U.S. patent application number 10/609003 was filed with the patent office on 2004-03-18 for beta titanium compositions and methods of manufacture thereof.
Invention is credited to Wu, Ming H..
Application Number | 20040052676 10/609003 |
Document ID | / |
Family ID | 30000905 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040052676 |
Kind Code |
A1 |
Wu, Ming H. |
March 18, 2004 |
beta titanium compositions and methods of manufacture thereof
Abstract
A composition comprises about 8 to about 10 wt % molybdenum,
about 2.8 to about 6 wt % aluminum, up to about 2 wt % vanadium, up
to about 4 wt % niobium, with the balance being titanium, wherein
the weight percents are based on the total weight of the alloy
composition. A method for making an article comprises cold-working
a shape from a composition comprising about 8 to about 10 wt %
molybdenum, about 2.8 to about 6 wt % aluminum, up to about 2 wt %
vanadium, up to about 4 wt % niobium, with the balance being
titanium, wherein the weight percents are based on the total weight
of the alloy composition; solution heat treating the shape; and
cooling the shape.
Inventors: |
Wu, Ming H.; (Bethel,
CT) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
|
Family ID: |
30000905 |
Appl. No.: |
10/609003 |
Filed: |
June 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60392620 |
Jun 27, 2002 |
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Current U.S.
Class: |
420/420 ;
148/671 |
Current CPC
Class: |
A63B 53/0445 20200801;
A61L 31/022 20130101; A63B 53/047 20130101; A63B 53/0416 20200801;
C22F 1/16 20130101; C22F 1/183 20130101; A61L 31/10 20130101; A63B
53/04 20130101; A63B 2209/14 20130101; A63B 60/00 20151001; C22C
14/00 20130101; A61L 27/04 20130101; A61L 27/34 20130101; A63B
2209/00 20130101; A61L 27/06 20130101 |
Class at
Publication: |
420/420 ;
148/671 |
International
Class: |
C22C 014/00 |
Claims
What is claimed is:
1. A composition comprising about 8 to about 10 wt % molybdenum,
about 2.8 to about 6 wt % aluminum, up to about 2 wt % vanadium, up
to about 4 wt % niobium, with the balance being titanium, wherein
the weight percents are based on the total weight of the alloy
composition.
2. The composition of claim 1, wherein the composition is cold
worked.
3. The composition of claim 2, wherein the composition, after cold
working, has an elastic recovery of greater than or equal to about
75% of the applied change in length when the applied change in
length is 2% of the original length.
4. The composition of claim 2, wherein the composition, after cold
working, has an elastic recovery of greater than or equal to about
85% of the applied change in length when the applied change in
length is 2% of the original length.
5. The composition of claim 2, wherein the composition, after cold
working, has an elastic recovery of greater than or equal to about
50% of the applied change in length when the applied change in
length is 4% of the original length.
6. The composition of claim 2, wherein the composition, after cold
working, has an elastic recovery of greater than or equal to about
75% of the applied change in length when the applied change in
length is 4% of the original length.
7. The composition of claim 2, wherein the composition, after cold
working, has a reduction in the elastic modulus of greater than or
equal to about 10% when compared with the elastic modulus of an
equivalent heat treated composition.
8. The composition of claim 2, wherein the composition, after cold
working, has a reduction in the elastic modulus of greater than or
equal to about 20% when compared with the elastic modulus of an
equivalent heat treated composition.
9. The composition of claim 2, wherein the composition, after cold
working, has a reduction in the elastic modulus of greater than or
equal to about 25% when compared with the elastic modulus of an
equivalent heat treated composition.
10. The composition of claim 1, wherein the composition exhibits an
elastic recovery of greater than or equal to about 50% of the
applied change in length when the applied change in length is 4% of
the original length.
11. The composition of claim 9, wherein the composition has a
.beta. phase or an .alpha. phase and a .beta. phase.
12. The composition of claim 11, further comprising solution
treating the composition.
13. The composition of claim 1, wherein the composition is cold
worked and shows an elastic recovery of greater than or equal to
about 75% of the initial strain when elastically deformed to a 2%
initial strain.
14. The composition of claim 1, wherein the composition is cold
worked and shows an elastic recovery of greater than or equal to
about 50% of the initial strain when elastically deformed to a 4%
initial strain.
15. An article manufactured from the composition of claim 1.
16. A composition comprising about 8.9 wt % molybdenum, about 3.03
wt % aluminum, about 1.95 wt % vanadium, about 3.86 wt % niobium,
with the balance being titanium.
17. The composition of claim 16, wherein the composition is cold
worked.
18. The composition of claim 16, having an elastic recovery of
greater than or equal to about 75% of the applied change in length
when the applied change in length is 2% of the original length.
