HIGH OXIDATION-RESISTANT ALLOY AND GAS TURBINE APPLICATIONS

USING THE SAME

DESCRIPTION TECHNICAL FIELD Embodiments of the subject matter disclosed herein relate primarily to a high oxidation-resistant alloy and gas turbine applications using the same.

BACKGROUND ART

For gas turbine applications (buckets, nozzles, shrouds, combustion chambers) nickel- base superalloys are used. However, in this field, nickel-base superalloys encounter one fundamental limitation, i.e. their oxidation resistance.

In this regard, it should be considered that creep damage in gas turbine components is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and micro-cracks. The formation of HfC and M23C6 carbides as grain boundary precipitates can also lead to grain boundary metal depleted zones which are susceptible to corrosive attack.

SUMMARY

Therefore, there is a general need for materials suitable for gas turbine applications, which show good properties in terms of thermal fatigue at the operating conditions, low density, flexural resistance, creep properties and fracture toughness, as well as an improved oxidation resistance.

An important idea is to provide an alloy wherein the selected elements in selected ranges allows to significantly increase the oxidation resistance by reducing the undesired formation of Hafnium carbide and precipitation of M23C6 carbides. This avoids the additional cost and process step of antioxidant coatings. This alloy can be produced by conventional processes such as Powder Metallurgy and Investment Casting, as well as innovative Additive Manufacturing technologies (e.g. Direct Metal Laser Melting processes).

First embodiments of the subject matter disclosed herein correspond to a high oxidation-resistant alloy having a nominal composition consisting of:

Co 9.00-9.50 wt%

W 9.30-9.70 wt%

Cr 8.00-8.70 wt%

Al 4.00-15.50 wt% Ti 0.60-0.90 wt%

Ta 2.80-3.30 wt%

Mo 0.40-0.60 wt%

Hf up tol .20 wt%

Mn up to 0.05 wt% Si up to 0.02 wt%

C up to 0.065 wt%

Re 0.00-4.00 wt%

Mg, B, Zr, Fe, O, N, S, or a mixture thereof up to 0.287 wt%

Ni balance based on the alloy weight.

In general, said alloy shows remarkably improved oxidation resistance properties with respect to conventional Ni super-alloys. Second embodiments of the subject matter disclosed herein correspond to a gas turbine component, such as a bucket, nozzle, shroud, and combustion chambers, made of the above alloy.

BRIEF DESCRIPTION OF DRAWINGS The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate exemplary embodiments of the present subject matter and, together with the detailed description, explain these embodiments. In the drawings:

Fig. 1 shows a micrograph taken by Optical Microscope of the oxidation surface of a conventional alloy 'Mar M247 LC, after 1000 h at 980°C; Fig. 2 shows a micrograph taken by Optical Microscope of the oxidation surface of the alloy of Example 1, after 1000 h at 980 °C; and

Fig. 3 shows the affected metal thickness at 980°C after different times (from 1000 to 4000 hours) for the alloy Mar M247 LC and the alloy of Example 1. The thickness is normalized with respect to the maximum affected thickness in the alloy Mar M 247 LC.

DETAILED DESCRIPTION

The following description of exemplary embodiments refers to the accompanying drawings.

The following description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. First embodiments of the subject matter disclosed herein correspond to oxidation-resistant alloy having a nominal composition consisting of:

Co 9.00-9.50 wt%

W 9.30-9.70 wt% Cr 8.00-8.70 wt%

Al 4.00-15.50 wt%

Ti 0.60-0.90 wt%

Ta 2.80-3.30 wt%

Mo 0.40-0.60 wt% Hf up tol .20 wt%

Mn up to 0.05 wt%

Si up to 0.02 wt%

C up to 0.065 wt%

Re 0.00-4.00 wt% Mg, B, Zr, Fe, O, N, S, or a mixture thereof up to 0.287 wt%

Ni balance based on the alloy weight.

It should be appreciated that the alloy above encompasses reduced amounts of Hafnium and Carbon so as to achieve an excellent oxidation resistance, as it will be demonstrated in the following working examples. Furthermore, the above alloy has an improved oxidation resistance due to the specific ranges of W and Cr. In some embodiments of the high oxidation-resistant alloy, Al is present in an amount of 4.00-10.50 wt%.

In other embodiments of the high oxidation-resistant alloy, Mg is present in an amount of up to 0.008 wt%, and Mo, B, Zr, Fe, O, N, S, or a mixture thereof in an amount of up to 0.879 wt%.

