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(2) The grain shape tends to remain approximately equant rather than reflect the
macroscopic strain, with no strong development of crystallographic preferred
orientation; but there is a marked tendency for grain growth.
These properties have been established by many observations on metallic and
ceramic materials (Baudelet and Suery
1985
; Chen and Xue
1990
; Edington and
Melton
1976
; Padmanabhan and Davies
1980
; Paton and Hamilton
1982
; Stowell
1983
).
The term ''superplasticity'' has also been applied in situations where consid-
erable ductility and/or a marked decrease in flow stress are exhibited under con-
ditions close to those at which a phase transformation occurs or under which large
internal stresses are generated. Such behaviour has been referred to as ''transfor-
mational'' or ''environmental'' superplasticity (Edington and Melton
1976
; Sam-
mis
and
Dein
1974
;
Sherby
and
Wadsworth
1990
).
We
shall
not
consider
transformational superplasticity here.
In rock deformation studies, where the uniaxial tensile test is little used and
predominantly compressive stress regimes are normally involved, it is useful to
extend the usage of '' superplasticity'' to embrace the general case of flow with
high strain rate sensitivity, under the presumption that, where this condition
applies, similar physical processes are involved and a potential for high ductility
exists. Thus, in the following sections, superplasticity is taken to be defined by a
high strain rate sensitivity of the flow stress, as measured by
d ln e
¼
n
, where r is
the stress, e is the strain rate and n may typically have a value between 1 and 2
(Paterson
1990
); note that Gilotti and Hull (
1990
) prefer a definition in terms of
''continuous'' deformation to very high strains, which is less restrictive in terms of
strain rate sensitivity and of implications for mechanisms. Superplasticity in this
sense has been observed experimentally in calcite, anhydrite, olivine and feldspar
aggregates by Schmid (
1976
), (Schmid et al.
1977
), Schwenn and Goetze (
1978
),
(Mueller et al.
1981
), Vaughan and Coe (
1981
), Brodie and Rutter (
1985
), Chopra
(
1986
), Ji and Mainprice (
1986
), Karato et al. (
1986
), Rutter and Brodie (
1988
),
Stretton and Olgaard (
1997
), Walker et al. (
1990
). It has also been frequently
proposed as being geologically important, at least in the sense that the flow stress
is grain-size-sensitive and therefore low in fine grained rocks (for example,
Boullier and Guéguen (
1975
), Guéguen and Boullier (
1976
), Twiss (
1976
), White
(
1976
), Etheridge and Wilkie (
1979
), Evans et al.
1980
), Schmid (
1983
), Behr-
mann (
1985
) and (Gilotti and Hull
1990
).
d ln r
7.3.2 Mechanisms of Temperature-Sensitive Granular Flow
The view is taken here that mechanistic models for superplastic flow can be based
appropriately on the concept of granular flow (
Sect. 7.1.1
),. In particular, the
approach in terms of granular flow is preferred to that of atom transfer models such
as Nabarro-Herring or Coble creep (
Chap. 5
) for the following reasons:
