GB2037322A - Super heat resistant alloys having high ductility at room temperature and high strength at high temperatures - Google Patents

Abstract

Super heat resistant alloys having high ductility at room temperature and high strength at high temperatures consisting mainly of Ni3A@ intermetallic compound phase containing at least one of B, Nb, Sr and Mo. These alloys are useful for structural materials exposing high temperatures for long time, such as gas turbine, jet engine, missile and the like. Where a single addition is present, up to 3% by weight may be used, and with two or more additives, a total of 4.5% by weight is permissible.

Description

SPECIFICATION Super heat resistant alloys having high ductility at room temperature and high strength at high temperatures The present invention relates to super heat resistant alloys having high ductility at room temperature and high strength at high temperatures consisting mainly of Ni3At base intermetallic compound phase.

For example, Ni-base super alloys have been broadly practically used as the super heat resistant alloys, which are used for structural materials to be exposed to high temperatures when using, such as gas turbine, jet engine or missile. These alloys are ones wherein Ni3At intermetallic compound which is called y' phase, or a Ni3Ae base intermetallic compound wherein a part of Ni or At in Ni3At is substituted with other elements, is dispersed in Ni solid solution ofy phase in Ni-At alloy phase diagram shown in Figure land have high oxidation resistance, sulfurization resistance and creep resistance in addition to the above described super heat resistance and are excellent practical alloys.

The above described Ni3At intermetallic compound forming the dispersion phase of these alloys forms L12-type super-lattice and Ni and At regularly position atface-centered cubic lattice and as mentioned above, Ni or At in such a lattice is partially substituted with an element of Ti, Si, Cr, Co, Cu, Mo, Ta, W, Nb, Fe, Zr, V, Mn, Hf and the like to form Ni3At-base intermetallic compounds, and it has been found that these alloys show that as the temperature increases, the yield stress showing the strength of the material becomes higher than that at room temperature. For example, the relation of the temperature to the yield stress (0.2% proof stress in compression) of Ni3Ae is as shown in Figure 2.

The characteristic of said Ni-base super-alloy consists in that the high stress level is maintained up to high temperatures as a whole by compensating the decrease of the yield strength of Ni solid solution with increase of temperature, with the positive temperature dependence of the yield strength of superlattice Ni3At which is dispersed in the Ni-solid solution, namely the unique phenomenon in which the yield strength increases with increase of temperature.

Recently, it has become possible owing to advance of melting and casting technique to add metals having higher affinity to oxygen or nitrogen, such as Ti, Zr or metals having high melting point, such as Mo, Wand the like in a larger amount or increase the amount of Ni3AW intermetallic compound dispersed, so that the alloys containing more than 50% by volume of the compound phase have been develped. However, if the above described dispersed amount increases, the workability is considerably deteriorated and when the amount reaches about 30-40%, the forging becomes infeasible.Especially in the precision forged articles, when Ni3At intermetallic compound phase exceeds 60%, the brittleness becomes considerably remarkable and there is fear that fracture occurs owing to small impact, so that the alloys having a large amount of Ni3At intermetallic compound phase can not be practically used at least as wrought materials.

The present invention aims to provide super heat resistant alloys having high ductility at room temperature and high strength at high temperatures, which obviate the defects of Ni-base super alloy having the above described dispersion phase of Ni3At base intermetallic compound wherein the workability is low and the brittleness is high, and consists in super heat resistant alloys mainly composed of Ni3At base intermetallic compound phase containing not more than 3.0% by weight of an element selected from the group consisting of B, Zr, Nb and Mo or containing not more than 4.5% by weight of the total amount of at least two elements selected from the group consisting of B, Nb, Zr and Mo.

A detailed explanation will be made with respect to the present invention.

For better understanding of the invention, reference is taken to the accompanying drawings, wherein: Figure 1 is a phase diagram of Ni-At binary alloy; Figure 2 is a curve showing the relation of the yield stress to the temperature of Ni3At intermetallic compound; Figure 3 is the curves showing the relation of the tensile strength to the elongation of the alloys of the present invention; Figure 4 shows the relation of the tensile strength to the elongation of Ni3At intermetallic compound not containing boron; Figure 5 is a photographic showing the test result of Ni3At intermetallic compound not containing boron after bending test;; Figure 6 is a photograph showing the test result of Ni3At intermetallic compound containing 0.1% by weight of boron after bending test; Figure 7 is a photograph showing the test result of Ni3At intermetallic compounds containing 0.1% by weight and 0.5% by weight of boron respectively after cold-rolling.

