Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature

Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of ∼26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and ∼24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.     

Silicon carbide filaments: Microstructure

The microstructure of chemically vapour deposited silicon carbide filaments has been examined using transmission electron microscopy. The filament bulk consisted of heavily faulted columnar subgrains of-SiC which were preferentially oriented such that {1 1 1} planes were parallel to the surface of the carbon fibre substrate. The protective coating on the filament surface was characterized by several microstructurally distinct layers, all of which consisted primarily of carbon. The first layers of the coating contained small crystallites of SiC in addition to turbostratic carbon, while the outer layers showed no evidence of SiC. Implications of the filament microstructure with respect to mechanical properties are discussed.     

Effect of thermal hysteresis on the gas permeation properties of 6FDA‐based polyimides

The effect of thermal hysteresis on the polymer chain packing and permeation properties of two 6FDA‐based polyimide isomers was investigated. Thermal quenching resulted in a small increase in the fractional free volume of the polyimides with respect to the samples that had been annealed. Quenching from above the glass‐transition temperature also resulted in larger increases in the permeabilities for both 6FDA–6FmDA and 6FDA–6FpDA with respect to annealed samples. Meta‐connected 6FDA–6FmDA exhibited a larger increase in the permeability after quenching than the para‐connected isomer, 6FDA–6FpDA. This larger increase in the permeability for 6FDA–6FmDA may have been due to differences in the effects of the increases in the free volume on the intersegmental resistance to chain motions. Although physical aging over a 3‐month period resulted in a reduction in the permeability of quenched samples of 6FDA–6FpDA, the quenched samples maintained higher permeabilities than the annealed samples. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1174–1182, 2004     

Nucleation and Growth Characteristics of Cavities during the Early Stages of Tensile Creep Deformation in a Superplastic Zirconia–20 wt% Alumina Composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.     

Cell-structure and mechanical properties of closed-cell aluminum foam

1　INTRODUCTIONMetalfoamsexhibitunusualmechanical,ther mal,acoustic ,damping ,electricalandchemicalprop ertiesthatcannotbefoundinsolidmaterials .Thosespecialpropertiesmayleadtoavarietyofapplicationsinstructuralandfunctionalproducts.Closed cellalu minumfoamisalight massstructuralmaterialofgreatpromise .Itsdensityisonlyafractionofthatofthecorrespondingbulkmetal,butitsstrengthissuf ficientlyhightobeusedintheautomobileindustry ,buildingindustryandtransportation .Examplesoftheirapplicationsinc…     

Trends in psychotherapy process research: Samples, measures, researchers, and classic publications.

Psychotherapy studies published in the Journal of Counseling Psychology (JCP) and the Journal of Consulting and Clinical Psychology (JCCP) between 1978 and 1992 were examined. Differences were found between the 2 journals. JCP published mostly process, outcome, and analog research, whereas JCCP published mostly outcome research. Most process and process-outcome studies across journals were of individual, brief therapy. Across the years, more diversity was evident in samples used in process research in terms of student status, gender, and race of clients and therapists. Three content areas were prominent in the process measures and classic studies: therapist techniques, therapist influence, and facilitative conditions. Lists are provided of the frequently used measures, most productive authors, and classic studies in process research. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Combination problems for commutative/monoidal theories or how algebra can help in equational unification

We study the class of theories for which solving unification problems is equivalent to solving systems of linear equations over a semiring. It encompasses important examples like the theories of Abelian monoids, idempotent Abelian monoids, and Abelian groups. This class has been introduced by the authors independently of each other as commutative theories (Baader) and monoidal theories (Nutt).We show that commutative theories and monoidal theories indeed define the same class (modulo a translation of the signature), and we prove that it is undecidable whether a given theory belongs to it. In the remainder of the paper we investigate combinations of commutative/monoidal theories with other theories. We show that finitary commutative/monoidal theories always satisfy the requirements for applying general methods developed for the combination of unification algorithms for disjoint equational theories.Then we study the adjunction of monoids of homomorphisms to commutative/monoidal theories. This is a special case of a non-disjoint combination, which has an algebraic counterpart in the corresponding semiring. By studying equations over this semiring, we identify a large subclass of commutative/monoidal theories that are of unification type zero. We also show with methods from linear algebra that unitary and finitary commutative/monoidal theories do not change their unification type when they are augmented by a finite monoid of homomor-phisms, and how algorithms for the extended theory can be obtained from algorithms for the basic theory.     

Dynamic Micro-Strain Analysis of Ultrafine-Grained Aluminum Magnesium Alloy Using Digital Image Correlation

Tensile tests were performed in situ on an ultrafine-grained (UFG) Al-Mg alloy using a micro-tensile module in a scanning electron microscope. The micro-strain evolution was tracked and measured using digital image correlation (DIC). A fine random speckle pattern was required to achieve high resolution and accuracy of strain measurement using DIC. To produce the speckle pattern, a patterning method was developed using electron beam lithography to deposit a gold speckle pattern. The nanoscale feature size of this gold pattern (45 nm) was useful for identifying the micro-strain among individual grains of the UFG Al-Mg alloy. Microstructural aspects of the UFG Al-Mg alloy were revealed by analysis of electron backscattered diffraction (EBSD) patterns. Finally, the effect of the UFG Al-Mg alloy microstructure on the nanoscale deformation mechanism was investigated by combining EBSD and DIC data in a contour map. This combined technique provides a method for direct measurement of micro-strain and is potentially useful for deformation studies of a wide range of nanostructured materials.     

Effects of Preprocessing on Multi-Direction Properties of Aluminum Alloy Cold-Spray Deposits

The effects of powder preprocessing (degassing at 400 °C for 6 h) on microstructure and mechanical properties of 5056 aluminum deposits produced by high-pressure cold spray were investigated. To investigate directionality of the mechanical properties, microtensile coupons were excised from different directions of the deposit, i.e., longitudinal, short transverse, long transverse, and diagonal and then tested. The results were compared to properties of wrought 5056 and the coating deposited with as-received 5056 Al powder and correlated with the observed microstructures. Preprocessing softened the particles and eliminated the pores within them, resulting in more extensive and uniform deformation upon impact with the substrate and with underlying deposited material. Microstructural characterization and finite element simulation indicated that upon particle impact, the peripheral regions experienced more extensive deformation and higher temperatures than the central contact zone. This led to more recrystallization and stronger bonding at peripheral regions relative to the contact zone area and yielded superior properties in the longitudinal direction compared with the short transverse direction. Fractography revealed that crack propagation takes place along the particle-particle interfaces in the transverse directions (caused by insufficient bonding and recrystallization), whereas through the deposited particles, fracture is dominant in the longitudinal direction.