U.S. patent application number 11/062805 was filed with the patent office on 2006-08-24 for electrochemical grain refining of a metal.
This patent application is currently assigned to National Research Council of Canada. Invention is credited to Benli Luan, Jianguo Yu.
Application Number | 20060185775 11/062805 |
Document ID | / |
Family ID | 36911391 |
Filed Date | 2006-08-24 |
United States Patent
Application |
20060185775 |
Kind Code |
A1 |
Luan; Benli ; et
al. |
August 24, 2006 |
Electrochemical grain refining of a metal
Abstract
A method for surface and subsurface grain refining of a bulk
hydrogen-absorbing metal includes the steps of cathodically
charging the bulk hydrogen-absorbing metal with an electric current
in the presence of a source of hydrogen to hydride the
hydrogen-absorbing metal, and, changing polarity of the electric
current to dehydride the hydrogen-absorbing metal. The method
results in improvement to hardness and/or wear resistance of the
metal, particularly titanium alloys such as Ti-6Al-4V. Metals
treated with this method are particularly useful for medical
implants and vehicle parts in which improved hardness and/or wear
resistance is required.
Inventors: |
Luan; Benli; (London,
CA) ; Yu; Jianguo; (Kitchener, CA) |
Correspondence
Address: |
NATIONAL RESEARCH COUNCIL OF CANADA;1200 MONTREAL ROAD
BLDG M-58, ROOM EG12
OTTAWA, ONTARIO
K1A 0R6
CA
|
Assignee: |
National Research Council of
Canada
|
Family ID: |
36911391 |
Appl. No.: |
11/062805 |
Filed: |
February 23, 2005 |
Current U.S.
Class: |
148/669 ;
205/322; 205/704 |
Current CPC
Class: |
C21D 10/00 20130101;
C21D 11/00 20130101; A61F 2310/00023 20130101; A61F 2/3094
20130101; A61F 2/30767 20130101; C22F 1/18 20130101 |
Class at
Publication: |
148/669 ;
205/322; 205/704 |
International
Class: |
C22F 1/18 20060101
C22F001/18 |
Claims
1. A method for surface and subsurface grain refining of a bulk
hydrogen-absorbing metal comprising: (a) cathodically charging the
bulk hydrogen-absorbing metal with an electric current in the
presence of a source of hydrogen to thereby hydride the
hydrogen-absorbing metal; and, (b) changing polarity of the
electric current to thereby dehydride the hydrogen-absorbing
metal.
2. The method of claim 1, wherein the electric current is AC.
3. The method of claim 1, wherein the electric current is DC.
4. The method of claim 1, wherein the hydrogen-absorbing metal is
nickel-free.
5. The method of claim 1, wherein the hydrogen-absorbing metal
comprises a titanium alloy.
6. The method of claim 5, wherein the titanium alloy is
Ti-6Al-4V.
7. The method of claim 1, wherein the source of hydrogen is an
aqueous acid or base.
8. The method of claim 1, wherein the source of hydrogen is an
aqueous inorganic acid or an aqueous inorganic base.
9. The method of claim 8, wherein the aqueous inorganic acid has a
concentration in a range of from 0.1 M to 10 M, and the aqueous
inorganic base has a concentration in a range of from 0.05 M to 6
M.
10. The method of claim 1, wherein the source of hydrogen is
aqueous sulfuric acid or aqueous potassium hydroxide.
11. The method of claim 2, wherein the AC during hydriding has a
current density (I.sub.1) in a range of from 0.01 to 100
mA/cm.sup.2 and a pulse period (t.sub.1) in a range of from 2 to
120 seconds, the AC during dehydriding has a current density
(I.sub.2) in a range of from 0.01 to 100 mA/cm.sup.2 and a pulse
period (t.sub.2) in a range of from about 2 to about 120 seconds,
and total time for hydriding/dehydriding is in a range of from 1
hour to 50 hours, temperature is in a range of from 0.degree. C. to
100.degree. C., and wherein (a) in an acidic environment, the AC
has a pulse potential for hydriding (E.sub.1) of from -1.3 to -0.5
V, a pulse potential for dehydriding (E.sub.2) of from about -0.5
to -0.1 V, or, (b) in a basic environment, the AC has a pulse
potential for hydriding (E.sub.1) of from -1.9 to -1.4 V a pulse
potential for dehydriding (E.sub.2) of from -1.1 to -0.5 V.
