Fcc Zr Research Articles

Understanding phase stability and deformation mechanisms of zirconium under high pressures from generalized stacking fault energy curves: A first-principles study

The mechanical properties of metals are strongly depend on their phase structures, in this work, to understand the phase stability and deformation mechanisms of zirconium(Zr) under high pressures, the generalized planar fault energy(GPFE) curves based on first-principles calculations are utilized to illustrate the deformation processes of α-Zr at high pressures. Firstly, it is found the prismatic slip system is more prominent toward α-Zr at zero pressure, while the slips based on basal plane undertake the main participant during the deformation processes of α-Zr at high pressures. Meanwhile, based on the basal slip, α-Zr phase containing the pre-existing stacking fault is found more favorable to generate twin fault instead of FCC fault at zero pressure; However, the deformation twinning is overwhelmed by the HCP-to-FCC transformation for the α-Zr when the external pressure is higher than 10.4 GPa. It is thus speculated that the slips of partial dislocations for forming FCC fault based on the basal plane (0001) act as a contributing participant during the phase transition processes of Zr under high-pressure. Additionally, the computed elastic constants and the HCP-to-FCC allotropic transformation under specified conditions have revealed the metastable nature of FCC Zr in comparison with HCP Zr.

Second Phase Particles in Zr–Sn–Nb–Fe Alloys: A Review

In the Zr–Sn–Nb–Fe quaternary alloys, the nature and evolution of the second phase particles (SPPs) is critical to the performance of the alloy in the extremely deteriorative environment. The main aimof this review consists in summarizing the fundamental results of the identification and characterization of the SPPs in the Zr–Sn–Nb–Fe alloys. Emphasis was placed on the importance of composition, identification,crystallographic structure, formation mechanism, and state and stability of several SPPs in these alloys. Identical compositions in ternary Zr–Nb–Fe intermetallics have been identified as C14 HCP Zr(Nb,Fe)2 orC15 FCC (Zr,Nb)2Fe structure. Zr(Nb,Fe)2 is often reported, while, the cubic phase (Zr,Nb)2Fe is easily distinguished.The reliability of R* parameter, which is specified by Nb/Fe physical ratio in determining the SPPs types in different composition range of Zr–Sn–Nb–Fe alloys, is discussed with reasons. The influence and the role of O and Cr in the formation and stability of ternary Zr–Nb–Fe intermetallics are also clarified. Finally, thermodynamic stability of SPPs was also taken into consideration in the current review.

Temperature Dependence of the Lattice Thermal Conductivity of Metastable Phases of FCC Ti and Zr

Temperature Dependence of the Lattice Thermal Conductivity of Metastable Phases of FCC Ti and Zr

Nucleation and growth of nanoscale precipitates and the effect on properties during secondary isothermal aging in thin-band of Cu–0.8Cr–0.1Zr alloy

The precipitation behavior during secondary isothermal aging of Cu–0.8Cr–0.1Zr alloy, especially the nucleation and growth of nanoscale body-centered cubic (bcc) Cr and orthogonal Cu4Zr precipitates, were clarified through high-resolution crystal structure and atomic-scale composition analysis. Results demonstrated that the crystal-nucleus of face-centered cubic (fcc) Cr and fcc Zr would be preferentially precipitated in a coherent orientation relationship (OR) with fcc Cu-based matrix. As they grew up, fcc Cr gradually developed into stable bcc structure, and the OR with matrix changed from N-W to incoherent. While fcc Zr would change lattice constant by atomic diffusion and segregation to evolve into stable Cu4Zr with orthogonal structure, during which they were precipitated on the Cu–Cr–Zr solid solution through a process of discharging surrounding Cr atoms. Abundant dispersed nanoscale precipitates that induced by the secondary aging contributed to produce well-matched comprehensive properties (σb=669 MPa, I = 84.3 %IACS).

Microstructures and Synthesis of Noble Metal-Based Bulk Metallic Glasses (Zr-Be-Ni-Ti)

High-energy ball-milling in hexane medium was employed to prepare Nobel Zr-based bulk metallic glasses (BMGs) alloy of three different nominal compositions Zr47Be23Ni15Ti15, Zr50Be20Ni15Ti15 and Zr52Be18Ni15Ti15, numbers indicate at.%). The glass forming ability was found to increase with decreasing Zr and increasing Be content, which can be ascribed to the enhanced atomic size mismatch of the constituents on Be addition. Amorphous Zr47Be23Ni15Ti15 powder undergoes two-stage crystallization with onset temperatures at 640 and 700 K and glass transition temperature Tg at 566 K. In contrast, the Zr50Be20Ni15Ti15 and Zr52Be18Ni15Ti15 samples remained crystalline to a certain extent even after prolonged milling and contained FCC Zr crystallites. Structural characterization was done by X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, thermal analyses were performed by means of differential scanning calorimetry (DSC) thermogram to justify the experimental findings.

