Field of invention The present invention concerns high strength silicon-containing titanium-based alloys with optionally additives of aluminium, boron, chromium, scandium and rare earth metals (Y, Er, and Ce and La containing misch metal).

Background art A variety of two phase α/β-titanium and near α-titanium alloys, such as TΪ-6AI-4V, IMI 834 (Ti-δ.8-AMSn-3Zr-0JNb-0.δMo-0.3δSi-0.06C) and TIMET 1100 (Ti-6AI- 2.7Sn-4Zr-0.4Mo-0.45Si) show great potential application in the air plane and space industry.

Among them Ti-6AI-4V exhibits the broadest application due to an optimum combination of high strength and fracture toughness and excellent fatigue properties at room and elevated temperature. These alloys have, however, some disadvantages such as a poor oxidation resistance above 475°C (α-case formation), insufficient creep strength at 600°C and higher temperatures and a poor wear resistance at room and elevated temperatures. The α-case causes crevice formation on the oxidised surface and has a detrimental effect on the fatigue properties. The arc melting process of these relatively high melting point alloy of about 16600C) and the necessary melt overheating to about 1750 to 17700C is a very energy consuming procedure for the manufacture of investment castings for the air plane and automotive industry, and engineering purposes in general.

Low silicon-containing titanium-based alloys are well known. Thus JP 2002060871 A describes a titanium alloy containing 0.2 - 2.3 wt % Si, 0.1 - 0.7 wt % O (total content oxygen), and 0.16 - 1.12 wt % N and 0.001 - 0.3 wt % B and remainder of titanium including unavoidable impurities, used for as cast products. These are e.g. golf club heads, fishing tackles and medical components such as tooth root, implants, bone plates, joints and crowns. The low silicon-containing titanium-based alloy does, however, suffer from a disadvantage, by forming small needle like Ti3Si precipates along grain boundaries, which decrease the fracture toughness and ductility of this material.

From the paper "Structures and properties of the refractory suicides, Ti5Si3 and TiS2 and Ti-Si-(AI) eutectic alloys", by Frommeyer et. al. published in May 2004, it is on page 301 described a hypereutectic Ti-Si7.5-AI1 alloy. It is further stated that with increasing silicon content up to about 9% by weight, the microstructure of the cast samples consists of fine dispersion of Ti5Si3 suicide particles within the α- Ti(Si) solid solution matrix.

The alloys described by Frommeyer et. al. have excellent hardness and flow strength. The warm strength of the Ti-Si-Al alloys is, however, moderate and there is no indication of the oxidation resistance at high temperature.

There is thus a need for an alloy that has a high strength at high temperatures, has a lower melting point than the Ti-Al-V alloys and has good casting properties.

Description of invention By the present invention it is provided Ti-Si alloys with relatively high silicon contents which exhibit a relatively low melting point due to their eutectic constitution, good casting properties and high strength at higher temperatures as well as a very high resistance to oxidation and creep deformation at high temperatures.

The present invention thus relates to a Ti-Si alloy comprising 2.5 - 12 wt % Si, 0 - 5 wt % Al, 0 -5 wt % Cr, 0 - 0.5 wt % B, 0 - 1 wt % rare earth metals and/or yttrium and/or Sc, the remaining except for impurities being Ti.

According to a preferred embodiment the alloy contains 0.3 - 3 wt % Al. According to another preferred embodiment the Ti-Si alloy contains 3 - 6 wt % Si and 1.2 - 2.5 wt % Al.

According to yet another preferred embodiment the alloy contains 0.001 to 0.15 wt % rare earth metals and/or scandium.

It has been found that the addition of rare earth metals and/or yttrium and/or scandium improves the warm strength and creep strength of the Ti-Si alloy up to at least 675°C,

The rare earths yttriym and scandium additions form a fine dispersion of thermo- dynamically stable oxides, such as Er2θ3, Y2O3 etc. in the Ti-Si alloy.

The alloy preferably contains 0.1 to 1.5 wt % Cr. The addition of Cr enhances solid solution hardening and therefore increases the strength and increase the oxidation resistance of the alloy.

In the as cast state, the Ti-Si alloy possesses fine-grained hypoeutectic, eutectic or slightly hypereutectic microstructures depending upon the silicon content. The microstructure of the eutectic Ti-Si alloy consists of finely dispersed Ti5Si3 suicide particles of discontinuous rod like shape within the hexagonal close-packed α- Ti(Si) solid solution matrix. The hypoeutectic microstructure consists of primary solidified α-Ti(Si) crystals and the surrounding eutectic.

The Ti-Si alloy according to the invention has with a yield stress of at least 800 MPa, a Brinell hardness of 350-400 HB and sufficient ductility and fracture toughness -stress intensity factor Kic . of more than 23 MPa Vm at room temperature and up to 5000C.

The Ti-Si alloy according to the invention further exhibits excellent oxidation resistance up to 65O0C and above depending upon the Si content and improved wear resistance both at room and elevated temperature. The yield strength at 65O0C will be of at least Rp0 2 >250 MPa and the tensile strength exceeds Rm =

450 MPa.

The hypereutectic microstructures consist of primary solidified TJsSi3 crystals of hexagonal shape within the fine-grained eutectic microstructure.

In the as cast state the hypoeutectic Ti-Si alloys exhibit at room temperature fractures toughness -K|C-values- of more than 23 MPa Vm , yield stress of more than 500 MPa with a plastic strain of more than 1.5 to 3 %.

