M₅B₃-Type Boride in Nickel-Base Superalloy

Abstract

No boride other than of M₃B₂-type has been reported to be present in nickel-base superalloys, and most of the M's in M₃B₂ are considered, for PHACOMP, to be molybdenum or tungsten.
However, tungsten-rich M₃B₂ is not probable to form in some recent nickel-base alloys containing tungsten instead of molybdenum, since the W-B system does not have a boride of this type.
In this experiment, high-boron cobalt-free Mar-M200 type alloys, containing tungsten but free from molybdenum, were examined to determine the type of boride. X-ray and chemical analyses of the residue indicated that the boride in the alloys was M₅B₃ which was considered to be formed by substituting a portion of chromium atoms in Cr₅B₃ by tungsten atoms, i. e. (Cr, W) ₅B₃. W₅B₃, however, is not present in the W-B system.