Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.

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Review: liquid phase sintering

Semisolid metal (SSM) processingis a relatively new technology for metal forming. Different from the conventional metal forming technologies which use either solid metals (solid state processing) or liquid metals (casting) as starting materials, SSM processing deals with semisolid slurries, in which non-dendritic solid particles are dispersed in a liquid matrix. Semisolid metal slurries exhibit distinctive rheological characteristics: the steady state behaviour is pseudoplastic (or shear thinning), while the transient state behaviour is thixotropic. All the currently available technologies for SSM processing have been developed based on those unique rheological properties, which in turn originate from their non-dendritic microstructures. Year 2001 marks the 30th anniversary of the concept of SSM processing. Today, SSM processing has established itself as a scientifically sound and commercially viable technology for production of metallic components with high integrity, improved mechanical properti...

Semisolid metal processing

Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.

Physical and mechanical properties of the B2 compound NiAl

Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Fundamental aspects of hot isostatic pressing: An overview

Direct microscopic evidence is presented in support of an explanation of R-curve behavior in monophase ceramics by grain-localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such “activity sites” along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface-adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal-zone microcracking and cracktip/internal-stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.

Crack-Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al2O3 at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al2O3, AI2O3 with anorthite, AI2O3 with anorthite and MgO, and Al2O3 with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al2O3.

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

Analysis of particle growth by coalescence during liquid phase sintering

Particle growth by coalescence during liquid phase sintering of Fe-Cu

The hot-pressing behavior of nanophase, 3-mol%-yttria-stabilized, tetragonal zirconia (Y-TZP) was studied in the temperature range 1290 to 1570 K. Powder was sol-gel synthesized using zirconium tetrachloride and hydrated yttrium chloride precursors. After drying and calcination, the particle size was 40 nm, coarsening to 70 nm upon subsequent annealing at 1273 K. On compact heating at 23-MPa applied stress, densification initiated near the calcination/annealing temperature. Theoretical density was obtained at densification temperature above 1490 K with particle coarsening consistent with classical coarsening kinetics. Microstructural change accompanying densification involved formation of dense regions 2 to 4 μm in size separated by irregularly shaped porous regions. The observed densification behavior complements calculated densification maps for fine-particle Y-TZP.

Sol–Gel Synthesis of Nanophase Yttria‐Stabilized Tetragonal Zirconia and Densification Behavior below 1600 K

Prediction of deformation behaviour during hipping is often difficult because data on complex material properties are required but are not usually available from the literature. From the present work, measurement of the deformation of a single sphere is proposed as a direct and simple method of predicting hipping behaviour. For C1018 steel, copper, and API nickel superalloy, the predictions made from deformation experiments with single spheres were in good agreement with measured hipping behaviour at hipping densities <93% theoretical. In addition, the creep parameters, such as the exponent n for the power law creep relation, of the powder material itself were measured during single sphere deformation. During hipping of C1018 in the density range above 90%, the use of lower pressures, which is favoured for economic reasons, showed that microstructural development and densification during the final stage of hipping are determined by relatively large pores. These large pores may result from irregul...

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Wolfgang A. Kaysser

Papers

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

Analysis of particle growth by coalescence during liquid phase sintering

Particle growth by coalescence during liquid phase sintering of Fe-Cu

Sol–Gel Synthesis of Nanophase Yttria‐Stabilized Tetragonal Zirconia and Densification Behavior below 1600 K

Microstructural development and densification during hipping of ceramics and metals