NICKEL BASE ALLOY

Abstract: A nickel base alloy, in particular used as a coating, essentially comprising: (measured in % by weight): Co 11 - 16, Cr 12.2 - 15.5, Al 6.5 - 7.2, Re 3.2 - 5.0, Si 1-0 - 2.5, Ta 1.5 - 4.5, Nb 0.2 - 2.0, Hf 0.2 - 1.2, Y 0-2 - 1.2, Mg 0 - 1.5, Zr 0 - 1.5, La and La-series elements 0 - 0.5, C 0 - 0.15, B 0 - 0,1. Remainder being Ni with impurities.

Description: NICKEL BASE ALLOY TECHNICAL FIELD The invention relates to a nickel base alloy in accordance with the preamble of the first claim. BACKGROUND OF THE INVENTION This invention relates to nickel-based alloys, especially for those used as a coating for high temperature gas turbine blades and vanes. Wide use of single crystal (SX) and directionally solidified (DS) components has allowed increased turbine inlet temperature and therefore turbine efficiency. Alloys, specially designed for SX/DS casting, were developed in order to make a maximum use of material strength and temperature capability. For this purpose modern SX alloys contain Ni and solid-solution strengtheners such as Re, W, Mo, Co, Cr as well as γ ' -forming elements Al, Ta, Ti. The amount of refractory elements in the matrix has continuously increased with increase in the required metal temperature. In a typical SX alloys their content is limited by precipitation of deleterious Re-, W-or Cr-rich phases. High temperature components are typically coated to protect them from oxidation and corrosion. In order to take full advantage of increased temperature capability and mechanical properties of SX/DS blade base material, coating material must provide now not only protection from oxidation and corrosion, but must also not degrade mechanical properties of base material and have a stable bond to substrate without spoliation during the service. Therefore requirements to advance coating are: -high oxidation and corrosion resistance, superior to those of the SX/DS superalloys; -low interdiffusion of Al and Cr into the substrate to prevent precipitation of needlelike phases under the coating; -creep resistance comparable to those of conventional superalloys, which can be achieved only with the similar coherent γ-γ ' structure; -low ductile-brittle transition temperature, ductility at low temperature; -thermal expansion similar to substrate along the whole temperature range. Coating described in US Patent 5 ' 043'138 is a derivative of the typical SX superalloy with addition of yttrium and silicon in order to increase oxidation resistance. Such a coatings have very high creep resistance, low ductile-brittle transition temperature (DBTT), thermal expansion equal to the substrate and almost no interdiffusion between coating and substrate. However, presence of such strengtheners as W and Mo, as well as a low chromium and cobalt content, typical for the SX superalloys, have a deleterious effect on oxidation resistance. EP Patent 0412397 describes the coating with significant addition of Re, which simultaneously improves creep and oxidation resistance at high temperature. However, combination of Re with high Cr content, typical for traditional coatings, results in undesirable phase structure of coating and interdiffusion layer. At intermediate temperatures (below 950-900°C), α-Cr phase is more stable in the coating than γ-matrix. This results in a lower thermal expansion compared to the base material, lower toughness and possibly lower ductility. In addition a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which makes it prone to precipitation of needlelike Cr-, W-and Re-rich phases. SUMMARY OF THE INVENTION Accordingly, one object of the invention is to provide an nickel base alloy which is designed to combine an improved ductility and creep resistance, phase stability of coating and substrate during service, phase structure and thermal expansion similar to the substrate and an excellent oxidation resistance. According to the invention, this is achieved by the features of the first claim. The core of the invention is therefore that the nickel base alloy, in particular used as a coating, essentially comprises: (measured in % by weight): Co 11 -16 Cr 12.2 -15.5 Al 6.5 - 7.2 Re 3.2 - 5.0 Si 1.0 - 2.5 Ta 1.5 - 4.5 Nb 0.2 - 2.0 Hf 0.2 - 1.2 Y 0.2 - 