Abstract:
A single-crystal seed, apparatus and process for producing a casting having a single-crystal (SX) microstructure. The seed has a geometry that includes a vertex capable of destabilizing an oxide film that forms at the interface between the seed and a molten metal during the casting process, and thereby promotes a continuous single-crystal grain growth and reduces grain misorientation defects that can initiate from the seed/metal interface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to materials and processes for producing directionally-solidified castings, and particularly to a process and apparatus capable of reducing defects in single-crystal (SX) castings, including but not limited to cast components of gas turbines and other high temperature applications. 
         [0002]    Components of gas turbines, such as buckets (blades), nozzles (vanes) and combustor components, are typically formed of nickel, cobalt or iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is an ongoing effort to develop components, and particularly turbine buckets, nozzles, and combustor components, that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, buckets, nozzles and other components employed in demanding applications are often cast by unidirectional casting techniques to have directionally-solidified (DS) or single-crystal (SX) microstructures, characterized by an optimized crystal orientation along the crystal growth direction to produce columnar polycrystalline or single-crystal articles. 
         [0003]    As known in the art, directional casting techniques for producing DS and SX castings generally entail pouring a melt of the desired alloy into an investment mold held at a temperature above the liquidus temperature of the alloy. One such process is represented in  FIGS. 1 and 2  as an apparatus  10  that employs a Bridgman-type furnace to create a heating zone  26  surrounding a shell mold  12 , and a cooling zone  42  beneath the mold  12 . The zones  26  and  42  may be referred to as “hot” and “cold” zones, respectively, which denote their temperatures relative to the melting temperature of the alloy being solidified. The mold  12  has an internal cavity  14  corresponding to the desired shape of a casting  32  ( FIG. 2 ), represented as a turbine bucket. As such,  FIG. 1  represents the cavity  14  as having regions  14   a ,  14   b  and  14   c  that are configured to form, respectively, an airfoil portion  34 , shank  36 , and dovetail  38  ( FIG. 2 ) of the casting  32 . The cavity  14  may also contain cores (not shown) for the purpose of forming internal structures such as cooling passages within the casting  32 . 
         [0004]    The mold  12  is shown secured to a chill plate  24  and initially placed in the heating zone  26  (Bridgman furnace). The heating zone  26  heats the mold  12  to a temperature above the liquidus temperature of the alloy. The cooling zone  42  is directly beneath the heating zone  26 , and operates to cool the mold  12  and the molten alloy  16  within by conduction, convection and/or radiation techniques. For example, the cooling zone  42  may be a tank containing a liquid cooling bath  46 , such as a molten metal, or a radiation cooling tank that may be evacuated or contain a gas at ambient or cooled temperature. The cooling zone  42  may also employ gas impingement cooling or a fluidized bed. 
         [0005]    An insulation zone  44  defined by a baffle, heat shield or other suitable means is between and separates the heating and cooling zones  26  and  42 . The insulation zone  44  serves as a barrier to thermal radiation emitted by the heating zone  26 , thereby promoting a steep axial thermal gradient between the mold  12  and the cooling bath  46 . The insulation zone  44  has a variable-sized opening  48  that, as represented in  FIG. 1 , enables the insulation zone  44  to fit closely around the shape of the mold  12  as it is withdrawn from the heating zone  26 , through the insulation zone  44 , and into the liquid cooling bath  46 . 
         [0006]    Casting processes of the type represented in  FIGS. 1 and 2  are typically carried out in a vacuum or an inert atmosphere. After the mold  12  is preheated to a temperature above the liquidus temperature of the alloy being cast, molten alloy  16  is poured into the mold  12  and the unidirectional solidification process is initiated by withdrawing the base of the mold  12  and chill plate  24  downwardly at a fixed withdrawal rate into the cooling zone  42 , until the mold  12  is entirely within the cooling zone  42  as represented in  FIG. 2 . The insulation zone  44  is required to maintain the high thermal gradient at the solidification front to prevent nucleation of new grains during the directional solidification processes. The temperature of the chill plate  24  is preferably maintained at or near the temperature of the cooling zone  42 , such that dendritic growth begins at the lower end of the mold  12  and the solidification front travels upward through the mold  12 . 