Figures 8 and 9 show the relation of the yield stress to the temperature of a sample of the present invention and a comparative sample; Figure 10 is the curves showing the relation of the stress to the elongation of a sample of the present invention and the result of a comparative sample in the tensile test, and the result of the compressive stress-strain curve of Ni3At polycrystal; Figure 11 is the curves showing the relation of the yield stress to the temperature of the samples of the present invention and a comparative sample; and Figure 12 (a) and (b) show the scanning electron micrographs of the fracture surface of binary Ni3At and Ni3A4 intermetallic compound containing 0.5% by weight of Mo.

As seen from Ni-At binary phase diagram of Figure 1, by selecting the content of At based on Ni, it is possible to obtain Ni-At base alloys composed of y phase and/or y' phase. The conventional Ni base super alloys are ones wherein the matrix is y phase, while the alloys of the present invention are ones wherein the matrix is y' phase consisting of Ni3At-base intermetailic compound and has no y phase or a very small amount of y phase.

The discovery of the alloys of the present invention has based on the following idea that in order to improve the heat resistant performance of Ni-base super alloys, it has been considered that if the matrix phase, which is considerably low in the strength at high temperature, is made to be as small as possible and ultimately consists of only y' phase, alloys having very high strength at high temperatures as shown in Figure 2 can be obtained.

However, as described in Metallurgical Transaction, 1970, Vol. 1, P. 207, the y' phase is brittle in poly-crystalline state similarly to the other intermetaliic compounds not only at low temperatures but also at high temperatures, and does not substantially show the elongation, so that it has never been contemplated to use only Ni3At polycrystal of y' phase.

As the result of the study concerning Ni3At for a long time, the inventors have found that Ni3At polycrystal is in fact a brittle material as mentioned above but Ni3At single crystal is not always brittle and shows a certain degree of ductility at room temperature and noticed that Ni3At itself has no brittle property and the brittleness of the polycrystal is due to the reason that any embrittling factor is present in the grain boundary.

The inventors have considered that if the embrittling factor in the grain boundary is removed, Ni3At polycrystal might be converted into the ductile materials and it has been attempted to add various third elements to Ni3A. As the result, it has been found that the alloys consisting mainly of Ni3At-base intermetallic compounds containing at least one of B, Nb, Zr and Mo within a given range are super heat resistant alloys having high ductility at room temperature and high strength at high temperature and the present invention has been accomplished.

The inventors have found that Ni3At-base intermetallic compounds containing Ca, Mg, Y, Ti, Si, Hf or rare earth elements in addition to B, Nb, Zr or Mo can also show some ductilities. The prevention of the brittleness is noticeable in B, Nb, Zr and Mo but the reason has not yet been clarified in detail.

Heretofore, Ni-base super alloys containing a small amount of B as in G 64, No 64 BC or Udimet (registered Trade Mark) 700 have been known. However, as seen from the fact that the content of At is about 6%, the total amount of Ni3At is small and the major part of the composition is the matrix consisting of y phase of solid solution and B is added in order to strengthen the matrix. Furthermore, even in Ni-base super alloys having a large amount of y phase of solid solution containing Nb, Zr or Mo, which has the content of At of not more than 6%, Nb, Zr or Mo is used as an element for strengthening the matrix.Namely, in the heretofore known above described Ni-base super alloys containing about 6% of At and a small amount of B, Nb, Zr and/or Mo, Ni3Af or Ni3At-base intermetallic compounds are dispersed in the matrix composed of y phase of Ni-solid solution strengthened with B, Nb, Zr and/or Mo. On the other hand, in the alloys of the present invention, a part of Ni and At in Ni3At-base intermetallic compounds is substituted with B, Nb, Zr and/or Mo and the alloys consisting mainly of Ni3At-base intermetallic compounds have never been heretofore known.