Defluoridation using biomimetically synthesized nano zirconium chitosan composite: Kinetic and equilibrium studies

The present study reports a novel approach for synthesis of Zr nanoparticles using aqueous extract of Aloe vera. Resulting nanoparticles were embedded into chitosan biopolymer and termed as CNZr composite. The composite was subjected to detailed adsorption studies for removal of fluoride from aqueous solution. The synthesized Zr nanoparticles showed UV–vis absorption peak at 420nm. TEM result showed the formation of polydispersed, nanoparticles ranging from 18nm to 42nm. SAED and XRD analysis suggested an fcc (face centered cubic) Zr crystallites. EDAX analysis suggested that Zr was an integral component of synthesized nanoparticles. FT-IR study indicated that functional group like NH, CO, CN and CC were involved in particle formation. The adsorption of fluoride on to CNZr composite worked well at pH 7.0, where ∼99% of fluoride was found to be adsorbed on adsorbent. Langmuir isotherm model best fitted the equilibrium data since it presented higher R2 value than Freundlich model. In comparison to pseudo-first order kinetic model, the pseudo-second order model could explain adsorption kinetic behavior of F− onto CNZr composite satisfactorily with a good correlation coefficient. The present study revealed that CNZr composite may work as an effective tool for removal of fluoride from contaminated water.

Amorphization Driven by Defect-Induced Mechanical Instability

Using ab initio molecular dynamics simulations, we perform a comparative study of the defect accumulation process in silicon carbide (SiC) and zirconium carbide (ZrC). Interestingly, we find that the fcc Si sublattice in SiC spontaneously and gradually collapses following the continuous introduction of C Frenkel pairs (FPs). Above a critical amorphization dose of ~0.33 displacements per atom (dpa), the pair correlation function exhibits no long-range order. In contrast, the fcc Zr sublattice in ZrC remains structurally stable against C sublattice displacements up to the highest dose of 1.0 dpa considered. Consequently, ZrC cannot be amorphized by the accumulation of C FPs. We propose defect-induced mechanical instability as the key mechanism driving the amorphization of SiC under electron irradiation.

An intermediate fcc Zr state observed in the Cu–Zr–Ni system upon ion beam mixing

A unique face-centered-cubic (fcc) Zr-based solid solution was obtained in the Cu10Zr76Ni14 multilayered films upon 200keV xenon ion beam mixing, accompanied by a novel phase transition sequence of Cu+Zr+Ni→dual-FCC→FCC→Amorphous while varying the irradiation dose. The energetic stability sequence, elastic stability and phonon behaviors derived from first-principle calculations jointly evidenced the metastability of fcc Zr from different perspectives. The experimental observations and theoretical calculations together justify the existence of an intermediate fcc Zr state under non-equilibrium conditions. Possible formation mechanisms for the metastable crystalline phase were also discussed in terms of the atomic collision theory.

Complex martensitic nanostructure in Zr nanowires: A molecular dynamics study

We use molecular dynamics to characterize the martensitic nanostructure that develops when bcc Zr nanowires are cooled down and transform to hcp (martensite). We find that size has a strong effect on nanostructure and even very small wires (a few nanometers in diameter) exhibit complex, multidomain structures with large internal strains (up to $\ensuremath{\sim}6\mathrm{%}$). Long and thin wires result in domains coexisting along their axes while those with small aspect ratios exhibit coexistence of domains within their cross-section. We also that find regions of fcc Zr that develop to bridge neighboring hcp domains.

Irradiation-Induced Growth and Microstructure of Recrystallized, Cold Worked and Quenched Zircaloy-2, NSF, and E635 Alloys

