In this paper, the effect of sub-zero temperatures on the activation of basal slip, {101‾2} tension twinning and <c+a> pyramidal slip in pure magnesium was investigated systematically at the micron ...length scale using micropillar compression. The micropillar compression was performed inside an SEM on selected grains of commercially pure polycrystalline magnesium between room temperature and ~ −94°C. A cryo-stub was designed and successfully employed to perform micropillar compression experiments below room temperature. Post-mortem SEM and TEM showed activation of basal slip in 112‾1 oriented micropillars, {101‾2} tension twinning in 13‾20 oriented micropillars and <c+a> pyramidal slip in 0001 oriented micropillars. CRSSs were derived from the stress-strain curves as a function of test temperature for the three deformation modes. The CRSSbasal increased with decrease in test temperature for micron-sized pillars in comparison to an opposite trend for CRSSpyra: CRSStwin showed no change from room temperature to ~ −94°C.
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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•We find that the {112} plane seems to be dominant for plasticity, although previous literature lists deformation on the {111} plane as the dominant mechanism for C15 Laves phases.•We ...could quantify the CRSS for both slip systems, namely {111}〈11¯0〉 and {112}〈11¯0〉, as approximately the same at ∼ 1 GPa.•Our TEM investigations confirm the 11¯0 Burgers vector type by using the g∙b analysis.•Together, these insights also allow us to explain why the crack and slip trace distribution around indentations varies systematically with crystal orientation.
The room temperature plasticity of the cubic C15 CaAl2 Laves phase was investigated using nanomechanical testing and electron microscopy. The correlation between slip traces in the vicinity of nanoindents and crystallographic orientation data allowed us to gain statistical data on the activated slip and crack planes for 10 different crystallographic orientations. Slip on {111} and {112} planes was found to be most favourable for all orientations, whereas cracks predominantly occurred on {112} planes. A constant hardness of 4.9 ± 0.7GPa and an indentation modulus of 85.5 ± 4.0 GPa for all investigated orientations for a constant strain rate and a strain rate sensitivity of 0.028 ± 0.019 were measured. Micropillar compression tests and transmission electron microscopy confirmed slip on {111} and {112} planes with a Burgers vector of <11¯0 > type. This allowed to determine the critical resolved shear stresses as 0.99 ± 0.03 GPa for {111}<11¯0 > and 0.97 ± 0.07 GPa for {112}<11¯0 > slip.
The deformation behavior of single crystals of Mo5SiB2 (so-called T2 phase) with the tetragonal D8l structure has been investigated by micropillar compression at room temperature as a function of ...crystal orientation and specimen size. The dissociation scheme and glide plane (actual atomic layers) of the identified dislocations have been investigated both experimentally through atomic-resolution scanning transmission electron microscopy imaging of their core structures and theoretically by first-principles calculations of the relevant generalized stacking fault energy curves. Although plastic flow is observed only above 1500 °C even for single crystals in the bulk form, plastic flow is observed at room temperature in a wide range of crystal orientation in the micropillar form, and three different slip systems, (001) , {110}<11¯0> and {01¯1} , are identified to be operative in Mo5SiB2 at room temperature. Although (001) slip was predicted by calculation in the past, the other two slip systems are identified for the first time. The values of critical resolved shear stress (CRSS) for the three slip systems are extremely high all exceeding 2 GPa. The CRSS value for each slip system increases with the decrease in the specimen size, following the inverse power-law relationship with an exponent much smaller than those reported for FCC and BCC metals. The dislocation on (001) and the <11¯0> dislocation on {110} are observed to dissociate into two collinear partials on their slip plane, while the 1/2 dislocation on {01¯1} does not make any apparent dissociation, all of which are consistently confirmed by experiment and theoretical calculation.
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An interstitial carbon induced-FCC-Ti was observed in a carbon-doped Ti37Nb28Mo28-C7 (atomic percentage) complex concentrated alloys (TNMC alloys). Not only the true crystal structure of this FCC-Ti ...was directly characterized using Cs-corrected STEM, but also the mechanical properties were tested by nanoindentation and micro-pillar compression for the first time. The FCC-Ti exhibited a high nano hardness of 17.8 GPa and a high elastic modulus of 233.1 GPa. The micropillar compression tests demonstrated the ultrahigh strengths of the interstitial C induced FCC-Ti (4.48 GPa at 1 1 1 direction, 2.67 GPa at 1 0 1 direction, and a CRSS of 1.12 ± 0.07 GPa). The nature of this FCC allotropic transformation was determined to be a rearrangement of lattice structure caused by interstitial C atoms, based on ab-initio studies. Findings in this study provide insights into the characteristics of FCC-Ti as well as the role of interstitial atoms in RCCAs which could open up unlimited possibilities in future material designing.
