Hitherto, for example, as shown in Japanese Unexamined Patent Application, First Publication Nos. S61-139633 and 2009-185352, a Ni-base alloy has been widely applied as a material of parts which are used in aircrafts, gas turbines, and the like.
Japanese Unexamined Patent Application, First Publication No. S61-139633 proposes that the amount of nitrogen present in a Ni-base alloy is set to be equal to or less than 0.01 mass %. The reason for this is considered to be as follows: a titanium nitride and other harmful nitrides tend to be formed in the presence of nitrogen and these nitrides cause fatigue cracks.
Japanese Unexamined Patent Application, First Publication No. 2009-185352 proposes that carbides and nitrides have a maximum particle diameter of 10 μm or less. It is pointed out that in the case where the particle diameter is equal to or greater than 10 μm, cracks occur from interfaces between the carbides and matrix phases and interfaces between nitrides and matrix phases during processing at room temperature.
In addition, in the iron and steel field, as shown in Japanese Unexamined Patent Application, First Publication Nos. 2005-265544 and 2005-274401, a method is proposed which estimates and evaluates a maximum particle diameter of nonmetallic inclusions, especially, oxides in a Fe—Ni alloy such as Fe-36% Ni and Fe-42% Ni.
However, in Japanese Unexamined Patent Application, First Publication No. S61-139633, although the upper limit value of the nitrogen amount is regulated, it is not associated with the maximum particle diameter of the nitrides. Therefore, there is a problem in that even when the nitrogen amount is reduced, a Ni-base alloy which has sufficient fatigue strength cannot be stably obtained.
In addition, Japanese Unexamined Patent Application, First Publication No. 2009-185352 specifies that the carbides and the nitrides have a maximum particle diameter of 10 μm or less. However, since the Ni-base alloy is used for aircrafts and gas turbine components for power generation, the degree of cleanliness must be extremely high. Therefore, in fact, it is difficult to grasp the maximum particle diameter by observation of all the sites. In the examples of Japanese Unexamined Patent Application, First Publication No. 2009-185352, the particle diameters of the carbides are measured, and in this regard, it is suggested that it is difficult to grasp the maximum particle diameter of the nitrides. In addition, in order to predict the maximum particle diameter of the nitrides, the maximum nitride particle diameter distribution in a field of view measured in practice is important. However, in Japanese Unexamined Patent Application, First Publication No. 2009-185352, there is no description with regard to this; and therefore, an estimated maximum particle diameter of the nitrides cannot be predicted.
In Japanese Unexamined Patent Application, First Publication Nos. 2005-265544 and 2005-274401, in the Fe—Ni alloy in which a large amount of relatively large nonmetallic inclusions are precipitated, oxides which easily increase in particle diameter is set as a measurement target. It is very difficult to estimate the maximum particle diameter of the nitrides in order to improve the fatigue strength in the Ni-base alloy, and various examinations are required. In addition, in the Ni-base alloy, an oxygen amount and a nitrogen amount are reduced due to re-melting, vacuum melting, and the like. Therefore, in the Ni-base alloy, the number of nonmetallic inclusions and their sizes are smaller than those in a steel material. Furthermore, since the Ni-base alloy includes various phases, analysis of emission intensities and observation of the nonmetallic inclusions cannot be performed in the same manner as in the iron and steel fields.
Therefore, even in the case where the method which is performed in the iron and steel field is simply applied, a relationship between the nitrides in the Ni-base alloy and the fatigue strength cannot be sufficiently evaluated.