Composite article, alloy and method

A mixed oxide ceramic product is made directly from a metal alloy of titanium, zirconium and/or hafnium and niobium, tantalum or hafnium, where the normally combustible alloy of titanium and zirconium or hafnium is passivated by the addition of more than about 7 atomic percent of niobium and/or tantalum and or vanadium which alloy can then be heated in air at atmospheric pressure to a temperature of from about 800 degrees C. to about 1500 degrees C. to produce an adherent monolithic ceramic containing product.

FIELD OF THE INVENTION 
The present invention relates to the manufacture of ceramic layers on 
metal, shaped ceramic bodies, cermet articles, composite articles, and 
alloys used in such manufacture and more particularly to the field of 
reaction formed ceramics, articles made thereby, and alloys for use in 
their manufacture. 
BACKGROUND OF THE INVENTION 
Many of the alloys of titanium with zirconium or hafnium are characterized 
by their extraordinarily rapid oxidation in air at only modestly elevated 
temperatures. This characteristic has severely limited the usefulness of 
such alloys for many applications that could otherwise advantageously use 
the other physical properties of those alloys. In particular, the light 
weight, high strength and corrosion resistance of the alloys, as well as 
their electrical conductivity properties, biocompatibility, ease of 
closed-die forming and other desirable properties have not been fully 
exploited due to the potential ignition of the alloy in air at relatively 
low temperatures. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide utility to a 
class of titanium and zirconium alloys. It is a further object of the 
present invention to provide a method of fabricating a class of reaction 
formed ceramics directly formed from such alloys and articles produced 
thereby. It is yet a further object of the present invention to provide a 
method for forming adherent surfaces on such metal alloys which are hard, 
smoother, substantially inert and suitable for a wide variety of 
applications from cuttlery to implantable prosthetic devices, and method 
for renewing such monolithic surfaces.

SUMMARY OF THE INVENTION 
The inventors have discovered that ceramic bodies and a variety of ceramic 
surface layers formed in a metal body can be achieved by providing a 
substrate comprised of an alloy containing titanium, zirconium, and/or 
hafnium together with metals in minor amounts selected from the group 
consisting of niobium, tantalum and vanadium. The oxides of the metals can 
be obtained in situ by the controlled oxidation of the surface of the 
metal alloy substrate. Further, the present invention is particularly 
effective where the titanium and zirconium alloy is passivated against 
rapid oxidation by the presence of minor amounts of niobium or tantalum. 
Still further, oxides containing the metals of the metal alloys selected 
can be reaction formed throughout the metal alloy structure to reaction 
form either a monolithic cermet or ceramic body or structure by the 
selection of appropriate oxidation conditions. 
DETAILED DESCRIPTION OF THE INVENTION 
As used in this specification and the appended claims, the terms below are 
defined as follows: 
"Ceramic" is not to be construed as being limited to a ceramic material in 
the classical sense, that is, in the sense that it consists entirely of 
non-metallic and inorganic materials, but rather refers to a material 
which is predominantly ceramic with respect to either composition or 
dominant properties, although the material may contain minor or 
substantial amounts of one or more metallic constituents. 
"Microporosity": Porosity in which pore diameters are of the order of 1-10 
.mu.m ("microns"), as opposed to "macroporosity" in which pores are 
considerably larger than 10 .mu.m. 
"Oxidation reaction product" generally means one or more metals in an 
oxidized state wherein a metal has given up electrons to or shared 
electrons with another element, compound or combination thereof. 
Accordingly, an "oxidation reaction product" under this definition 
includes the product of the reaction of one or more metals with an 
oxidant, such as those described in this application. 
"Oxidant" means one or more suitable electron acceptors or electron sharers 
which may be a solid, a liquid or a gas or some combination of these at 
the process conditions. 
"Parent metal" is intended to refer to relatively pure metals, commercially 
available metals with impurities and/or alloying constituents therein, and 
alloys and intermetallic compounds of the metals. When a specific metal is 
mentioned, the metal identified should be read with this definition in 
mind unless indicated otherwise by the context. 
A solid, liquid or vapor-phase oxidant, or a combination of such oxidants 
may be used, as noted hereinafter. For example, oxidants which may be 
emphasized include, without limitation, oxygen, nitrogen, ammonia, a 
halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, 
and compounds and combinations thereof, for example, silica (as a source 
of oxygen), methane, ethane, propane, acetylene, ethylene, and propylene 
(as a source of carbon), and mixtures such as air, cracked ammonia, 
N.sub.2 /H.sub.2, H.sub.2 /CH.sub.4 and other hydrocarbons H.sub.2 
/H.sub.2 O and CO/CO.sub.2, the latter two (i.e. H.sub.2 /H.sub.2 O and 
CO/CO.sub.2) being useful in reducing the oxygen activity of the 
environment. 
A vapor-phase (gas) oxidant is preferred, and specific embodiments of the 
invention are described herein with reference to the use of vapor-phase 
oxidants. If a gas or vapor oxidant is used, the term "vapor-phase 
oxidant" means a vaporized or normally gaseous material which provides an 
oxidizing atmosphere. For example, oxygen or gas mixtures containing 
oxygen, including air, are preferred for obvious reasons of economy. When 
an oxidant is identified as containing or comprising a particular gas or 
vapor, this means an oxidant in which the identified gas or vapor is the 
sole oxidizer of the parent metal at certain conditions under the 
conditions employed in the oxidizing environment utilized. For example, 
although the major constituent of air is nitrogen, the oxygen content of 
air is the oxidizer for the parent metal because oxygen is a significantly 
stronger oxidant than nitrogen. Air therefore falls within the definition 
of an "oxygen-containing gas" oxidant but not normally within the 
definition of a "nitrogen-containing gas" oxidant. 
