Plasticity of hard and brittle materials at micron-meter size scales

There are many hard materials that are considered to be candidates for structural applications under extreme conditions such as very high temperatures. This stems from the fact that many of them possess peculiar properties such as high hardness, high melting temperature, and so on. But, one of the common characteristics for these hard materials is their brittleness. They usually fail in cleavage without showing any plastic deformation at ambient temperature. So, even, fundamentals for plasticity such as operating slip systems and their CRSS values have yet to be known for many of them. If we assume that fracture in these hard materials occurs in a brittle manner at a pre-existing micro-crack, the effective defect size of the microcrack to cause fracture is believed to vary with the fracture toughness (KIC) at a given fracture stress. If the fracture stress is fixed at 1 GPa, the effective defect size of the micro-crack is calculated to be 320 nm for KIC of 1 MPam-1/2, but this value increases to about 8 mm if the KIC values is increased to 5 MPam-1/2. Then, there is a chance for these hard materials to plastically deform in the form of micropillars of the micron-meter size even at ambient temperature. We have investigated the compression deformation behavior of transition-metal silicides as typical examples of hard materials such in the micropillar form with the specimen size ranging from 0.5 to 10 mm at room temperature. Those hard materials include transition-metal (M) silicides of the M5Si3-type such as Mo5Si3, Nb5Si3 and Mo5SiB2 and those of the MSi2-type such as MoSi2, VSi2, CrSi2, NbSi2 and TaSi2. Although none of them listed above deform plastically at room temperature in the bulk form, plasticity is clearly observed at room temperature for all of them in the micropillar forms. This is very surprising in particular for transition-metal silicides of the M5Si3-type, since they usually need more than 1300°C for their plastic deformation to occur in the bulk form. Because of such a high temperature, slip systems have never been identified with confidence for these transition-metal silicides of the M5Si3-type. However, plasticity observed in the micropillar form at room temperature has made us to clearly identify their operative slip systems with their CRSS (critical resolved shear stress) values. For transition-metal silicides of the MSi2-type, slip systems operative at high temperatures in the bulk form are observed also to operate in the micropillar form at room temperature. The room-temperature bulk CRSS values for these slip systems can be obtained by extrapolating the power-law of the CRSS-specimen size dependence to the bulk size, which can be estimated to be 30-50 m. The room-temperature bulk CRSS values thus estimated are on the extension of the CRSS-temperature curve of the corresponding slip system for some silicides but not for other silicides. The origin of the latter behavior is proved to be due to a transition of deformation mechanisms

Micropillar compression deformation of single crystals of Fe₃Ge with the L1₂ structure

The plastic deformation behavior of single crystals of Fe₃Ge with the L1₂ structure has been investigated at room temperature as a function of crystal orientation by micropillar compression tests. In addition to slip on (010), slip on (111) is observed to occur in Fe₃Ge for the first time. The CRSS (critical resolved shear stress) for (111)[10overline{1}] slip, estimated by extrapolating the size-dependent strength variation to the ‘bulk’ size, is ~240 MPa, which is almost 6 times that (~40 MPa) for (010)[10overline{1}] slip similarly estimated. The dissociation scheme for the superlattice dislocation with b=[10overline{1}] is confirmed to be of the APB (anti-phase boundary)-type both on (010) and on (111), in contrast to the previous prediction for the SISF (superlattice intrinsic stacking fault) scheme on (111) because of the expected APB instability. While superlattice dislocations do not have any preferential directions to align when gliding on (010) (indicative of low frictional stress at room temperature), the alignment of superlattice dislocations along their screw orientation is observed when gliding on (111). This is proved to be due to thermally-activated cross-slip to form Kear-Wilsdorf locks, indicative of the occurrence of yield stress anomaly that is observed in many other L12 compounds such as Ni₃Al. Some important deformation characteristics expected to occur in Fe₃Ge (such as the absence of SISF-couple dissociation and the occurrence of yield stress anomaly) will be discussed in the light of the experimental results obtained (APB energies on (111) and (010) and CRSS values for slip on (111) and (010))

Direct observation of zonal dislocation in complex materials by atomic-resolution scanning transmission electron microscopy

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

Room-temperature deformation of single crystals of transition-metal disilicides (TMSi₂) with the C11b (TM = Mo) and C40 (TM = V, Cr, Nb and Ta) structures investigated by micropillar compression

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} and (0001), 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

Plastic deformation of single crystals of Pt_3Al with the L1_2 structure

The plastic deformation behaviour of single crystals of Pt3Al with the L12 structure having an off-stoichiometric composition of Pt–27 at% Al has been investigated in compression from 77 to 1073 K. The L12 structure is not stable below around 220 K, transforming into either a D0c or D0c′ structure. Slip occurs along 1 1 0 both on (001) and on (111) with slip on (001) being the primary slip system, which operates for most crystal orientations except for near [0 01], accompanied by a considerably lower CRSS (critical resolved shear stress). The CRSS tends to decrease gradually with increasing temperature for both slip in the temperature range where the L12 phase is stable, except for a moderate increase in CRSS observed above 673 K for slip on (001). Dislocations with b  = [1‾01] dissociate into two collinear superpartials with b  = 1/2[1‾01] separated by an APB on the corresponding slip plane for both slip on (001) and (111). For slip on (111), dislocations tend to align along their screw orientation at room temperature, suggesting the high Peierls stress for their motion. The possibility of showing the normal (large negative) temperature dependence of CRSS at low temperatures as well as the reason for the absence of the anomalous (positive) temperature dependence of CRSS for slip on (111) at high temperatures is discussed

Structure refinement of the δ1p phase in the Fe-Zn system by single-crystal X-ray diffraction combined with scanning transmission electron microscopy.

The structure of the δ1p phase in the iron-zinc system has been refined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy. The large hexagonal unit cell of the δ1p phase with the space group of P63/mmc comprises more or less regular (normal) Zn12 icosahedra, disordered Zn12 icosahedra, Zn16 icosioctahedra and dangling Zn atoms that do not constitute any polyhedra. The unit cell contains 52 Fe and 504 Zn atoms so that the compound is expressed with the chemical formula of Fe13Zn126. All Fe atoms exclusively occupy the centre of normal and disordered icosahedra. Iron-centred normal icosahedra are linked to one another by face- and vertex-sharing forming two types of basal slabs, which are bridged with each other by face-sharing with icosioctahedra, whereas disordered icosahedra with positional disorder at their vertex sites are isolated from other polyhedra. The bonding features in the δ1p phase are discussed in comparison with those in the Γ and ζ phases in the iron-zinc system

Room-temperature deformation of single crystals of the sigma-phase compound FeCr with the tetragonal D8b structure investigated by micropillar compression

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} 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 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

Improving the intermediate- and high-temperature strength of L12_{2}-Co3_{3}(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L1$_{2}$ compound Co$_{3}$(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L1$_{2}$ phase on the mechanical properties. The additions of Ni and Ta, both of which are L1$_{2}$-stabilizers that increase the L1$_{2}$ solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L1$_{2}$ phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L1$_{2}$ solvus temperatures. The increased strength of the L1$_{2}$ phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta

Plastic deformation of polycrystals of Co

The plastic behaviour of Co3(Al, W) polycrystals with the L12 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950 K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100 K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L12 compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co3(Al, W) and other L12 compounds, such as Ni3Al and Co3Ti, are discussed