19. The composition of claim 16, having an elastic recovery of
greater than or equal to about 50% of the applied change in length
when the applied change in length is 4% of the original length.
20. The composition of claim 16, wherein the composition, after
cold working, has a reduction in the elastic modulus of greater
than or equal to about 10% when compared with the elastic modulus
of an equivalent heat treated composition.
21. A composition comprising about 9.34 wt % molybdenum, about 3.01
wt % aluminum, about 1.95 wt % vanadium, about 3.79 wt % niobium,
with the balance being titanium.
22. The composition of claim 21, wherein the composition is cold
worked.
23. The composition of claim 21, having an elastic recovery of
greater than or equal to about 50% of the applied change in length
when the applied change in length is 4% of the original length.
24. The composition of claim 21, having an elastic recovery of
greater than or equal to about 75% of the applied change in length
when the applied change in length is 2% of the original length.
25. The composition of claim 21, wherein the composition, after
cold working, has a reduction in the elastic modulus of greater
than or equal to about 10% when compared with the elastic modulus
of an equivalent heat treated composition.
26. A method for making an article comprising:. cold working a
shape from a composition comprising about 8 to about 10 wt %
molybdenum, about 2.8 to about 6 wt % aluminum, up to about 2 wt %
vanadium, up to about 4 wt % niobium, with the balance being
titanium, wherein the weight percents are based on the total weight
of the alloy composition; solution heat treating the shape; and
cooling the shape.
27. The method of claim 26, wherein the solution heat treating is
conducted at a temperature below the isomorphic temperature for the
composition.
28. The method of claim 26, wherein the solution heat treating is
conducted at a temperature above the isomorphic temperature for the
composition.
29. The method of claim 26, wherein the cooling is conducted in
air.
30. The method of claim 26, wherein the shape is further heat aged
at a temperature of about 350 to about 550.degree. C.
31. The method of claim 30, wherein the heat ageing is conducted
for a time period of 10 seconds to about 30 minutes.
32. A method comprising: cold working a wire having a composition
comprising about 8 to about 10 wt % molybdenum, about 2.8 to about
6 wt % aluminum, up to about 2 wt % vanadium, up to about 4. wt %
niobium, with the balance being titanium, wherein the weight
percents are based on the total weight of the alloy composition;
solution treating the wire; and heat treating the wire.
33. The method of claim 32, wherein the cold working results in a
reduction in cross-sectional area of about 5 to about 85%.
34. The method of claim 32, wherein the wire diameter is about 0.1
to about 10 millimeters.
35. The method of claim 32, wherein the heat treating is conducted
at a temperature of about 500.degree. C. to about 900.degree.
C.
36. The method of claim 32, wherein the wire is solution treated at
a temperature of about 800 to about 1000.degree. C.
37. The method of claim 32, wherein the article has a .beta. phase
or an .alpha. phase and a .beta. phase.
38. The method of claim 32, wherein the article has an elastic
recovery of greater than or equal to about 75% of the applied
change in length when the applied change in length is 2% of the
original length.
39. The method of claim 32, wherein the article has an elastic
recovery of greater than or equal to about 50% of the applied
change in length when the applied change in length is 4% of the
original length.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application 60/392,620 filed Jun. 27, 2002, the entire contents of
which are incorporated herein by reference.
BACKGROUND
[0002] This disclosure relates to superelastic .beta. titanium
alloys, methods for manufacturing these alloys and articles derived
therefrom.
[0003] Alloys that undergo a martensitic transformation may exhibit
a "shape memory effect". As a result of this transformation, the
high temperature phase known as "austenite" changes its crystalline
structure through a diffusion-less shear process adopting a less
symmetrical structure called `martensite`. This process may be
reversible as in shape memory alloys and therefore upon heating,
the reverse transformation occurs. The starting temperature of the
cooling or martensitic transformation is generally referred to as
the M.sub.s temperature and the finishing temperature is referred
to as the M.sub.f temperature. The starting and finishing
temperatures of the reverse or austenitic transformation are
referred to as A.sub.s and A.sub.f respectively.
[0004] At temperatures below the A.sub.f, alloys undergoing a
reversible martensitic phase transformation may be deformed in
their high temperature austenitic phase through a stress-induced
martensitic transformation as well as in their low temperature
martensitic phase. These alloys generally recover their original
shapes upon heating above the A.sub.f temperature and are therefore
called "shape memory alloys". At temperatures above the A.sub.f,
the stress-induced martensite is not stable and will revert back to
austenite upon the release of deformation. The strain recovery
associated with the reversion of stress-induced martensite back to
austenite is generally referred to as "pseudoelasticity" or
"superelasticity" as defined in ASTM F2005, Standard Terminology
for Nickel-Titanium Shape Memory Alloys. The two terms are used
interchangeably to describe the ability of shape memory alloys to
elastically recover large deformations without a significant amount
of plasticity due to the mechanically induced crystalline phase
change.
