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Rapid Solidification Tecl~ologJ, BERNARD H. KEAR Ever since the pioneering work on "splat quenching," reported by Duwez et al. in 1960,~ 2 it has been known that rapid quenching from the molten state, i.e., rapid solidification, is a means to develop unusual, even novel microstructures, which frequently exhibit beneficial prop- erties. In order to exploit such structure/property advantages, much effort has been expended, at least since about the mid-1970s, on de- veloping new methods for (1) production and consolidation of rapidly solidified fine powders, (2) fabrication and utilization of rapidly solid- ified thin filaments or ribbons, and (3) rapid solidification surface treat- ments of materials. This paper will highlight some of the more exciting innovations that have occurred in these areas. As will be shown, major advances have been made on all three fronts, with real prospects for the widespread use of rapidly solidified materials in structural and mag- netic applications. Considerable progress has also been made in our understanding of rapid solidification behavior, including mechanisms and kinetics of rapid solidification, phase transformations, and structure/property/processing relationships. Although a complete discussion of such fundamental as- pects is clearly beyond the scope of this paper, some pertinent findings with respect to the influence of cooling rate on solidification microstruc- ture and the effects of subsequent heat treatment will be briefly ex- amined. Only selected aspects of rapid solidification technology are discussed here. For more complete information on the subject, the reader is re- ferred to recent publications.3~6 86
RAPID SOLIDIFICATION TECHNOLOGY MICROSTRUCTURAL CONSEQUENCES OF RAPID SOLIDIFICATION 87 The principal effects of rapid cooling from the molten state on the resulting solidification microstructure are summarized In Figure 1. Under ordinary casting conditions, with cooling rates of ~1 kelvin per second (K/s), the microstructure typically is very coarse and exhibits a high degree of chemical segregation. In passing from ordinary casting practice to cooling rates >102 K/s, there is a progressive refinement in the so- lidification microstructure, i.e., dendrites, eutectics, and other micro- constituents are all reduced in scale. This is because with increasing cooling rate there is much less time available for coarsening of the microstructure. The degree of segregation within these structures, how- ever, remains essentially the same, since local equilibrium is maintained at the solid/liquid interface during solidification, such that local tem- peratures and concentrations are given essentially by the equilibrium phase diagram. In other words, the indicated microstructural refinement is a consequence of differences in the growth process rather than of effects due to undercooling of the melt prior to the nucleation stage. With increasing cooling rates substantial undercoating of the melt can occur, and it is in this solidification regime that novel microstructures make their appearance. These are indicated in Figure 1 as extended solid solutions, metastable crystalline phases, and amorphous metallic solids. Large departures from local equilibrium at the solid/liquid in- terface can occur in this solidification regime, with the solid phase en- trapping supersaturated concentrations of solute and impurity atoms. In the limit, at sufficiently high cooling rates, the resulting solid will have exactly the same composition as that of the parent liquid. This Conventional | Coarse dendrites, -. . _ outoctice and other microconstituents / [ Melt - lo2 6104 10 10 Increasing cooling rate (K/s) _ micra _ structures ~ . Compositlon Refined and process Fine dendrites, _ micro- ~ eutoctics and other structures dependent microconstituents Novel micro- structures Extended solid solutions Microcrystalline structure Detestable Crystalline phases Amorphous solids Increasing homogonelty FIGURE 1 Microstructural consequences of rapid solidifications Reprinted with per- . . mission.
88 ADVANCES IN STRUCTURAL MATERIALS mode of solidification is called partitionless, segregationless, or massive solidification, and its characteristic feature is the formation of an ideally homogeneous structure. An additional complication at sufficiently high undercoating is the formation of a microcrystalline structure, due to the combined effects of higher nucleation rates and lower growth rates at the lower temper- atures. In general, with increasing cooling rate, conventional alloys fol- low 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 ~ amor- phous metallic solid. It should be emphasized that the picture of solidification behavior depicted in Figure 1 is very approximate. The more correct picture must take into account the operative temperature gradient in the liquid phase just ahead of the advancing solid/liquid interface, and the interplay between temperature gradient and solidification rate, or interface ve- locity. Steep temperature gradients tend to stabilize plane front growth, with compositional homogeneity, whereas steep solute gradients pro- mote cellular or dendntic growth. These effects can be negated at suf- ficiently high interface velocity, where plane front solidification can occur irrespective of the operative temperature gradient. An example of the refinement in dendntic structure observed in alu- minum (Al) alloys with increasing cooling rate is shown in Figure 2. . ~ Conventional Rapid solidification solidification 1000' 100 Segregate 10 spacing, Em 1.0 0.1 0.01 //~/~ /~' Ingots Molt spinning splat cooling surface melting it,,, 1 1 1 1 ,- ,-~ _ 1 , ,, , , . - __ Atomization ' I 1 1 1 1 1 10 3 1 103 106 109 Cooling rateK/s FIGURE 2 Effect of cooling rate on segregate spacing in aluminum alloys (dendritic mode of solidification). 3 Reprinted with permission.
