Interfacial dynamics in diffusion-driven phase transformations

The migration of ledges in a semicoherent α/β interface is considered to participate in solid-state transformations driven by diffusion. The advance of the ledge and/or the progress of the transformation can require the climb of misfit dislocations both in the ledge and in its path. The creation or annihilation of vacancies required for the transformation and the legde advance is provided by a combination of three vacancy sources or sinks: (a) the net vacancy flux to/from the interface resulting from the difference in lattice plane shift (Kirkendall effect) within the two contacting phases, (b) the climb of misfit dislocations from the interface into the bulk of the α and β phases, and (c) the climb of misorientation dislocations within the interface. Thus, the dynamic action of the interface during the phase transformation would include: (i) climb of misfit dislocations out of the interface, with ensuing dissociation into glissile dislocations which resupply the interface by return glide, and (ii) climb of misorientation dislocations in the interface necessitating a continuing arrival of such dislocations from sources in the bulk or in the interface.     

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.     

Stress enhanced grain boundary segregation of impurity elements in a low alloy steel

Grain boundary segregation isotherms determined by Auger electron spectroscopy (AES) have been employed to elucidate the effect of applied stress on grain boundary chemistry of a low alloy 2.6 NiCrMoV steel. A comparison of grain boundary segregation isotherms of low alloy steel isothermally heat treated at 773 K under constant load and unstressed conditions strongly indicates that stress accelerates grain boundary segregation of impurity elements N, P and S. This enhancement of grain boundary coverage of elements N, P and S is explained in terms of structure of grain boundary, described commonly as an array of linked deltahedra containing holes of varying size and shape.