Effect of increased length fraction of Ʃ3n special boundaries on OAIC response of cold rolled Ni-based alloy 718 thin sheets

Abstract

Among the well-established Ni-based superalloys, alloy 718 is one of the most widely used in the industry. However, its application is limited up to 650°C due to the occurrence of the oxidation assisted intergranular cracking (OAIC) phenomenon. As a general rule, an increased fraction of special boundaries (3 < Ʃ<29) along the grain boundary network, in replacement of the high energy random high angle boundaries (RHABs), is usually associated with enhanced resistance to several intergranular failure mechanisms. In the present investigation, thin sheets of alloy 718 were subjected to three different thermomechanical processing (TMP) routes, in order to manipulate the grain boundary character distribution (GBCD), aiming to achieve an increased fraction of low-Ʃ special boundaries and, consequently, to reduce the alloy's OAIC susceptibility. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to characterize the different microstructures and GBCD resulting from TMP. Thin sheet specimens underwent hot tensile tests at 650 °C under secondary vacuum at a strain rate of 3.2 × 10−4 s−1. Fractography analyses were performed to quantify the fraction of brittle areas on the fracture surface. The results indicate that the sample processed through iterative steps of cold rolling and solution annealing, followed by a double aging heat treatment, resulted in a microstructure resistant to OAIC, with 100% of ductile fracture. Such good resistance to OAIC was attributed to the higher percentage of special Ʃ3n boundaries, as well as to the best configuration of grain boundaries network, presenting increased fraction of special triple junctions 3CSL and 2CSL as well as a favorable triple junction ratio (TJR).