The first aspect of the present invention consists in super heat resistant alloys having high ductility at room temperature and high strength at high temperatures mainly composed of Ni3At-base intermetallic compound phase containing not more than 3.0% by weight of B, Zr, Nb or Mo.

The preferable range of B, Zr, Nb and Mo are 0.03-0.2%, 0.1-2%, 0.1-2% and 0.1-2% respectively.

The second aspect of the present invention consists in super heat resistant alloys having high ductility at room temperature and high strength at high temperatures composed mainly of Ni3A4base intermetallic compound phase containing a total amount of not more than 4.5% by weight, preferably 0.005-4.5% by weight of at least two elements of B, Nb, Zr and Mo.

The alloys of the present invention will be explained with respect to the experimental data hereinafter.

(1) By using highly pure Af (99.99%), highly pure Ni (99.9%) and Ni-8.7% B master alloy for adding B, the alloys containing B content shown in Sample Nos. 1-9 in the following Table 1 and composed of Ni3At base intermetallic compound phase were melted in argon arc melting furnace, cooled in the furnace and solidified to obtain cast materials. From the cast materials and the heat-treated materials obtained by heat treating the cast materials at 1,000"C for 5 days, test-pieces were cut off.

As the tensile test piece, the cast material was worked into the test piece wherein the gauge size was 1.5 x 1.5 x 23 mm, as the bending test piece, the cast materials was worked into a plate of 30 x 8 x 2 mm and as the rolling test piece, the cast material was worked into plates of 30 x 8 x 1.18 mm and 30 x 8 x 1.77 mm.

As the tensile test, stress-strain curve (S-S curve) at room temperature was measured by means of an Instron type tensile testing machine.

As the bending test, one end of the sample was cramped with a vice and bent by hammering.

As the rolling test, the sample was rolled by a small two-high rolling mill without intermediate annealing.

TABLE 1

Amount of Sample B added Stretching Bending Rolling No. (%) 1 no X X X 2 0.0001 A A A 3 0.01 0 0 0 4 0.05 0 0 0 5 0.1 0 0 0 6 0.5 0 0 0 7 1.0 0 0 0 8 3.0 A A 0 9 5.0 X X A.

(1) Stretching: X substantially not elongate A elongate less than 5% 0 elongate more than 30% (2) Bending: X break at once A slightly bend 0 bend more than 90 (3) Rolling: X impossible A roll upto 10% o roll more than 50% Table 1 shows the experimental results of each test. Sample No. 1 not containing B is very brittle and fractured immediately in a brittle manner in all the tests and is not different from the description in the above described publication.

However, the sample Nos. 2-8 containing B show the ductility as shown in Table 1 and this property has never been heretofore expected.

The inventors have thought that the reason why such excellent ductility which has never been heretofore known, is obtained, will be due to the change of the crystal structure and the alloys were analyzed by X-ray diffraction but it has been recognized that there was no change in the crystal structure and L12-type super lattice was maintained.

The relation of elongation (%) to tensile stress (kg/mm2) is shown in Figure 3 and Figure 4. The sample No. 1 shown in Figure 4, that is Ni3Ae not containing B does not show any elongation and as soon as the sample exceeds the elastic limit, the brittle fracture occurs. However, any one of Sample Nos. 3, 5 and 7 (the composition and Sample No. are same as in Table 1) shown in Figure 3 show the elongation of about 35% and it can be seen that the samples of the present invention have the surprising ductility.

Figure 5 and Figure 6 show the appearance photographs after the bending test of Sample No. 1 not containing B and Sample No. 5 containing 0.1% of B, respectively. Figure 7 shows the appearance photograph after Sample No. 5 (containing 0.1% of B) and Sample No. 6 (containing 0.5% of B) were subjected to the rolling test, the upper sample in this photograph corresponding to No. 5 and the lower sample corresponding to No. 6.

The above described test results are ones concerning the test pieces cut from these cast materials but these results were not different from the results concerning the heat-treated test pieces (1 ,000 C x 5 days).

