Thermal Stability of Electrodeposited Nanocrystalline Ni- and Co-Based Materials

Doktorsavhandling, 2007

The attractive properties associated with nanocrystalline materials are to a large extent a result of their high intercrystalline volume fraction. However, the intrinsic instability of the nanostructured state may compromise the gain in properties by the occurrence of grain growth during exposure at elevated temperatures. Thermal stability is therefore a fundamental materials issue for nanocrystalline materials. The aim of this project is to obtain a deeper and more fundamental understanding of microstructural development and texture in nanocrystalline materials upon annealing. This is achieved by applying a combination of techniques such as transmission electron microscopy, electron backscatter diffraction in the scanning electron microscope, 3D atom probe, calorimetry, and X-ray diffraction. In Co-P, the effect of solutes together with the allotropic phase transformation is investigated. Already in the as-prepared state, P is found at grain boundaries and further segregation occurs upon annealing. This segregation stabilizes the nanocrystalline structure by the combination of a thermodynamic effect (reduction in driving force for grain growth) and a kinetic effect (reduction in grain boundary mobility). Transmission electron microscopy and 3D atom probe reveal that precipitation takes place upon annealing. The P atoms in the grain boundaries are consumed during the formation of Co2P and CoP precipitates and grain growth can take place. In a material with higher P content, the thermal stability is slightly reduced due to earlier grain boundary saturation and precipitation. Nanocrystalline Ni and Ni-Fe are also analyzed. They have a lower thermal stability; initial grain growth occurs abnormally and is followed by normal grain growth at higher temperatures. In addition to chemical and morphological influences, the texture development during grain growth is investigated. It is found that during abnormal grain growth the initial <311>//ND changes to a more favorable <111>//ND orientation by twinning. Hence, the texture development is not a result of oriented nucleation. Furthermore, a Co-based nanocomposite is investigated with respect to the influence of the µm-sized boron carbides. Thermal stability is observed not to be affected by the presence of the carbides. In the thesis the investigated materials are also compared with other nanocrystalline electrodeposits and the stabilizing mechanisms discussed in a broader perspective.