Hexagonal Alloys Research Articles

Phase-field modeling of void anisotropic growth behavior in irradiated zirconium

A three-dimensional phase-field model was developed to study the effects of surface energy and diffusivity anisotropy on void growth behavior in irradiated Zr. The gamma surface energy function used in the phase-field model was developed with the surface energy anisotropy calculated from the molecular dynamics simulations. The model assumes that vacancies have larger mobility in the c-axis than in the a-axis and b-axis, whereas interstitials have larger mobility in the basal plane than in the c-axis. The equilibrium void morphology and the effect of defect concentrations and mobility anisotropy on void growth behavior were simulated using the developed model. The simulations demonstrated that (1) the developed phase-field model correctly reproduces the faceted void morphology predicted by the Wulff construction; (2) with isotropic diffusivity, the void prefers to grow on the basal plane; and (3) when the vacancy has large mobility along the c-axis and interstitial has a large mobility on the basal plane of hexagonal closed-packed Zr alloys, a platelet void grows in the c-direction and shrinks on the basal plane. These findings are in agreement with the observation of void morphology and growth behavior in irradiated Zr, and reveal that the strong mobility anisotropy of defects results in the anisotropic growth of voids in Zr.

Cryo-rolling enhanced inhomogeneous deformation and recrystallization grain growth of a zirconium alloy

This work focuses on the influence of cryo-rolling on subsequent recrystallization of hexagonal Zr-4 alloy. With initial recrystallized microstructure, the as-received Zr-4 sheets were rolled at liquid nitrogen temperature (LNT) and room temperature (RT). For each rolling temperature, two levels of strain, 20% and 40%, were applied. Then, samples, including as-received and rolled sheets, were annealed at 700 °C for 3 min–5 min. Microstructures of the deformed and annealed Zr-4 samples were characterized in detail by electron backscatter diffraction (EBSD) method. After rolling, no twin lamella is observed in Zr-4 sample. It is found that the intensity of the heterogeneous deformation in cryo-rolling is more severe than that in RT-rolling. After annealing, the recrystallized average grain size of Zr-4 decreases with the increases of the applied strain at both deformation temperatures. For a given rolling reduction, the recrystallized average grain size increases with decreasing deformation temperature. The texture orientation of Zr-4 was not remarkably changed during the rolling deformation and annealing process.

Structural and magnetocaloric properties in hexagonal MnNiGa alloys with Co doping

Hexagonal MnNiGe-based alloys are a series of novel functional materials with potential magnetostructural transitions (MSTs). Accordingly, it was investigated the magnetic features of bulk hexagonal MnNiGa alloy and attempted to partially substitute Mn by Co atoms to tailor its structural and magnetic properties. Nonetheless, the introduction of magnetic Co atom fails to bring about the first-order phase transition and gives rise to the emergence of second phase with cubic structure instead. For ternary MnNiGa parent alloy, the second-order nature of transition is confirmed by both the absence of thermal hysteresis and the standard Arrott plot. To the end, the values of isothermal entropy change are determined by Maxwell relation, and the maximal values follow the trend predicted by the mean-field theory. Its broad transition region (~53 K) leads to only a very small value of entropy change (~2.4 J·kg−1·K−1 at a field change of 3 T). In turn, the wide transition ensures a relative large refrigerant capacity (~89.4 J·kg−1), which is comparable to that of MnNiGe-based systems. Although the substitution of Co for Mn site is unsuccessful, the chemically modified MnNiGa is still a promising candidate for the application of magnetocaloric effect (MCE) with merits of higher magnetization and better mechanical performance than MnNiGe-based systems.

Quantitative characterization of the microstructure of heat-treated Zr-Excel by neutron line profile analysis

Neutron diffraction line profile analysis (DLPA) and transmission electron microscopy were used to characterize the components of the bimodal microstructure of Zr-Excel (Zr–3.5Sn–0.8Mo–0.8Nb), a nuclear structural material. The dual microstructure, consisting of equiaxed primary grains and martensitic domains both having hexagonal close-packed (h.c.p.) α crystal structure, forms when the as-received Zr-Excel alloy is heat treated at a high temperature and subsequently quenched,i.e. is solution treated. Because both microstructure components have the same crystal structure the reflections from the two components overlap significantly. The article presents how the multi-phase analysis capability of modern DLPA methods can be used to model the measured neutron diffraction patterns as the sum of two sub-patterns corresponding to the components of such a bimodal microstructure, which can be found in many hexagonal alloys relevant for industrial applications. The results show that the large equiaxed primary h.c.p. α grains have a highly correlated low-density dislocation structure and large sub-grains (∼300 nm), while the large martensitic domains have a randomly arranged very high density dislocation structure and sub-grains the size of ∼30 nm. The significantly different defect structures of the primary and martensitic phases manifest as large differences in the hardness and ductility of the individual components. As a result of this duality of the mechanical properties, solution-treated Zr-Excel materials can be considered as analogous to metal matrix composites where a softer ductile matrix contains a harder brittle reinforcing phase.

