Abstract
The lattice parameters of three iron-nickel alloys containing respectively 41.1, 32.2 and 24.2 per cent of nickel by weight have been measured at different temperatures. The alloys had to be carefully heat-treated before any spectra could be obtained with them. From the relations between parameter-value and temperature, curves were obtained showing how the coefficient of thermal expansion varied with temperature. The two alloys containing 41.1 and 32.2 per cent of nickel yielded curves which showed that the coefficient of thermal expansion had a somewhat low value at low temperatures. At a particular temperature the value increased suddenly to another value, which on further rise of temperature remained almost constant. These results are in agreement with the results obtained by other observers by other methods of measurement.
The expansion curve of an alloy containing 24.2 per cent of nickel showed the ordinary thermal-hysteresis effect found with the irreversible alloys. The thermal expansion of the components α and γ were measured. The mean atomic volume in the α phase is greater than the mean atomic volume in the γ phase at the same temperature. By the aid of these measurements it was possible to explain the form of the expansion curve obtained with a rod of the material, which shows a contraction when the α phase transforms to the γ phase.
Values are given for the lattice parameters of the pure α and γ phases at different temperatures. The coefficient of thermal expansion of the body-centred α phase is approximately 8.2 × 10-6 at 0° C. and 14.0 × 10-6 at 200° C., and that of the face-centred γ and γ' phases is 17.5 × 10-6 between 0° C. and 630° C.