12. The method of claim 3, wherein the metal is hydrided at a
current density in a range of from 0.01 to 20 mA/cm.sup.2 for a
period of time of from 1 to 200 hours per cycle at a temperature in
a range of from 0.degree. C. to 100.degree. C., and the metal is
dehydrided at a current density in a range of from 0.01 to 1
mA/cm.sup.2 for a period of time in a range of from 1 to 400 hours
per cycle at a temperature in a range of from 0.degree. C. to
100.degree. C.
13. The method of claim 1, wherein dehydriding is conducted
initially at a first rate and then subsequently at a second rate,
the second rate being lower than the first rate.
14. A method for surface and subsurface grain refining of a bulk
titanium alloy comprising: (a) cathodically charging the bulk
titanium alloy with an electric current in an aqueous inorganic
acid or an aqueous inorganic base to thereby hydride the titanium
alloy; and, (b) changing polarity of the electric current to
thereby dehydride the titanium alloy.
15. The method of claim 14, wherein the titanium alloy is
Ti-6Al-4V.
16. The method of claim 14, wherein the electric current is AC.
17. The method of claim 16, wherein the aqueous inorganic acid
comprises sulfuric acid having a concentration in a range of from
0.1 M to 10 M, and the aqueous inorganic base comprises potassium
or sodium hydroxide having a concentration in a range of from 0.05
M to 6 M.
18. The method of claim 17, wherein the AC during hydriding has a
current density (I.sub.1) in a range of from 0.01 to 100
mA/cm.sup.2 and a pulse period (t.sub.1) in a range of from 2 to
120 seconds, the AC during dehydriding has a current density
(I.sub.2) in a range of from 0.01 to 100 mA/cm.sup.2 and a pulse
period (t.sub.2) in a range of from about 2 to about 120 seconds,
and total time for hydriding/dehydriding is in a range of from 1
hour to 50 hours, temperature is in a range of from 0.degree. C. to
100.degree. C., and wherein (a) in the aqueous inorganic acid, the
AC has a pulse potential for hydriding (E.sub.1) of from -1.3 to
-0.5 V, a pulse potential for dehydriding (E.sub.2) of from about
-0.5 to -0.1 V; or, (b) in the aqueous inorganic base, the AC has a
pulse potential for hydriding (E.sub.1) of from -1.9 to -1.4 V a
pulse potential for dehydriding (E.sub.2) of from -1.1 to -0.5
V.
19. The method of claim 18, wherein dehydriding is conducted
initially at a first rate and then subsequently at a second rate,
the second rate being lower than the first rate.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to electrochemical grain
refining of a metal, particularly a titanium alloy.
BACKGROUND OF THE INVENTION
[0002] Medical (e.g. orthopedic) implants are used to replace or
augment existing biological structures, for example bones, in the
bodies of humans and other animals. Materials used to construct
such implants are in direct contact with the body through an
interface between the material's surface and the body's bones,
tissues and/or extracellular fluids. Therefore, the tribiological
(friction, wear, durability) properties of the implant are
important considerations, and the surface nano/micro structure of
the material used to construct the implant plays a very important
role in determining the tribiological properties of the implant.
For example, wear of the implant's surface can lead to loosening of
the implant and to releasing of debris into the body, therefore
increasing wear resistance and/or surface hardness of implant
materials is important.
[0003] There has been extensive work on surface modification of
materials to understand and enhance surface performance of
implants. One approach is to modify the surface topography by
creating a rough, or porous surface on the implant to increase the
surface area available for bone/implant apposition. A natural
consequence of these surface modifications, however, is an increase
in metal ion release due to increased surface contact with
corrosive media in the body. A further complication is an increase
in wear debris due to increased surface friction, which will also
result in increased metal ion release rates and loosening of the
implant.
[0004] Another approach is to provide the implant's surface with a
bioactive coating, for example bioactive glass. Bioactive glass is
intended to allow stable mechanical fixation of the implant to
bone. However, bioactive glass has a non-compatible thermal
expansion coefficient relative to the metal, for example Ti-based
alloys, used in the implant. Non-compatible thermal expansion
coefficients leads to cracking of the glass due to thermal stresses
that occur during the heating/cooling cycle used to apply glass
films to the metal. Additionally, bioactive glass rapidly dissolves
in body fluids when implanted.
[0005] Other surface coatings have been tried with the aim of
improving the interface bond between the implant and bone. These
include hydroxyapatite (HA) coatings produced by plasma spray or
ion implantation. While these methods were effective in improving
short-term bone-implant bonding without any fibrous tissue
formation, long-term performance of such coatings is severely
lacking due to adhesion problems of the coating on the metal of the
implant and due to poor control of dissolution rate of the coatings
in bodily fluids.
