Grain boundary precipitation phenomena in an alloy 33 (Cr-Fe-Ni-N) subjected to direct-aging treatments (700 °C and 900 °C)

Precipitation behavior of alloy 33 upon isothermally aging at 700 °C and at 900 °C was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (XEDS). The name, alloy 33, reflects the quasi-equal atomic content of the base elements Fe, Ni and Cr close to 33%, with the addition of Mo and N. The most remarkable features of the microstructural evolution resulting from the aging heat treatments are described as follows: aging at 700 °C leads to homogeneous and heterogeneous grain boundary precipitation, where the discontinuous mode is dominant at this temperature. Conversely, the opposite behavior was observed when the alloy is aged at 900 °C: predominantly homogeneous precipitation together with grain boundary precipitation, however with rare evidence for discontinuous precipitation (DP), even after longer aging times. In the present study, the topology and content of the DP colonies generated by the combined effect of grain boundary migration diffusion and precipitation under isothermal aging at 700 °C were particularly investigated. A variety of morphological features were observed: mainly (1) classical-type microstructure with straight and parallel lamellar precipitates with regular interlamellar spacing and (2) another microstructure with irregular and complex DP colonies. According to our analytical electron microscopy observations, the latter shows that there are more than one precipitate phase within the same colony, which is new to the current theory and experimental evidence on this subject.