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Advancing Materials Research
the range of structure-property relationships through microstructural refinement now constitutes a frontier of metallurgical research reaching down into the nanometer regime.
Dispersed precipitates are effective in obstructing dislocation motion and, hence, in raising the strength of alloys, but such precipitates can also be detrimental to toughness because of void initiation, which promotes shear localization and fracture. It has been estimated,32 however, that particles smaller than about 20 nm should be subcritical in size relative to void nucleation, whereas particles larger than about 1 nm should be capable of resisting dislocation motion. Thus, a “window of opportunity” exists in this size range for optimum dispersion strengthening and toughening. A further contribution to strength and toughness is obtainable from grain refinement of the matrix phase; in fact, no limit has yet been established in these favorable trends with diminishing grain size. Consequently, they should be exploited, or at least tested, by processing methods now available.
These guidelines have recently been adopted in a comprehensive initiative to increase the fracture toughness of ultrahigh-strength martensitic steels and, at the same time, to test the theoretical and practical limits of the relevant structure-property relationships due to microstructural refinement.33 In view of the obvious technological impact of such high-performance steels, several university, governmental, and industrial laboratories are now participating in this research endeavor.
Fine-grained microstructures can be advantageous for plastic-forming operations at elevated temperatures by inducing superplasticity. With rising temperature, grain boundaries in alloy systems tend to lose their strengthening capability and participate in new modes of plastic deformation. If grain growth can be inhibited to maintain a sufficiently fine grain size, typically through the presence of a second phase, as discussed earlier, extensive plasticity may be encountered—e.g., several hundred percent elongation in a tensile test.
Figure 19 summarizes the essential mechanical behavior characteristics of superplasticity.34 High strain-rate sensitivity (m) is required to achieve stable necking-free elongation, and this condition is favored by an optimum strain rate (usually rather slow), which can be increased with decreasing grain size and with increasing temperature (if grain growth is inhibited). An impressive example of superplastic forming is shown in Figure 20, which illustrates the closed-die forging of a superalloy gas-turbine wheel, with integral blades, in only two steps from a hot-extruded billet of RSP powders.20 Many other industrial shaping operations based on superplasticity are now in commercial practice.35