A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cracking

Abstract

International audienceIn Alloy 718, a sharp transition exists in the fracture path changing from an intergranular brittle mode to a transgranular ductile mode which is associated with a transition of flow behavior from smooth in the dynamic strain aging regime to a serrated one in the Portevin-Le Chatelier (PLC) regime. In order to better understand both deformation and rupture behavior, PLC phenomenon in a precipitation-hardened nickel-base superalloy was carefully investigated in a wide range of temperatures [573 K to 973 K (300°C to 700°C)] and strain rates (109^-5 to 3.2910^-2 s^-1 ). Distinction was made between two PLC domains characterized by different evolutions of the critical strain to the onset of the first serration namely normal and inverse behavior. The apparent activation energies associated with both domains were determined using different methods. Results showed that normal and inverse behavior domains are related to dynamic interaction of dislocations with, respectively, interstitial and substitutional solutes atoms. This analysis confirms that normal PLC regime may be associated to the diffusion of carbon atoms, whereas the substitutional species involves in the inverse regime is discussed with an emphasis on the role of Nb and Mo

Studying the influence of substitutional elements on mechanical behavior of Alloy 718

“…The tensile tests carried out on wide ranges of temperatures and strain rates in a previous study [13] permitted to distinguish between two domains of different types of PLC effects: a low temperature PLC effect, and a high temperature PLC effect. It is commonly agreed that for low temperature domain, the origin of flow instabilities is the interaction between mobile dislocations and C atoms.…”

“…In a previous study [13], we characterized the deformation modes of the alloy by means of tensile tests over a wide range of thermo-mechanical conditions. Typical true stress-true strain curves obtained in a previous study for different temperatures of test on aged alloy 718 are plotted on Fig.…”

“…From this map, various methods were employed to calculate the apparent activation energies of the mechanisms involved in both normal and inverse PLC areas. A detailed description of the results and of the methods used to evaluate the apparent activation energies is the object of a previous publication [13].…”

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

“…For instance, serrated plastic flow was reported in Waspalloy (United Technologies Corporation, Farmington, CT), [3,4] alloy 625 (Special Metals Corporation, Portland, OR), [5][6][7] Udimet 720 (Special Metals Corporation, Portland, OR), [8,9] Inconel 738 (Special Metals Corporation, Portland, OR), [10] and Inconel 718 (Special Metals Corporation, Portland, OR). [3,[11][12][13][14][15][16][17][18][19][20] Many studies have been carried out on this subject of interest contributing to a better understanding of the mechanisms and atomic species potentially involved in the occurrence of the phenomenon. The characterization of the PLC effect is based on the determination of the apparent activation energy for the occurrence of serrations on stress-strain curves and its comparison with the diffusion energy of different solute species in the alloy, which allows determination of the nature of the solute atom responsible for the interaction with mobile dislocations.…”

In Situ Diagnostics of Damage Accumulation in Ni-Based Superalloys Using High-Temperature Computed Tomography

“…Traditionally, such testing has emphasized measurements of force and displacement to deduce the stress and strain fields; this can be regarded as a form of macroscopic characterization. Microscopic analysis is usually carried out post mortem on, for example, failed [1][2][3][4][5] or interrupted test pieces [6][7][8][9][10][11] and most commonly at ambient temperature. However, a significant difficulty with this approach is that one can never be totally sure that damage deduced post mortem is truly representative of that driving deformation under operating conditions at much higher temperature.…”