CHROMIUM COATED CONTINOUS FIBER REINFORCED TIANTINUM METAL MATRIX COMPOSITE AND THE METHOD OF MAKING THEREOF

Abstract

The present disclosure relates to a titanium metal matrix composite with chromium coated fiber reinforcement and method of manufacturing thereof. The formulations and methods disclosed herein enable the manufacturing process to take place outside of specialized vacuum furnaces.

Claims

1. A metal matrix composite comprising: a fiber reinforcement comprising a fiber reinforcement coated with chromium; and a titanium alloy matrix comprising: from about 70 wt. % to about 85 wt. % Titanium (Ti), from about 0.05 wt. % to about 0.25 wt. % oxygen (O); from about 0.1 wt. % to about 0.4 wt. % carbon (C); not greater than 0.03 wt. %. nitrogen (N); a beta stabilizing element; and One or more of: no greater than 5 wt. % aluminum (Al); from about 5 to about 19 wt. % tin (Sn); no greater than 5 wt. % antimony (Sb); and no greater than 2 wt. % bismuth (Bi); Wherein the beta stabilizing element is one of: from about 1 to about 3 wt. total % of an element selected from the group consisting of molybdenum (Mo), Tantalum (Ta), vanadium (V), chromium (Cr), iron (Fe), manganese (Mn), or any combination thereof; or from about 1 to about 6 wt. % niobium (Nb).

2. The metal matrix composite of claim 1 wherein the sum of C, O, and N, is no greater than 0.45 wt. %; and the sum of Sn, Sb and Bi is no greater than 20 wt. %.

3. The metal matrix composite in claim 1 further comprising: a protective coating comprising either a glass or a flux coating surrounding the titanium alloy matrix, the fiber reinforcement, and the low melting elements.

4. The metal matrix composite of claim 1, wherein the fiber reinforcement consists of carbon fibers, carbides, boride, oxides, nitrides, or any combination thereof.

5. The metal matrix composite of claim 1, wherein the fiber reinforcement further comprises a titanium wire.

6. The metal matrix composite of claim 1, wherein the fiber reinforcement chromium coating is less than 0.002 thick.

7. The metal matrix composite of claim 1, wherein the fiber reinforcement is further coated with is either nickel (Ni) or copper (Cu); wherein the nickel or copper is between the chromium and the fiber reinforcement.

8. The method of claim 7, wherein the fiber reinforcement coating of nickel or copper is from about 0.0001 to about 0.0004 thick.

9. A method of preparing the metal matrix comprising: coating a fiber reinforcement with chromium; layering a titanium alloy on at least one side of the coated carbon fiber; and heating the layers to a temperature of from about 1600 F. to about 1800 F. such that the layers fully diffusion bond and consolidate to form a titanium alloy matrix.

10. A method of preparing the metal matrix composite comprising: coating a fiber reinforcement with chromium; layering a low melting element layer and titanium alloy on at least one side of the coated fiber reinforcement such that the low melting element is layered between the titanium alloy and the fiber reinforcement; heating the layers to a temperature ranging from about 400 F. to about 800 F. to melt the low melting elements; applying pressure to the layers such that the melted low melting elements fill the gaps between the titanium alloy and the coated fiber reinforcement; cooling the layers such that the low melting element solidifies, and the titanium alloy and the coated fiber reinforcement are soldered together forming a soldered matrix; and heating the soldered matrix to a temperature ranging from about 1600 F. to about 1800 F. such that the layers of the soldered matrix fully diffusion bond and consolidate to form a titanium alloy matrix.

11. The method of claim 9, wherein the method takes place in an electric furnace.

12. The method of claim 9, wherein the fiber reinforcement chromium coating is applied by either electroplating, by chemical vapor deposition, physical vapor deposition, or flash plating.

13. The method of claim 10, wherein the beta stabilizing element in the form of a thin mesh is layered between the LME layer and the Ti-alloy before the layers are heated.

