Sulfur containing refractory for resisting reactive molten metals

Improved refractories for resisting attack of molten titanium aluminum and similar metals are provided by the inclusion of sulfur. Metal, oxygen, and sulfur combinations, wherein sulfur is present at from 10 to 60 atomic percent, are particularly useful. Disclosed is a material having the atomic formula M.sub.a S.sub.b O.sub.c where O is oxygen, S is sulfur, and M is at least one metal selected from the scandium subgroup of the periodic table transition metals (scandium, yttrium and the rare earths) and aluminum. In an alternate material, M is comprised of at least two elements, the first selected as above and the second selected from the alkaline earth metal group. A preferred material is formed by mixing and firing CaS and Y.sub.2 O.sub.3 in proportions which results in (Ca+Y).sub.0.43 S.sub.0.14 O.sub.0.43.

BACKGROUND OF THE INVENTION 
1. This invention relates to the field of metal casting, in particular to 
crucibles and molds for casting of reactive metal alloys such as those of 
titanium and aluminum. 
2. Recently, there has been heightened interest in alloys of the 
titanium-aluminum system, most particularly those generally of the 
Ti.sub.3 Al (alpha-2) type and the TiAl (gamma) type. These alloys have 
the potential for serving in aircraft at higher temperatures than current 
titanium alloys and have much lower densities than the presently used 
nickel and iron based alloys. Experimental studies have shown that the 
titanium aluminum alloys present problems in melting and casting, insofar 
as reaction with materials heretofore known for containing titanium and 
aluminum. The alloys melt in the range of about 1450.degree. to 
1650.degree. C.; for casting several hundred degrees of super heat is 
often desired. Thus, they tend to present melting and casting problems 
analagous to titanium, rather than aluminum based alloys. 
As there has been little experience with casting titanium aluminum alloys, 
the prior art is only related to alloys comprised mostly of either 
titanium or aluminum. Both of these alloy systems have presented 
difficulty insofar as melting crucibles are concerned. Titanium alloys in 
particular have presented problems insofar as expendable molds are 
concerned. 
In the melting of titanium based alloys, only water cooled copper crucibles 
have been found to be commercially useful. The melting point and 
reactivity of the molten metal causes container degradation and 
contamination of the casting with virtually all common refractories. 
Studies, such as reported by Garfinkle et al., in Transactions of the 
American Society for Metals, Vol. 58, pages 520-530 (1965), indicate the 
reactivity of molten titanium with various carbides, borides and 
silicides. Garfinkle et al. found cerium sulfide to have the greatest 
resistance, but dissolution was still said to be significant. Undoubtedly, 
certain laboratory chemicals may be resistant to titanium. But for 
commercial success, a container material must additionally have a 
satisfactory cost and availability and be formable into desired shapes. 
None has met all these criteria heretofore. 
In conventional investment casting, the time of exposure of the mold 
material to molten alloy is relatively limited, compared to the crucible 
used for melting. Nonetheless, mold materials for casting titanium alloys 
still present a problem. When the mold materials usable with iron and 
nickel base alloys, such as metal oxides of silicon, zirconium and 
aluminum, are used for casting titanium alloys it is found that there is 
unacceptable interaction and introduction of debilitating oxygen into the 
casting. Molds of rammed graphite or metal oxide molds lined with graphite 
are usable for titanium alloys but excess carbon is found in an embrittled 
casting surface. Katz et al. U.S. Pat. No. 3,180,632 describes the use of 
a metal oxide such as yttria to coat a graphite mold and reduce 
interaction. Monolithic graphite containers present limitations on the 
types of shapes which can be formed; graphite-containing molds cannot be 
fired in conventional furnaces with oxidizing atmospheres. Molds with 
refractory metal linings, such as metal oxide molds having tungsten powder 
linings, and described in Brown et al. U.S. Pat. No. 3,537,949, present 
cost and manufacturing impediments. Other prior patent art on the 
foregoing types of molds is recited in Basche U.S. Pat. No. 4,135,030. 
Compared to titanium, aluminum melting and casting is somewhat easier. 
