ALPHA STABILIZED BLACK TITANIUM ALLOYS

BACKGROUND OF THE INVENTION

The term “black titanium” is used to describe titanium-based alloys containing enough zirconium to form a black oxide layer when the alloy is heated in environments containing air, oxygen, or nitrogen. This oxide layer can increase the wear resistance or the aesthetic appeal of the titanium surface, and articles formed through these processes have found wide commercial acceptance due in part to the aesthetic appeal of the black or blue/black appearance of the oxide layer. Further, the oxide layer can improve the coefficient of friction and other properties of the titanium alloy surface.

BRIEF SUMMARY OF THE INVENTION

Although well received, it would be beneficial to create a material with improved characteristics.

In accordance with an embodiment of the invention, there is provided a titanium alloy comprising a Ti—Zr—X—Y formulation, wherein X is a Group V or Group VI metal and Y is an alpha stabilizer selected from aluminum, tin, silicon, oxygen, carbon, and nitrogen; and, wherein said titanium alloy is capable of being blackened in an air, oxygen, or oxygen containing environment; where the alloy is one of Ti-30Zr-2Mo-1.5Sn-0.18O, 30Zr-2Mo-1.5Sn-0.26O, Ti-30Zr-4Mo-1.5Sn-0.18O, 30Zr-4Mo-1.5Sn-0.26O, Ti-35Zr-10Nb-0.24O, Ti-35Zr-10Nb-1Al-0.15O, Ti-35Zr-10Nb-2.5Al-0.23O, Ti-30Zr-10Nb-0.2O, Ti-30Zr-10Nb-2Al-0.21O, Ti-30Zr-10Nb-1Al-0.3O, Ti-30Zr-10Nb-0.5Al-0.22O, Ti-30Zr-10Nb-1.5Sn-0.20O, Ti-30Zr-10Nb-3Sn-0.28O, Ti-30Zr-10Nb-6Sn-0.20O, Ti-30Zr-3Cr-1.5Sn-0.25O, and Ti-30Zr-6V-1.5Sn-0.25O.

In accordance with an embodiment of the invention, there is provided a titanium alloy comprising a Ti—Zr—X—Y formulation where the alloy is capable of being blackened in an air, oxygen, or oxygen containing environment to a lightness value of 48 or lower on the CIELAB color space; wherein, Y is an alpha stabilizer selected from aluminum, tin, silicon, oxygen, carbon, and nitrogen; and wherein, X is a beta stabilizer selected from niobium, chromium, molybdenum, and vanadium; wherein when X is niobium, said alloy has a transus temperature of approximately 640° C. or greater; and when X is chromium, molybdenum, or vanadium, said alloy has a transus temperature of approximately 610° C. or greater.

In accordance with an embodiment of the invention, there is provided a titanium alloy comprising a Ti—Zr—X—Y formulation, wherein X is a Group V or Group VI metal and Y is an alpha stabilizer selected from aluminum, tin, silicon, oxygen, carbon, and nitrogen; and, wherein said titanium alloy is capable of being blackened in an air, oxygen, or oxygen containing environment. In such an embodiment, the following features may be included in all possible combinations: the alloy may have a yield strength of at least 100 ksi; Y may be oxygen and an oxygen level of the alloy may be above 1300 ppm; the Zr content may be greater than 22 weight percent or 25 weight percent; the alloy may have a lightness index of 42 or less on the CIELAB scale or 46 or less on the CIELAB scale; and/or the aluminum equivalent may be less than approximately 10; the transus temperature may exceed 610° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with features, objects, and advantages thereof, will be or become apparent to one with skill in the art upon reference to the following detailed description when read with the accompanying drawings. It is intended that any additional organizations, methods of operation, features, objects or advantages ascertained by one skilled in the art be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.

