Analysis of deformation mechanisms in superplastic yttria-stabilized tetragonal zirconia

There have been extensive experimental observations of changes in the apparent rate controlling creep parameters in studies on superplastic materials. The three most common explanations associated with these changes in the stress exponent, n, the activation energy Q and the inverse grain size exponent, p involve the effect of concurrent grain growth, the operation of a threshold stress or transitions in creep mechanisms. Each of these factors may influence experimental creep data in a similar manner. Therefore, a careful analysis of the consequences of all three factors must involve the development of a consistent set of experimental observations in order to adequately distinguish the effects of each. This paper discusses the role of concurrent grain growth, a threshold stress and transitions in creep mechanisms in superplastic materials. Specific attention is given to the analysis of data on superplastic yttria-stabilized zirconia ceramics for which an increase in n has been observed at low applied stresses. It is demonstrated that neither concurrent grain growth nor a threshold stress can account for all the relevant experimental observations in this material. It is concluded that the changes in rate controlling creep parameters are associated with the operation of two distinct sequential mechanisms as part of a grain boundary sliding process.

Microstructure and texture evolution during accumulative roll bonding of aluminium alloys AA2219/AA5086 composite laminates

Accumulative roll bonding of two aluminium alloys, AA2219 and AA5086 was carried out up to 8 passes. During the course of ARB, the deformation inhomogeneity between the two alloy layers results in interfacial instability after the 4th pass, necking of the AA5086 layers after the 6th pass and fracture along the necked regions after the 7th and 8th pass. The EBSD analysis shows deformation bands along the interfaces after 8 passes of ARB. The ARB-processed materials predominantly show characteristic deformation texture components. The weak texture after the 2nd pass results from the combination of a weakly-textured starting AA2219 layer and a strongly-textured starting AA5086 layer. A strong deformation texture forms due to the high imposed strain after a higher number of ARB passes. Subgrain formation and related shear banding induces copper/S components in the case of the small elongated grains, while planar slip leads to the formation of brass component in the large elongated grains.

Simple and efficient methods for discrimination of chiral diacids and chiral alpha-methyl amines

The three-component chiral derivatization protocols have been developed for H-1, C-13 and F-19 NMR spectroscopic discrimination of chiral diacids by their coordination and self-assembly with optically active (R)-alpha-methylbenzylamine and 2-formylphenylboronic acid or 3-fluoro-2-formylmethylboronic acid. These protocols yield a mixture of diastereomeric imino-boronate esters which are identified by the well-resolved diastereotopic peaks with significant chemical shift differences ranging up to 0.6 and 2.1 ppm in their corresponding H-1 and F-19 NMR spectra, without any racemization or kinetic resolution, thereby enabling the determination of enantiopurity. A protocol has also been developed for discrimination of chiral alpha-methyl amines, using optically pure trans-1,2-cyclohexanedicarboxylic acid in combination with 2-formylphenylboronic acid or 3-fluoro-2-fluoromethylboronic acid. The proposed strategies have been demonstrated on large number of chiral diacids and chiral alpha-methyl amines.