Creep cavitation in metals
Section snippets
Introduction to creep plasticity
Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 Tm, where Tm is the absolute melting temperature. The plasticity under these conditions is described in Fig. 1 for constant stress (a) and constant strain-rate (b) conditions. Several aspects of the curve in Fig. 1 require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where [in (a)]
Background
Creep plasticity can lead to tertiary or Stage III creep and failure. It has been suggested that creep fracture can occur by w or wedge-type cracking, illustrated in Fig. 2a, at grain boundary triple points. Some have suggested that w-type cracks form most easily at higher stresses (lower temperatures) and larger grain sizes (Waddington and Lofthouse, 1967) when grain boundary sliding is not accommodated. Some have suggested that the wedge type cracks nucleate as a consequence of grain boundary
Cavity nucleation
It is still not well established by what mechanism cavities nucleate. It has generally been observed that cavities frequently nucleate on grain boundaries, particularly on those transverse to a tensile stress (e.g., Chen and Argon, 1981a, Chen and Weertman, 1984, Lim and Lu, 1994, Arai et al., 1996, Ayensu and Langdon, 1996, Hosokawa et al., 1999). In commercial alloys, the cavities appear to be associated with second phase particles. It appears that cavities do not generally form in some
Diffusion—grain boundary control
The cavity growth process at grain boundaries at elevated temperature has long been suggested to involve vacancy diffusion. Diffusion occurs by cavity surface migration and subsequent transport along the grain boundary, with either diffusive mechanism having been suggested to be controlling, depending on the specific conditions. This contrasts with creep void growth at lower temperatures where cavity growth is accepted to occur by (e.g., dislocation glide-controlled) plasticity. A carefully
Creep crack growth
Cracks can occur in creeping metals from pre-existing flaws, fatigue, corrosion related processes and porosity (Ai et al., 1992, Sherry and Pilkington, 1993). In these cases, the cracks are imagined to develop relatively early in the lifetime of the metal. These contrast the case where cracks can form in a uniformly strained (i.e., unconstrained cavity growth and uniform cavity nucleation) metal where interlinkage of cavities leading to crack formation is the final stage of the rupture life.
Other considerations
As discussed earlier, Nix and coworkers (Goods and Nix, 1978a, Goods and Nix, 1978b, Nieh and Nix, 1979, Nieh and Nix, 1980b) produced cavities by reaction with oxygen and hydrogen to produce water-vapor bubbles (cavities). Other (unintended) gas reactions can occur. These gases can include methane, hydrogen, and carbon dioxide. A brief review of environmental effects was discussed recently by Delph (in press). The randomness (or lack of periodicity) of the metal microstructure leads to
Acknowledgements
Support from the Department of Energy, Basic Energy Sciences under grant DE-FG03-99ER45768 is gratefully acknowledged. The author also wishes to thank Professor Terry Delph of Lehigh for a careful review of this manuscript.
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