″Complete twin″ and ″the Transgranular Twin Line″ of Common Steels

Abstract

The microstructures of 20, 30, 40, 45, T9 and T11 commercial carbon steels were observed by optical microscopy, scanning electron microscopy and transmission election microscopy. It was found that the internal twins in martensite of common steels is basically a complete twin, and internal twin plane of mutually parallel martensite units in a martensite packet have a same crystallographic orientation, namely, forming ″the transgranular twin line″ or ″interpenetration twinline″. The underlying mechanism of ″complete twin″ and″ the transgranular twin line″ were explored. Both all are to form in the martensite transformation process. In order to promote the growth of martensitic nuclei in initial stage, the associated nucleus with twinning relationship had been formed at a semi-coherent end plane of nuclei which possesses high energy, and forming transformation twins through ″double change″. Therefore, transformation twins are usually all ″complete twin″. In the martensite packet, all the martensite plates of mutual parallelism are to form the associated nucleus with twin interface plane at a side of original ″ twin block″ of the martensite and grow up, so a martensite packet all have the identical ″trans-granular twin line″. In the formation process of ″complete twinning″ and″ trans-granular twin line″, don’t involve the mechanics properties of material, and mainly decide from nucleation work and nucleus growth work. It is thus clear that, having the complete twins in the martensite doesn’t mean that the critical resolved shear stress (CRSS) of slipping exceeded the CRSS of twinning.