HR: 11:45h
AN: T41F-06    [PDF]
TI: Hart's Mechanical Equation of State in Rock-Salt: Data and Theoretical Model Based on Subgrain Boundary Migration
AU: * Stone, D S
EM: dsstone@facstaff.wisc.edu
AF: University of Wisconsin-Madison, Department of Materials Science and Engineering 1509 University Ave, Madison, WI 53706
AU: Plookphol, T
EM: plookphol@wisc.edu
AF: University of Wisconsin-Madison, Department of Materials Science and Engineering 1509 University Ave, Madison, WI 53706
AU: Cooper, R F
EM: Reid_Cooper@Brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912
AB: There are a wide variety of materials that exhibit behavior consistent with Hart's mechanical equation of state. For such materials, constant structure "hardness curves" can be generated using load relaxation experiments, which sweep out a spectrum of stress vs. strain rate at nearly constant strain. Hardness curves generated at different levels of work hardening scale with each other: that is, they can be translated in log-log space along lines of fixed slope so that they fit on top of each other. Despite the ubiquity of scaling among many varied materials, there has never been an experimental effort to establish a microscopic basis for it until now. And until now, there has never been a careful effort to establish the relationship between data generated from load relaxation and those obtained from creep tests. One benefit of such efforts would be to provide experimenters a tool for extending experimental techniques to lower strain rates - short duration load relaxation experiments can routinely achieve 10$^{-8}$ s$^{-1}$ and, with effort, 10$^{-10}$ s$^{-1}$. Such efforts might also lead to a better understanding of power law creep itself, for which there is no universally accepted microscopic model. In this work we investigate mechanical equation of state in single crystal rock-salt at 673K and 973K deformed under compression in the [100] orientation. We find that different hardness states correspond to different average subgrain sizes and that subgrain size distributions obtained at different levels of stress are similar to each other, i.e., they have the same shape. We compare the experimental data with a model based on the partitioning of the crystal between dislocation glide in large subgrains and subgrain boundary migration in small ones; the distinction between "large" and "small" is based on strain rate. In the model, subgrain boundary migration is governed by the same kind of law that governs Nabarro-Herring creep (with subgrains substituting for grains), except that the numerical coefficient is much smaller in the case of subgrains owing to constraints between the migration of neighboring subgrain boundaries. Subgrain boundary migration is also responsible for dynamic recovery. The model is able to mimic all aspects of the data including load relaxation, transient creep, and steady state power law creep. Moreover, the model is able to account for a transition from power law creep at high temperature (973K) and low stress into power law breakdown at low temperature and high stress (673K) without resorting to additional mechanisms.
DE: 3900 MINERAL PHYSICS
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3919 Equations of state
DE: 3999 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting