GB2467312A

GB2467312A - An alpha-titanium alloy comprising aluminium, oxygen and carbon

Info

Publication number: GB2467312A
Application number: GB0901370A
Authority: GB
Inventors: Michael Thomas Cope
Original assignee: INTELLIGENT SOLUTIONS TECHNOLO
Current assignee: INTELLIGENT SOLUTIONS TECHNOLO
Priority date: 2009-01-28
Filing date: 2009-01-28
Publication date: 2010-08-04
Anticipated expiration: 2029-01-28
Also published as: WO2010086372A1; GB0901370D0; GB2467312B

Abstract

An alpha-titanium alloy containing (by weight): 6-10 % aluminium and 0.5-5.0 % of additional alpha stabilisers including carbon and oxygen. The alloy contains (by volume) : 0-20 o Ti3Al, 0-4 % beta phase, 0-4 o silicide, 0-5 carbide, with the balance being essentially alpha phase. The alloy preferably comprises (by weight): 6.5-9.5 aluminium, 0.1-0.5 % carbon, 0.1-0.5 % oxygen, 1.5-3.0 other alpha stabilisers (e.g. Zr, Hf), 0-1.5 % neutral alpha strengtheners (e.g. 0-0.5 % Si), 0-1.5 % beta stabilisers (e.g. Nb, Mo, Cr, V), with the balance being essentially titanium. The alloy may be produced as a finished or semi-finished article by extruding at a temperature below its beta transus temperature. Following extrusion the article can be solution treated and oil quenched and/or aged or not heat treated. Final extrusion can be preceded by an earlier extrusion step at a higher temperature.

Description

TITLE: TITANIUM ALLOY, A METHOD OF PRODUCING THE ALLOY

AND AN ARTICLE MADE OF THE ALLOY

FIELD OF THE INVENTION

The present invention relates to a titanium alloy, a method of producing the alloy and an article made of the alloy.

BACKGROUND OF THE INVENTION

Titanium alloys have an abundant resource base and can show attractive properties such as high strength, good corrosion resistance and low density. In consequence titanium alloys are widely used in the manufacture of quality engineering components for use in various industries. Such alloys are used for example in the aircraft industry in the manufacture of components of aircraft engines and aircraft structures, in the automotive industry in the manufacture of engine components and in the chemical industry for machine parts.

Titanium can exist in two principal crystalline forms, namely an alpha form (alpha phase) which is a closely packed hexagonal form and a beta form (beta phase) which is a body centred cubic form.

Transformation between the alpha form and the beta form takes place at a transition temperature known as the beta transus.

Known titanium alloys which show a good combination of strength and ductility normally include a high level of beta stabilisers which are alloying constituents which favour existence of the beta form by lowering the beta transus. However, the presence of such constituents normally causes the alloy to have a density which is higher than desired and to have properties which can be damaged at high temperatures.

Known titanium alloys which show a combination of high stiffness and low density normally include the intermetallic hAl and have a low ductility and are therefore difficult to process.

W02007/082,708 describes a titanium alloy designed to provide good mechanical strength when operating at high temperatures, e.g. 600°C and above. The alloy consists of the alpha-2 (Ti3A1) phase dispersed in the alpha phase, wherein the alloy contains between 40 per cent and 50 per cent by volume of the alpha-2 phase.

The alloy has a ductility and other properties which could be improved, particularly for operation at lower temperatures, e.g. lower than 600°C.

The purpose of the present invention is to provide a titanium alloy which can show a combination of properties, including strength, stiffness and ductility, which is superior to the combination of such properties obtainable in known alloys, especially for operation at temperatures lower than 600°C.

SUMMARY AND DESCRIPTION OF THE INVENTION

According to the present invention in a first aspect there is provided a titanium alloy as defined in claim 1 of the accompanying claims. That alloy will herein be referred to as the Improved Alloy' According to the present invention in a second aspect there is provided a method of production as defined in claim 17 of the accompanying claims.

According to the present invention in a third aspect there is provided an article as defined in claim of the accompanying claims.