14. The method of claim 10, wherein the soldered matrix is covered with a protective coating before heating.

15. The method of claim 9, wherein the titanium alloy matrix comprising: from about 70 wt. % to about 85 wt. % Titanium (Ti), from about 0.05 wt. % to about 0.25 wt. % oxygen (O); from about 0.1 wt. % to about 0.4 wt. % carbon (C); not greater than 0.03 wt. %. nitrogen (N); a beta stabilizing element; and One or more of: no greater than 5 wt. % aluminum (Al); from about 5 to about 19 wt. % tin (Sn); no greater than 5 wt. % antimony (Sb); and no greater than 2 wt. % bismuth (Bi); Wherein the beta stabilizing element is one of: from about 1 to about 3 wt. total % of an element selected from the group consisting of molybdenum (Mo), Tantalum (Ta), vanadium (V), chromium (Cr), iron (Fe), manganese (Mn), or any combination thereof; or from about 1 to about 6 wt. % niobium (Nb); the sum of C, O, and N, is no greater than 0.45 wt. %; and the sum of Sn, Sb and Bi is no greater than 20 wt. %.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0008] FIG. 1 is a Binary Phase Diagram of titanium and tin displaying the tin solubility limits within an alpha-titanium phase.

[0009] FIG. 2 is a Binary Phase Diagram of titanium and bismuth displaying the bismuth solubility limits within an alpha-titanium phase.

[0010] FIG. 3 is a Binary Phase Diagram of titanium and antimony displaying the antimony solubility limits within an alpha-titanium phase.

[0011] FIG. 4 is a graph displaying the correlation between aluminum and LME with respect to the resulting Ti-MMC desired ranges of elongation and yield strength.

[0012] FIG. 5 depicts one exemplary method for forming a titanium metal matrix composite.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present disclosure relates to a titanium metal matrix composite (Ti-MMC) having a fiber reinforcement coated with chromium such that the titanium alloy does not contact the fiber reinforcement. The present disclosure is also directed to a method of manufacturing Ti-MMC such that the manufacturing does not require a specialized vacuum furnace. One aspect of the method involves layering low melting elements (LMEs) with the titanium alloy and the fiber reinforcement such that the LMEs melt at a temperature lower than used conventionally and temporarily solder the layers together.

Composite

[0014] A metal matrix composite includes a metal matrix and a fiber reinforcement. The metal matrix is a continuous metallic phase which contacts the fiber reinforcement. The fiber reinforcement is fabric that provides strength to the composite. Traditionally, limited fibers (i.e., silicon carbon) have been used in Ti-MMC because titanium and certain fibers (i.e., carbon fibers) have a reaction that causes degradation of the composite.

[0015] The fiber reinforcement may be either continuous or discontinuous. The fiber reinforcement in the present composite may include carbon fiber, carbides, borides, oxides, nitrides, and any combination thereof. In one embodiment, the fiber reinforcement is a continuous carbon fiber. Examples of commercially available fiber reinforcements useful in the practice of the present disclosure include the carbon fiber reinforcement sold under the tradename Nishield 6Al-3K22 available from Conductive Composites with specifications of 240 grams per square meter cloth with a woven fabric thickness of 0.0126.

[0016] In a particularly suitable aspect of the present disclosure, the fiber reinforcement is coated with chromium. Chromium and titanium form a soft eutectic at 2575 F. The manufacturing process (described in full detail below) reaches a maximum diffusion bonding temperature from about 1600 F. to about 1800 F. Consequently, the chromium coating acts as a shield by preventing the titanium from contacting the reinforcement fiber. Chromium is highly oxidation resistant and protects the fiber reinforcement from oxidation.

[0017] The fiber reinforcement may also be coated with nickel, electroless nickel, copper, electroless copper, or any combination thereof, alone or in combination with the chromium as described above. Nickel and copper slow down the reaction between the fiber reinforcement fabric and the titanium. Nickel and titanium have a eutectic reaction at 1730 F. Copper and titanium have a eutectic reaction at 1625 F. Nickel and copper can melt at the manufacturing process diffusion bonding temperatures, allowing the titanium to react with the fiber reinforcement. Therefore, it is recommended when the reinforcement fiber is coated with nickel or copper it is further coated with chromium such that the chromium is on top of the nickel or copper.

