Method of producing titanium aluminide having superior oxidation resistance

Abstract: Ti powders and Aℓ powders are combined to prepare a mixture of 40 ∼ 55 at% of Aℓ and the balance of Ti. After CIP and degassing, plastic working by hot extrusion is applied to form a shaped mixture of Ti and Aℓ The shape is then processed by HIP to synthesize titanium aluminide while diffusing Aℓ into the Ti structure to form an Aℓ₂O₃ phase occurring from both the reaction between Aℓ and oxygen contained in the Ti structure and the oxides on the Aℓ surface, and to disperse the Aℓ₂O₃ to form the Aℓ₂O₃ protective film. With the reaction between Aℓ and oxygen contained in the Ti structure and with the "Pegging" effect, both the Aℓ₂O₃ phase formed at the grain boundaries of crystals or in the crystal grains of titanium aluminide and the Aℓ₂O₃ phase existing on the surface of raw material Aℓ powder peg the surface Aℓ₂O₃ film to the surface of the titanium aluminide body. This "Pegging" effect enhances the adhesiveness of the film and improves the oxidation resistance of titanium aluminide.

Description: FIELD OF THE INVENTION This invention relates to a method of producing titanium aluminide having superior oxidation resistance. More specifically, it relates to a method of producing titanium aluminide with improved oxidation resistance by forming a strongly adhesive Aℓ 2 O 3 film on the titanium aluminide at service temperatures, which is suitable for heat resistant components used in the fields of automobile, aircraft, space, and industrial equipment manufacture. BACKGROUND OF THE INVENTION Titanium aluminide (intermetallic compound of the Ti-Aℓ series) are expected to be useful materials for internal-combustion engine components such as inlet and outlet valves and piston pins because they are light weight material having superior rigidity and high temperature strength. For practical applications to such heat resistant components, the material should have high oxidation resistance as well as high temperature strength.Titanium aluminide alone, however, do not have sufficient resistance to oxidation, so attempts have been made to improve the oxidation resistance by adding alloying elements. For example, JP-A-1-246330 (the term "JP-A-" referred to herein signifies "unexamined Japanese patent publication") reports that the addition of 0.3 ∼ 5.0 % of Si to Ti-30 ∼ 45 wt% Aℓ improves the oxidation resistance. JP-A-1-259139 presents a Ti-Aℓ intermetallic compound having superior high temperature oxidation resistance, containing 22 ∼ 35 wt% of Aℓ and 5 ∼ 20 wt% of Cr, and it also notes that further improvement of high temperature oxidation resistance is achieved by adding 0.01 ∼ 3 wt% of Y, 0.01 ∼ 3 wt% of Re, 0.01 ∼ 0.2 wt% of C, 0.01 ∼ 1 wt% of Si, and 0.01 ∼ 0.2 wt% of B. JP-B-1-50933 (the term "JP-B-" referred to herein signifies "examined Japanese patent publication") states that the addition of 100 ∼ 1000 atPPM of P to a Ti-Aℓ intermetallic compound composed of 40 ∼ 50 at% of Ti and 60 ∼ 50 at% of Aℓ improves the oxidation resistance. JP-A-63-247321 discloses a process of producing TiAl intermetallics having excellent high temperature strength and oxidation resistance. The intermetallics are produced by mixing Al and Ti powders comprising 14-63wt% Al, compacting said mixture by cold isostatic pressing, heating the deaerated green compact to 455°C and pressing and extruding the compact composed of the intermetallic compounds Ti 3 Al, TiAl and TiAl 3 . Nevertheless, the addition of these alloying elements does not necessarily result in a sufficient improvement of oxidation resistance, and furthermore, when a specific property is intended to be boosted, other superior characteristics often suffer bad effects. SUMMARY OF THE INVENTION It is the main object of this invention to provide a method of producing titanium aluminide having a superior oxidation resistance. It is another object of this invention to provide a method of producing titanium aluminide having an improved oxidation resistance by forming a strongly adhesive Aℓ 2 O 3 film thereon without adding alloying elements. It is a further object of this invention to provide a method of producing titanium aluminide having increased adhesiveness of Aℓ 2 O 3 through the use of a Pegging effect. These objects are achieved by the sequential processing of Ti powder and Aℓ powder or Aℓ alloy powder as defined in claim 1, wherein these powders are combined and formed into shaped mixtures of Ti and Aℓ or Aℓ alloy using a plastic working method followed by a heat treatment in an inert atmosphere at a temperature of 300°C or above to synthesize titanium aluminide while diffusing Aℓ into the Ti structure and to form and disperse the Aℓ 2 O 3 phase occurring in both the reaction between Aℓ and oxygen in the Ti structure and the oxides on the Aℓ powder surface. Preferred embodiments of the claimed process are given in the dependent claims. Ti powder and Aℓ powder, both raw materials of titanium aluminide, are mixed at a composition of 40 ~ 55 at% of