Ingram, J. C., Jr.

English

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CURRENT STATUS OF REFRACTORY METALS FOR
STRUCTURAL APPLICATIONS
By T. Sgt. Jesse C. Ingram, Jr., USAF
Wright Air Development Division
INTRODUCTION
This paper is essentially a report on applied research programs
which cover the use of refractory materials as load-carrying primary
structural members. The Dyna-Soar glider employs refractory materials
in leading edges and heat-shield applications. In order to provide
growth capability in the Dyna-Soar glider, higher temperature load-
carrying structures would be desirable. This paper is a status report
on efforts to achievethis goal. In recent months the Wright Air
Development Division (WADD) has been cognizant of an urgent need for
immediate development in refractory materials for both structural and
heat-shield application@ and, since no one had really explored the
state of the art in refractory metals structural technology, it was
decided that fairly comprehensive programs in this area should be
initiated. Figure 1 depicts a nominal trajectory including transient
conditions for a typical skip reentry mission.
DISCUSSION
WADD began a number of research and development efforts including a
contract with McDonnell Aircraft Corporation for the design, fabrication,
and test of a representative, refractory metal, load-carrying, structural
component capable of efficient operation in the temperature range of
1,800 ° F to 2,500 ° F_ The General Electric Company, Flight Propulsion
Laboratory Division (Evendale, Ohio), was subsequently selected as the
main subcontractor. There are two other subcontractors, that is, Hughes
Tool Company (Culver City, Calif.), and Temco (Dallas, Texas). This
program was aimed at determining the state of the art and demonstrating
the feasibility of a refractory metal structure.
It is widely known that one of the primary requirements to produce
a refractory metallic end item, having high integrity, is to begin with
closely controlled extraction, refinement, melting, and ingot casting
practices. Even if the ore is extracted and refined carefully and a
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high purity sponge or other metallic form is obtained many problems
remain. An example of one of these problems is conversion of the
metal to a powder with which to make ingots for further processing. All
refractory metals have an affinity for one or more elements which
adversely affect the base metal properties. As a rule undesirable oxides,
hydrides, nitrides, etc., may be formed and more often thannot, it is
necessary to vacuum-arc melt the powder ingot (billet) to purify it.
Naturally, the impurity problem does not end here.
Another serious problem is segregation. Because of wide differences
in resistivity, conductivity, constituent melting points, and vapor
pressures, intolerable differences in chemistry, density, etc., may exist
from section to section within the same ingot. This problem has not yet
been overcome by powder metallurgy, but it is believed that a properly
conducted program on prealloying of powder can greatly alleviate this
problem. Also, the undesirable large grain size that is characteristic
of cast ingots can be somewhat overcome, initially, through powder
metallurgy techniques.
Next, before the ingot can be processed into bar formachining, or
still further processed into sheet, the cast structure must be broken
down and the ingot partially homogenized by hot work. Here, again,
problems are faced of an entirely different nature. Ashas been the
experience with A1, Mg, Ti, and steelj in order to achieve the ultimate
in physical and mechanical properties, hot work or "hot processing" of
the material above the recrystallization temperature must be possible
and should be started with the basic ingot. In order to accomplish this
with arc cast molybdenum (Mo)_ efficient controlled atmosphere furnaces
capable of reaching and maintaining equilibrium temperatures of 3,500 ° F
to 3,600 ° F are required. The best furnaces available will yield tem-
peratures between 2,700 ° F and 2,900 ° F and the capacity of any such
furnace is fairly small. In general, the higher capability furnaces
are smaller, cylindrical, induction-heated apparatus and therefore are
not very conducive to the processing of plate and sheet.
After the ingot is partially warm or hot-cold worked by extrusion
and maybe by further swaging into sheet bar, the recrystallization tem-
perature begins to lower. However, the problem of impurities still exists
and now, in particular, oxidation and nitriding of the material may occur.
Mo does not exhibit a high degree of oxygen penetration since, at
the temperatures under consideration, the oxide product (Mo03) is vola-
tile and, in processing, this problem could be overcome by either a good
vacuum or a reducing atmosphere such as high-purity dry hydrogen. Cracked
ammonia will severely nitride and embrittle Mo and commercially available
argon contains sufficient amounts of impurities to be quite deleterious.
With niobium (Nb), it is different in that the oxide product is both
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stable and porous and, therefore, oxygen will continue to penetrate.
Here, either a good vacuum or high-purity inert atmosphere is required.
Now, when an attempt is made to roll the sheet bar into sheet,
especially thinner gages, the aforementioned problems are magnified.
There are essentially no facilities available with necessary atmospheric
control. An exception to this statement is the Inert Fabrication (INFAB)
facility at Universal Cyclops Steel which is sponsored by the Navy. This
facility is a self-contained room with a minimum amount of necessary
processing equipment which will be operated in an atmosphere of argon
under slight pressure. At present, necessary, reliable, and adequate
quality control is almost nonexistent. This is an area that needs very
close attention; with it many of our problems, both present and future,
can be alleviated or even eliminated.
