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.back-stress hardening | heterogeneous lamella structure | ductility | strength | strain partitioning S trong or ductile? For centuries engineers have been forced to choose one of them, not both as they would like to. This is because a material is either strong or ductile but rarely both at the same time. High strength is always desirable, especially under the current challenge of energy crisis and global warming, where stronger materials can help by making transportation vehicles lighter to improve their energy efficiency. However, good ductility is also required to prevent catastrophic failure during service.Grain refinement has been extensively explored to strengthen metals. Ultrafine-grained (UFG) and nanostructured metals can be many times stronger than their conventional coarse-grained (CG) counterparts (1-5), but low ductility is a roadblock to their practical applications. The low ductility is primarily due to their low strain hardening (6-12), which is caused by their small grain sizes. To further exacerbate the problem, their high strengths require UFG metals to have even higher strain hardening than weaker CG metals to maintain the same ductility according to the Considère criterion. This makes it appear hopeless for UFG materials to have high ductility and it has been taken for granted that they are super strong but inevitably much less ductile than their CG counterparts. Microstructure of Heterogeneous Lamella StructureHere we report that a previously unidentified heterogeneous lamella (HL) structure possesses both the UFG strength and the CG ductility, which to our knowledge has never been realized before. The HL structure was produced by asymmetric rolling (13, 14) and subsequent partial recrystallization (see Materials and Methods for details). The asymmetric rolling elongated the initial equiaxed grains (Fig. 1A) into a lamella structure (Fig. 1B), which is heterogeneous with some areas having finer lamella spacing than others. This was due to the variation of slip systems and plastic strain in grains with different initial orientation ...

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Keywords:High specific strength steel Dual-phase nanostructure Strain hardening Ductility In situ high-energy X-ray diffraction a b s t r a c tWe report a detailed study of the strain hardening behavior of a Fee16Mne10Ale0.86Ce5Ni (weight percent) high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) during uniaxial tensile deformation. The dual-phase (g-austenite and B2 intermetallic compound) HSSS possesses high yield strength of 1.2e1.4 GPa and uniform elongation of 18e34%. The tensile deformation of the HSSS exhibits an initial yield-peak, followed by a transient characterized by an up-turn of the strain hardening rate. Using synchrotron based high-energy in situ X-ray diffraction, the evolution of lattice strains in both the g and B2 phases was monitored, which has disclosed an explicit elasto-plastic transition through load transfer and strain partitioning between the two phases followed by co-deformation. The unloadingreloading tests revealed the Bauschinger effect: during unloading yield in g occurs even when the applied load is still in tension. The extraordinary strain hardening rate can be attributed to the high back stresses that arise from the strain incompatibility caused by the microstructural heterogeneity in the HSSS.