Cutting Edge Technologies (1984) / Chapter Skim
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Rapid Solidification Technology
Rapid Solidification Technology
Pages 86-108
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From page 86... ...
Although a complete discussion of such fundamental aspects is clearly beyond the scope of this paper, some pertinent findings with respect to the influence of cooling rate on solidification microstructure and the effects of subsequent heat treatment will be briefly examined. Only selected aspects of rapid solidification technology are discussed here.
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From page 87... ...
_ micra _ structures ~ . Compositlon Refined and process Fine dendrites, _ micro- ~ eutoctics and other structures dependent microconstituents Novel microstructures Extended solid solutions Microcrystalline structure Detestable Crystalline phases Amorphous solids Increasing homogonelty FIGURE 1 Microstructural consequences of rapid solidifications Reprinted with per.
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From page 88... ...
In general, with increasing cooling rate, conventional alloys follow this sequence: coarse dendrites > fine dendrites ~ homogeneous or extended solid solutions ~ microcrystalline solid solutions. On the other hand, alloys that exhibit deep eutectic troughs tend to follow this sequence: coarse eutectic > fine eutectic > ultrafine eutectic ~ amorphous metallic solid.
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From page 89... ...
Thus, typically, annealing times are reduced from hours to seconds in many alloy systems where dendritic growth cannot be avoided even under the highest available cooling rates. Experience has shown that the attainment of an ideally homogeneous structure, irrespective of whether it is accomplished by massive solidification or by heat treatment of refined dendritic structures, imparts real property benefits to the alloy.
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From page 90... ...
. _ ~ Inert gas atomization Pressure Water cooled ~ copper chill, J_ ~ ~ Continuous I ~ ~ filament ~ ~ ~ or strip · Melt spinning Cooling gas ~ Rotary ~ n Fine atomizer ~U~ particles disk · Centrifugal atomization Laser or electron beam \~Melt pool Self quenched surface layer · Self-quenching FIGURE 3 Representative rapid solidification processes.3 Reprinted with permission.
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From page 91... ...
Since the heating and cooling rates are very fast, there is virtually no change in the microstructure of the material during compaction. As evidence for this it may be noted that dynamic compaction has been used successfully to produce amorphous solids from amorphous powders, splats, or ribbons.9 i0 Laser surface melting, in conjunction with continuous powder feed, has also been employed to fabricate bulk metastable structuresii (Figure 5~.
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From page 92... ...
Rapid quenching of the molten particles occurs when the droplets impact on the substrate. Cooling rates are typically 105 to 106 K/s, and the resulting microstructures are fine "rained (~0.5 Em)
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From page 93... ...
RAPIDLY SOLIDIFIED FILAMENTS AND RIBBONS Melt Spinning In melt spinningi9 (Figure 3) , thin filaments or ribbons are produced by forcing the melt through a small orifice directly onto the surface of a rapidly rotating copper disc, which may be water cooled for continuous operation.
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From page 94... ...
In centrifugal melt spinning22 the extended chill effect occurs quite naturally, since the melt is jetted onto the inner surface of a rapidly rotating copper drum. The problem here is not to extend the contact time but rather to induce filament detachment after completion of solidification and solid state cooling.
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From page 95... ...
Electron beam melting of the feedstock is a unique feature of the pendant-drop process, which makes it particularly useful for processing reactive and/or high melting point materials. Twin Roller Quenching The mechanics of twin roller quenching are similar to those of melt spinning, except that a pair of counter-rotating rolls replaces a single rotating chill for the purpose of melt quenching.25 26 Typically the melt stream is directed vertically downward between a pair of watercooled rolls, and thin filaments are formed by rapid quenching in the pinch of the rolls.
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From page 96... ...
Even higher cooling rates are possible using pulsed laser or electron beam sources, because of the higher available power densities.30 Reproducible and controllable surface melting and quenching using pulsed sources have been achieved in layers as thin as ~1,000 angstroms (it)
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From page 97... ...
In order to give some feeling for the scope of these activities, a few examples will be given of developing technologies that utilize rapidly solidified fine powders, thin filaments/ribbons, or thin surface layers. Powder Technology Most of the effort on rapid solidification powder technology has been concerned with the further development of high performance components for gas-turbine engines, such as blades, vanes, discs, rotors, combustors, and bearings.
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From page 98... ...
plasma spraying.~8 As mentioned earlier, this new technology offers a number of
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From page 99... ...
Thin FilamentlRibbon Technology Thin filament/ribbon technology has been developed mainly by Allied Corporation and Battelle Columbus Laboratory. Allied has favored the melt spinning process, whereas Battelle has favored the melt extraction process.
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From page 100... ...
A solution to this problem has been to prepare the thin ribbon material by melt spinning.34 The resulting amorphous or partially amorphous material makes an attractive interlayer for diffusion brazing because it possesses moderate ductility and can easily be bent or cut to comply with complex joint geometries. Considerable success has been achieved in utilizing melt spun nickelbase alloys (boron added as melt depressant)
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From page 101... ...
This complicated fabrication procedure contrasts with the simplicity of melt spinning, which produces ferromagnetic ribbon or tape directly from the melt at very high rates and at relatively low cost. In power transformers the properties of interest exhibited by amorphous magnetic alloys, such as Fe~OB20, are high saturation magnetization coupled with extremely low losses.36 Typically, losses are down
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From page 102... ...
Various electronic device applications have been considered for metallic glasses. The first of these applications was the use of high permeFIGURE 11 Demonstration amorphous cored power transformer.
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From page 103... ...
For laser glazing, a numerically controlled work station, with at least two axes of motion, is generally preferred, whereas for electron beam glazing, programmed electromagnetic beam deflection has proved to be more versatile (Figure 12~. Both laser and electron beam glazing treatments have been used to achieve beneficial modifications in the surface properties of materials.
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From page 104... ...
corrosion in chloride solutions. In M2 high speed steel,40 heat treatment of laser or electron beam glazed surfaces generates a uniformly fine distribution of hard carbide particles in an austenitic/martensitic matrix, which improves its cutting performance, e.g., in applications such as saw blades, drill bits, and end mills.
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From page 105... ...
Experimental work has also been conducted on the fabrication of bulk rapidly solidified structures by incremental solidification processing. Simple axisymmetric shapes, such as a demonstration turbide disc, have already been fabricated by this processii (Figure 14~.
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From page 106... ...
SUMMARY The technology of rapid solidification has evolved steadily since about the mid-1970s. Today's technology includes methods for the production and consolidation of rapidly solidified fine powders, fabrication and utilization of rapidly solidified thin filaments or ribbons, and rapid solidification surface modification of materials.
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From page 107... ...
4. Rapidly Solidified Amorphous and Crystalline Alloys, eds., B.H.
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From page 108... ...
Van Reuth, in Rapid Solidification Processing, 1980, p.
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