JP2009299187A - Heat treatment method for forming wavy grain boundary in nickel-based alloy, and alloy treated with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treatment method which enhances resistance to creep, fatigue and stress corrosion cracking. <P>SOLUTION: This heat treatment method includes: subjecting this nickel-based alloy to solution treatment in a high temperature region; immediately and slowly cooling the alloy to a moderate temperature region for aging treatment with a rate of 1 to 15°C/minute; then immediately subjecting the alloy to the aging treatment of keeping the alloy in the moderate temperature region for the aging treatment for a predetermined period of time; and subsequently air-cooling the alloy. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a nickel base alloy heat treatment method and alloy. More particularly, the present invention relates to a nickel-base alloy heat treatment method that increases resistance to breakage due to grain boundary cracks such as croup, fatigue, and stress corrosion cracking, and a nickel-base alloy having corrugated grain boundaries.

Nickel-based alloys have excellent workability, weldability, corrosion resistance, and high-temperature mechanical properties, and are used as materials for high-temperature components such as aircraft and power gas turbine power assemblies. .
During operation, such materials are exposed to severe environments such as heat and stress sustained or complex deformation cycles and high temperature corrosion, and are damaged and damaged mainly by creep, fatigue, stress corrosion cracking, etc. . Therefore, improving the resistance of creep, fatigue, stress corrosion damage, etc., which is the main cause of such material damage, is one of the important issues for manufacturers, parts processors and operators. I came.

  FIG. 1 shows a conventional heat treatment process applied to the manufacture and processing of a nickel-base alloy NIMONIC 263 widely used in a combustor liner, a transition duct, etc. of a gas turbine for power generation. In the method, solution treatment (1000 to 1200 ° C./5 minutes or more) is usually performed in a high temperature region, and then water cooling (50 ° C./second or more) is performed. Subsequently, after a lapse of a predetermined time, and after performing an aging treatment (700 to 900 ° C./5 hours or more) in an intermediate temperature range, a two-stage heat treatment step of air cooling is performed.

In the heat treatment step described above, after cold working, the coarse carbide and γ ′ precipitate phase in the γ matrix are simply dissolved and dissolved in the solution treatment step, and the carbide is crystallized in the subsequent aging treatment step. At the same time as pre-precipitation at the grain boundaries, the γ ′ precipitation phase is uniformly distributed in the matrix.
The purpose of this is to increase the high-temperature stability of the material and to reduce the degree of grain boundary sensitization to improve the strength of the material.
However, such a heat treatment method is not sufficiently satisfactory in resistance to creep, fatigue, and stress corrosion cracking, and has not been improved to a standard level.
Therefore, there is an urgent need for a heat treatment method that is economical, easy, and convenient by improving the above-described resistance.

Patent Document 1 discloses a nickel base alloy heat treatment method for improving corrosion resistance.
According to the above-mentioned Patent Document 1, after the solution treatment at a high temperature, the cooling rate is gradually cooled at 0.1 to 5 ° C./min in the whole temperature range or a partial range up to room temperature, and the aging treatment is performed. Depending on the method used, a heat treatment method has been proposed in which the grain boundary shape in the material is changed to a serration to improve the grain boundary damage resistance.
However, this method gradually cools at a relatively slow cooling rate in a wide temperature range, which is not only uneconomical due to the long heat treatment time, but also because it is exposed for a long time at high temperature. Becomes larger.
In addition, the precipitation strengthening phase γ 'is coarsened and various harmful phases are precipitated, which can improve the resistance to stress corrosion cracking, but for tensile properties and high temperature mechanical creep, fatigue, etc. Rather, it has an adverse effect. Thus, it is difficult to apply the above method to an actual industrial site.

Korean Patent Publication No. 10-1999-024668

The first technical problem to be solved by the present invention is to provide an economical, easy and convenient heat treatment method for a nickel base alloy by improving resistance to creep, fatigue and stress corrosion cracking. .
The second technical problem to be solved by the present invention is to provide a nickel-base alloy produced by the above method.