[0006] Yet another approach to surface modification of implants is
to coat the implant's surface with hard materials focussing on
increasing wear resistance. For example, titanium nitride may be
coated on to an implant surface by chemical vapor deposition (CVD)
and physical vapor deposition (PVD). Although these methods provide
the implant with excellent wear resistance, the deposited layers
suffer from lack of adherence as the interaction between the
coating and the substrate is not a requisite for coating growth in
either CVD or PVD.
[0007] Low energy nitrogen ion bombardment plasma nitriding is one
of the most recent methods for improving wear and corrosion
behavior of metallic alloys. In plasma nitriding, a Ti-based
substrate is directly involved in the formation of the coating,
which results in excellent adhesion of the coating to the
substrate. However, the inherently high cost of plasma nitriding
reduces cost-effectiveness of this method.
[0008] Hydrogenation/dehydrogenation techniques are very effective
in forming mesoscopic crystalline (.about.1 .mu.m),
submicrocrystalline (SMC, <1 am) and even nanocrystalline (NC,
<0.1 .mu.m) structures. Techniques such as thermal hydrogen
processing (THP) and a combination of electrochemical hydrogenation
and thermal dehydrogenation have been reported. In these
approaches, hydrogen is added to a metal alloy by controlled
diffusion from a hydrogen environment, which can be either gaseous
or electrolytic hydrogen environments. After processing, hydrogen
is then removed by a controlled vacuum anneal (a thermal
process).
[0009] THP processes have been shown to improve yield strength,
elongation, hardness, hot-workability and ductility of the metal.
It is thought that these improvements are due to modification of
grain microstructure of the metal. A typical THP process includes
subjecting a titanium alloy to a hydrogenation treatment, an
elevated temperature .beta. solution treatment, a moderate
temperature eutectoid decomposition and finally an elevated
temperature vacuum dehydrogenation treatment. It is believed that
after hydrogenation, .gamma. titanium hydride precipitates in a
phase of the hydrogenated specimen. During the eutectoid
decomposition treatment a mixture consisting of .alpha.+.gamma.
hydride nucleates from .beta..sub.H matrix. During the
dehydrogenation treatment, the .gamma. titanium hydrides gradually
lose their hydrogen content resulting in recrystallization of fine
.alpha./.beta. phases and refined microstructure.
[0010] A combination of electrochemical hydrogenation and thermal
dehydrogenation has been reported by Wu et al. in U.S. Pat. No.
5,178,694 issued Jan. 12, 1993 and in Metall. Trans. A, 24A,
1181-1185 (1993). An electrochemical technique was utilized to
dissolve hydrogen into a titanium alloy instead of a thermal
technique. A thermal technique was used to dehydride the alloy. The
results showed that a fine and equiaxed grain was formed. The
processed specimens showed an improvement in surface hardness,
which was attributed to associated recrystallization as in THP
processes.
[0011] Both THP and Electrochemical/THP processes use hydrogen as a
temporary alloying element to refine surface and subsurface grains
at the metal surface. However, both THP and Electrochemical/THP
processes require high temperatures (above 800.degree. C.).
Additionally, wear resistance and surface hardness of the treated
material could be improved.
[0012] While there has been considerable advance in the surface
modification of metals, the prior art methods all suffer from one
or more disadvantages including high capital investment, high
energy consumption, furnace chamber size limitations, complicated
operations and controls, and coating separation from substrates.
There remains a need in the art for a method of surface
modification that ameliorates one or a combination of the
disadvantages while providing a material with excellent
tribiological properties, for example wear resistance and surface
hardness.
SUMMARY OF THE INVENTION
[0013] There is provided a method for surface and subsurface grain
refining of a bulk hydrogen-absorbing metal comprising:
cathodically charging the bulk hydrogen-absorbing metal with an
electric current in the presence of a source of hydrogen to thereby
hydride the hydrogen-absorbing metal; and, changing polarity of the
electric current to thereby dehydride the hydrogen-absorbing
metal.
[0014] There is also provided a method for surface and subsurface
grain refining of a bulk titanium alloy comprising: cathodically
charging the bulk titanium alloy with an electric current in an
aqueous inorganic acid or an aqueous inorganic base to thereby
hydride the titanium alloy; and, changing polarity of the electric
current to thereby dehydride the titanium alloy.
[0015] Metals useful in the present invention have
hydrogen-absorbing capacity, although high sensitivity to hydrogen
is not required. Preferably, the hydrogen-absorbing capacity is in
a range of from about 50 mAh/g to about 999 mAh/g, more preferably
from about 150 mAh/g to about 999 mAh/g. The metal is provided in
bulk form, preferably in the form that will be used commercially.
