CRYSTALLOGRAPHY OF THE NIHFSI PHASE IN A NIAL (0.5HF) SINGLE-CRYSTAL ALLOY

Small additions of Hf to conventionally processed NiAl single crystals result in the precipitation of a high density of cuboidal G-phase alo ng with a newly identified silicide phase. Both of these phases form i n the presence of Si which is not an intentional alloying addition but is a contaminant resulting from contact with the ceramic shell molds during directional solidification of the single-crystal ingots. The mo rphology, crystal structure and orientation relationship (O.R.) of the silicide phase in a NiAl(0.5 at.%Hf) single-crystal alloy have been d etermined using transmission electron microscopy, electron microdiffra ction and energy dispersive X-ray spectroscopy. Qualitative elemental analysis and indexing of the electron microdiffraction patterns From t he new phase indicate that it is an orthorhombic NiHfSi phase with uni t cell parameters, a = 0.639 nm, b = 0.389 nm and c = 0.72 nm, and spa ce group Pnma. The NiHfSi phase forms as thin rectangular plates on {1 11}(NiAl) planes with an O.R. that is given by (100)(NiHfSi) parallel to (111)(NiAl) and [010](NiHfSi) parallel to [(1) over bar 01](NiAl). Twelve variants of the NiHfSi phase were observed in the alloy and the number of variants and rectangular morphology of NiHfSi plates are co nsistent with symmetry requirements. Quenching experiments indicate th at nucleation of the NiHfSi phase in NiAl(Hf) alloys is aided by the f ormation of [111](NiAl) vacancy loops that form on the {111}(NiAl) pla nes. Copyright (C) 1996 Metallurgica Inc.