JPS6366374B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high strength Ni ₃ Al alloy having ductility at room temperature, that is, room temperature workability.

It is well known that Ni-based superalloys are widely used in practical applications as super heat-resistant alloys used in structural materials that are exposed to high temperatures for long periods of time during use, such as in gas turbines, jet engines, and missiles. This material contains Ni ₃ Al in the Ni fixed body.
LI Type ₂ intermetallic compound is dispersed and precipitated in less than 60% as a dispersed phase, and is an excellent practical alloy with excellent oxidation resistance, sulfidation resistance, and creep resistance.

The characteristic of this alloy is that the yield strength of the Ni solid solution decreases as the temperature increases, but the yield strength of the dispersed ordered lattice phase Ni ₃ Al exhibits an inverse temperature dependence, which is contrary to the normal temperature increase. This is complemented by a unique phenomenon in which the strength increases as the temperature increases, and overall a high stress level is maintained even at high temperatures. As melting and casting technology progresses, the amount of active metals such as Ti and Zr and high melting point metals such as Mo and W increases, and the amount of Ni ₃ Al gradually increases, increasing the volume percentage.
There are some cases where it exceeds 50%.

However, as the amount of Ni ₃ Al increases, the workability deteriorates significantly, and when the amount reaches about 30 to 40%, it becomes impossible to cast, and even if precision casting is performed, if the Ni ₃ Al phase exceeds 60%, the material becomes brittle. Because of such high Ni ₃ Al
Those with phases have been considered impractical.

The present invention aims to provide a super heat-resistant alloy that completely changes the conventional wisdom regarding such Ni-based superalloys, and eliminates the r-phase as a matrix and uses only Ni ₃ Al-based intermetallic compounds. The aim is to provide a Ni-based super heat-resistant alloy that has achieved revolutionary ductility at room temperature by adding trace amounts of certain additive elements to the Ni ₃ Al-based intermetallic compound = r' phase. .

Ni ₃ Al-based intermetallic compounds form a so-called LI ₂ type ordered lattice, in which Ni and Al are arranged at regular positions in a face-centered cubic lattice, forming the main reinforcing phase r′ of the Ni-based superalloy.

Figure 1 shows the Ni side phase diagram of the Ni-Al binary system phase diagram, and it can be seen that the r phase and r' phase can be formed appropriately by selecting the amount of Al added to Ni. .

It is known that the yield stress of Ni ₃ Al has a pronounced positive temperature dependence over a wide temperature range, that is, as the temperature increases, the yield stress, which indicates the strength of the material, shows a higher value than at room temperature. It is being

Figure 2 is a diagram showing such temperature dependence, and it can be understood that the reason why Ni-based superalloys can maintain high strength at high temperatures is due to this property of Ni ₃ Al. Dew.

The inventors believe that in order to improve the heat resistance performance of Ni-based superalloys, the matrix r phase, which significantly reduces strength at high temperatures, should be eliminated as much as possible, and ultimately only the r' phase should be present, as shown in Figure 2. We thought that it might be possible to obtain an alloy that has significantly increased strength at such high temperatures.

However, this r' phase also has general properties as an intermetallic compound, and as has been reported in many previous reports, it is brittle not only at low temperatures but also at high temperatures and shows almost no elongation. It has been considered completely unthinkable to use it alone.

Here, as a result of many years of research on Ni ₃ Al, the inventors found that although Ni ₃ Al is indeed a brittle material as mentioned above, Ni ₃ Al single crystals are not necessarily brittle and exhibit a certain degree of ductility at room temperature. He realized that Ni ₃ Al itself was not brittle, but rather that the brittleness of polycrystals was due to the presence of some embrittling element at the grain boundaries. .

If the brittle factors at grain boundaries are removed, it may be possible to make Ni ₃ Al polycrystals into ductile materials. Taking note of this, the inventors attempted to add various third elements to Ni ₃ Al. As a result, they discovered that by adding 3% or less B, it was possible to obtain a Ni ₃ Al intermetallic compound with extremely high ductility, which is completely contrary to conventional wisdom.
55-58246).

As a result of subsequent experimental research, the inventors discovered that Zr also had the same effect as B, and filed a patent application in 1983.
It was proposed as No.-017699 (Japanese Unexamined Patent Publication No. 55-110748).

