Master alloys for beta 21S titanium-based alloys

Master alloys and methods of producing same are disclosed, wherein an intermetallic compound, for example Al.sub.3 Cb is first prepared via thermite processing, then size reduced, then mixed with other components in amounts yielding a mixture in the desired proportion for the master alloy. The mixture is compacted, then heated to produce the master alloy by fusion.

FIELD OF THE INVENTION 
The present invention relates to a master alloy, particularly for use in 
making beta Titanium-molybdenum alloys, and methods of making of such 
master alloys. 
BACKGROUND OF THE INVENTION 
Titanium-containing alloys find a broad range of applications in areas 
where low weight and strength are required, such as aerospace and military 
uses, as well as corrosion resistance and heat applications, including use 
in turbine blade jet engine parts, high speed cutting tools, and so on. 
Molybdenum is known to be difficult to diffuse uniformly in titanium, 
because of its higher melting point and higher density, which causes 
molybdenum-rich particles to drop to the bottom of a molten titanium pool 
where they sinter into agglomerates and form inclusions in the ingot 
produced. See, e.g., U.S. Pat. No. 3,508,910. The same problems of getting 
molybdenum to homogenize with titanium are also experienced with 
columbium, which like molybdenum, is also highly refractory. 
Matters are further complicated in that titanium alloys require relatively 
tight chemistries, and often the chemistry of the desired master alloy is 
poorly compatible with the homogenous alloying of the various components, 
due to differences in component solubility, melting point, density, etc. 
Furthermore, the chemistry of the alloy is frequently dictated by the 
alloying process used. 
Accordingly, it is an object of the invention to provide 
molybdenum/titanium alloys which may be readily formulated to be 
substantially free of high molybdenum inclusions. 
Another object of the invention is to provide columbium/molybdenum/titanium 
alloys which may be readily formulated to be substantially free of 
columbium inclusions. 
Still another object of the invention is to produce an alloy having 
relatively low aluminum. 
SUMMARY OF THE INVENTION 
In a preferred embodiment of the invention a thermite for use in preparing 
a Ti master alloy having low aluminum is produced, the master alloy 
comprising a predominant amount of Mo, and lesser amounts of Cb, Al, Si, 
O.sub.2, C, N.sub.2 and Ti. The master alloy of the invention comprises 
about 55-75% Mo, 6-16% Cb, 1-15% Al, 0.1-5% Si, 0-1% O.sub.2, 0-1%C, 0-1% 
N.sub.2 and balance Ti. In a most preferred embodiment of the invention, 
the master alloy comprises about 55-65% Mo, 6-16% Cb, 5-15% Al, 0.1-5% Si, 
0-1% O.sub.2, 0-1% C, 0-1% N.sub.2 and balance Ti. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A master alloy is an alloy of selected elements that can be added to a 
charge of metal to provide a desired composition or texture or to 
deoxidize one or more components of the mixture. 
According to the present invention, an intermetallic compound is first 
prepared using thermite processing. Thermite processing involves an 
exothermic reaction which occurs when finely divided aluminum mixed with 
metal oxides is ignited, causing reduction of the oxide and reaching 
temperatures of about 2200.degree. C., sufficient to propagate heat 
through the charge to homogenize the components comprising the resulting 
intermetallic compounds. 
Often, a simple thermite process uses a mixture of powdered iron (III) 
oxide, Fe.sub.2 O.sub.3 and powdered or granular aluminum. However, oxides 
of metals other than iron may be used, as discussed herein, and mixtures 
of these oxides may likewise be used. 
In practicing the invention, the mixed thermite components are charged to a 
furnace, typically a water-cooled, copper, below-ground reaction vessel, 
such as that described in "Metallothermic Reduction of Oxides in 
Water-Cooled Copper Furnaces," by F. H. Perfect, Transactions of the 
Metallurgical Society of AIME, Volume 239, August, 1967, pp. 1282-1286. 
See also U.S. Pat. No. 4,104,059, incorporated by reference herein. 
The mixture is thoroughly and intimately mixed prior to being charged to 
the furnace so the thermite reaction will occur rapidly and uniformly 
throughout the charge on ignition. 
The reaction vessel is preferably covered after the mixture is charged and 
the pressure within the vessel may be reduced, for example, to about 0.3 
mm Hg or less, followed by flooding the vessel with a high purity inert 
gas such as argon. Such evacuation and purging results in thermites of 
higher purity, lower nitrogen content. The thermite reaction is initiated 
with an igniter and allowed to proceed to completion. 
After the thermite is prepared using thermite processing, it is cooled and 
size reduced to powdered form using known methods, such as crushers, ball 
mills, pug mills, grinders, hydriding, etc. 
After size reduction, the intermetallic compound produced by the thermite 
process, typically Al.sub.3 Cb, is then mixed with at least one additional 
metal in powdered form, for example, Ti, to form a substantially uniform 
mixture. The resulting mixture is then pressed into a compact or 
briquetted with application of pressures of over about 7,000 psi and 
preferably of about 15,000-30,000 psi. Typically, such compacts are formed 
using an isostatic press. 
It is preferable, especially when forming large compacts, to place spacers 
at intervals within the compact in order to insure uniform compaction and 
produce more manageable compact sizes. Ten pound discs of compact are 
typically produced. The discs are then stacked in the furnace, under 
vacuum or inert gas and when the reaction starts, it tends to be 
semi-continuous and controlled rather than violent. The smaller compacts, 
when stacked, also help prevent melting of the compact, which is in some 
cases an undesirable result. 
The compacts or briquets are then heated, preferably with induction heat, 
to form the desired master alloy by fusion. No special pressure conditions 
are required for the fusion, which is generally carried out at atmospheric 
or a milli tor pressure and temperatures of about 600-1,700.degree. C., 
depending on the optimal fusion temperature of the compact. 
In a preferred embodiment of the invention, a master alloy for use in 
preparing a Ti (Beta 21S) alloy having low aluminum (i.e., less than about 
10% by weight aluminum) is prepared, comprising about 55-65% Mo, 6-16% Cb, 
5-15% Al, 0.1-5%, Si, 0-1% O.sub.2, 0-1%C, 0-1% N.sub.2 and balance Ti. In 
the thermite step the intermetallic compound Al.sub.3 Cb is produced, by 
mixing powdered aluminum fines with Cb.sub.2 O.sub.5 powder and at least 
one oxide, such as Fe.sub.2 O.sub.3 or SiO.sub.2. This thermite is then 
size reduced and mixed with powdered components, such as Mo and Ti, then 
compacted and fused. Most preferably, the master alloy so produced 
comprises about 60% Mo, 11% Cb, 10% or less Al, 0.4% or less Si, 0.25% or 
less O.sub.2, 0.02% or less C, 0-0.03% or less N.sub.2 and balance Ti. 
Unless otherwise specifically noted, all percentages set forth herein 
refer to weight percent. 
It is preferred to use alcohol to keep the mix from separating prior to 
compaction. As previously discussed, the resulting alloy may be hydrided 
to produce an end product in size reduced form, as is known. 
The master alloy is prepared as specified previously, then size reduced and 
mixed with sufficient Ti to yield a mixture, which upon compaction and 
melting yields an alloy comprising about 70-85% Ti, 10-20% Mo, 1-8% Al, 
1-8% Cb, 0-1% Si, 0-1% O.sub.2 and 0-1% Fe. (Beta 21S type alloy.)

