Influence of Powder Particle Size on the Oxidation Behavior of a PM Ni3Al Alloy

Abstract

The influence of particle size on the oxidationbehavior of Ni₃Al prepared by powdermetallurgy (PM) was investigated in the temperaturerange of 535 to 1020°C for exposures up to 200 hr.Four alloys were obtained, each one processed with a differentpowder particle size fraction (<25, 25-50, 50-100,and 100-200 μm). For temperatures below 730°C,the scale consists of an outer NiO layer, a thindiscontinuous intermediate nickel layer, and an internaloxidation zone. The lowest oxidation rate corresponds tothe material with the smallest particle size. Thisresults from its higher grain-boundary density; the boundaries act as easy-diffusion paths foraluminum leading to the rapid formation of a continuousinner alumina layer. At temperatures above 730°C, athree layered scale is observed consisting of an outer NiO layer, an intermediate layer that,depending on temperature, consisted of a mixture ofnickel and aluminum oxides orNiAl₂O₄, and an inner layer ofAl₂O₃, which accounts for thehigher oxidation resistance. The oxidation attack is characterized byintrusions of the scale into the alloy, the intrusionnumber increasing as the particle size decreases. It isassumed that oxide particles and impurities present at the original particle boundaries facilitatealumina growth along these regions. Thus, the lowestoxidation rate for the highest temperature rangecorresponds to the largest particle-sizematerial.