An oxide dispersion strengthened cobalt-chromium superalloy produced by mechanical alloying containing a refractory oxide, preferably yttrium oxide, and characterized by excellent corrosion resistance, high fatigue strength and high temperature stability; and prostheses formed from said superalloy.

BACKGROUND OF THE INVENTION 
This invention relates to a cobalt-based alloy containing a substantial 
proportion of chromium and referred to herein as a cobalt-chromium 
superalloy. More particularly the invention is concerned with certain 
cobalt-chromium oxide dispersion strengthened (O.D.S.) superalloys 
produced by a mechanical alloying process. 
The term "superalloy" is a term of art which generally signifies an alloy 
having particularly high strength, good mechanical and corrosion-resistant 
characteristics and a stable microstructure. Of particular interest are 
those alloys which additionally retain high strength properties (and 
stable microstructures) following thermal treatments at extremely high 
temperatures. 
The known alloy Vitallium.RTM. is a high corrosion-resistant 
cobalt/chromium alloy which is used successfully in numerous orthopaedic 
applications. A typical composition for Vitallium is the following: 
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Element % by weight 
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Carbon 0.25 
Silicon 0.75 
Manganese 0.70 
Chromium 28.00 
Molybdenum 5.50 
Cobalt 64.80 
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Because of its many favorable properties, particularly corrosion 
resistance, Vitallium is used extensively in orthopaedic applications, 
especially for prostheses. A particularly useful development in the area 
of orthopaedic implants is the provision of a porous coating in the form 
of multiple layers of spherical Vitallium particles on the surface of a 
Vitallium implant to encourage bone-growth in a cementless system. 
It is known that the properties of a given metal alloy are dependent upon 
its composition and also upon the manner in which the various alloying 
ingredients are formed into the final alloy. "Mechanical alloying" is a 
process which produces homogeneous composite particles with an intimately 
dispersed, uniform internal structure. The process is described in an 
article entitled "Dispersion Strengthened Superalloys by Mechanical 
Alloying" by John S. Benjamin, Metallurgical Transactions, Vol. 1, October 
1970, p. 2943. 
U.S. Pat. No. 3,591,362, issued July 6, 1971 to John S. Benjamin discloses 
a composite metal powder formed by the technique of mechanical alloying. 
Mechanical alloying is particularly advantageous for making superalloys 
that cannot be made readily by melting or by conventional powder 
metallurgy. Since the strength, and certain other properties, of 
superalloys depends ultimately on the presence of dispersions of 
intermetallic compounds and the utility of the superalloy is inherently 
limited by the stability of these components, the choice of dispersant 
materials, as well as alloying metals, is important for the performance of 
the final alloy. 
It is known that the inclusion of certain selected oxides in the alloy 
composition can improve the properties of the final alloy and oxide 
dispersion strengthened (O.D.S.) superalloys made by the mechanical 
alloying process exhibit high-temperature strength and stability as a 
result of the presence of stable oxide dispersions which resist thermal 
damage and permit much greater freedom in alloy design. 
It has now been found that improved cobalt-chromium superalloys made in 
accordance with O.D.S. mechanical alloying procedures not only have the 
high corrosion-resistant properties typical of Vitallium but also have 
excellent room temperature strength (tensile and fatigue) properties which 
are substantially retained after exposure to severe thermal conditions. 
These properties are substantially more advantageous than would be 
expected from prior art alloys made by similar techniques, for example the 
nickel-chromium, cobalt-chromium (of different composition to those 
specified herein), and iron-chromium systems disclosed in U.S. Pat. No. 
3,591,362. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a high strength, 
corrosion-resistant, high temperature stable, composite alloy produced by 
mechanical alloying and consisting essentially of the following percentage 
composition by weight: 
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chromium 26 to 30 
molybdenum 5 to 7 
manganese 0 to 1 
silicon 0 to 1 
iron 0 to 1.5 
nickel 0 to 2.5 
carbon 0 to 0.35 
refractory oxide 0.05 to 1.0 
aluminum 0.05 to 0.6 
titanium 0.05 to 0.6 
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and the balance cobalt, apart from trace amounts of incidental impurities; 
in which the refractory oxide is silica or an oxide of a metal of Group 
IIA, IIIA, IIB or IVB of the Periodic Table which forms high 
temperature-stable, non-accretive, fine particles and is present in the 
dispersed phase, and said alloy, after fabrication by mechanical alloying, 
consolidation and hot rolling, has an ultimate tensile strength of at 
least 250, preferably 280 to 300 k.s.i. (p.s.i..times.10.sup.3), a 0.2% 
offset yield strength of at least 220, preferably 235 to 275 k.s.i., an 
elongation of 2 to 5%, and a fatigue strength at 10.sup.7 cycles (Rotating 
Cantilever Beam) of at least 100 k.s.i. 