First-principles determination of band-to-band electronic transition energies in cubic and hexagonal AlGaInN alloys

We provide approximate quasiparticle-corrected band gap energies for quaternary cubic and hexagonal AlxGayIn1–x–yN semiconductor alloys, employing a cluster expansion method to account for the inherent statistical disorder of the system. Calculated values are compared with photoluminescence measurements and discussed within the currently accepted model of emission in these materials by carrier localization. It is shown that bowing parameters are larger in the cubic phase, while the range of band gap variation is bigger in the hexagonal one. Experimentally determined transition energies are mostly consistent with band-to-band excitations.

Giant exchange bias in Mn2FeGa with hexagonal structure

In this study, we present the experimental observation that polycrystalline Mn2+xFe1−xGa (x = −0.2, 0, 0.2, 0.4) compounds can be synthesized to be D019-type (Ni3Sn-type) hexagonal structure with space group P63/mmc. A giant exchange bias field up to 1.32 kOe was achieved in hexagonal Mn2FeGa alloy at 5 K. A cluster glass state is confirmed by ac susceptibility measurement under different driving frequencies. Interestingly, robust horizontal and vertical shifts in magnetic hysteresis loop were simultaneously observed at 5 K under high cooling field up to 90 kOe. The large exchange bias is originated from the large exchange anisotropy between cluster glass phase and ferrimagnetic matrix. The vertical shift is thought to be attributed to the incomplete reversal of frozen cluster spins.

Optical and Raman studies of Zn1-xMgxO ceramic pellets

The undoped and Mg-doped ZnO ceramics have been successfully synthesized using the conventional solid state sintering method. The doping effect of MgO content on the structural properties of ZnO/MgO composites has been investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns reveal that all the samples are polycrystalline and have a prominent hexagonal crystalline structure with (002) and (101) as preferred growth directions. The formation of the hexagonal ZnMgO alloy phase and the segregation of MgO-cubic phase took place for an MgO composition x≥20wt%. This finding is in good agreement with the Raman spectroscopy measurements which prove the existence of multiple-order Raman peaks originating from ZnO-like and MgO phonons. The band gap energy and the carrier concentration of ZnO pellets were found to be dependent upon the Mg doping whose values vary from 3.287 to 3.827eV and from 1.6×1017 to 5.2×1020cm−3, respectively.

Anomalous magnetotransport behavior in Fe-doped MnNiGe alloys

The electrical dc transport properties of hexagonal magnetic equiatomic alloys of nominal composition ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{NiGe}$ $(x=0.2\phantom{\rule{4pt}{0ex}}\text{and}\phantom{\rule{4pt}{0ex}}0.25)$ have been investigated experimentally as well as theoretically using first-principles electronic structure calculations. Thermal hysteresis in the magnetization data indicates that the alloys undergo a first-order martensitic transition. Both the alloys show unusual nonmetallic resistivity behavior and a noticeable amount of training effect in resistivity when thermally cycled through the first-order martensitic transition. We observe moderate negative magnetoresistance ($\ensuremath{\sim}\ensuremath{-}11.5$% for 150 kOe) at 5 K (well below the martensitic transition temperature) associated with clear virgin line effect for both the alloys. We have adapted different flavors of density functional theory approach to understand the experimentally observed nonmetallic transport behavior.

Effect of additional elements on compositional modulated atomic layered structure of hexagonal Co80Pt20 alloy films with superlattice diffraction

The effect of additional element on compositionally modulated atomic layered structure of hexagonal Co80Pt20 alloy films with superlattice diffraction was investigated. In this study it is found that the addition of Cr or W element to Co80Pt20 alloy film shows less deterioration of hcp stacking structure and compositionally modulated atomic layer stacking structure as compared to Si or Zr or Ti with Ku of around 1.4 or 1.0 × 107 erg/cm3 at 5 at.% addition. Furthermore, for O2 addition of O2 ≥ 5.0 × 10−3 Pa to CoPt alloy, compositionally modulated atomic layer stacking structure will be deteriorated with enhancement of formation of hcp stacking structure which leads higher Ku of 1.0 × 107 erg/cm3.

Molecular dynamics simulation of nanometer scale mechanical properties of hexagonal MgLi alloy

Nano mechanical behavior of MgLi nanowire is investigated under tension and compression to elicit property alteration due to Li alloying in Mg within hexagonal range. Embedded atom method (EAM) is employed to carry out present simulation work. Nanowire under consideration is supposed to be isotropic and mechanical behavior is uninfluenced by material texture. The elastic modulus, yield strength both in tension and compression is assessed with change in strain rate. Effects of temperature in tension and compression are studied. Results of present simulation work elicit serrated yielding under uniaxial tension, however, twin mediated deformation under compression is completely tuned with previously reported experimental works. This investigation bridges nanometer scale properties to microscale material response, which in turn can be applied for designing suitable robust processing routes of this material.

