EFFECT OF SILICON ON THE TRANSITION OF BRITTLENESS IN A NICKEL BASE WROUGHT SUPERALLOY

The tensile strength, ductility or impact value of a wrought nickel base super-alloy Ni-15Cr-6W-3Mo-2Al-2Ti depend markedly upon the silicon content of thealloy. On plotting these mechanical properties vs the silicon content which rangesfrom 0.1 to 0.89%, a saddle shown by the existence of a minimum at about 0.4-0.6% Si occurs. There is reason to believe that such a saddle behavior may be caus-ed by the variation of mechanical properties with the type, amount and morpho-logy of carbides as well as with the sequence of carbide precipitation in the alloyas influenced by its silicon content. Thus, the amount of M_6C and M_(23)C_6 precipi-tated along the grain boundaries would appear to proceed in four stages as follows: 1. For silicon content up to 0.1%, globular M_(23)C_6 may be the sole type ofcarbide formed; 2. In the range from 0.1 to 0.4% Si, besides M_(23)C_6, M_6C begins to appearand increases with increase of silicon content. The morphology of the carbidesseems to be different, however, being in a discontinuous, blocky form; 3. In the range from 0.4 to 0.6% Si, continuous films of M_6C were present.As might be expected, this type of carbide is very detrimental to the tensile andimpact properties at room temperature. The precipitation of M_(23)C_6 was slowed downor even suppressed; 4. In the range from 0.6 to 0.89% Si, M_6C may be altered to discontinuous,blocky form, whereas M_(23)C_6 may be very much limited. In conclusion, the detrimental effect of carbide is mainly associated with itsfilm-like morphology especially along the grain boundaries. Of course, carbides ingranular form are good strengtheners. According to prevailing view, criterion for the formation of either M_6C orM_(23)C_6 in nickel base superalloys would seem to be the relative Mo and W con-tents, e. g. if Mo+1/2W>6%, M_6C will be formed. Present work indicates that si-licon plays as equally important role in the formation of M_6C besides the Mo andW contents of the wrought nickel base superalloys.