In its preferred practice, the present invention will be described 
hereinafter with respect to alloys of titanium, zirconium or hafnium, or 
mixtures of the latter to which have been added minor amounts of other 
specific metals in sufficient quantity to render the overall alloy more 
resistant to rapid, combustion-like, oxidation at the temperatures 
described. It should be appreciated that wide composition limits are 
possible in the practice of the present invention when oxidation 
conditions are sufficiently controlled in any manner to prevent rapid 
reaction. For example, uncontrolled oxidation can be prevented where an 
inert gas diluent or reducing gas diluent is employed in a sufficient 
amount to prevent uncontrolled oxidation, or where the surface of the 
alloy is protected against unmoderated exposure to high concentrations of 
a gaseous oxidant. To obtain particularly advantageous metallurgical 
properties it may be necessary to conduct the oxidation reaction in air at 
elevated temperatures i.e., where combustion would normally occur. It may 
therefore be necessary to passivate the alloy of titanium, containing 
zirconium or hafnium, or mixture thereof, with minor amounts of a 
passivating metal or metal alloy, selected from the group consisting of 
niobium, tantalum or vanadium or mixture thereof. The foregoing also 
contemplates utilizing overpressures of oxygen where the Ti alloy has been 
passivated with niobium, tantalum or vanadium. 
As will be more fully described hereinafter, the titanium alloys described 
herein and the process of their conversion into oxides or oxidation 
reaction products can be regulated to produce a type of cermet or ceramic 
product which product differs radically from prior attempts to provide 
inert wear resistant surfaces or cermet or ceramic bodies by either the 
bonding of inorganic ceramic-like material to the surface of a metal or by 
surface passivation reactions with molten salts or the like; or by the 
reaction formation of a ceramic body by the oxidation of a molten metal 
either alone or by infiltrating a preform matrix with an oxidizable metal 
and forming the ceramic in situ. 
Typically, a rapid combustion-like reaction in air of some of the alloys 
described herein will produce a loose powder containing the single or 
mixed oxides of the parent metal and the alloy constituents. Such a result 
is, of course, unsatisfactory for the production of ceramic articles 
without further ceramic fabrication processing steps. The controlled 
oxidation of the present invention, directly produces unitary, monolithic 
adherent structural layers or rigid ceramic or cermet bodies. The 
processes of the present invention are suitable for the formation of 
shaped ceramic articles ceramic-metal composites and for the formation of 
smooth-hard impact resistant ceramic surfaces on metal articles. The 
present invention also contemplates the use of the alloys described as 
reinforcements in other ceramic articles which can be converted at high 
temperatures to the ceramic articles described herein in another ceramic 
body. 
The present invention in its preferred practice forms oxide layers on the 
surface of the passivated alloys described, in air at temperatures of from 
about 300.degree. C. to 800.degree. C. and most preferably from about 
500.degree. C. to about 800.degree. C. which layers are adherent, 
monolithic, hard, smooth, wear resistant surface layers. Moreover, since 
intricate shapes can be fabricated from the preferred metal alloy, the 
complete conversion of the alloy to oxides or mixed oxides can be 
controllably managed, preferably at temperatures of about 800.degree. C. 
to about 1500.degree. C. and for a sufficient period of time to produce 
shaped ceramic articles which have been substantially completely oxidized 
and typically contains uniformly distributed micro-porosity which varies 
in pore size when the process is conducted at different temperatures on 
the same starting material. 
Due to the moderation of the rate of oxidation imparted by the processes 
and compositions described herein, relatively thick oxide layers can be 
obtained on the parent metal or metal alloy at moderate temperatures and 
within relatively short times. This is shown most clearly in FIGS. 6-8. It 
is therefore now feasible to provide a hard surface on the parent metal 
while retaining the strength or toughness of the underlying metal 
substrate. For example, surface oxide thicknesses of 2.5 to 25 .mu.m can 
impart a hardness of approximately R.sub.c 70 to the cutting edge of a 
cutlery implement made from a titanium alloy comprising 35 wt. % zirconium 
and 10 wt. % niobium with the balance being titanium, by heating the 
implement in air at between about 300.degree. C. to about 800.degree. C. 
for a sufficient period of time to achieve the described hard, smooth 
oxide or ceramic layer. The process can even be repeated on a worn 
implement where a new hardened surface is desired using such readily 
available equipment as a conventional self-cleaning oven which is capable 
of reaching temperatures of about 300.degree. C. 
The utility of the present invention is further enhanced for many end use 
applications due to the excellent fabricability of the described titanium 
alloys. A wide range of mechanical properties is obtainable. Normally a 
titanium alloy containing 35 wt. % zirconium and 10 wt. % niobium exhibits 
a yield strength of only 2,000-3,000 psi at 1,350.degree. F. (with 200% 
elongation). Such an alloy can however be solution-treated, quenched and 
aged to obtain a room temperature yield strength of 140,000 psi. The alloy 
can be closed-die-forged to obtain articles of complex shape which can 
subsequently be surface hardened by the controlled oxidation process 
described herein while retaining some of the mechanical properties 
imparted by the foregoing mill practice. The article can of course be 
completely oxidized to produce a monolithic ceramic body as previously 
discussed. Likewise, it is possible to investment cast the alloy to 
achieve the desired shape and subsequently oxidize the shaped article in a 
partial or complete manner as described. In this manner, cutlery 
implements, dental castings, orthopedic prosthetic devices and the like 
can be fabricated. Small parts requiring great wear resistance, such as is 
necessary in certain firearms mechanisms can particularly benefit from the 
practice of this invention. 
An unexpected benefit is obtained by the practice of the present invention 
when making medical devices that in use are designed to bear against a 
plastic element, such as ultra high molecular weight polyethylene. The 
surface of the shaped article, after the controlled oxidation described 
herein to produce a smooth surface layer, is smooth enough to reduce 
attrition of the plastic or fretting of the plastic into small particles 
which is an undesirable phenomenon in some orthopedic prothesises 
implants. This advantage combined with the excellent biocompatibility of 
titanium-zirconium alloys and the possibility of forming materials having 
a generally low modulus of elasticity makes and the articles formed as 
described, highly desirable for such medial devices and in particular 
prosthetic devices. A low modulus better matches materials of relatively 
low stiffness such as mammalian bone. Likewise, the volume change upon 
conversion to a ceramic are acceptable for most applications and the 
problems normally encountered with mismatches in the coefficients of 
expansion are minimal. Undue experimentation is not believed to be 
necessary to optimize the desired properties by adjusting alloy 
composition and preparation. 