With respect to the drawings, FIG. 1 depicts a table providing specifications for a specific titanium alloy described in the Specification;

FIG. 2 depicts a graph showing the effects of zirconium content on lightness in a particular context described in the Specification;

FIG. 3 depicts a graph showing the blackening response of an aluminum stabilized material described in the Specification;

FIG. 4 depicts a table of mechanical performances for materials described in the Specification;

FIG. 5 depicts a table of mechanical performances for materials described in the Specification;

FIG. 6 depicts a graph showing the effect of different passivating elements on a material described in the Specification; and,

FIG. 7 shows a table providing transus temperatures for certain materials described in the Specification.

DETAILED DESCRIPTION

In the following are described the preferred embodiments of the ALPHA STABILIZED BLACK TITANIUM ALLOYS in accordance with the present invention. In describing the embodiments illustrated in the drawings, specific terminology will be used for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. Where like elements have been depicted in multiple embodiments, identical reference numerals have been used in the multiple embodiments for ease of understanding.

Throughout this disclosure it is to be understood that the term “articles of manufacture” is to be construed broadly and is inclusive of all manufactured articles which have conventionally been, or could be, formed from titanium using known techniques or techniques to be developed. Such articles may be considered articles of ornamentation or components therefor, such as fashion accessories, jewelry, eye glasses, watches, etc. Such articles may also be consumer products or components therefor, including wearable technology, phone components, firearms, etc. The articles may also be general mechanical components of any manufactured device used as a consumer product, industrial components, or the like. They may also include medical applications such as implantable devices or surgical instruments.

As discussed earlier, “black titanium” typically describes titanium-based alloys containing enough zirconium to form a black oxide layer when the alloy is heated in environments containing air, oxygen, or nitrogen. Conventionally, such alloys contain titanium (“Ti”), zirconium (“Zr”), and a third alloying element (“X”) that serves to passivate the rapid oxidation of the surface allowing a controlled oxidation to take place. This is often referred to as a Ti—Zr—X formula.

It is to be understood that the term passivate means to cause the metal to be less prone to rapid oxidation by altering the surface or coating the surface with a thin inert layer.

The proposed mechanism fox passivation is the transport of oxygen into the bulk of the Ti—Zr matrix through the third alloying element concentrated at the grain boundaries. Conventional wisdom proposed that elements having high oxidizing agent transport properties should be suitable for the third alloying element.

Alloys of this nature have also been used to achieve materials with a lower modulus. Indeed, titanium-based materials with a modulus close to bone are desirable for implanted prosthetic joints to reduce “stress shielding” caused by mis-matched modulus,

Articles of manufacture, including implant applications, utilize the Ti—Zr—X formula for alloy composition. For general articles of manufacture, the X component acts as a passivator and in the implant application the X acts as a beta stabilizer and potentially a passivator, and if chosen correctly contributes to a lower modulus.

All of these alloys are classified as beta alloys. Beta alloys have a lower beta transus than pure titanium, and typically need to be heat treated to develop optimal strength. This is a drawback because heat treatment adds expense and an opportunity for distortion of parts, adding further expense if additional machining or forming is required.

While titanium alloys have many desirable characteristics such as biocompatibility, high strength-to-weight ratio, and corrosion resistance, they also have poor wear resistance when compared to conventional materials. A black oxide surface can greatly improve the wear resistance of titanium material and overcome this limitation.

Because of its desirable characteristics titanium has found wide acceptance in critical use markets such as medical, aerospace, and defense. In these markets its lack of wear resistance has been overcome by incorporating other materials on areas that need better wear performance. Its adoption by consumer markets has been limited not only by the design burden of working around poor wear performance but also its high cost and difficulty in machining.

This invention overcomes these challenges by using improved black titanium alloys that enhance resultant mechanical properties, are more suitable for processing via powder metallurgy methods than conventional black titanium materials, and which respond more favorably to the blackening process and other thermal treatments such as hydrogen chemical treatment. The ability to process high performance titanium alloys via powder metallurgy can lower the manufacturing cost and thus increase adoption by more price sensitive markets such as wearables, consumer electronics, sporting goods or eyewear.