Then, the relation of the temperature to the strength of the alloys containing B according to the present invention was determined. However, Ni3At not containing B has no ductility, so that the relation of the tensile strength to the temperature can not be determined and therefore the relation of the yield stress (00.2) to the temperature was determined by compression test. Concerning Sample No. 5 containing 0.1% of B, the relation of the tensile yield stress to the temperature was determined and the result is shown in Figure 8. In Figure 8, Sample No. 1 is Ni3At not containing B.As can be seen in this figure, the temperature dependence of the tensile yield stress of Ni3At containing B is similar to that of the compressive yield stress of binary Ni3At not containing B. That is, as already mentioned in Table 1, the ductility of Ni3At can be improved by B-addition in an amount of 0.01-3.0% without deteriorating the yield strength. When B is contained in an amount of more than 3%, as seen from the result of Sample No. 9 in Table 1, the ductility is again deteriorated. This is probably because B itself precipitates at grain boundaries and the brittle phase is formed. Accordingly, it must be avoided to contain such a high concentration of B.

Concerning the lower limit of the content of B, even in the presence of 0.0001% of B as in Sample No. 2 in Table 1, there is more or less effect for improving the ductility and even in the presence of a very slight amount, the effect occurs.

(2) 99.99% purify Ael 99.9% purity Ni, 99.8% purity Zr and 99.8% purity Nb were used as the material. By using argon arc melting furnace, At and Ni of the amounts corresponding to the composition of Ni3At were firstly melted and then for adding ZR OR Nb, the given amount of Zr or Nb was charged into the furnace.

Then the melts were cooled in the furnace to obtain the cast materials of the alloys having the composition as shown in the following Table 2. After solidification, the cast materials were taken out. From the cast materials and the heat treated materials obtained by heat treating the cast materials at 1 ,0000C for 5 days, each test piece was cut off.

The tensile test piece, the bending test piece and the rolling test piece, each having the same dimension as described above, were prepared. The tensile test and the bending test and the rolling test were carried out in the same manner as described above.

The obtained results are shown in the following Table 2.

TABLE 2

Added element (%) Sample Stretching Bending Rolling No. Zr Nb 1 - - X X X 2 0.005 - X X A 3 - 0.005 X X A 4 0.005 0.005 A A 0 5 0.01 - A A O 6 - 0.01 A A o 7 0.05 - 0 0 0 8 0.05 0.03 0 0 Oo 9 0.1 - 0 O Oo 10 - 0.3 O O GO 11 0.5 - 0 GO 12 1.0 - O O o 13 - 1.5 O O O 14 2.0 - O @ O 15 - 2.0 O O O 16 2.0 0.5 0 0 0 17 3.0 - 0 0 0 18 - 3.0 0 0 0 19 3.0 1.0 0 0 0 20 1.0 3.0 0 0 0 21 5.0 - X X X 22 - 5.0 X X X TABLE 2 cont.

(1) Stretching: X not elongate A elongate less than 5% 0 elongate 5-10% O elongate more than 10% (2) Bending: X impossible A bend less than 30 0 bend 30-60 O bend 60-90 bend more than 90 (3) Rolling: X impossible A cause cracks at rolling reduction of less than 10% 0 roll 10-40% O roll 40-80% roll morethan 80% In Sample No. 1 not containing Zr or Nb or Sample Nos. 2 and 3 wherein the content of Zr or Nb is small, the brittle fracture immediately occurs in the tensile test, bending test and rolling test and these cases are not different from the prior report.But in each of Sample No. 5 and Sample No. 6 wherein the content of Zr or Nb is 0.01% respectively or in Sample No. 4wherein the sum of the content of Zr and Nb is 0.01%, it is apparent that a slight ductility is obtained as shown in Table 2. The ductility is more improved as the content of Zr and/or Nb increases and in the content of the range from 0.1% to 2.0%, the surprising ductility is obtained.

When the content of each component exceeds 3.0% or the total content exceeds 4.5%, the ductility is again deteriorated. This is probably because Zr or Nb forms the brittle phase at the grain boundary. Ni3Ae not containing Zr and Nb fractures in a brittle manner.

The relation of the temperature to the strength ofthe alloy (Sample Nos. 10 and 11 in Figure 9) containing Zr or Nb is compared with that of Ni3Af not containing Zr or Nb in Figure 9. In this case, Ni3At (No. 1 in Figure 9) not containing Zr and Nb has no ductility, so that the yield stress (0.2% proof stress) of this sample was determined by compression test. It can be seen from Figure 9 that the behavior of the yield stress at high temperatures of both Ni3At-base inter-metallic compounds containing the above described elements (Nos.

