Geology Reference
In-Depth Information
been put forward (for example, Burton
1982a
; Gittus
1975
; Sherby and Weertman
1979
; Spingarn et al.
1979
). Other creep models could be imagined to arise from
forest-cutting or cell-wall-trapping views of mutual dislocation interaction (for
example, Caillard and Martin
1982b
; for example, Caillard and Martin
1982a
,
1983
) where thermally activated intersection or reaction could play a rate-
controlling role. The latter role would be (Morris and Martin
1984b
) akin to a
quasi-viscous drag control, as considered in
Sect. 6.6.5
. Composite models that
incorporate an element of viscous drag control as well as mutual dislocation
interaction have been proposed by Gibbs (
1966
), Ahlquist et al. (
1970
), Lagneborg
(
1972
) and others (see Gittus
1975
, Chap. 3). In general, more definitive theoretical
developments require a more specific basis in microstructural observations and
models, taking into account where appropriate the organization of dislocations into
cell or subgrain structures and its stress dependence (although Weertman and
Weertman (
1983b
) make the point that an n
ΒΌ
3 power law tends to be predicted
regardless of model details if the dislocation structures at different stress are self-
similar in the sense that micrographs are superposable with change of magnification
only). In particular, postulates about internal stress fields lack real validity until
supported by observations such as those of Morris and Martin (
1984a
,
b
). In con-
clusion, it must be emphasized again (cf. Poirier
1985
, p. 114) that the determi-
nation of rheological parameters such as the stress exponent n and the experimental
activation energy Q gives poor constraint on the microscopic mechanisms of
deformation in the absence of microstructural observations.
6.6.7 Thermal Models for Precipitate and Particle Effects
The presence of fine precipitates or dispersed hard particles, especially with sub-
micron dimensions, can profoundly increase the creep strength of materials,
leading to important technological applications, as, for example, in the use of
nickel-based ''superalloys'' in gas turbines, or the development of the oxide-
strengthened metals such as sintered aluminum powder (SAP) and thoria-dispersed
(TD) nickel. Such materials can show peculiarities in creep behavior such as
difficulty in establishing a steady-state creep rate or a tendency for the appearance
of a marked tertiary or accelerated creep stage (the latter effect commonly arises
from instability of the material, for example, the re-solution of precipitates at
elevated temperature). Especially notable is the common tendency for the exper-
imentally determined stress exponent in power-law creep to be abnormally high
(values of 7-40 or more are reported), and the experimental activation energy may
also be abnormally high, substantially exceeding the activation energy for self-
diffusion. Because of these peculiarities, models for these cases are here dealt with
separately from the other models for behavior in the thermal regimes. Reviews of
creep in fine precipitate or particle-bearing materials have been given by Martin
(
1980
), Haasen (
1983
) and Strudel (
1983
).
