C11b Phase Research Articles

Effect of Fe and Mn on structure, mechanical properties, and oxidation resistance of lightweight intermetallic Al55Cr23Ti22 complex concentrated alloy

In this study, we analysed the effect of Fe, Mn, or Fe and Mn additions (5 or 10 at%) on the structure, mechanical properties, deformation behaviour, including microstructure evolution, and oxidation resistance of a lightweight (density of 3.92 g/cm3) intermetallic Al55Cr23Ti22 complex concentrated alloy (CCA) with a L12 + C11b structure. The additions of Fe, Mn, or Fe and Mn (at the expense of Cr) retained the density below 4 g/cm3 and resulted in forming a D8a phase (Th6Mn23-prototype; cF120; Fm-3m) in all the alloys, except for the Al55Cr18Ti22Mn5 alloy. The D8a phase nucleated with a different morphology within or adjacent to the C11b phase, and adopted an orientation relationship of (110)C11b || (4‾4‾ 0)D8a, [3 3‾ 1]C11b || [1‾ 11‾]D8a, which provided coherent C11b/D8a interfaces. Alloying with Mn or Fe and Mn had either neutral or negative effect on the mechanical properties, while the Fe additions boosted the strength along with some decrease in the compressive plasticity at room temperature. Specifically, the Al55Cr13Ti22Fe10 alloy showed a yield strength of 250 MPa at 1000°С, which was 60 % higher compared to the Al55Cr23Ti22 alloy. During plastic deformation, all the alloys demonstrated pronounced strain hardening at T ≤ 800 °C and steady state flow at 1000 °C. Post-deformation observation of the microstructure of the Al55Cr13Ti22Fe10 alloy showed the inhomogeneous distribution of the strain-induced defects between the L12 matrix and (C11b + D8a) regions at 800 °C and partial recrystallisation of these phases at 1000 °C. All the alloys exhibited complex oxidation behaviour with multistage oxidation kinetics. The addition of 5 at% of Fe or Mn was beneficial for the oxidation resistance, while the further increase in their contents intensified the mass gain after a certain time. The latter effect was more pronounced in the Al55Cr13Ti22Mn10 alloy than in the Al55Cr13Ti22Fe10 alloy, because of forming a loose Al2O3 oxide layer. All the alloys investigated exhibited a superior synergy of 1000°C-specific yield strength and compressive plasticity at room temperature, as well as a lower mass gain after 100 h at 1000 °C, compared to both lightweight and the most oxidation-resistant refractory CCAs.

A lightweight intermetallic Al-Cr-Ti medium-entropy alloy with good mechanical properties and oxidation resistance

Structure, mechanical properties, and oxidation behaviour of a Al55Cr23Ti22 (in at%) lightweight (ρ = 3.92 g/cm3) medium-entropy intermetallic alloy were studied. The initial as-cast structure consisted of an (Al, Ti)-rich L12 matrix and Cr-rich C11b lamellae with semi-coherent interfaces due to a (111)L12 || (110)C11b, [11̅0]L12 || [33̅1̅]C11b orientation relationship. The alloy demonstrated a combination of good plasticity (17–19%) and strength (600–610 MPa) both at 22 and 600 ℃; the latter gradually decreased to 155 MPa at 1000 ℃, while plastic strain exceeded 40%. Deformation at 600 and 800 ℃ led to the precipitation of (semi-) coherent C11b_1 and Al2Ti particles inside the L12 matrix and C11b phase, respectively, due to elemental redistributions within the initial phases and low lattice mismatch with a matrix. The alloy gained only 0.27 mg/cm2 after oxidation in the air at 1000 ℃ for 100 h thanks to an early formation of a protective Al2O3 layer. Compared to the current state-of-art lightweight and Al/Cr-containing refractory high/medium-entropy alloys, the Al55Cr23Ti22 alloy had a higher 1000 ℃-specific strength, room-temperature plastic strain, and demonstrated the best oxidation resistance. The obtained results suggest that the multicomponent L12-based trialuminides can be used as structural materials for potential high-temperature applications.

