HIGH STRENGTH, OXIDATION AND WEAR RESISTANT TITANIUM-SILICON BASED ALLOY

Description

Field of invention

[0001] 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

[0002] A variety of two phase α/β-titanium and near α-titanium alloys, such as Ti-6Al-4V, IMI 834 (Ti-5.8-Al-4Sn-3Zr-0.7Nb-0.5Mo-0.35Si-0.06C) and TIMET 1100 (Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si) show great potential application in the air plane and space industry.

[0003] Among them Ti-6Al-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 alloys of about 1660°C) and the necessary melt overheating to about 1750 to 1770°C is a very energy consuming procedure for the manufacture of investment castings for the air plane and automotive industry, and engineering purposes in general.

[0004] 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.

[0005] These are e.g. golf club heads, fishing tackle and medical components such as tooth roots, 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.

[0006] From the paper "Structures and properties of the refractory silicides, Ti5Si3 and TiS2 and Ti-Si-(Al) eutectic alloys", by Frommeyer et. al. published in May 2004, a hypereutectic Ti-Si7.5-Al1 alloy is described on page 301. 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 silicide particles within the α-Ti(Si) solid solution matrix.

[0007] 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.

[0008] 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

[0009] 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.

[0010] 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, 0.001 - 1 wt % rare earth metals and/or yttrium and/or Sc, the remaining except for impurities being Ti.

[0011] According to a preferred embodiment the alloy contains 0.3 - 3 wt % Al.

[0012] According to another preferred embodiment the Ti-Si alloy contains 3 - 6 wt % Si and 1.2-2.5 wt % Al.

[0013] According to yet another preferred embodiment the alloy contains 0.001 to 0.15 wt % rare earth metals and/or scandium.

[0014] 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,

[0015] The rare earths yttriym and scandium additions form a fine dispersion of thermodynamically stable oxides, such as Er₂O₃, Y₂O₃ etc. in the Ti-Si alloy.

[0016] 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.

[0017] 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 silicide 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.

[0018] 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 K_IC . of more than 23 MPa√m at room temperature and up to 500°C.

[0019] The Ti-Si alloy according to the invention further exhibits excellent oxidation resistance up to 650°C and above depending upon the Si content and improved wear resistance both at room and elevated temperature. The yield strength at 650°C will be of at least R_P0.2 ≥250 MPa and the tensile strength exceeds R_m = 450 MPa.

[0020] The hypereutectic microstructures consist of primary solidified Ti₅Si₃ crystals of hexagonal shape within the fine-grained eutectic microstructure.

[0021] In the as cast state the hypoeutectic Ti-Si alloys exhibit at room temperature fractures toughness -K_IC-values- of more than

yield stress of more than 500 MPa with a plastic strain of more than 1.5 to 3 %.

[0022] The eutectic alloy shows a fracture toughness of K_IC of

and the yield stress exceeds 850 MPa at room temperature. At 600°C and above the fracture toughness is increased to

and the strength is of the order of at least Rm = 450 MPa.

[0023] Oxidation tests with exposure to air at 600°C have resulted in an increase in mass of less than 5 mg/cm² after 500 hours. In comparison the conventional Ti-Al6-V4 alloy exhibits alpha case formation at 475°C during long term exposure on air.

[0024] The creep stress (applied stress at given temperature where the creep rate is ε̇ = 10⁷ s^-1) of the eutectic Ti-Si alloy according to the invention is higher than 200 MPa at 600°C. In contrast the Ti-Al6-V4 alloy with potential application in the air plane and space industry exhibits a creep stress of about 150 MPa at 450°C.

[0025] The Ti-Si alloy according to the invention has a low melting point of between about 1330 and about 1380°C. 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.

[0026] 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 1380°C and excellent castability.

[0027] 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

[0028] A hypoeutectic Ti-6Si-2Al alloy 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.

[0029] 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 R_P0.2 ≈ 725 to 750 MPa and the plastic strain exceeds -ε_pl 10 %. The fracture toughness was measured in a four point bend test. The stress intensity factor K_IC varies between 19 ≤ K_IC ≤ 21 MPa √m. At higher temperature of 650°C the flow stress is still 260 R_P0.2 275 MPa and the fracture toughness is about 32≤ K_IC ≤ 34 MPa √m. The weight gain in an oxidation test on air at 600°C was 4.5 mg/cm² after 525 hrs.

Example 2

[0030] A hypereutectic Ti-10Si alloy containing 0.2 wt % Al was also produced by arc melting technique as described above in Example 1.

[0031] 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 ≤

[0032] R_P0.2 ≤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_IC = 16 and 19 MPa √m.

[0033] At higher temperature of 650°C the yield stress is about 330 to 360 MPa. The fracture toughness is in between 25 and 28 MPa √m. The creep strength was determined at 600°C and exhibits values of 215 to 230 MPa in the coarse-grained state.

[0034] The oxidation on air at 650°C leads to a weight gain of about 3.8 mg/cm³ at 500 hrs exposure time.

Example 3

[0035] A hypoeutectic (near eutectic) oxide dispersion strengthened Ti-7Si-2AI 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 ε̇ =10^-7s^-1 showed a creep strength in between 235 and 255 MPa.

Example 4

[0036] A hypoeutectic oxide dispersion strengthened Ti-5.5Si-3.5Al.-1.5Cr-0.1Y 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.

