US20080260572A1

US20080260572A1 - Corrosion and oxidation resistant directionally solidified superalloy

Info

Publication number: US20080260572A1
Application number: US11/788,302
Authority: US
Inventors: Allister W. James; Douglas J. Arrell
Original assignee: Siemens Power Generations Inc
Current assignee: Siemens Energy Inc
Priority date: 2007-04-19
Filing date: 2007-04-19
Publication date: 2008-10-23

Abstract

A nickel-based superalloy having a good balance between corrosion and oxidation resistance. The alloy provides good mechanical properties. The superalloy is suited for directional solidification casting but can also be used for conventional or single crystal casting techniques. The superalloy is well suited for the hot section components such as blades, vanes and ring segments for gas turbine engines. The superalloys can be used with various thermal barrier coatings

Description

FIELD OF THE INVENTION

The invention relates to nickel-based superalloys usable to form hot section components of gas turbine engines.

BACKGROUND OF THE INVENTION

Nickel-based superalloys have a very good material strength at high temperatures. These properties permit their use in components for gas turbine engines where the retention of excellent mechanical properties at high temperatures is required. Hot section components include vanes, rotating blades and ring segments.
The metallurgy of superalloys is a sophisticated and well developed field. Optimization of the composition of superalloys consists of defining the amounts of elements which are desirably present, and the amounts of elements which are desirably absent. These impurities can in some cases be completely eliminated from the composition through the judicious selection of melt stock material; however some elements cannot be readily eliminated. One impurity which has long been recognized as being detrimental is sulfur. Sulfur was initially identified as being detrimental to mechanical properties, and its presence in alloy compositions was limited for that reason. However, the sulfur levels which do not present significant loss of mechanical properties, a bulk property, can in some cases still be highly detrimental to oxidation resistance, a surface property.
Oxidation resistance of superalloys is primarily due to the presence of an adherent surface oxide scale. The composition and nature of oxide scales depends on the composition of the alloy and the environment in which the superalloy component operates. Several major types of oxide scales exist, which include simple as well as complex oxides/spinels based primarily on aluminum, cobalt, nickel, and chromium. When certain rare earth elements (i.e., those elements with consecutive atomic numbers of 57 to 71, inclusive; also including yttrium, atomic number 39) are intentionally added to the superalloy in closely controlled amounts, the oxidation resistance of components made from such compositions is improved. This improvement is attributed to the ability of a rare earth element to reduce the residual sulfur content through the formation of sulfides and oxysulfides which stabilizes the oxide scale formed on the component surface improving the resistance of the scale and any coating thereon to spallation during use of the superalloy component.
The use of these superalloys at increasingly higher temperatures requires that a coating be applied to the superalloy component for thermal protection. The coating typically consists of applying a bondcoat to the superalloy and then a thermal barrier coating (TBC) to the bondcoat. Typical bond coats are alloys of the type MCrAlX where M is Ni, Co, or Fe and X is commonly Y, Zr, or Hf. The bondcoat tends to degrade during prolonged high temperature exposure. The degraded bondcoat does not adequately adhere the thermal barrier coating to the superalloy component and spallation of the TBC occurs with complete loss of thermal protection to the component. The rate at which the bondcoat degrades depends upon the composition of the superalloy to which it is applied. Generally alumina forming superalloys exhibit longer bondcoat lifetimes than chromia forming superalloys. However, it is often preferable to use high chromium containing superalloys for very high corrosion resistance. The formation of an alumina scale over that of a chromia scale can be enhanced by the presence of silicon.
Hence there remains a need for a superalloy with a lower propensity to promote bondcoat degradation and significantly enhance the resistance of the TBC to spallation.

