Patent Application
20180245191
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-034735 filed on Feb. 27, 2017, the contents of which are incorporated herein by reference.
The present invention relates to a Ni-based intermetallic alloy that contains, as basic elements, Ni, Al, and V that are in a range of a composition ratio that enables the formation of a dual multi-phase microstructure containing a primary precipitate Ni3Al phase (hereinafter referred to as the primary precipitate L12 phase) and an (Ni3Al+Ni3V) eutectoid microstructure (hereinafter referred to as the (L12+D022) eutectoid microstructure). The present invention further relates to a method for producing the Ni-based intermetallic alloy.
For example, a turbine or the like for a drive unit in an aircraft must be made from high-temperature structural materials that have a light weight, an excellent oxidation resistance, and sufficient strength and hardness (abrasion resistance) even in a high-temperature environment with a temperature of higher than 800° C. As a high-temperature structural material having such properties, a Ni-based intermetallic alloy having a dual multi-phase microstructure containing a primary precipitate L12 phase and an (L12+D022) eutectoid microstructure has been proposed in Japanese Laid-Open Patent Publication No. 2006-299403.
Furthermore, Ni-based intermetallic alloys containing Nb and Mo is described respectively in International Publication No. WO 2007/086185 and Japanese Laid-Open Patent Publication No. 2016-160495. As reported in International Publication No. WO 2007/086185 and Japanese Laid-Open Patent Publication No. 2016-160495, strength of the alloy in a high-temperature environment is improved by the addition of Nb, and hardness and tensile strength is improved by the addition of Mo.
As described in International Publication No. WO 2007/086185 and Japanese Laid-Open Patent Publication No. 2016-160495, it is possible to improve the strength of the Ni-based intermetallic alloy having the dual multi-phase microstructure. However, there is a demand for further improving the other properties.
A principal object of the present invention is to provide a Ni-based intermetallic alloy having a particularly excellent ductility.
Another object of the present invention is to provide a method for producing the Ni-based intermetallic alloy.
According to an aspect of the present invention, there is provided a Ni-based intermetallic alloy having a dual multi-phase microstructure containing a primary precipitate L12 phase and an (L12+D022) eutectoid microstructure. The Ni-based intermetallic alloy comprises Ni, Al, and V as basic elements, a composition of Ni, Al, and V being in a range that enables formation of the dual multi-phase microstructure, and further comprises at least one of Zr and Hf, a total composition ratio of the basic elements plus the at least one of Zr and Hf is 100 at %.
According to another aspect of the present invention, there is provided a method for producing a Ni-based intermetallic alloy having a dual multi-phase microstructure containing a primary precipitate L12 phase and an (L12+D022) eutectoid microstructure. The method comprises the steps of: mixing at least one of Zr and Hf with basic elements of Ni, Al, and V to prepare an alloy, a composition ratio of Ni, Al, and V being in a range that enables formation of the dual multi-phase microstructure wherein a total composition ratio of the basic elements plus at least one of Zr and Hf is 100 at %; subjecting the alloy to a first heat treatment, thereby forming a single-phase microstructure of an A1 phase (a face-centered cubic Ni solid solution phase); and subjecting the alloy to a second heat treatment, thereby forming a multi-phase microstructure containing the primary precipitate L12 phase and the A1 phase, and then decomposing the A1 phase to the (L12+D022) eutectoid microstructure to obtain the dual multi-phase microstructure.
Incidentally, in the present invention, the term “at %” means atomic percent.
The Ni-based intermetallic alloy has the dual multi-phase microstructure containing the primary precipitate L12 phase and the (L12+D022) eutectoid microstructure. In other words, the Ni-based intermetallic alloy contains the basic elements of Ni, Al, and V, a composition ratio of Ni, Al, and V being in a range that enables formation of the dual multi-phase microstructure. For example, the composition ratio is such that Al is 5.0 to 13.0 at %, V is 10.0 to 18.0 at %, and Ni is 60.0 at % or more (balance).