[0037] 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

At 650°C the tensile strength was measured to about R_m = 360 MPa, the fracture toughness was between 35 and

and the creep strength at the strain rate of ε̇ =10^-7s^-1 excerted 270 MPa.

[0038] Oxidation tests at 600°C 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 commercial Ti-6Al-4V alloy shows a mass gain of more than 20 mg/cm³ after 500 hours exposure in air at 600°C.

[0039] 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.

Claims

1. High strength, oxidation and wear resistant titanium-silicon base alloy characterized in that the alloy contains:

2.5-12wt% Si

0 - 5 wt% Al

0 - 0.5 % B

0-5% Cr

0.001 - 1 wt % rare earth metals and/or yttrium and/or scandium balance Ti with unavoidable impurities.

2. Alloy according to claim 1, characterized in that the alloy contains 0.3 to 3 wt % Al.

3. Alloy according to claim 1 or 2, characterized in that the alloy contains 0.001 - 0.15 wt % rare earth metal and/or scandium.

4. Alloy according to claims 1 - 3, characterized in that the alloy contains 0.1 to 1.5 wt % Cr.

5. Alloy according to claim 1 - 4, characterized in that the alloy contains 0.01 to 0.03 wt % B.

6. Alloy according to claims 1-5, characterized in that the alloy contains 3-6 wt % Si and 1.2-2.5 wt % Al.

7. Alloy according to claim 1, with near eutectic composition and related microstructure, characterized in that the alloy contains

6 - 9 wt % Si

1.2 - 2.5 wt % Al

0.001 - 0.15 wt % rare earth metals

with yield strength of more than 700 Mpa at room temperature, fracture toughness of more than K_IC = 15 and improved wear and oxidation resistance.

Ansprüche

1. Hochfeste, oxidations- und verschleißbeständige Titan-Silicium-basierte Legierung, dadurch gekennzeichnet, dass die Legierung enthält:

2,5 - 12 Gew.-% Si

0 - 5 Gew.-% Al

0 - 0,5% B

0 - 5% Cr

0,001 - 1 Gew.-% Seltenerdmetalle und/oder Yttrium und/oder Scandium, Rest Ti mit unvermeidbaren Verunreinigungen.

2. Legierung gemäß Anspruch 1, dadurch gekennzeichnet, dass die Legierung 0,3 bis 3 Gew.-% Al enthält.

3. Legierung gemäß Anspruch 1 oder 2, dadurch gekennzeichnet, dass die Legierung 0,001 - 0,15 Gew.-% Seltenerdmetalle und/oder Scandium enthält.

4. Legierung gemäß Ansprüchen 1 - 3, dadurch gekennzeichnet, dass die Legierung 0,1 bis 1,5 Gew.-% Cr enthält.

5. Legierung gemäß Anspruch 1 - 4, dadurch gekennzeichnet, dass die Legierung 0,01 bis 0,03 Gew.-% B enthält.

6. Legierung gemäß Ansprüchen 1-5, dadurch gekennzeichnet, dass die Legierung 3-6 Gew.-% Si und 1,2-2,5 Gew.-% Al enthält.

7. Legierung gemäß Anspruch 1 mit beinahe eutektischer Zusammensetzung und damit in Beziehung stehender Mikrostruktur, dadurch gekennzeichnet, dass die Legierung enthält:

6 - 9 Gew.-% Si

1,2 - 2,5 Gew.-% Al

0,001 - 0,15 Gew.-% Seltenerdmetalle

mit Streckgrenze von mehr als 700 Mpa bei Raumtemperatur, Bruchzähigkeit von mehr als

und verbesserter Verschleiß- und Oxidationsbeständigkeit.

Revendications

1. Alliage haute résistance à base de titane-silicium résistant à l'oxydation et à l'usure, caractérisé en ce que l'alliage contient :

2,5 à 12% en poids de Si

0 à 5% en poids d'Al

0 à 0,5% de B

0 à 5% de Cr

0,001 à 1% en poids de métaux de terres rares et/ou d'yttrium et/ou de scandium, le reste étant du Ti avec d'inévitables impuretés.

2. Alliage selon la revendication 1, caractérisé en ce que l'alliage contient 0,3 à 3% en poids d'Al.

3. Alliage selon la revendication 1 ou 2, caractérisé en ce que l'alliage contient 0,001 à 0,15% en poids de métal des terres rares et/ou de scandium.

4. Alliage selon les revendications 1 à 3, caractérisé en ce que l'alliage contient 0,1 à 1,5% en poids de Cr.

5. Alliage selon la revendication 1 à 4, caractérisé en ce que l'alliage contient 0,01 à 0,03% en poids de B.

6. Alliage selon les revendications 1 à 5, caractérisé en ce que l'alliage contient 3 à 6% en poids de Si et 1,2 à 2,5% en poids d'Al.

7. Alliage selon la revendication 1, avec une composition quasi-eutectique et une microstructure associée, caractérisé en ce que l'alliage contient

6 à 9% en poids de Si

1,2 à 2,5% en poids d'Al

0,001 à 0,15% en poids de métaux des terres rares

avec une limite d'élasticité dépassant 700 MPa à température ambiante, une ténacité à la rupture dépassant

et une résistance à l'usure et à l'oxydation améliorée.