Second Phase Strengthening

As in the case of conventional disordered alloys, NiAl alloys can be strengthened appreciably by second phases. Apart from the effects on strength, second phases may also be beneficial for ductility and toughness, as has been discussed with respect to NiAl-based intermetallic alloys (Noebe et al., 1991 Clemens and Bildstein, 1992). In any case, the effects of second phases depend on the properties of the respective phases and on the phase distribution. This is illustrated in the following sections by regarding various NiAl alloy systems which have been studied in some more detail. The creep behavior of NiAl alloys with strengthening second phases is addressed in Sec. 4.3.4. 

Besides strengthening, second phases may also be useful for improving the ductility and the toughness - in particular if they are soft, as is the case for the above-mentioned, disordered phases (Noebe etal., 1991). Even the two-phase eqilibri-um between NiAl and the other nickel alu-minide NijAl can be used to produce a two-phase NiAl-NijAl alloy with improved strength and toughness (Baker and... 

A strengthening second phase is embedded in a continuous matrix. 

The strengthening second phase and the matrix are initially separate materials and are joined during processing—the second phase is thus not produced by internal processes like precipitation. 

The volume fraction of the strengthening second phase is at least approximately 10%. 

Fig. 8.2. Representative data associated with four of the key strengthening mechanisms within a material, (a) Increase in flow stress as a function of concentration of substitutional impurities, (b) Dependence of flow stress on mean particle size for material in which there are second-phase particles, (c) Dependence of yield stress on mean grain size of material, (d) Relation between yield stress and mean dislocation density. (Adapted from (a) Neuhauser and Schwink (1993), (b) Reppich (1993), (c) Hansen (1985), (d) Basinski and Basinski (1979).)...

As noted above, our working hypothesis concerning the various hardening mechanisms is that chemical impurities, second-phase particles and even other dislocations serve as obstacles to the motion of a given dislocation. As a result of the presence of these obstacles, the intrinsic lattice resistance tp is supplemented by additional terms related to the various strengthening mechanisms. We further assume that the flow stress can be written as... 

Reinforcement by a fine, homogeneous dispersion of second-phase nanoscale particles is an effective way to achieve strengthening and stability. Oxide dispersion strengthened (ODS) superalloys (Ni-base alloys such as... 

In view of the ternary Ni-Al-Nb phase diagram (see Fig. 29) there are various possible ways to form two-phase or three-phase alloys on the basis of NiAl by alloying with Nb. For the strengthening of NiAl, second phases with not too low Al contents are preferred with respect to de sity and oxidation resistance. Al3Nb witl tetragonal DO22 structure, which has be discussed in Sec. 3.3.2, and the La> phase NbNiAl with a hexagonal C... 

Non-Heat-Treatable Alloys. Non-heat-treatable aluminum alloys are defined primarily by what they are not. They are not strengthened by second-phase particles and may be better described as non-precipitation-hardening alloys. The non-heat-treatable alloy classes are the Ixxx, 3xxx, and 5xxx alloys. 

A polymeric matrix is strengthened or stiffened by a particulate second phase in a very complex manner. The particles appear to restrict the mobility and deformability of the matrix by introducing a mechanical restraint, the degree of restraint depending on the particulate spacing and on the properties of the particle and matrix. In the simplest possible case, two bounds have been predicted for the composite elastic modulus (Broutman and Krock, 1967, Chapters 1 and 16 Lange, 1974 see also Section 2.6.4 of this book) ... 

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