In the present invention, at least one of Zr and Hf is added to the basic elements. Zr and Hf are capable of forming a compound particularly together with Ni. The compound is crystallized in a grain boundary. All or part of the grain boundaries in the dual multi-phase microstructure are replaced by an interface made up from the crystallized compound and the dual multi-phase microstructure, whereby intergranular cracking is prevented. As a result, the Ni-based intermetallic alloy has an excellent ductility.
Meanwhile, a certain amount of Zr and Hf are solid-dissolved in the dual multi-phase microstructure. As a result, the Ni-based intermetallic alloy has an excellent strength.
It is preferred that the total content of Zr and Hf is 1.5 at % or less. Thus, in the case of using only one of Zr and Hf and the case of using both of Zr and Hf, the maximum composition ratio is preferably 1.5 at %. When the composition ratio is more than 1.5 at %, it is possible that a coarse compound is generated, and the intergranular cracking cannot be prevented easily.
It is preferred that the Ni-based intermetallic alloy further contains at least one of Nb and Mo. In this case, the Ni-based intermetallic alloy can have a more excellent strength.
It is preferred that the total content of Nb and Mo is 2.5 at % or less.
It is preferred that the Ni-based intermetallic alloy further contains 1.5 at % or less of C. C together with Zr or Hf forms zirconium carbide or hafnium carbide. The carbide is also crystallized in a grain boundary, and acts to prevent the intergranular cracking. As a result, the Ni-based intermetallic alloy has a further improved toughness. When the content of C is more than 1.5 at %, it is possible that coarse carbide is generated, and the intergranular cracking cannot be prevented easily.
It is preferred that the Ni-based intermetallic alloy further contains B. B acts to prevent the intergranular cracking particularly at around room temperature, and thus to improve the ductility. It is preferred that the content of B is 0.02 to 0.1 at %. When the content of B is more than 0.1 at %, it is possible that a low-melting-point phase is formed, whereby the strength or the like of the Ni-based intermetallic alloy is often lowered at a high temperature.
As the second heat treatment, the alloy obtained from the first heat treatment may be subjected to natural cooling or continuous cooling at a predetermined cooling rate. Similarly, as the first heat treatment, the alloy obtained from the mixing step may be subjected to natural cooling or continuous cooling at a predetermined cooling rate.
In the present invention, the dual multi-phase microstructure containing the primary precipitate L12 phase and the (L12+D022) eutectoid microstructure is formed by the basic elements of Ni, Al, and V, and the dual multi-phase microstructure further contains at least one of Zr and Hf. At least one of Zr and Hf mainly together with Ni forms the compound in the grain boundary, whereby the intergranular cracking is prevented. Therefore, the resultant Ni-based intermetallic alloy has the excellent ductility. In addition, the resultant Ni-based intermetallic alloy has the excellent strength due to the above-described solid-dissolving.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.
A preferred embodiment of the Ni-based intermetallic alloy and the production method thereof of the present invention will be described in detail below with reference to the accompanying drawings.
A dual multi-phase microstructure 16 of a Ni-based intermetallic alloy 10 will be described with reference to
The Ni-based intermetallic alloy 10 has the dual multi-phase microstructure 16 containing a primary precipitate L12 phase 12 and an (L12+D022) eutectoid microstructure 14. The L12 phase composes Ni3Al, and the D022 phase composes Ni3V. Thus, the Ni-based intermetallic alloy 10 has the dual multi-phase microstructure 16 containing two kinds of intermetallic compound having the close-packing structure. Consequently, as compared with intermetallic compounds having single-phase structures, the Ni-based intermetallic alloy 10 has more excellent ductility and toughness and exhibits more excellent strength and hardness even in a high-temperature environment.