Understanding the reasons for the formation of the articles described 
herein from the alloys described is incomplete at the present time. It is 
postulated that the advantageous properties achieved, according to the 
practice of the present invention are achieved by the relatively rapid 
transport of dissolved oxygen into the binary alloy of titanium and 
zirconium by its solution into the third alloying metal constituent which 
is present at the grain boundaries of the binary alloy constituents. Such 
a mechanism would help to explain why the ceramic layer obtained at 
relatively low temperatures remains monolithic and adherent rather than 
forming oxides simply as a surface phenomenon as previously done, which 
could produce scales which are easily dislodged if the oxidation were 
performed as described herein. Alloying materials which have high 
oxidizing agent transport properties should then be suitable for use in 
the practice of the present invention. In the practice of the present 
invention it has been discovered niobium, tantalum and vanadium provide 
both passivation of the ignition characteristics of the Ti--Zr binary 
alloy and dissolved oxygen transport for ceramic formation. 
The following discussion is intended to advance further understandings of 
the invention while not being bound to any particular theory. 
While the extensive work of W. Wyder and M. Hoch (The System 
Niobium-Titanium-Zirconium oxygen at 1500.degree. C. Trans. Metallurg. 
Soc., AIME, Vol. 224, pp. 373-378, 1962) in constructing the Nb--Ti--Zr 
phase diagrams representing oxygen levels of 10, 20, 30, 40, 50 and 55 
atomic % is recognized as a classical accomplishment, the fact that it 
represents only the isotherm at 1500.degree. C. limits its utility in 
studying oxidation at the lower temperatures employed in the practice of 
the present invention. 
A more relevant phase diagram is that of the Zr--Ti--Nb system at 
570.degree. C. published in 1968 by F. Ishida, (Ishida, F., T. Doi and M. 
K. Tada Nippon Kinzoku Gakkaishi, Vol. 32, No. 7, pp. 684-685, 1968) 
although one must always bear in mind the effects of oxygen on the system 
which may be estimated from the metal-oxygen binary phase diagrams. 
The inventors herein have determined experimentally the boundary in the 
TiZrNb ternary system which separates alloys into "passive" and 
"ignitable" categories. The Ternary Oxidation diagram for Ti--Zr--Nb 
Alloys at 700.degree. C. with the ignitable and passive regions identified 
is shown in FIG. 21. The titanium rich portion of that diagram is shown in 
larger scale in FIG. 4. The weight gain shown graphically for Alloys 7, 8, 
and 9 in FIG. 1 and Alloys 8 and 9 in FIGS. 2 and 3 can be related to the 
data contained herein. In attempting to account for the differences 
between various TiZrNb alloys, it was observed that transformations in the 
metallic phases do not explain observed differences in oxidation behavior. 
This observation, coupled with the realization that all such alloys owe 
their oxidation resistance to protective oxide surface films, could lead 
one to conclude that studies of the oxide phases are more appropriate 
toward understanding oxidation behavior. The works of Kofstad, P. High 
Temperature Oxidation of Metals, John Wiley and Sons, Inc., New York, 
1966, and others present methods of identifying various mechanisms of 
oxidation based upon determining characteristic shapes of oxidation rate 
curves. An example of this approach is that of the oxidation kinetics of 
titanium at various temperatures. Referring to the general equation 
w.sup.n =kt (where w=weight gain per unit area, t=time, k=a constant), 
oxidation occurs at the following rates within indicated temperature 
regimes: 
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I: 100-400.degree. C. 
logarithmic 
II: 400-600.degree. C. 
transition (log - 
parabolic or cubic) 
III: 600-1000.degree. C. 
parabolic 
IV: 1000-1100.degree. C. 
linear 
______________________________________ 
Phase I is dominated by oxide film formation, II by oxygen dissolution in 
which cubic-versus-parabolic behavior is determined by pre-existing oxygen 
gradients in the metal, III by a combination of oxygen dissolution and 
scale formation, and IV by loss of protective behavior. 
In addition to reviewing empirical studies of oxidation kinetics, it is 
important to appreciate the large number of phases which may be observed 
in oxidation products of TiZrNb alloys. Although Zr in oxides is nearly 
always tetravalent, three allotropic forms of ZrO.sub.2 are observed: 
monoclinic (at .ltoreq.949.degree. C.), tetragonal 
(949.degree.-1221.degree. C.), and cubic (&gt;1221.degree. C.).sub.30. 
Titanium presents a much more extensive spectrum of oxides: 
______________________________________ 
Oxide Structure 
______________________________________ 
.alpha.-TiO monoclinic 
.beta.-TiO cubic 
Ti.sub.2 O.sub.3 hexagonal 
Ti.sub.3 O.sub.5 monoclinic 
TiO.sub.2 (anatase) tetragonal 
TiO.sub.2 (rutile) orthorhombic 
TiO.sub.2 (brookite) 
hexagonal 
______________________________________ 
Niobium oxides are nearly as diverse as those of titanium: 
______________________________________ 
Oxide Structure 
______________________________________ 
NbO cubic 
Nb.sub.2 O.sub.3 
NbO.sub.2 tetragonal 
Nb.sub.2 O.sub.5 
orthorhomic 
______________________________________ 
In understanding the possible phases which may be observed in TiZrNb 
oxidation products, one must not only consider the single oxides of each 
metal, but much more complex compounds formed in the quaternary system 
Ti--Zr--Nb--O shown in FIG. 5. (and more with description). 
The following examples further describe the processes, compositions, and 
products encompassed within the scope of the present invention. In the 
examples it is demonstrated that under certain conditions the binary alloy 
of titanium and zirconium is passivated from undergoing combustion-like 
oxidation by the addition of niobium or tantalum, or vanadium (or mixtures 
thereof) in modest amounts. 
