Solidification of grey cast irons is nowadays most commonly monitored by simple thermal analysis. By thermal analysis we can get assessment of the state of microstructure in terms of size, shape and distribution of microstructural components. Beside thermal analysis, the electrical resistivity measurements of metals can also be used to characterize alloys. These are an important part of analytical methods in the development and testing of materials. By electrical resistivity measurements it is possible to follow reactions such as precipitation, transformation and solidification of metals and alloys. In the master's thesis, we tried to determine the influence of the microstructure on the electrical resistivity of cast irons by measuring electrical resistance.
A measuring cell, which measured temperature and electrical resistance, was made using the Croning process. Several samples were made with different time after nodularisation, or with a different proportion of magnesium and/or carbon. Due to differences in the chemical composition of the alloys, changes in the microstructure and in the values of the electrical resistivity were achieved during solidification.
The electrical resistivity of the materials varies with temperature and depends on the microstructure. Microstructural phases, shape, amount and size of phases have a major influence on the electrical resistivity of materials. Larger phases or more lamellar graphite, brings higher electrical resistivity, as there are several obstacles for the movement of electrons. However, if the phases are smaller and more rounded, the electrical resistivity is lower. Matrix also have influence on the electrical resistivity . Resistivity is higher if the matrix is perlitic.
It was found that the shape of the graphite does not have a major influence on the electrical resistivity until graphite is completely lamellar. The change of electrical resistivity during the solidification of gray cast iron with a nodular graphite, is similar as of vermicular graphite. During solidification, the resistance in both cases decreases, while in the case of gray cast irons with lamellar graphite, electrical resistivity increases during solidification.
For cast iron samples with spheroidal and/or vermicular graphite, the specific electrical resistivity decreases by approximately 0,1 μΩm during solidification. At the solidification of cast iron samples with lamellar graphite, the specific electrical resistivity increases by approximately 0.3 μΩm.