In other embodiments of the high oxidation-resistant alloy, Mo is present in an amount of up to 0.60 wt%, preferably 0.40-0.60 wt%, and Mg, B, Zr, Fe, O, N, S, or a mixture thereof in an amount of up to 0.287 wt%.

In other embodiments of the high oxidation-resistant alloy, B is present in an amount of up to 0.015 wt%, preferably 0.005-0.015 wt%, and Mg, Mo, Zr, Fe, O, N, S, or a mixture thereof in an amount of up to 0.872 wt%.

In other embodiments of the high oxidation-resistant alloy, Zr is present in an amount of up to 0.015 wt%, preferably 0.005-0.015 wt%, and Mg, Mo, B, Fe, O, N, S, or a mixture thereof in an amount of up to 0.872 wt%. In other embodiments of the high oxidation-resistant alloy, Fe is present in an amount of up to 0.20 wt%, and Mg, Mo, B, Zr, O, N, S, or a mixture thereof in an amount of up to 0.687 wt%.

In other embodiments of the high oxidation-resistant alloy, O is present in an amount of up to 0.02 wt%, and Mg, Mo, B, Zr, Fe, N, S, or a mixture thereof in an amount of up to 0.867 wt%.

In other embodiments of the high oxidation-resistant alloy, N is present in an amount of up to 0.005 wt%, and Mg, Mo, B, Zr, Fe, O, S, or a mixture thereof in an amount of up to 0.882 wt%.

In other embodiments of the high oxidation-resistant alloy, S is present in an amount of up to 0.004 wt%, and Mg, Mo, B, Zr, Fe, O, N, or a mixture thereof in an amount of up to 0.883 wt%. In preferred embodiments, the high oxidation-resistant alloy has a nominal composition consisting of:

Co 9.00-9.50 wt%

W 9.30-9.70 wt%

Cr 8.00-8.70 wt%

Al 4.00-10.50 wt%

Ti 0.60-0.90 wt%

Ta 2.80-3.30 wt%

Hf up to l .20 wt%

Mn up to 0.05 wt%

Mg up to 0.008 wt%

Mo up to 0.60 wt%

Si up to 0.02 wt%

B up to 0.015 wt%

Zr up to 0.015 wt%

Fe up to 0.20 wt%

O up to 0.020 wt%

N up to 0.0050 wt%

S up to 0.0040 wt%

C up to 0.065 wt%

Re 0.0-0.4 wt% Ni balance based on the alloy weight.

In particularly preferred embodiments of the high oxidation-resistant alloy, Al is present in an amount of 5.25-5.75 wt%. In other particularly preferred embodiments of the high oxidation-resistant alloy, Hf is present in an amount of 1.00-1.20 wt%.

In other particularly preferred embodiments of the high oxidation-resistant alloy, Re is present in an amount of 0.0-3.0 wt%.

A particularly preferred embodiment corresponds to a high oxidation-resistant alloy having a nominal composition consisting of:

Co 9.07 wt%

W 9.36 wt%

Cr 8.43 wt%

Al 5.73 wt% Ti 0.65 wt%

Ta 2.93 wt%

Mo 0.51 wt%

Hf 1.02 wt%

Mn up to 0.001 wt% Mg up to 0.060 wt%

Si 0.06 wt%

B 0.010 wt% Zr 0.012 wt%

Fe 0.035 wt%

O 0.014 wt%

N 0.002 wt% S up to 0.010 wt%

C 0.043 wt%

Re 0.0 wt%

Ni balance based on the alloy weight. With reference to Fig. 1, the observed oxidation damage of conventional alloy 'Mar M 247 LC is characterized by spiking oxidation attack. The EDS analysis of the internal oxidation reveals that the AI2O3 is preferentially present along with Hf and Ta oxides. One possible explanation of this type of oxidation is that Hf carbides have greater affinity for oxygen than the matrix metal. Some literature studies show that Hf0 ₂ particles act as short circuit diffusion paths for oxygen transportation, due to the fact that the diffusivity of oxygen in Hf0 ₂ is several orders of magnitude higher than in AI2O3. This leads to preferentially localize scale thickening in the neighborhood of these particles, thus causing a deep penetration of the formed Hf0 ₂ scales into the substrate. The oxygen transported through this short circuit diffusion path reacts with Al atoms in the surrounding areas to form AI2O3 scales. Therefore, Hf0 ₂ 'pegs' scales surrounded by AI2O3 scales are formed.

Conversely, with reference with Fig. 2, the alloy herein disclosed shows a homogeneous oxide layer without preferentially localized scale thickening, but with a total affected layer that resulted half of that of conventional alloy 'Mar M 247 LC. Oxidation tests performed on the herein disclosed alloy demonstrated that its oxidation resistance is increased with respect to conventional alloy 'Mar M 247 LC, i.e. a comparative Ni-based superalloy, as shown Fig. 3.