[0005] Nitinol is a shape memory alloy comprising a
near-stoichiometric amount of nickel and titanium. When deforming
pseudoelastic nitinol, the formation of stress-induced-martensite
allows the strain of the alloy to increase at a relatively constant
stress. Upon unloading, the reversion of the martensite back to
austenite occurs at a constant, but different, stress. A typical
stress-strain curve of pseudoelastic nitinol therefore exhibits
both loading and unloading stress plateaus. However, since the
stresses are different, these plateaus are not identical, which is
indicative of the development of mechanical hysteresis in the
nitinol. Deformations of about 8 to about 10% can thus be recovered
in the pseudoelastic nitinol. Cold worked Nitinol also exhibits
extended linear elasticity. Nitinol compositions, which display
linear elasticity do not display any plateau but can recover a
strain of up to 3.5%. This behavior is generally termed "Linear
Superelasticity" to differentiate from transformation induced
"Pseudoelasticity" or "Superelasticity". These properties generally
make nitinol a widely used material in a number of applications,
such as medical stents, guide wires, surgical devices, orthodontic
appliances, cellular phone antenna wires as well as frames and
other components for eye wear. However, nitinol is difficult to
fabricate by forming and/or welding, which makes the manufacturing
of articles from it expensive and time-consuming. Additionally,
users of nickel containing products are sometimes allergic to
nickel.
SUMMARY
[0006] In one embodiment, a composition comprises about 8 to about
10 wt % molybdenum, about 2.8 to about 6 wt % aluminum, up to about
2 wt % vanadium, up to about 4 wt % niobium, with the balance being
titanium, wherein the weight percents are based on the total weight
of the alloy composition.
[0007] In another embodiment, a composition comprises about 8.9 wt
% molybdenum, about 3.03 wt % aluminum, about 1.95 wt % vanadium,
about 3.86 wt % niobium, with the balance being titanium.
[0008] In yet another embodiment, a composition comprises about
9.34 wt % molybdenum, about 3.01 wt % aluminum, about 1.95 wt %
vanadium, about 3.79 wt % niobium, with the balance being
titanium.
[0009] In yet another embodiment, a method for making an article
comprises cold working a shape from a composition comprising about
8 to about 10 wt % molybdenum, about 2.8 to about 6 wt % aluminum,
up to about 2 wt % vanadium, up to about 4 wt % niobium, with the
balance being titanium, wherein the weight percents are based on
the total weight of the alloy composition; forming the shape;
solution heat treating the shape; and cooling the shape.
[0010] In yet another embodiment, a method comprises cold working a
wire having a composition comprising about 8 to about 10 wt %
molybdenum, about 2.8 to about 6 wt % aluminum, up to about 2 wt %
vanadium, up to about 4 wt % niobium, with the balance being
titanium, wherein the weight percents are based on the total weight
of the alloy composition; cold working the shape; and heat treating
the shape.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graphical representation showing the effect of
molybdenum content on elastic recovery;
[0012] FIG. 2 is a graphical representation of the effect of aging
at 350.degree. C. on the elastic recovery of Sample 4 from Table
1;
[0013] FIG. 3 is a graphical representation of the effect of aging
at 350.degree. C. on the elastic recovery of Sample 5 from Table
1;
[0014] FIG. 4 is a graphical representation showing the effect of
aging at 350.degree. C. on the elastic recovery of Sample 6 from
Table 1;
[0015] FIG. 5 is a graphic representation showing the effect of
aging at about 250 to about 550.degree. C. for 10 seconds on the
elastic recovery of Sample 4 from Table 1;
[0016] FIG. 6 is a graphic representation showing the effect of
aging at about 250 to about 550.degree. C. for 10 seconds on the
elastic recovery of Sample 5 from Table 1;
[0017] FIG. 7 is a graphical representation showing the effect of
cumulative cold drawing reduction on the UTS of Sample 11 from
Table 2;
[0018] FIG. 8 is a graphical representation showing the effect of
cumulative cold drawing reduction on the Young's Modulus of Sample
11 from Table 2;
[0019] FIG. 9 is a graphical representation showing the effect of
tensile stress-strain curve for a wire having the composition of
Sample 11 from Table 2 with 19.4% drawing reduction, tested to 2%
strain;
[0020] FIG. 10 is a graphical representation showing the effect of
tensile stress-strain curve for a wire having the composition of
Sample 11 from Table 2 with 19.4% drawing reduction, tested to 4%
strain;
[0021] FIG. 11 is an optical micrograph showing the microstructure
of a cold drawn wire having the composition of Sample 10 from Table
2 with a 14% reduction;
[0022] FIG. 12 is an optical micrograph showing partially
recrystallized microstructure of a cold-drawn wire having the
composition of Sample 10 from Table 2 having a 14% reduction after
heat-treating at 816.degree. C. for 30 minutes;
[0023] FIG. 13 is an optical micrograph showing fully
recrystallized microstructure of a cold-drawn wire having the
composition of Sample 10 from Table 2 having a 14% reduction after
heat-treating at 871.degree. C. for 30 minutes;
[0024] FIG. 14 is an optical micrograph showing the microstructure
of a betatized Sample 10 from Table 2 after aging at 816.degree. C.