10 and 11) are not so different from that of binary Ni3At intermetallic compound (No. 1), though a slight solid solution hardening is observable in the former alloys.

(3) By using 99.99% purity awl 99.9% purity Ni and 99.5% purity Mo as the materials, Ni3At alloy containing 0.5% by weight of Mo was melted by weighing the necessary amount of the above described materials.

Namely, these materials were charged in Tamman tube in argon gas of high purity, after which the output of high frequency oscillator was raised to 7 kw and the materials were melted and then the output was lowered to 5 kw and the melt was sucked up into an opaque quartz tube having an inner diameter of 5 mmQ and then subjected to homogenizing at 1,000"C for 5 hours under vacuum of 1 x 1 10-3 torr and cooled in the furnace. Test pieces were cut off therefrom.

The tensile test piece was worked into the gauge size of 25 x 2.5 mm, the rolling test piece was worked into 2 x 3 x 30 mm and the bending test piece was used in the shape of the above described quartz tube sucking the alloy.

As the tensile test, the stress-strain curve was made at room temperature by means of an Instron type tensile machine.

As the bending test, one end of the test piece was cramped with a vice and the test piece was bent by hammering.

As the rolling test, a small two high rolling mill was used.

In the bending test which is the most simple test, the sample not containing Mo was not substantially bent and broken as heretofore known but the sample containing 0.5% of Mo could be bent into U-shape, and no crack was observed.

Figure 10 is one example of tensile or rolling test which was conducted at an initial strain rate of 1.4 x 10-3 sec-1 by using an Instron type testing machine. The sample not containing Mo (binary Ni3At, shown by x mark) fractures without showing any elongation, while the sample containing 0.5% of Mo shows an elongation of about 20% as shown by the solid line. The dotted line in Figure 10 is the compression stress-strain curve of binary Ni3At sample not containing Mo. As seen from Figure 10, in binary Ni3At polycrystal, the tensile deformation is impossible but the compression defromation is possible in a certain degree.By containing Mo, the large elongation is not only obtained, but also the yield strength is increased as seen from -marked curve in Figure 11. That is, the addition of Mo improves the ductility and concurrently increases the yield strength, so that Mo is said to be also the very preferable element in view of the structural material.

The fracture surfaces of binary Ni3At and Ni3At alloy containing 0.5% of Mo were observed by scanning electron microscope and the photographs are shown in Figure 12, (a) and (b) respectively. Figure 12, (a) shows that the binary Ni3At is fractured along the grain boundary, while Figure 12, (b) shows that the dimple pattern which is formed owing to formation and coalescence of microvoids at the fracture surface of Ni3At containing Mo, appears and shows the typical ductile transgranular fracture where the grains are elongated by the tensile stress and reach the limitation. Both the photographs show the distinct difference.

The test results of bending, rolling and stretching of the binary Ni3At not containing Mo and the alloys of the present invention containing various amounts of Mo are shown in the following Table 3.

TABLE 3

Sample Amount of Bending Rolling Stretching No. Mo added MO O X X X M1 0.005 X A A M2 0.01 X A A M3 0.05 A O A M4 0.1 0 0 A M5 0.3 8 0 O M6 0.5 O M7 0.8 0 0 0 M8 1.0 0 Q 0 M9 1.5 A 0 A M10 3.0 A A x M11 3.5 X X X (1) Bending:X impossible A bend less than 30 0 bend 30-60 O bend 60-90" bend more than 90" (2) Rolling: X impossible A cause cracks at rolling reduction of less than 10% 0 roll 10-30% O roll 30-50% 0 roll more than 50% (3) Stretching: X not elongate A elongate less than 5% 0 elongate 5-10% O elongate more than 10% It can be seen from the above Table 3 that in Sample MO not containing Mo or Samples M1 and M2 containing a small amount of Mo, the brittle fracture occurs in any one of bending, rolling and tensile tests and this result is not different from the prior report.

