Note: Descriptions are shown in the official language in which they were submitted.
CA 02847938 2014-03-31
Docket No: PLS-102
TITANIUM BASED CERAMIC REINFORCED ALLOY
FOR USE IN MEDICAL IMPLANTS
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a titanium based, ceramic reinforced alloy ingot for
use in
producing medical implants. More particularly, the invention pertains to a
ceramic reinforced alloy ingot comprising titanium, niobium and silicon. The
alloy has both an a crystal phase and a 3 crystal phase. The ingot has an
ultimate
tensile strength of about 940 MPa or more, and a Young's modulus of about 150
GPa or less.
Description of the Related Art
There is great commercial interest in the production of biocompatible,
medically
suitable implants for surgically jointing bone and implanting teeth. Medical
implants such as screws, pins, rods, bars, springs, coils, cables, staples,
clips,
plates and the like require materials with very high tensile strength and high
cyclic fatigue life while also having a modulus of elasticity low enough to be
compatible with bone. Common alloys include titanium, stainless steel and
cobalt
chrome alloys. Stainless steel and cobalt chrome alloys exhibit very high
tensile
strength, but both contain nickel and chromium which are known irritants to
the
body. In addition, these alloys have low ductility and a Young's modulus
approaching five times that of bone. This high tensile strength and Young's
modulus also makes it difficult to machine these components cost effectively
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using conventional techniques. Titanium and its alloys are especially popular
choices for orthopedic bone screws and plates commonly used for spinal
fixation.
Titanium alloys for a variety of applications are known in the art and there
are
numerous literature references disclosing a wide range of elements which are
used
to provide alloys having desired characteristics, such as increased tensile
strength
and ductility. Generally, titanium and its alloys may exist in one or a
mixture of
two basic crystalline structures, namely the a phase, which is a hexagonal
close-
packed structure, and the 13 phase which is a body-centered cubic structure.
The
commercially pure grades of titanium alloys have low tensile strengths but
show
no signs of tissue irritation. These alloys are commonly used for orthopedic
plates
which are implanted externally to the bone structure and can therefore have a
larger size. Ti6A1V4 alloys are commonly used for higher strength applications
such as fixation screws or plates which must be contained in a small area. One
known medically implantable alloy is disclosed in U.S. patent 6,752,882. It
provides a biocompatible low modulus, high strength titanium-niobium alloy
containing a phase as a major phase and consisting essentially of 10-30 wt %
of
Nb and the balance titanium. U.S. patent 5,954,724 relates to titanium alloys
suitable for use for medical implants and devices having a high-strength, low-
modulus, and high hardness with improved corrosion resistance due to the
addition of hafnium and molybdenum, and which additionally allow for surface
hardening of an implant made of this alloy. U.S. patent 7,892,369 provides a
method for modifying the microstructure of titanium alloys for use in the
manufacture of orthopedic prostheses. An orthopedic prosthesis is initially
formed
from a titanium alloy and subsequently subjected to a thermal treatment
followed
by rapid quenching. The microstructure of the titanium alloy in the prosthesis
has
improved resistance to fretting fatigue. U.S. patent 7,682,473 provides an
implant
prosthesis composed of a TiAlNb alloy having a modulus near that for bone to
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prevent stress shielding, and a tensile and compressive strength and fracture
toughness equal to or greater than that of bone. A key problem with other
alloys
which use aluminum and vanadium is the suspected effect of Al and V when
movement and fretting are involved. The release of Al and V into the blood
stream could cause irritation for the patient in the long term. Another issue
with
certain grades of titanium is the so called "notch effect" during cyclic
fatigue.
Prepared and polished samples of certain titanium alloys have been shown to
have
fatigue strength near the ultimate tensile strength. However, when a notch is
introduced to the sample, the fatigue strength can be lowered to 40% of the
ultimate tensile strength. Since implantable devices must be laser marked with
the
appropriate tracking information, a notch situation always exists and care
must be
taken not to exceed the notch fatigue strength.