Preferably the metal is biologically compatible, although for
non-biological purposes there is no requirement that the metal be
biologically compatible.
[0016] Some examples of hydrogen-absorbing metals are
titanium-based metals, lanthanum-based metals, magnesium-based
metals, cerium-based metals, zirconium-based metals and
cadmium-based metals. Cadmium-based metals are not particularly
preferred as cadmium is toxic and is therefore of limited value in
biological applications. Titanium alloys and lanthanum alloys (e.g.
Misch metals) are particularly preferred. Titanium alloys are more
preferred. Titanium alloys comprise titanium (Ti) and significant
quantities of one or more alloying elements, for example aluminum
(Al) and vanadium (V). Titanium alloys may contain small amounts of
impurities or incidental elements, for example, carbon (C),
nitrogen (N), iron (Fe), yttrium (Y), oxygen (O) and/or hydrogen
(H), although such impurities or incidental elements can sometimes
affect the overall properties of the alloy. A particularly
preferred titanium alloy is Ti-6Al-4V.
[0017] Electrochemical processes are typically conducted in an
electrochemical cell having a working electrode and a counter
electrode, and possibly a reference electrode, suspended in an
electrolyte. The general construction of electrochemical cells is
well known to one skilled in the art. The working electrode is made
of the hydrogen-absorbing metal to be treated. The counter
electrode may comprise any suitably conductive material, for
example platinum (Pt) or graphite. If a reference electrode is
required or desired, standard electrodes, for example saturated
calomel and Hg/HgO electrodes, may be used.
[0018] The electrolyte may comprise any suitable ionic species in
liquid or gaseous form. Preferably, the electrodes are suspended in
aqueous solutions of ionic species. Suitable ionic species may be
generated from salts (e.g. alkali metal halides such as sodium
chloride or potassium chloride), acids and/or bases. Preferably,
the ionic species are generated from acid or bases.
[0019] The hydrogen-absorbing metal is cathodically charged in the
presence of a source of hydrogen to hydride the metal. The source
of hydrogen may be any hydrogen-containing species that will
provide hydrogen atoms to be absorbed by the metal under cathodic
charging conditions. The source of hydrogen is preferably, an
aqueous acid or base since aqueous acids and based provide for both
the hydrogen source and the electrolyte. Aqueous acids are more
preferred.
[0020] Of the acids, inorganic acids are preferred. Inorganic acids
include, for example, hydrochloric acid, sulfuric acid, phosphoric
acid, nitric acid, perchloric acid, among others. Aqueous solutions
of the acid should contain a high enough concentration of hydrogen
ion (hydronium ion) to permit hydrogenation of the metal during
cathodic charging. Preferably, the concentration of hydrogen ion in
solution is in a range from about 0.1 to 10 M, more preferably from
about 2 to 6 M.
[0021] Of the bases, inorganic bases are preferred. Inorganic bases
include, for example, alkali metal hydroxides (e.g. sodium
hydroxide, potassium hydroxide) and alkaline earth hydroxides (e.g.
magnesium hydroxide and calcium hydroxide). Aqueous solutions of
the base should contain a high enough concentration of hydroxide
ion to permit hydrogenation of the metal during cathodic charging.
Preferably, the concentration of hydroxide ion in solution is in a
range from about 0.05 to 6 M, more preferably from about 1 to 6
M.
[0022] To dehydride the metal, the polarity of the electrochemical
cell may be changed. Typically, direct current (DC) has been used
in the art to perform hydrogenation. If direct current (DC) is used
to hydride the metal, a cycle or time course may be set up whereby
the DC cathodically charges the hydrogen-absorbing metal for a
first period of time, and then the polarity of the DC is reversed
to anodically charge the metal for a second period of time. Two or
more charging cycles may be conducted whereby the polarity of the
DC is periodically reversed to alternate between cathodic and
anodic charging of the metal. A simpler way of providing
alternation between cathodic and anodic charging (i.e. alternation
between hydrogenation and dehydrogenation) is to use alternating
current (AC), whereby the polarity is automatically changed on a
regular periodic basis. Charging programs for hydriding and
dehydriding the metal depend on the type of current used. The
process is generally faster using AC as the process is kinetically
sluggish using DC. The use of AC is preferred over DC.
[0023] For DC, hydrogenation is preferably performed at a current
density in a range of from about 0.01 to about 20 mA/cm.sup.2 for a
period of time in a range of from about 1 to about 200 hours.
