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OCR for page 86
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
OCR for page 87
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.
OCR for page 88
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 rate—K/s
FIGURE 2 Effect of cooling rate on segregate spacing in aluminum
alloys (dendritic mode of solidification). 3 Reprinted with permission.
OCR for page 89
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
OCR for page 90
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
OCR for page 91
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
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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.
OCR for page 93
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
OCR for page 94
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
OCR for page 95
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
OCR for page 96
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.
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98
ŘEXTRUDED
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
OCR for page 99
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
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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-
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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
OCR for page 102
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.
OCR for page 103
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
OCR for page 104
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
OCR for page 105
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.
OCR for page 106
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
OCR for page 107
RAPID SOLIDIFICA TION TECHNOLOGY
107
also an area of high potential payoff. Surface modification technology
is still in it infancy, although the benefits of rapid solidification laser or
electron beam glazing treatments have been amply demonstrated. How-
ever, areas of application have been targeted for development, including
hardfacing of tools, dies, and valve seats. The possible extension of this
technology to bulk rapid solidification processing has also been consid-
ered.
NOTES
1. P. Duwez, R.H. Willens, and W. Klement, J. Appl. Phys., 31:1136, 1960.
2. W. Klement, R.H. Willens, and P. Duwez, Nature, 187:869, 1960.
3. Rapid Solidification Processing: Principles and Technologies II, eds., R. Mehrabian,
B.H. Kear, and M. Cohen, Claitor's Publishing Division, Baton Rouge, La., 1980
(Proc. 2nd Int. Conf. at Reston, Va., March 1980) [hereafter cited as Rapid Solidi-
fication Processing].
4. Rapidly Solidified Amorphous and Crystalline Alloys, eds., B.H. Kear and B.C.
Giessen, Elsevier North Holland, New York, 1982 (Proc. Mater. Res. Soc. Meeting
at Boston, Mass., Nov. 1981, Symposium F).
5. Rapidly Quenched Metals IV, eds., T. Masumoto and K. Suzuki, Japan Institute of
Metals, 1982 (Proc. 4th Int. Conf. at Sendai, Japan, Aug. 1981~.
6. H. Jones, "Rapid Solidification of Metals and Alloys," Monograph No. 8, Institution
of Metallurgists, London, 1982.
7. E. War and W.M. Shafer, in Powder Metallurgy for High Performance Applications,
Syracuse University Press, New York, 1972, p. 57.
8. A.R. Cox, J.B. Moore, and E.C. Van Reuth, in Superalloys: Metallurgy and Man-
ufacture, eds., B.H. Kear, D.R. Muzyka, J.K. Tien, and S.T. Wlodek, Claitor's
Publishing Division, Baton Rouge, La., 1976, p. 45 (Proc. of 3rd Int. Symp., Seven
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10. D.G. Morris, Met. Sci., 15:116, 1981.
11. E.M. Breinan, D.B. Snow, C.V. Brown, and B.H. Kear, in Rapid Solidification
Processing, 1980, p. 440.
12. A.R.E. Singer, Met. Mater., 4:246, 1970.
13. R.G. Brooks, A.G. Leatham, and G.R. Dunston, Met. Powder Rep., 35:464, 1980.
14. N.J. Grant, private communication.
15. C.W. Chang and J. Szekely, J. Met., p. 57, Feb. 1982.
16. D. Apelian, M. Paliwol, R.W. Smith, and W.F. Schilling, International Metals Review,
American Society for Metals, Metals Park, Ohio, Dec. 1983.
17. S. Shanker, D.E. Koenig, and L.E. Dardi, J. Met., p. 13, Oct. 1981.
18. M.R. Jackson, J.R. Rairden, J.S. Smith, and R.W. Smith, J. Met., p. 23, Nov. 1981.
19. R.B. Pond and R. Maddin, TMS-AIME, 245:2475, 1969.
20. M.C. Narasimhan, U.S. Patent 4,142,571, 1979.
21. J. Bedell and J. Wellslager, U.S. Patent 3,862,658, 1975.
22. H.S. Chen and C.E. Miller, Mater. Res. Bull., 11:49, 1976.
23. R.B. Pond, R.E. Maringer, and C.E. Mobley, New Trends in Materials Processing,
American Society for Metals, Metals Park, Ohio, 1974, p. 128.
OCR for page 108
108
ADVANCES IN STRUCTURAL MATERIALS
24. E.W. Collings, R.E. Maringer, and C.E. Mobley, Tech. Rep. AFML-TR-80-70,
Battelle Columbus Laboratory, Columbus, Ohio, 1978.
25. H.S. Chen and C.E. Miller, Rev. Sci. Inst., 41:1237, 1970.
26. E. Babic, E. Girt, R. Krsnik, and B. Leontic, J. Phys. E: Sci. Instrum., 3:1014, 1970.
27. E.M. Breinan, B.H. Kear, C.M. Banas, and L.E. Greenwald, in Superalloys, 1976,
p. 435.
28. B. Lux and W. Hiller, Prakt. Metallogr., 8:218, 1977.
29. P.R. Strutt, M. Kurup, and D.A. Gilbert, in Rapid Solidification Processing, 1980,
p. 225.
30. H.J. Leamy and G.K. Celler, in Rapid Solidification Processing, 1980, p. 465.
31. R.E. Anderson, A.R. Cox, T.D. Tillman, and E.C. Van Reuth, in Rapid Solidification
Processing, 1980, p. 416.
32. J.F. Wooldridge and J.A. Easton, Ind. Heat., 45:44, 1978; 46:42, 1979.
33. D.S. Duvall, W.A. Owczarski, and D.F. Paulonis, Weld. J., 54:203, 1974.
34. B.H. Kear and W.H. King, U.S. Patent 4,250,229, Feb. 1981.
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1981.
36. F.E. Luborsky, Mater. Sci. Eng., 28:139, 1977, and in Amorphous Magnetism, Vol.
2, eds., R.A. Levy and R. Hasegawa, Plenum Press, New York.
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38. T.R. Anthony and H.E. Cline, J. Appl. Phys., 49:1248, 1978.
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40. Y.W. Kim, P.R. Strutt, and H. Nowotny, Metall. Trans., 10A:881, 1979.
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42. J.D. Ayers, T.R. Tucker, and R.J. Schaefer, in Rapid Solidification Processing, 1980,
p. 212.
Representative terms from entire chapter:
rapidly solidified
