53 research outputs found
On the tear resistance of skin.
Tear resistance is of vital importance in the various functions of skin, especially protection from predatorial attack. Here, we mechanistically quantify the extreme tear resistance of skin and identify the underlying structural features, which lead to its sophisticated failure mechanisms. We explain why it is virtually impossible to propagate a tear in rabbit skin, chosen as a model material for the dermis of vertebrates. We express the deformation in terms of four mechanisms of collagen fibril activity in skin under tensile loading that virtually eliminate the possibility of tearing in pre-notched samples: fibril straightening, fibril reorientation towards the tensile direction, elastic stretching and interfibrillar sliding, all of which contribute to the redistribution of the stresses at the notch tip
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Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy.
Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation
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Mechanical Competence and Bone Quality Develop During Skeletal Growth.
Bone fracture risk is influenced by bone quality, which encompasses bone's composition as well as its multiscale organization and architecture. Aging and disease deteriorate bone quality, leading to reduced mechanical properties and higher fracture incidence. Largely unexplored is how bone quality and mechanical competence progress during longitudinal bone growth. Human femoral cortical bone was acquired from fetal (n = 1), infantile (n = 3), and 2- to 14-year-old cases (n = 4) at the mid-diaphysis. Bone quality was assessed in terms of bone structure, osteocyte characteristics, mineralization, and collagen orientation. The mechanical properties were investigated by measuring tensile deformation at multiple length scales via synchrotron X-ray diffraction. We find dramatic differences in mechanical resistance with age. Specifically, cortical bone in 2- to 14-year-old cases exhibits a 160% greater stiffness and 83% higher strength than fetal/infantile cases. The higher mechanical resistance of the 2- to 14-year-old cases is associated with advantageous bone quality, specifically higher bone volume fraction, better micronscale organization (woven versus lamellar), and higher mean mineralization compared with fetal/infantile cases. Our study reveals that bone quality is superior after remodeling/modeling processes convert the primary woven bone structure to lamellar bone. In this cohort of female children, the microstructural differences at the femoral diaphysis were apparent between the 1- to 2-year-old cases. Indeed, the lamellar bone in 2- to 14-year-old cases had a superior structural organization (collagen and osteocyte characteristics) and composition for resisting deformation and fracture than fetal/infantile bone. Mechanistically, the changes in bone quality during longitudinal bone growth lead to higher fracture resistance because collagen fibrils are better aligned to resist tensile forces, while elevated mean mineralization reinforces the collagen scaffold. Thus, our results reveal inherent weaknesses of the fetal/infantile skeleton signifying its inferior bone quality. These results have implications for pediatric fracture risk, as bone produced at ossification centers during children's longitudinal bone growth could display similarly weak points. © 2019 American Society for Bone and Mineral Research
Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures
High-entropy alloys are an intriguing new class of metallic materials that
derive their properties from being multi-element systems that can crystallize
as a single phase, despite containing high concentrations of five or more
elements with different crystal structures. Here we examine an equiatomic
medium-entropy alloy containing only three elements, CrCoNi, as a single-phase
face-centered cubic (fcc) solid solution, which displays strength-toughness
properties that exceed those of all high-entropy alloys and most multi-phase
alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa,
failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2;
at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy
improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc
values of 275 MPa.m1/2. Such properties appear to result from continuous steady
strain hardening, which acts to suppress plastic instability, resulting from
pronounced dislocation activity and deformation-induced nano-twinning.Comment: 7 pages, 4 figure
Nanoscale Origins of the Damage Tolerance of the High-Entropy Alloy CrMnFeCoNi
Damage-tolerance can be an elusive characteristic of structural materials
requiring both high strength and ductility, properties that are often mutually
exclusive. High-entropy alloys are of interest in this regard. Specifically,
the single-phase CrMnFeCoNi alloy displays tensile strength levels of ~1 GPa,
excellent ductility (~60-70%) and exceptional fracture toughness (KJIc > 200
MPa/m). Here, through the use of in-situ straining in an aberration-corrected
transmission electron microscope, we report on the salient atomistic to
micro-scale mechanisms underlying the origin of these properties. We identify a
synergy of multiple deformation mechanisms, rarely achieved in metallic alloys,
which generates high strength, work hardening and ductility, including the easy
motion of Shockley partials, their interactions to form stacking-fault
parallelepipeds, and arrest at planar-slip bands of undissociated dislocations.
