MECHANICAL STUDY OF INSTABILITY OF AUSTENITIC FE-NI-C ALLOYS - EFFECTOF 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 tempera tures, are investigated considering three austenitic stares: water-coo led (gamma), cathodically hydrogen charged at 300 degrees C (gamma + 3 00 degrees C/H-2), and hear-treated at 300 degrees C (gamma + 300 degr ees C) for comparison. The true stress sigma versus true strain epsilo n 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 conten t beyond approximate to 0.2 wt% C this slope increase is due to strain induced martensite. As a consequence transformation induced plasticit y (TRIP) effect, confined to medium carbon contents, is observed. At t he same time the stress-strain diagrams exhibit instabilities in the f orm of serrated yieldings. The critical stress and strain of their ons et is correlated to the number of Frank-Read sources (FRS) activated b y the plastic flow. In the case of higher carbon alloys and higher str ains, the increased slope K-2 is thought to be due to another strength ening mechanism involving carbon atoms in the solid solution, associat ed with Portevin-Le Chatelier (PLC) effect. At low carbon content the effect of hydrogen in prior austenite is negligible, but at high conte nts the embrittlement and cracking of the strain induced martensite is immediate. Copyright (C) 1996 Acta Metallurgica Inc.