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

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

Minor additions of Sn suppress the omega phase formation in beta titanium alloys

A critical characteristic of β-Ti alloys is the inevitable formation of ω-precipitates during certain heat treatments which leads to embrittlement, or even to a complete loss of ductility. Therefore, alloy design with the goal to inhibit the elementary ω-formation process is of utmost importance. Here, we propose a design strategy for prototypical β-type Ti–Cr–(Mo) alloys to alleviate this problem using only minor additions of Sn. Upon addition of Sn, we observed an extensive deceleration or even suppression of the ω-formation kinetics during isothermal ageing. Furthermore, the internal friction response of the elementary formation process indicated a decisive reduction of potential ω-nucleation sites, while the activation energy of the process remained almost unchanged. The results show, that the addition of Sn can significantly increase the width of the time–temperature process window and the long-time ageing resistance of β-Ti alloys, opening up huge opportunities for advanced alloy design and manufacturing routes.</p

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

Influence of Oxygen on the Kinetics of Omega and Alpha Phase Formation in Beta Ti–V

A detailed understanding of the kinetics of phase formation in β-stabilised titanium is of decisive importance for the applicability of these materials. However, the complex nature and long timescales of the various transformations, calls for specialized measurement techniques. In this work high-stability isothermal laser dilatometry is used to study the temporal volume changes associated with the various phase formation processes. Distinctly different behaviours between samples of Ti–21 at. pct V with different solute oxygen content could be detected and quantified. Temperature regimes for both diffusionless and diffusion-assisted isothermal ω-formation as well as for ω-to-α-transformation were determined. Low oxygen contents promote the diffusionless ω-formation mechanism, but retard the diffusion-assisted one as well as the ω-to-α-transformation process. The results confirm recent findings of a clear distinction between the diffusionless and diffusion-assisted isothermal ω formation modes. Modelling of the ω-phase formation applying Austin–Rickett kinetics revealed the temperature-dependent formation rates, on the basis of which the isothermal TTT-diagrams were developed which reflect the strong influence of the oxygen content.</p

Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations

In titanium alloys, the ω(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.Tane M., Nishiyama H., Umeda A., et al. Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations. Physical Review Materials 3, 043604 (2019); https://doi.org/10.1103/PhysRevMaterials.3.043604

Crystal structure and thermoelectric properties of chimney–ladder compounds in the Ru2Si3–Mn4Si7 pseudobinary system

Phase relationships of manganese-substituted ruthenium sesquisilicide alloys have been investigated by using X-ray powder diffraction and scanning and transmission electron microscopy. A series of chimney–ladder phases Ru1−xMnxSiy (0.14 less-than-or-equals, slant x less-than-or-equals, slant 0.97, 1.584 less-than-or-equals, slant y less-than-or-equals, slant 1.741) are formed over a wide compositional range between Ru2Si3 and Mn4Si7. The compositions of these chimney–ladder compounds deviate slightly from the composition line connecting Ru2Si3 and Mn4Si7, which corresponds to the ideal composition line satisfying VEC (valence electron counting) = 14 rule. The occurrence of this compositional deviation is discussed in terms of the VEC rule and the atomic packing. The thermoelectric properties of the directionally solidified Ru1−xMnxSiy alloys (0.55 less-than-or-equals, slant x less-than-or-equals, slant 0.90) have also been investigated as a function of the Mn content and temperature. The dimensionless figure of merit (ZT) for those alloys with a high Mn content (x greater-or-equal, slanted 0.75) increased with the increase in Mn content. The ZT value for a crystal with x = 0.90 was as high as 0.76 at 874 K

Anisotropic elastic constants and thermal expansivities in monocrystal CrB2, TiB2, and ZrB2

The elastic constants and thermal expansivities in monocrystals of three transition-metal diborides with the AlB2 structure, CrB2, TiB2, and ZrB2, have been investigated in the temperature ranges from 300 to 1373 K and from 300 to 1073 K. The anisotropic parameters deduced from the measured elastic constants and thermal expansivities indicate that of the three diborides, the anisotropy is the most and least significant in CrB2 and ZrB2, respectively. The factors determining the significance in anisotropy in atomic bonding in AlB2-type diborides are analyzed by an approach similar to the valence-force-field method and are discussed in terms of the deformation of the electronic charge around the metal atoms occurring to fit themselves in the (0 0 0 1) basal plane