Thermal and thermodynamic properties of fully dense nanocrystalline Ni and Ni-Fe alloys.

摘要

A relatively new field of study in materials science involves the development of polycrystalline materials with ultra-fine grain sizes on the order of 100 nm or less. Such materials, commonly referred to as "nanocrystalline materials", are distinct from conventional polycrystalline materials in that novel mechanical, physical and chemical properties have been identified. Although several prominent methods of nanocrystalline material fabrication have been developed, early experiments, such as those performed on gas condensed and consolidated nanocrystalline materials with considerable amounts of entrained porosity, have shown greatly modified properties. However, there has been little published work that experimentally establishes the intrinsic properties of porosity-free nanocrystalline materials resulting from the large fraction of atoms associated with the interfacial regions. Such fully dense nanocrystalline materials can be fabricated, for example, by electrodeposition techniques.; In the present work, thermal and thermodynamic properties were measured on fully dense nanocrystalline electrodeposits and compared with results obtained for conventional polycrystalline material and other nanocrystalline materials containing porosity.; A comprehensive evaluation has been performed to examine atomic defect structure using positron lifetime spectroscopy, diffusivity using Rutherford Backscattering Spectrometry and Auger Electron Spectroscopy, thermodynamic functions including thermal expansion, heat capacity, grain growth activation energy, and some of the first direct measurements of volume averaged interfacial enthalpy. In contrast to porosity-containing gas condensed materials, these experiments have demonstrated no significant deviation in thermal expansion and heat capacity, no measurable enhancement of heterodiffusion coefficient and no resolvable contribution to defect structure from nanometre sized voids or other regions of excess free volume. However, thermal expansion was found to be sensitive to grain growth and showed a significant negative deviation beginning at temperatures coinciding with the grain growth temperature.; These experimental results demonstrate the true impact of a high interfacial volume fraction brought about by ultra-fine grain size on those properties of porosity-free electroplated nanocrystalline Ni and Ni-Fe alloys. Scanning calorimetry work performed with 90at.%Ni-10at.%Fe, 80at.%Ni-20at.%Fe and 52at.%Ni-48at.%Fe has provided evidence to support solid state phase transformations, including the possible ordering reaction from disordered fcc {dollar}gamma{dollar}(Ni,Fe) to ordered {dollar}rmgammaspprime(Nisb3Fe),{dollar} in the case of 80at.%/Ni-20at.%Fe. Shorter diffusion distances available with nanostructures may explain the rapid formation of the ordered intermetallic phase.; This research work establishes and advances the knowledge base for properties of fully dense, porosity-free nanocrystalline materials. This is required for a better fundamental understanding and for identifying future engineering applications. The work therefore provides an insight into the phenomenological processes which contribute in a novel way to properties observed in nanocrystalline materials as compared to the properties of conventional grain sized polycrystalline materials.