Fuping Yuan

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.

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

Significance For centuries it has been a challenge to avoid strength–ductility trade-off, which is especially problematic for ultrastrong ultrafine-grained metals. Here we evade this trade-off dilemma by architecting a heterogeneous lamella structure, i.e., soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The heterogeneous deformation of this previously unidentified structure produces significant back-stress hardening in addition to conventional dislocation hardening, rendering it higher strain hardening than coarse-grained metals. The high back-stress hardening makes the material as strong as ultrafine-grained metals and as ductile as coarse-grained metals. 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 strengthening and strain hardening in gradient structure

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. GRAPHICAL ABSTRACT

Origin of pulverized rocks during earthquake fault rupture

The origin of pulverized rocks (PR) in surface outcrops adjacent to the fault cores of the San Andreas and other major faults in Southern California is not clear, but their structural context indicates that they are clearly associated with faulting. An understanding of their origin might allow inferences to be drawn about the nature of dynamic slip on faults, including rupture mechanisms and their speed during earthquakes. In the present study, we use split Hopkinson bar recovery experiments to investigate whether PR can be produced under dynamic stress wave loading conditions in the laboratory and whether PR is diagnostic of any particular process of formation. The results of the study indicate that in Westerly granite for transition from sparse fracture to pervasive pulverization requires high strain rates in excess of 250/s and that the formation of PR may be inhibited at the larger burial depths. The constraint imposed by field observations of the relatively low strains (1-3%) in PR recovered from the field and the laboratory derived threshold for the critical strain rate (similar to 250/s and higher) together indicate that a dynamic supershear-type rupture may be necessary for the origin of pulverized rocks at distances of tens of meters away from the fault plane as observed in the field for both large strike-slip-type and the relatively small dip-slip-type fault ruptures in nature.

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 7 nm in diameter) were produced during electrodeposition, occupying only ~2.4% of the total volume. Yet the resulting Ni achieves a yield strength approaching 1.3 GPa, 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.

Size effects of primary/secondary twins on the atomistic deformation mechanisms in hierarchically nanotwinned metals

A series of large-scale molecular dynamics simulations have been performed to investigate the tensile properties of nanotwinned (NT) copper with hierarchically twinned structures (HTS). For the same grain size d and the same spacing of primary twins λ1, the average flow stress first increases as the spacing of secondary twins λ2 decreases, reaching a maximum at a critical λ2, and then decreases as λ2 becomes even smaller. The smaller the spacing for λ1, the smaller the critical spacing for λ2. There exists a transition in dominating deformation mechanisms, occurring at a critical spacing of λ2 for which strength is maximized. Above the critical spacing of λ2, the deformation mechanisms are dominated by the two Hall-Petch type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other GBs and twin boundaries (TBs); (b) partial dislocations emitted from TBs travel across other TBs. Below the critical spacing of λ2, the deformation mechanism is dominated by the...

Effect of high strain rates on peak stress in a Zr-based bulk metallic glass

The mechanical behavior of Zr41.25Ti13.75Cu12.5Ni10Be22.5 (LM-1) has been extensively characterized under quasistatic loading conditions; however, its mechanical behavior under dynamic loading conditions is currently not well understood. A Split–Hopkinson pressure bar (SHPB) and a single-stage gas gun are employed to characterize the mechanical behavior of LM-1 in the strain-rate regime of 102–105/s. The SHPB experiments are conducted with a tapered insert design to mitigate the effects of stress concentrations and preferential failure at the specimen-insert interface. The higher strain-rate plate-impact compression-and-shear experiments are conducted by impacting a thick tungsten carbide (WC) flyer plate with a sandwich sample comprising a thin bulk metallic glass specimen between two thicker WC target plates. Specimens employed in the SHPB experiments failed in the gage-section at a peak stress of approximately 1.8 GPa. Specimens in the high strain-rate plate-impact experiments exhibited a flow stress i...

Atomistic scale fracture behaviours in hierarchically nanotwinned metals

In the present study, a series of large-scale molecular dynamics simulations have been performed to investigate the atomistic scale fracture behaviours along the boundaries of primary twins in Cu with hierarchically nanotwinned structures (HTS), and compare their fracture behaviours with those in monolithic twins. The results indicate that crack propagation along [1 1 2] on the twin plane in monolithic nanotwins is brittle cleavage and fracture, resulting in low crack resistance and fracture toughness. However, the crack resistance along the boundaries of primary twins in HTS is much higher, and a smaller spacing of secondary twins (λ 2) leads to even higher fracture toughness. With large λ 2, the crack growth is achieved by void nucleation, growth and coalescence. However, considerable plastic deformation and enhanced fracture toughness in HTS could be achieved by the crack blunting and by the extensive dislocation accommodation ahead of the crack tip when λ 2 is small.

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

Identification of interfacial and bulk effects in modulating fatigue behaviors of Pb(Zr0.52Ti0.48)O3 thin films

The polarization fatigue behaviors of Pt/LaNiO3/Pb(Zr0.52Ti0.48)O-3/LaNiO3/Pt (Pt/LNO/PZT/LNO/Pt) and Pt/PZT/Pt structures under different temperatures T, voltages V-amp, and frequencies f are investigated in order to clarify defect-related interfacial and bulk effects. The fatigue endurance of the Pt/LNO/PZT/LNO/Pt structure is enhanced at higher T, larger V-amp, and lower f, whereas for the Pt/PZT/Pt structure a better antifatigue performance is obtained at lower T, smaller V-amp, and higher f. The defect chemistry as one of the origins of the switching fatigue is demonstrated by the predominant interfacial effect and bulk effect resulting in two types of markedly opposite fatigue responses. (c) 2007 American Institute of Physics.