Aℓ. Less than 40 at% of Aℓ addition results in an excessive amount of Ti 3 Aℓ in the product, which does not provide sufficient oxidation resistance. More than 55 at% of Aℓ addition significantly degrades ductility which is also an important characteristic. Mn is known as an element which improves the ductility of titanium aluminide (JP-B-62-215), but is also recognized to degrade oxidation resistance. The oxidation resistance mechanism of this invention is, however, effective to a composition containing one or more of the elements selected from the group of Mn, V, Cr, Mo, Nb, Si, and B. Therefore, this invention does not reject the addition of these metallic components to Ti powder and Aℓ powder, the raw materials of titanium aluminide. Elements of Mn, V, Cr, Mo, and Nb act as components to improve the ductility at room temperature. The preferred adding range of these elements is from 0.5 to 5 at%. Addition of less than 0.5 at% results in a rather weak effect on improving ductility, while more than 5 at% saturates the effect. Si acts as a component to further improve oxidation resistance. The preferred adding range of Si is from 0.1 to 3 at%. Less than 0.1 at% of Si results in a rather weak effect on improving ductility, while more than 3 at% degrades ductility at room temperature. B improves strength at a preferred adding range of 0.01 to 5 at%. Less than 0.01 at% of B results in a rather weak effect on improving ductility, while more than 5 at% degrades ductility at room temperature. A plastic working method is employed to form shaped mixtures of Ti and Aℓ from the mixed raw material powders. Extrusion, forging, or rolling can be applied as the processing means of the plastic working method. These techniques can be combined with pre-treatments such as powders compaction or vacuum degassing of powder mixture. The prepared shaped mixture is then subjected to heat treatment in a vacuum or inert gas atmosphere, such as Ar, at 300°C or higher preferably at 500°C or higher up to a practical upper limit of 1,460°C, for a period ranging from 0.5 to 500 hours, followed by compression processing. The heat treatment and compressing are preferably carried out with a HIP (Hot Isostatic Press) unit to obtain dense titanium aluminide. Furthermore, in order to obtain a uniform and dense titanium aluminide, the preferred HIP treatment conditions are a temperature range of 1,200 to 1,400°C and a processing period of 0.5 to 100 hours. When a shaped mixture of Ti and Aℓ is heated to 300°C or higher, Aℓ diffuses into the Ti structure. The diffusion becomes active at 500°C or higher temperature and is self-promoted accompanied by an exothermic reaction to form titanium aluminide. During the heat treatment process, the Aℓ 2 O 3 phase is formed in the titanium aluminide and is dispersed therein. The Aℓ 2 O 3 phase is generated by both the reaction between Aℓ diffused in the Ti structure and oxygen unavoidably existing in the Ti structure as well as the oxides on the Aℓ powder surface. The oxidation resistance of titanium aluminide is obtained by the formation of a protective film with strong adhesiveness on the surface thereof. Thus, the formation of a dense Aℓ 2 O 3 film by selective oxidation of Aℓ is preferred. Generally, however, an Aℓ 2 O 3 film formed during the initial stage of titanium aluminide oxidation does not necessarily have sufficient adhesiveness, so the film peels in the succeeding oxidation stage, which promotes a rapid oxidation denaturation of titanium aluminide as well as the formation of TiO 2 . Regarding the improvement of adhesiveness of protective film, the application of a "Pegging" mechanism is known to be effective. This mechanism improves the adhesiveness through an anchoring effect by pegging the surface protective film to the metallic body using oxide pegs, which grow into the metallic structure. [B. Lustman: Trans. Metall. Soc. AIME, 188 (1950), 995] According to this invention, the Aℓ 2 O 3 phase, which is formed or dispersed at the grain boundaries of crystals or at the phase boundaries or in the crystal grains of titanium aluminide and which is generated by both the reaction between Aℓ diffused in the Ti structure and oxygen unavoidably existing in the Ti as well as the oxides on the surface of the Aℓ powder, one of the raw materials, contributes to the formation of "pegs". These "pegs" act to enhance the interfacial adhesiveness by pegging the Aℓ 2 O 3 film formed by the initial oxidation in the heating stage up against the metallic body. In concrete terms, when Ti powder and Aℓ powder are mixed at a composition of 40 ~ 50 at% of Aℓ and the balance of Ti followed by plastic working to form a shaped mixture which is then heat treated in an inert atmosphere, Aℓ elements diffuse into the Ti structure, and Aℓ 2 O 3 is formed at the grain boundaries of crystals, at the phase boundaries, or in the crystal grains by the reaction between oxygen in the Ti and the Aℓ element. Ti powder, one of the raw materials, contains oxygen in a quantity sufficient to form "pegs" of Aℓ 2 O 3 . It is necessary to adjust the quantity of oxygen in the Ti powder in a range of 0.005 to 1 at%. Oxides are inevitably formed on the Aℓ powder surface and these oxides can be used as "Pegs"as well. Diffusion of Aℓ elements begins at 300°C or higher. In the heating stage at 500°C or higher, the rapid exothermic reaction between Ti and Aℓ activates the diffusion phenomenon to enhance Aℓ 2 O 3 formation. The Aℓ 2 O 3 formed during this stage also functions as "pegs". Fig. 1 is an illustration of the protective film which is formed by the method of this invention. In the illustration, the pegs 3 grow from the oxide film 2 on the Aℓ 2 O 3 phase formed on the surface of titanium aluminide 1 into the grain boundaries of crystals and the phase boundaries. This pegging effect enhances the interfacial adhesiveness. The above described adhesion mechanism is typical of the method wherein Aℓ elements diffuse into the Ti structure and wherein titanium aluminide is synthesized through the reaction between Ti and Aℓ, which comprises this invention. The formation of Aℓ 2 O 3 which can act as "pegs" in any titanium aluminide obtained from a melting and casting process is difficult and improved oxidation resistance cannot be expected. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the Aℓ 2 O 3 protective film formed by the method of this invention. Fig. 2 is an Auger analysis graph showing the concentration profiles of Ti, Aℓ, and oxygen in a range from the grain boundaries of crystals into the crystal grains. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is described by referring to examples and comparative examples. This invention is not limited, however, to these examples. Example 1 Ti powder containing 0.2 at% of oxygen was mixed with Aℓ-4 at% Mn alloy powder to prepare a mixture of Ti-48 at% Aℓ-2 at% Mn. The mixture was shaped through CIP (Cold Isostatic Press) followed by degassing at 450°C under 1.3 × 10 -4 Pa for 5 hours. The obtained degassed shape was sealed in a vacuum aluminum can, which was then extruded at 400°C to be cut into the predetermined size. The cut shaped mixture was subjected to a HIP process in an Ar gas atmosphere under conditions of 1,300°C, 152 GPa of pressure, and 2 hours of retention time to reactively synthesize titanium aluminide. The obtained titanium aluminide was measured to determine the presence of oxygen segregation into the grain boundaries of crystals, the weight gain resulting from oxidation, and the tensile breaking elongation. Auger analysis was applied to determine the oxygen segregation into grain boundaries of crystals, where the titanium aluminide was shock-broken within the analytical unit and the broken surface was subjected to Auger analysis. As for the determination of weight gain caused by oxidation, a sample sized 10 × 10 × 20 mm was cut from titanium aluminide and placed into a high purity alumina crucible, which was exposed to the ambient room atmosphere at 960°C for 2 hours, followed by weighing. Table 1 shows the result of measurements. Fig.2 shows the concentration profiles of Ti, Aℓ, and oxygen in a range from grain boundaries of crystals into crystal grains determined by Auger analysis. Fig. 2 clearly demonstrates oxygen segregation to grain boundaries of crystals, which corresponds to the formation of an Aℓ 2 O 3 phase at the grain boundaries. Example 2 Ti powder containing 0.15 at% of oxygen was mixed with Aℓ powder to prepare a mixture of Ti-43 at% Aℓ, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Example 3 Ti powder containing 0.1 at% of oxygen was mixed with Aℓ powder to prepare a mixture of Ti-45 at% Aℓ, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Example 4 Ti powder containing 0.04 at% of oxygen was mixed with Aℓ-3.5 at% Cr alloy powder to prepare a mixture of Ti-42.8 at% Aℓ-1.2 at% Cr, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Example 5 Ti powder containing 0.17 at% of oxygen was mixed with Aℓ-3.4 at% V-0.1 at% B alloy powder to prepare a mixture of Ti-42.8 at% Aℓ-1.16 at% V-0.03 at% B, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Example 6 Ti powder containing 0.05 at% of oxygen was mixed with Aℓ-3.0 at% Mo-0.5 at% Si alloy powder to prepare a mixture of Ti-42.8 at% Aℓ-1.02 at% Mo-0.17 at% Si, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Example 7 Ti powder containing 0.08 at% of oxygen was mixed with Aℓ-3.0 at% Nb alloy to prepare a mixture of Ti-42.8 at% Aℓ-1.02 at% Nb, and titanium aluminide was produced therefrom using the same procedure employed in Example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Comparative example 1 One hundred grams of titanium aluminide obtained in Example 1 were melted in a plasma-arc melting furnace. To prevent segregation, the ingot was repeatedly melted for a total of three times from the top surface and from bottom surface alternately, and a button-shaped ingot was produced. Characteristics of the obtained cast were determined with the same methods employed in Example 1. The results are listed in Table 1. Comparative example 2 Ti metal containing 0.15 at% of oxygen was blended with Aℓ metal, and the mixture was then melted in a plasma-arc melting furnace to obtain a ingot following the same procedure employed in Comparison example 1. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Comparative example 3 The raw material powders used in Example 2 were combined to prepare a mixture of Ti-33 at% Aℓ, and a titanium aluminide was obtained therefrom under the same synthetic condition as in Example 2. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Comparative example 4 The raw material powders used in Example 3 were combined to prepare a mixture of Ti-58 at% Aℓ, and a titanium aluminide was obtained therefrom under the same synthetic condition as in Example 3. Characteristics of the obtained titanium aluminide were determined with the same methods as in Example 1. The results are listed in Table 1. Embodiment Oxygen segregation into grain boundaries (positive/negative) Weight gain from oxidation (g/m 2 ) Tensile breaking elongation (%) Example 120 Positive 7.5 1.3 Example 2 Positive 3.2 1.2 Example 3 Positive 5.7 1.4 Example 4 Positive 6.3 1.1 Example 5 Positive 6.0 0.9 Example 6 Positive 2.5 0.9 Example 7 Positive 3.2 1.1 Comparative example 1 Negative 285 1.5 Comparative example 2 Negative 165 1.0 Comparative example 3 Negative 90 1.0 Comparative example 4 Positive 2.5 0.1 As clearly shown in Table 1, the titanium aluminides given in Example 1 through 7, which were produced by the method of this invention, offer oxygen segregation into grain boundaries of crystals, very slight weight gain from oxidation, and relatively good elongation at tensile breaking. In contrast, the titanium aluminides in Comparison examples 1 and 2, which were produced by melting-casting process, exhibit a large weight gain due to oxidation, indicating that they have no oxidation resistance. In the product of Comparative example 3, which has less than 40 at% of Aℓ, oxygen segregation into grain boundaries of crystals is observed but the weight gain from oxidation is extremely high, suggesting that no oxidation resistance is present. On the other hand, in the product of Comparison example 4, which has more than 55 at% of Aℓ, oxygen segregation into grain boundaries of crystals is observed and the weight gain from oxidation is also low, but the product suffers from reduced ductility. As described above, the production method of this invention provides a titanium aluminide which always has high oxidation resistance without degrading ductility by applying an exclusive mechanism of Aℓ 2 O 3 phase formation and of oxide film adhesion. Thus, the method of this invention is highly useful for the production of heat resistant components of internal-combustion engines, etc.

Claims: A method of producing titanium aluminide having a superior oxidation resistance, wherein said method comprises the steps of: (1) Ti powder and Al powders are mixed to prepare a mixture of 40 - 55 at% of Al, optionally 0.5 - 5 at% in total of one or more of the components selected from Mn, V, Cr, Mo or Nb, optionally one or more of the components selected from the group of 0.1 -3at% Si and 0.01 - 5at% of B, the balance being Ti, said Ti powder comprising 0.005 - 1 at% oxygen; (2) said prepared mixture is subjected to plastic working to form a Ti-Al shaped mixture; (3) said mixture shape is subjected to heat treatment in an inert atmosphere at 300°C or higher to react oxygen with Al by diffusing Al into the Ti structure and to form an Al 2 O 3 phase occurring from oxides on the Al powder surface and to disperse said Al 2 O 3 phase, followed by compression processing to synthesize titanium aluminide. The method of producing the titanium aluminide having a superior oxidation resistance of claim 1, wherein the powder mixture prepared in said process (1) contains one or more of the components selected from the group of 0.1 - 3 at% of Si, and 0.01 - 5 at% of B. The method of producing the titanium aluminide having a superior oxidation resistance of claim 1 or 2, wherein the heating and compressing processes employed in said process (3) are carried out at a temperature range of 500 to 1,460°C. The method of producing the titanium aluminide having a superior oxidation resistance of any of the preceding claims, wherein the heating and compressing processes employed in said process (3) are carried out in an HIP (Hot Isostatic Pressure) unit. The method of producing the titanium aluminide having a superior oxidation resistance of any of the preceding claims, wherein the heating and compressing processes employed in said process (3) are carried out with an HIP unit at a temperature range of 1,200 to 1,400° C for a retention time ranging from 0.5 to 100 hours.

Inventors: KIM MOK-SOON, KUMAGAI MASAKI, SHIBUE KAZUHISA, KIM, MOK-SOON, KUMAGAI, MASAKI, SHIBUE, KAZUHISA
Applicants: SUMITOMO LIGHT METAL IND, SUMITOMO LIGHT METAL INDUSTRIES, LTD.
Application year: 1992
Main classification: C22C32
IPC codes: C22C32, B22F3, C23C10, C22C1, C22C14, C23C8