Furthermore, it was revealed here that room-temperature workability can be ensured and strengthening through solid solution hardening can also be achieved by adding Mo.

A detailed explanation will be given below based on examples.

When Mo is added alone, the material is 99.99% Al, 99.9% Ni and
Using 99.5%Mo, weigh the required amount and get 0.5wt%Mo
A Ni 3 Al alloy containing Ni ₃ Al was melted.

In other words, after inserting these raw materials into the Tammann tube, the air was sufficiently replaced with high-purity argon gas, and the output of the high-frequency oscillator was increased to 7Kw.
After melting, reduce the output to 5Kw and reduce the inner diameter to 5mm.
It was sucked up into a φ opaque quartz tube.

After homogenization annealing at 1300K for 5 hours in a vacuum of 1×10 ^-5 Torr, the specimen was cooled and cut into test pieces.

The tensile test piece was shaped into a hanging type with a gage of 25 x 2.5 φ mm, rolled into a size of 2 x 3 x 30 mm, and bent into a quartz tube as-is.

For tensile strength, a stress-strain curve at room temperature was determined using an Instron type testing machine.

Bending was performed by holding one end of the test piece in a vise and hitting it with a hammer, and rolling was performed using a small two-high rolling mill without intermediate annealing.

In the bending test, which is a very simple test, the sample without Mo addition broke with almost no bending as previously reported, indicating that Ni ₃ Al is a brittle alloy with general characteristics as an intermetallic compound. I understand. However, this has 0.5%Mo
The brittle properties of the alloy completely changed and the alloy was transformed into a highly ductile alloy, with no cracks observed even when it was bent into a U-shape.

Figure 3 shows an example of the results of a tensile test conducted using an Instron type testing machine at an initial strain rate of 1.4 x 10 ^-3 S ^-1 . It is known that the sample without Mo added breaks with almost no elongation as in previous reports, but on the other hand, the sample with 0.5% Mo added shows approximately 20% elongation. The dotted line in the figure is the compressive stress-strain curve of the two-element sample without Mo addition.

Although binary Ni ₃ Al polycrystals cannot be deformed in tension, they can be deformed in compression to some extent. The point to note here is that the addition of Mo not only causes elongation, but also increases yield strength through solid solution hardening. i.e.
Addition of Mo improves ductility and increases yield strength at the same time, so it can be said that it is an extremely preferable element in terms of metal materials.

Figures 4a and 4b are scanning electron micrographs of the fracture surfaces of binary Ni ₃ Al and Ni ₃ Al alloy to which 0.5% Mo is added, respectively. The non-ductile binary alloy shown in Figure 4a suffers from fracture along grain boundaries, that is, intergranular fracture;
The sample shown in Figure b, whose ductility has been improved by adding Mo, shows a typical ductile intragranular fracture with a dimple pattern resulting from the formation and coalescence of microcavities, and there is a clear difference between the two. .

Table 1 shows the results of each test.

It can be seen that with M0 without Mo addition, or with M1 and M2 with a small amount of Mo added, brittle fracture occurs immediately in both bending and rolling, and there is no difference from previous reports.

However, it can be seen that when the Mo content is 0.05%, it clearly begins to show ductility. And its ductility improves more and more as the amount added increases, and shows surprising ductility in the range of 0.3% to 0.8%. However, the amount added is 2.5%
It was also found that ductility is inhibited again when the concentration becomes high enough to exceed . This is probably because the added elements form brittle phases at grain boundaries. Therefore, when Mo is added alone, the alloy is obtained by adding 0.05% to 2.0% Mo to Ni ₃ Al.

What is the mechanism by which something that would have completely brittle fractured in the case of no additives showed such high ductility when added in the range of 0.05 to 2.0%? , which is currently under investigation, is thought to have the effect of reducing the amount of harmful impurities on the grain boundaries or strengthening the grain boundaries themselves, thereby suppressing grain boundary cracking and improving ductility. It will be done.

By the way, it has already been proposed that the addition of B or Zr to Ni ₃ Al significantly improves the ductility (Japanese Patent Application No. 130765/1983 and Japanese Patent Application No. 54/1989).
017699), when Mo and these elements are added simultaneously, the ductility is further improved and the effective composition range of each element is expanded.

