US3915761A

US3915761A - Unidirectionally solidified alloy articles

Info

Publication number: US3915761A
Application number: US254125A
Authority: US
Inventors: Johann G Tschinkel; Anthony F Giamei; Bernard H Kear
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1971-09-15
Filing date: 1972-05-17
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

Unidirectionally solidified alloy articles, formed by high rate solidification techniques, are provided with a hyperfine dendritic structure and superior strength, ductility and fatigue resistance.

Description

United States Patent Tschinkel et a1. Get. 28, 1975 UNIDlRECTlONALLY SOLIDIFIED ALLOY 3,376,915 4/1968 Chandley 164/127 ux ARTICLES 3,485,291 12/1969 Piearcey 164/127 3,532,155 10/1970 Kane et a1 164/60 [75] Inventors: Johann G. Tschrnkel, South 3,564,940 2/1971 Thompson et a1... 75/134 P Glastonbury; Anthony F. Giamei, 3,620,288 11/1971 Barrow et a1. 164/127 X Middletown; Bernard H. Kear, 3,635,769 1/1972 Shaw 148/32 Madison all of Conn 3,671,223 6/1972 Thompson et a1. 75/l22 3,677,835 7/1972 Tien et al 148/325 Asslgneel Unlted Technologies Corporation, 3,690,367 9 1972 133111615 164/127 x Hartford, Conn. 3,783,033 1/1974 Tarshis .1 75/170 [22] Filed: May 17, 1972 [21] Appl. No.: 254,125 Primary Examiner- L. Dewayne Rutledge Assistant Examiner O. F. Crutchfield Relaed Apphcatlon Data Attorney, Agent, or FirmRichard N. James [63] Continuation-in-part of Ser. No. 180,597, Sept. 15,

1971, Pat. No. 3,763,926.

[52] US. Cl. 148/32; 75/170; 75/171; [57] ABSTRACT 164/ 127 [51] Int. Cl. C22C 19/03 Unidirectionally solidified alloy articles f by Field 0 arch 164/127, 60; 148/ 0 high rate solidification techniques, are provided with a 148/32; 75/171, 170 hyperfine dendritic structure and superior strength,

ductility and fatigue resistance. [56] References Cited UNITED STATES PATENTS 2 Claims, 8 Drawing Figures 3,248,764 5/1966 Chandley 148/100 X OOOQQOQ U.S. Patent Oct. 28, 1975 Sheet 2 of4 3,915,761

US. Patent Oct.28, 1975 Sheet 3 of4 3,915,761

a \w 1/ W f y S. Patent Oct. 28, 1975 Sheet4of4 3,915,761

UNIDIRECTIONALLY SOLIDIFIED ALLOY ARTICLES This application is a continuation-in-part of application Ser. No. 180,597 filed Sept. 15, 1971 now U.S. Pat. No. 3,763,926.

BACKGROUND OF THE INVENTION The present invention relates to directionally solidified superalloy castings and, more particularly, to superalloy castings characterized by a columnar grained or monocrystalline structure.

It is now well known that dramatic improvements in the properties of the superalloys can be achieved through unidirectional casting techniques. See, for example, the teachings of the U.S. Pat. Nos. of Ver Snyder 3,260,505 and Piearcey 3,474,709.

In the most conventional directional casting operations, solidification rates are typically limited to 4-12 inches per hour. While the ensuing columnar grained and monocrystalline castings possess near optimum grain morphology and superior overall properties for elevated temperature applications, the castings nevertheless exhibit substantial dendritic segregation. Depending on the particular superalloy chemistry such segregation can result in the formation of low melting point or brittle phases, nonuniform distribution of strengthening precipitates, interdendritic porosity and surface freckles. One or more of these structural manifestations of dendritic segregation can be undesirable.

The conventional methods for minimizing the presence or effects of dendritic segregation, including solid state diffusion heat treatments or mechanical working, are not truly feasible for use with the constitutionally complex superalloys as cast by normal directional solidification techniques.

