Processing of nickel aluminide material

Details Processing of nickel aluminide material Processing of nickel aluminide material

2001-11-07

2003-06-03

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C420S590000, C148S556000

Reexamination Certificate

active

06571857

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the processing of nickel aluminide intermetallic materials. More particularly, this invention relates to a process for producing a beta-phase nickel aluminide-based ingot, such as for use as a source material in physical vapor deposition (PVD) processes.
2. Description of the Related Art
Components within the turbine, combustor and augmentor sections of gas turbine engines are susceptible to oxidation and hot corrosion attack, in addition to high temperatures that can decrease their mechanical properties. Consequently, these components are often protected by an environmental coating alone or in combination with an outer thermal barrier coating (TBC), which in the latter case is termed a TBC system.
Diffusion coatings, such as diffusion aluminides and particularly platinum aluminides (PtAl), and overlay coatings, particularly MCrAlX alloys (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or another rare earth or reactive element), are widely used as environmental coatings for gas turbine engine components. Ceramic materials such as zirconia (ZrO
2
) partially or fully stabilized by yttria (Y
2
O
3
), magnesia (MgO) or other oxides, are widely used as TBC materials. Used in combination with TBC, diffusion aluminide and MCrAlX overlay coatings serve as a bond coat to adhere the TBC to the underlying substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the TBC to the bond coat.
More recently, overlay coatings (i.e., not a diffusion) of beta-phase nickel aluminide (&bgr;NiAl) intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being nickel. Notable examples of beta-phase NiAl coating materials include commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., which discloses a NiAl overlay bond coat optionally containing one or more active elements, such as yttrium, cerium, zirconium or hafnium, and commonly-assigned U.S. Pat. No. 6,291,084 to Darolia et al., which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly-assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta-phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed by Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia's beta-phase NiAl alloy contains zirconium. The beta-phase NiAl alloys of Nagaraj, Darolia et al., Rigney et al., and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby increasing the service life of the TBC system.
Suitable processes for depositing a beta-phase NiAl coating are thermal spraying and physical vapor deposition processes, the latter of which includes electron beam physical vapor deposition (EBPVD), magnetron sputtering, cathodic arc, ion plasma, and combinations thereof. PVD processes require the presence of a coating source material made essentially of the coating composition desired, and means for creating a vapor of the coating source material in the presence of a substrate that will accept the coating.
FIG. 1
schematically represents a portion of an EBPVD coating apparatus
20
, including a coating chamber
22
in which a component
30
is suspended for coating. A beta-phase NiAl overlay coating
32
is represented as being deposited on the component
30
by melting and vaporizing an ingot
10
of the beta-phase NiAl with an electron beam
26
produced by an electron beam gun
28
. The intensity of the beam
26
is sufficient to produce a stream of vapor
34
that condenses on the component
30
to form the overlay coating
32
. As shown, the vapor
34
evaporates from a pool
14
of molten beta-phase NiAl contained within a reservoir formed by crucible
12
that surrounds the upper end of the ingot
10
. Water or another suitable cooling medium flows through cooling passages
16
defined within the crucible
12
to maintain the crucible
12
at an acceptable temperature. As it is gradually consumed by the deposition process, the ingot
10
is incrementally fed into the chamber
22
through an airlock
24
.
The preparation of beta-phase NiAl for deposition by PVD typically requires the use of a vacuum induction melting (VIM) furnace in order to promote the purity of the composition by reducing the levels of residual elements such as oxygen. Other typical requirements for the ingot
10
include full density (e.g., pore-free), chemical homogeneity, mechanical integrity (e.g., crack-free), and dimensions and dimensional tolerances suitable for the particular PVD machine used. However, the casting and finish machining of beta-phase NiAl-based compositions are difficult to control as a result of the high melting point (1640° C.), very low room temperature ductility and low ambient fracture toughness (about 6 MPa·m
½
) of NiAl. The brittle nature of beta-phase NiAl-based materials particularly complicates the preparation of large ingots (e.g., diameters of about 2.5 inches (about 6.35 mm), lengths of about 20 to 30 inches (about 50.8 to 78.2 cm)) suitable for EBPVD processes, and machinable stock material required for cathodic arc processes. Also of concern is an exothermic reaction that takes place between nickel and aluminum when beta-phase NiAl is melted. When processing beta-phase NiAl in very small amounts, this exothermic reaction does not typically pose a significant problem. However, in the production of ingots of sufficient size for use in EBPVD processes, the exothermic reaction can be catastrophic to the processing equipment and therefore hazardous to personnel.
In view of the above, what is needed is a process for preparing, casting and processing an ingot of a beta-phase NiAl-based material that would be suitable for use in PVD coating processes, and particularly for creating relatively large cylindrical ingots for EBPVD processes and machinable stock material for cathodic arc and sputtering processes.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for preparing, casting and processing a beta-phase NiAl-based material, particularly for use in PVD coating processes. Materials produced by the process of this invention are preferably in the form of ingots that are crack-free, full density, chemically homogeneous, and capable of being machined to dimensional tolerances suitable for use in a PVD machine. In addition, the process is carried out so as to avoid the violent exothermic reaction between nickel and aluminum when beta-phase NiAl is melted.
The method entails melting a nickel-aluminum composition having an aluminum content below that required for stoichiometric beta-phase NiAl intermetallic so as to form a melt comprising nickel and Ni
3
Al. Aluminum is then added to the melt, causing an exothermic reaction between nickel and aluminum as the melt equilibrium shifts from Ni
3
Al to NiAl. However, the aluminum is added at a sufficiently low rate to avoid a violent exothermic reaction. The addition of aluminum continues until sufficient aluminum has been added to the melt to yield a beta-phase NiAl-based material, i.e., containing the NiAl beta-phase. The beta-phase NiAl-based material is then solidified to form an ingot, which is heated and pressed to close porosity and homogenize the microstructure of the ingot.
The process of this invention is capable of producing ingots of a variety of beta-phase NiAl intermetallic materials,

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