embrittlement at elevated operational temperatures (> 300 °C). The present study investigated the effect of thermal aging time on the electrochemical properties of lean and standard grade DSS that are exposed to a range of pressurized water reactors containing LiOH and H₃BO₃. The results indicated that the electrolyte chemistry plays a significant role in the corrosion behavior of the DSS alloys. Corrosion resistance decreased with thermal aging time for all DSS alloys; however, standard grade DSS (2205 and 2209-w) alloys showed better corrosion resistance than lean grades (2003, 2101, 2101-w). The presence of dissolved oxygen in the electrolytes resulted in a significant increase in corrosion rate for the DSS alloys, but it did not affect the general trend of corrosion rates with aging time. All DSS alloys became vulnerable to pitting corrosion due to chloride addition, but the pitting resistance decreased with increasing thermal aging time. Increased boron B content resulted in degradation of corrosion resistance of the DSS alloys, while minor changes in pH did not show a significant change in corrosion resistance. Mechanical and metallurgical characterization coupled with electrochemical characterization of the DSS alloys gave a comprehensive insight into the effects of thermal aging on the electrochemical response of the DSS.

Graphical abstract

To understand the corrosion mechanisms of structural materials in low-temperature components of direct supercritical CO2 power cycles, immersion experiments were performed in aqueous environments expected at these conditions. A ferritic-martensitic steel (UNS K91560) was selected as the candidate material. Steel specimens were fully submerged in H2O pressurized with 99% CO2 and 1% O2 to 8 MPa, and heated up to temperature (50°C, 100°C, 150°C, or 245°C), with a test duration of 500 h. Corrosion rates were calculated based on mass loss. Scanning electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy were used to characterize microstructure, phases, crystallinity, and composition of the corrosion product layer. Experimental results show that specimens exposed at 100°C had the highest corrosion rate, followed by the specimens exposed at 50°C. The specimens exposed at the highest temperature exhibited the lowest corrosion rate. An outer noncontinuous, nonprotective Fe-rich oxide layer and a well-adhered inner oxide layer containing both Fe and Cr formed on the specimen surfaces. The inner oxide layer changed from amorphous to crystalline as the temperature increased.

To understand the corrosion mechanisms of structural materials in low-temperature components of direct supercritical CO2 power cycles, immersion experiments were performed in aqueous environments expected at these conditions. A ferritic-martensitic steel (UNS K91560) was selected as the candidate material. Steel specimens were fully submerged in H2O pressurized with 99% CO2 and 1% O2 to 8 MPa, and heated up to temperature (50°C, 100°C, 150°C, or 245°C), with a test duration of 500 h. Corrosion rates were calculated based on mass loss. Scanning electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy were used to characterize microstructure, phases, crystallinity, and composition of the corrosion product layer. Experimental results show that specimens exposed at 100°C had the highest corrosion rate, followed by the specimens exposed at 50°C. The specimens exposed at the highest temperature exhibited the lowest corrosion rate. An outer noncontinuous, nonprotective Fe-rich oxide layer and a well-adhered inner oxide layer containing both Fe and Cr formed on the specimen surfaces. The inner oxide layer changed from amorphous to crystalline as the temperature increased.

The ion exchange and point defect models are two prominent models describing the role of anions, such as chlorides, in the degradation of passive oxide films. Here the thermodynamic feasibility of critical steps of Cl-induced degradation of a hydroxylated α-Cr₂O₃ (0001) surface, as proposed by these two models, are studied. Both models begin with Cl substitution of surface OH and H₂O, which becomes less favorable with increasing Cl coverage. The initial stages of Cl-induced breakdown of the α-Cr₂O₃ depend on Cl coverage and the presence of O vacancy near the surface as follows: (1) neither Cl insertion (supporting the ion exchange model) nor Cr vacancy formation (supporting the point defect model) is feasible at low Cl coverages except in the presence of O vacancies near the surface, where Cl insertion is thermodynamically feasible even at low coverages, (2) in the absence of O vacancies, Cr vacancy formation becomes feasible from 10/12 ML onwards whereas Cl insertion by exchange with subsurface OH only becomes feasible at full coverage. This implies that at higher coverages Cl-induced degradation first initiatesthrough a vacancy formation mechanism, but both insertion and vacancy formation would be feasible at full coverage.