The primary precipitate L12 phase 12 has an approximately cubic shape. The (L12+D022) eutectoid microstructure 14 is formed in a channel, i.e. a gap between the approximately cubic shapes of the primary precipitate L12 phase 12. Thus, in other words, the dual multi-phase microstructure 16 has an upper multi-phase microstructure containing the primary precipitate L12 phase 12 and the channel, and further has a lower multi-phase microstructure containing the L12+D022) eutectoid microstructure 14.
The Ni-based intermetallic alloy 10 contains Ni, Al, and V as basic elements, the composition ratio of Ni, Al, and V being in a range that enables the formation of the dual multi-phase microstructure 16. For example, the range of the composition ratio that enables the formation of the dual multi-phase microstructure 16 is that the content of Al is 5.0 to 13.0 at %, the content of V is 10.0 to 18.0 at %, and the content of Ni is 60.0 at % or more where each composition ratio has been defined with the total of all elements being 100 at % in the Ni-based intermetallic alloy 10.
The Ni-based intermetallic alloy 10 further contains at least one of Zr and Hf. Thus, the Ni-based intermetallic alloy 10 is an at least quaternary-system alloy.
The Ni-based intermetallic alloy 10 preferably contains C. It is more preferable for the Ni-based intermetallic alloy 10 to contain at least one of Nb and Mo. The Ni-based intermetallic alloy 10 may further contains B and/or another metal element such as Co.
Thus, in such composition, Zr or Hf forms a compound mainly with Ni. Furthermore, in the case of using C, C reacts with Zr or Hf to generate zirconium carbide or hafnium carbide. In some cases, Zr, Hf, and C may form complex carbide together. The compound or the carbide is a second phase particle in a grain boundary, and the diameter of the particle is generally 1 to 100 μm, typically 10 to 50 μm.
The second phase particle in the grain boundary acts to prevent so-called intergranular cracking. Therefore, the Ni-based intermetallic alloy 10 has an excellent ductility. In addition, Zr and Hf that do not participate in the formation of the carbide or the second phase particle in the grain boundary are solid-dissolved into the dual multi-phase microstructure 16. Therefore, the Ni-based intermetallic alloy 10 has also an excellent strength due to the solid solution strengthening.
It is preferred that the total content of Zr or Hf is 1.5 at % or less with respect to the total content.
As described above, C together with Zr or Hf forms the crystallized compound in the grain boundary and acts to prevent the intergranular cracking. In addition, a part of C solid-dissolves in the dual multi-phase microstructure 16. Thus, as well as Zr and Hf, C acts to improve the toughness and the strength of the Ni-based intermetallic alloy 10.
In a case where the Ni-based intermetallic alloy 10 contains Nb, the Ni-based intermetallic alloy 10 exhibits an improved strength at any temperature in a range from room temperature to high temperature. In a case where the Ni-based intermetallic alloy 10 contains Mo, the Ni-based intermetallic alloy 10 exhibits improved hardness and tensile strength. It is preferred that the total content of Nb or Mo is 2.5 at % or less with respect to the total content (100 at %) of all elements in the Ni-based intermetallic alloy 10. When C is present, Nb and Mo form carbide.
B acts to prevent the intergranular cracking particularly at around room temperature, and thus to improve the ductility. It is preferred that the content of B is 0.02 to 0.1 at % with respect to the total content (100 at %) of all elements in the Ni-based intermetallic alloy 10.
The Ni-based intermetallic alloy 10 may be produced by a melt casting method, a powder metallurgy method, etc.
First, for example, raw metals of the basic elements (Ni, Al, and V) and at least one of Zr and Hf are mixed in a manner such that the composition ratio of the elements falls within the above ranges. The mixture is melted to prepare a molten metal. It is to be understood that Nb, Mo, C, B, and the like may be added to the mixture in this step. In this case, the molten metal is cooled and solidified to prepare an alloy ingot.
