The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Particulate reinforced metal matrix composites — a review

The science governing the strength and fracture of structural ceramics has developed from a mostly empirical topic in 1965 into a mature discipline that now sets the standards in the field of mechanical behavior. The intent of this review is to provide a perspective regarding this evolution, followed by succinct descriptions of current understanding. The rapid developments in the field are considered to have commenced upon the first concerted attempt to apply fracture mechanics concepts to ceramics, beginning in the middle 1960s. This allowed a distinction between the separate contributions to strength from the flaws in the material and from the microstructure, as manifest in the fracture toughness. Another contribution that accelerated the learning process was the development of indentation techniques, which allowed trends in the damage resistance of new ceramics to be assessed on a routine basis. However, the most important development, which originated at about the same time, was the discovery of toughened zirconia alloys. The ensuing research on these alloys established two vital precedents.

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Perspective on the Development of High‐Toughness Ceramics

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

During the past decade considerable research efforts have been directed towards achieving balanced engineering properties of two-phase γ-titanium aluminide alloys for future applications as structural materials. For optimization of mechanical properties such as yield and creep strengths, tensile ductility and fracture resistance, a basic understanding of the temperature dependent micromechanisms of plasticity and fracture, and their interplay with various microstructural constituents is required. In this review article, the current knowledge on dislocation types and slip systems, the development of deformation substructures, factors controlling the mobility and multiplication of dislocations, interface related plasticity, solid solution and precipitate strengthening mechanisms as well as microscopic aspects of creep and fracture will be addressed. These topics will be related to specific microstructures and associated engineering properties.

Microstructure and deformation of two-phase γ-titanium aluminides

Recent advances in aerosol generation of materials are reviewed Gas-to-particle and spray processes (spray pyrolysis) for powder generation and various routes for film generation are discussed from the experimental and theoretical perspectives The range of materials generated by these routes has increased in recent years to include fullerenes and ceramic superconductors Many metals and various oxide and nonoxide ceramics have also been added to the list of materials generated by gas-phase routes Established aerosol routes such as vapor condensation have found widespread applications for generation of nanophase materials The formation of quantum dots via aerosol approaches has also been demonstrated The theoretical understanding of gas-to-particle conversion routes has advanced to include the finite rate of particle fusion or sintering occurring after collisions of particles The modeling of spray pyrolysis systems has provided insight into the control of particle morphology and reactor design In th

https://www.tandfonline.com/doi/pdf/10.1080/02786829308959650

Aerosol Processing of Materials

Phase equilibria and solidification in Ti-Al alloys

The solidification of undercooled spherical droplets with a discrete melting temperature is analyzed using both a Newtonian and a non-Newtonian (Enthalpy) model. Relationships are established between atomization parameters, the growth kinetics, the interface velocity and undercooling, and other important solidification variables. A new mathematical formulation and solution methodology is developed for simulating the solidification process in an undercooled droplet from a single nucleation event occurring at its surface. The computational mesh used in the enthalpy model is defined on a superimposed bispherical coordinate system. Numerical solutions for the solidification of pure aluminum droplets based on the enthalpy model are developed, and their results are compared to the trends predicted from the Newtonian model. The implications of single vs multiple nucleation events are also discussed. In general, the results indicate that when substantial undercoolings are achieved in a droplet prior to nucleation, the thermal history consists of two distinct solidification regimes. In the first, the interface velocities are high, the droplet absorbs most of the latent heat released, and the external cooling usually plays a minor role. The second regime is one of slower growth, and strongly depends on the heat extraction at the droplet surface. The extent of “rapid solidification”, as determined from the fraction of material solidified at temperatures below a certain critical undercooling, is a function of the nucleation temperature, the particle size, a kinetic parameter, and the heat translow as 10~4.

Heat Flow during Rapid Solidification of Undercooled Metal Droplets

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α2 (DO19) and γ (Ll0) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB2 (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB2 for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB2 are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α2 and later decompose to a lath structure consisting of alternating layers of γ and α2.

Microstructure evolution in tial alloys with b additions: Conventional solidification

Electrohydrodynamic atomization’has been adapted to produce Al2O3 powders ranging in size from 10 nm to 300 μm. Microstructural characterization using x-ray diffraction, scanning, and transmission electron microscopy reveals changes in phase selection as a function of particle size, hence supercooling. Amorphous powders are common below 100 nm in diameter. Cubic spinel γ is found in single phase form between 100 nm and 2μm, and partially transformed to δ between 2 and 20 μm. There is also evidence of σ and θ or a precursor of θ forming directly from the liquid above 5 μm. The stable corundum structure is consistently found above 20 μm but exhibits three different morphologies: faceted, dendritic, and cellular. Phase selection is examined on the basis of fundamental thermodynamic and kinetic considerations and results from computer models predicting the thermal history of the powders. It is concluded that metastable phases require the elimination of catalytic sites for the nucleation of a and are thus more likely to form in the smaller powders. Furthermore, submicron powders achieve sufficiently high cooling rates to preserve the metastable phases formed (γ), but those higher than ∼ 1 μm experience a thermal excursion long enough to transform γ to more stable forms of Al2O3.

R. Mehrabian

Papers

Phase equilibria and solidification in Ti-Al alloys

Heat Flow during Rapid Solidification of Undercooled Metal Droplets

Microstructure evolution in tial alloys with b additions: Conventional solidification

Phase selection in electrohydrodynamic atomization of alumina

A test procedure for characterizing the toughening of brittle intermetallics by ductile reinforcements