Microstructure and phase stability studies on Heusler phase Ni_2AlHf and G-phase Ni_(16)Hf_6Si_7 in directionally solidified NiAl-Cr(Mo) eutectic alloyed with Hf

摘要

Small additions of Hf to directionally solidifiedNiAl-Cr(Mo) eutectic resulted in precipitation of a high densityof Heusler phase Ni_2AlHf along with fine G-phaseNi_(16)Hf_6Si_7. The Heusler phase was mainly located on thegrain boundary region. The fine G-phase formed in the presenceof Si, which was a contamination resulting from contact withceramic shell molds during directional solidification of the alloy.These fine G-phases were cuboidal in shape and coherent withthe NiAl matrix. After hot isostatic pressing and aging treatment,the fine G-phases completely disappeared. The density of theHeusler phase was partially reduced, and the Heusler particlesprecipitated preferentially on the NiAl/Cr(Mo) interfaces andgrain boundaries of the NiAl matrix. Some Heusler particlesprecipitated locally within the NiAl matrix, and small amountsof them precipitated within the Cr(Mo) phase. The structures ofthe NiAl/Ni_2AlHf and NiAlINi_(16)Hf_6Si_7 interfaces wereinvestigated by high-resolution electron microscopy. The habitplane of the fine G-phase was {001}_(NiAl) This result was ingood agreement with calculation based on the linear elastictheory. The misfit dislocation network on the NiAl/Ni_2AlHf(1-bar10) interface was calculated from the 0-lattice model andcompared with the observation, which showed good agreement.