Patent Description:
Gas turbine engine hot section components are commonly formed of alloys, typically nickel- or cobalt-based superalloys. Many components, such as blades are formed of single-crystal (SX) alloys. In such single-crystal components, essentially the entire component is formed of a single continuous crystal lattice. Typically, the orientation of that lattice is predetermined to achieve desired properties of the component. The orientation may be assured by use of a grain starter or other casting techniques.

By way of example, <CIT> of Gemma and Dierberger identifies face centered cubic (FCC) single-crystal gas turbine blades wherein the [<NUM>] crystal axis is tangent to the blade airfoil surface in a critical crack-prone region just behind the leading edge of the airfoil at about <NUM>-<NUM>% of the airfoil span. <CIT> (the '<NUM> patent) of Wukusick and Buchakjian, Jr. , identifies a further single-crystal alloy. Table I (discussed below) includes comparative data from an alloy of the '<NUM> patent among others.

<CIT> discloses a prior art single-crystal Ni-based superalloy with high temperature strength, oxidation resistance and hot corrosion resistance.

<CIT> discloses prior art nickel-based superalloys with excellent mechanical strength, corrosion resistance and oxidation resistance.

<CIT> discloses a prior art Ni-based single crystal superalloy and turbine blade incorporating the same.

<CIT> discloses a prior art nickel-based superalloy.

From One aspect of the disclosure, there is provided an alloy as recited in claim <NUM>.

There is also provided a use of a substrate formed of the alloy as recited in claim <NUM>.

There is also provided a method of heat treatment of the alloy as recited in claim <NUM>.

In <FIG>, an engine turbine element <NUM> is illustrated as a blade having an airfoil <NUM> which extends between an inboard end <NUM>, and an opposing outboard end <NUM> (e.g., at a free tip), a distance therebetween extending substantially in the engine radial direction. The airfoil also includes a leading edge <NUM> and an opposing trailing edge <NUM>. A pressure side <NUM> and an opposing suction side <NUM> extend between the leading edge <NUM> and trailing edge <NUM>.

The airfoil inboard end <NUM> is disposed at the outboard surface <NUM> of a platform <NUM>. An attachment root <NUM> extends radially inward from the underside <NUM> of the platform.

The root <NUM> has an inner diameter (ID) end or face <NUM>, an upstream axial end <NUM>, a downstream axial end <NUM>, and first and second lateral sides <NUM> and <NUM>, respectively. The root <NUM> is complementary to a disk slot (not shown). When fully seated in the disk slot, the faces <NUM> and <NUM> may face exactly forward/upstream and rearward/downstream in the engine frame of reference. Depending on disk configuration (slot orientation), the sides may extend parallel to the engine centerline between the axial ends (root having a rectangular footprint/section) or may extend skew (root having a non-right parallelogram footprint) such as in the illustrated example.

The turbine blade is cast of a high temperature alloy, namely a single-crystal Ni-based superalloy discussed below.

The blade may also have a thermal barrier coating (TBC, e.g., one or more layer ceramic atop of one or more layer bondcoat) system along at least a portion of the airfoil.

The blade may have an internal cooling passageway system (not shown) extending from one or more inlets along a root to a plurality of outlets (along or mostly along the airfoil and platform surfaces).

Stress corrosion cracking (SCC) has been a concern in commercial engine turbine blades. Atmospheric salt and pollution appear to be contributing factors. A particular area of SCC is along platform undersides of high pressure turbine (HPT) blades.

A number of alloys were conceived as potentially offering improved resistance to stress corrosion cracking, particularly in salt environments. Table I shows test data for ten candidate alloys (Ex. <NUM>-<NUM>) and four prior art alloys (Prior Art <NUM>-<NUM>). Of the examples, Ex. <NUM>, <NUM>, and <NUM> show particular benefits discussed below.

<NUM> is an alloy according to the invention and Ex. <NUM>-<NUM> and <NUM> are reference examples.

In the present disclosure: <NUM>°F is <NUM>; 1ksi is <NUM> MPa and <NUM> inch is <NUM>.

Table I lists the post-solutioning heat treatments used. For example, "2050F/<NUM>+1975F/<NUM>+1650F/<NUM>" indicates <NUM>° Fahrenheit for four hours followed by <NUM>° Fahrenheit for four hours followed by <NUM>° Fahrenheit for four hours. The solutioning involved heat treatment at or above the alloy gamma prime solvus temperatures for <NUM> hours in the <NUM>° to <NUM>° Fahrenheit range.

Rupture tests were performed on creep-rupture specimens with nominal <NUM> inch gauge diameters tested isothermally at 1500F with constant load and low melting eutectic salt mixture brushed on the gauge section every <NUM> hours. The salt was a quaternary system of sodium, magnesium, potassium, and calcium sulfates with a melting point of about 1200F (e.g., 1175F-1225F or 1195F to 1205F). The salt represents an approximately equal blend of two ternary examples from the cited Du article below (rows <NUM> and <NUM> of Table <NUM> of the Du article). The air rupture test was performed in a furnace drawing in ambient laboratory air; whereas the salt rupture test was performed in a similar arrangement although salt was brushed on the gauge section periodically. Such salt and test procedure may be used as a reference in the tests of the claims below. Alternative reference salts are disclosed in <NPL> and <NPL>. The disclosures of said Bornstein and Du articles are incorporated by reference in their entireties herein as if set forth at length. Similar relative behavior of alloys and heat treatments would be expected when tested with the Bornstein and Du salts to the results obtained herein.