for 30 minutes;
[0025] FIG. 15 is an optical micrograph showing the microstructure
of a betatized Sample 10 from Table 2 after aging at 788.degree. C.
for 30 minutes;
[0026] FIG. 16 is a graphical representation showing the UTS of
betatized Sample 10 from Table 2 after aging at 500-900.degree. C.
for 30 minutes;
[0027] FIG. 17 is a graphical representation showing the ductility
of betatized Sample 10 from Table 2 after aging at 500-900.degree.
C. for 30 minutes;
[0028] FIG. 18 is a graphical representation showing a tensile
stress-strain curve tested to 4% tensile strain of a wire having
the composition of Sample 11 from Table 2 after strand annealing at
871.degree. C.; and
[0029] FIG. 19 is an optical micrograph showing the microstructure
of a wire having the composition of Sample 11 from Table 2 after
strand annealing at 871.degree. C.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] Disclosed herein is a .beta. titanium alloy composition
having pseudoelastic properties and linear-superelastic properties
that can be used for medical, dental, sporting good and eyewear
frame applications. In one embodiment, the .beta. titanium alloy
composition has linear elastic properties after solution treatment.
In another embodiment, the .beta. titanium alloy composition has
pseudoelastic properties that are improved with heat treatment. In
yet another embodiment, the .beta. titanium alloy composition
displays linear-superelastic properties after being cold worked.
The composition advantageously can be welded to other metals and
alloys. The articles manufactured from the .beta. titanium alloy
can also be deformed into various shapes at ambient temperature and
generally retain the high spring back characteristics associated
with superelasticity.
[0031] Pure titanium has an isomorphous transformation temperature
at 882.degree. C. The body centered cubic (bcc) structure, which is
called .beta.-titanium, is stable above the isomorphous
transformation temperature, and the hexagonal close packed (hcp)
structure, which is called .alpha. titanium is generally stable
below this temperature. When titanium is alloyed with elements such
as vanadium, molybdenum, and/or niobium, the resulting alloys have
an increased .beta. phase stability at temperatures less than or
equal to about 882.degree. C. (.beta. transus temperature). On the
other hand, when alloyed with elements such as aluminum or oxygen,
the temperature range of the stable .alpha. phase is increased
above the isomorphous transformation temperature. Elements which
have the effect of increasing the .beta. phase temperature range
are called the .beta. stabilizers, while those capable of extending
the .alpha. phase temperature range are called the .alpha.
stabilizers.
[0032] Titanium alloys having a high enough concentration of .beta.
stabilizers, generally are sufficiently stable to have a
meta-stable .beta. phase structure at room temperature. The alloys
showing such a property are called .beta. titanium alloys.
Martensite transformations are generally present in .beta. titanium
alloys. The martensitic transformation temperature in .beta.
titanium alloys generally decreases with an increasing amount of
.beta. stabilizer in the alloy, while increasing the amount of
.alpha. stabilizer generally raises the martensitic transformation
temperature. Therefore, depending on the extent of stabilization,
.beta. titanium alloys may exhibit a martensitic transformation
when cooled rapidly from temperatures greater than those at which
the .beta. phase is the single phase at equilibrium.
[0033] The .beta. titanium alloy generally comprises an amount of
about 8 to about 10 wt % of molybdenum, about 2.8 to about 6 wt %
aluminum, up to about 2 wt % vanadium, up to about 4 wt % niobium,
with the balance being titanium. All weight percents are based on
the total weight of the alloy. Within the aforementioned range for
molybdenum, it is generally desirable to have an amount of greater
than or equal to about 8.5, preferably greater than or equal to
about 9.0, and more preferably greater than or equal to about 9.2
wt % molybdenum. Also desirable within this range is an amount of
less than or equal to about 9.75, preferably less than or equal to
about 9.65, and more preferably less than or equal to about 9.5 wt
% molybdenum, based on the total weight of the alloy.
[0034] Within the aforementioned range for aluminum, it is
generally desirable to have an amount of greater than or equal to
about 2.85, preferably greater than or equal to about 2.9, and more
preferably greater than or equal to about 2.93 wt % aluminum. Also
desirable within this range is an amount of less than or equal to
about 5.0, preferably less than or equal to about 4.5, and more
preferably less than or equal to about 4.0 wt % aluminum, based on
the total weight of the alloy.
