Abstract:
Methods of processing compositions containing titanium and aluminum, especially titanium aluminide intermetallic compositions (TiAl intermetallics) based on the TiAl (gamma) intermetallic compound. The methods entail processing steps that include a hot isostatic pressing (HIP) cycle and a heat treatment cycle that can be performed in a single vessel. TiAl intermetallic compositions processed in this manner preferably exhibit a duplex microstructure containing equiaxed and lamellar morphologies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/614,751, filed Mar. 23, 2012, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to compositions containing titanium and aluminum and the processing thereof. More particularly, this invention relates to methods of processing cast titanium aluminide intermetallic compositions that entail hot isostatic pressing and heat treatment to close porosity and yield a desirable microstructure. 
         [0003]    Because weight and high temperature strength are primary considerations in gas turbine engine design, there is a continuing effort to create relatively light weight alloys/compositions that have high strength at elevated temperatures. Titanium-based alloy systems are well known in the art as having mechanical properties that are suitable for relatively high temperature applications. High temperature capabilities of titanium-based alloys has increased through the use of titanium intermetallic systems based on the titanium aluminide compounds Ti 3 Al (alpha-2 (α- 2 ) alloys) and TiAl (gamma (γ) alloys). These titanium aluminide intermetallic compounds (or, for convenience, TiAl intermetallics) are generally characterized as being relatively light weight, yet are known to be capable of exhibiting high strength, creep strength and fatigue resistance at elevated temperatures. Additions of chromium and niobium are known to promote certain properties of TiAl intermetallics, such as oxidation resistance, ductility, strength, etc. As a nonlimiting example, U.S. Pat. No. 4,879,092 to Huang discloses a titanium aluminide intermetallic composition having an approximate formula of Ti 46-50 Al 46-50 Cr 2 Nb 2 , or nominally about Ti-48 Al-2 Cr-2 Nb. This alloy, referred to herein as the 48-2-2 alloy, is considered to have a nominal temperature capability of up to about 1400° F. (about 760° C.), with useful but diminishing capabilities up to about 1500° F. (about 815° C.). In gas turbine engines used in commercial aircraft, the 48-2-2 alloy is well suited for low pressure turbine blade (LPTB) applications. 
         [0004]    The production of components from TiAl intermetallics is complicated by their relatively low ductility and the typical desire for these compositions to be extrudable, forgeable, rollable and/or castable. Hot isostatic pressing (HIP) is commonly performed to eliminate internal voids and microporosity in titanium aluminide intermetallic castings. Because uncontrolled cooling rates typically performed following HIP are not effective to generate a desired microstructure, responsiveness to post-HIP heat treatments is another desirable characteristic in order to achieve microstructures and mechanical properties needed for specific applications. 
         [0005]    HIP cycles are typically separate from the heat treatment cycle in the processing of castings. As an example, desired microstructures and mechanical properties have been obtained in castings of the 48-2-2 alloy using a process represented in  FIG. 3 . Following the production of the casting, a pre-HIP heat treatment is performed at a temperature within a range of about 1800 to about 2000° F. (about 980 to about 1090° C.) and for a duration of about five to twelve hours. Thereafter, the casting is cooled and transferred to a HIP chamber and then subjected to a high pressure HIP step (for example, 25 ksi (about 1720 bar) or more) at about 2165° F. for a duration of about three hours. The HIPed casting is then cooled, removed from the HIP chamber, and then subjected to a post-HIP solution treatment at a temperature of about 2200° F. for a duration of about two hours. This sequence requires the use of at least two different vessels and loading and unloading the casting three times from these vessels. In addition to incurring additional cost and cycle time, this process has been associated with the loss of aluminum from the casting surface, which leads to reduced environmental and/or mechanical properties. 
         [0006]    Unexpectedly, net-shape castings that have been produced, for example, by spin casting from the 48-2-2 alloy to produce low pressure turbine blades have not responded well to the heat treatment process described above, or to other processes employed with conventional TiAl castings, such as gravity casting and overstock casting. In particular, the 48-2-2 alloy net-shape castings processed by net-shape casting methods do not develop a desirable duplex microstructure containing equiaxed and lamellar gamma TiAl morphologies that improve the ductility of the casting, particularly when the volume fraction of the lamellar structure is about 10 to about 90 percent, particularly if the volume fraction of the lamellar structure is about 20 to about 80 percent and ideally about 30 to about 70 percent.  FIGS. 1 and 2  are photomicrographs showing desirable duplex microstructures present in two conventional TiAl castings. 
