Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

https://www.pnas.org/content/pnas/112/47/14501.full.pdf

Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility.

https://www.tf.uni-kiel.de/matwis/amat/iss/kap_9/articles/twip_raabe.pdf

Dislocation and twin substructure evolution during strain hardening of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel observed by electron channeling contrast imaging

We report significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and a procedure are developed to calculate back stress basing on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient. This induces strain gradient, which needs to be accommodated by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening to increase the ductility.Impact Statement: Gradient structure leads to high back stress hardening to increase strength and ductility. A physically sound equation is derived to calculate the back stress from an unloading/reloading hysteresis loop.

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2016.1153004

Back stress strengthening and strain hardening in gradient structure

https://research.birmingham.ac.uk/portal/files/45381603/Liu_et_al_Dislocation_network_in_additive_Materials_Today_2017.pdf

Dislocation network in additive manufactured steel breaks strength–ductility trade-off

Heterostructured materials have been reported as a new class of materials with superior mechanical properties, which was attributed to the development of back stress. There are numerous reports on ...

https://tandfonline.com/doi/pdf/10.1080/21663831.2019.1616331

X. Feaugas

Papers

On the origin of the tensile flow stress in the stainless steel AISI 316L at 300 K: Back stress and effective stress

CYCLIC DEFORMATION BEHAVIOUR OF AN α/β TITANIUM ALLOY—I. MICROMECHANISMS OF PLASTICITY UNDER VARIOUS LOADING PATHS

CYCLIC DEFORMATION BEHAVIOUR OF AN α/β TITANIUM ALLOY—II. INTERNAL STRESSES AND MICROMECHANIC MODELLING