The problems associated with designing an implantable device are specifically,
providing an alloy with high tensile strength, and a marginal Young's modulus
that contains no known irritants which can be economically machined with
conventional methods. The present invention addresses all these issues. The
invention provides an alloy of titanium, niobium and silicon. Titanium and
niobium alloys are known to form alloys with very low Young's modulus (50-
80GPa). A problem with these known alloys is that they do not have sufficient
strength for the manufacture of orthopedic devices such as bone plates and
fixation screws. This invention overcomes the limitations of conventional
alloys
by including within a solid solution of the metals, a glassy silicon ceramic
which
acts to absorb energy during crack propagation and retard dislocations during
applied stress. The atomic percent of this glassy silicon ceramic is
controlled as to
still allow for a moderately low Young's modulus and good formability. The
inventive alloy of primarily Ti with the addition of Nb and Si produces alloys
which have a complex alpha/beta structure with an amount of glassy material.
The
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resulting alloy has a higher strength then the titanium grades presently used
in
medical implants while retaining a comparable elastic modulus.
SUMMARY OF THE INVENTION
The invention provides an ingot comprising an alloy, the alloy comprising from
about 5 wt. % to about 35 wt. % of niobium, from about 0.5 wt. % to about 3.5
wt. % of silicon, and from about 61.5 wt. % to about 94.5 wt. % of titanium,
the
alloy having a hexagonal crystal lattice a phase of from about 20 vol % to
about
70 vol %, and a cubic body centered 13 crystal lattice phase of from about 30
vol.
% to about 80 vol. %, the ingot having an ultimate tensile strength of about
940
MPa or more, and a Young's modulus of about 150 GPa or less.
The invention also provides a method of forming an ingot which comprises
forming a molten alloy comprising a substantially uniform admixture of from
about 5 wt. % to about 35 wt. % of niobium, from about 0.5 wt. % to about 3.5
wt. % of silicon, and from about 61.5 wt. % to about 94.5 wt. % of titanium,
casting the molten alloy into a shape, and then cooling the shape into an
ingot, the
alloy having a hexagonal crystal lattice a phase of from about 20 vol % to
about
70 vol %, and a cubic body centered 13 crystal lattice phase of from about 30
vol.
% to about 80 vol. %, the ingot having an ultimate tensile strength of about
940
MPa or more, and a Young's modulus of about 150 GPa or less.
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DESCRIPTION OF THE INVENTION
An alloy is formed by combining commercially pure quantities of titanium,
niobium
and silicon. These may be obtained in the form of bars, wires, powders,
particles, or
any other convenient form. These are then heated until each is molten and
blended
into a substantially uniform admixture. The amount of titanium may range from
about 61.5 wt. % to about 94.5 wt. %, preferably from about 72.5 wt. % to
about
92 wt. %, and more preferably from about 78 wt. % to about 88.75 wt. %.
The amount of niobium may range from about 5 wt. % to about 35 wt. %,
preferably from about 7 wt. % to about 25 wt. %, and more preferably from
about
10 wt. % to about 20 wt. %. The amount of silicon may range from about 0.5 wt.
% to about 3.5 wt. %, preferably from about 1 wt. % to about 2.5 wt. %, and
more
from about 1.25 wt. % to about 2 wt. %. Preferably the alloy has no more than
2
wt. % of nitrogen, oxygen, or carbon. More preferably the alloy has about 1
wt.
% or less of nitrogen, oxygen or carbon. Still more preferably the alloy has
about
0.5 wt. % or less of nitrogen, oxygen or carbon. In a most preferred
embodiment,
the alloy comprises only these three elements such that the alloy has from
about 5
wt. % to about 35 wt. % of niobium, from about 0.5 wt. % to about 3.5 wt. % of
silicon, and the balance being titanium, apart from incidental impurities.
A method for preparing such a high strength, low modulus, biocompatible
titanium alloy involves mechanically blending the above components, and then
heating them until melted, one or more times.
