  This work explains the occurrence of transformation-induced plasticity via
stress-assisted martensite, when designing ultrafine-grained duplex steels. It
is found that, when the austenite is reduced to a fine scale of about 300 nm,
the initial deformation-induced microstructure can be dominated by parallel
lamellae of epsilon martensite or mechanical twinning, which cannot efficiently
provide nucleation sites for strain-induced martensite. Hence, alpha martensite
nucleation occurs independently by a stress-assisted process that enhances
transformation-induced plasticity in ultrafine-grained austenite. This
metallurgical principle was validated experimentally by using a combination of
transmission Kikuchi diffraction mapping, transmission electron microscopy, and
atom probe microscopy, and demonstrated theoretically by the thermodynamics
model of stress-assisted martensite.