EXAMPLE 1 
A. Oxidation of TiZrNb Alloys at 600.degree.-800.degree. C. 
Twenty vacuum-arc-melted alloy buttons (Table I) representing five 
different Nb concentrations (3.5, 5.7, 7.0, 10.5 and 14 at .%) at various 
Zr:Ti atomic ratios (2:1-1:3) were prepared for thermogravimetric studies 
of oxidation rate. Duplicate buttons were prepared and studied for five of 
the 15 different compositions. 
For thermogravimetric evaluation, 400-g buttons of each alloy were 
sectioned and machined to yield specimens of approximately 
10.times.18.times.18 mm. All buttons were given a vacuum heat treatment 
for one hour at 1500.degree. C. and furnace-cooled to ensure alloy 
homogeneity. Each specimen was weighed to the nearest 0.1 mg and measured 
dimensionally to determine total surface area. Oxidation was effected by 
placing sets 
TABLE I 
__________________________________________________________________________ 
Chemical Composition 
Calculated Values 
Actual Analysis 
Alloy 
Zr:Ti 
Atomic % Weight % Weight % 
No. At. Ratio 
Zr Ti Nb Zr Ti Nb Zr Ti Nb 
__________________________________________________________________________ 
1 2:1 64.5 
32 3.5 78 17.7 
4.3 77.3 
18.5 
4.26 
2A 15:1 48.3 
48.2 
3.5 62.6 
32.8 
4.6 62.8 
32.8 
4.35 
2B 1:1 48.3 
48.2 
3.5 62.6 
32.8 
4.6 62.6 
33.0 
4.43 
3 1:2 32 64.5 
3.5 46.1 
48.8 
5.1 46.4 
48.7 
4.90 
4 2:1 62 31 7 72.6 
19.1 
8.3 72.7 
19.6 
7.76 
5 1:1 46.5 
46.5 
7 59.6 
31.3 
9.1 59.6 
31.5 
8.86 
6 110:1.5 
37 56 7 50.3 
40 9.7 50.6 
40.3 
9.15 
7 1:2 31 62 7 43.9 
46.1 
10.0 
43.6 
47.0 
9.42 
8 1:2.5 
26.5 
66.5 
7 38.7 
50.9 
10.4 
37.3 
52.9 
9.77 
9 1:3 23.2 
69.8 
7 34.6 
54.7 
10.7 
34.8 
55.1 
10.1 
10A 2:1 59.7 
29.8 
10.5 
69.4 
18.2 
12.4 
69.7 
18.8 
11.4 
10B 125:1 
59.7 
29.8 
10.5 
69.4 
18.2 
12.4 
69.8 
18.8 
11.5 
11 1:1 44.8 
44.7 
10.5 
56.7 
29.7 
13.6 
57.4 
30.1 
12.5 
12A 1:2 29.8 
59.7 
10.5 
41.5 
43.6 
14.9 
42.6 
43.4 
14.0 
12B 1:2 29.8 
59.7 
10.5 
41.5 
43.6 
14.9 
42.5 
43.5 
14.0 
13 1:1 43 43 14 53.9 
28.3 
17.8 
54.7 
28.7 
16.6 
14A 220:1 
64.5 
29.8 
5.7 75 18.2 
6.8 75.4 
18.2 
6.39 
14B 2:1 64.5 
29.8 
5.7 75 18.2 
6.8 74.7 
19.0 
6.36 
15A 1:2 29.8 
64.5 
5.7 42.9 
48.8 
8.3 44.2 
47.9 
7.89 
15B 1:2 29.8 
64.5 
5.7 42.9 
48.8 
8.3 44.2 
48.6 
8.19 
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comprised of specimens of each alloy in an electric resistance furnace (air 
atmosphere) for times of 10, 20, and 30 minutes at temperatures of 
600.degree., 650.degree., 700.degree., 750.degree. and 800.degree. C., 
respectively. 
Visual observations of the specimens were made at approximately five-minute 
intervals in order to determine more accurate ignition times at particular 
furnace temperatures. Ignition was indicated by visible sparking and a 
distinct color change as the specimen temperature rapidly exceeded furnace 
temperature. During oxidation, samples were placed in random locations on 
a metal plate which rested on the bottom of the furnace and whose 
temperature was monitored by a thermocouple. Specimens were placed into 25 
ml fireclay crucibles in order to contain spalling oxidation product which 
might otherwise contaminate neighboring specimens. Included in each 
furnace cycle were coupons (1.5.times.25.times.5-0 mm) of pure Zr, Hf, Ti, 
and Nb, which served as standards with known oxidation kinetics. Following 
oxidation, weight-gain values were determined in units of mg/dm.sup.2, in 
accordance with customary corrosion rating convention. 
In an effort to obtain more data points by which to establish a boundary in 
the TiZrNb ternary system separating "passive" from "ignitable" alloys, 
six additional compositions were cast (Table II). 
TABLE II 
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Additional Alloys 
Atomic % Weight % 
Alloy No. 
Zr:Ti Zr Ti Nb Zr Ti Nb 
______________________________________ 
16 1:5.44 14.8 80.5 4.7 23.9 68.3 7.8 
17 1:5.45 14.3 77.9 7.8 22.7 64.8 12.5 
18 5 1:3.62 19.6 71.0 9.4 29.5 56.1 14.4 
19 1:1.86 31.2 58.0 10.8 42.5 41.5 15.0 
20 1.18:1 47.8 40.6 11.6 53.1 23.7 13.2 
21 1.38:1 54.0 39.1 6.9 66.2 25.2 8.6 
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Specimens of these compositions were tested only for ignitability at 
650.degree., 700.degree., and 800.degree. C. No time-dependent weight gain 
data were taken for these specimens. 
B. Oxidation of TiZrNb Alloys at 1200.degree.-1500.degree. C. 
Additional specimens of the experimental alloys were oxidized in air for 
1.0 hr at 1200.degree. C. This treatment was sufficient to obtain heavy 
oxide layers on even the most passive alloys. The oxides were evaluated by 
X-ray diffraction, metallography, and electron microprobe. 