SUMMARY OF THE INVENTION

This invention is directed to a nickel-based superalloy with a good balance of corrosion and oxidation resistance. The nickel-based superalloy is ideally suited to directionally solidified casting, but may also be produced by conventional casting or single crystal casting techniques. The superalloy is well suited for applications in the gas turbine engines as hot section components such as blades, vanes, and ring segments.
In one embodiment, the superalloy may be formed from materials in the following percentages: 9.5 to 14.0 Cr; 7.0 to 11.0 Co; 1.0 to 2.5 Mo; 3.0 to 6.0 W; 1.0 to 6.0 Ta; 3.0 to 4.0 Al; 3.0 to 5.0 Ti; 0 to 1.0 Nb; 0.05 to 0.2 Hf; 0.05 to 0.2 Si; 0.005 to 0.02 B; 0 to 0.1 Zr; 0.05 to 0.15 C, 0.001 to 0.1 of a mixture of two or more rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. A preferred superalloy may be formed from materials in the following percentages: 11.6 to 12.7 Cr; 8.5 to 9.5 Co; 1.65 to 2.15 Mo; 3.0 to 4.1 W; 4.8 to 5.2 Ta; 3.4 to 3.8 Al; 3.9 to 4.25 Ti; 0 to 0.5 Nb; 0.1 to 0.15 Hf; 0.1 to 0.15 Si; 0.005 to 0.015 B; 0 to 0.02 Zr; 0.05 to 0.11 C, 0.01 to 0.05 of a mixture of two or more rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. The most preferred superalloy may be formed from materials in the following percentages: 12.2 Cr; 9.0 Co; 1.9 Mo; 3.8 W; 5.0 Ta; 3.6 Al; 4.1 Ti; <0.2 Nb; 0.12 Hf; 0.12 Si; 0.01 B; 0.0075 Zr; 0.09 C, 0.02 of a mixture of two or more rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni.
An advantage of this invention is that the superalloy has good mechanical properties and provides a unique balance between good oxidation characteristics and good corrosion resistance.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a nickel-based superalloy with a good balance of corrosion and oxidation resistance. The nickel-based superalloy is ideally suited to directionally solidified casting, but may also be produced by conventional casting or single crystal casting techniques. The superalloy is well suited for applications in the gas turbine engines as hot section components such as blades, vanes, and ring segments.
The superalloy may promote a balance of corrosion and oxidation resistance suited for directionally solidified casting of hot section gas turbine engine components. In one embodiment, the superalloy may be formed from materials in the following percentages: 9.5 to 14.0 Cr; 7.0 to 11.0 Co; 1.0 to 2.5 Mo; 3.0 to 6.0 W; 1.0 to 6.0 Ta; 3.0 to 4.0 Al; 3.0 to 5.0 Ti; 0 to 1.0 Nb; 0.05 to 0.2 Hf; 0.05 to 0.2 Si; 0.005 to 0.02 B; 0 to 0.1 Zr; 0.05 to 0.15 C, 0.001 to 0.1 total of at least one rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni.
Chromium (Cr) is included to improve the alloy's high-temperature corrosion resistance. The reason for limiting the chromium content to 9.5 to 14.0 weight percent in the present invention is to assure a good level of corrosion resistance and is preferably from 11.6 to 12.7 weight percent. The desirable high-temperature corrosion resistance cannot be ensured at lower levels.
Cobalt (Co) replaces nickel (Ni) in the gamma-phase to strengthen the matrix in solid solution. Co is included in the range of 7.0 to 11.0 percent by weight in the present invention to strengthen the matrix in solid solution yet have Co below the level where the proportion of the gamma prime phase is too low to have good creep strength. A preferred range for Co is from 8.5 to 9.5 percent by weight.
Molybdenum (Mo) is a solid-solution strengthener of the gamma-phase, and can also promote the formation of raft structure, which strengthens the superalloy at high temperatures. In the present invention, Mo is included at 1.0 to 2.5 weight percent, and is preferably 1.65 to 2.15 weight percent. Mo at levels above 3.0 weight percent can be detrimental to the creep strength and low-cycle fatigue properties of the superalloy.
Tungsten (W) is a solid-solution strengthener of the gamma-phase. In the present invention, a W content is included at 3.0 to 6.0 weight percent and is preferably 3.0 to 4.1 weight percent.
Aluminum (Al) is an element of the gamma prime phase which also forms an aluminum oxide surface on the alloy to provide oxidation resistance. In the present invention, the Al content is 3.0 to 4.0 weight percent which is lower than that generally required to obtain a good oxidation resistance, however, the addition of rare earth elements, silicon (Si) and Hafnium (Hf) compensates for the lower amount of Al. The preferred range of Al is 3.4 to 3.8 weight percent.
Titanium (Ti) can replace some of the Al in the gamma prime phase to form Ni₃(Al,Ti), serving as a solid-solute strengthener of the gamma prime phase. In the present invention a Ti is included at 3.0 to 5.0 weight percent and is preferably 3.9 to 4.25 weight percent.
Tantalum (Ta) is primarily in the gamma prime phase in solid solution to strengthen the gamma prime phase and contributes to oxidation resistance. In the present invention, Ta is included at 1.0 to 6.0 weight percent and is preferably at 4.8 to 5.2 weight percent where it contributes positively to the creep strength of the superalloy.
Hafnium (Hf) improves the grain boundary strength of the superalloy. The present invention includes Hf at 0.05 to 0.2 weight percent where it also promotes the formation of an alumina surface and improves the oxidation resistance of the superalloy. At higher levels of Hf the melting point of the superalloy can be diminished. The preferred range of Hf is 0.1 to 0.15 weight percent.
Silicon (Si) is an element that forms an oxide, SiO₂on the surface of the resultant alloy which improves the oxidation resistance. Si is added to the superalloy of the present invention at a level of 0.05 to 0.2 weight percent. Si can inhibit other elements participating in the solid solution at levels higher than 0.2 weight percent. A preferred range of Si is 0.1 to 0.15 weight percent. The inclusion of Hf at levels similar to that of the silicon enhances the oxidation resistance provided by the Si.
Niobium (Nb) primarily partitions to and strengthens the gamma prime phase. Nb acts in concert with the Ta to increase the solution proportion of the gamma prime phase further enhancing the strength relative to a superalloy using Ta alone. In the present invention, Nb can be included at a level up to 1.0 weight percent and is preferably included at a level of less than 0.5 weight percent.
Carbon (C) improves the strength of grain boundaries. In the present invention, C is included at a range of 0.05 to 0.15 weight percent. Levels of C above this range can negatively affect the creep strength of the superalloy. The C is preferably included at a range of 0.05 to 0.11 weight percent.
Boron (B) is also included to improve the grain boundary strength. When the boron is added in excess of 0.05% the creep strength can be diminished. The content of B in the superalloy of the present invention is limited to 0.005 to 0.02 weight percent and preferably between 0.005 and 0.015 weight percent.
Zirconium (Zr) can also be included to improve the grain boundary strength of the superalloy. In the present invention, Zr can be included up to 0.1 weight percent and is preferably up to 0.02 weight percent. Higher levels of Zr can negatively affect the creep characteristics of the superalloy.
Rare earth elements Yttrium (Y), Lanthanum (La), Cerium (Ce), Gadolinium (Gd), Praseodymium (Pr), Dysprosium (Dy), Neodymium (Nd) and Erbium (Er) promote the formation of the Al₂O₃and SiO₂scale on the superalloy and improve adhesive property of this protective oxide layer. The presence of rare earth elements is believed to promote the diffusion of aluminum to the surface hence increasing the proportion of alumina in the scale. The inclusion of these rare earth elements also enhances the compatibility of the superalloy with various coatings. An excessive addition of the rare earths lowers the solubility of other elements and for this reason the combined rare earth elements are not included in excess of 0.1 weight percent. In the present invention one or more of the rare earths are included in a combined range of 0.001 to 0.1 weight percent. A preferred range for the combined rare earth elements is 0.01 to 0.05 weight percent.
The presence of the rare earth elements enhances the coating life. This enhancement is attributed to the ability of the rare earth elements to form sulfides and oxysulfides fixing sulfur impurities which prevents their diffusion to the surface and degrades the alumina scale on the superalloy.
As indicated above, a preferred superalloy for high corrosion resistance and an improved oxidation resistance may be formed from materials in the following percentages: 11.6 to 12.7 Cr; 8.5 to 9.5 Co; 1.65 to 2.15 Mo; 3.0 to 4.1 W; 4.8 to 5.2 Ta; 3.4 to 3.8 Al; 3.9 to 4.25 Ti; 0 to 0.5 Nb; 0.1 to 0.15 Hf; 0.1 to 0.15 Si; 0.005 to 0.015 B; 0 to 0.02 Zr; 0.05 to 0.11 C, 0.01 to 0.05 of one or more rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. A most preferred superalloy composition may be formed from materials in the following percentages: 12.2 Cr; 9.0 Co; 1.9 Mo; 3.8 W; 5.0 Ta; 3.6 Al; 4.1 Ti; <0.2 Nb; 0.12 Hf; 0.12 Si; 0.01 B; 0.0075 Zr; 0.09 C, 0.02 of a mixture of one or more rare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni.
The superalloy of the present invention is ideally suited for directionally solidification casting. However, it can be readily produced by conventional casting or single crystal casting techniques. The superalloy is well suited for the hot section components such as blades, vanes and ring segments for gas turbine engines. The superalloys can be used with various thermal barrier coatings.
Alternatives for the alloy composition and other variations within the range provided will be apparent to those skilled in the art. Variations and modifications can be made without departing from the scope and spirit of the invention as defined by the following claims.

