Method for making a coated base superalloy article of improved microstructural stability

Abstract: A coated Ni base superalloy article is provided with improved microstuctural stability under a coating at least partially diffused into an article surface portion, by avoiding the formation, after exposure to elevated temperatures, of a constituent called SRZ which can affect detrimentally mechanical properties of the article. Such avoidance can result from neutralizing stress in the article surface portion prior to coating, for example by physically removing the stressed portion or by recrystallizing the stressed portion to a fine grained structure which subsequently can be consumed by diffusing elements in a coating method. Also, such avoidance can result from maintaining a Re segregation factor, ReΔ, between dendrite cores and interdendritic regions of the alloy cast structure at no greater than about 40.

Description: This invention relates to a method for making a Ni base superalloy article, especially to such an article in the form of a single crystal, and, more particularly in one form to such an article including an environmental coating. The development of Ni base superalloys, particularly those intended to form single crystal articles for use at elevated temperatures such as 982°C (1800° F) and above, has identified certain phases or conditions which can form in the alloy structure and which can be detrimental to the useful life of the article. One such detrimental condition has been referred to as a Secondary Reaction Zone (SRZ). This formation consists essentially of an orthorombic "P" phase which is a Type II topographically close-packed (TCP) phase, and a gamma phase, both dispersed in a gamma prime matrix. The TCP phase and the gamma phase form needle-like structures within the blocky gamma prime matrix. Problems associated with the formation of SRZ after exposure of an alloy at temperatures of at least about 982°C (1800°F) have been discussed in U.S. Patent 5,270,123 - Walston et al, patented on December 14, 1993; and the formation of the TCP phase has been identified in U.S. Patent 5,151,249 - Austin et al, patented on September 29, 1992. Also, the TCP phase is addressed in U.S. Patent 5,173,255 - Ross et al, patented December 22, 1992, relating to an alloy including about 1.5 - 4 wt. % Re. The above mentioned Walston et al patent discloses that the potential for the formation of SRZ within the cast structure of a Ni base superalloy can be predicted and controlled by alloy content, particularly the element Re at greater than about the 4 wt. % shown in the above mentioned Ross et al patent, in the presence of other alloying elements. In solidification of Ni base superalloy single crystal castings, elements segregate either to the dendrites or interdendritic regions. Re segregates heavily to the dendrites and thus internal SRZ typically is observed in dendrites of such alloys including Re. Some Ni base superalloys, such as those which are identified in the Walston et al patent, have been developed to avoid and rarely do form internal SRZ. However, under certain conditions a small amount of SRZ has been observed in an article surface portion adjacent or under an environmental coating, such as an aluminide or a Pt-Al diffusion coating or an overlay coating at least partially diffused into a surface portion; for example, the well known MCrAIY type of coatings in which "M" generally is Ni, Co, or their combination. Such a condition can have a detrimental effect on mechanical properties of an article including SRZ in such surface portion. The present invention, in one form, provides a method for making a coated article of improved microstructural stability from a Ni base superalloy to avoid, after elevated temperature exposure, detrimental formation of SRZ in alloy microstructure in an article surface portion beneath a coating at least partially diffused into the article surface portion by a selected coating method. One form of the method includes the steps of neutralizing, such as by removing. stresses from the surface portion prior to coating by the coating method, and then coating the surface portion with a coating which will at least partially diffuse into the surface portion. The neutralizing of the stresses can be accomplished by application of surface material removal methods such as electropolishing, electrochemical material removal, or other well known material removal methods practiced in the metallurgical art and which result in a substantially stress free surface portion. Then the coating is applied. In another form of the present invention, neutralizing of stresses, particularly in a single crystal structure, can be accomplished by recrystallization of the surface portion including the stresses to a fine grained cellular structure (sometimes referred to as Type II recrystallization) which extends into the surface portion substantially no farther than a diffusion depth to which the coating is intended to diffuse. In the practice of the present invention, such recrystallization is accomplished by exposing the surface