Preferably, the current density remains constant during
hydrogenation. Dehydrogenation is preferably performed at a current
density in a range of from about 0.01 to about 1 mA/cm.sup.2 for a
period of time in a range of from about 1 to about 400 hours.
Preferably, the current density remains constant during
dehydrogenation. Preferably, hydrogenation is performed
continuously until maximum hydrogen absorption is achieved, and
then dehydrogenation is performed continuously until complete or
near complete dehydrogenation is achieved. One cycle comprises one
period of hydrogenation and one period of dehydrogenation. The
process may comprise more than one cycle and the number of cycles
depends on the desired depth of grain refinement.
[0024] For AC, hydrogenation and dehydrogenation recur periodically
as the polarity changes on a regular basis. Therefore, in using AC,
multiple hydrogenation/dehydrogenation cycles occur automatically.
The total charging time when using AC is preferably in a range of
from about 1 hour to about 50 hours, more preferably from about 20
hours to about 50 hours. During the total charging time,
hydrogenation is preferably performed at a pulse current density
(I.sub.1) in a range of from about 0.01 to about 100 mA/cm.sup.2
for a time period (t.sub.1) of from about 2 to about 120 seconds.
Dehydrogenation is preferably performed at a pulse current density
(I.sub.2) in a range of from about 0.01 to about 100 mA/cm.sup.2
for a time period (t.sub.2) of from about 2 to about 120 seconds.
The current densities change with change in potential, therefore,
there is no fixed value for the current density during hydriding
and dehydriding. Current density changes in relation to the amount
of hydrogen absorbed by the metal. As the metal absorbs hydrogen,
the current density drops, and as the metal loses absorbed hydrogen
the current density increases. Finally, the total time and the time
periods for hydrogenation and dehydrogenation depend in part on the
magnitude of the current density.
[0025] Pulse potentials in AC depend on the electrolyte. In acid
solutions, the pulse potential for hydrogenation (E.sub.1) is
preferably about -1.3 to -0.5 V. The pulse potential for
dehydrogenation (E.sub.2) is preferably about -0.5 to -0.1 V. In
base solutions, the pulse potential for hydrogenation (E.sub.1) is
preferably about -1.9 to -1.4 V. The pulse potential for
dehydrogenation (E.sub.2) is preferably about -1.1 to -0.5 V.
[0026] The present method may be conducted at any suitable
temperature. For aqueous electrolyte systems, the temperature is
preferably in a range of from about 0.degree. C. to about
100.degree. C. For simplicity, the method is preferably conducted
at ambient temperature. Lower temperatures slow the rate of
hydrogenation and dehydrogenation, whereas higher temperatures
drive dissolved hydrogen out of solution more rapidly. Optimal
temperature may be readily determined by one skilled in the art by
simple experimentation in light of the particular metal being
treated.
[0027] Once method parameters have been determined for a particular
treatment application, charging of the metal may occur for the
total time without having to change any of the parameters. Thus,
the electrochemical method of the present invention is very simple
to operate and control. The total charging time typically depends
on how thick of a treated layer is desired, which depends on the
particular commercial application. Longer charging time results in
a thicker hardened layer, i.e. a larger depth of refinement.
[0028] Other processing of the metal may be performed before and/or
after the electrochemical grain refining process of the present
invention. For example, the metal may be mill annealed, shaped,
polished, degreased, cleaned, etched, etc. In situations where it
is undesirable to grain refine all surfaces of the metal, certain
areas may be sealed to prevent treatment at those areas.
[0029] The present electrochemical method of grain refinement does
not require thermal treatment of the metal during either
hydrogenation or dehydrogenation, since hydrogenation and
dehydrogenation are both conducted electrochemically. Surprisingly,
such an electrochemical method leads to unexpectedly large
improvements in surface hardness and wear resistance of the metal.
Furthermore, such an electrochemical process is useful for metals
having low hydrogen sensitivity and for a wider variety of
hydrogen-absorbing metals than processes with a thermal component.
Other advantages of the present method in comparison to methods
involving thermal processes include: lower capital investment;
lower energy consumption; excellent scalability with lower scale-up
cost; and, simpler operation and controls. In addition, since the
present method does not require surface coating, there is no
problem of a surface layer coating separating from a bulk
material.
