Evaluation of classical precipitation descriptions for γ′′(Ni3Nb−D022) in Ni-base superalloys

The growth/coarsening kinetics of γ′′(Ni3Nb−D022) precipitates have been found by numerous researchers to show an apparent correspondence with the classical (Ostwald ripening) equation outlined by Lifshitz, Slyozov and (separately) Wagner for a diffusion controlled regime. Nevertheless, a significant disparity between the actual precipitate size distribution shape and that predicted by LSW is frequently observed in the interpretation of these results, the origin of which is unclear. Analysis of the literature indicates one likely cause for this deviation from LSW for γ′′ precipitates is the “encounter” phenomenon described by Davies et al. (Acta Metall 28(2):179–189, 1980) that is associated with secondary phases comprising a high volume fraction. Consequently, the distributions of both γ′′ precipitates described in the literature (Alloy 718) and measured in this research in Alloy 625 are analysed through employing the Lifshitz–Slyozov-Encounter-Modified (LSEM) formulation (created by Davies et al.). The results of the LSEM analysis show good far better agreement than LSW with experimental distributions after the application of a necessary correction for what is termed in this research as “directional encounter”. Moreover, the activation energy for γ′′ coarsening in Alloy 625 shows conformity with literature data once the effect of heterogeneous (on dislocations) precipitate nucleation at higher temperatures is accounted for

Sinking of Volcanic Ash in Uncompacted Sediment in Williams Lake, Washington

Volcanic ash from the eruption of Mount St. Helens on 18 May 1980 fell into Williams Lake in eastern Washington and was temporarily suspended at the sediment-water interface. After several months of compaction, the ash layer broke up and sank into lower density uncompacted lake sediment. Stratigraphic time displacements of several hundred years and a failure to recognize discontinuous ash layers in sediment cores are possible consequences of this process

Sedimentation in a Blast-Zone Lake at Mount St. Helens, Washington-Implications for Varve Formation

Sediment collected in traps in a newly formed lake in the blast-impact area at Mount St. Helens recorded a sediment yield that is about two orders of magnitude greater than for comparable basins with vegetation and similar precipitation. Most sediment was mobilized by storms and run off at the onset of the wet season. The sedimentation response to strongly seasonal precipitation, in the absence of vegetation, produced turbidites and graded annual couplets. The style of sedimentation suggests an alternate mechanism for the formation of long sequences of graded clastic varves

A Kinesthetic Model Demonstrating Molecular Interactions Involved in Anterior-Posterior Pattern Formation in Drosophila

Prerequisites for the Developmental Biology course at Augustana College are introductory courses in zoology and cell biology. After introductory courses students appreciate the fact that proteins have three-dimensional structures; however, they often fail to recognize how protein interactions with other cellular components can lead to specific cellular responses. One of the first topics covered in Augustana's Developmental Biology course is anterior-posterior axis determination in Drosophila. In the past, the subject was taught with a series of graphs demonstrating mRNA and protein concentrations along the anterior-posterior axis. However, this pedagogy was too conceptual for the majority of students enrolled in the course. To aid their understanding, a kinesthetic model of the molecular interactions involving bicoid, nanos, hunchback, and caudal transcripts and proteins utilizing colored pipe cleaners and beads was created. Students model molecular interactions between proteins (beads) and transcripts (pipe cleaners) by placing the appropriate bead on the appropriate pipe cleaner. After working with the model, the concept of molecular interactions became more concrete to students, and they were able to conceptualize anterior-posterior axis determination in Drosophila more clearly. Throughout the rest of the course, students were able to understand molecular interactions without the aid of additional models