  The nickel-base alloy heat treatment method of the present invention for achieving the first technical problem performs a solution treatment in a high-temperature region in a heat-treatment step after manufacturing and processing of the nickel-base alloy. Subsequently, immediately after the solution treatment, the solution is gradually cooled to an intermediate temperature range for aging treatment at 1 to 15 ° C./min. Immediately after the step of slow cooling, an aging treatment is performed while maintaining for a predetermined time in an intermediate temperature region for the aging treatment. Finally, air cooling is performed after the aging treatment.

  In the heat treatment method of the present invention, the step of gradually cooling includes the step of starting to form a corrugated grain boundary in a part of the flat grain boundary formed in the solution treatment stage, and the generated corrugated grain boundary. At the same time with a stable amplitude and period, and at the same time, the formation of corrugated grain boundaries gradually increases at the flat grain boundaries, and the stage where plate-like carbide starts to precipitate at the partial corrugated grain boundaries. it can. Further, in the aging treatment stage, the generated corrugated grain boundary is mostly grown on the corrugated grain boundary having a stable amplitude and period, and the precipitated carbide is above the corrugated grain boundary. In addition, it can grow stably in a plate shape with low interface energy.

  In a preferred method of the present invention, the solution treatment can proceed at 1000 to 1200 ° C. during the solution treatment time, and the aging treatment can proceed at 700 to 900 ° C. during the aging treatment time. .

  The alloy of the present invention for achieving the second technical problem has a corrugated structure in which plate-like carbides are arranged apart from each other in the crystal grain boundary, including the corrugated crystal grain boundary. At this time, the carbide becomes an interface aligned with one of the two crystal grains constituting the crystal grain boundary, and the remainder has a mismatched plate-like interface, but the carbide has the crystal grain boundary. For the two crystal grains constituting the interface, the interface creating the mismatch is arranged in a zigzag pattern alternately in the direction of the different crystal grains.

  According to the nickel base alloy heat treatment method and the alloy according to the present invention, the shape of the crystal grain boundary is changed to a waveform while maintaining the basic characteristics of the nickel base alloy, and the low density of the interface energy is low. Inducing carbide precipitation and increasing the bond between the grain boundaries and the matrix, improving resistance to intergranular crack failure such as creep, fatigue, and stress corrosion cracking, while saving time and money A heat treatment that can be performed can be performed.

It is a chart which shows the conventional heat treatment process. It is a chart which shows the heat treatment process by this invention. It is drawing for demonstrating conceptually the change of the micro structure by the process of FIG. 2A. It is a photograph which shows the fine structure of NIMONIC263 alloy obtained by the conventional heat processing method. It is a photograph which shows the fine structure of NIMONIC263 alloy obtained by the heat processing method of this invention. It is a photograph which shows a wave front after the normal temperature tension test of the NIMONIC263 alloy obtained by the conventional heat processing method. It is a photograph which shows a wave front after the normal temperature tension test of the NIMONIC263 alloy obtained by the heat treatment method of the present invention. It is a graph which shows the result of having performed the creep test on 760 degreeC / 295MPA conditions for the NIMONIC263 alloy obtained by the conventional heat processing method and the heat processing method of this invention. It is a graph which shows the result of having performed the creep test on the conditions of 815 degreeC / 180 MPa for the NIMONIC263 alloy obtained by the conventional heat treatment method and the heat treatment method of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments described below can be modified in various forms, and the scope of the present invention is not limited to the embodiments described below. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

In the present invention, first, the cause of major damage of a nickel-base alloy and a method for overcoming this will be described in detail, and then a heat treatment method for realizing the method will be described.
At this time, for convenience of explanation, creep, fatigue, stress corrosion cracking, etc., which are the main damage causes of the nickel-base alloy, will be defined as “grain boundary damage”.

Grain boundary damage, which is the main cause of damage in nickel-base alloys, propagates through cracks along all weak grain boundaries.
This reduces the grain boundary energy itself, increases the crack propagation path (distance), and changes the precipitation phase that precipitates at the grain boundary, for example, the shape and characteristics of carbides, thereby reducing the resistance to grain boundary damage. Can be increased.
The present invention suggests forming corrugated grain boundaries in order to lower the grain boundary energy and increase the crack propagation path to change the shape and properties of the carbide.

