Grain boundary serration tuning and its effect on hot workability of a wrought superalloy

This study aims to elucidate the underlying mechanism of grain boundary serration and investigate its effects on hot workability of U720Li superalloy. Industrial scale as-cast ingots were subjected to heat treatments at 1180 degrees C followed by different cooling rates (15 degrees C/s, 0.1 degrees C/s, 0.02 degrees C/s and 0.003 degrees C/s) prior hot deformation. With 15 degrees C/s fast cooling, samples possessed straight boundaries and fine L12 structured gamma' particles dispersion. When the cooling rate was decreased to 0.1 degrees C/s and below, continuous and dis-continuous precipitation of the gamma' phase led to the formations of Type-I and Type-II grain boundary ser-rations, respectively. Lamellar gamma/gamma' behind the mobile grain boundary was observed for Type-II boundary, which exhibited larger degree of undulation than that of Type-I boundary. With increasing fractions of Type-II boundary, the hot formability of U720Li was significantly improved during compression tests at 1000 degrees C; 70 % reduction ratio (true strain=1.2) could be achieved without crack formation. This work presents a simple process route to improve the hot deformability of U720Li fabricated on an industrial scale.(c) 2023 Elsevier B.V. All rights reserved.