CA2586974C

CA2586974C - Nickel-base superalloy

Info

Publication number: CA2586974C
Application number: CA2586974A
Authority: CA
Inventors: Mohamed Youssef Nazmy
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2004-11-18
Filing date: 2005-11-01
Publication date: 2013-06-25
Anticipated expiration: 2025-11-01
Also published as: JP2008520829A; EP1815035A2; WO2006053826A3; CA2586974A1; CN101061244A; WO2006053826A2; CN101061244B; JP5186215B2; US20070199628A1; AR051423A1

Abstract

The invention relates to a nickel-base superalloy. The alloy according to the invention is characterized by the following chemical composition (details in % by weight): 7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-750 ppm C, 50-400 ppm B, 0.1-5 ppm S, 5-100 ppm Y and/or 5-100 ppm La, remainder Ni and production-related impurities. It is distinguished by very good casting properties, a high resistance to oxidation and good compatibility with TBC layers applied to its surface.

Description

Nickel-base superalloy Field of the invention The invention deals with the field of materials science. It relates to a nickel-base superalloy, in particular for the production of single-crystal components (SX alloy) or components with a directionally solidified microstructure (DS alloy), such as for example .blades or vanes for gas turbines.
However, the alloy according to the invention can also be used for conventionally cast components.
Background of the invention Nickel-base superalloys of this type are known. Single-crystal components produced from these alloys have a very good material strength at high temperatures. This makes it possible, for example, to increase the inlet temperature of gas turbines, which boosts the efficiency of the gas turbine.
Nickel-base superalloys for single-crystal components, as are known from US 4,643,782, EP 0 208 645 and US 5,270,123, for this purpose contain alloying elements which strengthen the solid solution, for example Re, W, Mo, Co, Cr, as well as elements which form y' phases, for example Al, Ta, and Ti. The level of high-melting alloying elements (W, Mo, Re) in the base matrix (austenitic y phase) increases continuously with the increase in the temperature to which the alloy is exposed. For example, standard nickel-base superalloys for single crystals contain 6-8% of W, up to 6% of Re and up to 2% of Mo (details in % by weight). The alloys disclosed in the abovementioned documents have a high creep strength, good LCF (low cycle fatigue) and HCF (high cycle fatigue) properties and a high resistance to oxidation.

2 These known alloys were developed for aircraft turbines and were therefore optimized for short-term and medium-term use, i.e. the load duration is designed for up to 20 000 hours. By contrast, industrial gas turbine components have to be designed for a load time of up to 75 000 hours.
By way of example, the alloy CMSX-4 from US 4,643,782, when tested for use in a gas turbine at a temperature of over 10000C, has a considerably coarsened y' phase after a load time of 300 hours, and this phenomenon is disadvantageously associated with an increase in the creep rate of the alloy.
It is therefore necessary to improve the resistance of the known alloys to oxidation at very high temperatures.
A further problem of the known nickel-base superalloys, for example the alloys which are known from US 5,435,861, is that in the case of large components, e.g. gas turbine blades or vanes with a length of more than 80 mm, the casting properties leave something to be desired. The casting of a perfect, relatively large directionally solidified single-crystal component from a nickel-base superalloy is extremely difficult, since most of these components have defects, e.g. small-angle grain boundaries, freckles, i.e. defects caused by a series of identically directed grains with a high eutectic content, equiaxed limits of variation, microporosity, etc. These defects weaken the components at high temperatures, and consequently the desired service life or operating temperature of the turbine are not achieved. However, since a perfectly cast single-crystal component is extremely expensive, the industry tends to permit as many defects as possible without the service life or operating temperature being adversely affected.

3 One of the most common defects is grain boundaries, which are particularly harmful to the high-temperature properties of the single-crystal items. Whereas in small components small-angle grain boundaries in relative terms have only a minor influence on the properties, they are highly relevant to the casting properties and oxidation properties of large SX or DS
components at high temperatures.
Grain boundaries are regions with a high local disorder of the crystal lattice, since the neighboring grains collide in these regions, and therefore there is a certain misorientation between the crystal lattices.
The greater the misorientation, the greater the disorder, i.e. the greater the number of dislocations in the grain boundaries which are required for the two grains to fit together. This disorder is directly related to the properties of the material at high temperatures. It weakens the material if the temperature rises to above the equicohesive temperature (= 0.5 x melting point in K).
This effect is known from GB 2 234 521 A. For example, in a conventional nickel-base single-crystal alloy, at a test temperature of 871 C, the fracture strength drops greatly if the misorientation of the grains is greater than 6 . This has also been confirmed in single-crystal components with a directionally solidified microstructure, and consequently the viewpoint has generally been that misorientations of greater than 6 are unacceptable.
It is also known from the above-referenced GB 2 234 521 A that enriching nickel-base superalloys with boron or carbon during a directional solidification produces microstructures which have an equiaxed or prismatic grain structure. Carbon and boron