Two factors to be considered from the tests are absolute rupture life and consistency in rupture life. Because of the relatively close compositions of the examples and prior art, what would otherwise be a statistically insignificantly small sample size of one alloy (e.g., one or two tests) allows a statistically significant conclusion to be made when such small test numbers of many slightly different alloys are considered. Thus, running only two tests each of two alloys would not be statistically significant if one alloy had a <NUM>-hour test life and a <NUM>-hour test life while the other had a <NUM>-hour test life and a <NUM>-hour test life. But, such two-test per alloy data becomes significant when one looks a greater number of alloys. From such data one may statistically significantly determine a compositional range associated with the long life and/or consistent performance. In this case, a compositional range around Ex. <NUM>, <NUM>, and <NUM> is indicated as having an advantageous combination of life and consistency.

In contrast, consider Ex. <NUM>, <NUM> and <NUM> where Cr levels below <NUM>% produced instances of lower rupture lives in salted creep testing.

In Table I, the three targeted alloys (Ex. <NUM>, <NUM>, and <NUM>) have significant amounts of the elements Ni, Cr, Mo, W, Ta, Al, Co, and Re. As far as other elements, Ti, C, B, Y, and/or Hf can be added in small amounts to benefit alloy properties. S should be kept as low as possible to benefit oxidation resistance. Small Si additions will also benefit oxidation resistance. Combined Y + Hf + B + Si additions should be maintained below <NUM>% to achieve high levels of alloy incipient melting temperature. In terms of individual contents, in exemplary ranges one-to-all of a titanium content, if any, is no more than <NUM>%; a boron content, if any, is no more than <NUM>%; a yttrium content, if any, is no more than <NUM>%; a sulfur content, if any, is no more than 5ppm or no more than 1ppm; a hafnium content, if any is no more than <NUM>% or is in the range of <NUM>% to <NUM>%; a silicon content, if any, is no more than <NUM>% or <NUM>%; and a carbon content, if any, is no more than <NUM>%, by weight.

In various examples, a combined weight content, if any, of elements other than nickel, chromium, cobalt, aluminum, rhenium, tungsten, tantalum, molybdenum, if any, sulfur, if any, hafnium, if any, silicon, if any, and carbon, if any, is no more than <NUM>% or <NUM>% or <NUM>% or <NUM>%.

In various examples, the combined weight content of chromium, cobalt, and aluminum is <NUM>% to <NUM>% or <NUM>% to <NUM>% or <NUM>% to <NUM>%.

In terms of heat treatment, from Table I it can be further determined that an advantageous heat treatment involves full or close to full gamma prime solutioning at temperatures at or above 2375F, followed by intermediate heat treatments at 2050F and or 1975F for <NUM> hours with a final PHT aging heat treatment at 1650F for <NUM> hours.

As a further refinement, Table III involves exploration of heat treatment parameters with the Prior Art <NUM> alloy. Solution heat treat is at a temperature between solvus and incipient melting point. At commercial scale to have an effective heat treat, solutioning may be at a temperature of at least 20F over solvus or at least 25F or at least 30F. At the other end, to avoid errors (risk of incipient melting due to process variation), a target solution heat treat may be in the range of not more than 40F below incipient melting or not more than 35F or not more than 30F or not more than 25F.

The Prior Art <NUM> alloy has a 2440F incipient melting temperature and a 2370F solvus temperature. Exemplary solution heat treat is 2400F up to the incipient melting temperature. At commercial scale to avoid errors, a target solution heat treat may be in the range of 2380F to 2415F. Times of <NUM> minutes, <NUM> hours, and <NUM> hours are given. High consistency is seen at <NUM> hours. Alternative exemplary lower ends on ranges other than <NUM> hours are <NUM> hours and <NUM> hours. Much above <NUM> would seem to offer little benefit for cost.

Several pure precipitation heat treatments (PHT) are compared with several solution heat treatments. The latter are followed by a two-stage final heat treatment. In the same salt tests as those of Table I, it is seen that the increasing alloy homogenization by increasing the time at the solution heat treatment temperature reduces variability and increases average life. The post-solutioning final aging heat treatment (a precipitation heat treat (PHT)) also plays a significant role in increasing both the minimum and average rupture life in salt tests. Increasing both the PHT temperature to 1600F and the time to <NUM> hours produces the highest creep-rupture life in a salt environment.

When transferring this Prior Art <NUM> experience to the other alloys, adjustments may be made for differences in solutioning temperature and incipient melting point. For example, Examples <NUM>, <NUM>, and <NUM> have lower solvus and incipient meting temperatures which would have corresponding lower target heat treatment temperatures while preserving times.

Alloys to be used in turbine blade applications require good high temperature creep-rupture strength in an air environment. Key Table I examples also show good high temperature creep-rupture life as shown in testing conducted at 1800F/36ksi and 1850F/38ksi. Although lives are not as good as those in the alloy of Prior Art <NUM>, they are adequate to meet service turbine blade requirements. We infer that these good lives can be maintained by insuring that total W + Ta + Re content stays in the range of <NUM> to <NUM>% by weight. Total W + Ta content may more narrowly stay in the range of <NUM>% to <NUM>% by weight or more broadly <NUM>% to <NUM>% (with life compromise at the low end).

Claim 1:
An alloy comprising, by weight:
<NUM>% to <NUM>% chromium;
<NUM>% to <NUM>% molybdenum;
<NUM>% to <NUM>% cobalt;
<NUM>% to <NUM>% aluminum;
<NUM>% to <NUM>% rhenium;
<NUM>% to <NUM>% tungsten;
<NUM>% to <NUM>% tantalum;
<NUM>% to <NUM>% hafnium;
<NUM>% to <NUM>% titanium;
<NUM>% to <NUM>% boron;
<NUM>% to <NUM>% yttrium;
<NUM> to 5ppm sulfur;
<NUM>% to <NUM>% silicon;
<NUM>% to <NUM>% carbon;
<NUM>% to <NUM>% yttrium, lanthanum, and/or cerium combined; and
balance nickel.