The dendrites formed within the single crystal or the columnar grains in the cast article are distinguished from the surrounding material by differences in concentration of some constituents. Embedded carbide particles and eutectic microconstituents, for example, tend to accumulate in the normally weaker interdendritic regions and the strength of the alloy is decreased by such inhomogeneities. The size of the embedded particles and pools of such microconstituents is significantly reduced by a reduction in dendritic spacing in the casting. After the casting is completed, it is desirable to homogenize the cast alloys by heating them at a temperature close to the solidus temperature. Since diffusion in solids is a slow process, this homogenization of the alloy may require hundreds of hours where the dendritic spacing is relatively large so that normally complete homogenization of the conventional dendritic structure is not practical. The diffusion time for complete homogenization at a given temperature is proportional to the square of the distance between the dendrites so that a reduction in dendritic spacing by a factor of can reduce the annealing time by a factor of 100, thereby bringing the required time for complete difliision down to a few hours. In this way the homogenization treatment would become a practical procedure. The spacing of the dendrites is significantly reduced by more rapid solidification of the material being cast.

Another approach to the dendritic segregation prob lem is that suggested by Tien et al in copending application Ser. No. 81,229 filed Oct. 16, 1970, now U.S. Pat.

No. 3,677,835 for Homogeneous Nickel Base superalloy Castings where cellular/plane front solidification techniques provide a structure substantially free from dendritic segregation.

SUMMARY OF THE INVENTION Utilizing high solidification rates in excess of about 25 inches/hour, unidirectionally solidified nickel-base superalloy articles are provided having primary dendrite spacings of less than about 0.005 inch together with typical 'y/y' eutectic pool and MC carbide dimensions less than about 0.001 and 0.002 inches, respectively. As so provided, the hyperfine dendritic structures of the present invention, in addition to mechanical property improvements, provide possible economic advantages as well as reducing solutionizing heat treatment (eutectic dissolution) times from several hours to several minutes and making true homogenization (elimination of essentially all dendritic segregation) possible in feasible times. by appropriate heat treatment.

The very rapid heat removal from the mold of the apparatus disclosed herein in conjunction with a sharp transition between the hot and cold surroundings, in order to maintain a high thermal gradient, provides a high growth rate for making the cast article. One feature is the use of a liquid coolant into which the mold is immersed or which is gradually poured around the mold for the rapid extraction of heat from the mold, thereby obtaining the desired grain growth within the mold. Another feature is the use of this liquid coolant to surround all of the several molds in a multiple mold casting so that the heat removal from the several molds will be the same and, accordingly, the desired grain growth will be obtained within all of the molds. A particular feature of the invention is the control of the dendritic growth within the casting in such a way as to significantly reduce the distance between the dendrites and thereby the segregation of the microconstituents in the interdendritic regions.

According to the disclosure, the apparatus includes a heating chamber within which the mold is positioned for raising the mold to a high temperature above the melting temperature of the material to be cast, a container for a liquid bath below the heating chamber and in which the mold is immersed, a device for filling the mold, and a device for moving the mold relative to the chamber and container for gradually immersing the filled mold into the cooling liquid and simultaneously withdrawing it from the heating chamber. The process is carried out by heating the mold before filling to a temperature above the melting temperature of the material to be cast, pouring the molten material into the mold and then gradually withdrawing the mold from the heating area and simultaneously submerging it gradually into a liquid cooling bath, thereby establishing a steep thermal gradient in the material in the mold and causing a vertical solidification of the material in the mold from the base of the mold to the top at a controlled rate.

An alternative but less desirable form of the apparatus provides for gradually filling the container with a liquid coolant and optionally reducing the heat supplied to the mold as solidification proceeds from bottom to top. In either form of the invention, the filled mold is gradually surrounded with a cooling liquid from bottom to top of the mold and at the same time, the

heat supplied to the mold is gradually reduced from bottom to top by withdrawal of the mold or by a stepby-step reduction in the heat input to the mold as the level of the cooling bath effectively moves upward around the mold.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view through the preferred solidification apparatus.

FIG. 2 is a fragmentary vertical sectional view of a modification.

FIG. 3 is a fragmentary vertical view of another modification.

FIG. 4 is a transverse sectional view through a multiple mold showing the effectiveness of liquid cooling.

FIG. 5 is a vertical sectional view through a modified form of apparatus.

FIG. 6 is a transverse microstructure of a single crystal cast conventionally.

FIG. 7 is a similar view at the same magnification of a single crystal cast by the present invention.

FIG. 8 is a similar view at the same magnification of a single crystal cast at a faster cooling rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT by induction heating coils 10 by which the susceptor may be heated with the latter in turn heating the mold prior to the mold filling operation. Suitable heat shields 12 are positioned at the lower end of the susceptor sleeve 8 near the periphery of the chill plate and other heat shields 14 close the upper end of the chamber 16 formed within the susceptor 8 and within which the mold is positioned. These heat shields 14 are in the form of a removable cover. A pouring cup 18 may be positioned in the shield 14 at the top of the chamber.