Abstract This paper is devoted to the study of the effect of the texture, phase composition, and microstructure on the irradiation-induced growth strain (GS) of zirconium-based alloys. GS measurements and TEM microstructural examinations were performed on Zry-2, NSF, and E635 samples in the recrystallized, beta quenched and cold-worked (CW) conditions. The samples were irradiated in the BOR-60 reactor in the temperature range of 315–325°C up to a neutron fluence level of 1.1 ×1026 n/m2 (E>1 MeV), i.e., up to a damage dose of 23 dpa. Growth strains of NSF and E635 alloys in all states and in the longitudinal and transverse directions are lower as compared to those of Zry-2, and do not exceed 0.2 % even at the maximum fluence level. As for recrystallized Zry-2, the GS kinetics are characterized by the appearance of the accelerated growth stage. A combination of a certain amount of Nb, Fe, and Sn in the matrix content plays a key role in GS kinetics. The higher the degree of CW, the higher the irradiation growth but its rate of increase with increasing fluence is different for alloys of different compositions. The maximum GS, reaching 0.72 %, is observed in the 20 % CW Zry-2 samples. Texture, along with the alloy composition, is one of the main GS-determining factors. Irradiation growth of the transversal samples is lower as compared to the longitudinal ones because of texture. As for quenched alloys, the texture is practically isotropic and GS values are low, independent of the alloy composition. In CW materials, the density of <c>- dislocations greatly affects the irradiation growth strain. Particles of Zr(Fe,Cr)2 and Zr2(Fe,Ni) phases in Zry-2 as well as Zr(Nb,Fe)2 in NSF and E635 are depleted in iron under irradiation. The Fe goes into the matrix and modifies its properties. The HCP lattice structure in the Laves phases in NSF and E635 changes into BCC (β-Nb-type). FCC (Zr,Nb)2Fe precipitates preserve on the whole their composition and structure; no amorphization of the Nb-containing precipitates is observed. The Zr2(Fe,Ni) precipitates with a BCT lattice remain crystalline, and HCP Zr(Cr,Fe)2 precipitates undergo amorphization. The average particle size in the irradiated alloys is larger and the concentration is a little lower as compared to the unirradiated ones. Irradiation-induced fine dispersed precipitates about 3 nm in size, probably enriched in niobium, appear in NSF and E635. The observed changes of microhardness are discussed from the viewpoint of generation of radiation defects (clusters, dislocation loops), evolution of the initial dislocation structure, and matrix composition (enrichment in Fe, Cr, and, probably, Nb).

Phase stabilities and thermal decomposition in the Zr 1− xAl xN system studied by ab initio calculation and thermodynamic modeling

The ab initio density functional theory is used to calculate the lattice constants, total energy and bulk modulus of binary hexagonal close packed (hcp) and face-centered cubic (fcc) ZrN, AlN and ternary Zr 1− x Al x N phases. The calculated results are supported by previously reported experimental and theoretical data. The lattice stabilities of binary phases and demixing energies of ternary phases calculated by the ab initio method are then used in thermodynamic modeling to construct the Gibbs free energy diagram of the immiscible quasi-binary ZrN–AlN system at different temperatures. The results show that, for the composition x ⩽ 0.472, Zr 1− x Al x N solid solutions are more stable in fcc than in hcp structure, which is in a good agreement with the experimentally reported value of x < 0.43. The constructed chemically binodal and spinodal decomposition curves show that fcc Zr 1− x Al x N solution coatings should undergo phase decomposition into fcc ZrN and fcc AlN. However, considering the relatively large lattice mismatch between the fcc ZrN and the fcc AlN, the coherent spinodal decomposition may probably be hindered due to rapid phase transformation from fcc AlN to the stable hcp AlN. This is supported experimentally in that no intermediate fcc AlN has so far been observed in the Zr–Al–N system.

Thermal decomposition of Zr 1 − x Al xN thin films deposited by magnetron sputtering

The thermal stability of the metastable fcc Zr 1 − x Al xN thin films has been investigated by heating the samples in ultra high vacuum ( P < 10 − 7 Pa). For this purpose, two films with chemical composition Zr 0.65Al 0.35N and Zr 0.57Al 0.43N were grown by reactive magnetron sputtering. The films were heated for duration of 60 min between 400–850 °C by steps of 50 °C. The structural evolution of the annealed films has been investigated by using X-ray diffraction (XRD), transmission electron spectroscopy (TEM) and by atomic force microscopy (AFM). Results show that up to annealing temperature of 600 °C the pristine structural and mechanical properties of all the films are retained. At annealing temperatures above 600 °C, important structural modification result as deduced from the shift of the XRD peaks towards the low 2θ values indicating changes in the unit cell dimension. Structural modifications are accompanied by hardness enhancement. In Zr 0.57Al 0.43N films annealed at 850 °C, chemical analysis by X-ray energy dispersive spectrometry evidenced Al-rich regions inhomogeneously distributed in the plane of the film. The results are discussed in terms of the transformation of the fcc Zr 1 − x Al xN structure in a two-phase system composed of Al-poor Zr 1 − x Al xN and low crystallized h-AlN via spinodal decomposition.

