Promotion or Retention of Desired Metastable and Ultrafine Microstructures with an Electric Field or Current
Final rept. 22 Jul 2002-21 Jul 2005
NORTH CAROLINA STATE UNIV AT RALEIGH DEPT OF MATERIALS SCIENCE AND ENGINEERING
Pagination or Media Count:
The influence of an electric field on the equlibria and kinetics of grain growth and phase transformations in metals and the resulting mechanical properties were determined, with attention to ultrafine and nanometer microstructures. The phenomena and materials considered were a precipitation in Al-Mg-Si alloys, b phase coarsening in 60Sn40Pb solder joints, c grain growth in electrodeposited Cu and d dependence of the plastic flow stress on grain size. Regarding a, a field applied during solutionizing increased the solubility of the pertinent constituents and in turn the tensile properties in the naturally-aged temper. Thermodynamic considerations along with HRTEM and SAED studies indicated that the field reduced the Gibbs free energy of solution, increased the size of the naturally-aged precipitates and changed their crystal structure. Regarding b, a field retarded coarsening of the Sn and Pb phases and changed their volume fractions. The effect of the field on phase coarsening appeared to be through its effect on the diffusion coefficient. Regarding c, a field retarded grain growth. Whether the field retarded grain boundary mobility or enhanced the annihilation of the crystal defects responsible for the driving force was not clear. Regarding d, analysis of the effect of grain size d on the flow stress of metals and compounds over the range from nanometers to millimeters indicated three regimes Regime I d10-6m, Regime II d10-8-10-6m and Regime III d10-6m. Dislocations are active in I and II and obscent in III. The mechanism governing Regime III appears to be grain boundary shear accommodated by grain boundary diffusion. Unique pulsed laser vapor deposition techniques were employed to fabricate a metals and compounds with nanometer grain size, b La0.7Sr0.3MnO3ZnO heterostructures and c nanodot metals embedded in ceramic matrices, and their respective novel mechanical or electrical properties were determined.
- Electricity and Magnetism