Criteria of predicting phase formation for MSi2-Type High-Entropy refractory metal silicides

MSi2-type high-entropy refractory metal silicides (HERMS) have excellent oxidation resistance at medium and high temperatures due to high entropy effect, which is a promising ultrahigh temperature structural material. HERMS have two phase structures, C11b and C40, due to allotropic transformation. Compared to C40, C11b has better fracture toughness, but it is now difficult to synthesize single-phase C11b. The first-principle was used to systematically study phase formation ability of HERMS. Formation enthalpy (ΔHf), mixing enthalpy (ΔHmix) and mixing enthalpy difference (δmix) were calculated for 504 MSi2-type HERMS structures. δmix, VEC and ΔHmixC11b were identified to characterize and predict phase stability of MSi2-type HERMS, in which δmix is a decisive factor, and has general applicability and important guiding significance for material systems with allotropic transformation. The generalized criteria of predicting phase formation for MSi2-type HERMS were proposed. C11b phase formation criterion is VEC > 4.58, δmix < −0.05 eV/atom, ΔHmixC11b < −0.02 eV/atom. C40 phase formation criterion is VEC ≤ 4.58 and ΔHmixC11b > 0.09 eV/atom. Its accuracy was verified by experimental data to be 100%. This work is of great significance for composition design of HERMS and the further study of C11b-type HERMS.

Influence of Ti addition on the precipitation behavior and mechanical properties of Ni-7.9Cr-5.9W-22.2Mo superalloy

The Ni-7.9Cr-5.9W-22.2Mo-1.4Ti superalloy based on Ni-8Cr-6W-22.5Mo (the Haynes 244 alloy) was prepared to investigate the effect of Ti addition on the precipitation behavior and mechanical properties. The results indicate that the addition of Ti can facilitate the formation of C11b and D022 superlattice phases simultaneously after age-treated at 650 °C for 240 h. The yield strength is significantly improved to 1645 MPa compared with Ni-8Cr-6 W-22.5Mo with only the formation of C11b and a yield strength of 1297 MPa. The excellent strengthening effect of C11b and D022 superlattice phases are attributed to both order strengthening and coherency strengthening, and their calculated contributions to strengthening are 148 MPa and 439 MPa for D022 and 83 MPa and 208 MPa for and C11b, respectively. Together with solid-solution strengthening and grain refinement strengthening, the theoretically calculated yield strength matches well with the experimental results. In addition, the deformation mode changes from deformation slip to strain-induced twinning after the Ti addition.

Structural transitions at high pressure and metastable phase in Si0.8Ge0.2

The high-pressure behaviour of Si0.8Ge0.2 alloy is explored using in situ Raman spectroscopy, X-ray diffraction techniques and density functional theory (DFT) simulations. High pressure experiments revealed a pressure-induced transition from the stable cubic semiconducting phase (dc- Si0.8Ge0.2) to the tetragonal β-tin metallic phase (β-Si0.8Ge0.2) during compression. This sluggish transition is significantly accelerated at moderate temperature (<300 °C). Upon decompression, successive transitions towards metastable phases are observed. A first transition from the metallic β-Si0.8Ge0.2 toward the rhombohedral r8-Si0.8Ge0.2 phase is observed at 10.3 GPa followed by a partial transition to the body-centered cubic bc8-Si0.8Ge0.2 phase at 2.2 GPa. After releasing the pressure, r8 and bc8 phases coexist at ambient conditions. This transition pathway is similar to that followed by pure silicon and is consistent with the ab initio enthalpy calculations. This phase transition sequence is confirmed by in situ Raman spectroscopy, where signatures of r8 and bc8 phases are observed in the Raman spectra at decompression. An ab initio simulation method is proposed to assign the Raman spectrum of Si0.8Ge0.2 alloy using group theory and projection operators. The exploration of metastable states in these alloys is of major interest both in terms of applications (e.g. optoelectronics) and from a fundamental point of view to better understand the effects of alloying on the physical properties (e.g. vibrational).

Towards understanding the mechanism of vibration-assisted cutting of monocrystalline silicon by cyclic nanoindentation

Ultrasonic vibration cutting (UVC) is a promising technology that can promote the processing of hard-brittle materials in ductile mode. The feature of intermittent cutting in UVC is considered to promote the ductile responses of brittle materials. However, the effect of cyclic loading/impact on microscopic deformation behavior of the workpiece material in UVC is not clearly understood. This study used cyclic nanoindentation to reveal the UVC mechanism in monocrystalline silicon (Si). The surface morphology and subsurface microstructure of the residual indents were characterized and compared with those of the UVC-machined grooves. This work attempted to correlate the tool-workpiece contacts for cyclic nanoindentation and vibration cutting. Cyclic nanoindentation results show that cyclic loading promotes the extent of silicon amorphization and decreases the threshold for transitioning the β–Sn phase to the bc8 and r8 phases. As the number of indentation cycles increases, the subsurface microstructure changes from the amorphous phase to the bc8 and r8 phases. In UVC, both the critical cutting depth for brittle-to-ductile transition and the degree of amorphization in cutting chips increase in comparison to conventional cutting, which matches the trend of cyclic nanoindentation results. The findings inform the chip formation and subsurface deformation mechanisms of brittle materials during UVC.