1.2 Mg 0 - 1.5 Zr 0 - 1.5 La and La- ■series elements 0 - 0.5 C 0 - 0.15 B 0 - 0.1 Remainder being Ni with impurities The advantages of the invention can be seen, inter alia, in the fact that by optimisation of Al activity in the alloy and due to the specific phase structure, consisting of fine precipitates of γ ' and α-Cr in γ-matrix an improved ductility and creep resistance, phase stability of coating and substrate during service, phase structure and thermal expansion similar to the substrate and an excellent oxidation resistance can be obtained. To achieve the γ-γ ' -α-Cr-structure the relatively high but limited contents of Al and Cr were combined. To prevent coarsening of the α- Cr phase an addition of more than 3% Re was necessary. Further advantageous embodiments of the invention emerge from the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Fig. 1 Al activity vs. Al content in γ-γ ' -α-Cr system; Fig. 2 Al activity vs. Cr content in γ-γ ' -α-Cr system; Fig. 3 Al activity vs. Si content in γ-γ ' -α-Cr system; Fig. 4 Al activity vs. Re content in γ-γ ' -α-Cr system; Fig. 5 Phase structure of LSV-1 coating. Fine precipitates of α-Cr,Re (white due to high Re content and edge effect) phase; Fig. 6 Phase structure of LSV-6 coating. Undesirable chain-like distribution of β-(black) and σ-(gray) phases; Fig. 7 Phase structure of LSV-5 coating. Coarse pentagonal precipitates of α- Cr phase. Only those elements that are essential for an understanding of the invention are shown. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention describes a nickel base superalloy, whose essential composition range is shown in Table 2, which is particularly adapted for use as coating for advanced gas turbines blades and vanes. Generally, the alloy in this invention should be prepared with the elements in an amount to provide an alloy composition as shown in Table 1. Preferably, the alloy could be produced by the vacuum melt process in which powder particles are formed by inert gas atomisation. The powder can then be deposited on a substrate using, for example, thermal spray methods. However, other methods of application may also be used. Heat treatment of the coating using appropriate times and temperatures is recommended to achieve a good bond to the substrate and a high sintered density of the coating. The alloy chemical composition is specifically designed to combine an improved ductility and creep resistance, phase stability of coating and substrate during service, phase structure and thermal expansion similar to the substrate and an excellent oxidation resistance due to high activity of Al. This is achieved by optimisation of Al activity in the alloy (fig. 1-4) and due to the specific phase structure, consisting of fine precipitates of γ ' (55-65 vol.%) and α-Cr (1.5-3 vol.%) in γ-matrix (alloys LSV 1 ,3, fig. 5). To achieve this structure the relatively high contents of Al (about 7%) and Cr (about 13%) were combined. To prevent coarsening of the α-Cr phase an addition of more than 3% Re was necessary. Composition of experimental coatings are shown in Table 1. Table 3 represents results of experimental evaluation of several compositions of coatings with respect of their oxidation resistance and mechanical properties. Upon oxidation the alloy shows an increase in weight due to the uptake of oxygen. If the growing oxide scale is protective the weight gain as a function of oxidation time follows a parabolic rate law. Obviously, a small weight increase is indicative of a slowly growing oxide scale and, thus, is a desirable property. Presented in Table 3 are experimental data which show that the weight change is lowest for the preferred alloy composition (LSV 1 ,3) when compared to experimental alloys LSV 4,5,7,10,11. The oxidation resistance of the inventive alloy is determined by Al content (as reservoir of Al atoms for formation of protective AI 2 O 3 scale) by activity of Al in the system, by alloy phase structure, which determines Al diffusion and by control over oxide growth rate through controlled addition of active elements, i.e combination of Ta and Nb. Presence and content of other elements has a very strong effect on the activity of Al. Examples modelled for γ-γ ' -α-Cr