Microstructures and mechanical properties of NiAl-Ni2AlHf alloys

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Original languageEnglish
Pages (from-to)1189-1196
Journal / PublicationJournal of Materials Research
Issue number6
Publication statusPublished - Jun 1990
Externally publishedYes


The microstructure and mechanical properties of several Ni-Al-Hf alloys in the composition range between NiAl (β) and Ni2AlHf (Heusler phase) have been studied. The volume fraction of Heusler phase, Vƒ, varies from about 15 to 96%. The lattice misfit between the β and Heusler phases in two-phase alloys is larger than 5%, indicating no coherency between them. The yield strength increases with increasing Vƒ at all temperatures to 1000°C. Compressive ductilities of 4 and 7% were obtained for the alloy with Vƒ of 15% at room temperature and 500°C, respectively, but they decreased to 0% with increasing Vƒ to 96%. The fracture mode is basically transgranular cleavage. However, all the alloys can be deformed extensively without fracture at 1000°C. The hardness of the Heusler alloy is very high (8.3 GPa) at room temperature; it decreases gently with temperature to 600°C, followed by a rapid decrease to 1000°C. The brittleness and high hardness of the Ni2AlHf Heusler phase at low temperatures are interpreted in terms of internal lattice distortion resulting from its crystal structure. The thermally activated process of deformation takes place above 600°C, which is reponsible for the rapid drop in hardness.