Recent advances in titanium powder metallurgy have made the robust processing of complex high performance parts a reality. High performance titanium powder metallurgy techniques rely on developing a process that allows microstructural control and creates a final density that is effectively completely dense. The best route for achieving a combination of refined microstructure and high density is to sinter the article to closed porosity and then densify the article completely via a hot isostatic pressing (“HIP,” “HIPing,” or “HIPed”) operation. Not all applications require a HIPing step, but high-performance titanium alloys for demanding applications are almost always preferred to be completely dense. When a titanium alloy is less than completely dense, it can behave in a more brittle fashion and may also have much lower fatigue performance.

The alloys described are readily fabricated via powder metallurgy using any of the conventional forming techniques such as compaction, injection molding, extrusion or three-dimensional printing. Pre-alloy, master alloy, or blended elemental powder routes are all suitable raw material approaches for these alloys. After forming the powder into the desired shape, the article is sintered in an inert or vacuum atmosphere. After sintering the article may be hot isostatically pressed if higher density is desired.

In the conventional black alloy having a Ti—Zr—X structure, X is typically chosen from a Group V or Group VI metal such, as vanadium, niobium, tantalum, chromium, molybdenum or tungsten. Typically, 5 atomic percentage or more of a passivating element is added. All of these materials are strong beta phase stabilizers and because of this, all of the conventional black titanium alloys are classified as metastable beta alloys.

By also manipulating the black titanium alloy to stabilize and increase the alpha phase, the mechanical properties of the as-sintered or HIPed product can be greatly improved by both an increased presence of strengthened alpha phase and strengthened beta phase in the final product. Further, by stabilizing and increasing the alpha phase the transus temperature is elevated. The elevated transus temperature can aid in thermo-chemical heat treatments as well as protect the alloy from degradation of mechanical properties due to thermal blackening cycles. In the black titanium system, the alpha phase is stabilized by adding oxygen, tin, aluminum, or other alpha stabilizers.

There are many considerations to the level of alpha stabilizer or passivator used in an improved black titanium alloy composition. In addition to stabilizing the alpha phase via oxygen, tin, or aluminum additions, it may be desirable to adjust the zirconium content slightly to maintain the same color or blackening performance as well as allowing for adjustments to the amounts of other alpha stabilizers to optimize strength, wear, and cosmetic properties. With respect to zirconium, while it is an important consideration in determining the aluminum equivalent, it depresses the transus and from this perspective is considered a beta stabilizer. It is considered in the aluminum equivalent calculation because it concentrates the aluminum in the remaining titanium matrix. Aluminum, tin, oxygen, carbon, and nitrogen have an alpha stabilizing role and their effectiveness relative to aluminum is described in the aluminum equivalency equation:

Aluminum equivalent, wt %=Al+⅓Sn+⅙Zr+10(O+C+2N)

It is generally accepted that care should be taken not to exceed an aluminum equivalent approximately equal to 10. At high aluminum equivalent levels, detrimental precipitates, typically Ti₃X can form. Because of this there are limits to how much aluminum or other alpha stabilizers may be effectively added. If it is desired to stabilize more alpha phase it may be necessary to lower the zirconium content to enable a rise in the alpha stabilizer content.

Many conventional black titanium articles are fabricated from commercially available Ti-35Zr-10Nb alloy. This alloy is marketed as Tiadyne™ 3510 by Alleghany Technologies, Inc. of Pittsburgh, Pa. It is the only commercial titanium alloy designed for blackening known to the inventors at this time. The specification for this alloy shown in the table of FIG. 1.

Ti-35Zr-10Nb is a beta alloy. This is reflected in its low transus temperature of 635° C. Conventionally formed Ti-35Zr-10Nb alloy has a yield strength of 65 ksi when quenched from the beta annealed state. Working or heat treatment can be used to increase the yield strength to between 140-150 ksi. However, upon subjecting the material to a typical blackening treatment of heating in air to 550° C. for two hours, the yield strengths are degraded to 120 ksi or lower. While the oxygen in the specification could technically be considered an alpha stabilizer, the upper limit of 1300 ppm prevents it from substantially increasing the strength of the material.