         [0007]      FIGS. 1 and 2  represent a single-crystal seed  28  within a cavity  50  at the bottom of the mold  12 . The casting  32  epitaxially grows from the seed  28 , such that both the primary and secondary crystal orientations are controlled to yield a single-crystal casting. The seed  28  represented in  FIGS. 1 and 2  has a cylindrical shape, which is conventional for directional casting techniques, though other shapes are known.  FIGS. 1 and 2  further represent a crystal selector  30  coupling the seed cavity  50  to the mold cavity  14 , which ensures that a single crystal enters the cavity  14 . A bridge  40  connects protruding sections of the casting  32  with lower sections of the casting  32  so that crystal nucleation at these protruding locations can be suppressed and a unidirectional columnar single crystal forms substantially throughout the casting  32 . 
         [0008]    Mechanical properties of DS and SX castings depend, to a large degree, on the avoidance of grain misorientation defects, for example, high-angle grain boundaries, equiaxed grains, and other potential defects that may occur as a result of the directional solidification process. The avoidance of such defects in a SX casting depends primarily on whether the crystal orientation of the seed  28  can be successfully extended into the casting  32 . For this purpose, the seed  28  must be properly oriented at the bottom of the mold  12 . In an ideal situation, when the molten alloy  16  is poured into the mold  12  and makes contact with the seed  28 , a portion of the single-crystal seed  28  is re-melted. Then, as the mold  12  is slowly withdrawn from the hot zone  26 , continuous epitaxial grain growth occurs to yield a single crystal article with an orientation dictated by the single-crystal seed  28 . 
         [0009]    Although casting processes of the type represented in  FIGS. 1 and 2  are typically carried out in vacuum, a thin oxide film can form at the interface between the molten alloy  16  and the single-crystal seed  28  if the alloy and/or seed  28  contains elements capable of chemically reacting with residual oxygen in the vacuum chamber. It is understood that this oxide film is ceramic in nature and can prohibit continuous grain growth from the seed  28 , generate misoriented grains, and cause defects in the final casting  32 . The formation of an oxide film at the seed-alloy interface can be inhibited by reducing the availability of oxygen and reactive elements within the alloy. However, most nickel-base superalloys used to form single-crystal castings rely on the presence of aluminum to form Ni 3 Al (gamma prime) as the primary strengthening phase for alloys used to form articles subjected to high stresses in high temperature environments. For example, René N5 (U.S. Pat. No. 6,074,602) contains about 5 to about 7 weight percent aluminum, and CMSX-10 has a nominal aluminum content of about 5.7 weight percent. The oxide films that form during directional solidification of these alloys have been found to typically be aluminum oxide (Al 2 O 3 ) mixed with chromium oxide (Cr 2 O 3 ), nickel monoxide (NiO) and titanium oxide (Ti 2 O 3 ). Due to the high reactivity of aluminum with oxygen, Al 2 O 3  can form at partial pressures of oxygen being as low as 10 −18  torr at a pouring temperature of about 1500° C., which is equivalent to a vacuum of 10 −6  torr if water is assumed to be the only residual gas. However, the vacuum in a Bridgman system is typically not better than 10 −3  torr. Furthermore, oxygen may be present as an impurity in the molten alloy  16  and/or seed  28 , can be present in the mold  12 , and can also form as a result of reactions between the mold  12  and the molten alloy  16 . 