Consider joining for a moment. (See fig. 2.) It is generally con-
ceded that Mo is very brittle and, therefore, susceptible to fracture
in the cast state. This property makes welding a problem and so far
no welds, either fusion or resistance, have been seen that did not
exhibit severe grain growth and resultant brittle-type fractures. Nb
is more amenable to welding, if in the pure state or even alloyed with
certain elements. However, when the alloys which are most attractive for
the reentry temperatures involved are considered, problems arise. On
the USAF contract with McDonnell Aircraft Corporation the General Electric
developed F48 alloy was chosen to fabricate the end item which is a fin
and rudder with necessary hinge fittings and attachments but without a
leading edge. As of today, it is highly questionable as to whether it
is possible to weld it successfully either by fusion or resistance
methods. It should be added, however, that at least one of the experi-
ments conducted by the General Electric Company has shown promise in
spot welding the F48 alloy. With a thin titanium foll (about 1 mil)
inserted between the two sheets and by closely controlling the welding
cycle, the notch sensitivity around the shoulder of the nugget has been
noticeably reduced. Weld shear values have been increased and fractures
are apparently less brittle.
Reasonable success has been realized in riveting both Mo and Nb.
Marquardt Aircraft has been riveting and bolting Mo ramjet assemblies
for some time but, in general, the detail parts were not coated prior
to assembly. In fabricating the small test items under the McDonnell
Aircraft Corporation contract, it was deemed necessary to multiple coat
in some cases because of the configuration complexity. Figure 3 shows a
W2 coated M_Ti rivet after squeezing. This condition meant using
coated rivets, if possible, and this has been done. Even though it is
still essentially a hand operation, Mo rivets can be manufactured easily,
coated with a W2 or DurakMG type coa_ing, and squeezed successfully by
torch heating to 1,600 ° F or slightly above. Figure 4 shows some of the
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more easily manufactured fasteners other than rivets. Driving the rivets
by impacting appears to be risky at present. Manyother fastener con-
figurations are being investigated and evaluated including blind-mechanlcal
and explosive types. All the refractory metals which maybe candidates
for structural applications at present and in the foreseeable future are
susceptible to oxidation, which can be catastrophic, at the temperatures
involved. Molybdenum,which is under serious consideration, reacts
violentlywith oxygen above 1,500° F to 1,600o F; therefore, it must be
protected. So far, the only work or development in this area of any
great import has been the cementation pack process which has produced
a disiliclde (or variation thereof) type of coating. This is the best
of known available coatings for Mo. However, even though this maybe
suitable for short-tlme applications such as ramjet engines, it is not
considered at this time to be uniform, entirely reproducible, or wholly
reliable for long-time, multiple-mission structural applications. In
static oxidation tests rather encouraging results have been realized
at temperatures up to 2,900° F, but patching or repairing any kind of
break or defect appears to be impossible.
The potential of Nb is somewhatmore encouraging than Mo up to about
2,_00° F, in that the oxide product is stable and not volatile and that
Nb is inherently more oxidation-reslstant than Mobut It I still needs to
be protected. However, if a coating failure Ioccurs, the end product
should not be as catastrophic as that with Mo. Figure 5 is a time-
temperature static oxidation comparison between Nb and Mo alloys. From
a relative viewpoint andunder a given set of conditions, alloyed Nb can
be lOO times as oxldatlon-reslstant as Mo. Incidentally, so far, alloying
Mohas not enhanced its oxidation resistance. Again, work on the McDonnell
Aircraft Corporation- General Electric Companycontract has proven a need
for a Nb coating, and a fair amount of effort has been expended in this
direction. At present, it appears that an aluminum base cold slurry dip,
with subsequent heat treatment, may prove to be the best for this program
which of necessity must be limited in scope of coating development.
At this point, it is appropriate to mention somemore of the per-
verse characteristics of promising coatings for structural applications
which could possibly offer us the needed protection. First, they are
all brittle; at room temperature their impact value is essentially zero.
Second, as a rule, their coefficient of expansion is very low, and,
therefore, radically different and usually incompatible with the sub-
strate. These two inherent characteristics generally cause, at one
time or another, cracking, spalling, crazing, etc. The General Electric
Companyis investigating the possibility of a glass-llke material for
an overlay, on the Nb dip coating, which would becomeviscous at the
temperatures under consideration and_ thereby, fill and heal any cracks
that mayhave developed.
