Microstructural Development of Nanocrystalline Ni- and Co-Based Materilas upon Annealing

Licentiate thesis, 2004

Nanocrystalline materials are a relatively new group of materials that are constituted by crystals/grains of a few nanometers in size. Because their mechanical, physical and chemical properties are often superior compared to their microcrystalline counterparts, these materials are attractive for many industrial applications. However, the large volume fraction of grain boundaries in nanocrystalline materials results in grain growth when the temperature increases. Some of the attractive properties are then lost, limiting the service temperature of such materials. The aim of this project is to investigate how the microstructures of electrodeposited nanocrystalline Ni, Ni-Fe and Co-P change upon annealing. This is done by Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Field Ion Microscopy and Tomographic Atom Probe (FIM/TAP). DSC measurements show that all materials, except for Co- 3.2 at.% P, have a heat release in form of a pre-shoulder and a main heat release peak. TEM investigations of in-situ and ex-situ annealed samples show that abnormal grain growth occurs in the temperature range of the pre-shoulder, while normal grain growth takes place at temperatures above the onset of the main heat release peak. The investigations of Ni show that the presence of large grains in the as-prepared state do not result in earlier or more rapid grain growth. According to DSC measurements, Ni is stable up to ~333 K (60C), but no grain growth is observed at temperatures lower than ~473 K (200C) during TEM in-situ annealing experiments. In Ni- 20 at.% Fe, the thermal stability is influenced by the Fe additions and the ordering transformation of Ni3Fe. The investigations show that Ni-Fe is stable up to ~488 K (215C), and in addition to abnormal grain growth, the nanocrystalline matrix changes upon annealing. To get a better understanding of the combined effect of solutes and an allotropic phase transformation, Co-P alloys are investigated. In nanocrystalline Co- 1.1 at. % P the thermal stability is raised to ~720 K (447C). Moreover, there may be a synergistic effect between abnormal grain growth and the allotropic phase transformation. Hence, a Co-P alloy with higher P content (Co- 3.2 at.% P) is investigated. First TEM results show that although no pre-shoulder is observed in the DSC-curve, abnormal grain growth takes place in this alloy.