4 strengthen the grain boundaries, since C and B cause the precipitation of carbides and borides at the grain boundaries, and these compounds are stable at high temperatures. Moreover, the presence of these elements in and along the grain boundaries reduces the diffusion process, which is a primary cause of the grain boundary weakness. It is therefore possible to increase the misorientations to 10 to 12 yet still achieve good materials properties at high temperatures. However, these small-angle grain boundaries have an adverse effect on the properties in particular of large single-crystal components formed from nickel-base superalloys.
Document EP 1 359 231 Al describes a nickel-base superalloy which has improved casting properties and a higher resistance to oxidation than known nickel-base superalloys. Moreover, this alloy is, for example, particularly suitable for large gas turbine single-crystal components with a length of > 80 mm. It has the following chemical composition (details in 95 by weight):
7.7-8.3 Cr

5.0-5.25 Co 2.0-2.1 Mo 7.8-8.3 W
5.8-6.1 Ta 4.9-5.1 Al 1.3-1.4 Ti 0.11-0.15 Si 0.11-0.15 Hf 200-750 ppm C
50-400 ppm B
remainder nickel and production-related impurities.
However, its compatibility with TBC (thermal barrier coating) layers, which are used in particular in the gas turbine sector to protect components exposed to particularly high thermal stresses, still needs improvement.

Summary of the invention The aim of the invention is to avoid the abovementioned 5 drawbacks of the prior art. The invention is based on the object of further improving the nickel-base superalloy which is known from EP 1 359 231 Al, in particular with a view to achieving better compatibility with TBC layers to be applied to this superalloy combined with equally good casting properties and a high resistance to oxidation compared to the nickel-base superalloy which is known from EP 1 359 231 Al.
According to the invention, this object is achieved by the fact that the nickel-base superalloy consists of the following chemical composition in % by weight:
7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 0.11-0.15 Si, 0.11-0.15 Hf, 200-750 ppm C, 50-400 ppm B, < 5 ppm S, 5-100 ppm Y and 5-100 ppm La, remainder nickel and production-related impurities.
The advantages of the invention are that the alloy has very good casting properties, a high resistance to oxidation at high temperatures and is very compatible with TBC layers that are to be applied.

6 It is expedient if the alloy consists of the following composition in % by weight:

7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-300 ppm C, 50-100 ppm B, max 2 ppm S, 10-80 ppm Y and 10-80 ppm La, remainder nickel and production-related impurities.
An advantageous alloy according to the invention consists of the following chemical composition in % by weight:
7.7 Cr, 5.1 Co, 2.0 Mo, 7.8 W, 5.8 Ta, 5.0 Al, 1.4 Ti, 0.12 Si, 0.12 Hf, 200 ppm C, = 50 ppm B, 1 ppm S, 50 ppm Y, and ppm La, remainder nickel and production-related impurities.

This alloy is eminently suitable for the production of large single-crystal components, for example blades or vanes for gas turbines.
Ways of implementing the invention The invention is explained in more detail below on the basis of an exemplary embodiment.
Nickel-base superalloys which are known from the prior art (comparison alloys CA1 to CA5) and the alloy according to the invention Al having the chemical composition listed in Table 1 were tested (details in %
by weight):
CA1 CA2 CA3 CA4 CA5 Al (CMSX- (CMSX-6) (CMSX-2) (Rene N5) (in 11B) accordance with EP 1359231A) Ni Remainder Remainder Remainder Remainder Remainder Remainder Cr 12.4 9.7 7.9 7.12 7.7 7.7 Co 5.7 5.0 4.6 7.4 5.1 5.1 Mo 0.5 3.0 0.6 1.4 2.0 2.0 W 5.1 - 8.0 4.9 7.8 7.8 Ta 5.18 2.0 6.0 6.5 5.84 5.8 Al 3.59 4.81 5.58 6.07 5.0 5.0 Ti 4.18 4.71 0.99 0.03 1.4 1.4 Hf 0.04 0.05 - 0.17 0.12 0.12 C - - - - 0.02 0.02 B - - - 0.005 0.005 Si - - - - 0.12 0.12 Nb 0.1 - - - -Re - _ _ 2.84 - -S - - - -0.0001 ,Y - - - 0.005 La - - - - _- 0.001 Table 1: chemical composition of the alloys tested