A corrugated grain boundary increases resistance to grain boundary damage for the following reasons.
First, the degree of misalignment between crystal grains is lowered to increase the bonding force with the base, and at the same time, the grain boundary configuration is changed to lengthen the crack propagation path along the grain boundary.
Further, the carbide precipitated at the crystal grain boundary has a plate shape having a low density and stable interface energy.
Furthermore, even if the carbides are generated at the same grain boundary, the respective carbides share alignment with different crystal grains to form an interface, and eventually the mismatched interface of the carbide is crossed. Arranged in zigzag form.

As described above, the corrugation also changes the carbide characteristics so as to favor the grain boundary damage resistance.
That is, the mismatch interface density between the carbide and the matrix providing the main generation position for the grain boundary cavities or cracks is lowered and stabilized, and the speed at which the grain boundary cracks occur is reduced. Even if a crack occurs, the crossing misalignment interface delays the propagation velocity through the crack coalescence.

  The present invention presents a method for inducing plate-like precipitates by forming corrugated grain boundaries.

There are various mechanisms for the generation of corrugated grain boundaries. Recently, the present inventor has confirmed the fact that the grain boundaries themselves are generated by changing the shape to lower the total energy with temperature. did.
That is, in the high temperature region, the influence of surface energy is larger than the deviation between crystal grains, and a linear crystal grain boundary (this is called a flat grain boundary) develops in order to make the surface area as small as possible.
Below the intermediate temperature range, the shift between the crystal grains is relatively important, so that a wavy grain boundary is generated that is separated into several segments so that the crystal grain boundaries are advantageously arranged crystallographically.
Considering such a generation mechanism of corrugated grain boundaries, in order to obtain corrugated grain boundaries from the nickel-based alloy of the present invention, the following conditions must be satisfied.

First, carbide precipitation from grain boundaries must be delayed to the maximum. The reason for this is that carbide is hindered by the movement of grain boundaries due to the pinning effect of the grain boundaries, and it is difficult to improve the characteristics (density, appearance, etc.) of the precipitated carbides. Because there is. Thus, carbon supersaturation must be minimized.
Second, sufficient time and temperature must be provided so that the grain boundaries can move by themselves and come close to equilibrium.

In the present invention, in order to satisfy all the above-mentioned conditions, the nickel-base alloy is maintained for a predetermined time in a high-temperature region where carbides are dissolved and solid-dissolved, and then gradually lowered to an intermediate temperature or less where deviation between crystal grain boundaries becomes important. A method of aging treatment at that temperature immediately after cooling is presented.
In addition, the method maintained the basic characteristics required for the nickel-base alloy at the same time as the generation of the corrugated grain boundary. As a result, the heat treatment method is simpler than that of the existing method, and a new heat treatment method that meets the object of the present invention can be proposed.

The present invention presents optimum heat treatment conditions for inducing corrugated grain boundaries while maintaining the crystal grain size and the precipitation phase γ ′ fraction at a constant level through heat treatment tests under various conditions.
Looking specifically at the conditions, after maintaining for a predetermined time in the high temperature region for solution treatment, after slowly cooling to the medium temperature region for aging treatment, immediately after performing the aging treatment in the medium temperature region, soon Air-cool. At this time, the slow cooling to the middle temperature region is performed at 1 to 15 ° C./min.

A comparison of the heat treatment process of the present invention with the conventional method is as follows.
Conventionally, a two-step heat treatment method in which solution treatment is performed in a high temperature region (1000 to 1200 ° C.), then water cooling is performed to room temperature (50 ° C./second or more), and aging treatment is performed in a medium temperature region (700 to 900 ° C.). Apply.
On the other hand, the present invention is a one-stage heat treatment method in which, after solution treatment, immediately cooled to an intermediate temperature range, maintained at the aging treatment temperature, and then the heat treatment is terminated.

FIG. 2A is a chart showing a heat treatment process according to the present invention, and FIG. 2b is a diagram for conceptually explaining a change in microstructure by the process of FIG. 2a.
Here, the heat treatment temperature region and the heat treatment time merely exemplify typical conditions for performing the heat treatment, and do not limit the scope of the present invention. At this time, a nickel base alloy NIMONIC 263 rolled material was used as the target material.