The alloy herein disclosed can be obtained by processes known in the art, such as Powder Metallurgy, Investment Casting, Direct Metal Laser Melting (DMLM), Selective Laser Melting (SLM), Selective Laser Sintering (SLS), Laser Metal Forming (LMF) or Electron Beam Melting (EBM).

In general, the process of production of the alloy can be carried out until a desired thickness and shape of the alloy is achieved.

However, in preferred processes, the alloy is obtained by Direct Metal Laser Melting (DMLM), followed by a Hot Isostatic Press (HIP) process. The resulting alloy solution is then heat treated and allowed to cool and harden.

In some embodiments, the alloy is obtained by DMLM, wherein the power source has an energy power of 150-370 W, preferably of about 350W.

In other embodiments, the resulting powder layer thickness is preferably lower than 0.06 mm (i.e. 60 microns). Particularly preferred is a layer thickness of about 0.04 mm.

The power source scan spacing is preferably arranged in order to provide substantial overlapping of adjacent scan lines. An overlapping scan by the power source enables stress reduction to be provided by the subsequent adjacent scan, and may effectively provide a continuously heat treated material.

A Hot Isostatic Press (HIP) process is then performed in order to obtain the alloy with the desired characteristics. Good results have been obtained from 4 hours on, at 140 MPa and 1260°C with heat up and cool down rates of 8-15°C/min.

The resulting alloy solution is then heat treated and allowed to cool and harden. It should be understood that all aspects identified as preferred and advantageous for the alloy are to be deemed as similarly preferred and advantageous also for the respective processes of production.

Second embodiments of the subject matter disclosed herein correspond to a gas turbine component, such as a bucket, nozzle, combustion chambers, and shroud, made of the above alloy.

It should be also understood that all the combinations of preferred aspects of the alloy, and process of production, as well as their uses in gas turbine applications, as above reported, are to be deemed as hereby disclosed. While the disclosed embodiments of the subject matter described herein have been fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re- sequenced according to alternative embodiments. EXAMPLES

Example 1.

An alloy has been prepared having the following nominal composition: Co 9.07 wt% W 9.36 wt% Cr 8.43 wt%

Al 5.73 wt% Ti 0.65 wt%

Ta 2.93 wt%

Mo 0.51 wt%

Hf 1.02 wt% Mn up to 0.001 wt%

Mg up to 0.060 wt%

Si 0.06 wt%

B 0.010 wt%

Zr 0.012 wt% Fe 0.035 wt%

O 0.014 wt%

N 0.002 wt%

S up to 0.010 wt%

C 0.043 wt% Re 0.0 wt%

Ni balance based on the alloy weight.

The alloy was obtained by DMLM, wherein the power source had an energy power of about 350W. The resulting powder layer thickness was of about 0.04 mm. The power source scan spacing was preferably arranged in order to provide substantial overlapping of adjacent scan lines. An overlapping scan by the power source enabled stress reduction to be provided by the subsequent adjacent scan, and may effectively provide a continuously heat treated material.

A Hot Isostatic Press (HIP) process was then performed from 4 hours on, at 140 MPa and 1260°C with heat up and cool down rates of 8-15°C/min. The resulting alloy solution was then heat treated and allowed to cool and harden.

Example 2.

Oxidation resistance of the alloy of Example 1 has been evaluated by carrying out static oxidation tests at 870°C up to 4000 hours.

Tests were carried out on dish samples with a diameter of 25 mm and a thickness of 3mm.

Oxidated samples were cut in two parts and conventionally prepared for metallographic observation of their thickness. They were observed by Optical Microscope and the total affected layer by oxidation was measured.

OM microstructures for conventional Mar M247 LC (Fig. 1) and the alloy of Example 1 (Fig. 2) are reported. In particular, the oxidation damage of conventional Mar M247 LC characterized by spiking oxidation attacks due to Hafnium Carbides is well recognizable in Fig. 1. On the other hand, the alloy of Example 1 is characterized by a homogeneous oxidation layer.

Fig. 3 shows the affected metal thickness at 980°C after different times (from 1000 to 4000 hours) for conventional Mar M247 LC and the alloy of Example 1. The thickness is normalized with respect to the maximum affected thickness in Mar M 247 LC. It is well visible the better oxidation behavior of the alloy of Example 1 for which the affected metal thickness is halved with respect to conventional Mar M247 LC.