The invention relates to a shaped part consisting of an intermetallic gamma TiAl material (xcex3-TiAl, gamma titanium aluminide alloy) with 41-49 atom % Al. The invention also relates to a process for producing the part.
Gamma TiAl materials are frequently referred to as xe2x80x9cnear-gamma-titanium aluminidesxe2x80x9d. The metal structure in these materials consists primarily of a TiAl phase (gamma phase) and a small proportion of a Ti3Al (xcex12 phase). In some multi-component alloys, a small proportion of a beta phase may also be present. This phase is stabilized by such elements as chromium, tungsten, or molybdenum.
According to J. W. Kim (J. Met. 41 (7), pp. 24-30, 1989, J. Met. 46 (7), pp. 30-39, 1994), individual groups of advantageous alloy elements in gamma TiAl alloys can be described as follows (in atom %):
Tixe2x80x94Al45-48xe2x80x94(Cr, Mn, V)0-3xe2x80x94(Nb, Ta, Mo, W)0-5xe2x80x94(Si, B)0-1. Niobium, tungsten, molybdenum and, to a lesser degree, tantalum improve oxidation resistance, while chromium, manganese and vanadium have a ductilizing effect.
Due to their high strength/density ratio, their high specific Young""s Modulus, their oxidation resistance, and their creep resistance, intermetallic gamma TiAl materials present interesting possibilities for a wide range of different applications. These include, for example, turbine components and automotive engine or transmission parts.
The prerequisite for the use of gamma TiAl on an industrial scale is the availability of a technically reliable forming process which facilitates the cost-effective production of shaped parts with properties that meet the specific requirements of a given application.
Based on experience with the processing of titanium in casting operations, considerable effort has been made in recent years to develop a fine casting process for gamma TiAl materials.
It has been demonstrated that the coarse casting structure ordinarily achieved is highly disadvantageous with regard to the mechanical properties of gamma TiAl. Molded parts made of intermetallic gamma TiAl materials based on Tixe2x80x9445 atom % Alxe2x80x945 atom % Nb, produced using fine casting methods, exhibit an unacceptable coarse structure with a mean grain size of  greater than 500 xcexcm, whereby minimum and maximum grain sizes are distributed over a very broad range.
A molded part produced using fine casting methods with an alloy composition of 44 atom % Alxe2x80x941 atom % Vxe2x80x945 atom % Nbxe2x80x941 atom % B, remainder Ti (an alloy in conformity with European patent publication EP 0 634 496 and U.S. Pat. No. 5,514,333) exhibits a mean grain size in the range of 550 xcexcm and also has a broad grain-size range.
The following attempts to achieve a fine grain structure using different alloy compositions and production processes are described as representative of the many such experiments conducted in recent years.
U.S. Pat. No. 5,429,796 describes a cast article made of a titanium aluminide material consisting of 44-52 atom % aluminum, 0.05-8 atom % of one or more elements from the group chromium, carbon, gallium, molybdenum, manganese, niobium, silicon, tantalum, vanadium and tungsten and at least 0.5 vol. % of boride dispersoids with a yield strength of 55 ksi and a ductility of at least 0.5%. The achievable mean grain sizes in the preferred alloys produced using the processes cited in the patent, Tixe2x80x9447.7 atom % Alxe2x80x942 atom % Nbxe2x80x942 atom % Mnxe2x80x941 vol. % TiB2 Tixe2x80x9444.2 atom % Alxe2x80x942 atom % Nbxe2x80x941.4 atom % Mnxe2x80x942 vol. % TiB2 and Tixe2x80x9445.4 atom % Alxe2x80x941.9 atom % Nbxe2x80x941.6 atom % Mnxe2x80x944.6 vol. %, TiB2, ranged between 50 and 150 xcexcm, i.e. the structure was relatively fine. With an alloy composition of Tixe2x80x9445.4 atom % Alxe2x80x941.9 atom % Nbxe2x80x941.4 atom % Mnxe2x80x940.1 vol. %, TiB2, the mean grain size was 1000 xcexcm, i.e. the structure was relatively coarse.
