Effect of plasma spray processing variations on particle melting and splat spreading of hydroxylapatite and alumina

Splats of hydroxylapatite (HA) and alumina were obtained via plasma spraying using systematically varied combinations of plasma velocity and temperature, which were achieved by altering the primary plasma gas flow rate and plasma gas composition. Particle size was also varied in the case of alumina. Splat spreading was quantified via computer- aided image analysis as a function of processing variations. A comparison of the predicted splat dimensions from a model developed by Madejski with experimental observations of HA and alumina splats was performed. The model tended to underestimate the HA splat sizes, suggesting that evaporation of smaller particles occurred under the chosen experimental conditions, and to overestimate the observed alumina splat dimensions. Based on this latter result and on the surface appearance of the substrates, incomplete melting appeared to take place in all but the smaller alumina particles. Analysis of the spreading data as a function of the processing variations indicated that the particle size as well as the plasma temperature and velocity influenced the extent of particle melting. Based on these data and other considerations, a physical model was developed that described the degree of particle melting in terms of material and processing parameters. The physical model correctly predicted the relative splat spreading behavior of HA and alumina, assuming that spreading was directly linked to the extent of particle melting

Transient liquid-phase sintering of ceramic-reinforced Fe-based composites

The microstructural development of ceramic-reinforced iron-based composites has been studied. The composites were fabricated via powder metallurgy and liquid-phase sintering, a processing route which achieves near-net-shape with good ceramic particulate dispersion. Two matrix alloys were used, Fe-1 wt% C-1 wt% Si and Fe-2 wt% Cu; up to 30 wt% (≈36 vol%) yttria-stabilized zirconia in the form of ∼20 μm particles was added to these alloys. The microstructural evolution of these composite materials was studied by examining the densification rate and volume fraction of liquid phase as a function of time. Different particle/matrix interfaces developed in the two composites. A glassy silicon-rich layer formed in the Fe-1C-1Si-YSZ composites and a more limited crystalline layer was found in the Fe-2Cu-YSZ composites. © 1991 Chapman & Hall

The influence of interfacial structure on the mechanical properties of liquid-phase-sintered aluminum-ceramic composites

The effect of interfacial structure on the mechanical properties of aluminum-ceramic composite materials fabricated by liquid phase sintering was studied. The composites were based on two matrix alloys (powder metallurgy alloys 201 and 601) reinforced with either Al2O3 or SiC particulate. Characterization of the interfacial regions demonstrated that the SiC-matrix interfaces were faceted whereas the Al2O3-matrix interfaces had an incomplete layer of a silicon-rich amorphous phase. Preferential attack of the particles during sintering is believed to cause the crystallographic facets to form on SiC. Locally high silicon concentrations near Al2O3 particles led to the formation of a glassy phase from the reduction of Al2O3. The difference in interfacial structure resulted in a higher particle-matrix bond strength and therefore improved composite mechanical properties in the SiC-reinforced materials compared with the Al2O3-reinforced materials. © 1990

Liquid phase sintered metal matrix composite materials

Iron-base and aluminum-base composite materials reinforced with various ceramic particulates have been fabricated via powder metallurgy and liquid phase sintering. The advantage of this manufacturing route is that conventional powder metallurgy processing equipment can be used to fabricate metal matrix/ceramic composites. Furthermore, this approach makes it possible to manufacture these composites to near-net-shape. A number of matrix/ceramic combinations have been examined: Fe-C-Si and Fe-Cu with ZrO2 additions and a Al-Cu-Si-Mg alloy with SiC or Al2O3 additions. The interfacial structures were characterized and found to play a significant role in controlling the properties of the composites. Reinforcement was observed in several systems. However, a glassy interfacial layer forms when Si additions and oxide reinforcements are present; the resultant particle/matrix bond strength is weak and reinforcement. © 1990, Taylor & Francis Group, LLC. All rights reserved

The dependence of yield strength on lamellar termination density in Co-CoAl eutectic alloys

The compressive yield strength of directionally solidified Co-CoAl eutectic alloys was measured in specimens containing a range of lamellar termination densities but at approximately a constant interlamellar spacing. The yield strength was found to decrease as the transverse lamellar termination density increased. A linear dependence of yield strength on the distance between the terminations is observed provided the spacing between terminations is less than 500 μ. For spacings greater than this, the yield strength is approximately constant. These observations are attributed to a relaxation in constraint on deformation between the two phases due to the initiation of enhanced slip in the vicinity of the terminations. © 1985

The influence of γ-γ′ eutectic on the mechanical properties of conventionally cast MAR-M247

The influence of γ-γ′ eutectic on the yield strength and ductility of conventionally cast MAR-M 247 was examined. The amount of the γ-γ′ eutectic present at the interdendritic-grain boundary regions was varied by heat treatment below and above the γ′ solvus temperature. The room temperature yield strength increased and the ductility decreased as the amount of γ-γ′ eutectic was reduced. These results are attributed to the fact that the γ-γ′ eutectic possesses a lower strength and greater ductility than the dendrite cores do. As a consequence, the ductile γ-γ′ eutectic hinderes crack propagation and causes a changes in the fracture morphology as the amount of γ-γ′ eutectic is reduced. © 1988

Deformation and fracture in directionally solidified Co-CoAl eutectic

The effect of growth defects known as lamellar terminations on the yielding and fracture behaviour of Co-CoAl eutectic single crystals was studied using tensile tests and finite-element modelling. The yield strength and strain to fracture were found to decrease with increasing termination density. Observations of deformed surfaces and serial sectioning experiments on fractured tensile specimens revealed that crack initiation during the fracture process was enhanced by the presence of lamellar terminations. The fracture surfaces were found to have a staircase-type appearance, which indicated that the final fracture process was discontinuous with a step-wise propagation from one CoAl lamella to adjacent CoAl lamellae. A computer simulation was conducted to determine the stress distributions about lamellar terminations in model microstructures, since the experimental results suggested that the lamellar terminations behaved as stress concentrations in the microstructure. The finite-element calculation confirmed that lamellar terminations can influence the yielding process; the stress at which the first slip system was activated was found to decrease with increasing termination density

Dislocation substructures in doped sapphire (α-Al \u3c inf\u3e 2 O \u3c inf\u3e 3 ) deformed by basal slip

The dislocation substructures in sapphire (α-Al2O3) doped with isovalent (Cr3+ and Ti3+) and aliovalent (Ti4+ and Mg2+) solutes and deformed by basal slip at 1500-1520°C have been studied by transmission electron microscopy. The dislocation substructures in the Cr3+- and Ti3+-doped sapphire are similar to those in undoped sapphire; however, the aliovalent solutes (and their associated charge-compensating defects) cause changes in the dislocation substructures, particularly in the density and distribution of small dislocation loops produced by breakup of dislocation dipoles. Very few loops are present in Mg2+-doped sapphire, while prominent strings of loops are present in Ti4+-doped sapphire, although only at small strains. These and additional changes in the dislocation substructures are attributed to an increase in the bulk diffusion kinetics on doping with Mg2+ and a decrease in the bulk diffusion kinetics on doping with Ti4+. © 1982

Micromechanisms of deformation in high-purity hot-pressed alumina

A high-strength aluminum oxide was produced by vacuum hot pressing high-purity, submicron-size alumina powders. The uniaxial compressive fracture strength was strongly strain-rate sensitive and varied from 5.5 GPa at 10-4 s-1 to 8.3 GPa at 103 s-1. A Hugoniot elastic limit of about 11.9 GPa was determined from flyer plate impact tests. The deformation/fracture process was examined using both uniaxial stress and uniaxial strain conditions. Under a uniaxial stress condition, microplasticity was observed in the form of aligned dislocations that appeared similar to shear bands in metals. Under a uniaxial strain condition, extensive dislocation activity, grain boundary microcracking and occasional twins were observed. Based on the experimental results and microscopic observations, possible mechanisms responsible for the observed high strength and high strain-rate sensitivity in this alumina are discussed. (C) 2000 Elsevier Science S.A. All rights reserved