Studies on nanostructure for annealing twin in α-brass and transformation twin in high-carbonic martensite

Annealing twins usually form as a consequence of growth accidents or are presumed to form on stacking faults during the recrystallization of fcc metals and alloys. Therefore, the effects of annealing twins on mechanical properties are very important. It is desirable to determine the difference in strength contributions between general grain and twin boundaries. The results of hardness measurements have shown that the hardness of the twin boundary is a little lower (about 5Hv) than that of the general grain boundary. TEM micrographs indicated that slip lines can penetrate twin boundaries by cross-slip, or if obstructed, form ledges at the twin boundaries. The observations of the ledges at twin boundaries provided evidence for the dislocation dissociations. The energetically unfavorable dissociated reactions and the coalescent partial dislocations released at the twin boundary contribute to the maintained strength and excellent ductility of the twin boundaries. Additionally, a series of annealing treatments at different temperatures were carried out to measure twin density. The results show that annealing twin density depends on grain size. Pande''s experiential equation for calculating the annealing twin density (N=Kt ln(D/ D0)) agrees well with our experimental results. The material depending value (Kt value) in Pande''s experiential equation for C2600 brass was measured at about 0.3.
Transformation twins usually form in the high carbonic martensite transformation to maintain the inhomogeneous lattice - invariant. A typical characteristic of lenticular martensites is the appearance of an obvious high density twinned region (i.e., a midrib region). The midrib is considered to be the region where martensite transformation starts. In this work, it has been found that thin-plate and lenticular martensites co-existed in the specimens of Fe-1C-17Cr stainless steel. The substructures of thin plate martensites and lenticular martensites were examined using TEM, focusing on the details of the midrib region. The results of the DSC experiment and the course of the isothermal holding in the liquid nitrogen (-196˚C) indicated that the thin plate martensite formed first and lenticular martensite later. These results provide evidence to suggest that thin plate martensite can be transformed into lenticular martensite. Transmission electron microscopy revealed that thin plate martensite is composed of a set of internal transformation {112} twins crossing through the interior plate, while the lenticular martensite contained three subzones: the midrib region, extended twinned region, and untwinned region. The results obtained from TEM observations suggest that the transformations of thin plate martensite and lenticular martensite are initiated at the same midrib region. During the growth, the former keeps the lattice-invariant deformation mode of twinning, whereas the latter combines both twinning and slip modes.
Additionally, the result of tempering experiments indicated that the midrib region of the martensite contained a large amount of twinned boundaries, which is the preferential position for carbide precipitations. TEM observation showed that tempering treatment resulted in the release of stress at the midrib region, i.e., the stress-concentrated region, and caused the martensite crystal to rotate slightly. TEM results indicated that M3C type carbide was dominant after tempering at 600˚C for 0.5 hours, but M23C6 type carbides was frequent after tempering for 1 and 2 hours. Analysis of diffraction patterns revealed that in this Fe-1C-17Cr alloy, Bagaryatsky OR was found between ferrite and M3C carbide, and Kurdjumov-Sashs OR was found between ferrite and M23C6 carbide.