Fe–Cr is a model alloy for ferritic steels used in thermal power generation systems and envisaged as primary structural material for future fusion reactors. However, upon heating and, further, under ...irradiation, Fe–Cr can suffer from phase decomposition leading to the formation of Fe-rich and Cr-rich regions that induce simultaneous hardening and embrittlement. In this study, we examine the origins of the degradation in mechanical properties by performing room-temperature in situ micropillar compression tests on single-crystalline Fe–40wt.%Cr alloys in solid solution, and in the spinodally decomposed state obtained after annealing at 500 °C for 1008 and 2016 h, respectively. The compressed micropillars are subjected to correlative nanoscale structural characterization using transmission electron backscattered diffraction and transmission electron microscopy. Dislocation slip occurs unequivocally on the {110}〈11¯1〉 slip system for all conditions. While the 2016 h annealed state exhibits a more evolved nanoscale phase modulation than the 1008 h annealed condition, both microstructures display approximately double the yield strength of the solid-solution state without any concurrent loss of ductility. Our findings reveal a fundamental change in plasticity mechanism across the three different microstructures. Deformation in the solid-solution state is associated with kink-mediated local plasticity occurring on multiple glide systems, which activate sequentially based upon the largest instantaneous Schmid factor. This gradually transforms into a less localized Lüders-band like plasticity associated with single-slip activation in the 1008 h annealed state, while the deformation in the 2016 h annealed state is marked by uniform strain hardening related to a homogeneous polycrystalline-like dislocation motion occurring simultaneously on multiple slip systems. Correlations between the spatial and compositional fluctuations in Cr and the associated plasticity dynamics are established. It is shown that the spatial fluctuations in Cr strongly influence dislocation strengthening and the relative mobility of the edge and screw components across all microstructural states. It is further concluded that the phase-separation effect, despite promoting strengthening, does not act as the primary cause of embrittlement, but rather plays a contrary role of enhancing ductility.
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Although laser powder bed fusion (LPBF) can produce complex structures with promising properties, there still exists a concern on the diversity in materials, especially composites, for LPBF. This ...work innovatively used NiTi and Nb powders to prepare NiTi–Nb eutectic-type alloy by LPBF, which provides a new method to prepare NiTi–Nb shape memory alloy and could be an alternative for tailoring microstructure. The semi-molten Nb particle phase was retained during LPBF, so that β-Nb and the eutectic structure display a gradient distribution in the Nb diffusion concentration. The heat treatment process (annealed at 850 °C for 0.5 h) makes NiTi–Nb samples have high yield strength (~1640 MPa), high compressive strength (~2380 MPa), and high compressive strain (~39%), which are superior to the corresponding ones of its as-built and as-cast counterparts. Large Nb phase particles with a size of 10–30 μm are retained and accelerated the formation of eutectic phase and β-Nb precipitate phase. Overall, the presence of β-Nb phase has a beneficial effect on the alloy, and a large number of intertwined dislocations and stacking faults around the β-Nb phase promote the formation of martensite under stress loading. The eutectic phase makes an important contribution to the high strength and plasticity of the NiTi–Nb alloy.
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•The effect of Y and Ca on the critical resolved shear stress (CRSS) of 〈a〉 basal slip, 〈a〉 prismatic slip, 〈c + a〉 pyramidal slip and tensile twinning in Mg alloys is studied by means of micropillar ...compression tests.•The generalized stacking fault energy (GSFE) curves of different slip systems are calculated via the first-principles calculations using the Vienna Ab initio simulation package (VASP).•Y and Ca significantly change the CRSS for slip and tensile twinning in Mg-Y-Ca alloys and modify the dominant deformation mechanisms in polycrystals.•Tensile twinning is replaced by 〈a〉 prismatic slip during compressive deformation along the a-axis.•Prismatic 〈a〉 and piramidal 〈c + a〉 slips doniamted the plastic deformation of Mg-Y-Ca alloy, significantly improving the tensile ductility (∼32%).
The deformation mechanisms of an extruded Mg-5Y-0.08Ca (wt.%) alloy were analyzed by means of micropillar compression tests on single crystals along different orientations -selected to activate specific deformation modes- as well as slip trace analysis, transmission electron microscopy and transmission Kikuchi diffraction. The polycrystalline alloy presented a remarkable ductility in tension (∼32%) and negligible differences in the yield strength between tension and compression. It was found that the presence of Y and Ca in solid solution led to a huge increase in the CRSS for 〈a〉 basal slip (29 ± 5 MPa), 〈c + a〉 pyramidal slip (203 ± 7 MPa) and tensile twin nucleation (above 148 MPa), while the CRSS for 〈a〉 prismatic slip only increases up to 105 ± 4 MPa. The changes in the CRSS for slip and tensile twinning in Mg-Y-Ca alloys expectedly modify the dominant deformation mechanisms in polycrystals. In particular, tensile twinning is replaced by 〈a〉 prismatic slip during compressive deformation along the a-axis. The reduction of twinning (which generally induces strong anisotropy in the plastic deformation in textured alloys), and the activation of 〈a〉 prismatic slip (which provides an additional plastic deformation mechanism with limited hardening) were responsible for the large tensile ductility of the alloy.
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Micropillar compression was used to investigate whether Ag segregation to an asymmetric Σ5001 grain boundary will lead to measurable strength differences compared to the pure copper bicrystal. Ag ...segregation was accomplished by deposition and subsequent annealing of an Ag thin-film applied on the surface of the Cu bicrystal. Atom probe tomography analysis indicated Ag segregation at the grain boundary with a peak concentration of 2.3 at.%. While the pristine Σ5 grain boundary shows a yield strength of 288 ± 18 MPa when compressing 1 µm diameter pillars along 〈001〉, micropillars containing an Ag-segregated Σ5 grain boundary demonstrated an increased yield strength of 318 ± 17 MPa. In addition, post-deformation electron microscopy was carried out to examine the active slip systems and slip transmission across Ag-free and Ag-containing bicrystals. The results are compared to reference measurements of the adjacent single crystal grains. The 1 µm pillar diameter promoted deformation governed by dislocation-grain boundary interactions for the bicrystalline pillars. This is the first time that changes in flow stress associated with grain boundary segregation have been quantified locally without interference from other mechanisms such as solid solution strengthening, formation of precipitates or changes in stacking fault energy. The results clearly indicate that purely geometrical models for slip transmission are not sufficient as the local atomic structure and composition influence dislocation transmission through grain boundaries.
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•The evolution of the twinning mechanism was experimentally investigated from low to high strain rates at the micron-scale.•The competition of twinning dislocations and pyramidal ...dislocations allows the evolution of the twin boundary along a non-invariant twin plane.•At high strain rates, the prismatic to basal plane conversion resulting in a 90° lattice reorientation governs the entire twin process.•The 3D reconstruction of the twins allows to fully assess the spatial geometry of the twin boundaries.•The boundary lateral to the shear direction of deformation twins was investigated at different strain rates at the micron-scale.
We present a systematic investigation of {101¯2} extension twinning mechanism in single crystal magnesium micropillars deformed over seven orders of magnitude of strain rate, from 10–4 to 500 s−1, revealing how the accommodation of newly formed twins evolves with and depends on the kinetic compatibility of interfacial processes when high deformation rates are imposed. By combination of post-mortem 3D Electron Backscattered Diffraction, Transmission Kikuchi Diffraction and Transmission Electron Microscopy techniques, this work unveils the progressive evolution of the accommodating twin mechanisms from low to high strain rate, correlating differences in mechanical behavior with differences in twin crystallography. Away from quasi–static conditions, simple considerations of twinning shear do not suffice to describe unconventional twin morphologies, requiring the competition between newly activated dislocations and lattice distortions for allowing the evolution of the twin boundary along non–invariant twin planes. Under shock compressions, the basal/prismatic transformation establishing a lattice misorientation of 90° entirely governs the parent → twin conversion. The results illustrated here confirm that some of the recent interpretations deduced by particular twin morphologies are not universally valid and that deformation twinning is not only stress- but also strongly time–controlled.
