The structure and mechanical properties of metallic nanocrystals

Metallic nanocrystals are ultrafine-grained polycrystalline solids with grain sizes in the range of 1 to 10 nm in at least one dimension. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and thus, they possess novel, and often improved, properties over those of conventional polycrystalline or glassy materials. In comparison to more conventional materials, nanocrystalline materials show a reduced density; increased thermal expansion, specific heat, and strength; a supermodulus effect; and extremely high diffusion rates. Traditionally brittle materials can be made ductile by nano-structure processing. At present, there is considerabe confusion on the nature of the micro-structure and mechanical properties of the nanocrystalline materials, especially of the equiaxed (three-dimensional, 3-D) type. The present article reviews the current understanding of nanocrystals and evaluates the data available on structure and mechanical properties of nanocrystalline metals. This invited overview is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presesnted as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

The science and technology of advanced structural ceramics

In recent years, the pace of developments in structural ceramics has accelerated, bringing both monolithic and composite materials closer to broad application. Although an improved scientific understanding of these materials has led to substantial advancements, their inherent brittleness remains a major challenge to use in demanding applications. There are several methods under study that may enable the materials’ properties to be mastered, but costs will have to be brought in line with competing materials to ensure widespread use.     

Joining similar and dissimilar advanced engineered materials

The typically specialized property combinations associated with advanced materials, combined with a desire for monolithic structures to maximize efficiency and performance, requires their effective joining. Through proper joining process selection and parameter optimization, both similar and ultimately dissimilar combinations of materials can be joined to produce high-performance components and systems.     

Formation of Al–Y oxide during processing of γ-TiAl

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP’ing) were found to precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA’d) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).     

ADVANCED SYNTHESIS OF LIGHT METALS

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The current status of titanium rapid solidification

Rapid solidification can have a variety of effects on the microstructure, constitution and mechanical properties of titanium-base alloys. Numerous processing methods can be used to form a wide range of rapidly solidified compositions. Among the alloy classes studied— conventional alloys, rare-earth systems, metalloid systems, eutectoid systems and intermetallics— rare-earth systems and intermetallics hold the greatest promise for developing improved alloys. Such alloys are intended for elevated-temperature aerospace applications.