Fig. 2

Microstructure of reverted austenite after 3 (a) and 100 (b) γ-α-γ transformations. The electron microscopy observations have shown that in the reverted austenite, additional sub-boundaries are formed by dislocations introduced during the cyclic γ-α and α-γ martensitic transformations. The dislocation density was found to increase by three orders of magnitude even after the first γ-α-γ cycle. The following cycles hardly raised the dislocation density in line with findings described in [7]. It is important to note that crystalline defects appeared in the process of martensitic transformations are more thermally stable compared to ones formed in plastically deformed alloys. Consequently, they do not vanish within the temperature interval of the reverse transformation and, hence, can accumulate during the following cycling. It is evident that part of dislocations generated during the γ-α and α-γ transformations moves towards sub-boundaries, thus, determining their misorientation angle and size of new fragments. During certain stage of this process, the sub-boundaries form distinct fragments. After 10 cycles, the reflections present in the micrograph are split into 5 ÷ 7 parts that unambiguously indicate the formation of additional sub-boundaries. The sizes of the fragments of the reverted austenite decrease with the number of cycles