Fundamental Understanding of Deformation Mechanisms in Nanocrystalline Superplasticity

University Of California-Davis, Davis CA

Investigators

Abstract

TECHNICAL: This is a renewal project designed to gain a fundamental understanding of the deformation mechanisms of superplasticity in nanocrystalline materials. The phenomenon of sliding along the grain interfaces is one of the dominant rheological characteristics in superplasticity. The ultra-fine grained nanocrystalline materials contain a very large density of interfaces and, therefore, they offer a unique opportunity to study the role of grain boundary sliding in superplasticity. Using high pressure torsional straining to produce porosity and contaminant free nanocrystalline materials from ingot stock, the project outlines an experimental program to study the superplastic deformation of a model nanocrystalline material. Preliminary mechanical results on nanoscale materials suggest that the experimental goal is achievable. However, the theoretical micro-mechanisms for such observation of nanocrystalline superplasticity are far from being clear. Experimental investigations during the previous NSF grant have revealed high flow stress, extensive strain hardening, and a correlation between microstructural instability and onset of enhanced plasticity in nanomaterials. This renewal project attempts to uncover the underlying reasons for such observations. A specially designed and instrumented tensile testing device has been constructed for conducting precision mechanical tests under protective environment to prevent oxidation during testing. The nanostructure before and after deformation will be investigated using TEM/HREM. Particular emphasis will be paid to in-situ tensile TEM observation at the superplastic temperature range, which will be complimented at the University of Southern California by molecular dynamic simulation studies. The intellectual merit lies in our attempt to correlate the microstuctural information with the mechanical data obtained from such nanoscale materials in the context of elevated temperature plasticity in really diminished length scales. Special emphasis will be given to the difficulty of intra-granular dislocation generation inside the matrix in truly nanoscale structure and its implication to slip accommodation processes in current models of nanocrystalline plasticity at elevated temperatures. The role of grain boundary as a source of dislocations in the interface sliding process as the grain size decreases to nanoscale dimensions will be emphasized. NON-TECHNICAL: Determining whether the observed increase in superplastic strain rate and/or decrease in superplastic temperature with decreasing grain size is a general phenomenon or not, has important technological implications in our society. Increasing superplastic strain rate will decrease forming time and will make it an economically viable process. Lowering superplastic forming temperature will allow utilization of some of the existing forming technology for shop-floor practice in industrially significant intermetallic structural materials. Outreach activities would provide educational outreach to the Davis community through ongoing programs for high school students in the Davis/Sacramento area. The program has involved a number of lectures on the basics of Materials Science and Nanotechnology for students who came to visit campus and our Metallography in our laboratory. Another area of our outreach activity was targeted to the public at large of all ages through participation in Nanoscape project at the Exploratorium in San Francisco. This program aimed to increase public awareness of nanoscience by involvement in the creation of several large-scale, scientifically accurate, and visually compelling models of nanoscale molecules and crystals. Our task was to help the public understand how these geometries hold promise of new technologies.