Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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2nd Revised Pages

9.19 Creep in Ceramic and Polymeric Materials • 331

and, solving for the time,

22. 37 = 20 +logtr

tr=233h(9.7 days)

9.18 ALLOYS FOR HIGH-TEMPERATURE USE

There are several factors that affect the creep characteristics of metals. These in-

clude melting temperature, elastic modulus, and grain size. In general, the higher the

melting temperature, the greater the elastic modulus, and the larger the grain size,

the better is a material’s resistance to creep. Relative to grain size, smaller grains

permit more grain-boundary sliding, which results in higher creep rates. This effect

may be contrasted to the influence of grain size on the mechanical behavior at low

temperatures [i.e., increase in both strength (Section 8.9) and toughness (Section

9.8)].

Stainless steels (Section 13.2) and the superalloys (Section 13.3) are especially

resilient to creep and are commonly employed in high-temperature service appli-

cations. The creep resistance of the cobalt and nickel superalloys is enhanced by

solid-solution alloying, and also by the formation of precipitate phases. In addition,

advanced processing techniques have been utilized; one such technique is directional

solidification, which produces either highly elongated grains or single-crystal com-

ponents (Figure 9.40).

9.19 CREEP IN CERAMIC AND

POLYMERIC MATERIALS

Ceramic materials often experience creep deformation as a result of exposure

to stresses (usually compressive) at elevated temperatures. In general, the time-

deformation creep behavior of ceramics is similar to that of metals (Figure 9.35);

however, creep occurs at higher temperatures in ceramics.

Conventional casting

(a)

Columnar grain Single crystal

(b) (c)

Figure 9.40 (a) Polycrystalline turbine blade that was produced by a conventional casting

technique. High-temperature creep resistance is improved as a result of an oriented

columnar grain structure (b) produced by a sophisticated directional solidification technique.

Creep resistance is further enhanced when single-crystal blades (c) are used. (Courtesy of

Pratt & Whitney.)