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

Abstract

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 ...

Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

“…It usually appears for alloys of multi-phases with varying yield stresses, over a strain regime corresponding to heterogeneous elasto-plastic deformation [36,38]. Similar behavior of Q has also been observed recently in gradient structure [52,53] and heterogeneous lamella structure [41].…”

“…The development of internal stresses during deformation of an inhomogeneous microstructure with yield stress mismatch has been well described before [13,40,41]: upon tensile loading, plastic yield starts in the soft phase, and the applied load will be transferred from the soft phase to the hard one that is still in elastic state. Thus, internal stresses will build up at the phase interfaces.…”

“…The derivation of the formula and the calculation method can be found in Ref. [41]. The strain experienced by g, granular and lamellar B2, respectively, is about 27%, 7%, and 1%, respectively, at the total applied strain of~26%.…”

Synergetic strengthening far beyond rule of mixtures in gradient structured aluminum rod

“…In addition, the effect of residual stresses on the microhardness testing could skew the hardness due to the compression at the surface and tension in the core. Another factor that could increase the yield strength is the high back stress developed at the plastic-elastic interface of the gradient structure [18]. Also, although the linear relationship between hardness and yield strength was used to predict rule of mixtures calculations in this study, the high rate of strain hardening in aluminum could introduce error into this prediction especially in the measurements of the ductile core and this analysis was used only as an approximation for the gradient structure yield strength.…”

“…In general, yield strength is universally improved in gradient structures, though it seems ductility can either decrease [15], remain more or less unchanged [3], or increase [1]. The discrepancy, in part, must be explained by the mechanism or mechanisms by which gradient structures deform and has been attributed to grain growth, dynamic hardening, strain partitioning, and dislocation accumulation at grain boundaries, to name a few [1,2,[16][17][18]. One mechanism that has been experimentally verified is the evolution of a multi-axial stress state during tensile testing of the gradient structure [3].…”