The refractory oxide which provides the dispersed oxide phase in the 
improved ODS superalloy of the invention is suitably an oxide of a metal 
whose negative free energy of formation of the oxide per gram atom of 
oxygen at about 25.degree. C. is at least about 90,000 calories and whose 
melting point is at least about 1300.degree. C. Additionally the oxide 
must be adapted to form non-accretive fine particles in the dispersed 
phase. Examples of suitable refractory oxides are the oxides of silicon, 
beryllium, magnesium, calcium, aluminum, yttrium, cerium, titanium, 
zirconium, hafnium and thorium. 
The most preferred oxide for the purposes of the present invention and 
particularly when the superalloy is used for the production of prostheses, 
is yttrium oxide, Y.sub.2 O.sub.3 ; and the invention will be particularly 
described herein with reference to this preferred oxide. 
A preferred embodiment of the invention is a high strength, 
corrosion-resistant, high temperature stable, composite alloy as described 
above in which the percentage composition by weight is: 
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chromium 26.20 
molybdenum 5.40 
manganese 0.78 
silicon 0.67 
iron 0.18 
nickel 0.45 
carbon 0.025 
yttrium oxide (Y.sub.2 O.sub.3) 
0.50 
aluminum 0.59 
titanium 0.30 
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and the balance cobalt, apart from trace amounts of incidental impurities. 
Another preferred embodiment is an alloy as described above in which the 
percentage composition by weight is: 
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chromium 26.00 
molybdenum 
5.20 
manganese 
0.79 
silicon 0.76 
iron 0.17 
nickel 0.56 
carbon 0.03 
yttrium oxide 
0.54 
aluminum 0.27 
titanium 0.30 
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and the balance cobalt, apart from trace amounts of incidental impurities. 
The invention also provides a prosthethic device formed from an alloy as 
described. Preferably the alloy is forged into the required shape for the 
prosthetic device, for example, an artificial hip. 
DETAILED DESCRIPTION OF THE INVENTION 
The high strength, corrosion-resistant, high temperature stable, composite 
alloy of the invention is produced in powder form by mechanical alloying, 
which comprises subjecting a mixture of the metallic and oxide ingredients 
to dry, high energy milling in a grinding mill and continuing the milling 
under controlled conditions until a uniform distribution of oxide and 
metallic ingredients is achieved. The resulting homogeneous composite 
alloy in powder form is then consolidated, preferably by hot extrusion, 
and the resulting extruded solid alloy is further fabricated by hot 
rolling. 
Mechanical alloying is a dry, high-energy milling process that produces 
composite metal powders with controlled, extremely fine microstructures. 
The powder is produced in high-energy attrition mills or special large 
ball mills. For the production of the ODS alloy according to the present 
invention, a mixture of commercially available metal powders, master alloy 
alloy powders and the very fine oxide powder is charged into the grinding 
mill. 
Under controlled conditions powder particles first cold weld together, 
building up larger particles and then fracture, breaking down into the 
composite powder particle. The interplay between the welding and 
fracturing subdivides and kneads all the ingredients to provide a very 
uniform distribution of the oxide and the metallic components. 
The resulting oxide-containing powder is consolidated by extrusion and hot 
rolling prior to forging into the desired configuration for a prosthesis. 
Consolidation techniques other than extrusion, for example hot isostatic 
pressing or rapid omnidirectional compaction (ROC) (see U.S. Pat. No. 
4,142,888) may be used.

Referring to FIG. 1 of the drawings, this clearly illustrates that a 
superalloy piece made from a typical mixture of alloying ingredients 
according to the invention and fabricated by the mechanical alloying, 
extrusion and hot-rolling steps described herein rivals the strength 
levels of the ultra-high strength steels which have, as a family, the 
highest strength levels of any industrial alloy systems. 
The 10.sup.7 cycle rotating beam fatigue strengths for these ultra-high 
strength steels ranges from 105,000 to 135,000 psi. The ODS superalloy of 
the invention as-hot rolled far surpasses these materials with a 10.sup.7 
cycle value of 157,000 psi. Even the full sinter cycle annealed material 
is very competitive at 107,000 psi. 