In this specification an alpha stabiliser' refers

to an element (or a combination of elements) whose presence in a titanium alloy causes the beta transus to be raised thereby favouring the alpha phase.

In this specification a beta stabiliser' refers to

an element (or a combination of elements) whose presence in a titanium alloy causes the beta transus to be lowered thereby favouring the beta phase.

In this specification the expression the balance

being essentially the alpha phase' as used in claim 1 refers to the balance of the phase composition defined consisting of the alpha phase plus any incidental phases, such as a Laves phase, up to a total of two per cent by volume of the phase composition present.

In this specification the expression the balance

being essentially titanium' as used in claim 4 refers to the balance of the composition defined consisting of titanium plus any incidental elements and impurities up to a total not greater than 2,000 parts per million by weight of the composition.

The Improved Alloy can beneficially and unexpectedly show an attractive combination of desirable properties, namely strength, stiffness, ductility and density, especially at temperatures lower than 600°C. As illustrated later, the combination of such properties obtained can be better than the combination of such properties shown by titanium alloys of the prior art. As will be apparent to those skilled in the art, within the combination of properties obtainable, one or more of the properties of the Improved Alloy may be enhanced at the expense of one or more other of the properties of the combination of properties.

The Improved Alloy falls into the class of titanium alloys referred to as super' alpha alloys owing to the high level of alpha phase present as provided by the high level of alpha stabilisation elements present.

The Improved Alloy may contain at least 80 per cent, e.g. at least 90 per cent, in some cases at least per cent, by volume of the alpha phase. The Improved Alloy may contain not more than 20 per cent, e.g. not more than 10 per cent, in some cases not more than 2 per cent, by volume of titanium-aluminium intermetallic phase. Any titanium-aluminium intermetallic phase present consists essentially of the alpha-2 intermetallic Ti3A1. Thus, the intermetallic TiA1 is not present as the level of aluminium is too low.

As noted earlier, significant levels of Ti-Al intermetallic phases have an adverse effect on ductility of titanium alloys. Where such an intermetallic phase is used in such alloys in the prior art, it is normally included to improve the high temperature operating properties of the alloy. In the Improved Alloy, Ti3A1 may be present within the content limits described above, to improve strength and stiffness whilst reducing density especially for lower temperature applications, but at the expense of ductility.

The Improved Alloy may include a small amount, e.g. not more than four per cent, in some cases not more than two per cent, by volume of the beta phase and/or a small amount, e.g. not more than four per cent, in some cases not more than two per cent, by volume of silicide (one or more silicides) . The presence of silicon can be beneficial to form titanium and/or zirconium silicide giving precipitation on dislocations thus making plastic deformation more difficult at elevated temperatures. The latter property increases high temperature strength.

The Improved Alloy contains carbon which may form carbide in an amount of not more than five per cent, in some cases not more than 1.5 per cent, by volume of the phase composition of the Improved Alloy.

The Improved Alloy may have a composition (Selected Composition') selected to be of the following elemental components whose presence is expressed as a percentage by weight of the overall composition: 6.5 to 9.5 per cent of aluminium; 0.1 to 0.5 per cent of carbon; 0.1 to 0.5 per cent of oxygen; 1.5 to 3.0 per cent of additional alpha stabiliser; 0.0 to 1.5 per cent of neutral alpha strengthener; and 0.0 to 1.5 per cent of beta stabiliser; the balance being essentially titanium.

The additional alpha stabiliser of the Selected Composition may be selected from zirconium and hafnium and may have a content in the Selected Composition of from 1.5 to 2.5 per cent by weight.

The Selected Composition may contain the following alpha stabilisers whose presence is expressed as a percentage by weight of the overall composition: aluminium 8.5 to 9.5 per cent; zirconium 1.6 to 2.4 per cent; carbon 0.05 to 0.2 per cent; and oxygen 0.1 to 0.2 per cent.