The two alloys with a high proportion of TiB2 dispersoids tend to form coarse boride excretions at the grain boundaries during slow cooling following the casting process. These have a highly disadvantageous effect on the mechanical properties of the article. It is not possible to increase the cooling speed, as this induces thermal tensions which cause cracks to appear. The borides are added to the pre-alloy in a molten state. In order to reduce the unavoidable coarsening of the borides in the melt to the lowest possible level, the time interval between casting and the beginning of the hardening process must be kept short, which presents a further difficulty in the manufacturing process. In addition to these problems affecting the production process, high boride concentrations, which appear to be helpful in achieving effective grain size reduction, have a negative effect on the mechanical characteristics of the alloy.
The use of heat treatment to achieve a fine grain structure in intermetallic gamma TiAl materials is well known; see for example U.S. Pat. Nos. 5,634,992; 5,226,985; 5,204,058; and 5,653,828. With the aid of the heat treatments described in these patents, a degree of fineness is achieved in which the grain size of the cast structure is the most favorable that can be achieved through heat treatment. Ultimately, a degree of fineness that meets all the requirements of users cannot be achieved in a matrix structure produced in a casting process.
In addition to the coarse matrix structure, casting pores and blowholes have a disadvantageous effect on the mechanical properties of cast gamma TiAl articles. Consequently, recompression processes such as hot isostatic pressing or reforming processes must be applied in order to produce technically viable cast articles.
Due to the difficulties described above, the manufacture of shaped parts made of intermetallic gamma titanium aluminides using conventional casting processes such as fine casting has not been realized on an industrial scale.
As an alternative to casting, shaped parts with near-final form, shaped parts with final form and pre-material for further form processing are produced using standard powder-metallurgic processes such as hot isostatic pressing (see, for example, U.S. Pat. Nos. 4,917,858; 5,015,534; and 5,424,027). In those cases, powders produced using standard spray processes are used. Shaped parts produced using powder-metallurgy processes are significantly more fine-grained that those produced by casting. However, material produced using powder-metallurgy processes exhibits gas-filled poresxe2x80x94usually argon gas used in spray powder production. The pores have a negative effect on both creep deformation and fatigue resistance.
A satisfactory degree of grain fineness can be achieved in cast articles made of gamma TiAl with specially developed refining processes such as extrusion, forging, rolling and combinations of these processes. Thus industrial-scale production of gamma TiAl alloys ordinarily involves the use of VAR (vacuum arc remelting) base material which is converted to a fine-grained state through deformation and heat treatment. The actual forming of such products is effected following heat treatment in time-consuming mechanical processing which usually involves machining operations.
The entire manufacturing process for such shaped parts is thus expensive and restricts the range of possible applications due to cost considerations.
It is accordingly an object of the invention to provide an intermetallic gamma titanium aluminide alloy article, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, measured against the current state of the art as described above, provides a fine-grained shaped part that is as pore-free and ductile as possible on the basis of intermetallic gamma TiAl using comparatively economical production technology.
With the foregoing and other objects in view there is provided, in accordance with the invention, a shaped part formed of an intermetallic gamma TiAl alloy with 41-49 atom % Al, which exhibits a grain size of d95 less than 300 xcexcm and a pore volume of  less than 0.2 vol. %. The manufacture of the article comprises at least the following processing steps:
producing a semi-finished article involving a deformation process, with a degree of deformation greater than  greater than 65%;
shaping the semi-finished product in a solid-liquid phase state of the alloy in a mold applying mechanical forming forces during at least part of the process.
The processing of an alloy in the solid-liquid phase state is a semi-solid process. In semi-solid processes, ordinarily semi-liquid masses are processed in a thixotropic state, thixotropy is the state in which a material is highly viscous in the absence of external forces but assumes much lower viscosity under the influence of shearing forces. Thixotropic behavior is exhibited only by certain alloy compositions and within temperature ranges in which both solid and liquid phase components are present in the alloy. A semi-solid phase is desirable, in which regular, i.e. globular grains are present in the solid phase component and are surrounded by melt.
The processes used to form alloys using a semi-solid process are well known.
As a rule, molten liquid alloys are slowly cooled to a temperature within the dual-phase solid-liquid range using familiar stirring techniques such as MHD (magneto-hydrodynamic stirring) or mechanical stirring in this process. Stirring destroys the dendrites which separate from the melt. It gives the material maximum thixotropic properties and promotes the formation of globular primary crystals in the solid phase.
This process is described for intermetallic materials in U.S. Pat. No. 5,358,687, where TiAl is cited among other materials, although, in contrast to the present invention, no mention is made of subsequent forming processes using mechanical heat reforming steps. The achievable grain size was  greater than 50 xcexcm.