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Plasticity in body-centred cubic (BCC) metals, including dislocation interactions at grain boundaries, is much less understood than in face-centred cubic (FCC) metals. At low temperatures additional ...resistance to dislocation motion due to the Peierls barrier becomes important, which increases the complexity of plasticity. Iron-silicon steel is an interesting, model BCC material since the evolution of the dislocation structure in specifically-oriented grains and at particular grain boundaries have far-reaching effects not only on the deformation behaviour but also on the magnetic properties, which are important in its final application as electrical steel. In this study, two different orientations of micropillars (1, 2, 4 µm in diameter) and macropillars (2500 µm) and their corresponding bi-crystals are analysed after compression experiments with respect to the effect of size on strength and dislocation structures. Using different experimental methods, such as slip trace analysis, plane tilt analysis and cross-sectional EBSD, we show that direct slip transmission occurs, and different slip systems are active in the bi-crystals compared to their single-crystal counterparts. However, in spite of direct transmission and a very high transmission factor, dislocation pile-up at the grain boundary is also observed at early stages of deformation. Moreover, an effect of size scaling with the pillar size in single-crystals and the grain size in bi-crystals is found, which is consistent with investigations elsewhere in FCC metals.
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Basal slip and {011¯2} twinning are two major plastic deformation mechanisms in hexagonal closed-packed magnesium. Here we quantify the critical stresses associated with basal slip and twinning in ...single-crystal and bi-crystal magnesium samples by performing in situ compression of micropillars with different diameters in a scanning electron microscope. The micropillars are designed to favor either slip or twinning under uniaxial compression. Compression tests imply a negligible size effect related to basal slip and twinning as pillar diameter is greater than 10 μm. The critical resolved shear stresses are deduced to be 29 MPa for twinning and 6 MPa for basal slip from a series of micropillar compression tests. Employing full-field elasto-visco-plastic simulations, we further interpret the experimental observations in terms of the local stress distribution associated with multiple twinning, twin nucleation, and twin growth. Our simulation results suggest that the twinning features being studied should not be close to the top surface of the micropillar because of local stress perturbations induced by the hard indenter.
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•Plastic deformation behavior of α-Mn single crystals was investigated by micropillar compression at room temperature.•α-Mn single crystals are found to plastically deform by dislocation motion at ...room temperature, accompanied by very high yield stresses of the range of 4–6 GPa.•Slip along 111 and 001 are identified to operate.•Any low-indexed slip planes cannot be assigned for both slip, because of the significantly wavy nature of slip lines.
The deformation behavior of single crystals of α-manganese has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. When the specimen size is reduced to the micrometer-range, single crystals of α-manganese are found to plastically deform by dislocation motion at room temperature for the first time, accompanied by very high yield stresses of the range of 4–6 GPa. Slip along 111 and 001 are identified to operate for compression axis orientations near 001 and near 011 and 1¯11, respectively. Any low-indexed planes cannot be designated as the slip plane for both slip along 111 and 001, because of the significantly wavy nature of slip lines caused by the occurrence of frequent cross-slip. Slip along 111 tends to prefer the slip plane of {112} rather than {110}. Slip along 001, on the other hand, tends to occur on the maximum resolved shear stress plane. The 1/2111 dislocation carrying slip along 111 moves as a perfect dislocation without dissociating into partials and does not have any preferred orientation. The 001 dislocation carrying slip along 001 also moves as a perfect dislocation without dissociating into partials. Although the Peierls stress for the motion of these dislocations must be very high, there seems no deep Peierls valleys along particular directions, unlike the screw direction for the 1/2111 dislocation in body-centered cubic metals.
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The room-temperature deformation behavior of single crystals of transition-metal (TM) disilicides with the tetragonal C11b (TM=Mo) and hexagonal C40 (TM = V, Cr, Nb and Ta) structures has been ...investigated by micropillar compression as a function of specimen size, paying special attention to the deformation behavior of the equivalent slip ({110}<1¯11> and (0001)<21¯1¯0>, respectively for the two structures). In contrast to bulk single crystals, in which high temperature at least exceeding 400 °C is usually needed for the operation of the equivalent slip, plastic flow is observed by the operation of the equivalent slip at room temperature for all these TM disilicides in the micropillar form. The critical resolved shear stress (CRSS) value exhibits the ‘smaller is stronger’ behavior following an inverse power-law relationship for all these TM disilicides. The bulk CRSS values at room temperature estimated from the specimen size dependence are 620 ± 40, 240 ± 20, 1,440 ± 10, 640 ± 20 and 1,300 ± 30 MPa for MoSi2, VSi2, CrSi2, NbSi2 and TaSi2, respectively. Transmission electron microscopy reveals that the equivalent slip at room temperature occurs by a conventional shear mechanism for all TM disilicides, indicating the change in deformation mechanism from synchroshear in bulk to conventional shear in micropillars occurs in CrSi2 with decreasing temperature.
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