Heterostructure triggers inverse grain size effect on TRIP for strong and ductile stainless steel

In high-strength nanostructured metastable austenitic stainless steels (ASS), transformation induced plasticity (TRIP), a key ductility-enhancing mechanism, is often suppressed, causing drastic loss of ductility. Here, a heterogeneous lamellar structure is shown to induce an inverse grain-size effect on TRIP. The structure consists of soft ultrafine-grained lamellae and hard nanoscale lamellae. During tensile deformation, martensitic transformation initiates first in the hard nano-lamellae and subsequently in the soft ultrafine-grained lamellae, thereby prolonging TRIP and increasing the final martensite fraction, both of which enhance the ductility. Produced by one-step warm rolling, this heterostructure provides an industrially promising route to strong and tough ASS.