densation method for accessing ultrafine grain sizes in complex alloy systems has the unique feature of not depending on mutual solubility in the liquid state to carry out the alloying.

Modulated Structures

A different type of microstructural refinement is displayed by multilayered or modulated structures, and the resulting properties are most intriguing. In the solid-state community, the terms “artificially structured materials” and “strained-layer superlattices” have been adopted, particularly when semiconducting, optical, and magnetic phenomena are of primary interest. This field has been reviewed quite comprehensively in a recent report of the Solid State Sciences Committee of the National Research Council.39 For present purposes, however, we shall focus on the mechanical behavior of metallic systems, in line with the metallurgical emphasis of this chapter.

A number of vapor-deposited compositionally modulated alloy systems (e.g., copper-nickel, gold-nickel, copper-palladium, and silver-palladium) exhibit the strange elastic characteristics shown in Figure 24.40 The biaxial modulus plotted against the wavelength of layered Cu-Ni specimens is determined from a miniature bulge test and is found to peak sharply at a wavelength of about 20 angstroms.40 This “supermodulus effect” represents a phenomenal increase in stiffness, amounting to a factor of 2 to 4 over that of a homogeneous Cu-Ni alloy, and indicates that a significant change compared to stiffness in bulk materials has taken place.

FIGURE 24 Biaxial modulus versus wavelength of compositional modulations having approximately equal layers of vapor-deposited copper and nickel with {111} texture. From Tsakalakos and Hilliard.40 Reprinted with permission.

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