The plastic deformation behavior of single crystals of the quinary, equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy (HEA) with the face-centered cubic structure has been investigated in tension and ...compression as a function of crystal orientation and temperature from 10 K to 1373 K. The critical resolved shear stress (CRSS) for {111} slip at room temperature is 42−45 MPa. It does not depend much on crystal orientation (i.e., the Schmid law holds true) and the sense (tension vs. compression) of the applied load. The CRSS for {111} slip increases with the decrease in temperature, without showing any significant inertia effects at cryogenic temperatures below 77 K. Extrapolation from the measured yield stresses down to 10 K yields a CRSS value at 0 K of 168 MPa. At cryogenic temperatures, the measured strain-rate sensitivity of flow stress is consistent with a very small activation volume. The concept of stress equivalence holds true both for the temperature dependence of CRSS and the stress dependence of activation volume, indicating that solid-solution hardening is the major strengthening mechanism. Deformation twinning occurs at 77 K but not at room temperature, resulting in higher tensile elongation to failure at 77 K than at room temperature. Deformation twinning at 77 K occurs at a shear stress of 378 MPa on conjugate (1¯1¯1) planes in the form of Lüders deformation after large plastic strain (about 85%) achieved by the stage I (easy glide) and stage II (linear work-hardening) deformation.
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The anisotropic deformation behavior of single crystals of Ti3SiC2 has been investigated at room temperature as a function of loading axis orientation and specimen size by micropillar compression ...tests. Basal slip is found to be only the operative slip system at room temperature. The a/312¯10 basal dislocation responsible for basal slip dissociates into two partial dislocations of the Shockley-type and glides between Ti1 and Si atomic layers, as confirmed both experimentally and theoretically. The CRSS value for basal slip increases with the decrease in the specimen size, following an inverse power-law relationship with a very large power-law exponent of about 1.0, which is almost the upper limit of those reported for FCC metals. When the loading axis is parallel to the basal plane, kink band formation occurs only after the occurrence of basal-plane delamination and the subsequent activation of a-dislocations gliding on the basal plane. Kink bands consists of a/312¯10 basal edge dislocations of the same sign, each of which had glided between Ti1 and Si atomic layers. These results indicates that the kink formation can simply be interpreted by the classical dislocation-based kink-formation model.
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The deformation behaviors of rectangular-parallelepiped micropillars fabricated from 001-oriented Ni-based superalloys with negative and positive lattice misfits, respectively, were investigated with ...respect to the specimen size. The yield stresses of the larger specimens (side length L > ∼2 μm) of both considered alloy types were found to be nearly independent of the size, while the smaller specimens (L < ∼2 μm) exhibited “smaller is stronger” and “smaller is (slightly) weaker” trends for negative and positive lattice misfits, respectively. The internal stresses calculated based on the micro-elasticity theory revealed that the internal shear stress fields of the micropillars varied with their sizes. The size dependence of the calculated internal shear stress distribution agreed fairly well with the experimentally determined size dependence of the yield stress, indicating that the distribution of the internal misfit stress significantly contributed to the size-dependent yield stress of the smaller micropillars.
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Abstract
Micropillar compression tests of Ti
5
Si
3
single crystals were conducted at room temperature as a function of loading axis orientation and specimen size in order to investigate their room ...temperature plastic deformation behavior. Plastic flow by the operation of three deformation modes, {1
$${\overline{1}}$$
1
¯
00}0001, {2
$${\overline{1}}$$
1
¯
$${\overline{1}}$$
1
¯
2} < 2
$${\overline{1}}$$
1
¯
$${\overline{1}}$$
1
¯
$${\overline{3}}$$
3
¯
> and {1
$${\overline{1}}$$
1
¯
01} < 2
$${\overline{1}}$$
1
¯
$${\overline{1}}$$
1
¯
$${\overline{3}}$$
3
¯
> slip were observed in 2
$${\overline{2}}$$
2
¯
05-, 0001- and 4
$${\overline{3}}$$
3
¯
$${\overline{1}}$$
1
¯
0-oriented micropillar specimens deformed at room temperature, respectively. The CRSS values were evaluated to be very high above 2.7 GPa and were confirmed to increase up to about 6 GPa with the decrease in the specimen size. The fracture toughness values are evaluated to be 0.45 MPa m
1/2
(notch plane // (0001)) and 0.73 MPa m
1/2
(notch plane //(1
$${\overline{1}}$$
1
¯
00)) based on the results of micro-cantilever bend tests of chevron-notched specimens. The fracture toughness values are considerably lower than those for D8
l
-Mo
5
SiB
2
and D8
l
-Nb
5
Si
3
evaluated by the same method, indicating the inherent brittleness of binary Ti
5
Si
3
compared to the other transition-metal silicides of the TM
5
Si
3
type (TM: transition-metal).