Examples 
EXAMPLE 1 
It was desired to produce a master alloy having the chemistry 10% Al, 11% 
Cb, 60% Mo, 0.02% C, 0.003% N.sub.2, 0.11% O.sub.2, 0.4% Si balance Ti. An 
intermetallic compound Al.sub.3 Cb was produced using thermite processing 
as previously described. 5.5 pounds of this thermite, lot no. 42-096, 
comprising about 45.65% Al, 51.45% Cb, 2.32% Si, 0.015% C, 0.032% O.sub.2, 
0.004% S and 0.001% N.sub.2 was prepared via thermite processing as 
previously described and crushed to -50.times.200 mesh and mixed dry for 
five minutes with 15 pounds of -100 mesh Mo and 5.25 pounds of 
-100.times.325 mesh Ti. After five minutes of dry mixing, 65 ml of alcohol 
was added and the mixture was remixed for 15 minutes. The mixture was then 
packed into a CIP bag and isostatically pressed at 25,000 psi to produce a 
25.75 lb. compact 4.25" dia..times.10.75". The resulting compact was 
placed in a 200 lb. induction furnace graphite crucible and covered with a 
graphite lid, then purged with argon. The compact was heated to about 
1600.degree. C. for about 15 minutes. The argon flow was maintained while 
the fused compact cooled. The resulting master alloy was fully alloyed, 
was cleaned and crushed to -20 mesh, and analyzed as follows: 
______________________________________ 
RAI/McCreath 
______________________________________ 
Al - 10.10% 
Cb - 11.06% 
Mo - 60.08% 
Ti - 17.94% 
C - 0.057% 
N.sub.2 - 
0.130% 
O.sub.2 - 
0.263% 
Si - 0.40% 
S - 0.004% 
______________________________________