These very high strength levels far surpass anything ever observed for 
wrought cobalt-base superalloys, as illustrated in FIG. 1. In fact, even 
the full sinter cycle annealed material has strength levels that exceed 
the solution heat treated and aged properties. 
The bar stock resulting from hot rolling may be heat treated, forged to the 
desired prosthesis shape and then machined to produce a finished smooth 
surface prosthesis. 
If desired, the smooth surface prosthesis then may be further treated to 
provide a porous coated prosthesis. 
In FIG. 2 of the drawings the fatigue properties of a preferred alloy of 
the invention as illustrated in Example 1 are compared in the first graph 
with the fatigue properties of FHS Vitallium and cast Vitallium, 
respectively. The second graph shows that the alloy of the invention 
retains a fatigue strength comparable to that of FHS Vitallium even after 
thermal exposure. 
The following Examples illustrate the invention and the manner in which it 
may be performed. 
EXAMPLE 1 
A mixture of alloying ingredients comprising in percentage by weight: 
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chromium 26.20 
molybdenum 5.40 
manganese 0.78 
silicon 0.67 
iron 0.18 
nickel 0.45 
carbon 0.02 
ytrrium oxide 0.50 
aluminum 0.59 
titanium 0.30 
cobalt balance 
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was introduced into a grinding mill or attritor of the stirred ball mill 
type capable of providing agitation milling or high energy milling. This 
is a condition wherein sufficient mechanical energy is applied to the 
total charge so that a substantial portion of the attritive elements are 
continuously maintained in a state of relative motion. This type of 
milling is described in U.S. Pat. No. 3,591,362. 
The high energy milling in air was continued for about 24 hours until a 
substantially homogeneous powder of the oxide-containing superalloy was 
produced. 
The said superalloy powder was passed from the attritor into an extrusion 
can and then to an extrusion press where it was consolidated into an 
extrusion billet of about 4 inches diameter. 
The billet emerging from the extrusion press was then hot rolled to a rod 
of about 1 inch diameter. 
The as-hot rolled one inch diameter rod was subjected to the Krouse 
Cantilever rotating beam fatigue test and the resulting data are given in 
the first graph of FIG. 2. 
The rod was then given a full sinter cycle anneal. 
The fatigue properties of the superalloy subjected to the full sinter cycle 
anneal are illustrated in the second graph of FIG. 2. 
The as-hot rolled superalloy prepared above had a tensile strength of 299 
k.s.i.; a 2% offset yield strength of 259 k.s.i.; and a room temperature 
ductility (elongation) of 4.4%. 
The full sinter cycle annealed alloy had a tensile strength of 184 k.s.i.; 
a 2% offset yield strength of 150 k.s.i. and a ductility of 14.5%. 
EXAMPLE 2 
A mixture of alloying ingredients comprising in percentage by weight 
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chromium 26.00 
molybdenum 5.20 
manganese 0.79 
silicon 0.76 
iron 0.17 
nickel 0.56 
carbon 0.03 
yttrium oxide 0.54 
aluminum 0.27 
titanium 0.30 
cobalt balance 
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was processed in a similar manner to the mixture in Example 1. The 
resulting alloy powder was consolidated, hot-rolled and subjected to the 
same heat treatment steps as the alloy of Example 1 and the tensile 
properties of the resulting alloy were comparable to those of the alloy of 
Example 1. 
The alloys of both Examples 1 and 2 above are biocompatible and are 
particularly suitable for use in prostheses. Both comply with ASTM 
F799-82, "Standard Specification for Thermodynamically Processed 
Cobalt-Chromium-Molybdenum Alloy for Surgical Implants" which permits 1.0% 
maximum nickel and 1.5% maximum iron. 
The chemistry of the ODS superalloy of the invention is unique. Compared to 
FHS Vitallium.RTM. the superalloy of the invention has the specific 
alloying additions of yttrium oxide, aluminum and titanium which 
accomplish the desired purpose of retaining substantially high values of 
the tensile and fatigue strengths following thermal exposure. 
This alloy chemistry accomplishes the above by forming a combined 
dispersion (Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3) that, because of a close 
interparticle spacing, prevents significant growth in grain size and 
avoids resulting loss of properties. 
This unique cobalt-base alloy chemistry is particularly adapted for the 
specific application of porous coated prostheses that provide a new 
improved method of prosthesis fixation eliminating the use of bone cement.