C. Evaluation of a Ti35Zr10Nb (wt. %) Developmental Ingot 
A triple-melted, 4-inch-diameter (101.6 mm) version of Alloy No. 9 (Table 
I) was produced from Ti sponge, Zr sponge and Nb46Ti (wt. %) turnings by 
conventional consumable electrode vacuum arc melting methods. This alloy, 
had a chemical composition of 54.7Ti34.6Zr10.7Nb (wt. %), equivalent to 
69.8Ti23.2Zr7Nb (atomic %). After removal of the shrink pipe and minor 
sidewall machining, the ingot measured 3.95" dia..times.6.5" long and 
weighed 15.3 lb (100.3 mm.times.165.1-mm, 6.94 Kg). Forging at 800.degree. 
C. was performed in accordance with the following schedule: 
1) Upset 6.5" dimension to 4.5". 
2) Square to 4" tk.times.6" wide. 
3) Reheat 15 minutes at 800.degree. C. 
4) Reduce tk to 2". 
5) Reheat 15 minutes at 800.degree. C. 
6) Finish to 1.375.times.5".times.L. 
Samples were cut from top and bottom of the slab for chemical analyses. 
The forging was hot rolled at 800.degree. C. (17% reduction per pass) 
without reheating to 0.510" (12.95 mm) thick and cut in half. Following 
descaling and chemical cleaning (HF/HNO.sub.3), a portion of the plate was 
again heated to 800.degree. C. and rerolled to 0.260" (6.6 mm), again 
using 17% reductions without reheating. Samples representing the rolled 
condition were annealed for 1.0 hr at 800.degree. C. and the remainder of 
the material was then solution-treated for 60 minutes at 850.degree. C., 
followed by water quenching. The quenched plate was then sheared into 
specimens for precipitation hardening studies at 500.degree. C. for times 
of 15, 30, 60, 120 and 180 minutes. Specimens representing the as-quenched 
and 120-minute aged conditions were submitted for tensile testing. 
Young's modulus was determined for specimens of the as-rolled, as-quenched 
and variously-aged conditions. These measurements were obtained by a 
dynamic "impulse response analysis" (IRA) method. In this method, a 
specimen is supported in such a way as to allow free vibrational motion 
when a mechanical impulse is applied to it. The IRA instrument and 
accompanying software analyze the vibrational characteristics of the 
specimen and calculate elastic modulus. Although the method is capable of 
determining both flexural and torsional moduli, from which Poisson's ratio 
may be calculated, only flexural moduli were determined in the present 
studies. This method allows the same specimen to be non-destructively 
evaluated after various heat treatment operations in an expedient manner. 
Additional samples of 1/4" and 1/2" plate were oxidized under various 
conditions of temperature (500.degree.-1500.degree. C.) and time (1-40 
hours). Metallographic and SEM evaluations of the oxides were performed. 
Preliminary evaluations were conducted by machining cutting edges on a 
knife blank prepared from both as-quenched and precipitation-hardened 
plate, followed by various surface oxidation treatments in an attempt to 
determine the alloy's potential as a cutting implement. The knife blank 
was prepared by machining, heat treating and finishing of an existent 
knife blade design. This demonstration blade was produced using the 
following specific process: 
1) Solution-treat 1/4" plate for 60 min. at 850.degree. C., water quench. 
2) Precipitation harden for two hrs. at 500.degree. C., air cool. 
3) Descale, chemically clean. 
4) Machine blade profile to uniform 0.160" thickness. 
5) Grind contours and edge 
6) Oxidize for 90 min. at 650.degree. C. to a blue-black color. 
7) Assemble into finished knife. This knife was tested in a qualitative 
manner by many cycles of cutting various materials, resharpening and 
refinishing, the latter being accomplished by a variety of methods, 
including heating to up to about 500.degree. C. for one to two hours in a 
self-cleaning oven home appliance. This latter treatment being used to 
duplicate instructions given to a consumer when resharpening and 
rereacting a knife blade of this type at home. 
D. Evaluation of Larger Ingot of the Alloy 
A 9.0"-dia. (229 mm) ingot of approximately 110 lb was produced by triple 
vacuum-arc-melting in an analogous manner to that used for the previous 
4"-dia. developmental casting. Forging to 2".times.11" cross section was 
performed at 850.degree. C., followed by annealing for 30 min. at 
815.degree. C. Hot rolling to thicknesses of 0.310" (7.9 mm), 0.270" (6.85 
mm) and 0.180" (4.6 mm) was accomplished at 800.degree. C., with 17% 
reductions per pass. These sizes were produced for experimental production 
of two different types of knife blades and a forged version of plier jaws. 
In addition to these wrought products, scrap material was set aside during 
fabrication for input material to an investment-cast version of the plier 
jaws. Resulting products were evaluated by metallography and tensile 
testing. 
RESULTS 
Oxidation of TiZrNb alloys at 600.degree.-850.degree. C. 
Table I presents calculated and actual chemical composition of the initial 
set of 20 cast buttons. 
Oxidation weight gains (mg/dm2) are presented in Table III for the initial 
20 buttons, whereas only qualitative ratings for ignitability of the six 
additional compositions of Table II are presented in Table IV. Blank 
entries in Table III indicate that either catastrophic ignition of the 
sample occurred during the test, or that earlier cycles of lesser severity 
had resulted in ignition, eliminating the need for further evaluation. 
FIGS. 1 and 3 present comparisons between oxidation rates of Nb-containing 
alloys and corresponding binary TiZr alloys at 700.degree. C. and 
800.degree. C. 
A ternary oxidation diagram is presented in FIG. 4, which separate 
Zr--Ti--Nb alloys into primary categories of "ignitable" and "passive" 
behavior. Data points obtained from the alloys identified in Tables I and 
II were used to generate the areas and boundary lines. 