portion to temperatures ranging from about 27.8°C (50°F) below to about 222.2°C (400° F) below the gamma prime solvus temperature for a recrystallization time sufficient to recrystallize the surface portion. Thereafter, coating of the surface portion is conducted in a manner which substantially consumes the fine recrystallized cells within the diffusion coating depth of the article surface portion. Embodiments of the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a graphical presentation of %SRZ under a coating as a function of surface finish. Figure 2 is a photomicrograph at 500 X of a Ni base superalloy aluminide coated after introduction of stress by grit blasting, showing extensive, continuous SRZ under the coating. Figure 3 is a photomicrograph at 500 X of the Ni base superalloy of Figure 3 aluminide coated after grit blasting and then electropolishing to remove the stressed surface portion prior to coating, showing substantially no SRZ under the coating. The prevention of internal SRZ (away from a coating) can be accomplished through bulk alloy chemistry changes or control, or in reduction in elemental segregation. This is shown, in one form, in the above mentioned Walston et al patent 5,270,123. However, particularly in respect to diffusion type coatings at a surface portion of a Ni base superalloy article, especially one in the form of a single crystal and including greater than about 4 wt. % Re, it has been recognized that a stress condition in such surface portion can be a major cause of, or driving force toward, the formation of SRZ adjacent to or under such a coating. Such a surface portion stress condition can occur during manufacture of an article, such as a turbine blade for a gas turbine engine, as a result of processing such as cutting, grinding, polishing, abrading, aggressive grit blasting, etc., prior to coating, to obtain desired surface finishes or conditions. During evaluation of the present invention, it was recognized that the amount of SRZ under a diffusion coating, such as an aluminide diffusion coating of a type generally applied commercially to gas turbine engine blades, varies with the degree of surface finish: the finer the finish the more SRZ under the coating. One commercial form of such an aluminide diffusion coating, sometimes called Codep Coating, is disclosed in U.S. Patent 3,667,985 - Levine et al, patented June 6, 1972. In respect to the surface finish, a finer finish generally means that more surface stress has been introduced into the article surface portion. The graph of Figure 1 summarizes results of this evaluation on specimens of an alloy identified as Rene' (Registered Trade Mark) N6 and described in the above mentioned Walston et al patent. These data show that surface stress is directly related to SRZ formation. Therefore, one form of the present invention is a method which neutralizes such surface portion stress after article processing which can produce such stress and prior to coating of the surface portion. One method for neutralizing such stress in a surface portion is physically to remove that surface portion, prior to coating, by a means which does not introduce detrimental stress while it is practiced. Such substantially stress free material removal means can include surface electropolishing, electrochemical machining, low or stress free abrasion-type methods, as well as other substantially stress free material removal known in the metallurgical art. In tests associated with the present invention, specimens of an alloy identified as Rene' 162 alloy, described in the above mentioned Austin et al patent and including Re in the range of about 5.7 - 7 wt. %, were aluminide coated with the above identified Codep aluminide diffusion coating after grit blasting, with and without physical removal of the surface portion stressed by grit blasting. The physical removal was accomplished by standard, commercial electropolishing. After coating, the coated alloy specimens representing both treatment conditions were exposed in air to a temperature of 1121°C (2050° F) for 47 hours. Figures 2 and 3 compare the results of such testing. Figure 2 is a photomicrograph at 500 X of that Ni base superalloy Codep aluminide diffusion coated after the grit blasting introduced a detrimental amount of stress into the surface portion. In that Figure 2, an extensive, continuous SRZ formation is seen under the diffusion coating. By way of contrast, the Figure 3 photomicrograph at 500 X shows the condition under the coating after such grit blasting followed by removal of the stressed surface portion by electropolishing prior to coating. No detrimental SRZ is seen in Figure 3 under the coating as a result of having neutralized the stress by physical removal of the stressed portion prior to coating. Another form of the present invention relates to a particular kind of recrystallization to neutralize stress in a surface portion which is to be coated with a selected coating having a selected diffusion depth to which the coating is intended to progress. The diffusion depth generally results from the time and temperature