The eutectic alloy shows a fracture toughness of K|C of 15 - 18 MPa Vm and the yield stress exceeds 850 MPa at room temperature. At 6000C and above the fracture toughness is increased to 30 MPa Vm and the strength is of the order of at least Rm = 450 MPa.

Oxidation tests with exposure to air at 6000C have resulted in an increase in mass of less than 5 mg/cm2 after 500 hours. In comparison the conventional Ti-AI6- V4 alloy exhibits alpha case formation at 4750C during long term exposure on air.

The creep stress (applied stress at given temperature where the creep rate is έ = 107S"1) of the eutectic Ti-Si alloy according to the invention is higher than 200 MPa at 6000C. In contrast the Ti-AI6-V4 alloy with potential application in the air plane and space industry exhibits a creep stress of about 150 MPa at 4500C.

The Ti-Si alloy according to the invention has a low melting point of between about 1330 and about 138O0C. The alloy according to the invention has further excellent casting properties making it possible to cast virtually any size and shape. As a result of its spectrum of characteristics properties presented above, the Ti-Si alloy according to this invention are advantageously suitable for the manufacture of diverse components, subjected to high temperature, such as:

connecting rods, piston crowns, piston pins, inlet and outlet valves and manifolds of exhaust gas mains in internal combustion engines and diesel engines;

static blades in axial flow compressors and fan blades in jet engines;

wear resistant parts in textile machines -weaving looms- like shuttles and connecting shafts;

surgical implants, bone plates, joints;

hard facings and surface alloys used as coatings in surface engineering for improving wear resistance and to avoid fretting;

watch cases;

pump cases and impellers for the chemical and oil industry.

The Ti-Si alloy according to the invention is particularly suitable for as cast components because of their relatively low melting temperatures of about 1330 to 138O0C and excellent castability.

The Ti-Si alloy according to the invention can be produced in conventional way, such as by arc melting in a water cooled copper hearth. Detailed description of invention

Example 1

A hypoeutectic Ti-6Si-2AI alloy according to the invention was produced by arc melting using a non consumable tungsten electrode. Titanium sponge with a purity of more than 99.8 wt %, metallurgical grade silicon and aluminium granules with a purity of more than 99.8 wt % were used as starting materials. The alloy was kept during arc melting in a water cooled copper hearth by forming a thin solid skull on the copper hearth and was then cast into a copper mould in order to achieve rod like ingots. These were machined by turning and grinding to cylindrical compression and tensile test samples exhibiting a smooth surface finish.

The Brinell hardness was determined to be about 336 + 3 HB 187.5/2.5 applying a testing load of 187.5 kp. The flow stress was determined at room temperature in compression test to be about Rp0 2 « 725 to 750 MPa and the plastic strain

exceeds -εpι 10 %. The fracture toughness was measured in a four point bend test. The stress intensity factor Kic varies between 19 < Kic ≤ 21 MPa Vm. At higher temperature of 65O0C the flow stress is still 260 Rp0 2 275 MPa and the

fracture toughness is about 32< K|C < 34 MPa Vm. The weight gain in an oxidation test on air at 6000C was 4.5 mg/cm2 after 525 hrs.

Example 2

A hypereutectic Ti-IOSi alloy containing 0.2 wt % Al was also produced by arc melting technique as described above in Example 1. The macrohardness -Brinell- of this alloy was determined to be about 365 HB 187.5/2.5 and the yield stress at room temperature ranges between 930 < Rp <965 MPa depending upon the grain size of the alloy. The plastic strain in

compression is about 6 to 8 % and the fracture toughness is in between K|C = 16 and 19 MPa Vm. At higher temperature of 65O0C the yield stress is about 330 to 360 MPa. The fracture toughness is in between 25 and 28 MPa Vm. The creep strength was determined at 6000C and exhibits values of 215 to 230 MPa in the coarse-grained state. The oxidation on air at 65O0C leads to a weight gain of about 3.8 mg/cm3 at 500 hrs exposure time.

Example 3

A hypoeutectic (near eutectic) oxide dispersion strengthened Ti-7Si-2Al alloy with addition of 0.07 mass-% Y was also produced by the arc melting technique described in example 1. Metallic Yttrium was added to the melt and formed Y2O3 with the dissolved oxygen of about 1200 ppm. The Brinell hardness was determined to be 347 ± 2 HB 187.5/2.5. The measured yield strength was about 960 to 990 MPa. First creep experiments at 600°C with the creep rate of έ =10"V1 showed a creep strength in between 235 and 255 MPa.

Example 4 A hypoeutectic oxide dispersion strengthened Ti-5.5Si-3.5AI.-1.5Cr-0.1 Y alloy was produced by the melting method technique described in Example 1. Metallic yttrium was added to the melt and formed Y2O3 with oxygen dissolved in the melt.

The Brinell hardness was measured to 373±2 HB at a load of 187.5 Kp at room temperature and the fracture toughness stress intensity was measured to Kic = 21 MPa Vm . At 650°C the tensile strength was measured to about Rm = 360 MPa, the fracture toughness was between 35 and 40 MPa Vϊn and the creep strength at the strain rate of έ =10'V1 excelled 270 MPa. Oxidation tests at 6000C in air exhibits a mass gain of less than 8 mg/cm3 after an exposure time of 500 hours. For comparison, the oxidation tests of the c coommmmeerrcciiaall T TΪΪ--66AAII--44VV a allllooyy s shhoo^ws a mass gain of more than 20 mg/cm3 after 500 hours exposure in air at 600°C. These examples show that the Ti-Si alloys of the present invention have a surprisingly high warm strength and very good oxidation resistance at high temperatures.