For example, the Selected Composition may contain the following alpha stabilisers whose presence is expressed as a percentage by weight of the overall composition: aluminium 8.9 to 9.5 per cent; zirconium 2.0 to 2.4 per cent; carbon 0.05 to 0.15 per cent; and oxygen 0.10 to 0.16 per cent.

The Selected Composition may contain up to 0.5 per cent by weight of silicon, e.g. from 0.1 per cent to 0.5 per cent of silicon.

The Selected Composition may contain from 0.1 per cent to 1.5 per cent by weight of beta stabiliser. The beta stabiliser may be selected from niobium, molybdenum, chromium and vanadium. For example, the Selected Composition may contain from 0.2 per cent to 0.4 per cent by weight of molybdenum and/or from 0.5 per cent to 1.0 per cent by weight of niobium.

The Improved Alloy can show an unusually high strength which results in part from the relatively high level of the alpha stabilisation and strengthening elements present and in part from the unconventional way in which the material is thermally processed as described below. The high level of alpha stabilisation and strengthening elements present can provide, together with the method of thermally processing described later, a structural refinement which gives a fine achievable grain size and high levels of interstitial solute strengthening.

Unexpectedly and beneficially, the Improved Alloy may have a yield strength (0.2 per cent proof stress) measured at a temperature of 20°C of not less than 1200 MPa, preferably not less than 1300 MPa. The Improved Alloy may have a yield strength (0.2 per cent proof stress) measured at a temperature of 600°C of not less than 750 MPa, preferably not less than 850 MPa.

Unexpectedly and beneficially, the Improved Alloy may have a high elastic modulus. The elastic modulus is known to be a measure of stiffness of the Improved Alloy. For example, the Improved Alloy may have an elastic modulus, measured at a temperature of 20°C, of not less than 120 GPa, preferably not less than 130 GPa.

Furthermore, an acceptable or superior ductility can be obtained in the Improved Alloy, particularly by use of the unconventional thermal processing steps as described below. The Improved Alloy may have a ductility (tensile percentage elongation) at room temperature (20°C) of at least 4 per cent, preferably at least 6 per cent, e.g. from 8 per cent to at least 15 per cent.

The density of the Improved Alloy may be reduced by increasing the aluminium content. The Improved Alloy may have a density which is as low as possible whilst still showing an acceptable ductility and other properties.

The upper limit of the density may for example be chosen to be not greater than 4.4 g/cm3.

The improvement in the combination of properties in the Improved Alloy is shown to be most enhanced when the aluminium content is between 8 and 10 per cent, especially between 8.5 and 9.5 per cent, by weight of the composition. When the aluminium content is reduced to less than 8.0 per cent, the combination of properties obtained is less enhanced than for an aluminium content of between 8.0 and 10.0 per cent. However, in some cases when the aluminium content is reduced to the range from 6.0 per cent to 8.0 per cent by weight, the combination of properties obtained still shows an improvement and may still be acceptable in a number of applications. In such cases, a greater density is obtained as well as a reduction in mechanical properties such as stiffness, compared with an aluminium content of between 8.0 and 10.0 per cent.

Desirably, the Improved Alloy contains essentially no content (less than 0.1 per cent by weight) of any element such as yttrium which removes oxygen (an alpha stabiliser included in the composition) from the alloy matrix.

Desirably, the Improved Alloy contains essentially no content (less than 0.1 per cent by weight) of boron (which may be present in known alloys) The Improved Alloy may be produced by a thermal processing procedure which is not conventional. The conventional approach to producing high strength titanium alloys is to quench the alloy to produce a martensite structure and then to age the martensite structure. In contrast, no martensite structure is necessary to produce the Improved Alloy.

Formation of the Improved Alloy may be carried out using a procedure selected from one of the following, depending on the specific properties and temperature of operation desired in application of the resulting product: Procedure 1: The Improved Alloy may be formed by a procedure which includes extrusion at a temperature below the beta transus without any further heat treatment, e.g. to provide a solution treatment or an ageing process. This is a preferred procedure to obtain a low level of intermetallic phases, including a low level of the alpha-2 phase, as defined in the accompanying claims.