         [0007]    In view of the above, a method is needed that is capable of processing TiAl intermetallics, including but not limited to net-shape geometries of the 48-2-2 alloy, to yield a duplex microstructure containing equiaxed and lamellar morphologies. It would be further desirable if such a method did not require a sequence in which a casting is not required to be transferred between multiple different vessels. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    The present invention provides methods capable of processing compositions containing titanium and aluminum, and especially titanium aluminide intermetallic compositions (TiAl intermetallics) based on the TiAl (gamma) intermetallic compound, to yield desirable microstructures. The methods have the further capability of being performed in a single vessel, resulting in a less complicated process than conventional methods used to produce compositions that require void closure (for example, by HIPing) and heat treatment. 
         [0009]    According to a first aspect of the invention, a method of processing a titanium aluminide intermetallic composition includes hot isostatic pressing the composition at a temperature of at least 1260° C. (about 2300° F.), cooling the composition to a temperature of not less than 1120° C. (about 2050° F.), heat treating the composition at a temperature of about 1150 to about 1200° C. (about 2100 to about 2200° F.), and then cooling the composition to room temperature. Following the above procedure, the titanium aluminide intermetallic composition exhibits a desirable duplex microstructure containing equiaxed and lamellar morphologies of the gamma TiAl phase. 
         [0010]    According to a second aspect of the invention, an alternative method of processing a titanium aluminide intermetallic composition includes hot isostatic pressing the titanium aluminide intermetallic composition, cooling the composition, heat treating the composition at a temperature of at least 1260° C. (about 2300° F.) for about 2.5 to about 5 hours, cooling the composition to a temperature of not less than 1120° C. (about 2050° F.), holding the composition at a hold temperature of about 1150 to about 1200° C. (about 2100 to about 2200° F.) for a duration of about two to about six hours, and then cooling the composition to room temperature. Following this procedure, the titanium aluminide intermetallic composition exhibits a desirable duplex microstructure containing equiaxed and lamellar morphologies of the gamma TiAl phase. 
         [0011]    A technical effect of the invention is the ability to produce desirable duplex microstructures in TiAl intermetallics that may otherwise be difficult to obtain, particularly if produced by net-shape casting methods such as spin casting and possibly certain other casting techniques. Another technical effect is the ability to take advantage of the energy available for phase equilibration during cool down from a HIP step to assist in a subsequent heat treatment, which has been determined to eliminate the requirement for conventional pre- and post-heat treatment cycles that may cause aluminum to be lost from the casting surface as well as incur additional cost and cycle time. These advantages have been particularly observed with net-shape castings produced by net-shape casting methods, such as spin casting, in the aforementioned 48-2-2 alloy, though other TiAl intermetallic compositions also benefit from the processing methods provided by the present invention. 
         [0012]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1 and 2  are photomicrographs showing the microstructures of two castings formed of a TiAl intermetallic composition with a desirable duplex microstructure. 
           [0014]      FIG. 3  is a flow chart representing a method of processing castings formed of TiAl intermetallic compositions in accordance with a prior art HIP and heat treatment process. 
           [0015]      FIGS. 4 and 5  are flow charts representing two methods of processing castings formed of TiAl intermetallic compositions in accordance with embodiments of the present invention. 
           [0016]      FIGS. 6 and 7  are microphotographs showing the microstructures of two castings formed of the same TiAl intermetallic composition, wherein the casting of  FIG. 6  was processed in accordance with the prior art HIP and heat treatment process of  FIG. 3  and the casting of  FIG. 7  was processed in accordance with the HIP and heat treatment process of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIGS. 4 and 5  contain flow charts that represent two related methods by which TiAl intermetallic compositions, including but not limited to the 48-2-2 alloy, can be processed to yield a desirable duplex microstructure, with the additional benefit of avoiding the disadvantages of the prior art process summarized in  FIG. 3 . In particular, the methods of  FIGS. 4 and 5  avoid the pre- and post-HIP vacuum heat treatments that are believed to promote the loss of aluminum in TiAl intermetallic compositions. The invention also takes advantage of the high gas pressures and protective (inert) atmospheres used during HIP, the combination of which is believed to be capable of reducing the loss of aluminum in a TiAl intermetallic composition. Furthermore, each of the methods summarized in  FIGS. 4 and 5  provide for interrupted cooling from a HIP step ( FIG. 4 ) or a temperature that is believed to take advantage of the non-equilibrium phase distribution in TiAl intermetallic compositions following HIP ( FIG. 5 ) to generate (during a subsequent heat treatment) microstructures that are capable of providing desirable mechanical properties, especially if the TiAl intermetallic composition is a cast using a net-shape casting process, such as spin casting or other means. 