[0018] However, chromium also reacts with the carbon to form carbides. On the other hand, copper and nickel is compatible with carbon and do not readily form carbides. Therefore, copper plating is often used to prevent carbon diffusion into steel substrates during selective carburizing of steels. Copper and nickel can isolate and protect chromium from reacting with the fiber reinforcement. In some embodiments where the reinforcement is carbon fiber, chromium can form chromium carbide during diffusion, further described below, which can degrade the mechanical properties of the reinforcement.

[0019] The coating of chromium is less than 0.002 thick on the fiber reinforcement. The coating of nickel or copper are from about 0.0001 to about 0.0004 thick.

[0020] In one embodiment, a carbon fiber reinforcement is coated with chromium. In another embodiment a carbon fiber reinforcement is coated with nickel which is further coated by chromium.

[0021] When a high degree of homogeneity is desired in the resultant Ti-MMC, a metallic wire may be introduced to the composite. In one embodiment, the metallic wire is layered between the fiber reinforcement and the LME layer so that the LME layer, when the layers are heated as described in detail below, passes through the metallic wire openings and solders the fiber reinforcement to the Ti-alloy. In another embodiment, the metallic wire is comingled with the fiber reinforcement such that the metallic wire is woven with the fiber reinforcement. In suitable embodiments, the metallic wire is titanium or LME. Where a reinforcement included a metallic wire it is suitable for the reinforcement to have twisted carbon fiber filaments as the inter-cohesion of fibers due to twisting allows more resistance to separation of individual filaments. A suitable commercially available reinforcement comingled with metallic wire is the nickeled coated carbon fiber with titanium alloy wire sold under the tradenames 1AC1-12, 1BC1-12, 1CC1-12, and 1DC1-12 available from Conductive Composites. Other suitable reinforcement comingled with metallic wire are sold under the tradenames T300, T1000, T910S available from Toray.

[0022] In the present disclosure, the matrix for use with the fiber reinforcement is a titanium alloy matrix. The titanium alloy matrix for suitable use generally includes a titanium alloy, more suitably the titanium alloy matrix includes an alpha-titanium alloy.

[0023] In suitable embodiments, the titanium alloy matrix can include aluminum and/or low melting elements (LME).

[0024] Exemplary LMEs include tin, antimony, bismuth, gallium, and germanium. The inclusion of LME renders the alloy formulation more tolerant of interstitial elements, especially carbon. Thus, higher LME levels in the formulation allow for more interstitial element strength as compared to standard aluminum rich near alpha alloys.

[0025] In one embodiment, the composite includes a titanium alloy matrix. Suitably, the titanium alloy matrix includes one or more of the following: from about 70 to about 85 wt. % of titanium; from about 5 to about 19 wt. % tin (Sn), suitably, from about 5 to about 15 wt. %; no greater than 5 wt. % aluminum (Al), suitably, from about 2 to about 4.5 wt. %, in in particular suitable embodiments, no greater than 4 wt. %; no greater than 5 wt. % antimony (Sb); and no greater than 2 wt. % bismuth (Bi). Suitably, when used in the formulation, the sum of the LME is no greater than 20 wt. %; or from about 8 wt. % to about 20 wt. %. It is recommended to stay within the alpha-titanium phase so that the resulting Ti-matrix alloy has a yield strength of 120 to 160 ksi and ductility of over 10% (elongation). The solubility limits of Sn, Bi, and Sb within the alpha-titanium phase are set forth in titanium binary phase diagrams in FIGS. 1, 2, and 3, respectively. The maximum percentage of the respective element is represented by a vertical bold line within the diagram. The resulting Ti-MMC has a yield strength of 200 ksi to 300 ksi with over 1.5% elongation.

[0026] The percentage of components within the resulting titanium alloy matrix affects the performance of the composite; measured by parameters such as yield strength, ductility, elongation, UTS, etc. The resulting Ti-MMC has a desired yield strength range from about 120 and about 160 and a desired elongation range from about 10 to about 20. Turning to FIG. 4, a graph displaying the correlation between aluminum (y-axis) and LME (x-axis), wherein the carbon is approximately 8.2 wt. % and the oxygen is approximately 18.7 wt. %, with respect to the resulting Ti-MMC desired ranges of elongation and yield strength. As illustrated in FIG. 4, the aluminum weight percentage is inversely correlated with the weight percentage of tin (Sn) with respect to the desired ranges of elongation between 10% and 20% and yield strength (YS) between 120 ksi and 160 ksi. The other identified LME (e.g., bismuth and antimony) herein have similar correlations with the aluminum weight percentage as illustrated in FIG. 4.

