Microstructural study of intergranular M23C6 precipitates in Alloy 709

The microstructural properties of M23C6 (M: mainly Cr), the predominant precipitate phase of Alloy 709, were studied with respect to the correlation with the austenite matrix, γ-Fe. Alloy 709 specimens, aged at 550, 650, and 750℃ for 1 - 3000 hours, were investigated using electron microscopy and synchrotron X-ray diffraction (XRD). Intergranular M23C6 precipitates were first formed on high angle grain boundaries, followed by the formation of lamella-shaped M23C6 precipitates on twin boundaries. The precipitation time threshold of M23C6 was different from the known behaviors of other Fe-20Cr-25Ni austenitic steels so that the Time-Temperature-Precipitation diagram of the M23C6 precipitate phase in Alloy 709 was newly suggested. The presence of the intergranular M23C6 caused compressive stress on the austenite matrix and tensile stress on the M23C6 precipitates. With increasing aging time, the stress became higher due to the volumetric expansion of the M23C6 precipitates. The crystallographic characteristics of the intergranular M23C6 were investigated analyzing diffraction patterns of the intergranular M23C6 precipitates and the neighboring austenite. The intergranular M23C6 were formed on the grain boundaries in a cube - cube relationship with the austenite, [001]γ-Fe ‖ [001]M23C6 and [110]γ-Fe ‖ [110]M23C6. The intergranular M23C6 on the high angle grain boundaries grew toward the opposite side of the parent austenite grain while maintaining a lattice correspondence, 3·aM23C6 ≈ aγ-Fe. The lamella-shaped M23C6 precipitates on the twin boundaries also exhibited the analogous growing behavior. In situ synchrotron XRD tensile tests were performed to reveal the mechanical response of the austenite and the precipitates in Alloy 709. The lattice strain evolution of the austenite was anisotropic for all aging conditions. The softest planes in the austenite, {2 0 0}γ-Fe, yielded first, inducing compressive microstress on {2 2 0}γ-Fe. Brittle {1 1 1}γ-Fe with the highest elastic modulus did not yield over the entire strain range. The M23C6 precipitates preferentially hardened {2 0 0}γ-Fe that determines the yield of the austenite matrix during lattice strain development. The material was substantially strengthened due to the load partitioning by the M23C6, which was proportional to its volume fraction. The further increase of the load partitioning by the precipitates was limited due to the mechanical failure of the precipitates, which was confirmed by microcrack formations near the precipitates.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste