INSITU OBSERVATION OF STRAIN-INDUCED GAMMA-S.RA-EPSILON-S.RA-ALPHA' AND GAMMA-S.RA-ALPHA' MARTENSITIC TRANSFORMATIONS IN FE-CR-NI ALLOYS

Strain-induced gamma --> epsilon --> alpha' and gamma --> alpha' martensitic transformations in Fe-Cr-Ni alloys have been investigated by in-situ deformation with a high voltage electron microscope. The results obtained are summarized as follows: [A] gamma --> epsilon --> alpha' transformation: (1) Two sets of epsilon-crystals are formed along the primary and the conjugate slip planes in gamma-crystals oriented for multiple slip systems. Fine alpha'-crystals are frequently formed at the intersections between these epsilon-crystals. (2) In gamma-crystals oriented for a single slip system alpha'-crystals are also formed. In this case, cross-slip accompanying a stair-rod dislocation frequently occurs in local regions, and stacking faults corresponding to the secondary shear displacement are induced to form alpha'-crystals. (3) These alpha'-crystals grow to make coalescence with each other, and large alpha'-crystals are formed. [B] gamma --> alpha' martensitic transformation: (1) gamma --> alpha' transformation also occurs by "the double shear mechanism". (2) Formation processes of alpha'-crystals are classified into slow "Schiebung" type and rapid "Umklapp" type. The former is observed in gamma-grains oriented for a single slip system, and the latter in those oriented for multiple slip systems or in work-hardened gamma-grains. (3) In both cases, gamma-twins are always well formed prior to the formation of alpha'-crystals, and the secondary shear displacement is initiated at the interface of these gamma-twins. (4) Each alpha'-crystal grows easily along the gamma-twin plane. Nucleation and growth mechanisms of alpha'-crystals in these transformations are discussed on the basis of the experimental results.