         [0010]    Aside from excluding aluminum from the alloy being cast, attempts to inhibit the formation of an oxide film at the seed-alloy interface have included excluding aluminum from the seed, as reported in U.S. Pat. No. 6,740,176 and U.S. Published Patent Application No. 2010/0058977. However, if aluminum is a required constituent of the seed and/or the casting alloy, it is very difficult to prevent the formation of an oxide film at the seed-alloy interface. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0011]    The present invention provides a process of casting an alloy using a unidirectional casting technique to produce a casting having a single-crystal (SX) microstructure. The invention further provides a single-crystal seed whose geometry is able to automatically destabilize an oxide film that attempts to form at the interface between the seed and the molten metal during the mold filling process, and thereby promotes a continuous single-crystal grain growth and reduces and preferably eliminates grain misorientation defects that would otherwise initiate from the oxide film at the seed/metal interface. 
         [0012]    According to a first aspect of the invention, the seed includes a body having a single-crystal microstructure and at least a first surface region. The first surface region defines a vertex of the body that protrudes away from the body. The vertex is adapted to destabilize an oxide film attempting to form on the first surface region when the first surface region is contacted by a molten metal during a mold filling step of the casting process. The vertex is believed to destabilize the oxide film as a result of surface tension of the oxide film at the vertex being sufficiently high to cause the oxide film to collapse as the oxide film is forming on the first surface region during the casting process. 
         [0013]    According to other aspects of the invention, a casting apparatus and a casting process are provided that utilize the seed described above to cast an alloy. For example, such an apparatus may have a mold having a base and a mold cavity adjacent thereto. The mold cavity is adapted to contain a molten quantity of the alloy during solidification thereof to yield a unidirectionally-solidified casting defined by the mold cavity. A heating zone is provided to heat the mold and the molten quantity of the alloy therein to a heating temperature above the liquidus temperature of the alloy. A cooling zone is provided to cool the mold and the molten quantity of the alloy therein to a cooling temperature below the solidus temperature of the alloy to cause unidirectional solidification of the molten quantity of the alloy and thereby yield the unidirectionally-solidified casting. The single-crystal seed is disposed in the base of the mold and is coupled to the mold cavity so that the molten quantity of the alloy epitaxially solidifies based on a crystallographic orientation of the seed. 
         [0014]    According to another aspect of the invention, a process of casting an alloy includes providing a mold having a base and a mold cavity adjacent thereto, placing a single-crystal seed in the base of the mold, introducing a molten quantity of the alloy into the mold cavity, and then cooling the mold to cause unidirectional solidification of the molten quantity of the alloy within the mold and produce a unidirectionally-solidified casting having a columnar crystal structure. The seed comprises a body having a single-crystal microstructure and at least a first surface region. The first surface region defines a vertex of the body that protrudes away from the body. The molten quantity of the alloy contacts the seed so that the molten quantity epitaxially solidifies based on a crystallographic orientation of the seed. The vertex of the body of the seed destabilizes an oxide film attempting to form on the first surface region as a result of surface tension of the oxide film at the vertex being sufficiently high to cause the oxide film to collapse as the oxide film is forming on the first surface region. 
         [0015]    A technical effect of the invention is the ability to promote the mechanical properties of a casting, and particularly single-crystal castings, that depend primarily on the avoidance of potential defects that can occur during a unidirectional solidification process due to the formation of an oxide film at the interface between the molten metal and a single-crystal seed used to initiate the epitaxial growth required to produce a directionally solidified casting. In particular, a preferred aspect of the invention is that the seed has a geometry capable of destabilizing the oxide film to the extent that the film tends to collapse and does not interfere with the epitaxial grain growth from the seed during the casting process. Consequently, the seed is able to reduce grain misorientation defects that tend to initiate from the seed/metal interface and therefore can improve the yield of single-crystal castings produced by the process. 
         [0016]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1 and 2  represent sectional views showing two steps of a unidirectional casting (solidification) process to produce a single-crystal turbine blade. 
           [0018]      FIG. 3  schematically represents a single-crystal seed conventionally used in unidirectional casting processes in accordance with the prior art. 
