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Advancing Materials Research
are typically high in strength but are not “glass-brittle.” They tend to undergo localized shear (due to lack of strain hardening) when stressed beyond the yield strength, and thus they resist fracture even though they are not generally deformable like crystalline alloys. The metallic glasses based on metal (Fe, Ni, Co) and metalloid (B, Si, P, C) combinations are of special interest in view of their markedly low magnetic losses, and the processing is potentially inexpensive because of direct casting of the alloy liquid to final strip form. The technological impact of this metal-processing development is discussed below in the section on magnetic alloys.
The extensive refinement of dendritic structures caused by increased cooling rate or growth velocity during the solidification of crystalline alloys is shown by Figure 6.13 The reduction in dendritic-arm spacing not only improves the as-cast strength and ductility14 but also promotes compositional uniformity by decreasing the diffusion distances between the regions of microsegregation formed by solute buildup in the last pockets of liquid to solidify between the dendritic arms. Such compositional uniformity is advantageous in raising the incipient melting temperature of alloys intended for high-temperature service, as in the case of superalloys. At the same time, second phases that are likely to precipitate in the microsegregated regions tend to be finer in size and more uniformly distributed because of the rapid solidification. These second phases often appear as intermetallic compounds or nonmetallic inclusions that are embrittling when present in coarse or segregated form but that can be desirable when finely divided and well dispersed.
A beneficial consequence of uniform dispersions is that they pin grain boundaries of the matrix phase and thus inhibit grain growth, as shown in Figure 7.15 The effectiveness of this phenomenon is an inverse function of d/fv, where d is the average diameter of the precipitated particles and fv is their volume fraction. This means that for the grain boundary pinning to persist at very high temperatures, the distributed phase must be sufficiently
FIGURE 6 Dendritic-arm and microsegregate spacing in as-cast aluminum alloy microstructures as a function of cooling rate from the liquid state. From Cohen, Kear, and Mehrabian.13 Reprinted with permission.