selected for the coating as a funtion of mechanical properties desired to be maintained or developed. Generally there are two types of recrystallization typically observed in Ni base superalloys. One, sometimes called Type I recrystallization, occurs when a stressed specimen or article is exposed to temperatures approaching close to the gamma prime solvus of the alloy. The resulting microstructure generally consists of large recrystallized grains. Another, sometimes called Type II recrystallization, is referred to as cellular recrystallization and results from exposure at temperatures ranging from about 27.8°C (50° F) below to about 222.2°C (400° F) below the gamma prime solvus of the alloy. The cellular recrystallization consists of much finer grains or cells and is the type of recrystallization to which a form of this invention relates. This form of the invention for neutralizing stress in the surface portion by recrystallization, particularly of a single crystal structure, is to heat treat the surface portion in a manner which locally transforms the single crystal to a polycrystal by recrystallizing the stressed portion to a fine grained, cellular, Type II, structure extending into the surface portion to a depth substantially no greater than the selected diffusion depth. Then when the coating method is practiced, the diffusing coating consumes the fine, recrystallized grains within the diffusion depth, thereby maintaining the integrity of the original structure. The result is a coated surface portion from which has been eliminated the driving force for the formation of SRZ under or adjacent to the coating. One early series of tests was designed and conducted to evaluate the effect of heat treatment after the introduction of cold work into a surface portion of a specimen and prior to diffusion coating, to minimize SRZ formation. Specimens used were a single crystal form of Rene' N6 alloy, defined in the above incorporated Walston et al patent, nominally including Re at about 5.3 wt. %, and having a gamma prime solvus temperature of about 1304.4°C (2380° F). Other specimens were of an alloy, sometimes called Rene' N5, and nominally including about 3 wt.% Re. The specimens were processed initially to make all specimens uniform and comparable to standard production processing as a starting point. The specimens had experienced diffusion coating including standard production precoat heat treatment, grit blast at 378.6 x 10 3 Pa (55 psi), Pt electroplate without diffusion, diffusion aluminide, and post coating heat treatment. Such specimens then were acid stripped and again grit blasted at 378.6 x 10 3 Pa (55 psi) to remove the diffusion zone of the standard production processing and to standardize the surface portions. This assured uniform cold work in a new surface portion in an amount greater than that normally introduced in standard processing and which was guaranteed to result in the formation of SRZ. Thereafter, the specimens were given three different treatments: no heat treatment, and two heat treatments intended to recrystallize the stressed surface portion to a fine grained, Type II structure, one being at 1121°C (2050° F) for four hours and the other at 1149°C (2100° F) for four hours. Each of these treatments were followed by the standard production coating method of first electroplating with Pt without a diffusion cycle, then a standard production aluminide and post coating heat treatment. The Rene' N5 specimens, including about 3 wt % Re, in the range of about 1.5 - 4 wt. %, did not show the presence of SRZ. The amount of SRZ under the coating of each Rene' N6 alloy specimen. including nominally about 5.3 wt. % Re, was measured by photomicrographic evaluation. The averages of the results are shown in the following Table I: Average % SRZ Under Coating Test (in air) 100 hrs. No Recrystallization Recrystallization 1121°C (2050°F) 4 hrs. Recrystallization 1149°C (2100°F) 4 hrs. 982°C (1800°F) 53 41 28 1093°C (2000° F) 75 60 25 1149°C (2100° F) 56 58 25 The data of Table I, representing an aggressive, severe condition not generated in production but produced for this evaluation, demonstrates that, according to a feature of the present invention, a properly selected stress relief heat treatment which causes local surface cellular type recrystallization is effective in minimizing even excessive SRZ in the surface portion under or adjacent to a standard production Pt-Al type diffusion coating. Such a method can reduce the need to control the inherent less severe surface work normally introduced during the making of an article, such as in its manufacture or repair, prior to final coating. In this evaluation, one preferred recrystallization heat treatment was at 1149°C (2100° F) for 4 hours, about 155°C (280° F) below the gamma prime solvus temperature and representative of and within the range of about 27.8°C (50° F) below to about 222.2°C (400° F) below the gamma prime solvus temperature, of the Rene' N6 alloy. This treatment which was intended to reduce surface portion stresses prior to coating, provided a small layer of cellular