Influence of fiber alignment on creep in directionally solidified NiAla10Mo in-situ composites

A NiAlaMo eutectic having a nominal composition of Nia45Ala10Mo was directionally solidified in a floating-zone furnace at two different growth rates, 20 and 80 mm/h. At the slower growth rate, the Mo fibers in the composite are well-aligned with the growth direction, whereas at the higher growth rate cellular microstructures are observed. Creep testing at 900 degree C showed that the minimum creep rate is much higher for cellular than for well-aligned structures. In the cellular case a asofta cell boundary consisting primarily of binary NiAl surrounding aharda eutectic cell interiors seems to facilitate cell boundary sliding and therefore results in low creep strength. A microstructure-based composite model is used to explain the effects of fiber alignment on creep resistance.