[0030] Another surprising benefit of the present invention is that
the dehydriding step is sufficiently kinetically facile that there
is no need for the metal to contain any components to lower the
kinetic barrier to dehydriding. It is known in the art of
rechargeable metal hydride batteries that dehydriding
hydrogen-absorbing metals, especially titanium alloys, is often
kinetically sluggish. For this reason, additives (e.g. nickel in
the form of TiNi or Ti.sub.2Ni) are often added to titanium alloys
to lower the kinetic barrier to dehydrogenation in order for
dehydriding to occur sufficiently quickly. In the present process,
such additives are not required, although they could be present if
desired. Therefore, the metal may be nickel-free.
[0031] A further surprising benefit of the present invention is
that even with metals that are not highly hydrogen sensitive, it is
possible to conduct the dehydriding step at a low rate, referred to
as "trickling", towards the end of the dehydriding to ensure
complete dehydrogenation.
[0032] Metals treated by the present electrochemical method are
useful in any commercial application in which surface hardness
and/or wear resistance is desirable. For example, medical implants
(e.g. pace makers, orthopedic implants, etc.) and vehicle parts
(e.g. automotive, aircraft, aerospace, etc.) are two preferred
commercial applications. Surface and subsurface treatment of
medical implants, particularly orthopedic implants, is a
particularly preferred use of the present method.
[0033] Further features of the invention will be described or will
become apparent in the course of the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] In order that the invention may be more clearly understood,
embodiments thereof will now be described in detail by way of
example, with reference to the accompanying drawings, in which:
[0035] FIG. 1A is a schematic representation of an electrochemical
processing program for potential pulse;
[0036] FIG. 1B is a schematic representation of an electrochemical
processing program for direct current;
[0037] FIG. 2 is an optical micrograph showing optical morphology
of a cross-section of untreated Ti-6Al-4V;
[0038] FIG. 3 is an optical micrograph showing optical morphology
of a cross-section of Ti-6Al-4V treated with a process of the
present invention;
[0039] FIG. 4A is a photograph of indentations on the surface of an
untreated Ti-6Al-4V specimen after hardness measurements using a 10
g load;
[0040] FIG. 4B is a photograph of indentations on the surface of an
untreated Ti-6Al-4V specimen after hardness measurements using a 50
g load;
[0041] FIG. 5A is a photograph of indentations on the surface of a
Ti-6Al-4V specimen treated with a process of the present invention
after hardness measurements using a 10 g load; and,
[0042] FIG. 5B is a photograph of indentations on the surface of a
Ti-6Al-4V specimen treated with a process of the present invention
after hardness measurements using a 50 g load.
[0043] FIG. 6 is a graph comparing surface hardness (HV) of
untreated and treated Ti-6Al-4V specimens under 10 g, 50 g, 100 g,
200 g, 300 g and 400 g loads.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0044] In a process of the present invention, either DC or AC
(pulse current) may be used for the hydrogenation and
dehydrogenation steps in order to refine grain sizes of a metal
specimen. FIG. 1A graphically represents an electrochemical
processing program for potential pulse (AC). FIG. 1B graphically
represents an electrochemical processing program for a constant
current (DC) program.
[0045] Electrochemical cells used to conduct the inventive
processes in the following Examples comprised counter electrodes
and reference electrodes. The counter electrodes were platinum
(Pt), and the reference electrodes were saturated calomel electrode
for acidic solutions and Hg/HgO electrode for basic solutions.
[0046] In all Examples of the invention below, the
hydrogen-absorbing metal is a titanium alloy, namely, Ti-6Al-4V. In
all of the Examples, Ti-6Al-4V ELI sheets with a thickness if 1.83
mm manufactured by RMI Titanium company were mill annealed at
787.degree. C. (1450.degree. F.) for 15 minutes and then air
cooled. Specimens were then cut the desired shape and size for each
Example. The elemental composition of Ti-6Al-4V is provided in
Table 1. TABLE-US-00001 TABLE 1 Composition of Ti--6Al--4V (wt %) C
N Fe Al V Y O Ti H 0.02 0.008 0.213 6.16 3.92 <50 ppm 0.12
Balance 32 ppm
EXAMPLES
Example 1
Optical Microscopy
[0047] Two Ti-6Al-4V specimens were cut to a size of 10 mm.times.10
mm.times.1.83 mm and sealed with epoxy resin on one side of each
specimen. The specimens were abraded with emery papers of up to 600
grits, polished with 0.05 .mu.m alumina, degreased with acetone and
then ultrasonically cleaned with deionized water. One of the two
specimens was electrochemically treated by a process of the present
invention in an electrochemical cell and the other was left
untreated. The treated specimen was exposed to a 6 M KOH solution
at 22.degree. C. and treated with a potential pulse for 5 hours.
