Grain Boundary Nucleation of Displacive Phase Transformations in Steels

Abstract

Austenite Grain Boundaries (AGBs) are known to be the most preferable nucleation site for reconstructive ferrite transformation due to the presence of excess free-volume which provides a higher probability for satisfying the local atomic configuration appropriate for the nucleation by random phase fluctuations. On the other hand, it has also been reported that nucleation of displacive phase transformations, i.e. both the isothermal bainitic transformation and the athermal martensitic transformation, preferentially occurred at prior AGBs. However, no clear mechanism for the preferred nucleation of displacive phase transformations at AGBs has been successfully postulated. This is primarily due to the difficulties in experimental demonstration. For example, crystallographic features of a nucleation of bainitic ferrite at AGBs cannot be directly investigated in low-carbon, lean-alloyed steels due to the presence of martensitic transformation products which hinder the precise determination of the crystallographic orientation of the prior austenite. On the other hand, it is well known that both the bainite and martensite hold specific orientation relationships (OR) with respect to austenite, and that the transformations are confined to occur within a single austenite grain. These characteristic features of displacive transformation make it possible to determine the orientation of prior austenite grain and the structure of prior AGBs from the orientation distribution of transformed products measured by the Electron Back Scattered Diffraction (EBSD) technique. In the present study, the nucleation behavior of upper bainite in low-carbon, lean-alloyed steel was investigated by Scanning Electron Microscopy (SEM)-EBSD. It is shown that the bainitic ferrite nucleated at AGBs having the crystallographic orientation of a variant with a Kurdjumov-Sachs (K-S) OR with respect to one of the neighboring austenite grain. This variant selection rule indicates that the bainitic ferrite nucleation is enhanced by interfacial energy reduction as the initial AGBs are converted to low energy inter-phase boundaries (IPBs) in the process. As the nucleation of upper bainite is controlled by an interfacial energy reduction mechanism, the transformation behavior of upper bainite is strongly affected by B addition. It was observed that the addition of B clearly affected both the transformation kinetics of isothermal bainitic transformation and the morphology of the bainite. The addition of B increased the incubation time for the formation of a detectable amount of transformation product, and reduced the overall transformation kinetics. The bainite microstructure consisted of a bainitic ferrite matrix and the Martensite/Austenite (M/A) constituent. The M/A constituent had an elongated morphology in B-free steel, whereas the M/A constituent in B-added steel had a granular structure. In B-free steel, bainitic ferrite nucleated homogeneously within the austenite grains, and the overall transformation was controlled by the increase of transformed area within the initial austenite grain. In contrast, in B-added steel, the transformation was confined to the interior of the austenite grains, and the overall transformation kinetics was controlled by an increase in the number of the locally transformed grains. The variant of bainitic ferrite in B-added steel was selected within a group of K-S variants related to the same Bain variant. The characteristic bainite microstructure in B-added steel is therefore due to the inhibition of the bainitic ferrite nucleation at ABGs.The microstructure and crystallography of BCC α’-martensite formed in a sensitized AISI 304 stainless steel was also studied in detail by means of Transmission Electron Microscopy convergent beam Kikuchi line pattern analysis. It was suggested that GBs containing Intrinsic Grain Boundary Dislocation (IGBD) might act as preferential nucleation site according to a faulting mechanism of martensitic nucleation proposed by Olson and Cohen. Different from the nucleation behavior of an upper bainite in low carbon, lean-alloyed steels, the martensite variants do not appear to be selected to achieve interfacial energy reduction, or the easy accommodation of transformation strain by slip in the neighboring austenite grain. This implies that both the martensite and the upper bainite preferentially nucleate at prior AGBs, but the operating nucleation mechanisms are totally different. From the observations that interfacial energy reduction mechanism plays a pivotal role in the nucleation of upper bainite, specific alloy designs were made to produce ultra-high strength (UHS) martensitic grades in conventional continuous galvanizing lines. The thermal treatments related to the hot dip galvanizing and galvannealing processes needed to obtain a Zn or Zn-alloy coated martensitic steel did not degrade the mechanical properties significantly when the coating process was applied to the un-transformed austenite phase. Therefore, it is shown that martensitic steel designed according to the critical physical metallurgy principles can be successfully processed in galvanizing lines not originally intended to produce UHS grades.