Metallurgical and Materials Transactions A

, Volume 44, Issue 11, pp 5177-5191

First online:

On the Low Temperature Strain Aging of Bainite in the TRIP Steel

  • Ilana TimokhinaAffiliated withInstitute for Frontier Materials, GTP Research, Geelong Technology Precinct, Deakin University Email author 
  • , Hossein BeladiAffiliated withInstitute for Frontier Materials, GTP Research, Geelong Technology Precinct, Deakin University
  • , Xiang-Yuan XiongAffiliated withCentre for Electron Microscopy, Monash University
  • , Peter D. HodgsonAffiliated withInstitute for Technology Research and Innovation, Deakin University

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The aging behavior of a thermomechanically processed Mo-Al-Nb transformation-induced plasticity steel with ultrafine microstructure was investigated using transmission electron microscopy and atom probe tomography (APT). Strain aging at 73 K (200 °C) for 1800 seconds led to a significant bake-hardening response (up to 222 MPa). Moreover, aging for 1800 seconds at room temperature after 4 pct pre-strain also revealed a bake-hardening response (~60 MPa). The experimental results showed the formation of carbon Cottrell atmospheres around dislocations and the formation of carbon clusters/fine carbides in the bainitic ferrite during aging. It is proposed that this is associated with the high dislocation density of bainitic ferrite with formation of a complex dislocation substructure after pre-straining and its high average carbon content (~0.35 at. pct). The segregation of carbon and substitutional elements such as Mn and Mo to the retained austenite/bainitic ferrite interface during aging was observed by APT. This segregation is likely to be the preliminary stage for Mo-C particles’ formation. The aging after pre-straining also induced the decomposition of retained austenite with formation of ferrite and carbides.