Procedure 2: The Improved Alloy may be formed by a procedure which includes extrusion at a temperature below the beta transus followed by direct ageing (DA) Direct ageing involves heat treating the material, without an intermediate solution treatment, to provide a temperature between 500°C and 800°C for a period of time, for example between 1 and 8 hours. In the Improved Alloy, direct ageing forms a distribution of alpha-2 and silicides.

Procedure 3: The Improved Alloy may be formed by a procedure which includes extrusion at a temperature below the beta transus followed by a solution treatment, which may be a solution de-order (DO) treatment.

Procedure 4: The Improved Alloy may be formed by a procedure which includes extrusion at a temperature below the beta transus followed by a solution treatment plus ageing (ST + A) Procedure 1 is preferred to enhance the properties of the Improved Alloy at lower temperatures, i.e. temperatures lower than 600°C. Properties at higher temperatures, i.e. temperatures of 600°C or above, may be extended, at the expense of one or more lower temperature properties, by use of procedure 2 or procedure 4. One or more lower temperature properties, particularly ductility, may be enhanced by use of procedure 3.

Desirably, for the reasons given later, processing in the above procedures 1 to 4 is carried out by applying an extrusion step which is at a temperature not above 1050°C (1050 degrees Celsius) . Preferably, the extrusion step is carried out at a temperature not above 1035°C, e.g. a temperature in the range from 925°C to 1035°C. A temperature in the range from 1015°C to 1035°C is preferred.

Desirably, the temperature during the extrusion step, herein referred to as the extrusion temperature' is controlled to be within a temperature range having limits 10 Celsius degrees above and 10 Celsius degrees below a selected target temperature, e.g. a target temperature between 925°C and 1035°C, such as 1025°C.

Processing in the above procedures may include multiple extrusion steps, e.g. in which a cylinder produced by a first extrusion step is further extruded in a second extrusion step. In that case, the last of the extrusion steps is carried out at the selected target temperature as discussed above. The earlier extrusion step(s) may be carried out at a higher temperature. For example, a first extrusion step may be carried out at a temperature above 1050°C, e.g. at a temperature of 1080°C, and a second and last extrusion step may be carried out at a temperature not above 105000, e.g. a temperature of between 925°C and 1035°C, especially between 1015°C and 1035°C.

The extrusion step (or the last of multiple extrusion steps) may typically be carried out at an exit speed, i.e. speed at which extrudate leaves an extrusion die, which is not greater than 250 mm/sec (250 millimetres per second), preferably not greater than 200mm/sec. Such an exit speed is suitable for use with typical extrusion ratios. As known in the art, the extrusion ratio is the ratio of the cross-sectional area of the material being extruded before and after extrusion. Typical extrusion ratios are not greater than 20:1, e.g. from 10:1 to 20:1. Where it is acceptable to use even lower extrusion ratios, exit speeds greater than 250 mm/sec might be suitable for use.

The extrusion step (or at least one, e.g. the last, of multiple extrusion steps) may employ a die which has has a circular opening, although the opening may have an alternative shape, e.g. square, rectangular or elliptical.

Preferably, the extrusion step (or at least the last of multiple extrusion steps) uses a billet of material to be extruded and/or an extrusion die which is coated with a lubricant. The lubricant may be a solid lubricant such as one of those known in the alloy extrusion art which does not substantially react with titanium. Examples of such well known lubricants include stainless steel, low alloy steel, graphite, glass and copper. The lubricant may for example have a thickness equal to between 5 per cent and 60 per cent, e.g. between 10 per cent and 30 per cent, of the diameter of the extrudate (material formed by the extrusion) The purposes of limiting the extrusion exit speed and of using a lubricant are to avoid excessive heating owing to friction during the early stages of the extrusion leading to an unwanted excessive rise in temperature of the alloy being extruded. The presence of a lubricant may also beneficially avoid tearing and cracking of the material being extruded in addition to avoiding adiabatic heating above the beta transus temperature.