Model of ramp compression of diamond from ab initio simulations

Ramp compression experiments characterize high-pressure states of matter at temperatures well below those present in shock compression. However, because temperature is typically not directly measured during ramp compression, it is uncertain how much heating occurs under these shock-free conditions. Here, we performed a series of ab initio simulations on carbon in order to match the density-stress measurements of Smith et al. [Smith, et al., Nature 511, 330 (2014)]. We considered isotropically as well as uniaxially compressed solid carbon in the diamond and BC8 phases, with and without defects, as well as liquid carbon. Our idealized model ascribes heating during ramp compression to an initially uniaxially compressed cell transforming isochorically into an isotropically (hydrostatic equivalent) compressed state having lower internal energy, hence higher temperature so as to conserve energy. Multiple such heating events can occur during a single ramp experiment, leading to higher temperatures than with isentropic compression. Comparison with experiments shows that heating alone does not explain the equation of state measurements on diamond, instead implying that a significant uniaxial stress component remains present at high compression. The temperature predictions of our ramp compression model remain to be verified with future laboratory measurements.

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25–165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure–temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Body centered cubic carbon BC14: An all- sp3 bonded full-fledged pentadiamond

We report on the finding of a hard carbon structure in body centered cubic $(I{2}_{1}3)$ symmetry that possesses an extremely high bulk modulus (386 GPa) and Vickers hardness (60 GPa) comparable to that of $c$-BN and diamond. This carbon phase has a 14-atom primitive cell in an all-${sp}^{3}$ bonding network comprising five-membered rings, making it a truly full-fledged pentadiamond. Total energy calculations show that it is more stable than the previously reported diamondlike six-membered-ring bonded BC8 and BC12 carbon phases. Electronic band structure calculations show that it is an insulator with an indirect band gap of 5.64 eV. Simulated x-ray diffraction patterns and lattice parameters provide an excellent match to the previously unexplained distinct diffraction peaks found in carbon soot. The present results establish a distinct type of carbon phase and offer insights into its outstanding structural, mechanical, and electronic properties.

Formation of metastable bc8 phase from crystalline Si0.5Ge0.5 by high-pressure torsion

Si0.5Ge0.5 crystals were subjected to severe plastic deformation using high-pressure torsion (HPT). A metastable body-centered-cubic phase having a bc8 structure was formed by HPT processing with anvil rotations. High-resolution transmission electron microscopy analysis revealed that nanograins having diamond-cubic (dc) and bc8 phases were present in the HPT-processed sample. The bc8 phase was reverse transformed back to the dc phase after annealing. The resistivity of the crystalline Si0.5Ge0.5 (~9 × 10−3 Ω·cm) increased by roughly a factor of 6 after compression, and remained on the same order of magnitude but decreased slightly after 10 anvil rotations despite the grain refinement. These results indicate that the electrical behavior of bc8-Si0.5Ge0.5 is similar to that of bc8-Si.

First-principles investigation of the structural, dynamical, electronic, and elastic properties of WGe2 and W5Ge3

ABSTRACTWe have investigated the structural, dynamical, elastic, and electronic properties of WGe2 and W5Ge3 compounds in different phases. We have considered the C11b (tetragonal, space group I4/mmm) and C23 (orthorhombic, space group Pnma) strukturbericht phases for WGe2 compound and D81 (tetragonal, space group I4/mcm), D8m (tetragonal space group I4/mcm) strukturbericht phases for W5Ge3 compound. The structural parameters, formation enthalpies, phonon dispersion curves, elastic constants, mechanical modulus, anisotropic factors, thermal conductivities, and electronic structures have been investigated using generalised gradient approximation within in the plane wave pseudopotential density functional theory. The calculated lattice constants are in a good agreement with the experimental data. The considered phases for WGe2 and W5Ge3 compounds have a metallic character. The results indicated that all phases for compounds are both mechanically stable and dynamically stable except for W5Ge3-D81. The anisotropy in some mechanical modulus has been investigated using several elastic anisotropy indexes and directional dependence of compressibility, Young’s moduli, shear moduli, and Poisson’s ratio.