RAPID SOLIDIFICATION TECHNOLOGY 89 The indicated segregate spacings were derived from measurements of secondary dendritic arm spacings. The reduction in segregate spacing by two orders of magnitude due to rapid solidification is of particular significance from the viewpoint of achieving compositional homogeneity by subsequent heat treatment. Thus, typically, annealing times are re- duced 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 solidi- fication or by heat treatment of refined dendritic structures, imparts real property benefits to the alloy. For example, in nickel (Ni)-base superalloys, which are prone to dendritic segregation, until the advent of rapid solidification processing the full benefit of lye precipitation hard- ening was never achieved. It is now known that the optimum properties in such alloys can be realized only when the y~ precipitation hardening phase is uniformly distributed, which is possible only in homogenized material. Similar considerations apply to other alloy systems. In partic- ular, it may be noted that the effective dispersal of extraneous phases in many alloys due to rapid solidification can give rise to unexpected benefits. For example, the fine scale dispersal of manganese sulfides in steels prevents grain coarsening during austenitizing treatments.3 This result has also raised interesting questions concerning the possibilities for deliberately exploiting fine dispersions of sulfide phases in steels for hardening purposes. RAPIDLY SOLIDIFIED POWDERS Powder Production Inert gas atomizations and centrifugal atomizations (Figure 3) are the most widely used methods for producing bulk quantities of rapidly so- lidified powders. Inert gas atomization involves the interaction of a melt stream with a symmetrical arrangement of converging high velocity gas jets. Atomization occurs as a result of the dissipation of gas phase kinetic energy in the interaction zone. Most commonly the working fluid is steam, nitrogen, or argon. Centrifugal atomization employs a high speed rotating disc atomizer for particle generation and high mass flow helium (He) gas for quenching purposes. Good wetting between melt and disc surface is a prerequisite for efficient powder production. This is achieved by forming and maintaining a thin solid skull on the surface of the water- cooled copper disc. The forced convective cooling employed in centrif- ugal atomization generates high cooling rates, typically ~105 K/s for
go ADVANCES IN STRUCTURAL MA TERIALS melt Melt, Gas jets stream '(,''\,,Atomized /,, ~ \\pOWd~r . _ ;. . _ ~ 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. particles ~50 micrometers (~m) in diameter (dia.~. The cooling rate for a comparable particle size in inert gas atomization is ~104 K/s. Both processes yield spherical particles (Figure 4) in a size range of 20 to 100 Em dia. In conventional alloys the microstructure is typically refined dendritic. In certain alloys, amorphous structures can also be developed but only in smaller particles (<10 ,um dia.) that experience the highest cooling rates (~106 K/s). A more convenient method for fabricating high yields of amorphous powders is by pulverization of amorphous melt-spun ribbons. The resulting powders exhibit clean, smooth fracture surfaces and have a gritlike appearance (Figure 4~. Conventional Consolidation When the principal benefit of rapid solidification is perceived to be improvement in the homogeneity of the finished product, almost any convenient hot deformation processing technique may be employed for consolidation purposes. Thus, hot isostatic pressing of powders may be used for making near-net shape components or parts, whereas hot ex- trusion may be utilized for making preforms or billets (Figure 5~. On the other hand, when there is a need to preserve the initial metastable state of the powder, other methods of compaction must be employed. In dynamic compaction (Figure 5), consolidation is achieved by prop- agating a high intensity shock wave through the powder aggregate. Full densification is achieved when frictional heat generated between the particles is sufficient to cause surface localized melting and welding