A hexagonal close-packed high-entropy alloy: The effect of entropy

The formation of disordered solid solution in the hexagonal close-packed (hcp) structure in the GdHoLaTbY alloy and its mechanical properties were investigated in this study. The single hcp phase of the alloy in the as-cast state was confirmed by X-ray diffraction and scanning electron microscopy analyses. The compressive yield strength, fracture strength, and plastic strain of the alloy are 108MPa, 880MPa, and 21.8%, respectively, and the Vickers hardness is 96HV. The results show that the yield strength, fracture strength, and hardness of the alloy obey the rule of mixture, which indicates that there is no hardening effect from entropy. Although the high entropy of mixing stabilizes the solid solution against intermetallic compounds, lack of severe lattice distortion from elastic strain or electronic interactions between principal elements impacts little on the mechanical properties.

A high-specific-strength and corrosion-resistant magnesium alloy.

Ultra-lightweight alloys with high strength, ductility and corrosion resistance are desirable for applications in the automotive, aerospace, defence, biomedical, sporting and electronic goods sectors. Ductility and corrosion resistance are generally inversely correlated with strength, making it difficult to optimize all three simultaneously. Here we design an ultralow density (1.4 g cm(-3)) Mg-Li-based alloy that is strong, ductile, and more corrosion resistant than Mg-based alloys reported so far. The alloy is Li-rich and a solute nanostructure within a body-centred cubic matrix is achieved by a series of extrusion, heat-treatment and rolling processes. Corrosion resistance from the environment is believed to occur by a uniform lithium carbonate film in which surface coverage is much greater than in traditional hexagonal close-packed Mg-based alloys, explaining the superior corrosion resistance of the alloy.

Solid solution strengthening of hexagonal titanium alloys: Restoring forces and stacking faults calculated from first principles

The solid solution strengthening of α-Ti was investigated in respect of dislocation nucleation and dissociation in all four active glide modes. A series of Ti+X alloys (X = Al, Sn, V, Zr and O) was selected to analyze the impact of solute valence structure (Al, Sn – p type elements, V, Zr – d type elements) and lattice site (interstitial O) on the mechanisms responsible for variation of mechanical properties. The computational procedure relied on the generalized stacking fault energy (GSFE) concept combined with the nudged elastic band method that enables full atomic relaxation and determination of the true, minimum energy GSFE path. Additionally, various concentrations of solutes and their distance to the glide plane were considered as well. Our study revealed a strong, nonlinear influence of X position on GSFE and migration of O atoms during the crystal slip. These new phenomena allowed one to determine three solution strengthening mechanisms: (I) hindrance of <a> prismatic dislocation emission and reconfiguration of 1/3 <112¯0> screw dislocation cores (p type solutes), (II) hindrance of <a> prismatic dislocation emission (V) and SFE reduction in other modes (both d type solutes) and (III) suppression of dislocation nucleation in all modes caused by O. We found that the stacking faults formed by the single partial dislocations have a thickness of few atomic layers and exhibit a highly non-uniform structure. Their ability to accommodate the lattice deformation introduced by solute elements greatly affects the stacking fault energies of the α-Ti alloys.

Processing of β-treated Zircaloy-4 by warm compressions along three orthogonal axes

The purpose of this work is to refine the grains of Zircaloy-4 before it is processed into cladding tubes containing the nuclear fuel. This raises the question of the mechanisms by which the Widmanstatten lamellae inherited from the β-quenching transform into smaller crystallites. With a view to accelerate this transformation, strain steps of compression along three orthogonal axes were performed in the upper α-range (i.e. 550–810 °C). The effects of the temperature, the size of the strain steps, the strain rate and the number of cycles of three compressions were investigated. All have a direct influence on the mechanical behavior, the global texture and the microstructural evolution. The latter was studied mostly by electron backscattering diffraction. The best conditions to break the initial lamellae were found at high strain rate (4 s−1): small recrystallized grains around 10 μm were obtained after one cycle at 750 °C. But at moderate strain rates (0.1 s−1), good results were also reached after two cycles at 650 °C. The mechanisms of globularization are different in both cases: in the first, germination of new grains prevails while in the second, a progressive evolution of the subgrains into grains was observed. Some consequences of heat treatments are also briefly described, pointing out the sensitivity of Zircaloy-4 to undesirable grain growth. The conclusion emphasizes the interest of multiaxial forging in the thermomechanical treatment of hexagonal alloys.