Positioned below the heating chamber 16 is a tank 20 which holds a liquid 22. The tank 20 may have heating elements 24 surrounding it for raising the temperature of the bath to the desired temperature for immersion of the mold therein and the chamber is also preferably surrounded by cooling coils 26 adjacent the upper end of the tank for the purpose of maintaining the desired temperature within the bath of liquid especially as the mold is immersed therein during the solidification process. Suitable stirring means 28 may be provided to assure a circulation of the liquid bath when the casting process is being carried out. The tank may be secured to the wall of the vacuum chmnber in which the apparatus is positioned.

The position of the heating and cooling coils 24 and 26 around the tank serves to create and strengthen convective currents in the liquid bath to circulate the liquid and thereby maintain a more nearly constant temperature for the portion of the bath in which the mold is being immersed. The effect of the immersion of the mold is to heat the surrounding liquid rapidly causing an upward flow toward the surface. The cooling coils, near the top of the liquid bath, serve to cool the adjacent liquid and cause a downward flow along the inner surface of the tank toward the middle. At this point the liquid is again heated by the heating coils and an upward flow near the middle of the tank is caused. Thus, in some instances, the circulation of the liquid bath by the stirring device may be omitted. It will be understood that the drawing shows the several parts merely diagrammatically and suitable support means are provided for maintaining the tank 20 in a predetermined relationship with the heating chamber thereabove. The level of the liquid bath 22 is preferably such that it partially submerges the support plate 4 therein when the mold is within the heating chamber for the heating and pouring operation and in this way this plate serves as an effective chill plate without the need for a circulation of coolant through the plate.

The mold is preferably of the well-known shell mold type and as shown is a multiple mold and has two or more article forming portions 30 positioned on opposite sides of a central carrying cylinder 32. The article portions 30 are shown with a cavity in the shape of a turbine blade by way of example. The latter is of a dimension to fit around the vertical shaft 6 as shown. Between each of the article portions of the mold and the central sleeve 32 are vertically filling tubes 34 communicating at their top ends with a filling ring 36, the latter at one point being positioned directly below the pouring cup 18. Each article portion of the mold has an upward projecting riser portion 38 terminating at a point at least as high as the top of the filling ring 36. Below and communicating with the article forming portion of the mold is a growth zone including a crystal selector 40 which may be a helix defining therein a helical passage for selecting a single crystal to grow into the article portions. The helical passage terminates at the bottom in the main growth zone 42 in which columnar grains are grown. The filling tubes 34 communicate with the growth zone 42 as shown. Thus, when alloy is poured into the cup 18, it flows into the ring 36 through the tubes 34 into the growth zone and thence upward through the crystal selector to fill the article portion of the mold and upward into the riser. This mold arrangement is suitable for making single crystal articles.

Referring now to FIG. 2, a portion of a mold is shown which is adapted for making columnar grained cast articles instead of single crystal articles. To do this, the article mold 30' has the riser 38' at the top and a growth zone 42' at the bottom open to the chill plate. The crystal selector of FIG. 1 is omitted and the growth zone communicates directly with the bottom end of the article mold portion, the dividing line being represented by the dashed line 43, FIG. 2, and it is along this line that the growth portion of the casting would be severed from the article itself.

Crystalline structures of other orientations than 00l may be made by the use of a mold as shown in FIG. 3. In this arrangement, the article mold portion 30" has the riser 38" at the top, and a growth zone 42" at the bottom. This growth zone receives a single crystal slug 46 of the desired orientation and the base of this slug is preferably set into a recess 48 in the support plate 4 so that this slug will not be totally melted during heating of the mold. when the alloy is poured, single crystal growth occurs with the dendritic orientation throughout the article the same as that of the slug 46. In FIG. 1 the crystal selector is considered a part of the growth zone when making single crystal castings.

One particularly suitable liquid for use in the cooling process is tin because of its low vapor pressure and because of its low melting temperature (450F). A suitable temperature for the tin bathis about 500F since clearly the lower the temperature of the bath the faster the cooling rate will be. As above stated, the plate 4 is in the molten superalloy which could otherwise lead to a solidification defect known as freckles. In using a superalloy for making turbine blades, if the blade is 4 inches in length, for example, and the height partially immersed in the tin bath at the start of the casting operation and serves as a chill plate.