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Production problems should be pointed out which have been encountered
in procurement. An order can be placed for a lot of thin gage (below
0.020) material, having a given chemistry, exhibiting reasonable T bend
and elongation characteristics at room temperature, and being free from
scales, slivers, laminations, gouges, etc. With present technology, the
chemistry requirements would probably be met except for a few undesirable
segregations and possibly excessive amounts of contaminants. Thln-gage
material has been ordered with a promised delivery of 8 weeks and
8 months later the complete order has not been received. Figure 6 is
an ultrasonic trace recording of Mo sheet which has been fusion butt-
welded. Notice the discontinuities in the weld zone and also the parent
metal laminations. At present all material must be accepted on a best-
efforts, consigned basis. This year material has been shipped as "sup-
posedly acceptable" and, in the Mc_2Zr , more than a half-dozen surface
scales and slivers were evident in 1 square foot. Also, M_Ti has
been shipped as 0.010 gage in which the thickness varied from 0.0069 at
one end to 0.0105 at the other in a 10-inch by 28-inch piece. This is
not indicative of quality, quality control, reproducibility, or assurance
that the material needed can be obtained today. However, as the gage
thickness increases, problems decrease. For gage thicknesses of 0.050
either the Mo_lTi, M_Zr, or the TZM alloy could be madeup, available in
reasonable quantity and with reasonable quality, provided that necessary
process control is exercised. Inspection techniques and/or establish-
ment of acceptance standards and limits must still be optimized for these
alloys.
Another matter for consideration is expected or calculated yield.
The program at McDonnell - General Electric was initiated wlthmaterial
basic cost being estimated at approximately $60 per pound for Mo sheet
and $120 per pound for Nb sheet. First, with the more attractive alloys,
such as the Nb base F48 and Mo base TZC, no more than a 10- to 20-percent
yield from the ingot to thin-gage material has been realized. Second,
if desired quality requirements were imposed, probably 90 percent of
the yield would be rejected. Because of breakage, waste, rejections,
etc., our original cost estimates have increased by factors of 5 for
the thin-gage material and the materialwhich is ultimately desired for
fabrication into usable and reliable structures has not been obtained.
Figure 7 does show that with careful processing techniques it is possible
to hand fabricate small detailed parts.
CONCLUDING PJ_4ARKS
For structural applications other than heat shield and leading-edge
elements, problems associated with ingot production, sheet quality control,
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available assembly, coating, and material cost have been presented.
Because of these problems our confidence in structural applications of
refractory materials is poor; however, it is hoped that, at the conclusion
of the McDonnell - General Electric program in September, a satisfactory
test of componenthardware will be accomplished and the program will have
indicated the steps which industry as well as the USAFmust take to assure
reliable efficient refractory structures. The creep and rupture properties
shown in figure 8 are a very real reason why these refractory metals are
so attractive for elevated-temperature structural and heat-shield
applications.
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TYPICAL TIME-TEMPERATURE VARIATION FOR
REENTRY PORTION OF TRAJECTORY
3000 I TRANSIENT MANEUVERS
2400 __ _AF:" ' ATr ] r ]AT
222_' III Illrillll
,_o0 IIIAJ _1111
1400 " / I I I / %.
/ NOMINAL TRAJECTORY- -_
-O- CONDITIONS: I
DISTANCE FROM LEADING EDGE = 5 FEET
400 1 EFFECT OF INTERNAL RADIATION INCLUDED-
200 I
o
65 70 75 80 85 95
TIME AFTER BURNOUT, MIN
Figure l
MOLYBDENUM ALLO '
WELDING PROBLEMS
I. LOWDUCTILITYOF WELD DEPOSIT
a. LARGE G_IN SIZE IN WELD AREA
b CRATER CRACKING (OXYGEN)
c. GRAIN BOUNDARY CONSTITUENTS
2. POROSITYIN WELDDEPOSITS
3. ELECTRODESTICKING(RESISTANCEWELDING)
4. TENSILEPROPERTIESOF WELDS
5. POSTWELDCRACKING(RESIDUALSTRESSES)
• Figure 2
Figure 3
..........ili:
Figure 4
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2300 °.: OXIDATION
Mo
OY F50
I I I I I I I
2 4 O 8, I0 12 14
TIME -HOURS
Figure 9
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CREEP AND RUPTURE STRENGTH OF MOLYBOF..NUM ALLOYS
IO0-HR. EXPOSURE
70 xtO--r--q_ I
!
60 ; I
;
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,.', .-o.(._. -., .o
ee
_o I _ I I , , .
l _, J /--RUPTURE-O,5% Ti-.05% Zr ¢
I I I20 _ o.s% T, _o
I _ "_ I I------ (,TIM,TEDI kX.._,, I Io,T,souncz:_
,o I I_ _,-J _
, I "=,.I.. A.......I
ipoo ipoo •2,600 3,400 4,zoo
TEMPERATURE,°F
Fi_e7
Figure 8


Current status of refractory metals for structural applications

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