Powder forming routes for this alloy do not yield good results. Material subjected to a conventional HIP cycle exhibits a yield strength of 98 ksi, a very low value for titanium alloys. Commercially, titanium alloys usually have a yield strength of over 100 ksi to find practical application.

This invention improves the properties of black titanium alloys to not only exceed the performance of the known alloys formed by powder methods but to provide better performance than the known alloys when processed in their optimal methods such as heat treating or working. This can be achieved by many different changes to the alloy chemistry.

Improved alloys will may have an increased transus temperature. The elevated transus is indicative of greater amount of alpha in both the as-sintered product and the densified product. This increased alpha content provides for better mechanical properties without the need for mechanical working or further heat treatment. Additionally, the alloy's performance is not substantially degraded by the blackening process, and in some cases may be improved.

Because most alpha stabilizers will increase the transus temperature when added to the typical Ti—Zr—X formulation, there are many permutations of this improvement with respect to the actual formulation used. As a general rule, combinations of aluminum, tin, oxygen, carbon, or nitrogen as alpha stabilizers can be used to increase the transus, while reductions in beta stabilizing amounts or potency will also result in an increase in the beta transus.

The Ti-35Zr-10Nb alloy has an oxygen limit of 1300 ppm. While this allows for robust processing of the material by conventional methods, increasing the oxygen content in excess of this limit vastly improves the performance high zirconium alloys when processed using powder metallurgy routes.

Oxygen is known to have an embrittling effect on titanium alloys and this holds true for alloys of the Ti—Zr—X structure as well. However, a broad window exists for improving the powder metallurgy processability of these alloys that is above the current accepted oxygen limit and below the level that would impact functionality of the material. It has been discovered that the limit could be increased by two to four-fold, depending on the level of other alpha stabilizers and the zirconium level. Indeed, oxygen content can be pushed to 1500 ppm, 2000 ppm, 3000 ppm, 4000 ppm, or any range between 1300 ppm and 4000 ppm with suitable results. Within the range of 1300 ppm to 4000 ppm, it has been found that 1500 ppm to 2000 ppm is the most practical for commercial alloys.

While some helpful effects of oxygen at the surface such as increased wear resistance or fatigue performance are understood, adding oxygen to the bulk via oxidation is very challenging and time consuming, particularly because these alloys are designed to form coherent coatings at the surface. Further, introducing oxygen via diffusion of the oxygen from the surface will introduce a strong gradient or oxygen content across the section of the part.

The Ti—Zr—X—Y formulation where Y represents an alpha stabilizer selected from the group including, but not limited to, aluminum, tin, silicon, oxygen, carbon, and nitrogen may be further improved by lowering the level of Zirconium to allow for the addition of other stabilizers to increase the transus strength and alpha content. Stabilizers may also be combined.

An alloy with the composition Ti-35Zr-10Nb-1Al-0.2O exhibited an ultimate tensile strength of 166 ksi and a yield strength of 144 ksi with an elongation of 8 percent. An alloy with the composition Ti-30Zr-10Nb-1Al-0.3O exhibited an ultimate tensile strength of 170 ksi, a yield strength of 150 ksi and an elongation of 8 percent. An alloy with the composition Ti-30Zr-10Nb-1.5Sn-0.2O exhibited an ultimate tensile strength of 165 ksi, a yield strength of 143 ksi and an elongation of 8 percent. An alloy with the composition Ti-30Zr-2Mo-1.5Sn-0.26O exhibited an ultimate tensile strength of 155 ksi and a yield strength of 137 ksi with an elongation of 12 percent. An alloy with the composition Ti-30Zr-4Mo-1.5Sn-0.25O exhibited an ultimate tensile strength of 166 ksi, a yield strength of 151 ksi and an elongation of 10 percent. All of these are useful alloys having greater strength than the conventional Ti-35Zr-10Nb alloy. The mechanical performances of these materials are detailed in the table of FIG. 4.