[0027] In one particularly suitable embodiment, the resulting Ti-MMC titanium alloy matrix having higher tin and lower aluminum levels; that is, having levels of tin and aluminum as described herein, have improved ductility where the combined total amount of carbon, oxygen, and nitrogen remains under 0.045 wt. % while still resulting in high yield strength.

[0028] In some embodiments, the titanium alloy matrix can further include: from about 0.05 wt. % to about 0.25 wt. % oxygen (O); from about 0.1 wt. % to about 0.4 wt. % carbon (C); and no greater than 0.03 wt. %. nitrogen (N). Typically, the sum of C, O, and N in the matrix is no greater than 0.45 wt. % so as to prevent the reduction in ductility and toughness in the resulting metal parts. Carbon levels in the matrix can result in controlled levels of titanium carbides and increase strength.

[0029] In some embodiments, the titanium alloy matrix of the present disclosure further includes a beta stabilizing element for increasing strength. Suitable beta stabilizing elements include molybdenum (Mo), Tantalum (Ta), niobium (Nb), vanadium (V), chromium (Cr), iron (Fe), manganese (Mn), or any combination thereof. Some beta stabilizing elements such as niobium, Ta, and molybdenum greatly improve oxidation resistance and hot corrosion resistance thereby alloy the alloy to be used at higher application temperatures. In one embodiment, the formulation can include from about 1 to about 3 wt. % of any one or more of these beta stabilizing elements. In another embodiment, the beta stabilizing element is niobium (Nb) and the formulation can include from about 1 to about 6 wt. % of the beta stabilizing element.

[0030] Examples of commercially available stabilizing elements useful in the practice of the present disclosure include the stabilizing elements sold under the tradename Various available from McMaster-Carr.

[0031] The beta stabilizing elements can be in the form of a mesh. The mesh allows titanium to penetrate through openings and bond to the adjacent foils. Such mesh has wider opening size than their diameter to allow full encapsulation and bonding between top and bottom foils to the mesh and to each other.

Method of Manufacturing Ti-MMC

[0032] The present disclosure is further directed to methods of manufacturing the resulting Ti-MMC. In one embodiment, the method includes coating a fiber reinforcement with chromium. One embodiment of the method is depicted in FIG. 5.

[0033] In another embodiment, the method includes: coating the fiber reinforcement with chromium; layering the coated fiber reinforcement, titanium alloy, and low melting elements such that the process may take place in an electric furnace; and producing a Ti-MMC, which is considerably more durable and resistant to degradation compared to the currently marketed Ti-MMC.

[0034] One aspect of the method is coating the fiber reinforcement with chromium 505, and possibly one or more of nickel, electroless nickel, copper, and electroless cooper. The fiber reinforcement may be coated by the process of electroplating, chemical vapor deposition, physical vapor deposition, false metal plating, or any other coating process readily determined by those skilled in the art. In one embodiment, a continuous carbon fiber is coated with chromium by electroplating. In another embodiment, a continuous carbon fiber is coated with electroless nickel by flash copper plating.

[0035] Another aspect of the method is layering the coated reinforcement fiber, low melting elements (LME), and a titanium alloy (Ti-alloy), 510 and 515. The LME and the Ti-alloy should be in proper weight percentage such that the resulting Ti-MMC titanium metal matrix comprises the claimed concentrations after diffusion.

[0036] The LME layer can be in the form of foils or sheets. The LME layer may consist of antimony, bismuth, tin, gallium, germanium, or any combination thereof as described above.

[0037] The Ti-alloy can be in the form of a sheet or foil. The Ti-alloy comprises: no greater than 5 wt. % Aluminum; from about 0.05 wt. % to about 0.25 wt. % oxygen (O); from about 0.1 wt. % to about 0.4 wt. % carbon (C); and no greater than 0.03 wt. %, nitrogen (N). Typically, the sum of C, O, and N in the powder formulation is no greater than 0.45 wt. %.

