Ping Jiang

Extraordinary strain hardening by gradient structure

Significance Nature creates the gradient structure (GS) for a purpose: to make biological systems strong and tough to survive severe natural forces. For the grain-size GS, the deformation physics is still unclear. One wonders if the grain-size GS in the nanomicroscale would also benefit materials engineered by mankind. In this paper, a universal strain hardening mechanism is revealed in the GS. We discovered a unique extra strain hardening that is intrinsic to the GS. Its mechanism is the presence of strain gradient together with the stress state change. A superior combination of strength and ductility that is not accessible to conventional homogeneous materials is obtained. As a novel mechanism, extra strain hardening renders high ductility in the GS materials. Gradient structures have evolved over millions of years through natural selection and optimization in many biological systems such as bones and plant stems, where the structures change gradually from the surface to interior. The advantage of gradient structures is their maximization of physical and mechanical performance while minimizing material cost. Here we report that the gradient structure in engineering materials such as metals renders a unique extra strain hardening, which leads to high ductility. The grain-size gradient under uniaxial tension induces a macroscopic strain gradient and converts the applied uniaxial stress to multiaxial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby the accumulation and interaction of dislocations are promoted, resulting in an extra strain hardening and an obvious strain hardening rate up-turn. Such extraordinary strain hardening, which is inherent to gradient structures and does not exist in homogeneous materials, provides a hitherto unknown strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.

Nanodomained Nickel Unite Nanocrystal Strength with Coarse-Grain Ductility

Conventional metals are routinely hardened by grain refinement or by cold working with the expense of their ductility. Recent nanostructuring strategies have attempted to evade this strength versus ductility trade-off, but the paradox persists. It has never been possible to combine the strength reachable in nanocrystalline metals with the large uniform tensile elongation characteristic of coarse-grained metals. Here a defect engineering strategy on the nanoscale is architected to approach this ultimate combination. For Nickel, spread-out nanoscale domains (average 7u2009nm in diameter) were produced during electrodeposition, occupying only ~2.4% of the total volume. Yet the resulting Ni achieves a yield strength approaching 1.3u2009GPa, on par with the strength for nanocrystalline Ni with uniform grains. Simultaneously, the material exhibits a uniform elongation as large as ~30%, at the same level of ductile face-centered-cubic metals. Electron microscopy observations and molecular dynamics simulations demonstrate that the nanoscale domains effectively block dislocations, akin to the role of precipitates for Orowan hardening. In the meantime, the abundant domain boundaries provide dislocation sources and trapping sites of running dislocations for dislocation multiplication, and the ample space in the grain interior allows dislocation storage; a pronounced strain-hardening rate is therefore sustained to enable large uniform elongation.

Twin boundary spacing effects on shock response and spall behaviors of hierarchically nanotwinned fcc metals

Atomistic deformation mechanisms of hierarchically nano-twinned (NT) Ag under shock conditions have been investigated using a series of large-scale molecular dynamics simulations. For the same grain size d and the same spacing of primary twins lambda(1), the average flow stress behind the shock front in hierarchically NT Ag first increases with decreasing spacing of secondary twins lambda(2), achieving a maximum at a critical lambda(2), and then drops as lambda(2) decreases further. Above the critical lambda(2), the deformation mechanisms are dominated by three type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other boundaries; (b) partial dislocations emitted from twin boundaries (TBs) travel across other TBs; (c) formation of tertiary twins. Below the critical lambda(2), the deformation mechanism are dominated by two softening mechanisms: (a) detwinning of secondary twins; (b) formation of new grains by cross slip of partial dislocations. Moreover, the twin-free nanocrystalline (NC) Ag is found to have lower average flow stress behind the shock front than those of all hierarchically NT Ag samples except the one with the smallest lambda(2) of 0.71 nm. No apparent correlation between the spall strength and lambda(2) is observed in hierarchically NT Ag, since voids always nucleate at both GBs and boundaries of the primary twins. However, twin-free NC Ag is found to have higher spall strength than hierarchically NT Ag. Voids can only nucleate from GBs for twin-free NC Ag, therefore, twin-free NC Ag has less nucleation sources along the shock direction when compared to hierarchically NT Ag, which requiring higher tensile stress to create spallation. These findings should contribute to the understandings of deformation mechanisms of hierarchically NT fcc metals under extreme deformation conditions

Dynamically reinforced heterogeneous grain structure prolongs ductility in a medium-entropy alloy with gigapascal yield strength

Significance Back stress hardening is usually not obvious in single-phase homogeneous grains, but can be made unusually large and sustained to large tensile strains by creating an unusually heterogeneous grain structure in single-phase alloys with low stacking fault energy (SFE), as demonstrated here for the face-centered cubic CrCoNi medium-entropy alloy. The low SFE facilitates the generation of twinned nanograins and stacking faults during tensile straining, dynamically reinforcing the heterogeneity. Large uniform tensile strain can be achieved after yielding even at gigapascal stress, in the absence of heterogeneities from any second phase. Ductility, i.e., uniform strain achievable in uniaxial tension, diminishes for materials with very high yield strength. Even for the CrCoNi medium-entropy alloy (MEA), which has a simple face-centered cubic (FCC) structure that would bode well for high ductility, the fine grains processed to achieve gigapascal strength exhaust the strain hardening ability such that, after yielding, the uniform tensile strain is as low as ∼2%. Here we purposely deploy, in this MEA, a three-level heterogeneous grain structure (HGS) with grain sizes spanning the nanometer to micrometer range, imparting a high yield strength well in excess of 1 GPa. This heterogeneity results from this alloy’s low stacking fault energy, which facilitates corner twins in recrystallization and stores deformation twins and stacking faults during tensile straining. After yielding, the elastoplastic transition through load transfer and strain partitioning among grains of different sizes leads to an upturn of the strain hardening rate, and, upon further tensile straining at room temperature, corner twins evolve into nanograins. This dynamically reinforced HGS leads to a sustainable strain hardening rate, a record-wide hysteresis loop in load−unload−reload stress−strain curve and hence high back stresses, and, consequently, a uniform tensile strain of 22%. As such, this HGS achieves, in a single-phase FCC alloy, a strength−ductility combination that would normally require heterogeneous microstructures such as in dual-phase steels.

Shock and spall behaviors of a high specific strength steel: Effects of impact stress and microstructure

A series of plate-impact experiments were conducted to investigate the influences of impact stress and microstructure on the shock and spall behaviors of a high specific strength steel (HSSS). The HSSS shows a strong positive strain rate sensitivity on the yield strength. With increasing impact stress up to about 6u2009GPa, the spall strength is found to decrease significantly and then levels off with further increasing impact stress. This trend is proposed to be attributed to the accumulation damage within the target as the initial shock-induced compression wave propagates through the target. The microcracks are clearly observed to nucleate from the interfaces between γ-austenite and B2 phase and propagate along the interfaces or cut through the B2 phase in the HSSS during the spalling process. The Hugoniot elastic limit and the spall strength were found to be highly dependent on the microstructure. The spall strength was found to be higher when the density of the void nucleation sites is lower, indicating t...

Size effects of lamellar twins on the strength and deformation mechanisms of nanocrystalline hcp cobalt

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Annealing and strain rate effects on the mechanical behavior of ultrafine-grained iron produced by SPD

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Size effects of nano-spaced basal stacking faults on the strength and deformation mechanisms of nanocrystalline pure hcp metals

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