Processing of the Improved Alloy in the manner described above has the following beneficial effects: (i) controlling of the crystallographic texture, or preferential alignment of crystallographic planes, of the Improved Alloy to give increased stiffness; and (ii) refining the grain size of the Improved Alloy to give increased ductility and to allow further alloying to give increased strength. Such processing can further contribute to the beneficial combination of properties obtained in the resulting extruded material, as illustrated later.

The present invention in the third aspect defined above provides an article made of the Improved Alloy.

The article may be a finished or semi-finished product.

The article when finished may be suitable for use in one of the various engineering product applications known for titanium alloys, such as aircraft components, automotive engine components, chemical process equipment components, bicycle parts, surgical parts and various

others known in the field.

The Improved Alloy is particularly suitable for use in articles which in use have to have good properties when operating at lower temperatures, especially temperatures substantially lower than 600°C, although, as noted earlier, the Improved Alloy may also, or alternatively, be suitable for use in operation at high temperatures, e.g. 600°C or higher.

The Improved Alloy is particularly suitable for use in providing articles useful in notch sensitive applications. In such applications it is highly desirable to avoid cracking. Cracking can occur when the alloy is not sufficiently ductile. The ductility of the Improved Alloy and the method selected for producing the article containing the alloy, as described earlier, beneficially can be such as to avoid the problem of cracking. Examples of articles for use in notch sensitive application are gas turbine compressor blades and valves, valve train components, fasteners, gudgeon pins and threaded connecting rods for use in high performance internal combustion engines. Such articles may be manufactured by forming a semi-finished product by one or more extrusions steps in the manner described above and then treating the semi-finished product in a known manner, e.g. by refinement machining as known in the art, to obtain the required final product.

Where the Improved Alloy of the article according to the third aspect of the invention has (amongst other properties) a high specific stiffness (stiffness to density ratio), the article may be one requiring such a property in a particular application, e.g. a compressor blade of an aero engine.

The Improved Alloy is particularly suitable for use in forming articles by a multiple extrusion step process, such as that known in the art as stem back extrusion' . The latter procedure involves forming a bar or cylinder by a first extrusion step, optionally dividing, e.g. by slicing, the bar into billets, then using the bar or billet(s) in a second extrusion step in which only part of the length of the bar or billet is extruded to provide a first portion having a reduced diameter and a second portion comprising the unextruded part of the bar or billet. The article produced in this way may for example be an engine valve (or a semi-finished product which can be finished in the form of an engine valve) . The first portion of reduced diameter may for example provide a stem of the valve and the remainder of the bar or billet, the second portion, may provide a head of the valve.

Producing alloy engine valves using the Improved Alloy in a stem back extrusion process provides a way of making these products which may be more convenient than

the other prior art procedures and may give better

properties; for example superior extruded stem properties can be achieved compared to those obtained from products produced as hot rolled bars as in the

prior art.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of stress (measured in MPa) plotted versus process temperature (measured in °C) for various titanium alloys processed by extrusion at different temperatures illustrating beneficial strength properties obtainable by the Improved Alloy.

FIG. 2 is a graph of elongation (measured as a percentage) plotted versus process temperature (measured in °C) for various titanium alloys processed by extrusion at different temperatures illustrating beneficial elongations obtainable by the Improved Alloy.

FIG. 3 is a graph of elastic modulus (measured in GPa) plotted versus process temperature (measured in °C) for various titanium alloys processed by extrusion at different temperatures illustrating beneficial elastic moduli obtainable by the Improved Alloy.

ILLUSTRATIVE EXAMPLES

The following Examples illustrate the production and properties of alloys which are examples of the Improved Alloy defined earlier.

Example 1

A billet was formed of an alloy composition Al from the constituent elements (or one or more master alloys of them) of the composition Al by a conventional vacuum arc melting process. The alloy composition Al was shown to be: Ti (balance); 9.4 per cent by weight aluminium; 2.27 per cent by weight zirconium; 0.78 per cent by weight niobium; 0.33 per cent by weight molybdenum; 0.34 per cent by weight silicon; 0.10 per cent carbon; and 0.13 per cent oxygen; together with impurities: 20 ppm (parts per million by weight) nitrogen, 60 ppm hydrogen and 800 ppm iron.