In-situ high temperature micro-Raman investigation of annealing behavior of high-pressure phases of Si

Among the 13 polymorphic phases of Si, the ambient temperature stable body-centered cubic (bc8) and rhombohedral (r8) polymorphs have gained significant interest due to their attractive optical and electronic properties suitable for photovoltaic applications. Though ex situ methods were extensively employed previously to understand the pressure-induced phase transformation kinetics of Si, the limited number of available in situ studies has significantly improved the knowledge in this field and clarified uncertainties. Similarly, in this article, we attempt to understand the thermal annealing behavior of nanoindentation-induced r8 and bc8 phases of Si and their volume dependence using in situ high temperature micro-Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). A spherical diamond indenter of ∼20-μm radius was chosen to indent diamond cubic (dc) Si (100) at different peak loads (Pmax) to create different volumes of high-pressure phases. The Raman spectra, Raman imaging, and XTEM of the pre- and postannealed indents confirm complete annealing of r8/bc8 phases at 200 ± 10 °C, irrespective of the volume of indents. In contrast to the previous ex situ studies, no signature of the presence of the hexagonal diamond (hd)-Si phase was found at elevated temperatures and the overall transformation observed is directly from r8 → polycrystalline dc-Si and bc8 → polycrystalline dc-Si rather than through other metastable phases such as Si-XIII/hd-Si. The present systematic in situ study provides evidence for a few earlier predictions and clarifies ambiguities involved in understanding the annealing behavior and transformation pathways of two high-pressure phases of Si at elevated temperatures.

High-Temperature Deformation Behavior of (Mo&lt;sub&gt;0.85&lt;/sub&gt;Nb&lt;sub&gt;0.15&lt;/sub&gt;)Si&lt;sub&gt;2&lt;/sub&gt; Crystals with C40/C11&lt;sub&gt;b&lt;/sub&gt; Lamellar Microstructure

The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11b lamellar-structured (Mo0.85Nb0.15)Si2 silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11b duplex-phase crystals. The variant-1-type C11b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.

Ordering Transformation and Age Hardening in a Ni-Cr-W Superalloy

The microstructural changes occurring in a Ni-Cr-W superalloy during prolonged exposure to proper temperature have been investigated using transmission electron microscopy. It is demonstrated that nanometer-sized C11b (Pt2Mo-type) and DO22 superlattices can precipitate in the Ni-Cr-W alloy by means of a simple aging treatment at temperatures varying in the range of 773 K to 973 K (500 °C to 700 °C). The mechanism of transformation to long-range order has been revealed to accord with continuous mode based on transmission electron microscopy results and variation trend in Vickers microhardness. No signs of overaging and coarsening of C11b and DO22 phases with further aging have been found, which indicates that both of them have a high-thermal stability. The orientation relationships and interfaces between C11b/DO22 precipitates and Ni-based matrix have been investigated by high-resolution transmission electron microscopy, and the results reveal that the interfaces between C11b/DO22 precipitates and surrounding matrix are coherent at the atomic scale. Because of the high-density nanometer-sized C11b/DO22 precipitates, the microhardness of the alloy is improved remarkably.

Creep-deformation behavior of (Mo0.85Nb0.15)Si2 lamellar-structured C40/C11b two-phase crystals

The creep-deformation behavior of (Mo0.85Nb0.15)Si2 crystals that were composed of C40 and C11b two-phases with an oriented lamellar microstructure was examined over 1200–1400 °C under compression. The creep behavior of the crystals varied greatly depending on the orientation of the loading axis with respect to the lamellar interfaces. The steady-state creep strain rate (SSCR) when the loading orientation was parallel to the lamellar interfaces (0°-orientation) was approximately 2 orders of magnitude lower than that when the loading orientation was inclined by 45° (45°-orientation). Creep tests were performed also with single crystals of C40-single-phase (Mo0.85Nb0.15)Si2, and proved that the C40-phase effectively acts as a strengthening phase in the creep behavior of the C40/C11b duplex-phase crystals. The variant-1-type C11b phase grains, which have a loading orientation parallel to [001], were also found to act as an effective strengthening component. The SSCR of the 0°-orientation was succeeded to be further decreased by controlling the lamellar microstructure via the addition of Cr. The SSCR of the 1 at.%-Cr-added lamellar-structured crystal under 300 MPa in the 0°-orientation was approximately a third of that of the non-added ternary crystal.

Microstructure and fracture toughness in boron added NbSi2(C40)/MoSi2(C11b) duplex crystals

The effect of B-addition on the microstructure and fracture toughness of (Mo0.85Nb0.15)Si2 crystals with an oriented lamellar microstructure was investigated. B-addition led to an increase in the volume fraction of the C11b phase, which possesses different orientation relationship from that of the fine lamellae, and a reduction in their precipitation rate. The fracture toughness of the B-added crystal with the varied microstructure exhibited a value more than 4.0MPam1/2, that was significantly higher than that of the ternary crystal.

Phase competition mediated by composition and pressure in Zr2Cu1−xNix system

Phase competition between C11b, C16 and E93 phases under pressure (0–60GPa) in Zr2Cu1−xNix (x=0, 0.125, 0.25, 0.50, 0.75, 0.875 and 1) alloys has been investigated by first-principles pseudopotential calculations. The whole picture of phase competition mediated by both external pressure and composition has been unambiguously established by modeling using both supercell (SC) method and virtual crystal approximation (VCA) method, which sheds light on the pressure dependent glass stability and crystallization behavior of related alloy systems. It is revealed that both pressure and increasing Ni content stabilize the C16 phase over the other two. The underlying mechanism of external pressure and composition stabilizing the C16 phase is also analyzed. A feasible procedure for achieving metallic glass composites with tunable precipitated phases is proposed.

Structural, elastic, electronic, and thermodynamic properties of intermetallic Zr2Cu: A first-principles study

Three competing structures (C11b, C16 and E93) of intermetallic Zr2Cu have been systematically investigated by first-principles calculations and quasi-harmonic Debye model. Both the calculated equation of states (EOS) and pressure–enthalpy results indicate a structural phase transition from C11b to C16 phase at around 11–14 GPa. The calculated equilibrium crystal parameters and elastic constants are in consistence with available experimental or theoretical data. All three phases are mechanically stable according to the elastic stability criteria, and ductile according to Pugh's ratio, while the ambient-stable C11b phase shows a higher elastic anisotropy. Furthermore, differences in the nature of bonding between three competing structures are uncovered by electron density topological analysis. C11b Zr2Cu possesses an intriguing pseudo BaFe2As2-type structure with the charge density maxima at Zr tetrahedral interstices serving as Fe-position pseudoatoms; C16 Zr2Cu contains Zr-pair configurations bonded through bifurcated Zr–Zr bonding paths; while the E93 phase has only conventional straight bonding. Additionally, through quasi-harmonic Debye model, the pressure and temperature dependences of the bulk modulus, specific heat, Debye temperature, Grüneisen parameter and thermal expansion coefficient for three phases are obtained and discussed.

Control of microstructure and fracture toughness improvement of NbSi2/MoSi2 duplex lamellar silicides by TaC particles dispersion

The floating-zone melting method enabled the dispersion of TaC particles in a C40-(Mo0.85Nb0.15)Si2 single crystal. By annealing the C40 crystal, coarse C11b phases were formed around the TaC particles and the precipitation of the D88-(Mo,Nb,Ta)5Si3C phase was also induced, in addition to the development of a C40/C11b lamellar microstructure. Such a microstructure containing TaC particles led to a reduction in the orientation dependence of the fracture toughness of duplex silicide crystals.

Misfit strain affecting the lamellar microstructure in NbSi2/MoSi2 duplex crystals

The effect of including an additional element, like Re, Cr, V, Zr or Ir, in (Mo0.85Nb0.15)Si2 crystals, carried out with the aim of improving the thermal stability of the fine lamellar microstructures of the crystals, was examined with the help of first-principles calculations. The addition improved the thermal stability of the microstructures at 1400°C in all cases, and the effect was especially significant in crystals containing Cr and Zr additives. Quantitative evaluation by Moiré fringe analysis revealed that the misfit strain at the interfaces in crystals with added Cr and Zr was smaller than those in crystals with other elements added. The reduced misfit strain resulted in the suppression of the rapid growth of the C11b phase precipitates that did not possess the variant orientation relationship with the C40 matrix, and hence the thermal stability of the fine lamellar microstructures improved. The results further revealed that the ratio of the atomic size of the additive element to that of Mo, which is the main constituent element in the C11b phase, is an important factor that controlled the misfit strain at the lamellar interface by the result of Cr addition. However, the addition of Zr was found to result in a significant improvement in the thermal stability of the lamellar microstructure, despite the markedly larger atomic radius of Zr compared with that of Mo. This indicates that there may be other factors, in addition to the atomic radius of the additive, contributing to the thermal stability.