The process is desirably carried out in a vacuum or tus in the position of FIG. 1 and with the mold in position resting on and suitably secured to the support plate to prevent leakage of molten material from within the mold, the latter is heated by energizing the induction coils to raise the temperature of the mold itself at least to the melting temperature of the alloy and preferably to a temperature as much as about 300F above the melting temperature. Where the article to be cast is a turbine blade which is the shape shown for the article portion of the mold in FIGS. 1 and 2 of the drawings, one superalloy suitable for the purpose is MAR-M 200 although many other alloys are equally suitable as described, for example, in the US. Pat. Nos. to VerS- nyder 3,260,505 and Piearcey 3,494,709 and also in the US. Pat. No. to Gell et al 3,567,526.

The alloy to be cast is heated to a point about 300F above the normal melting point of the alloy so that it has a significant superheat. With the mold above the melting point of the alloy and with the alloy itself superheated to this extent, the alloy is poured into the mold, filling the mold at least to a point above the article portion of the mold and preferably substantially to the level of the pouring ring 36. Since the temperature of the support plate 4 is kept substantially at the temperature of the liquid bath, dendritic growth immediately begins within the growth zone 42 of the mold and as solidification continues upward through the growth zone, the grain growth becomes columnar as described in the Piearcey patent. Almost immediately after the alloy is poured and when grain growth has begun, the support plate with the mold thereon is gradually lowered from the heating chamber 16 so that the support plate is completely immersed and then the mold is gradually immersed within the liquid cooling bath. As the mold moves downward into the bath, the liquid coolant flows over the surface of the support plate and around the various portions of the mold. Since the coolant is in contact with all the outer surfaces of the mold, it completely surrounds the mold and rapidly removes heat from all portions of the mold thereby increasing the rate of solidification of the alloy in a vertical direction. The grain selector 40 functions in much the same manner as the crystal selector in the Piearcey patent to cause the growth of a single crystal from the main growth portion into the article forming portion of the mold.

The mold is gradually and continually moved downward into the liquid bath at such a rate that the level of the cooling bath does not precede the solidus level by any substantial amount so that the removal of heat from the mushy zone of the solidifying alloy is vertically downward and the liquid-solid interface will not have excessive curvature. This will assure the growth of a single crystal within the article portion of the mold and prevent nucleation of spurious grains along the surfaces of the mold. The high resulting axial thermal gradient of the growth Zone 42 is preferably at least an inch, the total height of the mold including the riser would be 8 inches. In a specific casting operafion in making a single crystal blade, this mold is heated to 2850F except for the portion closely adjacent to the support plate. The alloy is heated to 2850F and is then poured into the mold which is at this time positioned on the support plate 4 and within the heating chamber. The support plate and the mold thereon are held in the position shown for up to five minutes for the start of the columnar growth in the growth zone before a downward vertical movement of the chill and mold into the liquid tin at 500F is started. The downward movement of the chill and mold is carried out at a uniform rate of inches per hour until the mold is immersed to a point at least one inch above the top of the article portion of the mold thereby assuring a growth of a single crystal through the entire article forming portion of the mold.

Since the distance that the mold must move down ward to be immersed to this extent within the liquid tin bath is 6 inches, it will be apparent that the complete operation for adequate immersion of the mold requires only 3 minutes plus the holding time from thetime of pouring the mold for a completion of the solidification process. The power supplied to the induction coil 10 is then substantially reduced. The mold is then withdrawn upward and the device is preferably so constituted that the mold is drawn up through the heating chamber to a point thereabove, the heat shields 14 being carried upward therewith by a support collar 44 on the shaft. With the mold and support plate completely above the apparatus shown, removal of the mold from its position on the support plate is done by unscrewing the plate and retracting the mold from its position around the shaft. Any suitable mechanism not a part of this invention may be provided for this purpose. Obviously, the suspension shaft could be moved laterally to position the mold and plate over a suitable bench rather than over the hot chamber 16.

The heating coils are continuously energized during immersion and, therefore, the susceptor 8 is retained at its high heat during the downward movement of the mold into the chill plate so that, above the level of the bottom of the susceptor, the mold is still kept near the 2850F temperature. In this way a very high thermal gradient is maintained in the material within the mold between the level of the bottom of the susceptor and the top of the tin bath. That is to say, the mold is surrounded by a temperataure above the melting point of the alloy throughout the entire height of the susceptor and the lower portion of the mold is immersed in a cooling bath at 500F at a very short distance below the bottom end of the susceptor thereby establishing this very high thermal gradient. The steepness of the thermal gradient at the interface is determined to a great extent by the spacing of the susceptor above the surface of the bath, by the temperature and effectiveness of the bath, and by the alloy superheat.