system using known computer software (ThermoCalc and DICTRA), are presented on Fig. 1-4 (for varied Al, Cr, Si and Re respectively with fixed content of other elements, reference system Ni-13 Cr-12 Co-7 AI-3.5 Re-2 Si-3 Ta-1 Nb). Fig. 1 shows, that for the Al content higher than 6.5%, activity of Al (and therefore the oxidation resistance of the alloy) increases most efficiently. This is illustrated by comparison of properties of alloys LSV-1 and LSV-10 (Table 3). Their chemical composition is identical with exception of the Al level (7% and 6.1 % respectively). If Al content exceeds some particular level (7.2 % in the present system), the precipitation of β-and σ-phases with undesirable morphology reduce the low temperature ductility of alloys (alloy LSV-6, fig. 6, Table 3,4). Very tight control is also required for the Cr content. The low Cr content results not only in low corrosion resistance of the coating, but also in lower activity of Al and therefore considerably lower oxidation resistance. This illustrates Fig. 2, which shows, that the highest activity of Al in the alloy can be achieved at Cr content higher than 12%. Below this level the AI 2 O 3 scale is not dense and additional Ni and Cr oxides deleteriate the oxidation resistance. Comparison of properties of alloys LSV 1 , 3 and alloy LSV-11 from Table 3 proves it. For the other hand, Cr content higher then 15.5%, results in significant reduction in low temperature ductility of the alloy (alloy LSV-9, Table 1 ,3,4). At this concentration of Cr and other elements, the more thermodynamically stable at intermediate (below 900°C) temperatures α-Cr phase replaces to the large extend the ductile γ -matrix during the service exposure, which results in considerable enbrittlement of the coating. Resulting α-Cr-σ-γ ' -γ or α-Cr-β-γ ' -γ structures are much less ductile than the γ -γ ' structure with fine α-Cr precipitates, chosen for the coatings of the present invention. Co increases solubility of Al in γ -matrix. The relatively high Co level in alloys of the present invention allows to achieve the uniquely high concentration of both Al and Cr in γ -matrix without precipitation of the mentioned above undesirable β-and σ- phases, and therefore to increase the oxidation resistance of alloy without reduction in mechanical properties. Comparison of alloys ' properties for LSV-1 ,3 on one side and those of the alloy LSV-4, which is similar to compositions in the range of Pat. US 5035958, on the other side, confirms the beneficial role of high Co content (Table 3). High level of Co, more than 16%, results in significant lowering of the γ'-solvus temperature compared to the base alloy. Therefore at the temperature range above coating γ'-solvus and below substrate γ'-solvus, two materials have high thermal expansion mismatch, which leads to significant reduction in coating thermomechanical-fatigue-(TMF)-life. Re in the alloy replace other refractory elements such as W and Mo and provides high creep and fatigue resistance to the coating without deleterious effect on oxidation and corrosion resistance. Moreover, Re increases activity of Al in alloy and therefore is beneficial for oxidation resistance (Fig. 4). At the same time Re is responsible for the stabilising the fine morphology of γ ' particles which also considerably improves creep properties. These functions of Re are relatively linear to its content in alloy and are known from the state of art. What was found new in the present invention, is that in the γ -γ ' -α structure Re considerably changes α-Cr composition and morphology, but only after some particular level in the alloy. At the content up to 3 %, Re partitioning mostly in the γ -matrix, similar to it's behaviour in superalloys. α-Cr phase at low Re concentrations consist for 95 at.% of Cr with 1-2 at.% of each Ni, Re, Co. α-Cr precipitates have coarse pentagonal morphology with size in order of 3-6 μm (as in alloy LSV-5, fig. 7). The excess of Re and Cr in the matrix precipitates separately in the undesirable form of needlelike Re-rich TCP phases (so called r-and p-phases), especially on interface with substrate, and mechanical properties of the system falls down (Table 3, alloy LSV 5 compared to alloys LSV 1 , 3). At the Re content higher than 3%, the type of α- phase changes from Cr phase to mixed Cr-Re phase (with 15-20 at.