With the development of a hydrogen-based economy the demand for high strength materials with low hydrogen embrittlement susceptibility, i.e., suitable for hydrogen applications, increased. An exemplary class of materials are γ´ Ni3(Al, Ti) hardened face centered cubic Fe-Ni based alloys. They form coherent precipitates in a soft, austenitic matrix, which guarantee superior mechanical strength. The shape, spatial distribution, volumetric proportion, and average diameter of the precipitates determine, apart from the composition, crystal structure, and interface character of the precipitate, the mechanical properties and can be tailored by proper heat treatments. Presumably, the susceptibility to hydrogen embrittlement can also be affected by these variables.
Therefore, within this thesis, the precipitation evolution of two Fe-Ni based alloys, A286 alloy and an Invar-type alloy, was studied using electron microscopy and atom probe tomography. Two heat treatments for each material, i.e., under- and overaged, were investigated. The selection was based on their hardening curve at the chosen aging temperature.
The results show that the γ′ precipitates are dispersed in the austenitic matrix and that they are mainly composed of Ni, Ti, and Al, while the matrix is rich in Fe, Cr and Ni, irrespective of the applied aging treatment. In addition to the γ′-phase, irrespective of the condition, carbides and a small fraction of η-phase could be detected. The fraction of η-phase increased with prolonged aging times. Moreover, the morphology of the precipitates for under- and overaged state varied in shape and volume. Precipitates in both alloys and aging conditions showed a spherical shape with a mean diameter of 26-37 and 17-25 nm for A286 and Invar, respectively.
The effect of these aging treatments on the coherency of the precipitate-matrix interface will be analyzed in future experiments.