RAPID SOLIDIFICATION TECHNOLOGY 91 (a) 100X (b) 100X 1X FIGURE 4 Rapidly solidified fine particulates; (a) spherical powder (product of centrif- ugal atomization); (b) gritlike powder (product of pulverization of melt-spun ribbon); (c) flakelike powder (product of twin roller quenching). together of the particles. 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~. Spray Consolidation The average cooling rate in inert gas, or centrifugal atomization, may be increased by simply allowing the atomized spray to quench out on a water-cooled chill. The resulting splats experience cooling rates of ~106 K/s and may be removed from the chill by scraping them off as fast as they are formed. On the other hand, thick rapidly solidified deposits may be built up by continuous superposition and bonding together of splatted particles, or, in other words, by combining particle generation, quenching, and consolidation in a single spray consolidation operation. This requires very careful control of processing variables, including melt preheat, spray deposition rate, and heat transfer characteristics. In spray rollingi2 the spray deposition rate is adjusted so that the molten droplets experience efficient splat quenching prior to completion of densification in the pinch of the rolls. In spray forgingi3 the atomized spray is collected in a mold at a location in the atomizing chamber where many of the particles are in the partially solidified, or mushy, condition, which yields
92 ADVANCES IN STRUCTURAL MA TERIALS a preform with better than 95 percent of theoretical density. When the mold is full the hot preform is converted into a fully dense homogeneous product by closed die forging. In spray castingi4 massive ingots are formed by slowly filling the mold with a spray of fine particles generated by inert gas atomization. Plasma spray deposition, or "plasma spraying," also combines particle melting, quenching, and consolidation in a single operation.is i6 The process involves injection of powder particles into a high intensity plasma jet, which rapidly melts the particles and propels them toward the work- piece surface (Figure 6~. 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) and homogeneous. Conventional plasma spray deposition is nor- mally carried out at atmospheric pressure. Typically the deposits contain oxidation products, together with some porosity due to incomplete melt- ing, wetting, or fusing together of deposited particles. The problem of oxidation can be minimized by shielding the plasma arc with an inert gas atmosphere. An alternative approach is to enclose the entire plasma spraying unit in an evacuated chamber, which is main- tained at about 30 to 60 tort inert gas pressure by high speed pump- ing.~7 i~ Under such "vacuum plasma spraying" conditions, plasma gas velocities are much higher (typically in the Mach 2 to 3 range), due to the higher permissible pressure ratios. Other advantages include (1) higher particle velocities, which give rise to denser deposits (often >98 percent of theoretical density), (2) broader spray patterns, which pro- duce larger areas of relatively uniform deposition, and (3) transferred Sealed L~ \ Container ~ i Heat pressure '/ - Hot isostatic pressing _ Powder_ Ram ', pressure ~ billet .~;;.,';'jI ,' ~ I) . it;, ~,,~ · Hot extrusion Laser or electron beam t] Powder or sRI jp..td~leyd \ V ~ wire Projectile Powder Substrate4 - pressure ];: ~ Gu Incremental solidification ~ Dynamic compaction FIGURE 5 Representative consolidation methods.3 Reprinted with permission.