The pulse parameters were E.sub.1=-1.93 V, E.sub.2=-1.13 V,
t.sub.1=10 s, t.sub.2=10 s. The 6 M KOH solution was prepared with
analytically pure KOH and deionized water.
[0048] The untreated and treated specimens were etched with Kroll's
reagent and observed with an optical microscope. FIG. 2 is an
optical micrograph showing optical morphology of a cross-section of
the untreated specimen. In FIG. 2, the light area is the metal and
the dark area below the light area is not part of the metal. The
surface of the metal is at the interface between the light and dark
areas. FIG. 3 is an optical micrograph showing optical morphology
of a cross-section of the treated specimen. The gray area labeled
as L in FIG. 3 is a layer in which grain refinement has occurred
due to the electrochemical treatment of the metal. It is evident
from the micrographs that extensive grain refinement has occurred
in the treated specimen in comparison to the untreated specimen. It
can be seen from the scale in the upper left corner of FIG. 3, that
the refined layer is about 10 .mu.m thick, i.e. the depth of the
hardened layer is about 10 .mu.m. The clear demonstration of grain
refining effect of the present method indicates that further grain
refining of the grain structure of hydrogen-absorbing metals to
nano grain or even amorphous structure is achievable using the
present method.
Example 2
Hardness
[0049] Ti-6Al-4V specimens were cut to a size of 10 mm.times.10
mm.times.1.83 mm and sealed with epoxy resin on one side of each
specimen. The specimens were abraded with emery papers of up to 600
grits, polished with 0.05 .mu.m alumina, degreased with acetone and
then ultrasonically cleaned with deionized water. Some of the
specimens were electrochemically treated by a process of the
present invention in an electrochemical cell and the others were
left untreated. The treated specimens were exposed to a 1 M
H.sub.2SO.sub.4 solution at 80.degree. C. and treated with a
potential pulse. The 1 M H.sub.2SO.sub.4 solution was prepared with
analytically pure H.sub.2SO.sub.4 and deionized water. The pulse
parameters for the treated specimens were based on a trickling
design and were E.sub.1=-0.9 V, E.sub.2=-0.5 V, t.sub.1=10 s,
t.sub.2=10 s for 1 hour, and then E.sub.1=-0.5 V, E.sub.2=-0.1 V,
t.sub.1=10 s, t.sub.2=10 s for 2 hours.
[0050] To measure hardness of the specimens, a Vickers
microhardness tester was used with 10 g, 50 g, 100 g, 200 g and 300
g loads for 20 seconds. Due to roughness of the electrochemically
treated specimens, the treated specimens were slightly polished
with 0.05 .mu.m alumina before the hardness was measured. FIGS. 4A
and 4B show photographs of indentations on the surface of the
untreated specimens after hardness measurements using a 10 g load
(FIG. 4A), and a 50 g load (FIG. 4B). FIGS. 5A and 5B shows
photographs of indentations on the surface of treated specimens
after hardness measurements using a 10 g load (FIG. 5A), and a 50 g
load (FIG. 5B). It is evident from the photographs that treated
specimens are harder than untreated specimens as the indentations
are not as deep on the treated specimens.
[0051] Table 2 lists surface hardness values (HV) of the untreated
specimens (Untreated 1-5), the electrochemically treated specimens
(Treated 1-5), untreated comparative specimens (Untreated A-B) and
treated comparative specimens (Comp A-B). The comparative specimens
are Ti-6Al-4V specimens treated with electrochemical/thermal
methods as described in U.S. Pat. No. 5,178,694 (Comp A) and in
Metall. Trans. A, 24A, 1181-1185 (1993) (Comp B). The hardness
values for Untreated A and Comp A at 400 g are taken from U.S. Pat.
No. 5,178,694. The hardness values for Untreated B and Comp B at
100 g are taken from the Metall. Trans. A paper, with Comp B being
the average of the sixteen hardness values listed in Table III of
the Metall. Trans. A paper. Table 2 also provides the numerical
difference (A) between the treated and untreated specimens.
