On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

Abstract

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d<100 nm), ultrafine (d % 100 to 500 nm), and submicron (d % 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)-high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocationcell-type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell-type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell-type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperaturetime annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell-type structure in lieu of lath-type facilitates NG/UFG structure with a high strength-high ductility combination. Interestingly, the yield strength follows the Hall-Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.In the context of obtaining high strength-high ductility combination, we recently described a novel processing route of developing nanograined/ultrafinegrained (NG/UFG) structure in a metastable austenitic stainless steel (AISI 301LN) involving controlled phase reversion annealing of the cold-deformed austenite. [1][2][3][4] In this approach, severe deformation of adequately metastable austenite at room temperature leads to strain-induced transformation of austenite (fcc c) to martensite (bcc a¢). On annealing, the severely deformed strain-induced martensite reverts back to austenite [1][2][3][4] via diffusional or shear reversion mechanism. [5,6] The optimal phase reversion annealing sequence resulted in the structure that was characterized by a combination of high yield strength and excellent elongation of 800 to 1000 MPa and 30 to 40 pct, respectively. [1] In a diffusional reversion mechanism, the phase reversion process and the ''final'' microstructure are not only a function of diffusion rate in bcc (martensite) and fcc (austenite) phase, but also depend on the martensite structure and density of defects. These two characteristics may accelerate the transformation characteristics or provide an increased number of nucleation sites. Thus, in sequel to our earlier work, there is a need to...

Interplay between grain structure, deformation mechanisms and austenite stability in phase-reversion-induced nanograined/ultrafine-grained austenitic ferrous alloy

“…The annealing experiments were carried out on strips of 120 Â 25 mm (thickness $0.6 mm). The phase reversion process is described in detail elsewhere [9][10][11][12]. Room temperature tensile properties were determined using specimens that were machined to a profile of 25 Â 25 mm with a 20 mm gage length.…”

“…But the differences between the two methods were observed to be small, suggesting that either of the two methods were appropriate in our case. The weighted average grain size ( d w ) of CG steel was 22 lm, while that of NG steel (cold rolled to 62% reduction and annealed at 800°C for 10 s) was 225 nm [9][10][11][12].…”

“…In the context of obtaining nanograined (NG) structure, we have recently obtained a high strength, high ductility combination in NG austenite stainless steels using the innovative concept of a phase-reversion-induced nano/ submicron-grained structure [9][10][11][12]. The controlled deformation-annealing approach to obtain NG material with a high strength, high ductility combination involved cold deformation ($60-80%) of metastable face-centered cubic (fcc) austenite (c) to strain-induced body-centered cubic (bcc) martensite (a').…”

“…The controlled deformation-annealing approach to obtain NG material with a high strength, high ductility combination involved cold deformation ($60-80%) of metastable face-centered cubic (fcc) austenite (c) to strain-induced body-centered cubic (bcc) martensite (a'). Upon annealing at 700-800°C for a short duration of $10-100 s, martensite transforms back to austenite via a diffusional reversion or martensitic shear mechanism, depending on the chemical composition of the steel, without affecting the texture [9][10][11][12]. The success of this approach in obtaining NG structure depends on the predominance of dislocation cell-type structure in the severely deformed austenite [10].…”

“…Upon annealing at 700-800°C for a short duration of $10-100 s, martensite transforms back to austenite via a diffusional reversion or martensitic shear mechanism, depending on the chemical composition of the steel, without affecting the texture [9][10][11][12]. The success of this approach in obtaining NG structure depends on the predominance of dislocation cell-type structure in the severely deformed austenite [10]. The concept enables us to explore deformation mechanisms in a single material from the NG to the coarse-grained (CG) regime, depending on the processing parameters (cold deformation and annealing temperature-time sequence).…”