RAPID SOLIDIFICATION TECHNOLOGY WATER COOLED COPPER ANODE \^ ~ POWDER INLET / tNEARsT_ F//~///~ /A / TU Nit CATHODE 1~ _ r,,, ~ A . ~ ~ i ma// ~ ~~ L GUN POWER TRANSFERRED ARC SUPPLY POWER SUPPLY PLASMA 93 ,~ HIG ~ VELOCITY / MOLTEN PARTICLES An' RSPD MATERIAL SUBSTRATE FIGURE 6 Schematic of plasma spray systemic In low pressure plasma spraying, the entire system is enclosed in an evacuated chamber. Reprinted with permission. arc heating of the substrate, which improves deposition characteristics (Figure 6~. In addition, the process can be automatically regulated to make controlled deposits of complex geometries at high deposition rates (up to 50 kilograms per hour [kg/hr], and in large section thicknesses (>5 centimeters tcmi), without sacrificing quality. In other words, the addition of a vacuum environment to plasma spraying has created new opportunities for near-net shape processing of bulk rapidly solidified materials. 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. Current practice favors a downward directed jet (0.3 to 1.5 millimeters tmm] dia.), inclined at 15° to the disc radius, with the nozzle tip located about 3 mm from the disc surface and set back about 25 mm from its crest. The disc is typically 15 to 45 cm dia. and rotates at up to 20,000 revolutions per minute (rpm). Provided that the melt properly wets the surface of the disc, this simple jetting technique readily pro- duces rapidly solidified filaments up to about 3 mm in width, with thick- nesses that range from 25 to 100 ,um. Cooling rates are in the range of 1os to 106 K/s, depending primarily on ribbon thickness. Ribbons up to
94 ADVANCES IN STRUCTURAL MA TERIALS ( ~/'~ Substrate FIGURE 7 Planar flow casting process.20 '\ \~N 15 cm in width can also be produced by melt spinning, but this requires careful positioning and design of the nozzle. An optimal arrangement appears to be one in which the slotted nozzle has an angled tip, which is positioned almost in contact with the surface of the rotating disco (Figure 7~. Such an arrangement stabilizes the melt pool that is formed under steady state conditions in contact with the disc. The time of contact of the solidifying material on the copper chill is of decisive importance in the fabrication of amorphous ribbons. If the ribbon detaches from the disc too soon, crystallization and phase de- composition will occur during cooling in the solid state. In extended chill melt spinning2i this is avoided by deliberately increasing the ribbon contact time by employing a spring-loaded auxiliary disc in contact with the main melt spinning disc. 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. This can be done most effectively by using an inclined chill surface on the inside edge of the rotating drum, since centrifugal forces acting on the sloping surface encourage detachment of the filament. Filament contact time decreases as the slope of the inclined chill surface increases. On the other hand, the steeper the slope, the greater the tendency to form a ribbon of unequal thickness across its width. Melt Extraction Melt extraction is slightly different from melt spinning in that the melt source is stationary, and the edge of a rotating disc picks up the melt
RAPID SOLIDIFICATION TECHNOLOGY 95 to form a rapidly quenched filament.23 24 Cooling rates are somewhat slower than those attainable by melt spinning, typically about 5 x 104 K/s. The melt may be contained in a crucible, or a special arrangement may be employed that does not require a crucible, e.g., as in pendant- drop melt extraction. A typical disc for thin filament production is about 20 cm dia. and has a wedge-shaped edge. Notched or serrated discs have been used to make short fibers, or particulates. To achieve steady state processing, the melt is fed to the edge of the disc in a continuous manner by raising the molten bath in the crucible process, or by lowering the feedstock in the pendant-drop process. 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. In order to produce filaments of uniform thickness, the roll surfaces and shafts must be machined to close tolerances, and precision bearings must be used. Ribbons from 50 to 200 ,um in thickness are formed when the rolls initially are in contact under some pressure. Thicker ribbons can be made by expanding the roll gap. Owing to the limited contact time of the solidified material with the rolls, twin roller quenching is not as efficient as melt spinning in producing amorphous materials. However, it is quite suitable for making extended solid so- lutions or metastable phases and has the advantage that the material can be obtained in thicker sections. RAPIDLY SOLIDIFIED SURFACE LAYERS Surface Melting (Glazing) Surface modification by rapid solidification is most readily accom- plished by laser or electron beam surface melting (glazing) techniques, which exploit the principle of self-substrate quenching27 29 (Figure 3~. Typically a high power density beam is rapidly traversed over the ma- terial surface so as to induce surface localized melting with high melting efficiency, i.e., melting occurs at such a high rate that there is little time