The ion exchange and point defect models are two prominent models describing the role of anions, such as chlorides, in the degradation of passive oxide films. Here the thermodynamic feasibility of critical steps of Cl-induced degradation of a hydroxylated α-Cr₂O₃ (0001) surface, as proposed by these two models, are studied. Both models begin with Cl substitution of surface OH and H₂O, which becomes less favorable with increasing Cl coverage. The initial stages of Cl-induced breakdown of the α-Cr₂O₃ depend on Cl coverage and the presence of O vacancy near the surface as follows: (1) neither Cl insertion (supporting the ion exchange model) nor Cr vacancy formation (supporting the point defect model) is feasible at low Cl coverages except in the presence of O vacancies near the surface, where Cl insertion is thermodynamically feasible even at low coverages, (2) in the absence of O vacancies, Cr vacancy formation becomes feasible from 10/12 ML onwards whereas Cl insertion by exchange with subsurface OH only becomes feasible at full coverage. This implies that at higher coverages Cl-induced degradation first initiatesthrough a vacancy formation mechanism, but both insertion and vacancy formation would be feasible at full coverage.

The ion exchange and point defect models are two prominent models describing the role of anions, such as chlorides, in the degradation of passive oxide films. Here the thermodynamic feasibility of critical steps of Cl-induced degradation of a hydroxylated α-Cr₂O₃ (0001) surface, as proposed by these two models, are studied. Both models begin with Cl substitution of surface OH and H₂O, which becomes less favorable with increasing Cl coverage. The initial stages of Cl-induced breakdown of the α-Cr₂O₃ depend on Cl coverage and the presence of O vacancy near the surface as follows: (1) neither Cl insertion (supporting the ion exchange model) nor Cr vacancy formation (supporting the point defect model) is feasible at low Cl coverages except in the presence of O vacancies near the surface, where Cl insertion is thermodynamically feasible even at low coverages, (2) in the absence of O vacancies, Cr vacancy formation becomes feasible from 10/12 ML onwards whereas Cl insertion by exchange with subsurface OH only becomes feasible at full coverage. This implies that at higher coverages Cl-induced degradation first initiatesthrough a vacancy formation mechanism, but both insertion and vacancy formation would be feasible at full coverage.

Often, addition of BiMO₃ to BaTiO₃ (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO₃ composition, that is, Bi(Zn_1/2Ti_1/2)O₃ (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO₃ ceramics, to make them better suited for dielectric or other applications.

Often, addition of BiMO₃ to BaTiO₃ (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO₃ composition, that is, Bi(Zn_1/2Ti_1/2)O₃ (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO₃ ceramics, to make them better suited for dielectric or other applications.

The corrosion behavior of 347H stainless steel was investigated in supercritical CO₂ (sCO₂) containing H₂O and O₂ simulating heat exchanger conditions that would exist in direct sCO₂ power cycles. Thermodynamic properties of oxygenated sCO₂ aqueous systems related to the corrosion phenomena was determined using NIST developed software REFPROP. The exposure tests of the corrosion samples were performed at a pressure of 80 bar and two temperatures, 50 °C and 248 °C up to 1500 hours. The samples were exposed to oxygenated H₂O-containing CO₂, and oxygenated sCO₂-containing H₂O. The corrosion rate of the alloys was determined by mass change measurements. The surface microstructure and composition of the corrosion films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD results for the corrosion products on the sample surfaces formed in the oxygenated CO₂-containg H₂O and water-containing CO₂ and O₂ at 248°C revealed the presence of Fe₂O₃ and Fe₃O₄,-rich phase, respectively. The XPS results revealed that the samples had thicker films after exposure to 0.01 O₂/ 0.04 H₂O ratio in CO₂ at 248°C than at 50°C. The outer film layer contained Fe and O and the inner layer Cr and O at 248°C. Mostly Cr and O were found at 50°C.

