The status of rapid solidification is discussed in terms of recent progress in modelling methods of achieving solidification at high cooling rates and its effects on alloy constitution and microstructure. Applications currently in view are reviewed under the following headings: high-strength structural materials; tool and bearing materials; high-temperature materials, corrosion-resistant, catalytic and storage materials; and electrical and magnetic materials. It is concluded that the future of rapid solidification is not identifiable with any one process, activity or application and that many unresolved questions remain to challenge both the scientific and business communities.

The status of rapid solidification of alloys in research and application

It has been over eight years since the subject of rapid solidification processing (RSP) has been given a general review. The early reviews of RSP were undertaken before the very rapid expansion of the late 1970s and early 1980s and the most recent general review was published in 1984, at approximately the height of RSP activity in the USA. More recent specialized reviews have been prepared for magnesium, titanium and aluminium alloys. The expansion of research and application testing resulted from the first major attempts to employ RSP on a large scale under both industrial and government sponsorship. The objective here is to develop the historical background of RSP, and then to update the present theoretical base for this technology. Examples of recent work on the rapid solidification of metals and ceramics will be given, along with examples of applications. Finally, an assessment of the future directions and prospects for RSP will be presented.

Rapid solidification processing

Titanium alloys parts are ideally suited for advanced aerospace systems because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent corrosion resistance. Despite these features, use of titanium alloys in engines and airframes is limited by cost. The alloys processing by powder metallurgy eases the obtainment of parts with complex geometry. In this work, results of the Ti-6Al-4V and Ti-13Nb-13Zr alloys production are presented. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 900 up to 1500 °C, in vacuum. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. It was shown that the samples were sintered to high densities and presented homogeneous microstructure from the elements dissolution with low interstitial pick-up.

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Production of titanium alloys for advanced aerospace systems by powder metallurgy

Titanium alloys have several advantages over ferrous and non-ferrous metallic materials, such as high strengthto-weight ratio and excellent corrosion resistance. A blended elemental titanium powder metallurgy process has been developed to offer low cost commercial products. The process employs hydride-dehydride (HDH) powders as raw material. In this work, results of the Ti-35Nb alloy sintering are presented. This alloy due to its lower modulus of elasticity and high biocompatibility is a promising candidate for aerospace and medical use. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by isochronal sintering between 900 up to 1600 °C, in vacuum. Sintering behavior was studied by means of microscopy and density. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Samples sintered at high temperatures display a fine plate-like a structure and intergranular b. A few remaining pores are still found and density above 90% for specimens sintered in temperatures over 1500 °C is reached.

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Production of a low young modulus titanium alloy by powder metallurgy

A method is described for the manufacture of articles in defect immunized materials in which the defects are eliminated, or broken up and oriented in such manner as to minimize their harmful effects on the article. Referring to FIG. 1e, a rotor disc for a gas turbine engine is formed to an approximate shape by stacking together "sticks" 5 of material in an evacuated container and bonding them together by a hot isostatic pressing process. The "sticks" 5 are produced by extruding a starting body, for example, of powder material, to produce an elongation of up to twenty times and then cutting them to the appropriate length. By this means any non-metallic inclusions in the powder are broken up, inspection of the sticks and rejection of defective ones becomes easier, and the sticks can be oriented so that the effects of any remaining defects can be minimized.

Method for eliminating, or minimizing the effects of, defects in materials

Throughout the past decade a number of new powder metallurgy processes have appeared which offer con siderable promise for superior aeroengine combustor, blade, and disc alloys. Furthermore, several features of these processes can be exploited for improved material utilization such that total manufacturing costs are contained to combat steep increases in basic alloying element prices. The processes include gas atomization of superalloy powder for critical rotating parts, rotating electrode atomization for titanium powder in similar components, controlled thermomechanical processing of attrited powders to produce oxide dispersion strengthened superalloy sheet and airfoil parts, and finally plasma-sprayed gas-atomized powder for advanced hot-section overlay and thermal barrier coatings. Considerable alloy and process development work has already been undertaken on all these systems by aeroengine material suppliers and advantages accruing in terms of superior properties and/or lower processing costs ...

H. Harrison

Papers

Powder Metallurgical Innovations for Improved Hot-Section Alloys in Aeroengine Applications