TABLE-US-00002 TABLE 2 Surface Hardness (HV) 10 g 50 g 100 g 200 g
300 g 400 g Specimen load load load load load load Untreated 1 315
-- -- -- -- -- Treated 1 358 -- -- -- -- -- .DELTA.1 43 -- -- -- --
-- Untreated 2 -- 292 -- -- -- -- Treated 2 -- 363 -- -- -- --
.DELTA.2 -- 71 -- -- -- -- Untreated 3 -- -- 295 -- -- -- Treated 3
-- -- 359 -- -- -- .DELTA.3 -- -- 64 -- -- -- Untreated 4 -- -- --
307 -- -- Treated 4 -- -- -- 379 -- -- .DELTA.4 -- -- -- 72 -- --
Untreated 5 -- -- -- -- 298 -- Treated 5 -- -- -- -- 365 --
.DELTA.5 -- -- -- -- 67 -- Untreated A -- -- -- -- -- 325 Comp A --
-- -- -- -- 340 .DELTA.A -- -- -- -- -- 15 Untreated B -- -- 325 --
-- -- Comp B -- -- 349 -- -- -- .DELTA.B -- -- 24 -- -- --
[0052] The data in Table 2 is represented graphically in FIG. 6. It
is evident from Table 2 and FIG. 6 that the increase in hardness
(A) is greater in Ti-6Al-4V specimens treated by the
electrochemical process of the present invention compared to
Ti-6Al-4V specimens treated by processes of the prior art. In
particular, the results at a load of 100 g show that the increase
in hardness value (A) of a treated specimen over an untreated
specimen for a specimen treated by a process of the present
invention (Treated 3) is over 2.5 times greater than the increase
in hardness value (A) of a specimen treated with an electrochemical
hydrogenation step and a thermal dehydrogenation step (Comp B). The
results above demonstrate that the present electrochemical
hydrogenation/dehydrogenation method improves the surface hardness
of Ti-6Al-4V. The results also demonstrate that the present
electrochemical method can result in a greater improvement in
surface hardness in comparison to a method that involves thermal
dehydrogenation.
Example 3
Wear Resistance
[0053] Three Ti-6Al-4V specimens were cut to a size of 20
mm.times.20 mm.times.1.83 mm and sealed with epoxy resin on one
side of each specimen. The specimens were abraded with emery papers
of up to 600 grits, polished with 0.05 .mu.m alumina, degreased
with acetone and then ultrasonically cleaned with deionized water.
Two of the three specimens were electrochemically treated by a
process of the present invention in an electrochemical cell and the
other was left untreated. The treated specimens were exposed to a 1
M H.sub.2SO.sub.4 solution at 80.degree. C. and treated with a
potential pulse. The 1 M H.sub.2SO.sub.4 solution was prepared with
analytically pure H.sub.2SO.sub.4 and deionized water. The pulse
parameters for both of the treated specimens were based on a
trickling design and were E.sub.1=-0.9 V, E.sub.2=-0.5 V,
t.sub.1=10 s, t.sub.2=10 s for 0.5 hour, and then E.sub.1=-0.5 V,
E.sub.2=-0.1 V, t.sub.1=10 s, t.sub.2=10 s for 2.5 hours.
[0054] Wear resistance of the untreated and treated specimens was
tested using a pin-on-disk apparatus based on ASTM G99-95a:
Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus.
The untreated and treated specimens were used as the disk. The
material of the pin was WC-6% Co and the diameter of the pin ball
was 1/8 inch. The load applied to the disk was 100 g. The rotation
speed of the disk was 60 rpm an the total rotation number was 2200.
The track radius was 5 mm. The wear tracks on the specimens after
pin-on-disk tests was analyzed using profilometry, and the wear
loss was calculated based on the following formula assuming that
there was no significant pin wear. Disk .times. .times. volume
.times. .times. .times. loss , mm 3 = .pi. .times. .times. ( wear
.times. .times. track .times. .times. radius , mm ) .times. ( track
.times. .times. width , mm ) 3 6 .times. ( sphere .times. .times.
radius , mm ) ##EQU1##
[0055] Table 3 lists wear loss data of the untreated and treated
specimens. TABLE-US-00003 TABLE 3 Wear Loss Specimen Wear Loss
(mm.sup.3) Untreated 0.089 Treated 1 0.071 Treated 2 0.068
[0056] Wear resistance increase for treated specimen 1 is
(0.089-0.071)*100/0.089=20%. Wear resistance increase for treated
specimen 2 is (0.089-0.068)*100/0.089=24%. The average wear
resistance increase was about 22%. Thus, the results have
demonstrated that the electrochemical method of the present
invention can result in an initial increase in wear resistance of
over 22% of Ti-6Al-4V in comparison to untreated specimens.
[0057] Other advantages which are inherent to the invention are
obvious to one skilled in the art. The embodiments are described
herein illustratively and are not meant to limit the scope of the
invention as claimed. Variations of the foregoing embodiments will
be evident to a person of ordinary skill and are intended by the
inventor to be encompassed by the following claims.
* * * * *