Simultaneous improvement of tensile strength and ductility in micro-duplex structure consisting of austenite and ferrite

Abstract

a b s t r a c tA micro-duplex structure consisting of austenite and ferrite was produced by equal channel angular pressing and subsequent intercritical annealing. As compared to coarse-grained (CG) counterpart, the strength and ductility of micro-duplex samples are enhanced simultaneously due to smaller grain sizes in both phases and more uniformly distributed austenite in ferrite matrix. The average yield stress and uniform elongation are increased to 540 MPa and 0.3 as compared to 403MPa and 0.26 of its CG counterpart respectively. The Hall-Petch coefficients of austenite and ferrite grain boundaries were quantitatively measured as 224.9 and 428.9 MPa μm 1/2 respectively. In addition, a Hall-Petch type coefficient was used to describe the ability of phase boundary to obstruct dislocation motion, which was measured as 309.7 MPa μm 1/2 . Furthermore, the surface-to-volume ratio of phase boundary in microduplex structure was estimated to be 1.17 Â 10 6 m À 1 , which is increased by an order of magnitude as compared to 1.2 Â 10 5 m À 1 of its CG counterpart. Based on the strain gradient theory, a model was proposed to describe the effect of surface-to-volume ratio of phase boundary on strain hardening rate, which shows a good agreement with the experimental results.

Grain Size Effect on the Mechanical Behavior of Metastable Fe-23Cr-8.5Ni Alloy

“…The fitted line shows a slope of 315.2 MPa μm 2 and flow stress of 145 MPa at an infinite grain size. This slope is comparable to that (334.7 MPa μm 2 ) reported for a Fe-24Cr-4Ni-0.4Mo-0.1N duplex alloy [9]. However, the Fe-24Cr-4Ni-0.4Mo-0.1N alloy shows much higher flow stress (308 MPa) at an infinite grain size than the present alloy, which can be attributed to the differences in chemical compositions, especially to the solution strengthening effect imposed by nitrogen.…”

“…Note that in the micrometer-grained scale, a grain refinement from 2 to 1 μm leads to a slight increase of ductility. Similar results were also reported for both ferritic-martensitic and ferritic-austenitic dual phase alloys [9,38,39]. The increased densities of grain boundaries and phase interfaces due to structural refinement can increase the number of dislocation sources, giving rise to rapid dislocation multiplication and enhanced strain hardening [38], which is responsible for the observed improved ductility.…”

“…However, the Fe-24Cr-4Ni-0.4Mo-0.1N alloy shows much higher flow stress (308 MPa) at an infinite grain size than the present alloy, which can be attributed to the differences in chemical compositions, especially to the solution strengthening effect imposed by nitrogen. The present study combined with previous research [9,40] on Fe-Cr-Ni based duplex alloys, clearly reveal an extension of the Hall-Petch relationship from the micrometer scale to the sub-micrometer scale. Many studies on metastable austenitic steels [15][16][17] indicate that the mechanical stability of austenite is increased with decreasing grain size.…”

“…The differences in mechanical properties between constituent phases lead to an inhomogeneous stress and strain partitioning between them, resulting in a heterogeneous distribution of plastic strains across phase interfaces [8]. This interaction between two phases requires the generation of geometrically necessary dislocations (GNDs) at phase interfaces to accommodate the plastic strain gradient [9], which has a potential to improve the dislocation storage, and hence, contributes to the strain hardening.…”

Managing both strength and ductility in duplex stainless steel with heterogeneous lamella structure

“…For the present DSS, austenite usually has higher hardness than ferrite due to its high nitrogen content [8] and a smaller average grain size. Under plastic deformation, the ferrite will deform preferentially, results in a large accumulation of dislocations along phase boundaries, which further leads to the generation of geometrically necessary dislocations (GNDs) to accommodate the strain incompatibility between the two phases [9][10]. This produces a long-range back stress [11][12] to make it difficult for dislocations to transfer from ferrite to neighboring austenite until the latter starts to yield at a larger total strain, and as a result, it strengthens DSS.…”