8 The alloy Al is a nickel-base superalloy for single-crystal components, the composition of which is covered by the patent claim of the present invention. The alloys CA1, CA2, CA3, CA4 are comparison alloys which are prior art known under the designations CMSX-11B, CMSX-6, CMSX-2 and Rene N5. Inter alia, they differ from the alloy according to the invention primarily by virtue of the fact that they are not alloyed with C, B, Si and Y and/or La. The comparison alloy CA5 is known from EP 1 359 231 Al and differs from the alloy according to the invention in terms of the S, Y and/or La content.
Carbon and boron strengthen the grain boundaries, in particular also the small-angle grain boundaries which occur in the <001> direction in SX or DS gas turbine blades or vanes made from nickel-base superalloys, since these elements cause the precipitation of carbides and borides at the grain boundaries, and these compounds are stable at high temperatures. Moreover, the presence of these elements in and along the grain boundaries reduces the diffusion process, which is a primary cause of the grain boundary weakness. This considerably improves the casting properties of long single-crystal components, for example gas turbine blades or vanes with a length of approx. 200 to 230 mm.
The addition of 0.11 to 0.1595 by weight of Si, in particular in combination with Hf in approximately the same order of magnitude, significantly improves the resistance to oxidation at high temperatures compared to the previously known nickel-base superalloys CA1 to CA4.
Restricting the composition according to the invention to a sulfur content of < 5 ppm produces very good properties, in particular good bonding of the TBC layer which has been applied to the surface of the

9 superalloy, for example by thermal spraying. If the sulfur content is > 5 ppm, this has an adverse effect on the TBC bonding, and the layer quickly flakes off in the event of fluctuating thermal stresses.
The addition of Y and/or La in the specified range (in each case 5 to 100 ppm), i.e. in total, that is to say Y + La, 10 to 200 ppm, if both elements are present produces very good bonding of the ceramic thermal barrier coating (TBC layer) which is to be applied.
The Y content of 50 ppm and the La content of 10 ppm specified for the alloy Al is particularly advantageous, since Al is particularly compatible with the TBC layers to be applied. Moreover, these two elements also increase the resistance to environmental influences. The addition of these elements in these low ranges stabilizes the aluminum/chromium oxide scale layer on the alloy surface and produces a significant resistance to oxidation. Y and La are oxygen-active elements which improve the bonding strength of the scale layer on the base material. The resistance to spalling during cyclic oxidation is the key factor for the stability of the TBC layer.
Table 2 in each case lists the number of cycles which it takes for the A1203 and other oxide layers formed to flake off under cyclic oxidation at 10500C/1h/air cooling to room temperature for the alloys listed in Table 1:
Alloy Number of cycles until spelling occurs CA1 < 30 Al 2500 Table 2: number of cycles until spalling occurs The alloy according to the invention Al, compared to 5 the alloys which are known from the prior art, has by far the highest number of cycles before the oxide layer flakes off. This implies a high stability of a TBC
layer which is to be applied to the surface of the superalloy, for example by thermal spraying.
If, in other exemplary embodiments, by way of example, nickel-base superalloys with higher C and B contents (at most 750 ppm of C and at most 400 ppm of B) are selected in accordance with claim 1 of the invention, it is also possible for the components produced from these alloys to be cast conventionally; i.e. without them producing single crystals.

Claims

1. A nickel-base superalloy, consisting of the following chemical composition in % by weight :
7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-750 ppm C, 50-400 ppm B, < 5 ppm S, 5-100 ppm Y and 5-100 ppm La, remainder nickel and production-related impurities.

2. The nickel-base superalloy as claimed in claim 1, for the production of single-crystal components, consisting of the following chemical composition in 96 by weight:
7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-300 ppm C, 50-100 ppm B, max. 2 ppm S, 10-80 ppm Y and 10-80 ppm La, remainder nickel and production-related impurities.

3. The nickel-base superalloy as claimed in claim 2, consisting of the following chemical composition, in % by weight:
7.7 Cr, 5.1 Co, 2.0 Mo, 7.8 W, 5.8 Ta, 5.0 Al, 1.4 Ti, 0.12 Si, 0.12 Hf, 200 ppm C, 50 ppm B, 1 ppm S, 50 ppm Y, and ppm La, remainder nickel and production-related impurities.