[0038] In a suitable embodiment the Ti-alloy is a Beta21S titanium alloy. In another suitable embodiment, the Ti-alloy is pure titanium such as Grade 1, Grade 2, or Grade 4.

[0039] Carbon powder can be painted or sprayed over the Ti-alloy with organic binders and pre-cured to help control carbon levels in the Ti-MMC titanium metal matrix. Carbon levels which exceed 0.1 wt. % results in formation of titanium carbide within the titanium alloy matrix that can further strengthen the matrix and reduce the Coefficient of Thermal Expansion (CTE) differential between the matrix and the fiber reinforcement. Additionally, carbides can greatly reduce grain growth and improve creep resistance.

[0040] Traditionally carbides reduce ductility for conventional titanium alloys. However, in the present disclosure the negative impact of carbides on ductility by reducing aluminum and excluding vanadium and zirconium.

[0041] The present method is directed to layering the LME foil and the Ti-alloy on at least one side of the coated fiber reinforcement such that the LME foil is layered between the Ti-alloy and the coated fiber reinforcement.

[0042] If present, the beta stabilizing element as described more fully above is placed between the LME layer and the Ti-alloy to facilitate the titanium to penetrate through openings and bond to it and the LME. Beta stabilizing elements may be in the form of thin mesh or sieve having an opening size wider than their diameter to allow full encapsulation and bonding between the layers.

[0043] When a high degree of homogeneity is desired in the resultant Ti-MMC, a metallic wire may be introduced to the composite. In one embodiment, the metallic wire is layered between the fiber reinforcement and the LME layer so that the LME layer, when the layers are heated as described in detail below, passes through the metallic wire openings and solders the fiber reinforcement to the Ti-alloy. In another embodiment, the metallic wire is comingled with the fiber reinforcement. In suitable embodiments, the metallic wire comprises titanium or LME.

[0044] One aspect of the method is cleaning the fiber reinforcement and the layers prior to laying them together. The fiber reinforcement may be cleaned with acetone, MPK, or any other suitable solvent cleaner. The metallic foils can be mechanically cleaned, such as using a stainless steel wool. and subsequently cleaned with a solvent cleaner or acid cleaner. A suitable acid cleaner is a dilute nitric acid or hydrofluoric acid at room temperature.

[0045] The layers may be placed in a tooling that can be readily determined by those skilled in the art. The layers are heated to a temperature from about 400 F. to about 800 F. such that the LME layer melts and temporarily solder the Ti-alloy and the fiber reinforcement, 520. The layers may be heated in a method that can be readily determined by those skilled in the art. In one embodiment, the layers are heated in an electric furnace.

[0046] Pressure is applied to the temporarily solder layers such that the LME fill in the gaps between the Ti-alloy and the coated fiber reinforcement, 525. Pressure is applied by using inert gas in the furnace or mechanical pressure in the tooling.

[0047] The layers are cooled at room temperature such that the LME solidifies and the Ti-alloy and fiber reinforcement are solder together forming a soldered matrix, 530.

[0048] The soldered matrix may be removed from the tool and covered with a protective coating to seal the surface, 535. The protective coating prevents diffusion of excessive interstitial elements into the titanium alloy matrix eliminating the need for the process to take place in a vacuum or inert atmosphere chamber. The protective coating may be an anti-oxidation resistant material such as a forging glass or a flux.

[0049] The soldered matrix, with or without the protective coating, can be placed in a tooling and heated in an electric furnace such that the metals in the layers are consolidated in to a homogenous solution to form the Ti-MMC. The temperature of the furnace is between 1600 F. and 1800 F., 540. The pressure in the furnace is applied through inert gas or mechanical pressure inside the tooling.

[0050] The Ti-MMC can be heat treated, cleaned, inspected, drilled, trimmed, certified, and made ready for assembly into a structure. The cleaning of the Ti-MMC may involve removing the protective coating, etched, and pickled. The inspection of the Ti-MMC may include the chemical composition, metallurgical, and NDT.

[0051] Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

EXAMPLES

[0052] The following non-limiting examples are provided to further illustrate the present invention.

