Dislocation Strengthening without Ductility Trade-off in Metastable Austenitic Steels

Abstract

Strength and ductility are mutually exclusive if they are manifested as consequence of the coupling between strengthening and toughening mechanisms. One notable example is dislocation strengthening in metals, which invariably leads to reduced ductility. However, this trend is averted in metastable austenitic steels. A one-step thermal mechanical treatment (TMT), i.e. hot rolling, can effectively enhance the yielding strength of the metastable austenitic steel from 322 ± 18 MPa to 675 ± 15 MPa, while retaining both the formability and hardenability. It is noted that no boundaries are introduced in the optimized TMT process and all strengthening effect originates from dislocations with inherited thermal stability. The success of this method relies on the decoupled strengthening and toughening mechanisms in metastable austenitic steels, in which yield strength is controlled by initial dislocation density while ductility is retained by the capability to nucleate new dislocations to carry plastic deformation. Especially, the simplicity in processing enables scaling and industrial applications to meet the challenging requirements of emissions reduction. On the other hand, the complexity in the underlying mechanism of dislocation strengthening in this case may shed light on a different route of material strengthening by stimulating dislocation activities, rather than impeding motion of dislocations.

Figure 1. Microstructural characterization of warm-rolled metastable austenitic steels.

Two-beam bright field TEM images of TMT samples with (a) 10% and (b) 20% thickness reduction. Zone axis is (110) andg = (111). Scale bar: 100 nm. (c) (EBSD) image of the TMT sample with 20% thickness reduction. Scale bar: 100 um. (d) The responding pole figure of (c) showing weak texture characteristics.

Figure 2. Strengthening of metastable austenitic steels by TMT treatment.

(a) Stress-strain curves for the TMT samples. (b) Comparison of the uniform elongation versus yield strength for typical steels.

Figure 3. Phase boundary as effective dislocation source.

(a,b) TEM image sequence of an in-situ deformed solid solution treated metastable austenitic steel. The electron beam is along the [011]α. Scale bar: 200 nm. (c,d) TEM image sequence of an in-situ deformed TMT samples with 20% thickness reduction. The phase boundary are marked by the solid lines. The dashed line in (d) locates the original phase boundary in (c). The direction of the step-wise propagation is indicated by the arrows in (d). Scale bar: 200 nm.

Figure 4. Dislocation pileup against the phase boundary promotes α-martensite transformation.

(a–c) TEM image sequence of an in-situ deformed metastable austenitic steel reveals the irregular growth of α-martensite. The dashed lines delineate the phase boundary. The arrows in (a,b) point to the direction of dislocation movements. The dislocation array piles up against the phase boundary. The preferential α-martensite growth is indicated by the arrow in (c). Scale bar: 100 nm. (d) The two-beam bright field TEM image shows typical dislocation arrangements in an α-martensite. Inset is the corresponding SAED pattern from the α-martensite with g = (01–1). Scale bar: 100 nm.

Figure 5. Microstructural evolution under tension.

EBSD images of warm rolled samples with tensile strain of (a) 15%, (b) 30%, (c) 50% and (d) 67%. All TMT samples have the same rolling reduction in thickness of 20%. EBSD images of solid solution treated samples with tensile strain of (e) 15%, (f) 30% and (g) 60%. (h) The XRD-measured volume fraction α-martensite as a function of tensile strain for both TMT and solid solution treated samples. The rolling reduction in thickness of TMT samples is 20%. The tensile tests were performed at strain rates 1.0×10⁻³ s⁻¹ and room temperature. Supplementary Fig. S6 shows the corresponding stress-strain curves.