Microplastic deformation of polycrystalline silver chloride containing a small volume fraction of hard inclusions

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.Vita.Includes bibliographical references (p. 131-138) and index.Silver chloride is a ductile ionic solid which has mechanical properties typically associated with metals. A new method is presented for making composites of silver chloride doped with 500 ppm copper chloride (CuC1) and 0.01 volume percent of 1 to 5 micron glass spheres and particles. After rolling, recrystallizing and tensile testing, composite samples are exposed to UV light. This "decorates" dislocations in the top 40 microns of a sample with metallic silver so they can be seen with an optical microscope. Dislocation decoration is used to investigate the nucleation of dislocations in polycrystals. In one set of experiments, precisely determined thermal misfit strains are generated between the AgC1 and spherical inclusions by quenching. The critical sphere diameter dp required for punching compares favorably with the predictions of Ashby and Johnson (1969). For spheres larger than the critical diameter, the data are well-fit by a stochastic model for heterogeneous nucleation which predicts that the probability of one or more loops nucleating is 1 - exp(-cdp). In other experiments, dislocation decoration is used to reveal dislocations and slip bands in samples loaded to strains of 90 to 250 microstrain. The equilibrium shape of prismatic loops under stress compares favorably to a dislocation-based model which incorporates the matrix friction stress and the orientation dependence of a dislocation's line energy. Additionally, the role that grain triple junctions and inclusions play in the nucleation of slip is considered. Statistical tests show slip is independent of inclusion location for diameters less than 5 microns. Conversely, triple junctions are positively correlated with slip (p-value= 0.00004). A simple model predicts 46% of slip bands should initiate at triple junctions, vs. 56% observed. In conclusion, slip bands do not nucleate at inclusions in grain interiors but at grain boundaries and in the largest grains first. Preliminary results obtained by straining AgCl with an NaCl epilayer support the conclusion that elastic compatibility effects between large features are more important in determining the initial yielding behavior than a small volume fraction of 1-5 micron inclusions.by Robert Bensen Calhoun.Ph.D