Abstract
Microstructure and mechanical properties of polycrystalline Ni₇₈Al₂₂ alloy prepared by directional solidification (DS) at three growth rates of 18, 60 and 108 mm/h were studied. After DS the microstructure consisted of coarse columnar γ’(Ni₃Al) grains, which solidified directly from the melt, fine γ’ grains formed from primary γ-phase (Ni-based solid solution) and γ’ nanoparticles formed in the γ channels surrounding fine and coarse γ’ grains. Volume fraction of the γ’-phase increased from 70 to 75 vol.% with decreasing growth rate from 108 to 18 mm/h. The highest room temperature tensile ductility was measured in the specimens with the highest volume fraction of the γ’-phase, which was prepared at the lowest growth rate of 18 mm/h. After fracture the proportion of the dislocation density in the gauge section to that in the heads was by an order of magnitude higher in the specimens showing the highest ductility than that in the specimens showing the lowest ductility. High values of room temperature ductility result from an intensive multiplication of dislocations during tensile deformation.

Key words
nickel aluminides, Ni₃Al, crystal growth, mechanical properties, microstructure

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