The billet of the alloy composition Al was extruded in the manner described earlier using at a controlled temperature of 1025°C and using a stainless steel lubricant coating. A suitable exit speed of about mm/second was used to avoid excessive adiabatic heating. An extruded alloy product P1 was produced from the billet. The product P1 was an extruded stem cylinder having a diameter of about 12 mm.

The phase composition of various batches of the alloy product P1 was measured using X ray diffraction and a known peak separation procedure. The alloy product P1 was found to be an example of the Improved Alloy.

There was a small variation of the phase composition obtained for different batches, but the results showed the following in each case: alpha phase content of more than 95 per cent by volume; and Ti3A1 content of from about 0.1 to about 1.0 per cent by volume.

Mechanical properties of the alloy product P1 were measured using industry standard methods at temperatures of 20°C, 500°C and 600°C. Fatigue data obtained was axial (with stress along the axis of the tested specimen), R = -l (fully reversed stress, e.g. -500MPa to +500MPa) at a frequency of 100Hz in air. (It is known to those skilled in the art that fatigue data will show a small variation according to the production procedure used.) The average results obtained for the measured properties of the product P1 are given in Table 1 as follows:

Table 1

Temperature 20 500 600 (°C) ________ 0.2% Proof 1350 910 920 Stress (MPa) Ultimate 1360 1040 980 Tensile Stress (UTS) (MPa) Tensile 9 20 20 elongation (%) Elastic 135 ---119 modulus (GPa) cycle 725 640 580 Fatigue Strength (MPa) 106 cycle 700 610 520 Fatigue Strength (MPa) l0 cycle 675 580 460 Fatigue Strength (MPa) Density (g/cm3) 4.35 The results given in Table 1 show that the alloy product P1 produced from the alloy composition Al, giving an Example of the Improved Alloy, is especially suitable for use in engineering applications, particularly applications requiring high alloy strength and stiffness and low density, such as in the production of inlet valves, gudgeon pins and connecting rods of internal combustion engines.

Example 2

The alloy composition Al was formed into several products each in the manner described in Example 1, but using various different process temperatures to carry out the (final) extrusion step in each case. The process temperatures used were as follows: 1025°C (as for the product P1 in Example 1), 1050 °C, 1075°C, 1100°C and 1150°C. Some of the important properties of the resulting product obtained in each case were measured using industry standard measurement procedures. The results obtained by the measurements are plotted graphically in Figures 1 to 3.

Figure 1 is a graph of stress (measured in MPa) plotted versus process (extrusion) temperature (measured in °C) for the various products obtained. The results of the stress measurements shown are of two kinds, namely: (i) 0.2% proof stress (yield strength) measurements denoted by diamond shaped indicators in Figure 1; and (ii) UTS (ultimate tensile strength) measurements denoted by larger square shaped indicators in Figure 1 (see also the key to the indicators in Figure 1) . For the process temperature of 1050°C the results obtained for 0.2% proof stress and UTS are very close, so the indicators in Figure 1 are shown to overlap for that temperature.

Figure 1 illustrates that the strength results obtained are best at the selected process temperature of 1025°C and fall to a minimum at a process temperature of about 1100°C. The beta transus for the composition Al is at a temperature of about 1130°C SO the alloy processed at 1150°C is one processed in the beta phase. This shows a better strength than the alloy processed at 1100°C, but has an unacceptably poor elongation as shown in Figure 2.

Figure 2 is a graph of elongation (measured as a percentage) plotted versus process temperature (measured in °C) for the various products obtained.

Figure 2 illustrates that the best elongation result for the various products is obtained at the selected process temperature of 1025°C. Figure 2 shows that as the process temperature is raised above 1025°C the corresponding elongation obtained is unfortunately reduced. The elongation obtained at the process temperature of 1150°C is the poorest of the results shown in Figure 2.