[0035] Within the aforementioned range for niobium, it is generally
desirable to have an amount of greater than or equal to about 2,
preferably greater than or equal to about 3, and more preferably
greater than or equal to about 3.5 wt % niobium, based on the total
weight of the alloy.
[0036] In one exemplary embodiment, it is generally desirable for
the .beta. titanium alloy to comprise 8.9 wt % molybdenum, 3.03 wt
% aluminum, 1.95 wt % vanadium, 3.86 wt % niobium, with the balance
being titanium.
[0037] In another exemplary embodiment, it is generally desirable
for the .beta. titanium alloy to comprise 9.34 wt % molybdenum,
3.01 wt % aluminum, 1.95 wt % vanadium, 3.79 wt % niobium, with the
balance being titanium.
[0038] In one embodiment, the .beta. titanium alloy may be solution
treated and/or thermally aged. In solution treating the .beta.
titanium alloy, the alloy is subjected to a temperature greater
than or equal to about 850.degree. C., the .beta. transus
temperature for the alloy. The solution treatment of the alloy is
normally carried out in either vacuum or inert gas environment at a
temperature of about 850 to about 1000.degree. C., preferably about
850 to about 900.degree. C., for about 1 minute or longer in
duration depending on the mass of the part. The heating is followed
by a rapid cooling at a rate greater than or equal to about
5.degree. C./second, preferably greater than or equal to about
25.degree. C./second, and more preferably greater than or equal to
about 50.degree. C./second, by using an inert gas quench or air
cooling to retain a fully recrystallized single phase .beta. grain
structure. In some instances, it is preferred that the quenched
alloy is subsequently subjected to an ageing treatment at about 350
to about 550.degree. C. for about 10 seconds to about 30 minutes to
adjust the amount of a fine precipitate of the .omega. phase.
[0039] In another embodiment, the .beta. titanium alloy may be
solution treated at a temperature below the .beta. transus
temperature of about 750 to about 850.degree. C., preferably about
800 to about 850.degree. C., for about 1 to about 30 minutes to
induce a small amount of .alpha. precipitates in the recrystallized
.beta. matrix. The amount of the .alpha. precipitates is preferably
less than or equal to about 15 volume percent and more preferably
less than or equal to about 10 volume percent, based on the total
volume of the composition. This improves the tensile strength to an
amount of greater than or equal to about 140,000 pounds per square
inch (9,846 kilogram/square centimeter).
[0040] The .beta. titanium alloy in the solution treated condition
may exhibit pseudoelasticity. The solution treated .beta. titanium
alloy generally exhibits a pseudoelastic recovery of greater than
or equal to about 75% of the initial strain when elastically
deformed to a 2% initial strain, and greater than or equal to about
50% of the initial strain when elastically deformed to a 4% initial
strain. The initial strain is the ratio of the change in length to
the original length of the alloy composition.
[0041] The .beta. titanium alloy in the solution treated condition
may exhibit linear elasticity. The solution treated .beta. titanium
alloy generally exhibits a linear elastic recovery of greater than
or equal to about 75% of the initial strain when elastically
deformed to a 2% initial strain, and greater than or equal to about
50% of the initial strain when elastically deformed to a 4% initial
strain. The initial strain is the ratio of the change in length to
the original length of the alloy composition.
[0042] In another embodiment, the .beta. titanium alloy may be cold
worked by processes such as cold rolling, drawing, swaging,
pressing, and the like, at ambient temperatures. The .beta.
titanium alloy may preferably be cold worked to an amount of about
5 to about 85% as measured by the reduction in cross-sectional area
based upon the original cross sectional area. Within this range it
is desirable to have a cross sectional area reduction of greater
than or equal to about 10, preferably greater than or equal to
about 15% of the initial cross sectional area. Also desirable
within this range is a reduction in cross sectional area of less
than or equal to about 50, more preferably less than or equal to
about 30% based on the initial cross-sectional area. The .beta.
titanium alloy in the cold worked state (also referred to as the
work hardened state) exhibits linear superelasticity where greater
than or equal to about 75% of the initial strain is elastically
recoverable after deforming to a 2% initial strain, and greater
than or equal to about 50% of the initial strain is elastically
recoverable after deforming to a 4% initial strain. In one
exemplary embodiment related to cold working, the elastic modulus
of the .beta. titanium alloy is reduced through cold working by an
amount of greater than or equal to about 10, preferably greater
than or equal to about 20 and more preferably greater than or equal
to about 25% based upon the elastic modulus, after the alloy is
heat treated.
[0043] It is generally desirable to use shape memory alloys having
pseudo-elastic properties, and which are formable into complex
shapes and geometries without the creation of cracks or fractures.
In one embodiment, the .beta. titanium alloy having linear elastic,
linearly superelastic, pseudoelastic or superelastic properties may
be used in the manufacturing of various articles of commerce.