Claims

1. A nickel-based superalloy expressed in weight percentages consisting essentially of:

9.5 to 14.0 Cr;

7.0 to 11.0 Co;

1.0 to 2.5 Mo;

3.0 to 6.0 W;

1.0 to 6.0 Ta;

3.0 to 4.0 Al;

3.0 to 5.0 Ti;

0 to 1.0 Nb;

0.05 to 0.2 Hf;

0.05 to 0.2 Si;

0.005 to 0.02 B;

0 to 0.1 Zr;

0.05 to 0.15 C;

0.001 to 0.1 of a mixture of two or more rare earth metals selected from the group of consisting of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and

balance formed from Ni.

2. The superalloy of claim 1, wherein the superalloy consisting essentially of:

11.6 to 12.7 Cr;

8.5 to 9.5 Co;

1.65 to 2.15 Mo;

3.0 to 4.1 W;

4.8 to 5.2 Ta;

3.4 to 3.8 A1;

3.9 to 4.25 Ti;

0 to 0.5 Nb;

0.1 to 0.15 Hf;

0.1 to 0.15 Si;

0.005 to 0.015 B;

0 to 0.02 Zr;

0.05 to 0.11 C;

0.01 to 0.05 of a mixture of two or more rare earth metals selected from the group of consisting of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and

the balance formed from Ni.

3. The superalloy of claim 1, wherein the superalloy consisting essentially of:

12.2 Cr;

9.0 Co;

1.9 Mo;

3.8 W;

5.0 Ta;

3.6 Al;

4.1 Ti;

<0.2 Nb;

0.12 Hf;

0.12 Si;

0.01 B;

0.0075 Zr;

0.09 C;

0.02 of a mixture of two or more rare earth metals selected from the group consisting of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and

the balance formed from Ni.