Dissecting the Mechanism of Martensitic Transformation via Atomic-Scale Observations

Abstract

Martensitic transformation plays a pivotal role in the microstructural evolution and plasticity of many engineering materials. However, so far the underlying atomic processes that accomplish the displacive transformation have been obscured by the difficulty in directly observing key microstructural signatures on atomic scale. To resolve this long-standing problem, here we examine an AISI 304 austenitic stainless steel that has a strain/microstructure-gradient induced by surface mechanical attrition, which allowed us to capture in one sample all the key interphase regions generated during the γ(fcc) → ε(hcp) → α′(bcc) transition, a prototypical case of deformation induced martensitic transformation (DIMT). High-resolution transmission electron microscopy (HRTEM) observations confirm the crucial role of partial dislocations, and reveal tell-tale features including the lattice rotation of the α′ martensite inclusion, the transition lattices at the ε/α′ interfaces that cater the shears, and the excess reverse shear-shuffling induced γ necks in the ε martensite plates. These direct observations verify for the first time the 50-year-old Bogers-Burgers-Olson-Cohen (BBOC) model, and enrich our understanding of DIMT mechanisms. Our findings have implications for improved microstructural control in metals and alloys.

Time, stress, and temperature-dependent deformation in nanostructured copper: Stress relaxation tests and simulations

“…10(d) /2 101 on (¯) 111 planes, respectively, as shown in Fig. 10(e), which causes the geometrical misorientation (Yang et al, 2014(Yang et al, , 2015. The discovered stress-dependent creep mechanisms are also shown by the MD snapshots in Fig.…”

Combining gradient structure and TRIP effect to produce austenite stainless steel with high strength and ductility

“…Two types of a 0 -martensite are observed in the shear bands. One is granular, as indicated by blue triangles, which is nucleated at the intersections of ε-martensite bands [37,44]. The other is plateshaped, which is located inside and along ε-martensite bands, as indicated by red arrows shown in Fig.…”

“…The initial constituent phase in these bands is ε-martensite, which was formed at low strains by the planar slip of partial dislocations due to low stacking fault energy of 304ss [37,38]. These ε-martensite bands subdivide the original coarse grains into a large number of submicron-sized parallelepiped domains with austenite interior [37,38,44,50]. Fig.…”

Considerations in the Design of Formable Austenitic Stainless Steels Based on Deformation-Induced Processes

“…Even in cases where it is difficult to identify intersection of planar glide features, such as the example of Figure 5d where the α′-martensite appears to have formed in association with deformation twins, the activation of at least one conjugate partial dislocation glide system is quite likely. After all, the most widely accepted models proposed for the α′-martensite formation rely on the occurrence of two shears in directions compatible with the glide direction of partial dislocations [32][33][34][35]. The magnitudes of the shears needed to cause the lattice change to bcc/bct are only fractions of the full twinning shear.…”