Referring to FIGS. 2A and 2B, the heat treatment method of the present invention is divided into a solution treatment stage (a stage), a slow cooling stage (bc stage), an aging treatment stage (d stage), and an air cooling stage. Is done.
That is, first, in order to perform the solution treatment, the solution treatment time is maintained at 1000 to 1200 ° C. which is a high temperature region, for example, 5 minutes or more.
Then, it is gradually cooled at a rate of 1 to 15 ° C./min to an intermediate temperature range that is an aging treatment temperature (700 to 900 ° C.).
Subsequently, after maintaining the aging treatment temperature at 700 to 900 ° C. which is an aging treatment temperature, for example, 5 hours or more, the heat treatment is terminated by air cooling.

In the solution treatment step, in accordance with the object of the present invention, the solution treatment sufficiently occurs in the alloy, that is, the carbide and γ ′ precipitation phase in the material are sufficiently dissolved and dissolved, but the grain growth is It proceeds during the solution treatment time to the extent that it does not occur.
At this time, the grain boundary of the material is a flat grain boundary (20) having a flat shape by the solution treatment as in step a.

The corrugated grain boundary starts to be formed in the process of slow cooling at 1 to 15 ° C./min to the intermediate temperature region. As for the wavy grain boundary, a wavy grain boundary (22) in which the shape of the waveform is partially visible as in the b stage appears. This is called the initial stage of slow cooling.
At this time, the corrugated grain boundary (22) in the initial stage of slow cooling is in a state where the amplitude and period are not fully developed (this is referred to as an incomplete corrugated grain boundary for convenience).

On the other hand, in the c stage, an incomplete corrugated grain boundary (22) is subsequently formed, and a part of the grain boundary is grown to a complete corrugated grain boundary (24) showing a stable corrugated form. This is called the final stage of slow cooling.
In other words, in the final stage of slow cooling, an incomplete corrugated grain boundary (22) that has not yet fully developed, a complete corrugated grain boundary (24) that has grown and fully developed, and the form of the corrugation is still A partially flat grain boundary (20) that does not appear coexists.
At this time, the plate-like carbide (30) begins to precipitate at the incomplete corrugated grain boundary (22) and the complete corrugated grain boundary (24), and the precipitation hardening phase γ 'begins to be generated in the crystal grain. The portion where the plate-like carbide (30) is in contact with the corrugated grain boundary (22, 24) is aligned with one crystal grain, and the remainder is not aligned.

After a gradual cooling stage, when the aging process stage is started, a certain time has passed and the corrugated grain boundary grows as shown in the d stage, and largely changes to a complete corrugated grain boundary (24). At such a perfect corrugated grain boundary (24), the precipitated carbide (30) grows to form a plate-like carbide (32).
The carbide (32) grown here becomes a mismatch interface to the crystal grain side opposite to the matching interface while the carbide is perfectly aligned with one crystal grain constituting the corrugated grain boundary (24). Growing while.
At this time, due to the change in crystallographic variation of the corrugated grain boundary itself, the array that forms a mismatched interface between the carbide and the base having a relatively high interface energy is crossed in a zigzag form.

In accordance with the object of the present invention, the aging treatment step is performed in such a manner that the γ ′ precipitation phase of the alloy is uniformly distributed in the matrix and the carbides are grown on the grain boundaries, and (700 to (900 ° C.) Even when exposed, the aging treatment proceeds sufficiently during the aging treatment time such that there is no change in the structure.
At this time, the plate-like carbide (32) grows stably at the complete corrugated grain boundary (24).

The carbides (32) that have been subjected to the aging treatment are plate-like carbides and are arranged apart from each other by the corrugated grain boundaries (24).
Even if the carbides (32) are generated at the same corrugated grain boundary (24), the respective carbides (32) share alignment with different crystal grains to form an interface. 32) mismatched interfaces are crossed and arranged in a zigzag form.

In short, since it changes from a flat grain boundary (20) to a complete corrugated grain boundary (24), the interfacial energy of the grain boundary itself can be made very low, and carbides deposited on the corrugated grain boundary with low interfacial energy. (32) also has a low density, grows in a stable plate shape, and significantly reduces the mismatch interface energy of the carbide (32).
At the same time, due to the crystallographic transformation change of the corrugated grain boundary itself, the inconsistent interfaces of the respective carbides are arranged so as to intersect in a zigzag form.

