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Ductility-dip cracking mechanism in Ni-Cr-Fe alloys

Author(s):
Jimy Unfried Silgado
Total Authors: 1
Document type: Doctoral Thesis
Institution: Universidade Estadual de Campinas. Faculdade de Engenharia Mecânica
Defense date:
Examining board members:
André Paulo Tschipstchin; Carlos Angelo Nunes; Itamar Ferreira; Sergio Tonini Button
Advisor: Antonio José Ramírez Londoño
Abstract

Ductility-dip cracking (DDC) is a high temperature fracture phenomenon, which affects several face centered cubic (FCC) metallic materials, such as Nickel alloys, Copper alloys, and stainless steels. DDC is observed as a drastic reduction of ductility that leads to intergranular fracture at homologous temperature range between 0,4 and 0,8 under tensile stress. Diverse theories related to the grain boundary sliding and to carbides and carbonitrides precipitation were proposed to describe DDC behavior in Ni-alloys; however, the fundamental mechanism of DDC is not clear yet. In this work is investigated the fundamental mechanism of DDC, as well as the role of carbonitride precipitates and metallurgical characteristics on this phenomenon in aswelded solid-solution strengthened Ni-Cr-Fe alloys. Experimental alloys were designed by means of Calphad methodology using the alloy 690 chemical composition as the start point. Five compositions with Nb, Mo and Hf additions were subsequently fabricated and evaluated. The DDC evaluation was performed using electron microscopy characterization techniques, experimental measurements of stacking fault energy (SFE) using synchrotron radiation, and a scanning electron microscopy thermo-mechanical in situ test that allows a strain mapping from digital images. The in situ test has allowed obtaining at real time information about of DDC phenomenon on the temperature range between 500 °C and 1000 °C. Evidences of grain boundary sliding (GBS) were obtained through high temperature experiments, consequently allowing the recognition of DDC stages characteristics. Wavy grain boundaries were obtained in Ni-Cr-Fe alloys with Nb and Hf additions due to the high frequency and homogeneous distribution of fine intergranular primary precipitates. Mo, Nb, and Hf additions contributed for a perceptible SFE reduction. Several authors suggested that wavy grain boundaries block GBS, while the Mo presence in the crystal lattice leads to SFE reduction, which is related to the restriction of dislocations mobility at high temperature and to the increase of DDC resistance. Finally, a new fundamental mechanism of DDC is proposed based on experimental evidences for as-welded structures of Ni-Cr-Fe alloys, which is similar to the creep mechanism without diffusion, involving grain boundary sliding mechanism proposed by Rachinger. (AU)

FAPESP's process: 06/05661-1 - Study of intergranular fracture mechanism by ductility reduction in Ni-Cr-Fe alloys
Grantee:Jimy Unfried Silgado
Support type: Scholarships in Brazil - Doctorate