Crystallization behavior of the Zr 61Al 7.5Cu 17.5Ni 10Si 4 amorphous alloy

The ribbons of amorphous Zr 61Al 7.5Cu 17.5Ni 10Si 4 alloy were studied by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD), high-temperature optical microscope and transmission electron microscopy (TEM). The activation energy for such the crystallization of the amorphous phase was about 360 and 370 kJ mole −1 calculated by Kissinger and Avrami plots, respectively. Kinetics for the crystallization was analyzed by means of Johnson–Mehl–Avrami equation and discussed regarding to the value of Avrami exponent. The average value of the Avrami exponent n was measured about 2.43 at the range from 698 to 713 K. In addition, the cube of crystal size as a function of isothermally annealing time presents a linear relationship for the Zr 61Al 7.5Cu 17.5Ni 10Si 4 alloy. When the Zr 61Al 7.5Cu 17.5Ni 10Si 4 alloy was isothermally annealed at 698 K, a Zr 2Cu crystal with an average size of 105 nm was first observed at the early stage (30% crystallization) of crystallization. In addition, both nano-crystals of ordered fcc ZrCu (∼100 nm) and ordered fcc Zr 3Al (∼30 nm) were observed to precipitate from the amorphous matrix upon the middle stage (50% crystallization) of crystallization.

Electronic stability of clusters in devitrification phases of Zr-based amorphous alloys

Abstract The discrete variational method is used to calculate the electronic structure of the clusters such as Zr 9 Ni 4 and Zr 10 Cu 5 in the primary devitrification phases such as fcc–Zr 2 Ni and bct–Zr 2 Cu in Zr-based amorphous alloys. In order to reveal the effect of Al on the stability of these clusters, Al atoms are introduced to replace Ni or Cu. The calculated density of states curves show that the Zr 9 Ni 3 Al cluster is more stable than Zr 9 Ni 4 but the stability deteriorates with more Al substitutions. For the Zr 10 Cu 5 cluster, one Al stabilizes this cluster but more Al substitutions destabilize it because more electrons are activated towards high energy levels. This stability mechanism explains the superior stability of the Zr–Al–Ni and Zr–Al–Cu amorphous alloys with respect to the binary Zr–Cu and Zr–Ni amorphous alloys.

Change in primary phase from icosahedral quasicrystal to fcc Zr 2Ni by mechanical disordering in Zr–Al–Ni–Cu–Pd glassy alloy

Change in the primary crystallization from a single icosahedral quasicrystalline phase into the fcc Zr 2Ni phase by mechanical disordering was investigated in a melt-spun Zr 65Al 7.5Ni 10Cu 12.5Pd 5 glassy alloy. The transition of the primary phase is attributed to the mechanical strain induced in the icosahedral local structure in the glassy state.

Quasicrystals from glass devitrification

It was found that the nano-icosahedral phase was formed in the devitrification stage of Zr- and Hf-based glassy alloys. The primary phases are metastable fcc Zr 2Ni and fcc Hf 2Ni phases in the Zr 65Al 7.5Ni 10Cu 17.5 and Hf 65Al 7.5Ni 10Cu 17.5 glassy alloys, respectively. By substitution of 5 at.% Pd for Cu, the primary phase changes to an icosahedral quasicrystalline phase in both alloys. The addition of elements, that have a positive or weak chemical affinity with one of the constitutional elements in the Zr–Al–Ni–Cu and Hf–Al–Ni–Cu glassy alloys is effective for the precipitation of the icosahedral phase. The icosahedral phase also precipitated with a slight deviation from the composition with a high glass-forming ability. Since an icosahedron is contained as a structure unit in the icosahedral, fcc Zr 2Ni and fcc Hf 2Ni phases, it is concluded that these phases are correlated with the local icosahedral order of the glass. The high-resolution transmission electron microscopy images of the as-spun Zr 65Al 7.5Ni 10Cu 7.5Pd 10, Zr 70Al 7.5Ni 10Cu 12.5 and Hf 65Al 7.5Ni 10Cu 12.5Pd 5 alloys reveal a possibility of the existence of the icosahedral ordered regions. It is, therefore, concluded that the existence of icosahedral short- or medium-range order stabilizes the glassy state in the Zr- and Hf-based multicomponent alloys.