96 ADVANCES IN STRUCTURAL MATERIALS for thermal energy to penetrate into the solid substrate. Under these conditions very steep thermal gradients are developed in the melt zone, which promote rapid solidification. The actual quench rate is ultimately dependent on the melt layer thickness, with cooling rates of 104 to 108 K/s readily attainable in appropriately thin sections. Using available continuous wave carbon dioxide (CO2) gas lasers, experience has shown that melt depths can be controlled down to ~25 ,um, corresponding to an average maximum cooling rate of ~108 K/s. In practice, in order to exploit the microstructural/property advantages of such high cooling rates, processing must be carried out in two steps. First, the surface of the material is thoroughly homogenized by a "deep penetration" homogenizing pass, with cooling rates of ~ 104 K/s. Second, the same region of the surface is subjected to another surface melting pass, using much higher incident power density and shorter interaction time to achieve the desired higher cooling rate in a very thin surface layer. Even higher cooling rates are possible using pulsed laser or electron beam sources, because of the higher available power densities.30 Re- producible and controllable surface melting and quenching using pulsed sources have been achieved in layers as thin as ~1,000 angstroms (it). Typical operating conditions in this regime of processing are power densities of ~5 x 107 watts per square centimeter (W/cm2) and inter- action times of ~10-8 s. Surface Alloying Surface alloying using high power density lasers and electron beams has also been investigated. Two distinct approaches have been evalu- ated: (1) preplacement of alloying material on the workpiece surface prior to melting and (2) continuous delivery of alloying material (wire, ribbon, or powder) to the interaction, or melt zone. In incremental solidification processingii (Figure 5), prealloyed powder is fed contin- uously to the interaction zone as the mandrel rotates. Thus, a much thicker, even bulk, rapidly solidified structure can be built up gradually as one deposited layer fuses to another in a continuous manner. Good interlayer bonding and epitaxial growth from layer to layer can be achieved under proper operating conditions. A critical parameter is the location of the powder impingement point with respect to the laser melt zone. Since the mandrel is rotating, feedstock impingement must occur slightly ahead of the laser beam for stable, steady state deposition. The process has great potential as a hardfacing treatment.
RAPID SOLIDIFICATION TECHNOLOGY APPLICATIONS 97 Rapid solidification processing has been successfully applied to many different alloys, including nickel (Ni)-, iron (Fe)-, aluminum (All-, and titanium (Ti)-base alloys for structural applications and certain special alloys for magnetic and electronic applications. 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 compo- nents for gas-turbine engines, such as blades, vanes, discs, rotors, com- bustors, and bearings. This work has been conducted primarily at Pratt & Whitney Aircraft (P&WA) and General Electric (GE). The work carried out at P&WA is illustrative of the more conventional approach to powder metallurgy processing, wherein powder production and consolidation are two distinct operations. Thus, at P&WA powder is produced by the proprietary "RSR" centrifugal atomization process (Figure 3), and subsequent consolidation is achieved by hot extrusion (Figure 5~. After extrusion the billet is subjected to gradient annealing in order to develop a textured, columnar "rained structure. Such a textured, homogeneous structure exhibits outstanding high temperature creep properties. For example, the much longer creep rupture life ex- hibited by RSR185 (new powder alloy), compared with directionally solidified PWA1422, is equivalent to ~70°C improvement in the metal temperature capability of this alloy. Similar improvements in creep prop- erties, coupled with enhanced oxidation/hot corrosion resistance, have been found in derivative alloys produced by the same process. In order to exploit such property benefits, this same processing method has been applied to the fabrication of a demonstration "radial wafer" turbine blade.3i The wafer blade represents an advanced concept in air cooling design, which combines internal convective cooling with external transpiration, or film cooling. The sequence of steps involved in the manufacture of such a blade is shown schematically in Figure 8. As illustrated in this figure, rapidly solidified superalloy powder is produced by the RSR process and consolidated by hot extrusion, with extrusion parameters adjusted to give an intrinsically superplastic prod- uct. The billet is rolled superplastically into sheet stock by controlling temperature and deformation rate. The resulting sheet product is pho-
