Abstract
The precipitation of z-phase (NbCrN) in a commercially available, high-nitrogen stainless steel, Rex 734, was investigated using SEM, TEM, and X-ray diffraction. The phase precipitates during isothermal aging at temperatures between 700° and 1000 °C and also during slow cooling from elevated temperatures. Precipitation was observed at grain boundaries, twin boundaries, and intragranularly in association with matrix dislocations. Grain boundary particles were equiaxed, whereas twin boundary and intragranular precipitates had a cuboidal morphology. The phase has a tetragonal unit cell (a = 303 pm, c = 737 pm) and precipitates in accordance with a previously observed orientation relationship. The mechanical properties of Rex 734 were affected by the precipitation of z-phase in two ways. First, intragranular precipitation caused significant age hardening, and secondly, grain boundary precipitation lead to an intergranular failure during impact loading at low temperature.
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