Combining 2D and 3D characterization techniques for determining effects of HIP-rejuvenation after fatigue testing of SX microstructures

Abstract

Geometrically complex turbine blades made of single crystal (SX) Ni-base superalloys operate at high temperatures and high alternating stresses. During service, pores and precipitates act as stress concentrators leading to crack initiation and propagation until failure [1]. Elimination or healing of such damage can be achieved by applying a hot isostatic pressing (HIP) rejuvenation treatment. In this work, a correlative approach is presented by combining 2D and 3D characterization techniques for determining microstructural heterogeneities in SX microstructures after both fracture in high temperature low cycle fatigue (LCF) and after subsequent HIP rejuvenation. The material of the specimens was processed either by casting or electron beam powder bed fusion (EPBF), respectively [2]. LCF tests were performed on miniature specimens at 950 °C in tension-tension with a load ratio of 0.62 to 0.65 with a frequency of 0.25 Hz [3, 4]. Half of each fractured specimen was prepared for scanning electron microscopy (SEM) on a 2D cross section. The opponent half was used to monitor the spatial arrangement of the accumulated damage status in 3D via X-ray tomography, post LCF and after HIP-rejuvenation, respectively. Subsequent to X-ray tomography after the HIP-rejuvenation the same specimen was subjected to series of metallographic cross sections for achieving complementary 2D information at higher resolution by means of large-scale SEM panorama micrographs. By combining the 3D and 2D data statistical volume related quantities were achieved while detailed characteristics were assigned to individual defects by fitting 2D SEM micrographs into the corresponding 3D volume as Fig. 1 highlights. This technique is in general appropriate for length-scale bridging microstructural investigations. References [1] P. Caron, C. Ramusat, MATEC Web of Conferences, 2014, 14, 13002. [2] C. Körner, M. Ramsperger, C. Meid, D. Bürger, P. Wollgramm, M. Bartsch, and G. Eggeler Metallurgical and Materials Transactions A, 2018, 49(9), 3781-3792. [3] C. Meid, U. Waedt, A. Subramaniam, J. Wischek, M. Bartsch, P. Terberger, R. Vaßen Materialwissenschaft und Werkstofftechnik, 2019, 50, 777–787. [4] C. Meid, A. Dennstedt, M. Ramsperger, J. Pistor, B. Ruttert, I. Lopez-Galilea, W. Theisen, C. Körner, M. Bartsch Scripta Materialia 168, 2019, 124–128.