98 ØEXTRUDED BLANK RSR POWDER ADVANCES IN STRUCTURAL MATERIALS - PHOTO-ETCHED WAFER SUPERPLASTICALLY ROLLED SHEET Dl FFUSION BONDED I WAFERS I I N DUCTION ECM M ELT machined blade l ~ J ~ 7 DIRECTIONALLY RECRYSTALLIZED BLANK FIGURE 8 Sequence of steps involved in the fabrication of a transpiration-cooled wafer blades Reprinted with permission. toetched to form a multiplicity of thin shaped wafers, which are diffusion bonded together in a predetermined arrangement. This is the critical step in the process, since it generates the desired network of internal cooling passages. Following directional recrystallization of the bonded structure, the actual blade profile is formed by electrochemical machin- ing. This design concept, coupled with the higher metal temperature capability of RSR185 or a similar alloy, has the potential for increasing the turbine inlet temperature by ~350°C in the next generation of ad- vanced engines. The experimental air cooled blade shown in Figure 8 has already been successfully tested in an advanced engine. Using this same technology, P&WA has demonstrated significant im- provements in the properties of Al-base alloys and bearing steels. Thus, certain Al-Fe-Mo alloys exhibit higher strengths than those of conven- tional aluminum alloys at temperatures in the range of 180°C to 350°C. Such alloys are promising candidates for integral vane and case assem- blies in the cooler compressor section of the engine as replacements for the more expensive titanium alloys. Improvements in the rolling contact fatigue resistance of M50 bearing steel by rapid solidification processing also presents an opportunity for advancing the performance of high speed bearings. This beneficial effect has been related to refinement of the carbide phases in M50 steel. In contrast to the work at P&WA, GE's effort has been concerned with combining particle melting, consolidation, and shaping in a single operation, utilizing advances in vacuum (low pressure) plasma spray- ing.~8 As mentioned earlier, this new technology offers a number of
RAPID SOLIDIFICATION TECHNOLOGY 99 (a) (b) FIGURE 9 Prototype aircraft engine components made by low pressure plasma spraying of Rene 80X; (a) thin-walled engine combustor (0.05 cm thick x 10 cm dial; (b) massive turbine disc (10 cm diary Reprinted with permission. advantages over conventional plasma spraying, including more uniform spray patterns, higher deposit densities, and higher deposition rates. Vacuum plasma spraying of high performance coatings has become rou- tine practice, with applications in industrial gas turbines and jet engines. Progress has also been made in the fabrication of a thin-walled com- bustor and a massive turbine disc (Figure 9), making use of the unique thick section capabilities of the process. Laboratory tests have shown that deposited materials, such as Rene 80, have superior resistance to thermal fatigue, which is a prerequisite for combustor applications. Cur- rently, efforts are being made to apply this technology to the near-net shape fabrication of general engineering components, such as extrusion dies, valve bodies, pipes, casings, and sleeves. 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. Many areas of application have been identified; some have already been commercialized. Thus, today, rapidly solidified thin fila- ments/ribbons are being used (1) as reinforcing elements in ceramic matrix composites, (2) as interlayers for conventional and diffusion braz- ing, and (3) in a variety of magnetic applications. Castable refractories are widely used in furnaces and reactors.32 The incorporation of steel fibers (typically 0.2 to 0.4 mm2 cross section x 20 to 40 mm long) in castable ceramics increases their resistance to
100 ADVANCES IN STRUCTURAL MA TERIALS thermal and mechanical cycling, thereby increasing service life. Con- ventional processing of steel fibers involves repeated shear-cutting of continuously drawn wires, and final embossing of the fibers to improve adhesion. The cost of processing is high, so that the 2 volume percent of fibers normally introduced into the ceramic can cost several times that of the ceramic. Battelle was first to recognize the potential for melt extracted steel fibers in this application. High aspect ratio fibers are readily and inexpensively produced by melt extraction, using a notched wheel. The resulting fibers tend to have expanded ends (dog-bone shaped), which facilitates reinforcement of the ceramic matrix. Another advan- tage is that melt composition is no longer limited by mechanical working considerations, so that even low grade scrap can be used for melting. Resulting savings in production costs have been substantial, and many thousands of tons of melt extracted steel fibers are used today in castable ceramics. This same process is being considered for making steel fibers for reinforcing concrete. Diffusion brazing is a method of joining materials that combines the essential features of both conventional brazing and diffusion bonding.33 Typically the process employs an interlayer that closely matches the composition of the workpiece, except for the addition of an appropriate melt depressant to form a low melting point eutectic. The filler material is placed between the mating surfaces of the workplace and is permitted to alloy with it at a temperature where only the eutectic melts. Under isothermal conditions the melting point of the filler material gradually rises as the melt depressant diffuses away into the workpiece. Bonding is judged to be complete when no melt remains. Subsequent heat treat- ment is employed to erase all traces of the original junction. Success in diffusion brazing depends not only on good design of filler material, but also on the ability to produce the material in a usable form. A particular problem has been encountered in the preparation of thin ribbon material (25 to 50 ,um thick x 2 to 5 cm wide), which is very difficult, if not impossible, to produce by conventional hot working methods because of the limited ductility of the eutectic alloy. 