Callister - An introduction - 8th edition

270 • Chapter 8 / Failure
Figure 8.33 (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 singlecrystal
blades (c) are used. (Courtesy of Pratt &
Whitney.)
(a)
Conventional casting
(b)
Columnar grain
(c)
Single crystal
better 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 7.8) and toughness
(Section 8.6)].
Stainless steels (Section 11.2) and the superalloys (Section 11.3) are especially
resilient to creep and are commonly employed in high-temperature service applications.
The creep resistance of the 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 components
(Figure 8.33).
SUMMARY
Introduction
• The three usual causes of failure are
Improper materials selection and processing
Inadequate component design
Component misuse
Fundamentals of Fracture
• Fracture, in response to tensile loading and at relatively low temperatures, may
occur by ductile and brittle modes.
• Ductile fracture is normally preferred because
Preventive measures may be taken inasmuch as evidence of plastic deformation
indicate that fracture is imminent, and
More energy is required to induce ductile fracture than for brittle fracture.
• Cracks in ductile materials are said to be stable (i.e., resist extension without an
increase in applied stress).
• For brittle materials, cracks are unstable—that is, crack propagation, once started,
will continue spontaneously without an increase in stress level.