Transformation in the initial crystallization stage of Zr–Al–Ni–Cu glassy alloys made with low oxygen concentrations

We investigated the transformation behavior of glassy phase in the Zr 65+ x Al 7.5Ni 10Cu 17.5− x ( x=0–9) alloys with low oxygen concentrations below 500 mass ppm. In the initial crystallization stage, metastable fcc Zr 2Ni, Zr 2Cu and Zr 6NiAl 2 phases are precipitated in the alloy where x=0. A weak diffraction peak corresponding to an icosahedral phase is found in addition to the mixture phases of fcc Zr 2Ni, Zr 2Cu and Zr 6NiAl 2 in alloys with x=1 and 2. The primary phase changes to the single icosahedral phase in the range of x=3–6 and a bcc Zr phase is precipitated above x=7. The icosahedral particles have a dendritic morphology with a grain diameter ranging from 500 to 2000 nm for the Zr 70Al 7.5Ni 10Cu 12.5 glassy alloy. A slight concentration redistribution leading to the enrichment of Zr and the rejection of Ni in the icosahedral phase is confirmed. The nucleation rate of the icosahedral phase at the crystallization temperature is calculated to be 1.5×10 14 m −3 s −1, which is 10 2 times lower than that of the fcc Zr 2Ni phase. It is suggested that the formation of the icosahedral phase with such a low nucleation rate is attributed to the existence of icosahedral clusters in the glassy state, and the high grain growth rate of the icosahedral phase is due to the slight redistribution of constituent elements during quasicrystallization.

Icosahedral quasicrystalline phase formation in Zr–Al–Ni–Cu glassy alloys by the addition of V, Nb and Ta

The formation of an icosahedral quasicrystalline phase as a primary phase was found in Zr 65Al 7.5Ni 10Cu 12.5X 5 (X=V, Nb or Ta) glassy alloys. The icosahedral phase was precipitated during the first exothermic reaction in a two-stage crystallization process after a distinct glass transition. The diameters of the icosahedral particles are as small as 10–50 nm, similar to those found in Zr-based glassy alloys made with the addition of noble metals. Based on a Johnson–Mehl–Avrami analysis, we suggest that nano-quasicrystallization proceeds by a diffusion-controlled growth mode. The icosahedral phase in the Zr 65Al 7.5Ni 10Cu 17.5− x Ta x ( x=0–4) glassy alloys was formed at a Ta concentration of as little as 1 at.%, the primary crystallization phases were the fcc Zr 2Ni and icosahedral phases. For Ta concentrations above 3 at.%, a nearly single icosahedral phase was formed as the primary phase. The formation of a nano-scale icosahedral phase in Zr–Al–Ni–Cu glassy alloys with the addition of V, Nb or Ta, as well as noble metals, indicates that an increasing nucleation rate contributes to the precipitation of the icosahedral phase, leading to the concept that icosahedral short- or medium-range order exists in the glassy state of the Zr–Al–Ni–Cu alloy.

Atomic-scale Ab-initio study of the Zr-H system: I. Bulk properties

Bulk properties of the Zr-H system were studied in the framework of the density functional theory. The local density approximation (LDA) is found to be insufficient for a proper description of interactions between Zr and H atoms and the generalized gradient approximation (GGA) is required. In αZr, H atoms preferentially occupy tetrahedral (T) sites at low temperatures, and can be regarded as being independent of each other up to very short distances, except for repulsive interactions between dumbbells in the same interstitial site. The Zr density of electronic states is perturbed by the presence of H, which induces the emergence of localized states. H diffusion occurs along the c → axis preferentially in octahedral (O) sites, and in the basal plane by alternate jumps into T and O sites. In the γ(ZrH), δ (ZrH 1.5) and ε(ZrH 2) hydrides, H-H interactions cannot be neglected, the nearly equal formation energies of these compounds indicate that their relative stabilities probably depend on mechanical and thermal contributions to free energies, and in fcc Zr, H atoms tend to adopt planar arrangements for compositions close to ZrH.

Phases formed during crystallization of Zr 55Al 10Ni 5Cu 30 metallic glass containing oxygen

In the present work the Zr 55Al 10Ni 5Cu 30 alloy has been processed by melt spinning and die-casting into a chilled copper mold. The ribbon was fully amorphous (X-ray diffraction and transmission electron microscopy) and cast samples partially crystalline. The amorphous ribbon was crystallized and analyzed by differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. The smaller supercooled liquid range of the glassy ribbons (60 K) and partial crystallization of a larger cast sample during quenching indicate the presence of oxygen in both samples (∼0.8 at.%). Although the crystallization process of the glassy ribbon, a glass transition followed by a single crystallization peak, showed the same crystallization process as reported by the literature, the heat release and the phases formed differed. The main phases formed were Zr 2Cu, a complex `big-cube' fcc Zr 4Cu 2O-type phase and a tetragonal phase, not reported before to our knowledge, containing a lattice parameter of 2.8 nm.