The application of this process to gamma TiAl does not permit economical manufacture, as mechanical wear of the stirrer is too high.
In previous years, semi-finished products consisting of individual steel alloys were produced with extruders on a laboratory scale with structures that exhibited thixotropic properties during subsequent processing in the dual-phase solid-liquid range (dissertation by H. Mxc3xcller-Spxc3xa4th, RWTH Aachen, 1999). However, no encouraging improvements in quality or cost-effectiveness have been achieved in this way.
Unlike steel alloys, intermetallic materials are difficult to handle in deformation processes. The degree of microstructure consolidation achievable in gamma TiAl, in particular, is less than satisfactory. This is reflected in the fact that the deformed and dynamically recrystallized matrix regularly exhibits a banded structure and chemical inhomogeneities resulting from segregation.
Those of skill in the art could not have foreseen that, according to the invention, gamma TiAl alloys reformed into semi-finished products in an initial heat-reforming process would exhibit thixotropic behavior after being heated to a temperature within the solid-liquid range for further shaping processing. Yet the prerequisite is a degree of deformation of  greater than 65%. The deformation degree is defined as follows:
Degree of deformation={(cross-sectional area prior to deformationxe2x88x92cross-sectional area in the deformed state)/cross-section area prior to deformation}xc3x97100 [%].
The level of thixotropic behavior is not satisfactory at low degrees of deformation.
Proof of the advantages described was obtained using a processing sequence that is described in greater detail in the examples for various gamma TiAl alloys.
Gamma TiAl base material produced by VAR (vacuum arc remelting) was deformed via extrusion with a degree of deformation of  greater than 65%. The semi-finished product in the form of a roughly shaped billet was then heated inductively to a temperature between solid and liquid. In this state, the semi-finished product exhibited a sufficient degree of xe2x80x9chandlingxe2x80x9d stability that it could be formed using a thixo-casting process. For this purpose, it was placed in the fill chamber of a die casting machine and pressed into the adjacent die by the pressure cylinder. Under the resulting shearing load, the alloy took the form of a viscous suspension that could be used to form complexly designed parts. This process of pressing the material into the die must take place without material flow turbulence in order to ensure that the material expands without forming pores and blowholes within the casting die.
The use of this shaping process made it possible to eliminate or substantially reduce the need for subsequent mechanical machining, which meant that, in addition to the outstanding structural and mechanical properties of the material, the shaped parts according to the invention could be produced very economically. Compared to molded parts cast directly from a molten mass in a final mold, the advantage of parts made according to the invention lies in their significantly more fine-grained matrix structure and a lower incidence of pore formation.
In order to establish a standard for the grain size of the molded parts manufactured in this way, grain size distribution was determined using the intercepted-segment method and the value d95. This means that 95% of the grains analyzed exhibited a diameter smaller than the value indicated. It should be noted in this context that the grain size of d95 produced a much higher numerical value than would be the case if the value were expressed as the mean grain size.
In matrices with a broad particle-size distribution range, however, d95 is a much more reliable value. Depending upon the composition of the gamma TiAl material and the semi-solid process used, the achievable d95 grain sizes lie with a range of  less than 100 xcexcm to  less than 300 xcexcm.
Molded parts produced for purposes of comparison by fine casting and not further processed through heat-reforming exhibit a matrix with five times the grain size of shaped parts produced in accordance with the invention.
The difference in grain size is especially marked when, in accordance with the preferred embodiment of the invention, alloys with a niobium content of between 1.5 and 12 atom % are used. These alloys exhibit structures that are from 7 to 16 times as fine-grained as those achieved through conventional manufacture using fine casting.
The best results were achieved with gamma TiAl alloys consisting of between 5 and 10 atom % of niobium. An additional degree of fineness was achieved by adding carbon and boron to the alloy in concentrations of up to 0.4 atom %.
Acceptable alternative forming processes for gamma TiAl alloys in accordance with the invention in the solid-liquid phase include thixo-forging and thixo-lateral extrusion, each of which is a familiar, tested process. In thixo-forging, the semi-liquid billet is laid in an open tool or die. The part is formed by a subsequent tool operation, in a forging press, for example.
The thixo-lateral extrusion process is a modified form of thixo-casting. Here, a plug driven by a punch is diverted at a 90xc2x0 angle on its way from the casting chamber to the die or the forming tool.
The invention is described in greater detail with reference to examples of production sequences: