Rajiv Asthana

Pressure infiltration technique for synthesis of aluminum–fly ash particulate composite

Abstract Beds of nickel coated and uncoated cenosphere fly ash can be successfully infiltrated by molten aluminum under very low pressures. The density of the resultant composite is ≈1.4 and 1.2 g cm−3 respectively, compared to 2.68 g cm−3, the density for aluminum. The threshold pressure was found to be between 20.68 and 27.58 kPa for infiltration of molten pure aluminum into uncoated fly ash, and around 6.7 kPa for Ni-coated fly ash. These data were used to back calculate an effective wetting angle between molten pure aluminum and fly ash using Young’s and Washburn equations. The average value of the wetting angle calculated for uncoated fly ash is 111°. The microstructures and chemical composition of the composites made with coated and uncoated fly ash were studied using optical and scanning microscopy; they show good infiltration except for regions between contacting cenospheres. They also show transport of nickel from the bottom to the top of the sample when nickel coated particles were used.

A map for wear mechanisms in aluminium alloys

A quantitative wear map for aluminium and its alloys has been constructed using normalized test variables and the physical modelling approach proposed by Lim and Ashby for steels. New model equations based on a different state-of-stress criterion suitable for aluminium alloys have been developed and found to match well with reported experimental wear data on aluminium alloys. The field boundaries between various interfacing wear mechanisms were constructed by using critical values of experimental wear data which manifest themselves in discontinuities in the slope of wear curves. However, within a given wear regime, the model equations developed here agreed fairly well with the reported wear data. The wear mechanisms successfully modelled here include oxidation dominated wear, delamination wear, severe plastic deformation wear, and melt wear.

The engulfment of foreign particles by a freezing interface

The interactions of second-phase particles, liquid droplets or gas bubbles with a solidification front form the basis of various materials synthesis and purification processes and the design of microstructures in cast metal-matrix composites, as well as frost heaving and biological cell interactions. The physical mechanisms of this interaction phenomenon are based upon surface thermodynamic factors, solidification parameters, and fluid dynamic effects such as fluid drag and buoyancy. An overview is presented of the role of various factors which determine the nature as well as the kinetics of foreign particle-solidification front interactions, and the current status and limitations of the various theoretical models of the phenomenon.

Reinforced cast metals: Part I Solidification microstructure

The solidification process in fibre- and particle-reinforced metals is modified due to solute screening, thermal shielding, heterogeneous nucleation, fluid damping, particle pushing and morphological instabilities at the liquid–solid–fibre boundary. These factors lead to considerable variations in grain size, grain morphology, microsegregation and macro-segregation, and reinforcement distribution in the matrix. This article examines the roles of the above factors in the evolution of solidification microstructure in composites under controlled growth as well as under normal casting conditions.

Reinforced cast metals : Part II Evolution of the interface

Interface evolution in metal-matrix composites is a thermodynamic necessity and interface design a kinetics challenge. The synergistic interaction between processing science and surface engineering has led to considerable progress in understanding, modelling and tailoring the fibre–matrix interface at the microstructural, crystallographic and atomic levels. The chemical, morphological, crystallographic and thermoelastic compatibilities between the fibre and the matrix influence the interfacial adhesion strength. This article examines the role of material properties and fabrication conditions in chemical interactions between the fibre and the matrix in metal-matrix composites synthesized using the solidification and casting techniques.

Interfaces in cast metal-matrix composites

Abstract A detailed analysis of possible interfaces in cast aluminum silicon base reinforced particle composites containing SiC, Al 2 O 3 and C indicates that several different kinds of interfaces can form. The reinforcements may be totally surrounded by primary-phase, or primary silicon, or by the eutectic between Al and Si. In addition, some of the original coatings or their reaction products in the case of coated particles (such as nickel or nickel-aluminum intermetallics in nickel coated reinforcements Cu or Cu-aluminum intermetallics in Cu coated reinforcements) may also form the interface. The reaction between dispersoids and the alloy itself can form a complex interface. These different interfaces have also been experimentally observed in the microstructures of cast particulate composites, with the exception of primary α-aluminum surrounding the reinforcement. The absence of α-aluminum on the reinforcements is attributed to possible lack of nucleation, persistent lateral growth and a thermal lag between the reinforcement and the matrix. Estimates of works of adhesion for the different interfaces observed in cast composites have been made using London-van der Waal equation, correlated to the properties of the composites, and used to identify the possibilities of further improving these properties.

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.

Interfacial and capillary phenomena in solidification processing of metal-matrix composites

Abstract Chemical and hydrodynamic aspects of wetting and interfacial phenomena during the solidification processing of metal-matrix composites are reviewed. Significant experimental results on fibre-matrix interactions and wetting under equilibrium and non-equilibrium conditions in composites of engineering interest have been compiled, based on a survey of the recent literature. Finally, certain aspects of wetting relevant to stir-casting and infiltration processing of composites are discussed.

Energetics of particle transfer from gas to liquid during solidification processing of composites

The energetics for equilibrium transfer of solid particles in the shape of prolate spheroids across a stationary gas-liquid interface is computed as a function of the path of submersion. By considering the energy for submersion as comprising surface, buoyancy, and potential energies, equations have been derived for these energies in terms of particle size, degree of submersion, contact angles, surface tension, and densities of particle and liquid. The results for wetting particles show that certain intermediate configurations of partially submerged particles can be energetically stable even though this is not reflected in the calculations based only on initial and final configurations. The results of the analysis are discussed in the context of synthesis by stir casting of metal-matrix composites where the externally added particles or fibers must cross over in succession a gas-oxide interface, oxide-melt interface, and melt-solidification front interface.

The solidification of metal-matrix particulate composites

The simplicity, economy and flexibility of solidification processes make them attractive methods for the production of particle-reinforced metal-matrix composites. At present, however, there is limited understanding of the phenomena occurring during solidification of these advanced materials. Nucleation and refinement of crystalline phases, physical and chemical interactions between dispersed particles and solidifying interfaces, and buoyancy-driven movement of the particles are areas where a knowledge base is beginning to be formed. Ultimately, the understanding of solidification processes in metal-matrix composites must become complete enough that microstructures can be tailored for specific applications.