[0053] Example 1.In this example, various titanium alloys are used. The Ultimate Tensile Strength was determined using ASTM-E8. The formulas are as follows:

TABLE-US-00001 Aluminum Carbon Oxygen Total LME Element Element Element Ultimate Components Component Component Component Tensile in Alloy in Alloy in Alloy in Alloy Strength for Formulation Formulation Formulation Formulation the Alloy (wt. %) (wt. %) (wt. %) (wt. %) Formulation 5 1 .02 0.2 108 5 2 0 0.4 125 9 3 0.3 0.2 156 9 1 0.2 0.2 123 3 4 0.2 0.2 127 13 1 0.26 0.2 144 5 2 0.2 0.2 125 2 4 0.2 0.2 124 4 4 0 0.4 147 10 2 0.2 0.05 127 5 3 0 0.4 132 5 5 0 0.2 125 3 4 0.1 0.2 123 1 4 0.2 0.2 120 16 2 0 0.2 145 13 2 0.2 0.2 153 9 3 0.2 0.2 134 16 1 0 0.2 131 5 3 0 0.3 139 9 3 0.2 0.2 142 10 2.5 0.2 0.2 146 13 1 0 0.4 140 10 3 0.2 0.2 146 19 1 0 0.2 147 1.6 5 0.25 0.05 149 2.5 5 0.2 0.2 135 19 2 0 0.2 151 2.5 5 0.2 0.2 134 4 5 0 0.4 137

[0054] Example 2In this example, after analyzing the data listed in Example 1, it was determined there was a strong correlation among the element concentrations and their interactions. This led to the development of the combination of the elements for a titanium metal matrix to increase the strength and ductility of a titanium metal matrix composite. The combination of titanium metal matrix elements are as follows: [0055] from about 70 wt. % to about 85 wt. % Titanium (Ti), [0056] from about 0.05 wt. % to about 0.25 wt. % oxygen (O); [0057] from about 0.1 wt. % to about 0.4 wt. % carbon (C); [0058] not greater than 0.03 wt. %. nitrogen (N); and [0059] One or more of: [0060] no greater than 5 wt. % aluminum (Al); [0061] from about 5 to about 19 wt. % tin (Sn); [0062] no greater than 5 wt. % antimony (Sb); [0063] no greater than 2 wt. % bismuth (Bi); and [0064] Either: [0065] 1 to about 3 wt. total % of the beta stabilizing element, wherein the beta stabilizing element is selected from the group consisting of molybdenum (Mo), Tantalum (Ta), vanadium (V), chromium (Cr), iron (Fe), manganese (Mn), or any combination thereof; and [0066] 1 to about 6 wt. % of the beta stabilizing element, wherein the beta stabilizing element comprises niobium (Nb); and [0067] Wherein [0068] the sum of C, O, and N, is no greater than 0.45 wt. %; and [0069] the sum of Sn, Sb and Bi is no greater than 20 wt. %.

[0070] Example 3 Method of manufacturing a titanium metal matrix composite

[0071] A nickel-coated continuous carbon fiber reinforcement and titanium alloy metallic foils are cleaned with acetone. The continuous carbon fiber network is electroplated with 0.0002 chromium.

[0072] A titanium alloy foil is cleaned with MPK solvent.

[0073] A tin sheet weighing 15% of the total matrix alloy formulation is place on either side of the electroplated continuous fiber reinforcement. A titanium alloy foil is place on either side on either side of the tin sheet.

[0074] The layers are placed in a tooling and inserted into an electric furnace. The layers are heated to 700 F. for 20 minutes such that the tin foil melts. Pressure is applied to the layers by pressing down on the tooling such that the tin fills the space between the titanium alloy and the carbon fiber reinforcement.

[0075] The layers are removed from the electric furnace and cooled for 2 hours such that the tin solidifies and solders the layers together.

[0076] The solder layers are then covered with a protective coating of fiber glass.

[0077] The layers are placed back in the tooling, placed in the electric furnace, and heated to 1700 F. for 1 hour. Pressure is mechanically applied to the tooling. The layers diffuse and consolidate into to a homogeneous mixture resulting in a titanium metal matrix composite.

[0078] When introducing elements of the present invention or the various embodiments(s) thereof, the articles a, an, the and said are intended to mean that there are one or more of the elements. The terms comprising, including and having are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0079] In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

[0080] As various changes could be made in the above processes without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.