Interrelationships between microstructures and mechanical properties of physically vapor-deposited NiAl 2O 3 alloys

Abstract

Thick free-standing films ( t = 250 μm) of Ni+Al 2O 3 deposits were prepared by high rate (4 μm min −1) electron beam evaporation and condensation in the temperature range 0.3−0.5 T m. Fine dispersions of Al 2O 3 particles, 20–70 Å in size, were randomly dispersed throughout the fine-grained near equiaxed structure ( d ̄ = 0.3 μm) in deposits with Al 2O 3 contents ranging from ƒ = 0.25 vol.% to ƒ = 0.76 vol.% . The presence of the Al 2O 3 particles stabilizes the grain size, produces strengthening effects and reduces the creep ductility. Compared with conventionally processed Ni, the Ni+Al 2O 3 deposits creep at a faster rate owing to the fine grain sizes. Low temperature creep deformation involves dislocation-controlled processes, whereas high temperature ( T ≈ 535 and 625°C) deformation appears to exhibit a change in deformation mode from dislocation glide at low stresses to grain boundary sliding at high stresses. The contribution from the two processes is determined by a comparison of the total creep elongations with the mean grain elongations along the strain axis as measured in the transmission electron microscope. The high temperature deformation is characterized by a high ductility and a low stress exponent ( n = 25) at higher than strain rates, indicating superplastic-like behavior.