Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy

Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments

Basis for Calculating Cross Sections for Nuclear Magnetic Resonance Spin-Modulated Polarized Neutron Scattering

In this work we study the potential for utilizing the scattering of polarized neutrons from nuclei whose spin has been modulated using nuclear magnetic resonance (NMR). From first principles, we present an in-depth development of the differential scattering cross sections that would arise in such measurements from a hypothetical target system containing nuclei with non-zero spins. In particular, we investigate the modulation of the polarized scattering cross sections following the application of radio frequency pulses that impart initial transverse rotations to selected sets of spin-1/2 nuclei. The long-term aim is to provide a foundational treatment of the scattering cross section associated with enhancing scattering signals from selected nuclei using NMR techniques, thus employing minimal chemical or isotopic alterations, so as to advance the knowledge of macromolecular or liquid structure

B812: Dairy Farmer Indebtedness in Maine

The dairy industry in Maine is an important contributor to the agricultural sector and general economy. In 1982 there were 750 employees processing dairy products in Maine drawing a 12 million dollar payroll (Maine Bureau of Labor). The 1983 farm-gate value of milk produced in Maine totaled 108 million dollars, higher than any other single commodity\u27s farm-gate value (Maine Department of Agriculture, Food and Rural Resources). For the past several years the farm level price of milk has remained fairly steady while production costs inflated. This price-cost squeeze worsened in 1983 with a 50 cent per hundredweight decrease in the price received by farmers which was authorized by the Dairy and Tobacco Adjustment Act of 1983. Limited public information is available concerning the financial health of Maine\u27s dairy farming sector. This aspect is of crucial concern to policy makers in the state. Toward this end the Maine Dairy Industry Association requested that the University of Maine at Orono, in cooperation with the Maine Department of Agriculture, Food and Rural Resources, conduct a study to provide an accurate overall picture of the financial structure and business management practices of Maine\u27s dairy farms.https://digitalcommons.library.umaine.edu/aes_bulletin/1111/thumbnail.jp

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Thesis (Ed.M.)--Boston Universit

Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centered cubic (fcc) solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa, failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa.m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.Comment: 7 pages, 4 figure

Model for Screened, Charge-Regulated Electrostatics of an Eye Lens Protein: Bovine GammaB-Crystallin

We model screened, site-specific charge regulation of the eye lens protein bovine gammaB-crystallin (γB) and study the probability distributions of its proton occupancy patterns. Using a simplified dielectric model, we solve the linearized Poisson-Boltzmann equation to calculate a 54 × 54 work-of-charging matrix, each entry being the modeled voltage at a given titratable site, due to an elementary charge at another site. The matrix quantifies interactions within patches of sites, including γB charge pairs. We model intrinsic pK values that would occur hypothetically in the absence of other charges, with use of experimental data on the dependence of pK values on aqueous solution conditions, the dielectric model, and literature values. We use Monte Carlo simulations to calculate a model grand-canonical partition function that incorporates both the work-of-charging and the intrinsic pK values for isolated γB molecules and we calculate the probabilities of leading proton occupancy configurations, for 4 \u3c pH \u3c 8 and Debye screening lengths from 6 to 20 A. We select the interior dielectric ˚ value to model γB titration data. At pH 7.1 and Debye length 6.0 A, on a given ˚ γB molecule the predicted top occupancy pattern is present nearly 20% of the time, and 90% of the time one or another of the first 100 patterns will be present. Many of these occupancy patterns differ in net charge sign as well as in surface voltage profile. We illustrate how charge pattern probabilities deviate from the multinomial distribution that would result from use of effective pK values alone and estimate the extents to which γB charge pattern distributions broaden at lower pH and narrow as ionic strength is lowered. These results suggest that for accurate modeling of orientation-dependent γB-γB interactions, consideration of numerous pairs of proton occupancy patterns will be needed

Fine Particulate Matter Constituents Associated with Cardiovascular Hospitalizations and Mortality in New York City

Bac k g r o u n d: Recent time-series studies have indicated that both cardiovascular disease (CVD) mortality and hospitalizations are associated with particulate matter (PM). However, seasonal patterns of PM associations with these outcomes are not consistent, and PM components responsible for these associations have not been determined. We investigated this issue in New York City (NYC), where PM originates from regional and local combustion sources. Obj e c t i v e: In this study, we examined the role of particulate matter with aerodynamic diameter ≤ 2.5 µm (PM2.5) and its key chemical components on both CVD hospitalizations and on mortality in NYC. Met h o d s: We analyzed daily deaths and emergency hospitalizations for CVDs among persons ≥ 40 years of age for associations with PM2.5, its chemical components, nitrogen dioxide (NO2), carbon monoxide, and sulfur dioxide for the years 2000–2006 using a Poisson time-series model adjusting for temporal and seasonal trends, temperature effects, and day of the week. We estimated excess risks per interquartile-range increases at lags 0 through 3 days for warm (April through September) and cold (October through March) seasons. Re s u l t s: The CVD mortality series exhibit strong seasonal trends, whereas the CVD hospitalization series show a strong day-of-week pattern. These outcome series were not correlated with each othe