Suitable examples of such articles are eyewear frames, face inserts
or heads for golf clubs, medical devices such as orthopedic
prostheses, spinal correction devices, fixation devices for
fracture management, vascular and non-vascular stents, minimally
invasive surgical instruments, filters, baskets, forceps, graspers,
orthodontic appliances such as dental implants, arch wires, drills
and files, and a catheter introducer (guide wire).
[0044] The superelastic .beta. titanium alloy generally provides an
adequate spring-back for eyewear applications. It is generally
desired to use superelastic .beta. titanium alloy having a minimum
recovery of about 50% of the initial strain, when the alloy is
deformed to an outer fiber initial strain of about 4% in a bend
test. It is preferable to have a minimum recovery of greater than
or equal to about 75% of the initial strain when the alloy
composition is deformed to about 4% of the outer fiber initial
length in a bend test. It is also generally desirable for the
superelastic .beta. titanium alloy to have a minimum recoverable
strain of about 50% of the initial strain, when the alloy
composition is strained to about 4% initial tensile strain. It is
preferable to have a minimum recovery of greater than or equal to
about 75% of the initial tensile strain, when the alloy is strained
to about 4% initial strain in a tensile test. The strain recovery
is measured as a function of the initial bending strain and the
initial bending strain is expressed as a percentage of the ratio of
the change in length to the original length.
[0045] The following examples, which are meant to be exemplary, not
limiting, illustrate some of the various embodiments of the .beta.
titanium alloy compositions described herein.
EXAMPLES
Example 1
[0046] All of the sample alloys discussed below were prepared by a
double vacuum arc melting technique. The ingots were hot rolled and
flattened to sheets having a thickness of 1.5 millimeter (mm). The
sheets were then heat treated at 870.degree. C. for 30 minutes in
air and air cooled to ambient temperature. Oxides on the sheets
were removed by double-disc grinding and lapping to a thickness of
1.3 mm. Heat aging experiments were conducted at 350.degree. C.
using a nitride/nitrate salt bath.
[0047] Permanent deformation and pseudo-elastic recovery strains
were determined using bend tests. Specimens having dimensions 0.51
mm.times.1.27 mm.times.51 mm were cut from the sheets. The
specimens were bent against a rod of approximately 12.2 mm in
diameter to form a "U" shape to yield an outer fiber or outer
surface strain close to 4%. The angles between the straight
portions were measured afterwards and the strain recovery
calculated by using the formula:
e(rec)-e(180-a)/180;
[0048] where "a" is the unrecovered angle and "e` is the
outer-fiber bending strain.
[0049] Tensile strain recovery was measured by tensile elongation
to a strain of 4% followed by unloading to zero stress. Tensile
specimens with a cross sectional dimension of 0.90 mm.times.2.0 mm
were used and the strain was monitored using an extensometer. An
environmental chamber with electrical heating and CO.sub.2 cooling
capabilities provided a testing capability from -30.degree. C. to
180.degree. C.
[0050] Nine .beta. titanium alloys having the compositions listed
in Table 1 were examined. The percentage of the elastic recovery
strain with respect to the total bend strain was measured for
comparison.
1TABLE 1 Sample # Titanium Molybdenum Niobium Vanadium Aluminum 1
Balance 7.63 3.98 2.05 3.10 2 Balance 8.03 3.89 2.03 3.09 3 Balance
8.40 3.83 1.94 3.03 4 Balance 8.97 3.86 1.95 3.03 5 Balance 9.34
3.79 1.95 3.01 6 Balance 10.35 3.83 1.99 3.02 7 Balance 10.83 3.88
2.01 3.02 8 Balance 11.48 4.00 2.04 3.15 9 Balance 11.68 3.89 1.98
3.07
[0051] In the Table 1 above Sample 1 and Samples 6-9 are
comparative examples. The results of elastic recovery after bending
to approximately 4% outer fiber strain is graphically demonstrated
in FIG. 1. The figure shows a maximum elastic strain recovery at
about 9 wt % molybdenum, where the alloy after solution heat
treatment and subsequent air cooling, exhibits an elastic recovery
strain of approximately 80% of the applied 4% deformation strain.
Increasing or decreasing the molybdenum content from 9 wt %
generally results in decreasing elastic recovery. It may also be
seen that an aging treatment at 350.degree. C. for a short duration
of 10 seconds results in an improved elastic recovery, for titanium
alloys having a molybdenum content between 8.4 and 11 wt %. The
optimal elastic strain recovery after heat aging at 350.degree. C.
for 10 seconds for the alloy having about 9 wt % molybdenum is
approximately 90% of the applied 4% bend strain. Alloys with a
molybdenum content less than 8.4 wt % exhibit a different aging
characteristic. Aging at 350.degree. C. may degrade elastic strain
recovery as for alloy 2 having about 8.03 wt % molybdenum, or has
no significant effect as for alloy 1 having about 7.63 wt %
molybdenum.