TABLE III 
__________________________________________________________________________ 
Oxidation Weight Gain Data (Units: mg/dm.sup.2) 
600 650 700 750 800 
10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 
__________________________________________________________________________ 
192 466 
1029 
-- -- -- -- -- -- -- -- -- -- -- -- 
2A 127 288 
-- 196 
-- -- -- -- -- -- -- -- -- -- -- 
2B 125 -- 314 
-- -- -- -- -- -- -- -- -- -- -- -- 
3 34 138 
25 94 200 
303 
-- -- -- -- -- -- -- -- -- 
4 112 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
5 78 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
6 55 219 
241 
168 
-- -- -- -- -- -- -- -- -- -- -- 
7 25 88 104 
105 
242 
428 
279 
508 
-- -- -- -- -- -- -- 
8 18 58 66 39 89 136 
100 
216 
354 
341 
552 
791 
466 
771 
9 13 44 49 28 72 110 
84 180 
259 
254 
440 
579 
371 
575 
10A 
102 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
10B 
136 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
11 108 215 
-- -- -- -- -- -- -- -- -- -- -- -- -- 
12A 
61.7 
-- -- -- -- -- -- -- -- -- -- -- -- -- -- 
12B 
24.8 
123 
75 84 -- -- 161 
-- -- -- -- -- -- -- -- 
13 40 178 
-- 117 
-- -- -- -- -- -- -- -- -- -- -- 
14A 
149 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
14B 
159 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
15A 
32 111 
80 68 202 
305 
201 
495 
-- -- -- -- -- -- -- 
15B 
31 133 
70 41 216 
306 
237 
656 
-- -- 
Zr 4.3 16 21.7 
17.4 
41.1 
51.1 
44.6 
65.1 
78.3 
74.8 
94.6 
120.9 
107.3 
144.5 
179.4 
Hf 0 0 1.2 
0.8 
4.3 
4.3 
4.7 
6.2 
11.2 
0 12.0 
15.5 
14.0 
20.2 
26.0 
Ti 0.4 1.6 
0 0.8 
4.3 
4.3 
4.2 
6.2 
8.9 
12.8 
16.7 
23.6 
20.2 
31.4 
47.3 
Nb No data 
371 
88 158 
226 
181 
371 
489 
415 
618 
864 
668 
1062 
1316 
__________________________________________________________________________ 
TABLE IV 
______________________________________ 
Oxidation Behavior of Additional Alloys 
Ignition 
Alloy 
Ignition, Temp., .degree.C. 
Time, min. Comments 
______________________________________ 
16 None observed to 800.degree. C. 
-- Uniform 
oxidation 
17 None observed to 800.degree. C. 
-- Uniform 
oxidation 
18 None observed to 800.degree. C. 
-- Uniform 
oxidation 
19 800.degree. C. (?) (specimen displayed 
elevated temperature but no 
spalling) 
20 650.degree. C. 15 Combustion 
ceased after 
removal 
21 650.degree. C. 10 Combustion 
continued after 
removal 
______________________________________ 
Biomedical Materials 
The Ti--Zr alloys and their surface oxidation products exhibit many of the 
properties sought by the biomedical community for use in implanted 
prosthetic devices. Some of these properties are well known e.g. low 
toxicity, high-corrosion resistance, high strength, etc., It is desirable 
also to be able to match elastic modulus of implanted materials more 
closely to that of bone, and to provide porous surfaces to which tissue 
may bond by "ingrowth". At the present time, five metallic elements (Ti, 
Zr, Ta, Nb and Pt) have been demonstrated to have little or no adverse 
effects when implanted. 
In reviewing the literature addressing metallic orthopedic implants, it is 
evident that in the past a wide variety of material and design selections 
have been utilized. Some examples involve the indiscriminate mixing of 
components such as screws and pins made of different alloys. This of 
course resulted in galvanic corrosion of the more anodic components as 
would be expected of dissimilar metals in the presence of an electrolyte 
(i.e., body fluids). Such corrosion is not only detrimental from the 
standpoint of reduced strength in the prosthesis, but also from release of 
corrosion products (metallic ions and inorganic debris) into the human 
body. In some cases, intimate contact of the dissimilar metals is not 
necessary for galvanic corrosion to occur. It is sufficient to merely 
locate such materials in the same local region of the body. 
Ranking the suitability of various alloys for implant use by means of both 
in vitro and in vivo corrosion evaluations has shown that corrosion 
resistance increases in the order of 316L, Co--Cr alloys, and Ti6Al4V, all 
of which are presently widely utilized for implants. Another means of 
ranking materials, is to study electrochemical effects on 
biocompatibility, as determined by actual healing in human bone. When 
current densities in fast-reacting redox system, K.sub.4 Fe(CN).sub.6!/K3 
--Fe(CN).sub.6 ! were measured, gold was highest, followed by stainless 
steel, Co--Cr, and Ti6Al4V. The same order was noted in observing the 
degree of disturbance in the initial bone healing process; i.e., gold 
interfered with healing to the greatest degree, while Ti6Al4V caused very 
little interference. The explanation which has been offered for these 
results is that surface oxides formed on some of these alloys are believed 
to serve to prevent the exchange of electrons and thereby suppress redox 
reactions at the implant surface. All of the alloys mentioned above owe 
their corrosion resistance to the presence of protective oxide films which 
serve as barriers to further diffusion of a variety of chemical species. 
There have been many successful techniques for imparting protective oxide 
films to such materials including Ti--Zr alloys. Destruction of these 
protective films by, for example, mechanical abrasion and the ease with 
which films reform are important factors in their selection for such uses. 
It has been demonstrated that there is a transition from corrosion control 
to electron exchange control of polarization resistance during spontaneous 
passivation of film-forming alloys. An extremely complex series of 
transient conditions may therefore be encountered when abrasive wear and 
subsequent reformation of the passivating film are considered. Abrasion 
may not only degrade corrosion resistance but may also result in 
deposition of debris in joints. This is the primary reason that Co--Cr 
alloys are presently utilized for knee joint prostheses, whereas Ti6A+4V 
may be superior in hip joint replacements which do not involve such severe 
shear/abrasion stresses. Because of the poor resistance to "fretting wear" 
displayed by conventional Ti alloys, a demonstrable need exists for a 
titanium containing alloy which is capable of having formed thereon a 
protective oxide surface layer which is capable of resisting abrasive, 
fretting wear. 