recrystallization no greater than about 25 µm (1 mil) in size, and neutralized the residual stresses due to cold work. This small cellular recrystallized structure, extending into the surface portion to a depth no greater than a diffusion depth to which the diffusion of the coating will extend, then was consumed, according to a form of this invention, during coating by the advancing, diffusing coating elements The diffusion depth is established by selecting appropriate coating parameters which provides the article with desired mechanical properties. In this way, the tendency for SRZ formation is reduced or eliminated. For example, in this preliminary evaluation, at the 1093°C (2000° F) test temperature for 100 hours in air, the heat treatment at 1149°C (2100° F) for 4 hours reduced the SRZ formation from about 75% to about 25%, a significant, unexpected change. Therefore, under the conditions of the above evaluation, a heat treatment of about 1149°C (2100° F) for about 4 hours to neutralize surface portion stresses can be selected for Rene' N6 alloy prior to coating to minimize SRZ nucleation under such a coating. At the same time the heat treatment can be selected as an effective age for such a material to produce desired mechanical properties. Therefore, one form of the present invention to minimize the formation, or the tendency for the formation of SRZ, particularly under a diffusion coating, is to neutralize stresses in a surface portion of an article into which such coating is intended to be diffused. Such neutralization can be accomplished either by physically removing the surface portion which includes the potentially harmful stresses without introducing additional harmful stress, or by recrystallizing the stressed surface portion to a fine grained cellular structure which then can be consumed by the diffusion of elements of a subsequently applied coating. The present invention provides a method for making a coated article of improved microstructural stability from a Ni base superalloy by avoiding the formation of a detrimental amount of SRZ under or adjacent to a coating.

Claims: In a method for making a coated article of improved microstructural stability from a Ni base superalloy, to avoid, after elevated temperature exposure, detrimental formation of a Secondary Reaction Zone ( SRZ ) in alloy microstructure of an article surface portion under a coating at least partially diffused into the article surface portion by a selected coating method, the article, prior to coating having experienced at least one condition which has developed stress in the article surface portion, the steps of: neutralizing stress in the surface portion after exposure to conditions which will develop stress in the surface portion and prior to the coating method; and then. coating the surface portion with the coating method. The method of claim 1 wherein the neutralizing of stress in the surface portion is by physically removing the surface portion which includes the stress prior to coating and without introducing additional detrimental stress into the surface portion. The method of claim 1 wherein: the neutralizing of stress in the surface portion is by recrystallizing the surface portion which includes the stress to generate a fine grained cellular structure of a cellular size substantially no greater than a size which can be consumed by the selected coating method; and then, coating the surface portion by the selected method in which diffusion of the coating substantially consumes the the recrystallized fine grained structure. The method of claim 3 for making a coated single crystal article from a Ni base superalloy having a gamma prime solvus temperature and including greater than 4 wt. % Re, in which: the recrystallizing is by heating the article surface portion which includes the stress in a combination of a recrystallization temperature range of from 27.8°C (50° F) below to 222.2°C (400° F) below the gamma prime solvus temperature of the superalloy and for a recrystallization time sufficient to recrystallize the stressed surface portion to a fine grained cellular structure, with cells extending into the surface portion substantially no farther than a diffusion depth to which the coating will diffuse into the surface portion during a selected diffusion coating method, and of a cellular size substantially no greater than a size which can be consumed by the selected coating method; and then, diffusion coating the surface portion by the selected diffusion coating method in which the diffusion coating substantially consumes the recrystallized fine grained cells in the surface portion. The method of claim 4 in which the diffusion coating includes Al. The method of claim 5 in which the diffusion coating includes a combination of Al and Pt. The method of claim 4 in which the recrystallized cells are no greater than 25 µm (1 mil) in size.

Inventors: MURPHY WENDY HOWARD, WALSTON WILLIAM SCOTT, MURPHY, WENDY HOWARD, WALSTON, WILLIAM SCOTT
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1996
Main classification: C23C10
IPC codes: C23C10, B32B15, C22F1, C22C19