         [0018]    As noted above, the processes summarized in  FIGS. 4 and 5  are believed to be particularly beneficial to the 48-2-2 alloy, whose composition is based on the gamma (TiAl) intermetallic compound. Castings of the 48-2-2 alloy exhibit improved ductility and other desirable properties if they contain a duplex microstructure containing equiaxed and lamellar gamma phase morphologies.  FIGS. 6 and 7  are representative of LPTB castings produced from the 48-2-2 alloy. Both castings were produced by spin casting, the casting in  FIG. 6  was processed by a HIP and heat treatment procedure corresponding to that represented in  FIG. 3 , and the casting in  FIG. 7  was processed by a modified HIP and heat treatment procedure corresponding to that represented in  FIG. 4 . The microstructure of the heat treated casting shown in  FIG. 6  possesses an excessive amount of equiaxed gamma phase and an inadequate amount of the lamellar phase (less than 10% volume fraction of the lamellar phase). Such a microstructure would yield a component with insufficiently high temperature creep strength. The microstructure of the heat treated casting shown in  FIG. 7  has acceptable amounts of the equiaxed gamma phase and the lamellar phase (about 20% volume fraction of the lamellar phase), the sole exception being at the outermost surface of the casting where titanium levels are depleted. However, the outermost surface can be removed by conventional techniques, such as abrasive blasting or chemical milling, with the result that the entire remaining casting would contain acceptable amounts of the equiaxed gamma phase and lamellar phase. 
         [0019]    While the invention has been shown to yield particularly advantageous results with the 48-2-2 alloy, the invention is believed to be more generally applicable to titanium aluminide intermetallic compositions, particularly TiAl (gamma) intermetallic compositions modified with elements that are intended to promote various properties. For example, the invention has also been shown to be effective with TiAl intermetallic compositions that contain tantalum. Particular compositions that have been successfully evaluated include TiAl compositions that contain chromium, niobium and/or tantalum, for example, about 1.8 to about 2 atomic percent chromium, up to about 2 atomic percent niobium, and up to about 4 atomic percent tantalum. Specific compositions that were successfully evaluated contained, in atomic percent: about 47.3% aluminum, about 1.9% chromium, about 1.9% niobium and the balance titanium and incidental impurities (roughly corresponding to the 48-2-2 alloy); or about 47.3% aluminum, about 1.8% chromium, about 0.85% niobium, about 1.7% tantalum and the balance titanium and incidental impurities; or about 47.3% aluminum, about 2.0% chromium, about 4.0% tantalum and the balance titanium and incidental impurities. More generally, the levels of titanium and aluminum in these TiAl intermetallic compositions are selected to yield a casting whose predominant constituent is the TiAl (gamma) intermetallic compound. While the compositions evaluated all contained about 47.3 atomic percent aluminum and about 46.7 to 48.9 atomic percent titanium, those skilled in the art will appreciate that aluminum and titanium levels beyond these amounts can be used to yield a casting that is entirely or predominantly the TiAl intermetallic compound, and such variations are within the scope of the invention. Furthermore, those skilled in the art will recognize that other alloy constituents could be included to modify the properties of the TiAl intermetallic compound, and such variations are also within the scope of the invention. 
         [0020]    During investigations leading to the present invention, solidification modeling was conducted that suggested that areas of low pressure turbine blade (LPTB) castings formed by net-shape casting, including spin casting, solidified in less than a few seconds. It was concluded that, compared to other casting methods and/or other types of castings, such a rapid solidification rate may modify the route through the Ti-Al phase diagram that the alloy/composition takes during solidification and may lead to unexpected responses to conventional heat treatments that are subsequently performed on the castings. These unexpected results negatively impact the uniformity of the microstructure of net-shape cast and heat treated components, such as the chemistry and uniformity of the microstructure over the full chord and span in net-shape TiAl airfoils. The process represented in  FIG. 4  combines a HIP cycle with a heat treatment without cooling to room temperature therebetween, which reestablishes phase equilbria that are capable of developing a duplex microstructure that provides desirable mechanical properties. 