[0052] The percent of the elastic recovery to the total deformation
during thermal aging at 350.degree. C. for Samples 4, 5 and 6
respectively, are plotted in the FIGS. 2, 3 and 4 respectively.
From the FIGS. 2, 3 and 4 it may be seen that the elastic
recoveries of all three alloys reach a maximum after aging for
about 10 to about 60 seconds. Aging beyond 15 minutes (900 seconds)
degrades the elastic recovery.
[0053] The percents of the elastic recovery to the total
deformation during thermal aging at about 250 to about 550.degree.
C. for 10 seconds for Samples 4 and 5 respectively are plotted in
the FIGS. 5 and 6, respectively. An optimal for Sample 4 appears at
350.degree. C., which improves the elastic recovery to a percentage
close to 90% while aging at temperatures equal to or higher than
400.degree. C. degrade elastic recovery to about 40%. For Sample 5,
aging in this temperature range generally improves the elastic
recovery. The maximum improvement occurs at about 450.degree. C.
where the elastic recovery is improved to 90%.
[0054] The alloys shown in Table I also exhibit linear
superelasticity after cold working with a reduction of greater than
or equal to about 30% in the cross-sectional area. For example, a
wire fabricated from an ingot having a composition of 11.06 wt %
molybdenum, 3.80 wt % niobium, 1.97 wt % vanadium, 3.07 wt %
aluminum with the remainder being titanium exhibited an elastic
recovery strain of 3.5% after bending to a total deformation of 4%
outer fiber strain, when the reduction in the cross sectional area
after cold working was 84%.
Example 2
[0055] In this example, the .beta. titanium alloys were
manufactured by double vacuum arc melting. Chemistries of the
alloys were analyzed using inductively coupled plasma optical
emission spectrometry (ICP-OE). The results are tabulated in Table
2. The ingot was hot-forged, hot-rolled and finally cold-drawn to
wire of various diameters in the range of about 0.4 to about 5 mm.
Inter-pass annealing between cold reductions was carried out at
870.degree. C. in a vacuum furnace for wires having a diameter of
larger than 2.5 mm or by strand annealing under inert atmosphere
for the smaller diameters. Tensile properties were determined using
an Instron model 5565 material testing machine equipped with an
extensometer of 12.5 mm gage length. Microstructures were studies
by optical metallography using a Nikon Epiphot inverted
metallurgical microscope.
2TABLE 2 Sam- Alu- ple # Titanium Molybdenum Niobium Vanadium minum
Mo.sub.Eq 10 Balance 11.06 3.80 1.97 3.07 10.37 11 Balance 9.59
3.98 1.99 3.13 8.91
[0056] The strand-annealed wires generally have a higher ultimate
tensile strength (UTS) around 1055 mega Pascals (MPa) than vacuum
annealed wires and sheets, the typical UTS of which is about 830
MPa. FIG. 7 plots the UTS of wires drawn from an annealed 1.0 mm
diameter Sample 11 wire stock as a function of reduction in
cross-section area. After a 49% reduction, the UTS was elevated
from 1055 MPa to only 1172 MPa indicating a fairly weak strain
hardening effect. Young's Modulus was determined by tensile testing
the wire to 1% strain and measuring the linear slope of the
stress-strain curve. As shown in FIG. 8, cold-drawn wires generally
have a lower modulus than does annealed wire. The modulus, of
approximately 65.9 gigapascals (GPa) for the annealed wire,
decreases with increasing accumulative amount of reduction and
stabilizes at approximately 50 GPa after cold drawing with a
cumulative reduction greater than 20%.
[0057] Similar to alloys in Table 1, Samples 10 and 11 exhibit
linear superelasticity after cold working. Loading and unloading
stress-strain curves tested to 2% and 4% tensile strains of a cold
drawn, 0.91 mm diameter wire of Sample 11 with a 19.4% reduction
are plotted in FIGS. 9 and 10, respectively. As may be seen in FIG.
7, after unloading, following a 2% tensile elongation, the wire
recovers the majority of the deformation leaving only a small
plastic deformation of 0.1% strain. When deformed to a 4% tensile
elongation, the residual strain after unloading increases to 1.4%.
The wire recovers a strain of 2.6%. The residual strain decreases
with increasing drawing (cross-sectional area) reduction. However,
when the reduction exceeds 20%, specimens failed before reaching a
4% tensile elongation. As this data suggests, cold drawn .beta.
titanium alloy wires exhibit linear superelasticity and are capable
of recovering large deformations greater than the typical elastic
limit for conventional metallic alloys. The mechanical property of
cold-drawn wire appears to be insensitive to chemical composition
as the cold-drawn Sample 10 exhibits similar mechanical properties.