Further, the rate of the upward movement of the liquid-solid interface, the growth rate, is determined by the rate of downward movement of the mold into the liquid bath. Since the bath is in contact with the outer surfaces of the mold, the rate of heat withdrawal from the mold and thus from the alloy at and below the surface of the liquid bath by conduction will be extremely rapid. It is desirable to have a relatively thin mold wall thereby to improve the heat transfer rate and thus the wall thickness of the mold will be limited by the strength needed to withstand the pressure of the material within the mold during the casting process.

Instead of withdrawing the mold from the heating chamber and immersing it in a liquid coolant bath, the mold may be cooled gradually by pouring the cooling liquid into a chamber surrounding the mold. As shown in FIG. 5, the mold 50, which is shown as a single article mold, rests on a chill plate 52 and is surrounded by a susceptor 54. The foot 56 of the mold is extended to overlie the entire chill plate and to extend under the susceptor at the periphery of the chill plate. The susceptor is held to the mold foot by cement 57 to form a liquid tight connection at this point. The susceptor is surrounded by an induction heater 58 which consists of several axially. ailgned coils so that the energy supplied to the coils may be gradually reduced from the bottom to the top of the susceptor.'A pipe 60 provides for admission of a supply of liquid coolant into the chamber surrounding the mold. In use, the mold having been heated to the desired temperature as above described, is filled with the superheated molten alloy and solidification is started at the chill plate by the supply of coolant to the passages in the chill plate. After a short period for the columnar growth to be established in the mold at the chill plate, a cooling liquid is supplied to the chamber and simultaneously the lowermost heating coil is turned off. The coolant surrounds the mold and rapidly extracts heat from the mold and the alloy therein to cause upward solidification of the alloy. The

rise of coolant in the chamber for submerging the mold is at the same rates given above for the downward movement of the mold in FIG. 1. Except for the need for the tin to absorb heat from the susceptor, the efiect is the same in the submergence of the mold by the poured-in coolant as in the immersion technique of FIG. 1. As the level of the coolant rises within the chamber, successive coils are shut down so that only the portion of the susceptor above the level of the coolant continues to be heated.

The rate of solidification is limited by the rate of removal of heat from the alloy which in its most desirable form produces no excessive curvature of the solidus surface. Since the size of the dendrites grown is a function of the rate of cooling, the shorter the local solidification time, the more refined will be the dendritic structure. In experimentation, growth rates as high as 300 inches per hour have been realized and such rates or higher are not unreasonable in casting, for example, blades and vanes for gas turbine engines. The growth rate depends on the cross-sectional area of the material in the mold and also the shape of the article since, for example, a blade shape has a greater surface area than a circle for the same cross-sectional area and will therefore lose heat faster.

As above stated, the thermal gradient is controlled by several parameters such as the amount of superheat in the molten alloy at the time of pour, the temperature of the liquid bath and the spacing between the bottom of the heating chamber and the surface of the liquid bath. The thermal gradient may be quite steep and gradients as high as 500F per inch have been obtained. Thermal gradients as high as 1000F per inch seem feasible with the present invention.

The limiting growth rate, i.e., the maximum rate at which the solidification front can move upward, while maintaining a unidirectional structure, is determined essentially by the maximum rate atwhich heat can be removedfrom the mold. With a thin mold wall, the heat removal is a function of the cross-sectional area of the alloy compared to the surfacearea, the rate at which the mold is immersed in the bath and the ability of the bath to accept the heat removal without a significant increase in temperature. This last parameter is thus affected by the volume of the bath, the specific heat of the material of v the bath, the circulation of the bath to keep the liquid close to the mold in motion and the external cooling means for temperature maintenance. Due to the high available rate of heat removal, the thermal gradient is high and constant over a wide range of growth rates so that both gradient and rate may be adjusted independently for optimum results.

The efiect of the high solidification rate and the high thermal gradient of this invention is emphasized in FIGS. 6, 7, and 8. FIG. 6 is a transverse microstructure of a single grain at magnification of MAR-M 200 alloy cast by directional solidification techniques as in the VerSnyder patent. This shows the large dendrites with comparable large dendrite spacing; the white areas being eutectic microconstituents forming areas of inhomogeneity that decrease the strength of the alloys. FIG. 7 shows a similar microstructure of the same alloy cast by the present techniques at 25 inches per hour immersion rate with obviously a much finer dendritic structure and closer interdendritic spacing and smaller embedded carbide particles and eutectic microconstituents. The alloy is thus inherently stronger and more resistant to fatigue. The smaller dendritic structure and spacing also extends throughout the cast article and thus provides much more uniform mechanical properties such as fatigue strength, stress rupture and yield strength in all areas of the casting. This minimizes the scatter of mechanical properties characteristic of more conventionally cast articles.

