Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility, Wu, Yang, Yuan, Wu Wei, Huang, Zhu, PNAS, 2015
Novelty/impact/significance:
A novel heterogeneous lamella structure in Ti is created, which is as strong as the ultrafine-grained Ti and simultaneously as ductile as the coarse-grained Ti, with higher strain hardening than the coarse-grained Ti.
The process and mechanisms promote the manufacturing of advanced metal materials that surpass current ones in achieving the commonly exclusive properties of strength and ductility.
Scientific question:
How to make ultrafine-grained, high-strength metals meanwhile possessing high ductility?
Key of how:
A heterogeneous structure with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix generates heterogeneous deformation; the high strength results from significant back stress developed from heterogeneous yielding, while the high ductility comes from the extraordinary strain-hardening rate resulted from back-stress hardening and dislocation hardening; three key structure features are the lamella geometry, the full constraint around the soft lamellae by hard matrix, and the high density of interface.
Major points:
1. There is always a conflict in obtaining strength and ductility in materials; ultrafine-grained (UFG) and nanostructured metals can be super strong, but the ductility is lower than conventional coarse-grained (CG) counterparts, due to low strain hardening.
2. A heterogeneous lamella structure is produced by asymmetric rolling (some areas are finer than others) and partial recrystallization (finer lamellae form soft microlamellae while others maintain UFG structure). It shows alternating, lamellar arrangements of the recrystallized grains (RGs) almost free of dislocations and the UFGs containing high densities of dislocations. The volume fraction of RGs (20-30%) decrease through the depth but the sizes increase slightly, while the UFGs show constant sizes.
3. Tensile tests show that the heterogeneous lamella 60 and 80 Ti (HL60 and HL80) (sampled from the central 60µm- and 80µm-thick layers) are as strong as UFG Ti (3 times stronger than CG) and as ductile as CG Ti (by the necking strain and by the uniform tensile elongation). (the tensile loading is parallel to the rolling/lamella orientation)
The high ductility in HL Ti comes from their high strain-hardening rate (higher than the CG Ti). They show a two-stage behavior, a drop followed by a steep upturn (typical of discontinuous yielding, shortage of mobile dislocations [in CGs], higher stress needed to move dislocations faster, dislocations quickly multiply and entangle [in UFG]) and a continuous rising, which drastically differs from the monotonic decreasing in rate of UFG (with short strain range) and CG metals.
The RGs are hardened by and the heterogeneous mechanical properties remain after the plastic deformation (by the hardness variation along the depth).
4. The high strength of HL Ti is due to the constrained deformation in the soft CG by the surrounding hard UFG, where the back stress produced by the dislocations piling up and blocking. The soft CGs have to wait to deform until the UFG lamellae start to yield at a larger global strain. (PS: It will be tricky to determine an optimal amount/position of UFGs for getting the similar strength of UFG metals).
5. Investigating the Baushinger effect by load-unload-reload tests, (1) the back stress is ~400 MPa near the yield point, which the soft CG lamellae need to overcome additionally for plastic deformation; thus, the high yield strength of the HL Ti results from the high back stress. (2) the back stress increases with plastic strain at the early strain stage, resulting in the increasing strain hardening with global strain and the higher strain hardening than CG Ti; thus, the high strain hardening originates from both back stress hardening and dislocation hardening.
The HL structure promotes the geometrically necessary dislocations (GNDs) and the incidental type dislocations; the latter does not produce back stress, but the stress state changes may increase the incidental dislocation density (accumulation, interaction, more slip systems), thus strengthening.
6.. The back stress hardening, caused by the (GNDs)piling up, also causes dislocations confined within the soft CG lamellae and stops the dislocation from emitting more dislocations, making the soft lamellae stronger than when they are not constrained.
The full constraint by the hard matrix surrounding the soft lamellae is a prerequisite for this.
7. Strain partitioning/inhomogeneous plastic strain occurs within the HL Ti accompanying the back-stress hardening, as the soft and hard lamellae carry high and low plastic strains, respectively (estimated by measuring the grain geometry changes before and after test).
8. The HL structure is more effective in producing strain hardening than conventional bimodal structures, in (1) lamellar structure (lamellae are in favor of higher strain hardening and mutual constraint between the soft and the hard), (2) full constraint of soft lamellae by hard matrix (more effective to constrain the plastic deformation of the soft lamellae for higher back stress), (3) high density of interlamella interfaces (facilitate dislocation pile-up and accumulation to enhance stress hardening and dislocation hardening).
It is a very interesting and enlightening work.
Here is the link of the fulltext: https://www.pnas.org/content/112/47/14501