All the loading/unloading tensile test results for Sample 10 are
tabulated in Tables 3.
3 TABLE 3 Cold Work Amount (%) 21 37 50 61 69 Tested to 2% tensile
strain Elastic Strain (%) 1.9 1.8 1.8 1.9 2.0 Plastic Strain (%)
0.1 0.2 0.2 0.1 0.0 Tested to 3% tensile strain Elastic Strain (%)
2.5 2.6 2.6 2.7 2.7 Plastic Strain (%) 0.5 0.4 0.4 0.3 0.3 Tested
to 4% tensile strain Elastic Strain (%) -- 2.8 2.9 3.1 3.2 Plastic
Strain (%) -- 1.2 1.1 0.9 0.8
[0058] A micrograph in FIG. 11 reveals the cold-worked
microstructure of the Sample 10 wire after a 14% cold working
reduction in cross sectional area. The recrystallized
microstructures of the wire after heat-treatments at 816.degree. C.
and 871.degree. C. for 30 minutes are shown in FIGS. 12 and 13,
respectively. It is apparent that the material was not fully
betatized after the heat-treatment at 816.degree. C. as .alpha.
phase was present in the microstructure. As may be seen in FIG. 11,
a fully recrystallized .beta. grain structure was obtained after
the heat-treatment at 871.degree. C. for 30 minutes.
[0059] Sample 10 wires hot-rolled to 8.6 mm in diameter were
further drawn down to 6.0 mm diameter. After being fully betatized
at 871.degree. C. for 30 minutes the 6.0 mm diameter wires were
again aged at temperatures of about 500 to about 850.degree. C. for
30 minutes. As can be seen in FIG. 14, the .beta. structure was
preserved after aging at 816.degree. C. When the aging temperature
was lowered to 788.degree. C., intragranular .alpha.-phase
precipitates began to appear in the microstructure as may be seen
in FIG. 15. The amount of intragranular .alpha.-phase precipitate
increased with decreasing aging temperature. .alpha.-phase
precipitates eventually appeared along the grain boundary when aged
at 649.degree. C. and below.
[0060] The ultimate tensile strength (UTS) and tensile ductility (%
reduction in cross-section area) of betatized Sample 10 from Table
2 after aging at a temperature of about 500 to about 900.degree. C.
for 30 minutes are plotted in FIGS. 16 and 17, respectively. Fully
betatized specimens such as solution-treated specimens and those
aged at 816.degree. C. and above, exhibited a low UTS of about 800
MPa and a good tensile ductility of about 25 to about 30% in
reduction in cross-section area (RA). As the aging temperature
decreased, there was a drastic increase in UTS with a significant
reduction in tensile ductility, presumably due to an increasing
amount of .alpha.-precipitates. The peak of 1400 MPa in UTS
coincides with the low in ductility (5% RA) and both appeared at
approximately 500.degree. C. of aging temperature.
[0061] The Sample 11 composition in solution treated condition
exhibits pseudoelasticity. Their mechanical properties are highly
sensitive to solution heat treatment and subsequent aging at a
temperature of about 350 to about 550.degree. C. It was discovered
that Sample 11 wires after strand annealing at 870.degree. C.
exhibit well-defined pseudoelasticity. An example is presented in
FIG. 18, which shows a 4% tensile stress-strain curve of a
strand-annealed, 0.4 mm diameter Sample 11 wire. After deforming to
a 4% elongation, the wire specimen was able to go through a
pseudoelastic recovery recovering a 3.4% tensile strain and leaving
a residual strain of only 0.6% after unloading.
[0062] A transverse cross-sectional view of the wire microstructure
is shown in a micrograph of FIG. 19. Instead of the anticipated
.beta. structure, the microstructure consists of equiaxial .alpha.
precipitates in .beta. matrix. It appears that the short duration
of strand annealing did not allow the wire to fully recrystallize
into the .beta. grain structure. Without being limited by theory,
it is believed that this may explain why strand-annealed wire
generally has a higher UTS when compared to that of a fully
betatized material.
[0063] As may be seen from the above experiments, the .beta.
titanium alloys can display an elastic strain recovery of 88.5%,
when subjected to an initial bending strain of 4%. The strain
recovery is measured as a function of the initial bending strain
and the initial bending strain is expressed as a percentage of the
ratio of the change in length to the original length. These alloys
may be advantageously used in a number of commercial applications
such as eyewear frames, face insert and heads for golf clubs,
orthodontic arch wires, orthopedic prostheses and fracture fixation
devices, spinal fusion and scoliosis correction instruments,
stents, a catheter introducer (guide wire) and the like.
[0064] While the invention has been described with reference to
exemplary embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention.
* * * * *