The corrosion behavior of 347H stainless steel was investigated in supercritical CO₂ (sCO₂) containing H₂O and O₂ simulating heat exchanger conditions that would exist in direct sCO₂ power cycles. Thermodynamic properties of oxygenated sCO₂ aqueous systems related to the corrosion phenomena was determined using NIST developed software REFPROP. The exposure tests of the corrosion samples were performed at a pressure of 80 bar and two temperatures, 50 °C and 248 °C up to 1500 hours. The samples were exposed to oxygenated H₂O-containing CO₂, and oxygenated sCO₂-containing H₂O. The corrosion rate of the alloys was determined by mass change measurements. The surface microstructure and composition of the corrosion films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD results for the corrosion products on the sample surfaces formed in the oxygenated CO₂-containg H₂O and water-containing CO₂ and O₂ at 248°C revealed the presence of Fe₂O₃ and Fe₃O₄,-rich phase, respectively. The XPS results revealed that the samples had thicker films after exposure to 0.01 O₂/ 0.04 H₂O ratio in CO₂ at 248°C than at 50°C. The outer film layer contained Fe and O and the inner layer Cr and O at 248°C. Mostly Cr and O were found at 50°C.

Mechanical property degradation due to an ordering phase transformation is a concern for alloys based on the Ni-Cr binary system (e.g., 690, 625), particularly in nuclear power applications, such as stream generator tubing, reactor pressure vessel and head control rod drive mechanism penetrations, where component lifetimes can exceed 40 years. In the present research, the disorder-order phase transformation has been studied in Ni-Cr model alloys with varying stoichiometry by experimental methods. In this paper, the effect of composition on ordering is characterized via X-ray diffraction.

Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate temperatures (~475°C) via α-α' phase separation limits upper service temperatures for many applications. The development of low Cr and Ni equivalent lean grade alloys potentially increases the upper service temperature of these alloys by delaying the onset of α-α' phase separation. The present work assesses the thermal stability of a relatively new lean grade of duplex stainless steel, alloy 2003. In this paper, alloy 2003 has been compared to the most widely used duplex alloy, 2205, through a series of isothermal agings between 260°C and 538°C for times between 1 and 10,000 hours. The thermal stability of these alloys was primarily characterized by changes in microhardness. The microhardness data were fit to a JMA-type equation to quantify embrittlement rates and predict microstructural changes out to 50 years. Additionally, as-received specimens were characterized with the scanning electron microprobe to quantify the chemistry within the ferrite grains relative to the bulk material. Alloy 2003 was shown to be much more resistant to thermal embrittlement than alloy 2205. For 50 years of service at 288°C, it is predicted that alloy 2003 components will have a change in microhardness of about 25 HK where alloy 2205 components would increase approximately 175 HK, which indicates significant embrittlement. These findings show that lean grade alloys will have a greater service temperature range than standard grades. However, additional data, characterization, and modeling are needed to better predict embrittlement kinetics over component lifetimes.

With potential energy shortages and increasing electricity demand, the nuclear energy option is being reconsidered in the United States. Public opinion will have a considerable voice in policy decisions that will “roadmap” the future of nuclear energy in this country. This report is an extension of the last author’s work on the “safety culture” associated with three engineered systems (automobiles, commercial airplanes, and nuclear power plants) in Japan and the United States. Safety culture, in brief is defined as a specifically developed culture based on societal and individual interpretations of the balance of real, perceived, and imagined risks versus the benefits drawn from utilizing a given engineered systems. The method of analysis is a modified scale analysis, with two fundamental eigenmetrics, time- (τ) and number-scales (N) that describe both engineered systems and human factors. The scale analysis approach is appropriate because human perception of risk, perception of benefit and level of (technological) acceptance are inherently subjective, therefore “fuzzy” and rarely quantifiable in exact magnitude. Perception of risk, expressed in terms of the psychometric factors “dread risk” and “unknown risk”, contains both time- and number-scale elements. Various engineering system accidents with fatalities, reported by mass media are characterized by τ and N, and are presented in this work using the scale analysis method. We contend that level of acceptance infers a perception of benefit at least two orders larger magnitude than perception of risk. The “amplification” influence of mass media is also deduced as being 100- to 1000-fold the actual number of fatalities/serious injuries in a nuclear-related accident.