A Study Of Crystallography Of Surface Austenite Precipitates In A Duplex Stainless Steel

Crystallography of the phase transformation is a key aspect for understanding the microstructure, and is essential to theory-based materials design. In the present work the phase transformation near surface of a duplex stainless steel is investigated for understanding the complex morphology. The theoretical model has been extended to give reasonable explanations of the observed crystallographic features. This in-depth study has enriched the knowledge of the precipitate crystallography.3D morphology and the crystallography in 00Cr25Ni7Mo3 has been systematically measured with scanning electron microscope (SEM), electron backscattering diffraction (EBSD) and atomic force microscope (AFM). The basic unit of the precipitates transformed near the surface is for single needle as well as for complex dendrite shaped precipitates. The detailed crystallographic features of the needle shaped precipitates vary with the surface orientation. Three types have been identified, according to the orientation relationship (OR), the long axis and the interface orientation, as measured with transmission electron microscope (TEM): the OR for the first type is near the K-S OR, and the Burgers vector is the near parallel close packed direction (CPD), and a pair of twin precipitates is often observed; the second is also near K-S OR, while the Burgers vector is not the near parallel CPD; the third is near N-W OR.Surface effect has been taken into consideration to modify the O-line model and a new criterion is proposed: the deviation of the invariant line from the surface should be minimized (Smin). The O-line solution of the Smin provide good explanation of the typical phenomena only observed on the surface, such as the deviation of the corresponding Burgers vectors, the deviation of the interfacial dislocation and the long axis, and the double habit plane with"buffer matrix"and so on. The"buffer matrix"was proposed to be the result of recovery in the local matrix nearby the interface, during the evolution of microstructure from a rational OR to the irrational OR. The"double habit planes"structure is a unique phenomenon in the surface layer between the precipitates and the matrix, which can be achieved in different way: such as the structure matrix / precipitate 1 / precipitate2 / matrix of the twining laths and the the structure matrix1 / precipitate / matrix2 of the lath with"buffer matrix". The precipitation of austenite on the surface is associated with the surface relief.A tracking measurement combining EBSD with AFM reveals the structure of the surface relief: the double-tilt surface relief is related with the twinning austenite pair and the single-tilt surface relief is related with the single precipitate, no matter the OR is near K-S or near N-W.