FUNDAMENTAL UNDERSTANDING OF SUPERPLASTICITY IN NANOCRYSTALLINE METALS

1. FUNDAMENTAL UNDERSTANDING OF SUPERPLASTICITY IN NANOCRYSTALLINE METALS Amiya K. Mukherjee, University of California-Davis, DMR 0703994 Electrodeposition parameters were set to produce single-phase Ni-Fe samples with modulated microstructures consisted of alternate nano- and ultrafine- grained layers with 1:1 thickness ratio. The individual layer thickness in these laminated samples varies from 20nm to 5 µm, that provide a unique opportunity to control the mechanical properties. Figures 1 and 2 demonstrate representative TEM images of layered microstructures in samples with average layer thickness of 100nm and 50 nm, respectively. It has been observed that at smaller layer thicknesses, significantly higher strength can be obtained, Figure 3. The enhanced strength is related to presence of the alternating layers and the role of the layer interfaces in determining the deformation mechanisms. Different proposed strengthening mechanisms for layered structures including Hall-Petch, Orowan, and Koehler are candidates for a detailed interpretation of the observed microstructure-mechanical properties relationships. Elevated temperature deformation characteristics of the multilayered samples are being studied via strain rate jump tests. As an example, Figure 4 demonstrates strain rate jumps in a sample with 5µm layer thickness. Our analysis for this sample revealed a strain-rate sensitivity value of 0.131 that suggests a deformation mechanism related to power law creep may be operative in this region of strain rate (7x10-5-4x10-4). Microstructure Deformation under tensile load Figure 3 Tensile strength vs. layer thickness. Figure 1 Bright field TEM image of Ni-Fe multilayer sample with an average layer thickness of 100 nm. Figure 2 Bright field TEM images of Ni-Fe sample with an average layer thickness of 50 nm Figure 2 Elevated temperature strain rate jump tests.

2. FUNDAMENTAL UNDERSTANDING OF SUPERPLASTICITY IN NANOCRYSTALLINE METALS Amiya K. Mukherjee, University of California-Davis, DMR 0703994 Outreach • Training / Education: • Dr. Troy Holland, Postdoctoral Researcher at UCD, presented a poster, “Deformation Mechanisms and Processes in Nanometric, Bimodal Nickel” at International Workshop on the Plasticity of Nanocrystalline Metals sponsored by the German Science Foundation (DFG). • Collaborating with National Center for Electron Microscopy (NCEM) to develop stress normalized observations of deformation mechanisms in-situ. Proposal #1276. • Developed high pressure dies/apparatus (>1.5GPa pressures) for full consolidation of nanometric powders at low temperatures. This equipment is and will be used in training graduate students and visiting scholars. • Dr. Hamed Bahmanpour (current) Postdoctoral Researcher will be presenting our work on NiFe nanolaminates at the Fall meeting of MRS at Boston, MA. Postdoctoral Fellow Dr. Troy Holland developed a sintering apparatus for high pressure consolidation for use in both producing superior part densities, and training lab personnel and visiting scholars.