           [0019]      FIG. 4  schematically represents a single-crystal seed suitable for use in a unidirectional casting process in accordance with an embodiment of this invention. 
           [0020]      FIGS. 5 and 6  schematically represent single-crystal seeds evaluated during investigations leading to the present invention. 
           [0021]      FIGS. 7 through 10  are microphotographs showing in cross-section four castings produced using the seeds of  FIGS. 3 through 6 , respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention can be employed to produce various castings from a wide variety of alloys, including but not limited to nickel-base, cobalt-base and iron-base superalloy. Certain capabilities of the invention are particularly well suited for producing castings having a columnar single-crystal microstructure (SX). In some cases, a preferred single-crystal direction is &lt;001&gt;, though crystalline structures having orientations other than &lt;001&gt; are also within the scope of the invention. The capabilities of the invention are also particularly well suited for producing castings from alloys that contain levels of reactive elements above incidental or trace amounts that may otherwise be present. Most notably, an alloy may contain aluminum at a level of 0.5 weight percent or more, which renders the alloy reactive to oxygen in the casting environment, including the surrounding atmosphere as well as any oxygen that might be available in the alloy being cast and the mold and cores used to cast the alloy. Other reactive elements of potential concern include titanium, yttrium and rare-earth elements. In addition to aluminum, these elements are commonly found in alloys used to produce cast articles suitable for such applications as the hot gas flow path components of a gas turbine, including but not limited to buckets and nozzles of land-based gas turbines, blades and vanes of aircraft gas turbines, as well as shrouds found in both types of gas turbines. While the advantages of this invention will be described with reference to SX components of a gas turbine, the teachings of this invention can be applicable to other components that may benefit from being unidirectionally cast. 
         [0023]    A SX casting can be produced with the present invention from a melt of the desired alloy, for example, prepared by known vacuum induction melting techniques. The melt is then cast in a mold, in particular an investment mold such as the shell mold  12  used with the apparatus  10  represented in  FIGS. 1 and 2 . As such, the previous discussion of the apparatus  10  can also be applied to the discussion of the present invention though, as discussed below, with at least one notable exception being the single-crystal seed  28  represented in  FIGS. 1 and 2 . The present invention proposes modifications to the seed  28  that are capable of promoting the metallurgical and mechanical properties of the casting  32  beyond what can ordinarily be achieved with conventional unidirectional casting techniques. The invention does not necessarily restrict or otherwise modify other aspects of the apparatus  10 . For example, the mold  12  may be formed of conventional mold materials such as alumina or silica, and cores may be positioned within the mold cavity  14  to form internal passages/features in the casting. Furthermore, liquid metal can be introduced into the mold cavity  14  through a gating system (not shown), and a riser (not shown) may be used to feed the solidification shrinkage of the casting. As such, the following discussion will refer to the apparatus  10  described in reference to  FIGS. 1 and 2 , and aspects of the casting process and apparatus  10  not discussed in any detail below can be, in terms of structure, function, materials, etc., essentially as was described in reference to  FIGS. 1 and 2 . 
         [0024]    As noted above, the present invention is primarily directed to the use of a single-crystal seed that differs from the cylindrical-shaped seed  28  represented in  FIGS. 1 and 2 . The conventional seed  28  is schematically represented in isolation in  FIG. 3 . The seed  28  has a planar circular-shaped upper surface region  50  and a cylindrical lower surface region  52 . As evident from  FIGS. 1 and 2 , the molten alloy  16  poured into the mold cavity  14  comes in contact with the upper surface region  52  of the seed  28 , causing the seed  28  to melt at this surface region  52  and initiate epitaxial growth that is consistent with the orientation of the single-crystal seed  28  as the mold  12  is slowly withdrawn from the hot zone  26  of the apparatus  10 . In this manner, the seed  28  controls both the primary and secondary crystal orientations of the casting  32  characteristic of a single-crystal casting. 
