When a Ni-8A1 (wt pct) alloy is aged at 1020 °C (30 °C below the solvus temperature of about 1050 °C) by cooling directly from the solution-treatment temperature, theγ′ precipitates undergo Ostwald ripening from the early stage of the aging treatment because the nucleation rate is relatively high. Theγ′ precipitates are initially spheres and, as they coarsen, become cubes which then split into eight smaller cubes (octets) or clusters with square and rectangular faces. The observed critical sizes for these shape transformations are fairly close to those predicted by Khachaturyanet al. on the basis of the elastic strain effect arising from lattice misfit if the lattice parameters measured by Hornbogen and Roth are used. As the octets coarsen, they appear to transform into doublets of plates. Because shapes other than those considered by Khachaturyanet al. are also observed, only the sphere-cube and cube-octet transformations can be compared with the theoretical prediction without ambiguity. At temperatures above 1020 °C, dendritic growth occurs, and at low temperatures, the precipitate number density is so high that apparently the elastic fields of the precipitates overlap, rearranging the precipitates into aligned cubes or plates. These observations show that the shape evolution of the isolated precipitates can be observed only in a narrow aging temperature range where the nucleation rate is moderately high but where the precipitate volume fraction is relatively low.

The use of aluminum alloys for automotive heat exchangers has increased considerably in the last 15 to 20 years, and in parallel, new alloys have been developed to meet the increased demand for higher strengths and improved corrosion resistance. An Al-Mn alloy, X800, has been developed by Alcan to significantly increase the corrosion resistance of radiator tubes when subjected to typical service environments. Conventional alloy tubes, 3xxx or 6xxx, fail by intergranular attack, whereas X800 utilizes the diffusion of Si during brazing to form a sacrificial layer between core and cladding and thus prevent penetration through the core. The Si penetrates up to a depth of 70 µm into the core alloy and combines with both the Mn in solid solution and the coarse constituent particles to form theα-AlMnSi phase. In contrast to the core, the interface layer exhibits a high dispersoid density, a modified coarse particle chemistry, and a lower Mn level in solid solution after brazing. Three layers remain after brazing; anα-Al residual cladding, the interface layer with a band of dense precipitates (BDP), and the X800 core. Free corrosion potential measurements confirmed the lowering of the potential within the BDP by about 30 mV compared to —710 mV for the brazed X800 core.

This article describes the development of elastic and plastic constitutive relations as functions of relative density for partially consolidated —100 mesh aluminum powder. First, measurements of yield stress as a function of stress state and relative density are described. Measurements of the plastic strain increments associated with yielding in unconstrained compression tests and elastic properties, both as functions of relative density, are also described. The experimental results are combined with the associated flow rule to show that the yield surface is asymmetric with respect to hydrostatic tension and compression. Second, it is shown that the yield stress results can be represented by a two-part (capped Drucker-Prager) yield surface. The consoli-dation yield surface moves along the hydrostatic stress axis during densification, while the shear yield surface approaches the Mises yield surface. For the Al powder used in the present inves-tigation, superposition of shear stress on a hydrostatic stress state aids the densification process. However, the hydrostatic stress requirement was found to be reduced by only about 20 pct for relative densities below 0. 98.

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σys E)1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.

The relationships between the longitudinal and thickness strains and between the apparent plastic Poisson's ratio and the relative density achieved during hot rolling of porous nickel strips of initial relative densities 0.36, 0.48, and 0.67 are experimentally determined. The experimental data have been found to agree well with the results predicted from analytical relationships. Hot rolling of such porous metal strips can be divided into three distinct stages. In stage I, the powder particles rearrange and restack as the rolling progresses and the longitudinal elongation and apparent plastic Poisson's ratio are negligibly low. The end of this stage is marked by the situation when no further densification without longitudinal flow is possible by hot rolling. In the present case, stage I continues until a relative density of 0.7 has been achieved in the strip, resulting into a predominantly orthorhombic pattern of particle arrangement. Stage II,i.e., densification from the relative density from 0.70 to 0.95, is associated with the continuous increase in interparticle contact area, longitudinal elongation, and apparent plastic Poisson's ratio as the hot rolling progresses. In this stage, the interconnected porosity changes to the isolated one. In stage III,i.e., densification beyond the relative density of 0.95, pore fragmentation and collapsing of the opposite pore surfaces take place until all the porosity has been eliminated.

The solidification of undercooled Ti3Sn melts was investigated using electromagnetic levitation and electrohydrodynamic atomization experiments followed by extensive microstructural char- acterization. The study was motivated by several reports on the kinetic preference for the body- centered cubic (bcc) phase over more closely packed disordered and ordered structures during competitive crystallization from undercooled melts. At low undercoolings, Ti3Sn melts yield the equilibrium ordered hexagonal DO19 structure, which is retained without change upon cool- ing. Undercoolings between ~100 and ~300 K yield primary dendrites with hexagonal sym- metry but a final microstructure which is clearly martensitic in origin. Two previously unknown metastable forms of Ti3Sn were identified: an ordered base-centered orthorhombic derived from the α martensite and an ordered monoclinic phase related to the face-centered orthorhombic martensite observed in the Ti-V system. Both phases are believed to evolve from the solid state transformation of a high temperature β phase, but the dendritic structure clearly indicates the formation of a hexagonal phase different from DO19,i.e., α. The latter forms in preference to β, which has a larger driving force in at least part of the undercooling regime studied. It is proposed that the primary α transforms to β as a consequence of recalescence, which subse- quently transforms martensitically and orders to yield the observed metastable forms of Ti3Sn.

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α 2 lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.

Pertinent failure processes in unidirectional fiber metal-matrix composites (MMCs) have been identified and analyzed. The critical conditions for interface delamination in several composites are compared with the theoretical delamination diagram proposed by He and Hutchinson, in which interface delamination and fiber fracture are delineated on the basis of their relative tough-ness values. It is shown that the delamination diagram does not provide information about the extent of interface cracking or the onset of fiber bridging. An alternative failure diagram that depicts composite fracture processes, such as matrix yielding followed by fiber fracture, inter-face cracking, and fiber bridging, is proposed. Development of the composite failure diagramvia micromechanical modeling of individual mechanisms is presented together with experimental results from the literature. Good correlation between theory and experiment suggests that the composite failure diagram might be used for tailoring composite properties through control of the dominant fracture mechanism.