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 nickel- base alloys (boron added as melt depressant) for diffusion brazing of gas-turbine engine components, such as blades, vanes, and even entire stator rings (Figure 104. Standard brazing alloys (e.g., those based on Ni) also contain sub-
RAPID SOLIDIFICATION TECHNOLOGY 101 FIGURE 10 Compressor vane and case assembly for the P&WA JT8D engine (diffusion brazed with amorphous tape). stantial amounts (~20 weight percent) of melt depressants, such as phosphorus (P), boron (B), and silicon (Si). Again such eutectic alloys are essentially unworkable but are amenable to glass formation by rapid quenching from the melt.35 Thus, entirely new classes of brazing alloys are now available in convenient tape form. Amorphous brazing tapes have the advantages of convenience in form, chemical uniformity, and cleanliness (no binders to pyrolyze, as in conventional brazing mate- rials), and they are relatively inexpensive to produce. Commonly used soft magnetic alloys include Fe-3.2 percent Si for cores of power transformers and motors, and special nickel-iron alloys for electronic devices. Sheet material ~0.3 mm in thickness is used in transformer cores and motors, whereas tape 25 to 100 ,um in thickness is employed in electronic devices. These materials are normally pro- duced by a complicated sequence of rolling operations, with critical intermediate annealing steps to develop the optimal crystallographic texture and magnetic properties. Subsequent processing may involve stress relieving and coating with polymers. 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 amor- phous magnetic alloys, such as Fe~OB20, are high saturation magneti- zation coupled with extremely low losses.36 Typically, losses are down
102 ADVANCES IN STRUCTURAL MA TERIALS by a factor of 4 compared with the best textured iron-silicon alloy. In a finished transformer this translates into substantial energy savings over the lifetime of the installation. It has been estimated that about $200 million dollars now wasted annually as heat in transformers can be saved by substituting amorphous FeB for the best textured FeSi. Thus, there is a real incentive for pushing forward with the development of amor- phous cored transformers despite certain technical drawbacks related to the thin gauge of the sheet. Prototype systems have already been fab- ricated (Figure 11) and are now being evaluated in actual field tests. Various electronic device applications have been considered for me- tallic glasses. The first of these applications was the use of high perme- FIGURE 11 Demonstration amorphous cored power trans- former.
RAPID SOLIDIFICATION TECHNOLOGY 103 ability amorphous alloys, e.g., Fe40Ni40P14B6, for magnetic shielding purposes. Large sheets for shielding were made by simple weaving and coating with polymers. Cylindrical shields made from these woven fab- rics compared favorably in performance with conventional 80Ni-20Fe permalloy foil, except at very low fields where metallic glass loses its high permeability. The main advantage claimed for the metallic glass fabric was its ability to be formed into the required shape without altering shielding performance. Another application that takes advantage of high permeability, coupled with high electrical resistance, mechanical hard- ness, and resistance to corrosion and wear is for audio and video recorder heads. The preference in this application is for zero magnetostriction high-cobalt compositions with B and Si as glass farmers, and twin roller quenching to produce smooth surfaces on both sides of the tape. Overall performance is claimed to be superior to conventional ferrites and similar materials. Other applications being considered include "stress trans- ducers," which exploit the high stress sensitivity of the magnetic prop- erties in amorphous alloys, and "acoustic delay lines," which make use of the very large values of magnetomechanical coupling and change in Young's modulus with applied field that are found in metallic glasses.37 Delay lines are essential elements in all signal processing equipment. Surface Modification Technology Laser or electron beam surface melting (glazing) has been employed to modify the surface structure and properties of very thin edges of samples using a single pass of a sharply focused beam. On the other hand, to obtain continuous surface coverage of glazed material it has been necessary to generate a multiplicity of overlapping passes by scan- ning the focused beam over the workpiece surface or by indexing the workpiece with respect to a fixed beam. A laser beam may be scanned by making use of special coupled arrangements of mirrors, whereas an electron beam may be scanned by electromagnetic means. For laser glazing, a numerically controlled work station, with at least two axes of motion, is generally preferred, whereas for electron beam glazing, pro- grammed electromagnetic beam deflection has proved to be more ver- satile (Figure 12~. Both laser and electron beam glazing treatments have been used to achieve beneficial modifications in the surface properties of materials. In sensitized 304 stainless steel,38 laser glazing has the effect of reso- lutionizing harmful carbide phases at the grain boundaries and restores the resistance to stress corrosion cracking. In 614 aluminum bronze,39 laser glazing homogenizes the surface, which increases its resistance to