         [0025]      FIG. 4  schematically represents a single-crystal seed  58  in accordance with an embodiment of the present invention. The seed  58  can be seen to be in the form of a body having an upper surface region  60  with a protruding conical shape and, similar to the seed  28  of  FIG. 3 , a lower surface region  62  having a cylindrical shape. The body of the seed  58  is represented as being unitary, though it is foreseeable that lower portions of the seed  58  could differ in shape and composition from the remainder of the seed  58 , and particularly its conical-shaped upper surface region  60 . As a result of its conical shape, the upper surface region  60  defines a vertex (apex)  64  of the body that extends or protrudes away from the remainder of the body, including the cylindrical surface region  62  of the body. When placed in the seed cavity  50 , the vertex  64  faces the mold cavity  12  such that the molten alloy  16  placed in the cavity  14  first comes into contact with the upper surface region  60 , causing initial melting of the seed  58  to occur at the upper surface region  60  and initiate epitaxial growth that results in the single-crystal casting  32 . Notably, whereas the epitaxial growth direction is normal to the flat upper surface region  50  of the conventional seed  28  of  FIG. 3 , the epitaxial growth direction is not normal to any part of the conical-shaped upper surface region  60  of the seed  58  of  FIG. 5 . 
         [0026]    According to a preferred aspect of the invention, the vertex  64  of the upper surface region  60  is capable of destabilizing an oxide film that attempts or begins to form on the interface defined by and between the seed  58  and the molten alloy  16 . Due to a very large surface tension believed to be present at the vertex  64  of the seed  58 , any oxide film that begins to form on the surface region  60  tends to collapse at the vertex  64 , with the result that any oxide film that has formed on the remainder of the surface region  60  will collapse under surface tension. In contrast, an oxide film is able to remain stable as it forms on the flat upper surface region  50  of the conventional seed  28  of  FIG. 3 . 
         [0027]    Due to its conical shape, the upper surface region  60  of the seed  58  is a surface of revolution formed by rotating a segment of a first line around a second line that intersects the first line. In geometric terms, the upper surface region  60  can be described as a lateral surface of the conical portion of the seed  58 . The upper surface region  60  is represented in  FIG. 4  as a right circular cone, in that an axis  66  that passes through the vertex  64  (and therefore about which the upper surface region  60  has rotational symmetry) also passes through the center of the base  68  of the cone at a right angle, and the base  68  is a circle. However, it is foreseeable that the upper surface region  60  could have other conical shapes, such as an oblique cone in which the axis  66  does not pass perpendicularly through the center of the base  68 . Furthermore, the base  68  is not required to be circular, but may have any shape, including rectilinear. The upper surface region  60  is preferably disposed at an angle of about 20 to about 40 degrees from the axis  66 , though lesser and greater angles are foreseeable. In addition, the height of the conical shaped defined by the upper surface region  60  (as defined by the distance between the vertex  64  and the base  68 ) can vary depending on the size of the seed  58  and the particular application in which the seed  58  is to be used, though a suitable height is believed to be in a range of about 0.5 to about 1.5 centimeters. 
         [0028]    Preferred crystallographic orientations for the seed  58  will depend on the particular application, though for producing single-crystal castings it may be preferred that the &lt;001&gt; crystal axis of the seed  58  is oriented parallel to the axis  66 . Similarly, preferred materials for the seed  58  will depend on the particular application, including the particular alloy being cast. Generally, the predominant constituent of the casting alloy will also be the predominant constituent of the seed, for example, the seed  58  will have a nickel-base alloy composition when casting a nickel-base alloy. Notably, the effectiveness of the vertex  64  to destabilize the formation of an oxide film allows for the seed  58  to be formed of an alloy that contains one or more reactive elements, such as aluminum, titanium, yttrium, rare-earth metals, and other potentially reactive elements that would otherwise be of concern to form an oxide film. 
