Oxidation induced intergranular cracking and Portevin-Le Chatelier effect in nickel base superalloy 718

The mechanical behaviour of nickel base superalloy 718 was investigated by means of constant extension rate tensile (CERT) tests performed at 5 x 10(-7) s(-1), in air and under secondary vacuum, in the temperature range 400-600 degreesC. Below 470 degreesC, the Portevin-Le Chatelier (PLC) effect occurs and leads to shear fracture. Above 500 degreesC, the PLC effect disappears and oxidation-induced intergranular cracking takes place. The transition from serrated flow to continuous plastic flow occurs abruptly in a narrow temperature domain (470-500 degreesC) and coincides with a transition from ductile to intergranular cracking. Thus, the interactions between oxidation and plasticity at the crack tip were investigated by means of a specific CERT test conducted at 500 degreesC up to 4% plastic strain and at 470 degreesC up to fracture in air. Results show that the local plastic strain rate may control the oxidation induced intergranular cracking process. (C) 2001 Elsevier Science B.V. All rights reserved.