Dislocation Strengthening without Ductility Trade-off in Metastable Austenitic Steels.

Jiabin Liu,Yongbin Jin,Yuzeng Chen,Hongtao Wang,Jian Lu,Qiong Feng,Wei Yang,Xiaoyang Fang,Yujie Wei,Xiaowei Liu,Youtong Fang,Tianlin Huang,Feng Zhao,Tao Suo,Liang Meng,Xi Wang,Chenxu Chen

doi:10.1038/srep35345

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.

Highlights

Strength and ductility are mutually exclusive if they are manifested as consequence of the coupling between strengthening and toughening mechanisms
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
We report that 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

Summary

Results

Microstructural characterization and tensile behaviors of TMT metastable austenitic steel. It is generated during the phase transformation process, instead of deforming the BCC lattice. Further TEM characterization of the fractured samples reveals that the α-martensite grains have the same orientation and are aligned in strips with sub-micron sizes and non-equal-axed shapes, separated by austenite phase (Supplementary Fig. S5). The only notable structural difference of two types of samples is the high density of initial dislocations in the TMT samples This result clearly demonstrates that the pre-existed dislocations promote α-martensite phase transformation

Discussion

Methods

Limitations