Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
  • C22C32/001—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with only oxides
  • C22C32/0015—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with only oxides with only single oxides as main non-metallic constituents
  • C22C32/0031—Matrix based on refractory metals, W, Mo, Nb, Hf, Ta, Zr, Ti, V or alloys thereof

Definitions

• 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).
• T ⁇ -6AI-4V A variety of two phase ⁇ / ⁇ -titanium and near ⁇ -titanium alloys, such as T ⁇ -6AI-4V, IMI 834 (Ti- ⁇ .8-AMSn-3Zr-0JNb-0. ⁇ Mo-0.3 ⁇ Si-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.
• 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 ( ⁇ -case formation), insufficient creep strength at 600°C and higher temperatures and a poor wear resistance at room and elevated temperatures.
• the ⁇ -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 1660 0 C) and the necessary melt overheating to about 1750 to 1770 0 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.
• JP 2002060871 A describes a titanium alloy containing 0.2 - 2.3 wt % Si, 0.1 - 0.7 wt
• % O total content oxygen
• 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 Ti 3 Si precipates along grain boundaries, which decrease the fracture toughness and ductility of this material.
• 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.
• 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.
• the alloy contains 0.3 - 3 wt % Al.
• the Ti-Si alloy contains 3 - 6 wt % Si and 1.2 - 2.5 wt % Al.
• the alloy contains 0.001 to 0.15 wt % rare earth metals and/or scandium.
• the rare earths yttriym and scandium additions form a fine dispersion of thermo- dynamically stable oxides, such as Er 2 ⁇ 3 , Y 2 O 3 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.
• the Ti-Si 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 Ti 5 Si 3 suicide particles of discontinuous rod like shape within the hexagonal close-packed ⁇ - Ti(Si) solid solution matrix.
• the hypoeutectic microstructure consists of primary solidified ⁇ -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 500 0 C.
• the Ti-Si alloy according to the invention further exhibits excellent oxidation resistance up to 65O 0 C and above depending upon the Si content and improved wear resistance both at room and elevated temperature.
• the hypereutectic microstructures consist of primary solidified TJsSi 3 crystals of hexagonal shape within the fine-grained eutectic microstructure.
• hypoeutectic Ti-Si alloys exhibit at room temperature fractures toughness -K
• 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 600 0 C 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 600 0 C have resulted in an increase in mass of less than 5 mg/cm 2 after 500 hours.
• the conventional Ti-AI6- V4 alloy exhibits alpha case formation at 475 0 C during long term exposure on air.
• the Ti-AI6-V4 alloy with potential application in the air plane and space industry exhibits a creep stress of about 150 MPa at 450 0 C.
• the Ti-Si alloy according to the invention has a low melting point of between about 1330 and about 138O 0 C.
• the alloy according to the invention has further excellent casting properties making it possible to cast virtually any size and shape.
• the Ti-Si alloy according to this invention are advantageously suitable for the manufacture of diverse components, subjected to high temperature, such as:
• 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 138O 0 C 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
• 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 Rp 0 2 « 725 to 750 MPa and the plastic strain exceeds - ⁇ p ⁇ 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 65O 0 C the flow stress is still 260 Rp 0 2 275 MPa and the fracture toughness is about 32 ⁇ K
• the weight gain in an oxidation test on air at 600 0 C was 4.5 mg/cm 2 after 525 hrs.
• 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.
• 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 600 0 C and exhibits values of 215 to 230 MPa in the coarse-grained state.
• the oxidation on air at 65O 0 C leads to a weight gain of about 3.8 mg/cm 3 at 500 hrs exposure time.
• 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 Y 2 O 3 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 ⁇ 10 " V 1 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 Y 2 O 3 with oxygen dissolved in the melt.

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• Chemical & Material Sciences (AREA)
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• Organic Chemistry (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)
• Turbine Rotor Nozzle Sealing (AREA)
• Heat Treatments In General, Especially Conveying And Cooling (AREA)

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