Although the metal is quite reactive and reduces its own otherwise stable 
oxide, aluminum base alloys on the whole have considerably lower melting 
points than titanium alloys. Clay bonded silicon carbide and certain oxide 
materials are found to be suitable. But aluminum technology does not 
provide any useful materials for alloys containing substantial titanium, 
including titanium aluminum alloys, probably because of the higher melting 
point and reactivity of such alloys. 
Thus, there is a need for improved materials for melting and casting of 
titanium aluminum alloys and other similarly reactive materials. An 
improved container material will be either unreactive, or have products of 
reaction which are not deleterious to the alloy, and will have a cost and 
availability which will make it commercially feasible. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a material which is nonreactive 
with molten titanium aluminum alloys or similarly reactive metals and 
which is fabricable into many shapes for melting casting. 
According to the invention, vessels for containing or receiving molten 
metals contain one or more metal from the alkaline earth group and 
scandium subgroup of transition metals of the periodic table, with oxygen 
and sulfur. One manner of forming a preferred material is to admix an 
oxide of a scandium subgroup metal, such as oxides of yttrium, scandium 
and the rare earths, with an alkaline earth metal sulfur compound, such as 
sulfides of calcium and strontium. The general atomic formula for a 
material of the invention will be M.sub.a S.sub.b O.sub.c where M 
represents one or more metal, S is sulfur, and O is oxygen and where the 
subscripts are atomic proportions. Useful ranges of values of the 
subscripts are: a, 0.40 to 0.45; b, 0.1 to 0.6; c, 0.2 to 0.6. If only a 
single metal is present, then the metal is selected from the scandium 
subgroup. If more than one metal is present, then the first metal is 
selected from the scandium subgroup, and other metals are selected from 
the group consisting of the scandium subgroup, alkaline earth metals, and 
aluminum; preferred subscript ranges will be a, 0.41 to 0.45; b, 0.02 to 
0.24; c, 0.31 to 0.57. 
A useful particular combination for casting titanium aluminum alloys, 
comprises the designation of the element M as yttrium. In such a yttrium, 
sulfur, and oxygen combination, it is further possible to diminish the 
oxygen content to the point where essentially only yttrium and sulfur are 
present. Another preferred material in accord with the above formulae is 
the combination of calcium, yttrium, sulfur, and oxygen, where the 
specific formula is 
EQU (Ca+Y).sub.0.43 S.sub.0.14 O.sub.0.43 
To fabricate containers in the practice of the invention various 
conventional powder processing techniques can be used. One manner of 
forming a material of the aforementioned Ca-Y-S-O type is: admix fine 
powders of calcium sulfide and yttrium oxide, press the mixture to a 
shape, and fire the shape. Another approach comprises the use of a 
nonaqueous slurry of the starting constituents to coat the internal 
surfaces of a conventional metal oxide investment casting mold, followed 
by firing of the coated mold prior to the introduction of the casting 
metal. 
When containers are used for the casting of titanium aluminum alloys, 
contamination from mold material is advantageously reduced. Specifically, 
oxygen and sulfur contaminations appear reduced below those attainable by 
the separate use of a metal sulfur compound or a metal oxide compound. It 
is believed that the invention will be usable for casting of other high 
temperature reactive metals, particularly those of the titanium subgroup 
of the periodic table. Further, through the use of metal oxides with metal 
sulfides, containers are lower in cost than those made of useful metal 
sulfides alone, and thus are more likely to be commercially feasible.

cl DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment and the scope of the invention are described in 
terms of performance with titanium aluminum alloys, most particularly the 
alloy TiAl, comprising 54 atomic percent aluminum. However, it is believed 
that the invention will be useful for other alloys containing titanium and 
aluminum with or without the addition of still other elements. It is also 
expected that the invention may be useful with a variety of other alloys 
where conventional metal oxide mold materials are unsatisfactory due to 
high temperature interaction; included in this group are alloys based on 
zirconium and hafnium. 
