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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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.
•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 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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The deformation behavior of single crystals of the sigma-phase compound FeCr with the tetragonal D8b structure has been investigated by micropillar compression at room temperature as a function of ...crystal orientation and specimen size. In spite of the repeatedly reported brittleness, plastic flow is observed at room temperature for all loading axis orientations tested. Three slip systems, {100}001, {100} and {111}<01¯1> are newly identified to be operative at room temperature depending on the loading axis, in addition to {110}001 slip we previously identified. The CRSS values for all the identified slip systems are very high exceeding 1.3 GPa and decrease with increasing specimen size, following an inverse power-law relationship with a very small power-law exponent. Similarly to {110}001 slip, {100}001 slip is confirmed to be carried by the motion of 001 zonal dislocations through atomic-resolution scanning transmission electron microscopy imaging of their core structures. dislocations gliding on {100} are confirmed to dissociate into two collinear partial dislocations, while <01¯1> dislocations gliding on {111} to dissociate into three collinear partial dislocations. The fracture toughness values estimated by micro-cantilever bend tests of chevron-notched micro beam specimens are indeed very low, 1.6∼1.8 MPa·m1/2 (notch plane // (001) and (100)), indicating significant brittleness of sigma FeCr.
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