Study of Persistent Slip Bands Formed by Low Cycle Fatigue on Nickel-based Superalloys at Room Temperature

The ability of Ni-based superalloys to withstand temperatures in excess of 11000C makes them useful for applications in the hottest components in jet engines, gas turbines, and thrust engines. Increasing the efficiency of these gas turbine engines helps to reduce the fossil fuel consumption and the production of greenhouse gasses. A common mode of failure in these Ni-based superalloys is low cycle fatigue, in which narrow regions of high dislocation density, which are known as persistent slip bands (PSBs), can develop and lead to crack initiation. A detailed understanding of the formation and structure of PSBs has eluded researchers. In order to better understand PSBs, we have subjected a single crystal Ni-based superalloy PWA1484 to low cycle fatigue using a strain amplitude of 0.6%. The specimen was interrupted every few 1000 cycles to polish the surface in order to remove surface steps and prevent early failure. This ensured that the single crystal specimen would develop numerous PSBs for us to characterize. We utilized scanning electron microscopy (SEM) based electron channeling contrast imaging (ECCI) to examine the formation of PSBs and to study the dislocation behavior. Using ECCI, we were able to rapidly identify the presence of PSBs under optimal imaging conditions. The structure, morphology, and orientation of the PSBs will be presented, and the application of ECCI in rapidly identifying PSBs will be discussed

Development of high temperature refractory-based multi-principle-component alloys by thermodynamic calculations and rapid alloy prototyping

Recently, new refractory-based high entropy alloys (HEAs) have been investigated for potential use as high temperature structural alloys, and some alloys exhibit excellent high temperature strength and ductility. While the high entropy alloy community is generally concerned with obtaining single phase solid-solution phases, secondary strengthening phases are usually required to achieve an adequate balance of mechanical and physical properties for structural applications. This contribution will report on new Mo,Nb-based alloys that have been developed using HEA design guidelines, as well as new tools that enable thermodynamic property predictions and rapid alloy prototyping and assessment. An elemental palette of Mo-Nb-Hf-Ta-Ti-V-W-Zr was chosen in order to promote the formation of a single body-centered cubic (BCC) solid-solution phase upon solidification, which facilitates homogenization heat treatments. Al, Cr, and Si were also included to promote secondary phase formation. These 11 elements were then used to calculate the phases present and their reaction temperatures of 3-, 4-, 5-, and 6-component alloy compositions from all of the available PandatTM databases. Mo and Nb were required to be present in each alloy composition in order to maintain modest alloy costs and densities. Please click Additional Files below to see the full abstract

The cellular senescence response and neuroinflammation in juvenile mice following controlled cortical impact and repetitive mild traumatic brain injury.

Traumatic brain injury (TBI) is a leading cause of disability and increases the risk of developing neurodegenerative diseases. The mechanisms linking TBI to neurodegeneration remain to be defined. It has been proposed that the induction of cellular senescence after injury could amplify neuroinflammation and induce long-term tissue changes. The induction of a senescence response post-injury in the immature brain has yet to be characterised. We carried out two types of brain injury in juvenile CD1 mice: invasive TBI using controlled cortical impact (CCI) and repetitive mild TBI (rmTBI) using weight drop injury. The analysis of senescence-related signals showed an increase in γH2AX-53BP1 nuclear foci, p53, p19ARF, and p16INK4a expression in the CCI group, 5 days post-injury (dpi). At 35 days, the difference was no longer statistically significant. Gene expression showed the activation of different senescence pathways in the ipsilateral and contralateral hemispheres in the injured mice. CCI-injured mice showed a neuroinflammatory early phase after injury (increased Iba1 and GFAP expression), which persisted for GFAP. After CCI, there was an increase at 5 days in p16INK4, whereas in rmTBI, a significant increase was seen at 35 dpi. Both injuries caused a decrease in p21 at 35 dpi. In rmTBI, other markers showed no significant change. The PCR array data predicted the activation of pathways connected to senescence after rmTBI. These results indicate the induction of a complex cellular senescence and glial reaction in the immature mouse brain, with clear differences between an invasive brain injury and a repetitive mild injury

The influence of stacking fault energies and solute segregation on high temperature creep strength in L12-containing Co-based Superalloys