For example, in a case where the melting is conducted in a small arc furnace, the molten metal is cooled at a relatively high cooling rate (solidifying rate) until the molten metal becomes the ingot, whereby the microstructures and the constituent elements could be non-uniformly distributed in the ingot. Therefore, the ingot is subjected to a first heat treatment. In the first heat treatment, the ingot is solution-treated (the constituent elements are mixed) and homogenized. Thus obtained alloy has a single-phase microstructure of a face-centered cubic (fcc) A1 phase. In other words, in the first heat treatment, the conditions of the temperature, the holding time, and the like may be such that the mixture and the homogenization proceed to form the single-phase microstructure of the A1 phase. Incidentally, the A1 phase is a Ni solid solution phase that does not have an ordered structure (i.e. has a disordered structure).
Then, the obtained alloy is subjected to a second heat treatment. Thus, the solution-treated and homogenized alloy ingot is cooled to a temperature at which the ingot has both of the primary precipitate L12 phase 12 and the A1 phase or has all of the primary precipitate L12 phase 12, the A1 phase, and the D022 phase, and is further cooled to a temperature equal to or lower than the eutectoid temperature. In this step, the primary precipitate L12 phase 12 is precipitated from the A1 phase, and the A1 phase remaining in the gap (channel) of the primary precipitate L12 phase 12 is transformed by a eutectoid reaction to the D022 phase and the L12 phase.
As a result, as shown in
It is apparent also from
The first and second heat treatments may be carried out successively. In this case, after the first heat treatment, the alloy may be cooled to the eutectoid temperature at a predetermined rate in a heating furnace.
Alternatively, the Ni-based intermetallic alloy 10 may be produced by a casting method such as a vacuum induction melting method.
The upper and lower multi-phase microstructure can be each formed by further cooling the alloy to the temperature equal to or lower than the eutectoid temperature in the second heat treatment. The Ni-based intermetallic alloy 10 having the dual multi-phase microstructure 16 can be obtained in this manner.
In any of the production methods above, the alloy may be maintained in two stages at different temperatures under the second heat treatment. In this case, the holding temperature of the first stage is set to be higher than the eutectoid temperature, and the holding temperature of the second stage is set to be lower than the eutectoid temperature. The upper multi-phase microstructure is formed at the holding temperature of the first stage, and the lower multi-phase microstructure is formed at the holding temperature of the second stage.
In the first and second heat treatments, the solidified alloy may be left to cool naturally or may cool continuously at an arbitrary cooling rate.
Metals of Ni, Al, V, Zr, Hf, Nb, Mo, C, Ti, Co, Cr, and B were mixed to obtain each of compositions of Examples 1 to 20 shown in
An alloy piece, which did not contain Zr and Hf, contained 3 at % of Nb, and had the same shape as above, was produced as a test sample which is used as Comparative Example.
As the first heat treatment, the alloy was held at 1280° C. for 5 hours under vacuum in the heating furnace. After the first heat treatment, as the second heat treatment, the alloy was continuously cooled at a cooling rate of 10° C./minute.
Each of the test samples was observed with an SEM.
In
In
Then, ductility of the test samples of Examples 1 to 20 and Comparative Example were evaluated. Specifically, each test sample was subjected to a compression test at 800° C. and a strain rate of 8.3×10−5 s−1, and the crack initiation strain and the 0.2% proof stress were measured. The crack initiation strain means a strain amount measured when the test sample was cracked. A sample having a larger crack initiation strain has a more excellent ductility and is more resistant to fracturing, and thus has a more excellent toughness.
The results are shown in
It is also clear from
In addition, each of the test samples of Examples 5 and 6 was subjected to a tensile test at 800° C. in vacuum at a strain rate of 1.66×10−4 s−1. Furthermore, each of the test samples of Examples 5 and 6 and Comparative Example was subjected to a tensile test at 800° C. in atmospheric air at a strain rate of 8.3×10−5 s−1. The elongations of the test samples are shown in the graph of
As is clear from
The present invention is not particularly limited to the above embodiment. Various changes and modifications may be made to the embodiment without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-034735 | Feb 2017 | JP | national |