  In the present invention, the reason for limiting to 1 to 15 ° C./min in the slow cooling to the aging treatment temperature immediately after the solution treatment is that when the cooling rate is less than 1 ° C./min, the exposure time is high. Since the length becomes longer, the crystal grains and the precipitation hardening phase γ ′ are coarsened, and the basic mechanical properties may be deteriorated. Further, when the cooling rate exceeds 15 ° C./min, the crystal grain boundaries become corrugated, so there is no sufficient time margin, and carbide is precipitated, so that the corrugated grain boundaries cannot be obtained.

On the other hand, after solution treatment, when slowly cooling at 1 to 15 ° C / min in the whole temperature range from that temperature to room temperature, γ 'precipitation and high temperature stability are not sufficient, and the raw material is used as it is It is not possible to do this, and another aging treatment must be performed, which is a heavy burden of time and money.
In the unlikely event that the solution is annealed at a temperature other than the aging treatment temperature of the present invention from 1 to 15 ° C./min from the temperature, not only does the grain boundary not occur, but also the aging treatment. The problem arises that must be done again.

In addition, if a rapid cooling process is performed after passing through the slow cooling stage shown by this invention, and an aging process is performed, a fine structure will be visible at the above-mentioned c stage.
That is, at this stage, an imperfect corrugated grain boundary (22), a perfect corrugated grain boundary (24), and a flat grain boundary (20) coexist.
Further, when the carbide (30) is supersaturated by being rapidly cooled and subjected to an aging treatment, an incomplete corrugated grain boundary (22), a complete corrugated grain boundary (24), and a flat grain boundary (rather than that) Also in 20), carbides are precipitated in a massive form. In this case, the interfacial energy is higher than that of the present invention.

FIG. 3 is a photograph showing the microstructure of the NIMONIC 263 alloy obtained by a conventional heat treatment method.
Here, the following photograph is an enlargement of the vicinity of the crystal grain boundary.
In the heat treatment, solution treatment was performed at about 1150 ° C./30 minutes, water cooling to room temperature (50 ° C./second or more), aging treatment was performed at about 800 ° C./8 hours, and air cooling was performed.
As shown in the figure, it can be seen that in the microstructure of the conventional alloy, small granular carbides are precipitated at a high density at flat grain boundaries and crystal grain boundaries in a linear form. At this time, it was confirmed that the crystal grain size was 60 to 70 μm.

FIG. 4 is a photograph showing the microstructure of the NIMONIC 263 alloy obtained by the heat treatment method according to the embodiment of the present invention.
Here, the following photograph is an enlargement of the vicinity of the crystal grain boundary.
In this case, that is, after heat treatment is performed at about 1150 ° C./30 minutes, immediately cooled to 10 ° C./min to 800 ° C., which is an aging treatment temperature, and then maintained at 800 ° C. for 8 hours. Air-cooled.

  According to FIG. 4, it can be seen that the microstructure according to the embodiment of the present invention has well-developed corrugated grain boundaries, and plate-like carbides having low interface energy are precipitated at a low density at the crystal grain boundaries. At this time, the size of the crystal grains was 70 to 80 μm similar to the structure obtained by ordinary heat treatment.

Next, the characteristics of the alloy manufactured by the conventional heat treatment method shown in FIG. 3 and the alloy manufactured by the manufacturing method of the present invention shown in FIG. 4 will be verified.
Table 1 shows test results obtained by performing a tensile test on each alloy at room temperature.

As can be seen from Table 1, the alloys of the present invention show similar levels of yield and tensile strength compared to conventional alloys.
However, it can be seen that the ductility is increased to a very high level, from 23.3% to 38.1% in the conventional alloy.

FIG. 5 and FIG. 6 are photographs showing wave fronts after a normal temperature tensile test of a NIMONIC 263 alloy obtained by the conventional heat treatment method and the heat treatment method of the present invention, respectively.
At this time, the heat treatment is as described above.
As shown in FIG. 5, the conventional alloy can confirm that the grain interface is fragilely separated and fractured without any special plastic deformation as shown in FIG.

In the alloy of the present invention, dimples and shear marks were observed at the corrugated grain interface as shown in FIG.
This shows that the alloy of the present invention is ruptured through sufficient plastic deformation until just before the rupture.
That is, the alloy of the present invention means that the bonding force between the crystal grain boundary and the base is relatively higher than that of the conventional alloy.
Such a result can be judged by one of the factors causing the increase in ductility in Table 1.