104 II I ELECTRON a, BEAM ~71f~ _ ~ 1' Hi X-Y DEFLECTION ill AXIS YOKE 'll l/1 11 11 /1 1 11 ; ' GLAZED / I TRACE (a) ADVANCES IN STRUCTURAL MATERIALS (b) 400X FIGURE 12 (a) Schematic of electron beam surface melting (glazing), using electro- magnetic beam deflection; (b) cross-sectional view of glazed M2 steel showing overlapping passes.29 Reprinted with permission. 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. In a pseudobinary Fe-TiC alloy,4i electron beam glazing and tempering produce a threefold increase in the wear life in tests performed on a fully hardened M42 steel counter- face material. Laser glazing has also been applied to eutectic-type alloys that are ready glass farmers. Thus, amorphous surface layers have been developed on crystalline substrates in Pd-4.2Cu-5.lSi and in the tech- nically more interesting Fe40Ni40P~4B6 alloy, which exhibits exceptional mechanical properties and corrosion resistance. The high hardness and corrosion resistance of metallic glasses containing P (and chromium [CrJ), together with their ability to accept and maintain a sharp cutting edge, suggests such uses as surgeon's scalpels and even long-life razor blades. Laser glazing in conjunction with surface compositional modification is also an area of obvious high potential. Methods of processing typically involve preplacement of alloying material (powder, electrodeposit, etc.) on the workpiece surface prior to glazing, or particle injection during glazing. Carbide particle injection into alloy substrates has been used to develop wear resistant surfaces42 (Figure 13~. Much thicker deposits
RAPID SOLIDIFICATION TECHNOLOGY 'I // SUITE ~ POO// piano (a) 105 FIGURE 13 Surface melting (glazing) and particle injections; (a) schematic of apparatus; (b) Ti-6Al-4V alloy injected with TiC particles. Reprinted with permission. have also been laid down by the continuous delivery of prealloyed pow- der to the interaction, or melt zone (Figure 5~. Surface alloying by this means is being developed for a wide range of applications, including hardfacing of valve seats, turbide blade tips, bearing surfaces, and gas- path seals. Experimental work has also been conducted on the fabri- cation of bulk rapidly solidified structures by incremental solidification processing. Simple axisymmetric shapes, such as a demonstration tur- bide disc, have already been fabricated by this processii (Figure 14~. Typically the deposited material exhibits a pronounced columnar "rained dendritic structure, with grains extending through many successive layers of material. The inherently strong tendency for epitaxial growth between (a) {b) FIGURE 14 Demonstration gas-turbine disc (10 cm dia.) produced by laser glazing with continuous powder feeder; (a) as-glazed condition; (b) after machining. Reprinted with permission.
106 ADVANCES IN STRUCTURAL MATERIALS Laser beam wit IZ Powder / feed Zeal ~ __X TW^.:3Y;C martin_ LIZ w2~ W~141Z Three-axis motion - Y~ ~ LIZ W2 ~ FIGURE 15 Examples of laser glazing of near-net shapes, using continuous powder feeder Reprinted with permission. layers ensures good mechanical strength at the interfaces between layers, even when the composition is deliberately changed, e.g., by changing the composition of the powder feed. Applications for this process are currently limited by the requirement that the deposited material possess good weld-cracking resistance and by the need to improve the shape- defining capabilities of the process. As indicated in Figure 15, the fab- rication of more complex shapes requires the use of a numerically con- trolled work station, which is capable of simultaneous motion about two or three axes. 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 so- lidification surface modification of materials. Powder technology has been applied to the fabrication and coating of high-performance com- ponents for gas-turbine engines. This same technology is also being applied to airframe structural materials, such as high specific strength aluminum alloys. Thin filament/ribbon technology continues to evolve impressively, with several applications already to its credit, including the use of high aspect ratio filaments as reinforcing elements in castable ceramics and of wide ribbons (tapes) as interlayers for conventional, or diffusion brazing, purposes. The anticipated use of amorphous soft mag- netic alloy ribbons in the cores of power transformers and motors is
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