Figure 3 is a graph of elastic modulus (measured in GPa) plotted versus process temperature (measured in °C) for the various products obtained. As will be appreciated by those skilled in the art, the elastic modulus for a given product is a measure of stiffness of the product.

Figure 3 illustrates that for process temperatures below 1150°C, the best elongation result for the various products is obtained at the selected process temperature of 1025°C. Figure 3 shows that as the process temperature is raised above 1025°C until the temperature of 1100°C is reached, the corresponding elastic modulus obtained is unfortunately reduced. The elastic modulus is increased again at the process temperature of 1150 °C but, as noted above, the alloy product obtained at that temperature has an unacceptably poor elongation as shown in Figure 2.

Figures 1 to 3 show that, for the process temperatures used in Example 2, the selected process temperature of 1025°C as used in Example 1 unexpectedly gives the best combination of strength, elongation and elastic modulus for the product obtained.

Example 3

A billet having the alloy composition Al used in Example 1 was single extruded in a manner similar to the procedure used in Example 1 but using a die having a rectangular opening to form a rod having a rectangular cross-section. The opening had dimensions of about 75mm and 25mm.

The rod so formed was further treated by a solution treatment as follows. An oil quench was applied for about 1 hour at a temperature of about 1085°C.

The solution treatment was performed to remove the Ti3A1 intermetallic content, and to dissolve the primary silicides in order to improve the ductility by taking more of the silicon into solution within the alpha phase. This was achieved at the expense of some of the very high strength, relative to Examples 1 and 2.

The product P2 produced following the solution treatment was suitable for use as a connecting rod of a high performance internal combustion engine owing to a useful balance of strength and ductility (elongation), in combination with the high stiffness and low density of the product P2. Typical mean values of measured tensile properties achieved by use of this processing route were a 0.2% proof stress of 935 MPa, a UTS of 1100 MPa, an elongation of 16% and elastic modulus of 132 GPa. These mean values were based on the results from four heat treatment batches.

The combination of results obtained for the Improved Alloy when made into a product by extrusion at a suitable temperature in the manner described in the Examples above, particularly at an extrusion temperature selected to be 1050°C or lower, e.g. from 1015°C to 1035°C, is a combination which can be superior to the comparable combination of results obtainable from known alloys, including typical alloys made according to the specific procedures and compositions described in