Co-based superalloys strengthened by the γ’-(L12) phase exhibit comparable and, in some cases, superior high temperature creep resistance to 1st-generation Ni-based superalloys. Despite the comparable creep resistance between Co- and Ni-based superalloys, the high temperature creep deformation modes are markedly different: the γ’ phase in Ni-based superalloys is typically sheared via coupled a/2\u3c110\u3e matrix dislocations, whereas the γ’ phase in Co-based superalloys is sheared via Shockley superpartial a/3\u3c112\u3e dislocations, which leave superlattice intrinsic stacking faults (SISF) behind in their wake. Previous investigations have shown that the creep strength of Co-based alloys increases with increasing SISF energy. In this contribution, the SISF energies for Co3(Al,W,X) and Co3(Al,Mo,X) compounds (X = Cr, Ta, Ti, Nb, and V) are calculated using density functional theory and special quasi-random structures (SQS) in order to assess the potency for quaternary alloying additions to increase the SISF energy, and thus the high temperature creep strength. In all alloy systems except Co-Al-W-Ti, quaternary compositions exhibited higher SISF energies compared to binary or ternary compositions. This implies that higher-order alloying additions that partition to the γ’ phase will always aid to increase the SISF energy and the high temperature creep strength as well. Recent work incorporating vibrational entropy in order to determine temperature-dependent SISF energies will be presented. Additionally, recent observations via high resolution microscopy and atom probe tomography of solute segregation at the SISFs will be presented. The relationship between solute segregation, SISF energy, and high temperature creep strength will be exemplified. Please click Additional Files below to see the full abstract

Mass uptake during oxidation of metallic alloys: literature data collection, analysis, and FAIR sharing

The area-normalized change of mass ($\Delta$m/A) with time during the oxidation of metallic alloys is commonly used to assess oxidation resistance. Analyses of such data can also aid in evaluating underlying oxidation mechanisms. We performed an exhaustive literature search and digitized normalized mass change vs. time data for 407 alloys. To maximize the impact of these and future mass uptake data, we developed and published an open, online, computational workflow that fits the data to various models of oxidation kinetics, uses Bayesian statistics for model selection, and makes the raw data and model parameters available via a queryable database. The tool, Refractory Oxidation Database (https://nanohub.org/tools/refoxdb/), uses nanoHUB's Sim2Ls to make the workflow and data (including metadata) findable, accessible, interoperable, and reusable (FAIR). We find that the models selected by the original authors do not match the most likely one according to the Bayesian information criterion (BIC) in 71% of the cases. Further, in 56% of the cases, the published model was not even in the top 3 models according to the BIC. These numbers were obtained assuming an experimental noise of 2.5% of the mass gain range, a smaller noise leads to more discrepancies. The RefOxDB tool is open access and researchers can add their own raw data (those to be included in future publications, as well as negative results) for analysis and to share their work with the community. Such consistent and systematic analysis of open, community generated data can significantly accelerate the development of machine-learning models for oxidation behavior and assist in the understanding and improvement of oxidation resistance

Multi-year Observations of Mid-latitude Middle Atmospheric Winds, Waves, and Temperature associated with SSW Events over Northern Utah

We investigate the behavior of 14 years of wind, wave, and temperature observations in the middle atmosphere over northeastern Utah, USA during periods of sudden stratospheric warming events. This systematic review of the impacts of sudden stratospheric warming events on the middle atmosphere at a northern mid-latitude site is conducted using ground based measurements from imaging Doppler interferometry and meteor wind radar and Na and Raleigh lidar and space based measurements made by the Sounding of the Atmosphere Using Broadband Emission Radiometry sensor onboard the NASA sponsored Thermosphere Ionosphere mesosphere Energetics Dynamics Mission

Patent Foramen Ovale: Current Pathology, Pathophysiology, and Clinical Status

Patent foramen ovale (PFO) is experiencing increased clinical interest as a congenital cardiac lesion persisting into adulthood. It is implicated in several serious clinical syndromes, including stroke, myocardial infarction, and systemic embolism. The PFO is now amenable to percutaneous interventional therapies, and multiple novel technologies are either available or under development for lesion closure. The PFO should be better understood to take advantage of emerging percutaneous treatment options. This paper reviews PFO anatomy, pathology, pathophysiology, and clinical impact and discusses current therapeutic options