FIG. 8 is a transverse microstructure also at lOO magnification of the satne alloy as in FIGS. 6 and 7 but solidified by the present techniques with a 180 inches per hour immersion rate. When solidified at this rate, the dendritic structure and spacing is very much smaller than at the slower immersion rate of FIG. 7, and the carbide particles and eutectic rnicroconstituents are also much smaller by reason of the closer dendrite spacing. As in the casting from which the showing of FIG. 7 was made, this microstructure prevails throughout the casting, thus assuring uniform mechanical properties throughout the cast article.

The pools of eutectic microconstituents shown in these microstructures may be minimized or eliminated by heating the alloy close to the solidus temperature to diffuse the materials. If the dendrite spacing is large as in FIG. 6, the cast articles must be held at this temperature for a long time since the diffusion time is proportional to the square of the distance between the dendrites. The structure of FIG. 7 can be homogenized by only a few hours of heating making such a treatment practical. The structure of FIG. 8 would require a significantly shorter time than FIG. 7 because of the smaller dendritic spacing. Superalloys are not normally Table l Although the eutectic pool size and MC particle size have decreased, little change is seen in the overall volume fraction of eutectic and carbide. Since the eutectic pools are only due to segregation and can be solutionized, these may be caused to disappear by heat treatment. Thus, such pools are often referred to as a nonequilibrium microconstituent. The MC carbide is, however, an equilibrium phase which has its origin in the Spacing of Primary Dendrites and Secondary Dendrite Arms Under Various solidification Conditions Growth Solidus Secondary Process Rate Gradient Primary Spacing Spacing Power-Down 4"/hr 30F/inch 15.00 milli-inch 5.00 milli-inch Withdrawal 12 180 10.00 3.00 LMC (Pour) 264 4.55 1.44 LMC (Pour) 180 286 1.40 0.40

The values in Table I refer to a level 5 inches above the fixed chill plate in all cases. The primary spacing is the average spacing between adjacent dendrite cores as observed on a polished and etched surface taken perpendicular to the growth direction. The secondary spacing is the average spacing between adjacent secondary dendrite arms as observed on a section containing the growth direction. It should be emphasized that both types of spacings are mean values and are of limited statistical significance since they were determined experimentally from a small number of observations of quantities which vary by at least i 25% from the mean value. However, they do clearly show the trend to smaller spacings at higher rates. Higher gradients also refine secondary dendrite arm spacings, and apparently even primary dendrite spacing since these two quantities decrease approximately proportionately.

Although the MC carbides and 'y/' eutectic pools are somewhat difficult to characterize due to a lack of regularity in shape, typical sizes are given in Table II.

Table 11 Typical Microconstituent Dimensions melt and its volume fraction is independent of solidification rate.

As previously observed, it is desirable to approach complete homogenization with the nickel-base alloys in order to improve their mechanical properties. As a result of this invention this now becomes feasible. Furthermore, mechanical properties such as transverse rupture life in single crystals will be more sharply defined and reside near the upper extreme of the normally observed scatter band.

Although the invention has been described in connection with certain examples and preferred embodiments, the invention is not limited thereto and departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

We claim:

1. A unidirectionally solidified superalloy article of columnar grained or monocrystalline morphology having a hyperfine dendritic structure as cast characterized by a primary dendrite spacing of less than about 0.005 inch.

2. A unidirectionally solidified nickel-base superalloy article of columnar grained or monocrystalline morphology having a hyperfine dendritic structure as cast characterized by a primary dendrite spacing less than about 0.005 inch and average 'yl'y' eutectic pool and MC carbide particle dimensions, where present, less than about 0.001 and 0.002 inch respectively.

Claims

2. A unidirectionally solidified nickel-base superalloy article of columnar grained or monocrystalline morphology having a hyperfine dendritic structure as cast characterized by a primary dendrite spacing less than about 0.005 inch and average gamma / gamma '' eutectic pool and MC carbide particle dimensions, where present, less than about 0.001 and 0.002 inch respectively.