The alloys are preferably made by mechanically blending accurately weighed
portions of the pure elements and melting the blend in a furnace such as a
plasma
arc furnace or vacuum arc furnace, and remelting as necessary to achieve
uniformity, and then casting and cooling. One example of a method of melting
includes combining the components in a commercially available arc-melting
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vacuum pressure casting system. A melting chamber is first evacuated and
purged
with an inert gas such as argon. An argon pressure of, for example 1.5 kgf/cm2
may be maintained during melting. The appropriate amounts of titanium,
niobium and silicon are prepared by electron beam skull melting with induction
stirring of the melt. The resulting mixture may optionally be re-melted
multiple
times to improve homogeneity. The molten alloy is then cast, or drawn out of
the
crucible by a water cooled rod to form a cylindrical ingot, with cooling. The
alloy
has a combination crystal lattice structure of both a and 13 phases. In
particular,
the alloy has a hexagonal crystal lattice a phase of from about 20 vol % to
about
70 vol %, and a cubic body centered 13 crystal lattice phase of from about 30
vol. % to about 80 vol. %. Preferably the alloy has a hexagonal crystal
lattice a
phase of from about 40 vol. % to about 70 vol. %, and a cubic body centered i3
crystal lattice phase of from about 30 vol. % to about 60 vol. %. More
preferably
the alloy comprises a hexagonal crystal lattice a phase of from about 45 vol.
% to
about 65 vol. %, and a cubic body centered 13 crystal lattice phase of from
about
45 vol. % to about 60 vol. %.
The resulting ingot has an ultimate tensile strength of about 940 MPa or more,
usually from about 1000 MPa to about 1400 MPa, and more usually from about
1100 MPa to about 1300 MPa. The resulting ingot has a Young's modulus of
about 150 GPa or less, usually from about 100 GPa to about 150 GPa, and more
usually from about 110 GPa to about 140 GPa.
The resulting ingot may then be formed into the desired medial implant shape,
such as those in the form of a screw, pin, rod, bar, spring, coil, cable,
staple, clip,
plate, or the like. The implant may also be form into customized shapes such
as
those conforming to hip joint stems, femoral heads, knee femoral components,
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. ,
knee tibial components, intramedullary nails, inner ear vent tubes, spinal
plates,
spinal disks, pelvic plates, dental implants, cardiovascular implants,
compression
hip screws, and the like. Such forming may be done by the use of customary
machine tooling. Optionally either the cast ingot or the machined medical
implant
may be annealed for additional strength, polished or anodized by well known
methods. Annealing may be done by heating at temperatures ranging from about
500 C. to about 1200 C., preferably from about 750 C. to about 1000 C.,
for
from about 20 minutes to about 360 minutes, preferably from about 40 minutes
to
about 120 minutes. Polishing may be done by mechanical burnishing.
Anodizing may be done by electrochemically oxidizing the surface.
The following non-limiting examples serve to illustrate the invention.
EXAMPLES
Three alloys were formed and tested in both the as cast condition, and after
annealing at 950 C for 1 hour in vacuum. The alloys were prepared by electron
beam skull melting with induction stirring of the melt. The resulting material
was
drawing out of the crucible by a water cooled rod to form a cylindrical ingot.
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,
Young's Yield
Alloy UTS Modulus Strength
Elongation
Test Condition Nb Si (Mpa) (GPa) (Mpa) (%)
1 As Cast 10 1.1 1012 113 940 4.6
2 As Cast 13 1.5 995 112 937 6.2
3 As Cast 21 1.25 1022 110 960 4.8
la Annealed 10 1.1 1006 81 945 4.5
2a Annealed 13 1.5 1008 83 957 3.3
3a Annealed 21 1.25 957 76 850 3.25
The sample ingots were subjected to machinability tests, polishing tests and
color
anodizing. The composition performed excellently in all cases, with the
polishing
and anodizing exceeding the characteristics of commercially available Grade 4
and Grade 23 titanium. Detailed chemical and phase analysis of the
Ti-21Nb-1.25Si material was performed. The phase analysis shows a roughly
55/45 alpha/beta structure. XPS analysis confirmed that there existed a large
number of atoms in a glassy phase with 1.6at % of the material existing as
SiC.
This SiC glassy ceramic is deposited at the grain boundaries. Given the high
fracture toughness of the carbide these interstitial components act not only
prohibit dislocation movement but also absorb energy in the case of crack
propagation. The presence of carbon in the alloy used to form the SiC is
present in
the starting raw material as a typical impurity.
While the present invention has been particularly shown and described with
reference to preferred embodiments, it will be readily appreciated by those of
ordinary skill in the art that various changes and modifications may be made
without departing from the spirit and scope of the invention. It is intended
that
the claims be interpreted to cover the disclosed embodiment, those
alternatives
which have been discussed above and all equivalents thereto.
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