Thermal stability of electrodeposited nanocrystalline Ni- and Co-based materials

Journal article, 2007

The attractive properties associated with nanocrystalline materials are to a large extent a result of their high inter-crystalline volume fraction. However, the intrinsic instability of the nano-structured 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 and grain growth is a complex phenomenon. To better understand the microstructural development upon annealing and to determine the mechanisms operative in the stabilization of Ni- and Co-based nanocrystalline electrodeposits, a wide range of advanced characterization techniques has been applied (transmission electron microscopy, electron backscatter diffraction in the scanning electron microscope, 3D atom probe, calorimetry, and X-ray diffraction). In pure materials like Ni and Co and in single-phase alloys like Ni-Fe, grain growth occurs at relatively low temperatures due to the lack of stabilizing additions; initial grain growth occurs abnormally and is followed by normal grain growth at higher temperatures. The formation of the first grown grains is described to occur by a sub-grain coalescence model similar to the one known from recrystallisation. Thermal stability of nanocrystalline materials can be enhanced significantly with addition of solutes. In a strongly segregating system like Co-P, the effect of solutes together with the allotropic phase transformation is investigated. Already in the as-prepared state, P is found in the grain boundaries and further segregation occurs upon annealing. 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 addition to chemical and morphological influences, the texture development during grain growth is investigated in nanocrystalline Ni and Ni-Fe. The dominant orientation of the first grains changes from <411>//ND to a faster growing/energetically more favourable <111>//ND orientation by twinning. The results are the basis for a model description in which it is stated that segregation leads to an improved thermal stability of 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). A texture with fast growing/energetically favourable orientations has to be avoided.