Atom probe tomography (APT) and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) have been used correlatively to explore atomic-scale local structure and ...chemistry of the exactly same area in the vicinity of growth front of a long-period stacking ordered (LPSO) phase in a ternary Mg-Al-Gd alloy. It is proved for the first time that enrichment of Gd atoms in four consecutive (0001) atomic layers precedes enrichment of Al atoms so that the formation of Al
Gd
clusters occurs only after sufficient Al atoms to form Al
Gd
clusters diffuse into the relevant portions. Lateral growth of the LPSO phase is found to occur by 'ledge' mechanism with the growth habit plane either {1Formula: see text00} or {11Formula: see text0} planes. The motion of ledges that give rise to lateral growth of the LPSO phase is considered to be controlled by diffusion of Al atoms.
Since the high entropy concept was proposed at the beginning of the millennium, the research focus of this alloy family has been wide ranging. The initial search for single-phase alloys has expanded ...with the aim of improving mechanical properties. This can be achieved by several strengthening mechanisms such as solid-solution hardening, hot and cold working and precipitation hardening. Both single- and multiphase high- and medium-entropy alloys can be optimized for mechanical strength via several processing routes, as is the case for conventional alloys with only one base element, such as steels or Ni-based superalloys.
Graphical abstract
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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Based on recent findings in ultrafine grained metals, we proposed a strategy for overcoming the strength-ductility trade-off in structural metallic materials. Sequential nucleation of different ...deformation modes, such as unusual slip systems, deformation twinning, martensitic transformation, etc., would regenerate strain-hardening ability of the material, leading to high strength and large tensile ductility. For discussing the activation of different deformation modes in atomistic scales, the concept of plaston which considered local excitation of atoms under singular dynamic fields was proposed.
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Abstract
The plastic deformation behavior of single crystals of two transition-metal diborides, ZrB
2
and TiB
2
with the AlB
2
structure has been investigated at room temperature as a function of ...crystal orientation and specimen size by micropillar compression tests. Although plastic flow is not observed at all for their bulk single crystals at room temperature, plastic flow is successfully observed at room temperature by the operation of slip on {1
$${\bar{1}}$$
1
¯
00}<11
$${\bar{2}}$$
2
¯
3> in ZrB
2
and by the operation of slip on {1
$${\bar{1}}$$
1
¯
00} and {1
$${\bar{1}}$$
1
¯
00}<11
$${\bar{2}}$$
2
¯
0> in TiB
2
. Critical resolve shear stress values at room temperature are very high, exceeding 1 GPa for all observed slip systems; 3.01 GPa for {1
$${\bar{1}}$$
1
¯
00}<11
$${\bar{2}}$$
2
¯
3> slip in ZrB
2
and 1.72 GPa and 5.17 GPa, respectively for {1
$${\bar{1}}$$
1
¯
00} and {1
$${\bar{1}}$$
1
¯
00}<11
$${\bar{2}}$$
2
¯
0> slip in TiB
2
. The identified operative slip systems and their CRSS values are discussed in comparison with those identified in the corresponding bulk single crystals at high temperatures and those inferred from micro-hardness anisotropy in the early studies.
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