         [0021]    The process of  FIG. 4  generally entails preparing a TiAl intermetallic composition. A preferred but not limiting example entails spin casting an appropriate melt containing the desired constituents of the TiAl intermetallic composition. The composition (casting) is then loaded in a suitable HIP chamber and heated in a protective atmosphere (for example, argon or another inert gas) to a temperature at which the casting is to undergo HIPing. According to a preferred aspect of the invention, the HIP temperature (T HIP1 ) is at least 2300° F. (about 1260° C.), more preferably at least 2350° F. (about 1290° C.), and most preferably in a range of about 2375 to about 2425° F. (about 1300 to about 1330° C.). The pressure applied to the casting during the HIP cycle is intended to eliminate internal voids and microporosity in the castings. For this purpose, pressures of at least 15 ksi (about 1030 bar) are believed to be sufficient, with pressures of about 18 ksi (about 1240 bar) and higher believed to be particularly preferred. The duration of the HIP cycle may vary depending on the particular composition and pressure used, but suitable results are believed to be obtained with HIP cycles having durations of about 2.5 to about 5 hours, and particularly about 2.5 to about 3.5 hours. 
         [0022]    Following the HIP cycle, the casting is cooled to a temperature of not less than 2050° F. (about 1120° C.), more preferably not less than 2100° F. (about 1150° C.), and most preferably about 2100 to about 2150° F. (about 1150 to about 1175° C.). The cooling rate may vary, but rates of about 5 to about 20° F./minute (about 3 to about 11° C./minute) have been found to be acceptable. Without needing to be removed from the HIP chamber, the casting then undergoes a heat treatment at a temperature of about 2100 to about 2200° F. (about 1150 to about 1200° C.), for example, about 2100 to about 2150° F. (about 1150 to about 1175° C.). The duration of this heat treatment may vary depending on the particular composition and HIP treatment used, but suitable results are believe to be obtained with heat treatment cycles having durations of about two to about six hours, and especially about 4.5 to about 5.5 hours. 
         [0023]    Following heat treatment, the casting can be cooled directly to room temperature (about 20 to about 25° C.) at any desired rate. At the result of this process, the TiAl intermetallic casting preferably exhibits a duplex microstructure of the type seen in  FIG. 7 . From the above, it should be evident that the casting is not required to be removed from the HIP chamber during the steps identified in  FIG. 4 , and that the casting can be continuously exposed to the inert atmosphere of the HIP chamber throughout the process represented in  FIG. 4 . 
         [0024]    The process set forth in  FIG. 5  differs from that set forth in  FIG. 4  by the allowance of a full cool down (to room temperature) between the HIP cycle and the heat treatment. The process of  FIG. 5  additionally involves heating the casting to the T HIP1  temperature prior to the heat treatment. This process is believed to allow more flexibility in the temperature used for the HIP cycle, in that HIPing is not required to be performed at the T HIP1  temperature of  FIG. 4 , but instead can be at a temperature (designated as T HIP2 ) that can be higher or lower than the temperatures within the ranges stated above for T HIP1 . 
         [0025]    In view of the above, the process set forth in  FIG. 5  generally entails HIPing a TiAl intermetallic composition (typically a casting) at a suitable temperature (T HIP2 ), which can be followed by cooling the casting to essentially any temperature (including room temperature). Thereafter, the casting is heat treated at the T HIP1  temperature (for example, at least 2300° F. (about 1260° C.)) for a duration sufficient to ensure the entire casting is at T HIP1 . The casting can then be cooled at a suitable rate (for example, about 5 to about 20° F./minute (about 3 to about 11° C./minute)) to a temperature of not less than 2050° F. (about 1120° C.), more preferably not less than 2100° F. (about 1150° C.), and most preferably about 2100 to about 2150° F. (about 1150 to about 1175° C.). The casting can then be subjected to the same heat treatment as described for the process of  FIG. 4 , after which the casting can be cooled directly to room temperature (about 20 to about 25° C.). As the result of this process, the TiAl intermetallic casting preferably exhibits a duplex microstructure of the type seen in  FIG. 7 . As with the process of  FIG. 4 , it should be evident that the casting is not required to be removed from the HIP chamber for any step of  FIG. 5 , and that the casting can be continuously exposed to the inert atmosphere of the HIP chamber throughout the process represented in  FIG. 5 . 
         [0026]    While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.