On the quest for the strongest materials LLorca, Javier
Science (American Association for the Advancement of Science),
04/2018, Letnik:
360, Številka:
6386
Journal Article
Recenzirano
Odprti dostop
Diamond nanoneedles have strength approaching the theoretical maximum
The strength of a material is a measure of its ability to withstand a load without breaking. Scientists in search of the ...strongest materials have recently turned their attention to nanomaterials, which have few of the defects that typically reduce a material's strength. On page 300 of this issue, Banerjee
et al.
(
1
) show that when nanoscale single-crystal diamond needles are elastically deformed, they fail at a maximum local tensile stress of ∼89 to 98 GPa, which is very close to the theoretical limit for this material.
We briefly review the state-of-the-art in phase-field modeling of microstructure evolution. The focus is placed on recent applications of phase-field simulations of solid-state microstructure ...evolution and solidification that have been compared and/or validated with experiments. They show the potential of phase-field modeling to make quantitative predictions of the link between processing and microstructure. Finally, some current challenges in extending the application of phase-field models within the context of integrated computational materials engineering are mentioned.
The mechanical behavior of polymer–matrix composites unidirectionally reinforced with carbon or glass fibers subjected to compression perpendicular to the fibers was studied using computational ...micromechanics. The stress–strain curve was determined by the finite element analysis of a representative volume element of the microstructure idealized as a random dispersion of parallel fibers embedded in the polymeric matrix. The dominant damage mechanisms experimentally observed – interface decohesion and matrix plastic deformation – were included in the simulations, and a parametrical study was carried out to assess the influence of matrix and interface properties on the stress–strain curve, compressive strength, ductility and the corresponding failure modes. It was found that the composite properties under transverse compression were mainly controlled by interface strength and the matrix yield strength in uniaxial compression. Two different fracture modes were identified, depending on whether failure was controlled by the nucleation of interface cracks or by the formation of matrix shear bands. Other parameters, such as matrix friction angle, interface fracture energy or thermo-elastic residual stresses, played a secondary role in the composite mechanical behavior.
The deformation and crack initiation mechanisms were analyzed in a textured AZ31B-O Mg alloy subjected to fully-reversed, strain-controlled cyclic deformation along the rolling direction after 50 ...cycles (approximately 33% of the fatigue life). Distinct deformation bands corresponding to pyramidal slip or tensile twins were found in 538 grains out of 2100 grains. Slip trace analysis showed that 72.3% were pyramidal slip bands and 18.4% were twin boundaries. Both pyramidal slip and twinning was only found in 9.1% of the grains with deformation bands. Cracking was widespread after 50 cycles. Grain boundary cracks were found in ≈15% of the small grains (< 20 µm) and they were mainly associated with high angle grain boundaries (>40º). Cracking was also found to occur by transgranular cracks parallel to the pyramidal slip bands or twin boundaries in large grains (>45 µm). The majority (>60%) of these large grains presented transgranular cracks after 50 cycles.
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The effect of Ca and Zn in solid solution on the critical resolved shear stress (CRSS) of basal slip, tensile twinning and <c+a> pyramidal slip in Mg alloys has been measured through compression ...tests on single crystal micropillars with different orientations. The solute atoms increased the CRSS for basal slip to ~ 13.5 MPa, while the CRSS for pyramidal slip was lower than 85 MPa, reducing significantly the plastic anisotropy in comparison with pure Mg. Moreover, the CRSSs for twin nucleation and growth were very similar (~ 37 MPa) and the large value of the CRSS for twin growth hindered the growth of twins during thermo-mechanical processing. Finally, evidence of prismatic slip and cross-slip between basal and prismatic dislocations was found. It is concluded that the reduction of plastic anisotropy, the activation of different slip systems and cross-slip and the weak basal texture promoted by the large CRSS for twin growth are responsible for the improved ductility and formability of Mg-Ca-Zn alloys.
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The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning ...electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness.
Transverse cracking in cross-ply carbon/epoxy and glass/epoxy laminates in tension is analyzed by means of computational micromechanics. Longitudinal plies were modeled as homogenized, anisotropic ...elastic solids while the actual fiber distribution was included in the transverse plies. The mechanical response was obtained by the finite element analysis of a long representative volume element of the laminate. Damage in the transverse plies was triggered by interface decohesion and matrix cracking. The simulation strategy was applied to study the influence of ply thickness on the critical stress for the cracking of the transverse plies and on the evolution of crack density in 02/90n/2s laminates, with n=1, 2, 4 and 8. It was found that the transverse ply strength corresponding to the initiation and propagation of a through-thickness crack was independent of the ply thickness and that the transverse strength of carbon/epoxy laminates was 35% higher than that of the glass fiber counterparts. In addition, the mechanisms of crack initiation and propagation through the thickness as well as of multiple matrix cracking were ascertained and the stiffness reduction in the 90° ply as a function of crack density was computed as a function of the ply thickness.
•Nucleation and growth of extension twins with low or even negative Schmid factor was reported by in-situ EBSD.•Anomalous extension twins nucleated mainly at the onset of plastic deformation at or ...near triple point grain boundaries.•Their nucleation associated with localized compressive stresses and/or severe strain incompatibility between neighbor grains.•The anomalous twins grew with the applied strain due to the continuous activation of 〈a〉 basal slip in neighbor grains.•These results reveal the complexity of the local deformation mechanisms in Mg alloys when deformed along hard orientations.
A large number of anomalous extension twins, with low or even negative twinning Schmid factors, were found to nucleate and grow in a strongly textured Mg-1Al alloy during tensile deformation along the extruded direction. The deformation mechanisms responsible for this behaviour were investigated through in-situ electron back-scattered diffraction, grain reference orientation deviation, and slip trace-modified lattice rotation. It was found that anomalous extension twins nucleated mainly at the onset of plastic deformation at or near grain boundary triple junctions. They were associated with the severe strain incompatibility between neighbour grains as a result from the different basal slip-induced lattice rotations. Moreover, the anomalous twins were able to grow with the applied strain due to the continuous activation of basal slip in different neighbour grains, which enhanced the strain incompatibility. These results reveal the complexity of the deformation mechanisms in Mg alloys at the local level when deformed along hard orientations.
The micromechanisms of plastic deformation and void growth were analyzed using discrete dislocation dynamics in an isolated FCC single crystal deformed in-plane strain in the
(
1
¯
1
0
)
plane. Three ...different stress states (uniaxial tension, uniaxial deformation and biaxial deformation) were considered for crystals oriented in different directions and with a different number of active slip systems. It was found that strain hardening and void growth rates depended on lattice orientation in uniaxial tension because of anisotropic stress state. Crystal orientation did not influence, however, hardening and void growth when the crystals were loaded under uniaxial or biaxial deformation because the stress state was more homogeneous, although both (hardening and void growth rates) were much higher than under uniaxial tension. In addition, the number of active slip systems did not substantially modify the mechanical behavior and the void growth rate if plastic deformation along the available slip systems was compatible with overall crystal deformation prescribed by the boundary conditions. Otherwise, the incompatibility between plastic deformation and boundary conditions led to the development of large hydrostatic elastic stresses, which increased the strain hardening rate and reduced the void growth rate.
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