The invention herein was discovered after much experiment on the 
interaction of materials with molten TiAl. To give some appreciation of 
both the scope and advantage of the invention, some of this experimental 
work will now be described. Basically, the testing comprised contacting 
molten TiAl at 1550.degree. C. (70.degree. C. over its melting point) with 
a test material for one half to one hour under an argon atmosphere. After 
the test period, the metal was allowed to solidify in contact with the 
experimental material and an initial evaluation was made using 
metallography. In particular instances, more refined evaluation was 
undertaken by X-ray image electron probe microanalysis (hereafter, X-ray 
imaging). The following summarizes the results. 
Severe reaction was observed between the melt and both graphite and 
vitreous carbon; silicon carbide, silicon nitride and boron nitride were 
almost completely dissolved after one half hour. Yttria stabilized 
zirconia faired better but was still poor; a high degree of wetting of the 
material by the melt was observed and there was an extensive interaction 
zone in the TiAl; this contained both aluminum and yttrium rich particles. 
A similar degree of wetting and interaction was observed with a calcia 
stabilized zirconia. 
Commercially pure alumina and yttria appeared more compatible with molten 
TiAl, with little metallographic evidences of reaction. In one respect, 
the yttria somewhat out performed the alumina in that there appeared to be 
less wetting and less tendency for the melt to climb the wall of the 
container. However, while metallography gave no indication of an 
interaction, X-ray imaging of the TiAl-yttria specimen revealed extensive 
diffusion of aluminum into the yttria and yttrium into the TiAl. Extensive 
tests with alumina crucibles of various qualities tended to show that 
increased surface roughness and porosity increased the degree of attack of 
the container. Further tests indicated favorable results with sapphire 
(monocrystalline corundum) and LUCALOX (dense polycrystalline alumina, of 
the General Electric Company, Fairfield, Connecticut). Thus, while these 
high density, smooth surfaced materials produced reasonably good 
performance, it is not however practical to use them in most applications 
because of their lack of adaptability to easy shaping as required for most 
expendable molding. Furthermore, a quantitative oxygen analysis of the 
TiAl revealed that the oxygen content had been increased during the 
metal's contact with alumina. In addition, closer scrutiny by electron 
microprobe showed very fine particulate inclusions that were predominantly 
aluminum and oxygen. 
Prior work had shown that the addition of periodic table Group IIIB 
transition elements resulted in the formation of oxides in a TiAl alloy 
during melting and solidification by conventional practice. (It was 
further observed that the addition of yttrium in modest amounts decreased 
the surface tension of the TiAl melt and reduced the wetting of metal 
oxide crucibles, thereby indicating its potential for reducing 
interaction.) Thus, it was anticipated that Group IIIB metal oxides might 
effectively reduce reactions when included with alumina. 
Yttria-alumina and lanthana-alumina materials were prepared by blending 
fine powders, cold pressing and sintering at 1350.degree. C. Compositions 
evaluated included Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 in 40/60 and 65/35 
molar proportions. The surface of the materials was glazed by heating the 
sintered test piece above about 1850.degree. C. The 40/60 composition was 
better than the 65/35 composition but both revealed interaction. Tests 
with 10/90 and 20/80 mixtures were somewhat inconclusive but appeared to 
indicate somewhat more interaction. Tests with scandia produced an 
interaction. Tests with various proportions of lanthana and alumina 
mixtures produced results similar to the yttria-alumina results. Pure 
lanthana produced a severe reaction, as did calcia. Magnesia and thoria 
produced less reaction but the melt evidenced oxygen pickup. 