However, it is apparent from the above Table 3 that Sample M3 containing 0.05% of Mo has the ductility, even if low. And the ductility is improved as the content increases and within the range of 0.3-0.8%, the ductility is surprisingly improved. However, it has been found that when the content exceeds 3%, the ductility is again deteriorated. Accordingly, in the alloys of the present invention containing Mo alone, the content of Mo must be not more than 3.0% and the content of more than 0.1% is advantageous.The mechanism why the alloys of the present invention containing not more than 3.0% of Mo have the high ductility as mentioned above, although binary Ni3At not containing Mo causes the complete brittle fracture, has not yet been fully understood, but it is considered that Mo serves to decrease the amount of harmful impurities on the grain boundary or to strengthen the grain boundary to restrain formation of cracks at the grain boundary and to improve the ductility.

(4) By using 99.99% purity A, 99.9% purity Ni, Ni-8.7% B master alloy, 99.8% purity Zr and 99.8% purity Nb as the materials, test pieces of the alloys of the present invention containing various contents of Mo and at least one element of B, Nb and Zr were prepared in the same manner as described in the above described item (3) and the bending test, rolling test and tensile test were carried out with respect to these test pieces.

The obtained results are shown in the following Table 4.

TABLE 4(a)

Sample Mo B Nb Zr Bending Rolling Stretching No. (wt%) (wt%) (wt%) (wt%) 1 - 0.005 - - X X X 2 0.01 0.002 - - A A t 3 , 0.01 0.005 - - 0 0 A 4 0.01 0.01 - - 0 0 0 5 0.01 1.0 - 6 0.01 3.0 - - A 0 A 7 0.01 5.0 - - X X X 8 - - 0.005 - A A A 9 0.01 - 0.002 - A A A 10 0.01 - 0.005 - 0 0 A 11 0.01 - 1.0 - O 12 0.01 - 3.0 - 0 O 0 13 0.01 - 4.0 - A 0 A 14 0.01 - 5.0 - X X X 15 0.01 - 0.002 0.003 0 0 0 16 0.01 - 0.001 0.001 A A A 17 0.01 0.002 0.002 0.002 O O 0 18 0.01 0.001 0.001 0.001 A A X 19 0.01 0.5 0.5 - @ O O O 20 0.01 - 1.0 2.0 0 O o 21 0.01 1.0 1.0 1.0 O 0 0 22 0.01 2.0 1.0 1.0 0 0 A 23 0.01 2.0 2.0 2.0 X X X TABLE 4(b)

Sample Mo B Nb Zr Bending Rolling Stretching No. (wt%) (wt%) (wt%) (wt%) 24 0.01 1.0 1.0 2.0 A O x 25 0.01 1.0 3.0 2.0 X X X 26 0.5 0.001 - - O O O 27 0.5 1.0 - - O O O 28 0.5 3.0 - - 0 Q O 29 0.5 4.0 - - X X X 30 0.5 - 0.05 - 0 0 0 31 0.5 - 2.0 - O O t 32 0.5 - 4.0 - 0 0 A 33 0.5 - 5.0 - X X X 34 0.5 - - 0.1 O 35 0.5 - - 2.0 O 36 0.5 - - 3.0 0 Q O 37 0.5 - - 4.0 0 0 A 38 0.5 - - 5.0 X X X 39 0.5 2.0 2.0 - 0 0 A 40 0.5 1.0 1.0 1.0 0 0 41 0.5 1.0 2.0 2.0 X X X 42 2.0 0.001 - - 0 0 0 43 2.0 0.05 - - O O O 44 2.0 1.0 - - 0 0 0 45 2.0 2.0 - - A 0 A 46 2.0 3.0 - - X X X TABLE 4(c)