Zirconium is considered the agent that creates the black color upon blackening, and it is understood that increasing the zirconium content will increase the degree of blackening. It has been discovered however, that similar levels of blackening can be achieved with less zirconium when aluminum is present. Between zirconium levels of 20 and 33 percent, it was found the addition of aluminum increased the blackening performance such that the aluminum containing materials blackened equivalently or better than the no aluminum containing counterparts. This is important because in addition to strengthening the alloy, the aluminum can also serve to decrease the density and reduce the raw material cost. The graph of FIG. 2 depicts decreasing lightness with increasing zirconium content. Lightness is measured using the Standard CIELAB color space method and decreasing lightness is interpreted as increasing blackness. At all points in this range, the aluminum decreases the lightness. Alternatively stated, aluminum can increase the effectiveness of the zirconium as a blackening agent.

During development it was determined that items having a lightness index of 46 or lower were acceptable from an aesthetic viewpoint. While this is obviously a subjective number, and others may find an acceptable lightness index limit less than 46, for example 42, it is helpful to guide the evaluation of different formulations. Likewise, others may find a lightness index of 48 to be acceptable. The effects of zirconium content on lightness in this context is shown in the graph of FIG. 2.

In addition to the alloys having the formulation Ti—Zr—X, similar blackening behavior can be achieved with binary alloys of Ti—Zr. These alloys are generally more prone to ignition during blackening and require more complex processing conditions such as tightly controlled atmospheres or use of molten salt baths to control the oxidation. By adopting the Ti—Zr—Y formulation, similar gains in mechanical performance and powder metal processability can be achieved with the elevation of the transus via additions of alpha stabilizers such as aluminum, tin, oxygen, nitrogen and carbon.

The choice of passivating elements can be made based on several factors, among them perceived biocompatibility, beta solution strengthening effects, economy, or different blackening behavior. Preferred passivating elements include niobium, molybdenum, chromium, or vanadium. Other elements may work as passivating elements, as an example, tungsten or tantalum might be used, but the high cost and high density of these material would preclude them from being preferred passivating elements. Testing has indicated that chromium, molybdenum, and vanadium, are all more effective passivating agents than niobium. At an atomic percentage of 6.5 percent chromium, molybdenum, and vanadium, all samples yielded lower lightness index values at the same zirconium content.

Although niobium has been used predominantly in the Ti—Zr—X alloys, this may be because many of the prior examinations were from the perspective of suitability for medical implants. Molybdenum, chromium, or vanadium are all suitable choices for passivating agents and can lower the cost of the alloy without effecting the hypoallergenic nature of titanium alloys. Further, the passivating elements can be combined to manipulate the blackening behavior.

The use of molybdenum, although it lowers the beta transus temperature, is especially advantageous in these alloys as it can function as a passivator and a strong beta solution strengthened. This advantage is further developed as molybdenum containing alloys show increased blackening behavior over alloys with the same atomic percentage of niobium. Since alloys containing molybdenum can achieve the same degree of blackness using less zirconium, more room for alpha stabilization via alpha stabilizing agents is enabled, and at the same time providing more beta solution strengthening due to the higher potency of molybdenum over elements such as niobium and vanadium.

A typical Ti-35Zr-10Nb alloy undergoes a heat treatment cycle in which the quenched beta phase is aged to enhance the strength of the material. The material can then be subject to many different and possibly repeated blackening processes, potentially degrading the designed mechanical response of the alloy. In an alloy densified by powder processing methods, the full microstructure has been developed upon sintering or HIPing and further heating cycles to blacken the material will only serve to coarsen grains, which will not have a large effect on mechanical properties if properly controlled, especially if the alloy has a higher beta transus temperature due to the addition of alpha stabilizers.