When Mo and B or Mo and Zr are added in combination,
Table 2 shows examples in which Mo is further added with B or Zr.

The experimental method was the same as in the case of adding Mo alone.

The first notable effect is that when using Mo alone, the lower limit of ductility is 0.05% as shown in Table 1 above, but when combined with Mo, the lower limit of Mo addition increases to 0.01%. .
In this case, the lower limit of the amount of each B and Zr added that exhibits such a synergistic effect is 0.05 for B.
%, and in the case of Zr it is 0.15%. If the amount is less than this, the effect will decrease.

As the amount of composite addition increases from the above lower limit, a significant improvement in ductility is seen due to the synergistic effect with Mo, as shown in Table 2, but if it is not too large, the ductility improvement effect disappears. The upper limit is 3.0% for B and 4.0% for Zr.

However, even in this case, if the total amount including Mo exceeds 4.5%, the total amount added becomes too large and becomes brittle.

This is thought to be due to the appearance of some kind of brittle phase at the grain boundaries.

However, in the above range i.e. Mo0.01~2.0%,
B0.05 to 3.0 with B or Zr each alone
%, Zr0.5 to 4.0%, and in this case, if added so that the total with Mo does not exceed 4.5%, these added elements will have a synergistic effect, and the ductility improvement effect will be even greater. I found out what to do.

Moreover, it not only has an effect of improving ductility, but also moves the overall stress level upwards compared to when using Mo alone, and also moves the temperature at which stress is maximum on the characteristic curve showing the inverse temperature dependence explained earlier to the high temperature side. It was confirmed that it also has a stimulating effect. In other words, as explained earlier, Fig. 2 is a diagram showing the inverse temperature dependence of Ni ₃ Al, but the ductility of Ni 3 Al with 0.5% Mo addition is also increased due to the solid solution effect. At the same time as the improvement, a remarkable increase in yield stress was observed, and this
It can be seen that when 1.0% Zr is added in combination, in addition to improving the ductility described above, the yield stress increases significantly, and at the same time, the temperature of the maximum stress also shifts to the higher temperature side.

Such an effect could be more or less confirmed for B as well.

In addition, in the test shown in Figure 2, Mo
Since tensile testing is not possible for the case without additives, the stress-strain curve for compression was determined and the yield stress (0.2% proof stress) was determined, and for the others, that for tension was determined and plotted in the figure.

As described above, the alloy according to the present invention has higher strength at high temperatures than at room temperature.
By focusing on the temperature dependence of Ni ₃ Al-based interalloy compounds, we are able to directly create ductility in the intermetallic compounds, and use the intermetallic compounds themselves, which have excellent properties at high temperatures, as high-temperature materials. We were able to provide an epoch-making alloy that made it possible to process

It is essential that the demand for ultra-heat-resistant structures for turbines, jet engines, nuclear power-related applications, etc. will continue to increase in the future.

In that case, the alloy of the present invention, which has room temperature workability and high ductility at room temperature, will attract even more attention and will be able to widely contribute to the development of industry.

[Brief explanation of drawings]

Figure 1 is the Ni side phase diagram of the Ni ₃ Al base system phase diagram, and Figure 2 is
The figure is a diagram showing the temperature dependence of the Ni ₃ Al sample, Figure 3 is the stress-strain curve showing the mechanical properties of the Ni ₃ Al polycrystalline material at room temperature, and Figure 4 is the fracture surface of the sample material. FIG. 2 is a diagram showing a scanning electron microscope structure of the material, in which a shows the case without any additive element and b shows the case with 0.5% Mo added.

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Claims (1)

[Claims] 1. A Ni ₃ Al alloy with improved cold ductility and strength, which is obtained by adding 0.05 to 2.0% Mo to a Ni ₃ Al intermetallic compound expressed as LI ₂ type. 2 Add 0.01 to 2.0% Mo to the Ni ₃ Al intermetallic compound expressed as LI type ₂ , and add B or
Ni with improved cold ductility and strength by adding 0.05 to 3.0% Zr in the case of B and 0.5 to 4.0% in the case of Zr, and adding them so that the total with Mo does not exceed 4.5%. ₃ Al-based alloy.