We further show that crack propagation is impeded by twinned, nano-scale
bridges that form between the near-tip crack faces and delay fracture by
shielding the crack tip.Comment: 6 figures, 4 figure
Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism
We believe this article is of broad interest to the materials science and engineering community. Bulk-metallic glasses (BMGs) are currently considered candidate materials for numerous structural applications. A major limitation in their use as engineering material is the often poor and inconsistent fatigue behavior. Although recently developed BMG composites provide one solution to this problem, fatigue remains a main issue for monolithic metallic glasses. The authors report unexpectedly high fatigue resistance in a monolithic Pd-based glass arising from extensive shear-band plasticity, resulting in a very rough and periodic “staircase” crack trajectory. The research both reveals a unique mechanism in fatigue of a monolithic metallic glass and demonstrates that this mechanism mitigates previous limitations on its use as an engineering material
Submission 238 to: Australia's Science and Research Priorities - Conversation Starter
Australia plans to obtain, build, and operate nuclear propelled submarines. The immense challenges brought about by this change in national defence strategy, along with the ambition to create a 20,000 strong nuclear science and engineering capable workforce, must be reflected in the National Science and Research Priorities, and the National Science Statement. We therefore recommend the inclusion of Nuclear Technology as a contemporary National Science and Research Priority, for consideration by the Science Strategy and Priorities Taskforce
Ni-Nb-P-based bulk glass-forming alloys: Superior material properties combined in one alloy family
Ni-Nb-based bulk glass-forming alloys are among the most promising amorphous metals for industrial applications due to their incomparable combination of strength, hardness, elasticity and plasticity. However, the main
drawback is the limited glass-forming ability, narrowing the field of application to solely small components. In
this study, we show that minor additions of P to the binary Ni-Nb system increase the glass-forming ability by
150 % to a record value of 5 mm. P can be easily added by using an industrial Ni-P pre-alloy which is readily
available. The partial substitution of Nb by Ta further boosts the glass-forming ability to values 200 % higher
than that of the binary base alloy. Besides conventional X-ray diffraction measurements, the amorphous nature of
the samples is verified by high-energy synchrotron X-ray diffraction experiments. Moreover, the mechanical
properties of the new alloy compositions are characterized in uniaxial compression tests and Vickers hardness
measurements, showing a high engineering yield strength of 3 GPa, an extended plastic regime up to 10 % strain
to failure and an increase of the hardness to a maximum value of 1000 HV5. Additionally, calorimetric measurements reveal that the modified alloys feature an extended supercooled liquid region up to 69 K upon heating,
permitting thermoplastic micro molding of amorphous feedstock material
Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys close to liquid helium temperatures
Medium- and high-entropy alloys based on the CrCoNi-system have been shown to
display outstanding strength, tensile ductility and fracture toughness
(damage-tolerance properties), especially at cryogenic temperatures. Here we
examine the JIc and (back-calculated) KJIc fracture toughness values of the
face-centered cubic, equiatomic CrCoNi and CrMnFeCoNi alloys at 20 K. At flow
stress values of ~1.5 GPa, crack-initiation KJIc toughnesses were found to be
exceptionally high, respectively 235 and 415 MPa(square-root)m for CrMnFeCoNi
and CrCoNi, with the latter displaying a crack-growth toughness Kss exceeding
540 MPa(square-root)m after 2.25 mm of stable cracking, which to our knowledge
is the highest such value ever reported. Characterization of the crack-tip
regions in CrCoNi by scanning electron and transmission electron microscopy
reveal deformation structures at 20 K that are quite distinct from those at
higher temperatures and involve heterogeneous nucleation, but restricted
growth, of stacking faults and fine nano-twins, together with transformation to
the hexagonal closed-packed phase. The coherent interfaces of these features
can promote both the arrest and transmission of dislocations to generate
respectively strength and ductility which strongly contributes to sustained
strain hardening. Indeed, we believe that these nominally single-phase,
concentrated solid-solution alloys develop their fracture resistance through a
progressive synergy of deformation mechanisms, including dislocation glide,
stacking-fault formation, nano-twinning and eventually in situ phase
transformation, all of which serve to extend continuous strain hardening which
simultaneously elevates strength and ductility (by delaying plastic
instability), leading to truly exceptional resistance to fracture.Comment: 31 pages, 10 figures, including Supplementary Informatio
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A Highly Fatigue-Resistant Zr-Based Bulk Metallic Glass
The strength-normalized fatigue endurance strength of the bulk metallic glass (BMG) Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ (Vitreloy 105) has been reported to be the highest for any BMG; however, to date, there has been no explanation of why this material is so much better than other Zr-based compositions. In this study, the fatigue-crack growth behavior of Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ was compared in ambient air vs dry nitrogen environment. The excellent fatigue life behavior is attributed to a relatively high fatigue threshold (Kₜₕ ≈ 2 MPa√m) and a lack of sensitivity to environmental effects on fatigue-crack growth in ambient air, as compared to other Zr-based BMGs. Fatigue life experiments conducted in ambient air confirmed the excellent fatigue life properties with a 10⁷-cycle endurance strength of ~0.24 of the ultimate tensile strength; however, it was also found that casting porosity, even in limited amounts, could reduce this endurance strength by as much as ~60 pct. Overall, the BMG Zr₅₂.₅Cu₁₇.₉Ni₁₄.₆Al₁₀Ti₅ appears to have excellent strength and fatigue properties and should be considered as a prime candidate material for future applications where good mechanical fatigue resistance is required
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