The room-temperature plastic deformation behavior of 6H–SiC single crystals has been investigated by uniaxial compression of micropillar specimens as a function of crystal orientation and specimen ...size. Plastic flow is observed even at room temperature by basal and prism slip, latter of which have never been observed in the bulk. The CRSS values for basal and prism slip are as high as above 5 and 6 GPa at the specimen size of 5 μm, respectively, each of which increases with decreasing specimen size, following an inverse power-law relationship with a relatively small power-law exponent of ∼0.10 and ∼0.21, respectively. The CRSS values for basal slip are not virtually affected by the existence of basal dislocations introduced at 1300 °C prior to micropillar compression tests at room temperature. The majority of basal dislocations observed after micropillar compression are perfect (undissociated) screw dislocations, and they are considered to be introduced in the shuffle-set plane during micropillar testing, unlike widely dissociated dislocations introduced in the glide-set plane in the bulk during high-temperature deformation. Prism dislocations are observed also to glide as perfect (undissociated) dislocations and tend to align strongly along their screw orientation. The fracture toughness values are estimated to be 1.37 ± 0.13 and 1.57 ± 0.13 MPa m1/2 by three-point bend tests for chevron-notched single crystalline specimens with a notch plane being parallel to (0001) and {011¯0} planes, respectively.
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Aluminum alloys are widely used in engineering structures due to light weight and corrosion resistance but aluminum has low yielding and flow strengths. Here we reported super-strong Al-30 vol%SiC ...composites with a flow strength of about 1.18 GPa up to a uniform strain of 16.0%. Micromechanical tests revealed a flow strength of 0.73 GPa associated with the nano-spaced SiC nanowires strengthening, and additional flow strength of 0.45 GPa associated with high-density stacking faults (SFs) that are rarely formed and stabilized in Al due to high stacking fault energy (SFE). More surprisingly, SFs possess excellent thermal stability up to 320 °C and can be regained by thermal cooling even after they are eliminated during annealing at 600 °C. Microscopy characterizations and theoretical analysis revealed that thermal mismatch induced high stress during cooling promotes the formation of SFs, and the segregation of Si into SFs and dislocation cores enables the thermal stability of wide SFs. This work demonstrated an approach to creating high-density and thermo stable SFs in high SFE metals via microstructure-enabled thermal stress.
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•High density and thermal stable SFs can be generated in bulk aluminum by introducing nano-spaced SiC nanowires to form composites.•The formation mechanisms of SFs are associated to extended dislocations with high thermal mismatch stress and Si segregation.•SFs can significantly strengthen aluminum matrix without sacrificing plasticity and thus achieving ultra-strong Al–SiC composites.
Dislocation glide to carry plastic deformation in simple metals and alloys is a well-understood process, but the process in materials with complex crystal structures is not yet understood completely ...as it can be very complicated often involving multiple atomic planes during dislocation glide. The zonal dislocation is one of the examples predicted to operate in complex materials, and during glide it involves multiple atomic planes called shear zone, in which non-uniform atom shuffling occurs. We report direct observation made by Z-contrast atomic-resolution microscopy of the zonal dislocation in the σ phase FeCr. The result confirms the zonal dislocation indeed operates in this material. Knowledge gained on the dislocation core structure will lead to improved understanding of deformation mechanisms in this and other complex crystal structures and provide ways to improve the brittleness of these complex materials.
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We have performed in-situ micropillar compression to investigate the local strain rate sensitivity of single α phase in dual-phase Ti alloy, Ti–6Al–2Sn–4Zr–2Mo (wt%). Electron backscatter diffraction ...(EBSD) was used to identify two grains, anticipated to primarily activate a slip on the basal and prismatic plane respectively. Comparative micropillars were fabricated within single α laths and load-hold tests were conducted with variable strain rates (on the order of 10−2 to 10−4 s−1). Local strain rate sensitivity exponent (i.e. m value) is determined using two types of methods, constant strain rate method (CSRM) and conventional stress relaxation method (SRM), showing similar rate sensitivity trends but one order higher magnitude in SRM. We thus propose a new approach to analyse the SRM data, resulting in satisfactory agreement with the CSRM. Significant slip system dependent rate sensitivity is observed such that the prism slip has a strikingly higher m value than the basal. Fundamental mechanisms differing the rate sensitivity are discussed with regards to dislocation plasticity, where more resistance to move dislocations and hence higher hardening gradients are found in the basal slip. The impact of this finding for dwell fatigue deformation modes and the effectiveness of the present methodology for screening new alloy designs are discussed.
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