% of Re and up to 8 at.% of Co, Table 4,5). The new phase has much finer morphology (size is 1 μm and smaller) and its presence prevents also precipitation of needle-like Re- rich r-and p-phases, as solubility range of Re and Co in the α-Cr-Re phase is relatively wide. The condition, when the desirable Cr-Re α-phase precipitates is described (for Al range 6.5-7.2 % and in presence of Ta, Nb, Si; W + Mo = 0; Re > 3%) as (Re + 0.2 Co) / 0.5 Cr = 0.9, {1} where Re, Co, Cr are contents of elements in alloy in wt.%. At (Re + 0.2 Co) / 0.5 Cr < 0.9 the coarse α-Cr and needle-like Re-rich TCP phases precipitate. Typically, MCrAIY coatings contain 0.3 to 1 wt% Y which has a powerful effect on the oxidation resistance of the alloy. In some fashion, Y acts to improve the adherence of the oxide scale which forms on the coating, thereby substantially reducing spallation. A variety of other so-called oxygen active elements (La, Ce, Zr, Hf, Si) have been proposed to replace or supplement the Y content. Patents which relate to the concept of oxygen active elements in overlay coatings include U.S. Pat. Nos. 4,419,416 and 4,086,391. in the present invention Y is added in amounts on the order of 0.3 to 1.3 wt%, La and elements from the Lanthanide series in amounts ranging from 0 to 0.5 wt%. In the present invention Nb and Ta were found increasing oxidation resistance through reducing the rate of oxide growth, with their cumulative effect stronger than the influence of any one of them taken separately. Even small amounts of Nb on the order of 0.2-0.5 wt% in the presence of Ta has found to have a significant effect on oxidation resistance (preferred composition results vs. LSV-7, Tab. 3). Si in alloy increases oxidation resistance by increasing the activity of Al (Fig. 4). The Si effect on Al activity becomes significant first at Si content higher than 1 %. At the same time the Si content higher than 2.5 % results in precipitation of brittle Ni (Ta, Si) Heusler phases and in embrittlement of the γ-matrix. The range of composition for Hf, Y, Mg, Zr, La, C and B is optimized for oxidation lifetime of the coating. The invention is of course not restricted to the exemplary embodiment shown and described. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. Table 1 : Composition of experimental coatings LSV-4* : W = 2.5 wt.%, Mo = 1 wt.% Table 2: Preferred range of the alloy according to the invention La* = La and La-series elements Table 3: Experimental evaluation of coatings Table 4: Phase volume fraction in structure of experimental coatings, vol.% Table 5: Phase composition of α phase in experimental coatings, at.%

Claims: PATENT CLAIMS 1. A nickel base alloy, in particular used as a coating, essentially comprising: (measured in % by weight): Co 11 -16 Cr 12.2-15.5 Al 6.5-7.2 Re 3.2 - 5.0 Si 1-0-2.5 Ta 1.5-4.5 Nb 0.2-2.0 Hf 0.2-1.2 Y 0-2 - 1.2 Mg 0-1.5 Zr 0-1.5 La and La- •series elements 0-0 C 0-0.15 B 0-0.1 Remainder being Ni with impurities 2. A nickel base alloy as claimed in claim 1 , wherein (Re + 0.2 Co) / 0.5 Cr not less than 0.9 and Y + Zr + La (+La-series) 0.3 - 2.0. 3. A coating or a nickel base alloy as claimed in claim 1 and 2, having a phase structure consisting of fine precipitates of γ ' and α-Cr in a γ- matrix. 4. A coating as claimed in claim 3, wherein the fine precipitates of γ ' ranges from 55 to 65 vol.% and the α-Cr ranges from 1.5 to 3 vol.% in γ-matrix. 5. A nickel base alloy or a coating as claimed in claim 1 to 4, used as a coating for gas turbine components. A nickel base alloy or a coating as claimed in claim 1 to 4, used as a coating for gas turbine blades and vanes.

Inventors: BOSSMANN HANS-PETER, HOLMES PETER, KONTER MAXIM, SOMMER CHRISTOPH, TOENNES CHRISTOPH, BOSSMANN, HANS-PETER, HOLMES, PETER, KONTER, MAXIM, SOMMER, CHRISTOPH, TOENNES, CHRISTOPH
Applicants: ABB RESEARCH LTD, BOSSMANN HANS PETER, HOLMES PETER, KONTER MAXIM, SOMMER CHRISTOPH, TOENNES CHRISTOPH, ABB RESEARCH LTD., BOSSMANN, HANS-PETER, HOLMES, PETER, KONTER, MAXIM, SOMMER, CHRISTOPH, TOENNES, CHRISTOPH
Application year: 1997
Main classification: C22C19
IPC codes: C22C1, C22C19, C23C30