Sample Mo B Nb Zr Bending Rolling Stretching No. (wt%) (wt%) (wt%) (wt%) 47 2.0 - 1.0 - 0 0 0 48 2.0 - 2.0 - O 0 A 49 2.0 - 3.0 - x x x 50 2.0 - - 0.5 O 0 0 51 2.0 - - 2.0 0 0 0 52 2.0 - - 3.0 X X X 53 2.0 0.05 0.05 0.05 0 0 0 54 2.0 1,0 1.0 1.0 X X X (1) Bending: X impossible A bend less than 30 0 bend 30-60 O bend 60-90 bend more than 90" (2) Rolling:X impossible A cause cracks at rolling reduction of less than 10% 0 roll 10-30% O roll 30-50% GO roll more than 50% (3) Stretching: X not elongate A elongate less than 5% 0 elongate 5-10% O elongate more than 10% elongate more than 30% As seen from the above Table 4, Sample Nos. 5, 11, 19,20 and 21 are very excellent in the bending, rolling and elongation but the Mo content in any one of these alloys is 0.01%. On the other hand, as shown in Table 3, any Ni3At alloy containing 0.01% of Mo alone is poor in the bending, rolling and elongation, while it has been found that N i3At alloys containing at least one of B, Nb, and Zr in addition to Mo can reduce the lower limit of Mo content to 0.01%.

As the amount of B, Nb and Zrto be combined to Mo increases, the ductility is greatly improved by the combination effect of these elements to Mo as seen in Table 4 but when the amount to be combined is too large, the effect for improving the ductility becomes low.

In the alloy of the present invention containing each element of B, Nb, Zr and Mo, the upper limit of the content of B, Nb, Zr and Mo must be defined to be 3.0% but in the alloys of the present invention containing at least two elements of B, Nb, Zr and Mo, if the total amount of these elements is more than 4.5%, the alloys become brittle, so that the above described total amount must be not more than 4.5%.

Furthermore, such combined use not only improves the ductility, but also transfers the maximum point of the tensile yield stress toward the higher temperature.

Namely, Figure 11 is the curves showing the relation of the yield stress to the temperature as already explained but as compared with Ni3At not containing Mo, the alloy containing 0.5% of Mo improves the ductility and concurrently increases the yield stress. Moreover, when such as alloy additionaly contains 1.0% of Zr, the yield stress is considerably increased in addition to the above described improvement of the ductility and the temperature of the maximum stress moves to the higher temperature side.

Such an effect was more or less confirmed with respect to B and Nb.

In the test shown in Figure 11, it was impossible to do the tensile test with respect to the binary Ni3At not containing Mo, so that the yield stress (0.2% proof stress) of this alloy was determined by rolling test and compared with the tensile yield stresses of the new alloys as shown in Figure 11.

The above described data were obtained by examination using the materials having the high purity but the substantially same results as described above were obtained in the experiment using the materials having low purity.

It has been found that the alloys composed of Ni3At-base intermetallic compound and having L124ype super lattice wherein a part of Ni or At is substituted with Ti, Si, Cr, Co, Cu, Mo, Ta, W, Nb, Fe, Zr, V, Mn or Hf and a given amount of B, Nb, Zr and/or Mo is contained, or the alloys consisting mainly of said Ni3At-base intermetallic compounds, wherein a small amount of y phase might be present in admixture, have the satisfactory ductility at room temperature and the high strength at high temperatures.

As mentioned above, the alloys consisting mainly of Ni3Ae intermetallic compound phase containing a given amount of at least one element of B, Nb, Zr and Mo are particularly higher in the strength at high temperatures than the heretofore known Ni-base super alloys consisting mainly of y phase and containing a small amount of Ni3At-base intermetallic compound phase and further are excellent in the workability at room temperature.

Accordingly, it is expected that the alloys of the present invention are used for super heat resistant structural materials for turbine, jet engine, atomic power instruments.

Claims (5)

1. Super heat resistant alloys having high ductility at room temperature and high strength at high temperatures consisting mainly of Ni3At-base intermetallic compound phase containing not more than 3.0% by weight of B, Zr, Nb or Mo.

2. The super heat resistant alloys as claimed in claim 1, wherein the amount of B, Zr, Nb or Mo is 0.005-3.0% by weight.

3. Super heat resistant alloys having high ductility at room temperature and high strength at high temperatures consisting mainly of Ni3At-base intermetallic compound phase containing a total amount of not more than 4.5% by weight of at least two element of B, Nb, Zr and Mo.

4. The super heat resistant alloys as claimed in claim 3, wherein said total amount is 0.005-4.5% by weight.

5. A super heat resistant alloy as claimed in any of the preceding claims and substantially as hereinbefore described in any of the foregoing examples with reference to the accompanying drawings.