Specifically, the carbide (32) that has been subjected to the aging treatment with the alloy of the present invention is a plate-like carbide, and is disposed apart from each other on the stable corrugated grain boundary (24).
The carbide (32) grows only in the direction of one crystal grain in the two crystal grains constituting the corrugated grain boundary (24), and is not an inconsistent interface array arranged only in one direction. Zigzags are formed alternately in the crystal grain direction (4a, 4b).
Thereby, as described above, the formation of the corrugated grain boundary (24) also advantageously changes the carbide (32) characteristics to the grain boundary damage resistance.

  That is, the density of the mismatched interface between the carbide (32) providing the main generation position for the grain boundary cavities or cracks and the matrix is lowered, the energy is stabilized, and the grain boundary crack generation rate is reduced. Even if a crack occurs, the misalignment interface arranged in a crossing manner delays the propagation speed through the crack coalescence, and sufficient plastic deformation appears until just before the fracture.

  FIG. 7A and FIG. 7B are graphs showing the results of performing a creep test on the NIMONIC 263 alloy obtained by the conventional heat treatment method and the heat treatment method of the present invention under the conditions of 760 ° C./295 MPA and 815 ° C./180 MPA, respectively.

Referring to FIGS. 7A and 7B, it was confirmed that the heat treatment of the present invention exhibited much superior creep characteristics regardless of the test conditions.
Specifically, when the test was conducted at 760 ° C./295 MPa, the creep time increased from about 129 hours to about 178 hours, and the creep strain also increased from about 6% to about 11%.
Further, when the test was conducted under the condition of 815 ° C./180 MPa, the creep time increased from about 181 hours to about 252 hours, and the creep strain also increased from about 17% to about 20%.

  The present invention has been described in detail with reference to the preferred embodiments. However, the present invention is not limited to the above-described embodiments, and within the scope of the technical idea of the present invention, those skilled in the art can Various modifications are possible.

Claims (7)

In the heat treatment process after manufacturing and processing nickel-base alloys,
Performing a solution treatment in a high temperature region;
Immediately after performing the solution treatment, gradually cooling at 1 to 15 ° C./min to an intermediate temperature region for aging treatment;
A heat treatment of a nickel-based alloy for corrugated grain boundaries, comprising a step of performing an aging treatment by maintaining for a predetermined time in a medium temperature region for the aging treatment immediately after the step of gradual cooling, and a step of air cooling after the aging treatment Method. The step of slow cooling comprises:
Forming an incomplete corrugated grain boundary in a part of the flat grain boundary formed in the solution treatment step; and the imperfect corrugated grain boundary grows in a stable waveform, 2. The heat treatment method for a nickel-base alloy according to claim 1, further comprising a step in which an incomplete corrugation is formed and a plate-like carbide starts to precipitate at the corrugated grain boundary.   In the aging treatment stage, the imperfect corrugated grain boundary is mostly transferred to a stable corrugated shape, and the precipitated carbide is aligned with any crystal grain constituting the corrugated grain boundary. However, the nickel-base alloy heat treatment method according to claim 2, wherein the crystal grain side on the opposite side grows away in a plate shape to form a mismatch interface.   2. The solution treatment according to claim 1, wherein the solution treatment proceeds at a solution treatment time of 1000 to 1200 ° C., and the aging treatment proceeds at a temperature of 700 to 900 ° C. during the aging treatment time. Heat treatment method of nickel base alloy for corrugated grain boundaries.   A nickel-based alloy having a corrugated grain boundary including a corrugated crystal grain boundary in which plate-like carbides are arranged apart from each other at the crystal grain boundary.   6. The nickel-base alloy according to claim 5, wherein the carbide becomes a matching interface with any one of the crystal grains constituting the crystal grain boundary, and grows to a crystal grain on the opposite side to become a mismatch interface. .   6. The carbide according to claim 5, wherein the carbides are alternately arranged in different crystal grain directions with respect to two crystal grains constituting the crystal grain boundary, and misaligned interfaces are arranged in a zigzag form. Nickel-based alloy for corrugated grain boundaries.