The increased alpha stabilization of these alloys also allows them to better respond, to thermochemical treatments such as hydrogen-chemical-treatments and others. This advantage is developed as the beta transus temperature of the alloy is increased due to the presence of the alpha stabilizers. This allows better control over the equilibrium levels of hydrogen during the hydrogenation step of the thermochemical treatment, which must be kept below the point at which titanium hydride precipitates form, cracking the part. The elevated levels of the beta transus are also beneficial during the de-hydrogenation step of a hydrogen-chemical-treatment, where the highest temperature that is below the beta transus is desired to aid in hydrogen removal. This step of hydrogen removal is difficult at temperatures equal to or below that of typical beta stabilized alloys containing only zirconium and passivating elements. Due to the nature of the thermal profiles used in powder processed materials, the ability to employ these types of strengthening treatments furthers the degree of microstructural control possible in the net shape manufacture of articles and also increases the mechanical properties, including dynamic fatigue performance.

While both tin and aluminum can be used as an alpha stabilizer there are nuances that can dictate which might be a better choice in certain conditions. An important consideration in these materials is blackening response, or how readily the material becomes black during oxidation, and how dark the black is. At lower zirconium levels materials using aluminum as an alpha stabilizer exhibit better blackening response than materials using tin as an alpha stabilizer, however this trend is reversed at higher zirconium levels. In both cases the alpha stabilized material preforms better or equivalent to the beta stabilized material.

As shown in the graph of FIG. 3, aluminum stabilized material exhibits better blackening response from about 20 weight percent zirconium up to about 29 weight percent, including 22 weight percent and 25 weight percent, at which point materials stabilized with tin begin to exhibit better blackening behavior. That said, 35 weight percent zirconium has been found to be a practical upper limit for commercial titanium. All curves are for systems using 10 weight percent. Niobium as a passivator.

Improved blackening performance can also be interpreted as improved wear performance. While the thicknesses of the black oxide layers are difficult to measure, their wear resistance can be gauged by the nano-indentation measurements, As an example, a Ti-30Zr-10Nb alloy demonstrated a nano-indentation hardness of 1800 on the Vickers scale while a Ti-1.5Sn-30Zr-6Mo alloy demonstrated a hardness of 2000 on the Vickers scale. Other Ti—Zr—X and Ti—Zr—X—Y alloys may demonstrate similar hardness values.

As previously discussed, molybdenum is a potent beta stabilizer and also provides for better blackening performance than material stabilized with niobium. It can be used at lower levels and is substantially less expensive than niobium. In a system with using 1.5 weight percent tin and 0.3 weight percent oxygen as the primary alpha stabilizers, molybdenum works well over a range of usage levels and provides flexibility in the materials performance. The mechanical performances of these materials are detailed in the table of FIG. 4.

Additions of molybdenum as a beta-stabilizer and passivator steadily increase the strength of the alloy. As shown in the table of FIG. 4, the range of two to four percent molybdenum provides strengths at least as good as the Ti-6Al-4V alloy. As is to be expected, elongation decreases slightly as strength increases. Unlike the Tiadyne™ 3510 Annealed and Blackened alloy, these alloys demonstrated similar yield strengths after blackening. These are tabulated in the table of FIG. 5.

Molybdenum also performs well as a passivating agent and when combined with tin as an alpha stabilizer outperforms niobium on a weight basis with respect to nano-indentation values, which are used as an indication of wear resistance. As an example, a Ti-30Zr-10Nb alloy develops a Vickers hardness of 1800 after blackening, but a Ti-1.5Sn-30Zr-6Mo alloy develops a Vickers hardness value of 2000.

To evaluate the individual effects of passivator elements on blackening behaviors, several formulations using different passivating elements were tested. The result of the effect of the different passivating element are shown in the graph of FIG. 6. All of the element tested were added at 10 atomic weight percent.

Also considered were titanium alloys containing zirconium and X, and an alpha stabilizer selected from aluminum, tin, silicon, oxygen, carbon, and nitrogen, capable of being blackened in an air, oxygen, or nitrogen containing environment to a lightness value of 48 or lower on the CIELAB color space and having a beta transus temperature of 610° C. or greater, where X is shown in the table of FIG. 7.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

ALPHA STABILIZED BLACK TITANIUM ALLOYS

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