The foregoing results of several dozens of tests confirmed that the 
containment of TiAl presents a substantial problem. Since the presence of 
oxygen in TiAl castings would degrade properties, more promising materials 
than those described above were sought. As mentioned, there was some prior 
work which indicated that a rare earth sulfide, CeS, exhibited improved 
resistance to titanium. Furthermore, many sulfides of rare earths were 
assessed to have high free energies of formation and melting points, 
making them conceptually attractive. Thus, tests were run on cerium 
sulfide (Ce.sub.2 S.sub.3), yttrium sulfide Y.sub.2 S.sub.3 and yttrium 
oxysulfide Y.sub.2 O.sub.2 S which had been pressed and sintered in vacuum 
at 1350.degree. C. The cerium sulfide exhibited substantial interaction 
according to the standards we applied in our work. Using metallography, in 
several tests, there was only an extremely slight or no reaction with 
yttrium sulfide. Less conclusively, the yttrium oxysulfide exhibited 
similar behavior. Oxygen contents of the TiAl showed a small increase over 
the baseline levels and were by far the best of the materials heretofore 
tested. Some dispersed sulfides, presumably titanium sulfides, were found 
interspersed with the minor amounts of oxides of yttrium and aluminum in 
the TiAl melted in yttrium sulfide. Thus, it was discovered that yttrium 
sulfide and yttrium oxysulfide are useful materials for casting TiAl. 
However, yttrium sulfide reacts with water vapor during storage. Further, 
both materials are quite expensive. Therefore, further improvements were 
sought. 
Attempts to produce a face coat of yttrium sulfide on an alumina substrate 
were not successful since the materials separated during sintering at 
1350.degree. C. in vacuum. An equimolar mixture of yttrium sulfide and 
alumina was fabricated using the sintering technique mentioned above and 
produced dramatically reduced reaction and oxygen in the TiAl compared to 
prior alumina tests. Some sulfur was found interdendritically within the 
TiAl. 
Thus, it was discovered that the inclusion of alumina with yttrium sulfide 
as a mixture was feasible and resulted in TiAl with reduced sulfides, but 
somewhat increased oxygen, compared to yttrium sulfide alone. Although the 
amount of testing and analysis was limited and the conclusions were 
preliminary, they nonetheless formed the basis for discovering more 
improved materials. 
Because of its greater thermodynamic stability, yttria was used as a 
substitute for alumina in further testing. Sulfides of aluminum and zinc 
were attempted to be included with yttria but their evaluation was 
abandoned before any melting trials due to processing problems involving 
reaction with water vapor and generation of hydrogen sulfide. An equimolar 
mixture of yttria and calcium sulfide (CaS) was pressed, sintered and 
fired into a test piece. Analysis showed that the molten TiAl increase in 
oxygen content was very low, of the order of 0.1%, comparable to the best 
results with yttrium sulfide, and below that expectable with yttrium oxide 
alone. Although there was a slight degradation of the container surface, 
the melt was characterized by an absence of substantial sulfides; sulfur 
content was only about 0.004%. Further testing involving 5 hours of molten 
TiAl alloy contact indicated that the oxygen content rose to only about 
0.3%. Consequently, it was shown that the combination of calcium sulfide 
and yttria constituted a new and useful material. Furthermore, the nominal 
weight distribution of the equimolar mixture was 75% yttria and 25% 
calcium suffide, producing a substantial reduction in cost compared to 
yttrium sulfide alone. Combinations of a metal, oxygen, and sulfur are 
further advantageous because they are likely to be stable during storage 
or processing in the presence of water vapor and oxygen, compared to the 
sulfides which tend to be reactive. 
To form the container material from the admixture, sintering was undertaken 
in vacuum at 1350.degree. C. Sintering was also done in air in other 
trials and may be done in other atmospheres as well. The object of 
sintering is to form a stable complex compound from the constituents. 
Since there was no significant evolution of gas or other products during 
firing, it is reasonably concluded that the sintered material has the 
constituents of the original compounds. The temperature and time of 
sintering may be varied: The temperature may range from at least 
1150.degree. C. to 1650.degree. C. The time of sintering during experiment 
was nominally one hour, although longer times are of course acceptable and 
shorter times may be permissible, depending on the fineness and 
homogeneity of the particulate mixture and the sintering temperature. 
