Mechanical study of instability of austenitic Fe-Ni-C alloys - Effect of hydrogen

The mechanical properties of different Fe-Ni-C alloys, in which Ni and C contents are correlated in order to ensure roughly equal Ms temperatures, are investigated considering three austenitic stares: water-cooled (gamma), cathodically hydrogen charged at 300 degrees C (gamma + 300 degrees C/H-2), and hear-treated at 300 degrees C (gamma + 300 degrees C) for comparison. The true stress sigma versus true strain epsilon are approximated by = Ki + K-2 epsilon(1/2). Except for 0.006 wt %C, the fitting displays two or three domains of strain characterized by higher values of the slope K-2 at high deformations. For carbon content beyond approximate to 0.2 wt% C this slope increase is due to strain induced martensite. As a consequence transformation induced plasticity (TRIP) effect, confined to medium carbon contents, is observed. At the same time the stress-strain diagrams exhibit instabilities in the form of serrated yieldings. The critical stress and strain of their onset is correlated to the number of Frank-Read sources (FRS) activated by the plastic flow. In the case of higher carbon alloys and higher strains, the increased slope K-2 is thought to be due to another strengthening mechanism involving carbon atoms in the solid solution, associated with Portevin-Le Chatelier (PLC) effect. At low carbon content the effect of hydrogen in prior austenite is negligible, but at high contents the embrittlement and cracking of the strain induced martensite is immediate. Copyright (C) 1996 Acta Metallurgica Inc.