Then, the member X is heated at 800???-1100??? C. in an atmosphere of an inert gas such as Ar or H2, or in a gas whose composition are the same as those of a gas produced when the above powder is heated at 800???-1100??? C. During this heating step, the cycle of increasing the heating temperature from 800??? C. to 950??? C. and reducing it from 950??? C. to 800??? C. is repeated at least twice. (T