Referring to FIG. 21 and the photomicrographs, a typical Ti--Zr alloy 
containing more than about 10-20 wt % Zirconium will have the undesirable 
ignitable property described herein unless that rapid oxidation behavior 
is moderated with the addition of small amounts of niobium, tantalum or 
vanadium. These ignitable alloys have previously been unsuitable for many 
uses, in particular in the fabrication of metal prosthetic devices, and 
the study of their other properties has therefore been largely ignored. 
While there are many schemes to provide passive oxide surfaces on the 
non-ignitable alloys containing Titanium and Zirconium only alloys 
containing Ti--Zr--Nb that contained less than about 20 wt % Zr have been 
reported. The surface oxides formed have been imported by molten salt 
baths and the like which results in a very thin layer to resist abrasion 
in use. 
The exemplary TiZrNb alloy described herein i.e. 35 wt % Zr 10 wt % Nb when 
oxidized for short times at various temperatures in air, produces a thick 
monolithic adherent oxide layer in the surface of the metal. Fore example, 
FIG. 6 shows oxidation for 10 minutes at 750.degree. C., FIG. 7, 20 
minutes at 750.degree. C. and FIG. 8, 30 minutes at 750.degree. C. The 
resultant oxide surface layer shown in each photograph is dense and 
adherent. Other oxidation behavior was also studied. FIG. 9 shows a 
nitride layer on pure Zirconium formed after the 1 hour at 1200.degree. C. 
FIG. 10 shows a nitride layer on pure Titanium formed after 1 hour at 
1200.degree. C. These two layers can be visually compared with a nitride 
layer on 35 Zr 10 Nb wt % formed after 1 hour at 1200.degree. C. (FIG. 
12). 
An additional value is obtainable from the alloys and products of the 
present invention because of the low modulus of elasticity that can be 
achieved with these alloys. Low modulus materials are now preferred for 
the following reasons. 
Because of problems in attaching prostheses to bone with adhesives, a trend 
has developed toward the use of cementless, interference fit methods. 
Viewing the human femur from the front or rear, a significant curvature 
(concave downward) is noted as the bone deviates from the generally 
straight lower segment of the femur toward the hip socket. In a normal 
bone, the body's weight tends to flex this region so that the curvature 
increases as weight is shifted to that side of the body. The Young's 
elastic modulus of cortical bone is about 31 GPa (4.5 Mpsi), far below 
values for common engineering alloys-. When a relatively stiff prosthesis 
is inserted into the intramedullary canal of the femur, the curved region 
in question may become "shielded" from flexural stresses. As is true of 
other biological structures (e.g., muscles), the absence of stress and 
flexure may result in a form of atrophication. In the specific case of 
bone "shielding", the body resorbs and weakens the bone, which may 
ultimately cause loosening or fracture of the prosthetic stem. Attempts to 
design the prosthesis so that the curved region of the femur experiences 
increased stresses by increasing the length of the curved neck have 
created excessive moments at other points, causing fractures. A logical 
approach to this problem would appear to be to develop suitable alloys 
with lower elastic modulus values, while preserving adequate strength, 
corrosion resistance and other required attributes. 
Significant reduction in bone resorption have been demonstrated in dogs and 
sheep with low modulus hip implants. Finite element analyses have also 
confirmed that healthy femurs are more closely simulated by low modulus 
materials. Strain gauge analyses have also confirmed this finding. In 
human patients, bone resorption, loosening, and the pain which has clearly 
been attributed to excessive prosthetic stiffness has been shown to be 
reduced in frequency and severity by using low modulus hips. Metallic 
materials are felt to be preferable to other materials, such as polymer 
composites, because of cost considerations and the poor wear resistance of 
these composites. 
Titanium alloys recently promoted for medical prosthetics include 
Ti5Al2.5Fe, Ti6Al7Nb and Ti11.5Mo6Zr2Fe. The first two of these still have 
relatively high modulus (105-115 GPa versus 120 GPa for Ti6Al4V) and 
contain the potentially undesirable element aluminum. The second contains 
relatively high concentrations of Mo and Fe, both of which have been 
demonstrated to cause severe tissue reactions. The alloys of the present 
invention contains only biocompatible elements and are therefore more 
desirable. 
As will be more fully described hereinafter, the partial or complete 
conversion at higher temperatures, of the alloys of the present invention 
into their corresponding oxides and mixed oxides in the form of a reaction 
formed ceramic can also be useful as materials for prosthetic devices due 
to the inert character of the ceramic, the various pore sizes obtainable 
that may promote tissue ingrowth and the physical properties of the 
underlying metal in the case of partial conversion--no para ceramics, etc. 
Ceramics formed from the subject TiZrNb alloys will be of interest as 
replacements or reinforcements for bone and teeth. Various other 
approaches to obtain porous ceramic structures into which bone may grow to 
form stable interfaces have been evaluated. These include sintered 
powders, foamed ceramics, preferential etching to remove included phases, 
and calcining natural materials such as coral. It has been found that 
controlling pore size and uniformity is essential to obtain optimum 
properties in these materials. Bone ingrowth requires pore diameters of at 
least 100 .mu.m in order for nourishment to be continuously supplied to 
living cell structures, while excessive porosity or unnecessarily large 
pore diameters tend to weaken the ceramic prosthesis. The homogeneous 
distributions of predicatable uniformly-sized grains and pores associated 
with oxides formed from the subject TiZrNb alloys makes them desirable for 
such medical use. 