Prediction of the electronic structures, thermodynamic and mechanical properties in manganese doped magnesium-based alloys and their saturated hydrides based on density functional theory

The crystal structures, electronic structures, thermodynamic and mechanical properties of Mg2Ni alloy and its saturated hydride with different Mn-doping contents are investigated using first-principles density functional theory. The lattice parameters for the Mn-doped Mg2Ni alloys and their saturated hydrides decreased with an increasing Mn-doping content because of the smaller atomic size of Mn compared with that of Mg. Analysis of the formation enthalpies and electronic structures reveal that the partial substitution of Mg with Mn reduces the stability of Mg2Ni alloy and its saturated hydride. The calculated elastic constants indicate that, although the partial substitution of Mg with Mn lowers the toughness of the hexagonal Mg2Ni alloy, the charge/discharge cycles are elevated when the Mn-doping content is high enough to form the predicted intermetallic compound Mg3MnNi2.

Spin-glass–like ground state and observation of exchange bias in Mn0.8Fe0.2NiGe alloy

The ground-state magnetic properties of a hexagonal equiatomic alloy of nominal composition Mn0.8Fe0.2NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes a first-order martensitic transition below 140 K with simultaneous development of long-range ferromagnetic ordering from the high-temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic-like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, but it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley i) the observation of exchange bias at low temperature, and ii) spin-glass–like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior, the spin glass state is confirmed by the zero-field–cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic-like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where a similar phase separation can lead to a glassy ground state.

Peak broadening anisotropy in deformed face-centred cubic and hexagonal close-packed alloys

The broadening of diffraction peaks representing different families of grain orientations has been measured for a number of deformed metals: austenitic stainless steel 316, nickel 200 and the titanium alloy Ti-6Al-4V. These measurements have been compared with predictions that explain differences in broadening in terms of the contrast factor of dislocationsviatwo different approaches. This was done in order to understand the effect the contrast factor has on the results of diffraction peak profile analysis methods and the cause of broadening anisotropy. An approach that considers all grains and orientations to behave similarly was found to be unsuccessful in explaining the large variations of broadening in different peaks. These variations can be explained, and errors reduced, by adopting an approach that uses a polycrystal plasticity model. However, if the plasticity based approach is used to solely calculate changes in the contrast factor, it only partly explains changes in broadening. Instead, factors such as variations in the dislocation density and crystallite size in different orientations, the number of dislocations that are mobile, and the number of edge and screw dislocations need consideration. The way to incorporate these additional factors is difficult, but their contribution to broadening anisotropy can be as important as that of the contrast factor.

Formation of MgZnO alloy under thermodynamic conditions

MgxZn1−xO ceramics with x=0.10, 0.15, 0.20, 0.25 and 0.30 were sintered at Ts=700–1250°C. Photoluminescence (PL) and PL excitation (PLE) spectra as well as X-ray diffraction patterns were measured at 293K. Bandgap width was evaluated from the position of PLE peak. Non-monotonous dependence of PLE peak position on Ts was observed for all used x. This effect was concluded to be caused by the formation of cubic phase side by side with hexagonal one and the enhancement of this process with increasing Ts, which resulted in the decrease of Mg content in the hexagonal phase at Ts>1000°C. Temperature range of 1000–1050°C was found to be the optimum one for the formation of hexagonal MgZnO alloy at used x. It was shown that x=0.20 was the solubility limit of MgO in hexagonal MgxZn1−xO ceramics.

Hexagonal core-shell and alloy Au/Ag nanodisks on ZnO nanorods and their optical enhancement effect.

Au and Ag hybrid hexagonal nanodisks were synthesized on ZnO nanorods' (0002) surface via a new two-step deposition-annealing method. The structural, compositional, as well as optical investigations were carried out systematically to find out the nanodisks' formation mechanism and optical enhancement effect. It was shown that the core-shell Au/Ag nanodisk can be formed under rapid annealing temperature of 500°C, while Au/Ag alloy nanodisks are formed if higher temperatures (>550°C) are applied. The optical effect from these nanodisks was studied through photoluminescence and absorption spectroscopy. It was found that the carrier-plasmon coupling together and carrier transfer between metal and ZnO contribute to the emission enhancement. Furthermore, the results suggest that the composition of nanodisk on the vicinity of metal/ZnO interface plays an important role in terms of the enhancement factors.

High-pressure phase diagram of O2 and N2 binary system: formation of kagome-lattice of O2

High-pressure phase diagram of O2 and N2 binary system has been investigated using an x-ray diffraction method and the pressure-composition phase relation has been proposed up to 12 GPa. From the result, a hexagonal alloy phase reported previously was stable in relatively wide pressure and composition ranges. The structure of this phase was also determined to be a hexagonal lattice (space group: P6/mmm) with seven molecules in the unit cell from the Rietveld refinement. In this phase, formation of a new magnetic lattice of O2: the kagome-lattice has also been proposed.