Optimization of toughness and strength in multiphase intermetallics

Abstract

We have examined the effects of a fine dispersion of precipitates in the matrix phase of two multiphase NiAl-based alloys on strength and toughness. The first system is a directionally solidified Ni–30Fe–23Al alloy composed of a B2 matrix reinforced with a ductile fcc-based second phase. Spinodal decomposition leads to fine-scale bcc precipitates within the B2 phase, resulting in a 50% increase in room-temperature strength, but with reduced ductility and toughness compared to similar alloys without the strengthening precipitates. The increase in strength limits matrix plasticity prior to cleavage crack initiation, but some slip transfer still occurs from the fcc-based phase to the (B2+bcc) matrix. A mixed dendritic and lamellar microstructure also contributes to lower toughness. The second system is a directionally solidified NiAl–31Cr–3Mo eutectic composed of a B2-NiAl matrix reinforced with a Cr(Mo) phase. Small additions of Hf and Si to this material result in the precipitation of a fine cuboidal G-phase in the NiAl matrix. Reduced toughness in this modified alloy relative to unalloyed NiAl–Cr(Mo) is attributed to the lack of plasticity in the precipitate-strengthened matrix and partial loss of the aligned lamellar microstructure by Hf and Si alloying. Observations of the deformation and fracture mechanisms in these alloys are used as a basis to discuss microstructural design of multiphase intermetallics with optimized strength and toughness.