Bainite self accomodating shear strains


However, morphological differences do exist that require a TEM to see.Under a simple light microscope, the microstructure of bainite appears darker than martensite due to its low reflectivity.Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.The temperature range for transformation to bainite (250–550 °C) is between those for pearlite and martensite.The martensitic transformation is a shear-dominant diffusionless solid-state phase transformation occurring by nucleation and growth of the martensitic phase from a parent austenitic phase.

The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder than it normally would be.The microstructures of martensite and bainite at first seem quite similar.This is a consequence of the two microstructures sharing many aspects of their transformation mechanisms.The martensitic transformation possesses well-defined characteristics that distinguish it among other solid state transformations: is an important factor in characterizing shape memory behavior.When the SMA is heated from the martensitic phase in the absence of stress, the reverse transformation (martensite-to-austenite) begins at the temperature ) is associated with the energy dissipated during the transformation.ABSTRACT: Since the major strengthening mechanisms in nanocrystalline bainitic steels arise from the exceptionally small size of the bainitc ferrite plate, accurate determination of this parameter is fundamental for quantitative relating the microstructure to the mechanical properties.In this work, the thickness of the bainitic ferrite subunits obtained by different bainitic heat treatments was determined in two steels, with carbon contents of 0.3 and 0.7 wt.%, from SEM and TEM micrographs.As these measurements were made on 2D images taken from random sections, the method includes some stereological correction factors to obtain accurate information.Finally, the determined thicknesses of bainitic ferrite plates were compared with the crystallite size calculated from the analysis of X-ray diffraction peak broadening.High resolution measurements of the displacements caused by the formation of bainite in a steel have confirmed that the surface relief due to each platelet in any given sheaf is identical to that of the others in a sheaf, and that the relief of each subunit conforms with the general features of an invariant plane strain.The maximum observed shear strain wasfound to be about 0·26, which is consistent with the magnitude expected from the phenomenological theory of martensite crystallography.

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