Materials for Cutlery 
Although wear resistance may be a primary consideration in selecting 
materials for cutlery implements, other properties such as fracture 
toughness, strength, and elastic modulus are also important. The poor 
corrosion resistance of plain carbon and low alloy steels resulted in 
large commercial markets for a martensitic class of stainless steels which 
is generally described by the relationship % Cr-(17X%C)&lt;12.5. These alloys 
are capable of transforming from austenite to martensite upon quenching to 
yield relatively high hardness (50-55 R.sub.c), whereas alloys in which % 
Cr-(17-X%C)&gt;12.5 are strictly ferritic and are therefore not hardenable. 
Recent interest in reducing weight in cutlery items has resulted in 
evaluation of titanium-based alloys. There are also special applications 
in which increased corrosion resistance in seawater and other chemical 
environments make titanium a potentially desirable material. Certain 
military applications require materials which are non-magnetic and low in 
surface reflectivity, or which do not display ductile-to-brittle 
transitions at low temperature. Still, other applications require high 
impact resistance or low elastic modulus to increase flexibility in such 
implements as fileting knives. 
Although many of these desired properties may be provided by conventional 
titanium alloys such as Ti6Al4V, adequate hardness for cutting edges is 
difficult and expensive to obtain, often involving surface diffusion 
processes conducted for long times at high temperatures and in special 
environments such as autoclaves. Furthermore, the hard surface films 
(e.g., oxides, nitrides, carbides, borides) are impossible for the 
consumer to repair when resharpening becomes necessary. The oxidation 
characteristics of the subject TiZrNb alloys offers a potential solution 
to these problems. 
It should be noted that many considerations applicable to consumer goods 
such as high-volume cutlery are non-technical in nature. A sportsman, for 
example, may prefer a softer knife which is easily sharpened to one which 
will "hold an edge" longer, while a housewife may prefer a kitchen knife 
which is not overly sharp and which does not need frequent sharpening. 
The morphology of the transformation of the Ti--Zr alloys which have been 
rendered non-ignitable according to this invention, into ceramic oxides is 
complex and not easily described. Referring again to the photomicrographs, 
FIG. 13 shows a sample of the typically examined 35Zr10Nb alloy oxidized 
in air for 10 minutes at 800.degree. C. This is a temperature only 
slightly higher than the 750.degree. C. results shown in FIGS. 6-8 for the 
same time. The oxide layer is shown at A, B is an area of alph-stabilized 
metal c is an area of mixed phases and D is the relatively unaffected 
metal alloy. 
Referring to FIG. 14, the same metal alloy is shown after oxidation in air 
for 30 minutes at 800.degree. C. The oxide layer A is still visually well 
characterized while the alph-stabilized layer is larger and the mixed 
phase area is larger and less defined. In FIG. 15, a sample of 35Zr10Nb 
was oxidized in air at 1300.degree. C. A much more complex picture of the 
conversion process is developed here. The metal substrate 1 originally was 
nearly as thick as the resultant oxide covered structure. The first layer 
of oxide obtaining material remains rich in niobium and is believed to be 
comprised of several oxide or suboxide species. The next layer 3 moving 
outward from the metal represents a transformation zone where conversion 
to the final oxide specie found in the outer surface 4 is taking place. 
Pores are being formed in layer 4 possibly as a result of this conversion 
process and larger pores form where the conversion is carried out at 
higher temperatures. FIG. 19 and FIG. 20 show respectively under 200 power 
magnification that the dark areas (pores) are smaller where the 
temperature of formation was 1400.degree. C. (FIG. 19) than the larger 
pores formed at 1500.degree. C. (FIG. 20). This ability to form uniformly 
distributed pores and to control the pore size by adjusting the 
temperature of the ceramic oxide formation while maintaining nearly 
constant pore volume percent porosity, can be useful in the fabrication of 
many articles including catalyst supports and the like where pore sizes is 
important to the intended performance of the finished article. 
FIGS. 16 and 17 visually show that the adjustment of both time and 
temperature can be critical to different results being obtained in the 
final oxidized product. While oxide layer formation such as shown in FIGS. 
6-8 can be achieved at relatively low temperatures and only short times, 
modestly higher temperatures and longer times produce radically different 
phenomenon. In FIG. 16 the metal substrate 10 was subjected to oxidation 
at 1000.degree. C. for 39 hours. The 200 power photomicrograph shows the 
metal 10 with two visually identifiable intermediate zones or areas 11, 
and 12 underlying the final oxide 13. 
In FIG. 17 the metallic substrate has at least three visually identifiable 
zones 21, 22 and 23, underlying the final oxide layer 24. Zone 21 appears 
to be a metallic zone with internal and grain boundary oxidation. FIG. 17 
is a 200 power photograph of a 35Zr10Nb sample oxidized at 1000.degree. C. 
for 39 hours. 
FIG. 18 is yet another example of the possible variations in the morphology 
of formation of the ceramic described herein. The 35 Zr10Nb alloy was 
oxidized in air for 64 hours at 1100.degree. C. The 200 power 
photomicrograph clearly shows a metallic zone A, a metallic zone B and 
intermediate zone C and the surface oxide D. 
Referring to FIG. 5 a very complex spectrum of possible oxide species are 
possibly formed during the ceramic conversion process. 
The present studies of oxides formed at 1400.degree.-1500.degree. C. 
confirm the presence of TiO.sub.2 (rutile form) which contains an 
appreciable amount of Nb, and a Zr-rich phase (probably TiZrO.sub.4) which 
contains only a trace of Nb. The moderated oxidation rate of 35Zr10Nb may 
therefore be attributable to some ability of Nb to prevent interactions 
between the Zr-rich and Ti-rich phases which could otherwise lower the 
monoclinic &lt;-- --&gt; tetragonal zirconia transformation temperature. 
As previously described, tantalum and vanadium have been successfully 
substituted in the alloy of the present invention for the niobium, 
likewise the nitridation of the alloys reacts generally similarly and 
produces results similar to the air oxidation described herein. 
This invention has been described with respect to its preferred embodiments 
and contemplated utility. Variations can be made without undue 
experimentation by those skilled in the art with the expected results 
being obtained without departing from the spirit and scope of the 
invention described in the appended claims as interpreted in view of the 
applicable prior art.