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

Abstract

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.