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.
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•Measurement of silicon segregation in solution strengthened ferritic ductile iron.•Micropillar compression tests and trace analysis.•Determination of CRSS as a function of the ...silicon content in the ferrite matrix.•Comparison of the measured CRSS with macroscopic yield limits under tension and compression.
Solid solution strengthened ferritic ductile iron (SSFDI) exhibits improved mechanical properties compared to conventional ductile cast iron (DCI) grades, however, its potential widespread application is hindered by unpredictable brittle fracture which might be attributed to microstructural silicon segregation and associated superstructure formation. The aim of the present study is therefore to deepen the understanding on the effect of local silicon segregation on the mechanical properties of SSFDI, which is crucial especially for the common applications of DCI in cyclically loaded structures. Micropillar compression tests were carried out on three different casts to investigate the solution strengthening effect of silicon in the ferritic matrix. An almost perfect linear relationship between critical resolved shear stress (CRSS) and global silicon content was found. It was also found that the variation of CRSS with silicon content corresponds well to the variation of the macroscopic yield limits (under tension and compression) with global silicon content of different SSFDI alloys. This indicates that the ferritic matrix dominates the yield limit of the DCI alloys investigated in this study, while the morphology of the graphite nodules plays a minor role under monotonic loading conditions.
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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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The plastic deformation behavior of commercially pure Ti single crystals has been investigated by uniaxial micropillar compression tests as a function of crystal orientation and specimen size at room ...temperature. {101¯1} (first-order) pyramidal c+a slip and prism a slip are activated in micropillar specimens with the 0001 and 21¯1¯0 orientations, respectively. {101¯1} pyramidal c+a slip has never been observed to operate as a major deformation mode in compression tests of ‘bulk’ single crystals at room temperature, in which {112¯2}<112¯3¯> twinning is usually observed. The CRSS values for {101¯1} pyramidal c+a slip and prism a slip increase with the decrease in the specimen size, following an inverse power-law relationship with a power-law exponent of about 0.06 and 0.59, respectively. The extrapolation of the inverse power-law relationship up to the ‘bulk’ specimen size estimated from the CRSS values of prism a slip gives the ‘bulk’ CRSS value for {101¯1} pyramidal c+a slip to be 580-635 MPa, which is by far higher than those for any other deformation modes operative at room temperature.
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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 influence of solute atoms (Al and Zn) on the deformation mechanisms and the critical resolved shear stress for basal slip in Mg alloys at 298 K and 373 K was ascertained by micropillar ...compression tests in combination with high-throughput processing techniques based on the diffusion couples. It was found that the presence of solute atoms enhances the size effect at 298 K as well as the localization of deformation in slip bands, which is associated with large strain bursts in the resolved shear stress (τRSS)-strain (ε) curves. Deformation in pure Mg and Mg alloys was more homogeneous at 373 K and the influence of the micropillar size on the critical resolved shear stress was much smaller. In this latter case, it was possible to determine the effect of solute content on the critical resolved shear stress for basal slip in Mg-Al and Mg-Zn alloys.
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