Optimization of toughness and strength in multiphase intermetallics

We have examined the effects of a fine dispersion of precipitates in the ma trix phase of two multiphase NiAl-based alloys on strength and toughness. T he first system is a directionally solidified Ni-30Fe-23Al alloy composed o f a B2 matrix reinforced with a ductile fcc-based second phase. Spinodal de composition leads to fine-scale bcc precipitates within the B2 phase, resul ting in a 50% increase in room-temperature strength, but with reduced ducti lity and toughness compared to similar alloys without the strengthening pre cipitates. The increase in strength limits matrix plasticity prior to cleav age crack initiation, but some slip transfer still occurs from the fcc-base d phase to the (B2 + bcc) matrix. A mixed dendritic and lamellar microstruc ture also contributes to lower toughness. The second system is a directiona lly solidified NiAl-31Cr-3Mo eutectic composed of a B2-NiAl matrix reinforc ed with a Cr(Mo) phase. Small additions of Hf and Si to this material resul t in the precipitation of a fine cuboidal G-phase in the NiAl matrix. Reduc ed toughness in this modified alloy relative to unalloyed NiAl-Cr(Mo) is at tributed to the lack of plasticity in the precipitate-strengthened matrix a nd partial loss of the aligned lamellar microstructure by Hf and Si alloyin g. Observations of the deformation and fracture mechanisms in these alloys are used as a basis to discuss microstructural design of multiphase interme tallics with optimized strength and toughness. (C) 2001 Elsevier Science Lt d. All rights reserved.