On an atomic basis, the equimolar yttria calcium sulfide material can be 
represented by the atomic formula: 
EQU (CA+Y).sub.0.43 S.sub.0.14 O.sub.0.43 
where Ca=calcium, Y=yttrium, S=sulfur and O=oxygen and where the subscripts 
indicate the relative amounts of the elements present. Based on the 
performance of the aforementioned material, expectable variations in 
normal practice, and experience with other combinations of materials for 
resisting molten metals, it is sound to project the range of atomic 
percentages which are useful. We believe that useful materials are at 
least defined by the general formula: 
EQU (Ca+Y).sub.a S.sub.b O.sub.c 
where a ranges from about 0.41 to 0.45, b ranges from about 0.02 to 0.24 
and c ranges from about 0.31 to 0.57. These ranges are reflective of the 
composition which results when the molar percentage of calcium sulfide in 
a CaS-Y.sub.2 O.sub.3 mixture is varied between 10 and 70%, as further 
discussed below. Of course, minor impurities, such as are present in 
commercially pure constituents may also be present and the exact atomic 
formula would be accordingly adjusted. Fundamentally, the combination of 
calcium sulfide and yttria constitutes the combination of a scandium 
sub-group metal oxide and an alkaline earth metal sulfide. Based on the 
periodicity of the chemical elements, other alkaline earth sulfides may be 
substituted for calcium sulfide. Among those alkaline earths most likely 
to be useful would be sulfides of strontium and barium, although it is 
likely the others will be usable as well. Calcium, strontium and barium 
are more reactive than beryllium and magnesium, having greater affinities 
for oxygen, and thus are preferred. It was pointed out that yttrium oxide 
produced better results in combination with the sulfide than did aluminum 
oxide. Based on considerations similar to those discussed for the sulfide 
compounds, it would appear that oxides of other elements of the scandium 
sub-group (including the rare earth lanthanides and actinides) of the 
transition elements would be useful. 
The experimental work shows that it is the combined presence of sulfur and 
oxygen in a metal complex compound which provides an improved material. As 
noted previously above, yttrium oxysulfide was found to be an improvement 
over any metal oxide. And the alumina-yttrium sulfide compound was also 
found advantageous. Consequently, the common feature which prevails is, in 
combination, a metal with a high affinity for oxygen and sulfur, together 
with oxygen and sulfur. If only one metal is present, it is one chosen 
from the scandium sub-group. If more than one metal, one is from the 
scandium sub-group while the others are chosen from the group consisting 
of the alkaline earths, scandium sub-group, and aluminum, or mixtures 
thereof. Also, it is quite expectable that combinations of one or more 
different scandium sub-group metal oxides will work favorably in 
combination with one or more alkaline earth metal sulfides. The oxidation 
states of the alkaline earth metals are similar; only the +2 ions of these 
elements are commonly observed. For the metal oxides which appear 
analagously useful, the scandium sub-group metals are present as +3 ions, 
although they are capable of combining at other valances. Given this, the 
range of useful formulae will comprise one or more metals from the 
alkaline earth group or scandium sub-group of transition metals, in 
combination with oxygen and sulfur. Thus, this aspect of our invention can 
be stated as the general formula: 
EQU (M).sub.a S.sub.b O.sub.c 
where M is at least one metal chosen according to the foregoing rules and 
the ranges of the subscripts are a from about 0.4 to 0.45, b from about 
0.02 to 0.6, and c from about 0.2 to 0.6. 
The ranges set forth for our useful formulations are based on reasonable 
projection from the experiments which were undertaken. At one limit, 
yttrium sulfide was usable; and this would constitute an oxygen portion of 
zero. At the other limit, it was shown that entirely metal oxide was not 
an improvement; this would constitute a sulfur portion of zero. The 
improvement in the performance of a metal oxide refractory is imparted by 
the incorporation of sulfur. If there is inadequate sulfur, no improvement 
will result. On the other hand, an entirely metal-sulfur compound without 
oxygen, while feasible, has a high cost and the material is hygroscopic 
and hard to handle. So, it may be said that the incorporation of oxygen in 
a metal sulfur compound improves its stability in moist environment. In 
summary, there must be sufficient sulfur to impart to the refractory 
increased thermodynamic stability and improved resistance to molten metal 
attack, but insufficient sulfur to reach the point of environmental 
instability. Accordingly, the practical limits for the CaS-Y.sub.2 O.sub.3 
material combination are judged to be more than 10% but less than 70% 
molar CaS. These limits are represented by (Ca+Y).sub.0.41 O.sub.0.57 
S.sub.0.02 and (Ca+Y).sub.0.45 O.sub.0.31 S.sub.0.24. It is also 
noteworthy that Y.sub.2 O.sub.2 S is also represented by the formula 
Y.sub.0.4 O.sub.0.4 S.sub.0.2 and Y.sub.2 S.sub.3 is represented by 
Y.sub.0.4 S.sub.0.6. Further, mixtures and other more complex compounds 
are formulatable. Thus, it is within these contexts that the 
aforementioned formulae subscript ranges have been derived. 