Claims

CLAIMS1. A titanium alloy containing from 6 to 10 per cent by weight of aluminium and from 0.5 to 5.0 per cent by weight of additional alpha stabiliser including carbon and oxygen, the alloy having a phase composition containing from zero to not more than twenty per cent by volume of titanium-aluminium intermetallic alpha-2 phase Ti3A1, from zero to not more than four per cent by volume of the beta phase, from zero to not more than four per cent by volume of silicide, from zero to not more than five per cent by volume of carbide, the balance being essentially the alpha phase.
2. An alloy according to claim 1 which contains not less than eighty per cent by volume of the alpha phase.
3. An alloy according to claim 1 or claim 2 which contains not more than ten per cent by volume of the alpha-2 phase Ti3A1.
4. An alloy according to any one of the preceding claims which has a composition of the following components whose presence is expressed as a percentage by weight of the overall composition: 6.5 to 9.5 per cent of aluminium; 0.1 to 0.5 per cent of carbon; 0.1 to 0.5 per cent of oxygen; 1.5 to 3.0 per cent of additional alpha stabiliser; 0.0 to 1.5 per cent of neutral alpha strengthener; and 0.0 to 1.5 per cent of beta stabiliser; the balance being essentially titanium.
5. An alloy according to claim 4 wherein the additional alpha stabiliser is selected from zirconium and hafnium and has a content in the composition of from 1.5 to 2.5 per cent by weight.
6. An alloy according to claim 5 wherein the composition contains the following alpha stabilisers whose presence is expressed as a percentage by weight of the overall composition: aluminium 8.5 to 9.5 per cent; zirconium 1.6 to 2.4 per cent; carbon 0.05 to 0.2 per cent; and oxygen 0.1 to 0.2 per cent.
7. An alloy according to claim 6 wherein the composition contains the following alpha stabilisers whose presence is expressed as a percentage by weight of the overall composition: aluminium 8.9 to 9.5 per cent; zirconium 2.0 to 2.4 per cent; carbon 0.05 to 0.15 per cent; and oxygen 0.10 to 0.16 per cent.
8. An alloy according to any one of the preceding claims wherein the composition contains up to 0.5 per cent by weight of silicon.
9. An alloy according to claim 8 wherein the wherein the composition contains from 0.1 per cent to 0.5 per cent of silicon.
10. An alloy according to any one of the preceding claims wherein the composition contains from 0.1 per cent to 1.5 per cent by weight of beta stabiliser.
11. An alloy according to claim 10 wherein the beta stabiliser is selected from niobium, molybdenum, chromium and vanadium.
12. An alloy according to claim 11 wherein the composition contains from 0.2 per cent to 0.4 per cent by weight of molybdenum.13. An alloy according to claim 11 or claim 12 wherein the composition contains from 0.5 per cent to 1.0 per cent by weight of niobium.
13. An alloy according to any one of the preceding claims which has a density not greater than 4.4 g/cm3.
14. An alloy according to any one of the preceding claims which has a tensile elongation at a temperature of 20°C which is not less than 4 per cent.
15. An alloy according to any one of the preceding claims which has a yield strength at a temperature of 20°C which is not less than 1200 MPa.
16. An alloy according to any one of the preceding claims which has an elastic modulus at a temperature of 20°C which is not less than 125 GPa.
17. A method of producing an alloy according to any one of the preceding claims which comprises an extrusion step in which a material suitable to provide the composition of the alloy is extruded at an extrusion temperature below the beta transus of the composition.
18. A method according to claim 17 wherein the extrusion is carried out through a die having a circular, square, rectangular, elliptical or other shaped opening.
19. A method according to claim 17 or claim 18 wherein the extrusion step follows at least one previous extrusion applied to the composition.
20. A method according to claim 19 wherein the previous extrusion is carried out at a temperature higher than that of the extrusion temperature of the extrusion step.
21. A method according to any one of claims 17 to 20 wherein, following the extrusion step, no further heat treatment is applied to material extruded.
22. A method according to any one of claims 17 to 20 wherein following the extrusion step a solution treatment is applied to material which has been extruded.
23. A method according to claim 22 wherein the solution treatment comprises an oil quench which removes Ti3A1 intermetallic content and dissolves primary silicide.
24. A method according to claim 22 or claim 23 wherein the extrusion step comprises extrusion through a die having a rectangular opening.
25. A method according to any one of claims 17 to 20 wherein following the extrusion step an ageing procedure is applied to material which has been extruded.
26. A method according to any one of claims 17 to 25 wherein the extrusion step is carried out at an extrusion temperature not above 1050°C.
27. A method according to any one of claims 17 to 26 wherein the extrusion temperature is controlled to be within a range of temperatures having limits not more than 10 Celsius degrees from a target extrusion temperature.
28. A method according to claim 27 wherein the target extrusion temperature is between 1015°C and 1035°C.
29. A method according to any one of claims 17 to 28 wherein in the extrusion step a billet of material is extruded through a die and a lubricant is employed to coat a surface of the billet or a surface of the die or both.
30. An article made of the alloy defined in any one of claims 1 to 16.
31. An article according to claim 30 which is a finished or semi-finished extruded product.
32. An article according to claim 31 which has been produced by the method according to any one of claims 17 to 29.
33. An article according to claim 31 or claim 32 which is a finished or semi-finished valve, rod or other component for use in an internal combustion engine.
34. An alloy according to claim 1 and substantially as herein described with reference to one of the Examples.
35. A method according to any one of claims 17 to 29 and substantially as herein described with reference toone of the Examples.