A composite flow law for wet polycrystalline halite to capture the transition from dislocation creep to solution-precipitation creep

For wet or dry polycrystalline halite, the creep behaviour observed in laboratory
experiments at relatively high temperatures and strain rates is generally considered to be
controlled by dislocation mechanisms. For fine grained wet materials at low temperature and
strain rates, solution-precipitation creep is suggested to dominate. We studied if the transition
between these mechanisms can be observed in laboratory experiments, and if so, at what
strain rate. We used synthetic and natural wet polycrystalline halite (starting grain sizes ~0.3
and ~4.0 mm, respectively), and deformed these in multiple strain rate step experiments at in
situ PT conditions of 50 MPa and 125 °C. We also applied the stress relaxation technique, to
achieve strain rates approaching 10-9 s-1. For higher stresses and strain rates, we found a
power law stress exponent n~11, while towards lower stress and strain rate, the n-value
decreased to ~1. This transition took place over the strain rate interval 10-8-10-9 s-1. We interpret
this behaviour as a transition from glide-controlled dislocation creep at high n to solutionprecipitation creep at n~1, made possible by grain size adjustment through fluid-assisted
dynamic recrystallization. We defined a first-order creep law combining a power law and a
solution-precipitation law to cover the transition.