The green pressed preform is sintered in the range of 
1400.degree.-1600.degree. C. to produce a container of good density 
suitable for melting and casting. As an alternate technique, a slurry of 
the calcium sulfide and yttrium oxide powders can be used to coat the 
interior surfaces of a conventional investment casting mold of the 
alumina, silica, zircon types used for nickel alloy casting. Generally, 
the techniques used will be those such as are described in Brown et al. 
U.S. Pat. No. 3,537,949, the prior art for coating the interior of molds 
with refractory metal compounds. A slurry in volume comprised of 9 parts 
methanol and 1 part mixed powders has been found satisfactory to produce a 
nominal 0.1 mm coating on the interior of an alumina base mold. After 
evaporation of the methanol, the mold is fired in air or other atmosphere 
at temperatures from 1150.degree.-1600.degree. C. to produce a coated mold 
ready for introduction of metal. The coating thickness may be varied from 
that indicated to economize on material or provide increased resistance in 
difficult applications. Other liquid vehicles may be used but water and 
other materials which react with the sulfide are to be avoided. 
The fabrication of articles useful for the melting and casting of titanium 
aluminum alloys may be accomplished with techniques which are conventional 
for the forming of complex metal oxide compounds into shapes. By way of 
example, powders of calcium sulfide and yttrium oxide having a maximum 
particle size of less than 44 micrometers are blended without a binder and 
cold pressed with sufficient pressure to a desired shape. Optionally, 
binders which are volatilized during sintering or remain benignly present 
in the final material may be used. Also, the particle size and 
distribution may be varied to suit the requirements of the final article 
and fabrication process as is well known in current ceramic technology. 
Generally, fine powders in the range of 5 to 45 micrometer particle size 
are preferred to produce homogeneous compounds of good density; they will 
also be preferred when the improved material is applied as a coating. 
Coarser powders may be lower in cost and permit the more rapid accretion 
of structures made by particle accumulation. 
It is also anticipated that techniques for constructing investment molds, 
as are conventionally used for nickel and iron based alloy casting, will 
be usable. The inventive material may be used to make an entire mold, or 
to only construct the first metal contacting portions. In such instances, 
it is expectable that a certain amount of binder such as colloidal silica, 
alumina, or yttria, from 10 to 30 percent, may be included in the improved 
composition to improve the integrity of the material accreted during the 
formation of a mold. And while we generally conceive our improved 
materials to preferably be used unadulterated, occasion may nonetheless 
arise, as for economic or structural reason, when they may be included in 
combination with other materials to provide advantage to the combination. 
Our improved composition may also be formulated from other combinations of 
materials containing the essential elements. Examples of this would be the 
use of metal sulfate compounds in conjunction with metal or metal oxides, 
the use of elemental metals with metal-oxygen-sulfur compounds, the 
addition of sulfur to metal oxide complexes, the addition of oxygen to 
metal-sulfur compounds, and the use of compounds containing extraneous 
elements which are liberated during the formation of the final product. 
Other means may reveal themselves as the economic construction of improved 
containers according to the invention is pursued. And while the preferred 
process described herein comprises the mixing of particulates, it should 
be evident from the foregoing recitation that other means and reactions 
may be used to formulate an improved material in a useful form. 
Although this invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in this art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.