         [0029]    As with the apparatus  10  and process described in reference to  FIGS. 1 and 2 , casting processes performed with the seed  58  of  FIG. 4  are preferably carried out in a vacuum or an inert atmosphere. The mold  12  is preheated prior to introducing the melt of the desired alloy, preferably to a temperature equal to or above the melting temperature of the alloy, and more particularly above the liquidus temperature of the alloy, after which unidirectional solidification is initiated by withdrawing the chill plate  24  and the base of the mold  12  downwardly at a fixed rate through the insulation zone  44  where solidification is initiated, and then into the cooling zone  42  where solidification is completed. The cooling zone  42  may contain a liquid metal cooling bath  46 , or a vacuum or ambient or cooled air for radiation cooling. Depending on particular conditions, a single unidirectional columnar crystal (SX) forms substantially throughout the casting  32 . For example, the seed  58  can be oriented with the seed cavity  50  so that epitaxial growth occurs with the &lt;100&gt; orientation. From the above, it should be appreciated that the overall sequence of the unidirectional solidification process performed with the seed  58  can be similar to unidirectional solidification processes performed with other traditional Bridgman furnaces. 
         [0030]    In investigations leading to the present invention, a melt of an aluminum alloy containing about 5 weight percent copper was prepared, along with single-crystal seeds configured according to the conventional cylindrical seed  28  of  FIG. 3 , the seed  58  of  FIG. 4 , and two additional seeds whose geometries are schematically represented in  FIGS. 5 and 6 . Each seed was formed of essentially the same Al—Cu alloy as the melt. Each of the seeds represented in  FIGS. 5 and 6  has an outer cylindrical shape and an inward conical recess defined in its upper surface, and is therefore essentially the inverse of the outward conical protrusion of the seed  58  represented in  FIG. 4 . The seed of  FIG. 6  differed from that of  FIG. 5  by including a small amount of silica (SiO 2 ) powder in its conical recess. Each of the four seeds had a total height of about 2.0 centimeters from top to bottom, and the cylindrical surface region of each seed had a diameter of about 0.6 centimeter. The height of the conical shape of the upper surface region  60  of the seed  58  was about 0.5 centimeter, and the upper surface region  60  was disposed at an angle of about 30 degrees to the axis  66  of the seed  58 . The depth of the conical shape of each recessed surface region of the seeds shown in  FIGS. 5 and 6  was about 0.5 centimeter, and the recessed surface regions were disposed at an angle of about 30 degrees to the axes of the seeds. 
         [0031]    All four seeds were employed in the same or otherwise identical molding apparatus, and roughly the same amounts of the Al—Cu alloy were unidirectionally solidified using essentially identical processes, including the same growth velocity and temperature gradient. Sections of castings produced with the seeds of  FIGS. 3 through 6  are shown in  FIGS. 7 through 10 , respectively. In  FIG. 7 , corresponding to the conventional cylindrical-shaped seed  28  of  FIG. 3 , an oxide film can be clearly seen at the interface between the cast Al—Cu alloy and the remainder of the seed  28  (following partial melting of its upper surface region  50 ). An oxide film can be similarly seen at the same interface for the Al—Cu alloy castings produced with the seeds of  FIGS. 5 and 6 . In contrast, no oxide film is evident at the interface (or elsewhere) for the Al—Cu alloy casting produced with the seed  58  of  FIG. 4 . From these results, it was concluded that a seed having an upper surface region that defines a vertex is capable of preventing the formation of an oxide film through some mechanism by which the oxide film breaks and/or collapses as it attempts to form. As such, the seed and its vertex have the ability to reduce grain misorientation defects that can initiate from the seed/metal interface. 
         [0032]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the seed  58 , the apparatus  10 , and castings formed therewith could differ from those shown, and the seed  58  could be used in a casting process that differs from what was described above in reference to the apparatus  10 . Therefore, the scope of the invention is to be limited only by the following claims.