Tetragonal Zr2Ni Research Articles

Multistage Phase Decomposition Behavior and Kinetics for Zr65Ni30Pd5 Ternary Amorphous Alloy

We have examined heating-induced phase decomposition and its kinetics for Zr65Ni30Pd5 amorphous alloy in an argon atmosphere. The amorphous phase decomposes through three stages in the order of amorphous → icosahedral quasicrystal (I-Q) + amorphous → approximant cubic Zr2(Ni,Pd) → tetragonal Zr2Ni + tetragonal Zr2Pd phases. The approximant crystalline (APC) Zr2(Ni,Pd) phase includes icosahedral-like local atomic arrangements and the structural similarity between the I-Q and APC phases seems to enable the multistage phase decomposition through the precipitation of the APC phase. The primary precipitation of the I-Q phase from the amorphous phase occurs through nucleation and growth mechanisms through two-dimensional (2D) growth mode at lower heating rate and 3D growth mode at a high heating rate.

Influence of Ag replacement on supercooled liquid region and icosahedral phase precipitation of Zr65Al7.5Ni10Cu17.5-xAgx (x = 0–17.5 at%) glassy alloys

The glass transition phenomenon for Zr65Al7.5Ni10Cu17.5-xAgx (x = 5–17.5 at%) glassy alloys was observed in the Ag content range below about 15% and the icosahedral phase precipitated as the heating-induced primary phase in the limited Ag content range. In the higher Ag content range where glass transition is not clearly observed, the primary precipitation phase changed to mixed phases of tetragonal Zr3Ag + tetragonal Zr2Ni + hexagonal Zr4Al3. The good correspondence between the appearance of glass transition and the icosahedral phase precipitation was recognized in the multicomponent glassy alloys containing the immiscible type atomic pairs of Ni-Ag and Cu-Ag. Although neither glass transition nor supercooled liquid region is observed for Zr65Al7.5Ni10Ag17.5 and no icosahedral phase is formed, a modified Zr65Al7.5Ni17.5Ag10 alloy with higher Ni/Ag content ratio exhibits glass transition and supercooled liquid region and the primary precipitates also changes to an icosahedral phase. The good correspondence can be interpreted on the basis of the previous knowledge that the appearance of glass transition and supercooled liquid state originates from icosahedral-like medium range ordered atomic configurations. Besides, the 6%Ag-containing alloy keeps high glass-forming ability which is high enough to form a bulk glassy alloy rod of 6 mm by suction casting. The close correlation between the appearance of glass transition phenomenon and the primary precipitation of icosahedral phase is expected to provide a useful knowledge on the role of icosahedral-like atomic configuration in the achievement of bulk glass-forming ability through the stabilization of supercooled liquid.

Formation and mechanical properties of Zr-Nb-Cu-Ni-Al-Lu bulk glassy alloys with superior glass-forming ability

Glass-forming ability (GFA) and mechanical properties of (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)100-x Lu x (x = 0-3 at%) alloys have been investigated. The GFA of Zr58Nb3Cu16Ni13Al10 alloy is dramatically enhanced by adding Lu. The (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)98Lu2 alloy possesses the highest GFA in the studied Zr-Nb-Cu-Ni-Al-Lu alloys, with its critical diameter for glass formation reaching 20 mm by copper-mould casting method, while that of the Lu-free Zr58Nb3Cu16Ni13Al10 alloy is 7 mm. The critical diameters of (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)100-xLux (x = 1 at% and 3 at%) alloys are 15 mm and 12 mm, respectively. The Lu addition to Zr58Nb3Cu16Ni13Al10 alloy induces the change of initial crystallization phases from face-centred-cubic Zr2Ni and tetragonal Zr2Ni phases for the Lu-free Zr58Nb3Cu16Ni13Al10 alloy to an icosahedral quasi-crystalline phase for the Lu-doped alloys, which may be the origin for the enhanced GFA of the Lu-doped alloys. The compressive fracture strength and plastic strain of the bulk glassy (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)98Lu2 alloy are 1 610 MPa and 1.5%, respectively.

Synthesizing and Characterizing of a Novel Zr90Ni6Pd4 Bulk Metallic Glassy Alloy Obtained by Spark Plasma Sintering of Mechanically Alloyed Powders

A single phase of metallic glassy Zr90Ni6Pd4 powders was synthesized by mechanical alloying approach of the elemental powders, using a low-energy ball mill. The solid-solution hcp-ZrNiPd phase obtained after 25 h of the milling time transformed into a single amorphous phase upon ball milling for 100 h to 150 h. This synthesized amorphous alloy transformed into a metallic-glass at a glass transition temperature of 552.8°C. A small volume fraction of this glassy phase transformed into a mixture of two metastable phases of i-phase + big-cube upon annealing at 649.1°C. The supercooled liquid region of the metallic glassy Zr90Ni6Pd4 alloy powders was 69.7°C. A complete crystallization was achieved at a temperature ranged from 649.1°C to 682.2°C through a sharp exothermic reaction with an enthalpy change of crystallization of -76.3 J/g. After this temperature, the formed metallic glassy phase was transformed to polycrystalline mixture of tetragonal Zr2Ni and Zr2Pd phases. The powders obtained after 150 h of milling were subsequently consolidated at 600°C, using spark plasma sintering technique. The sizes of the obtained bulk metallic glassy buttons ranged were 15 mm and 50 mm in diameter with different thicknesses in the range between 0.25 mm to 20 mm. This consolidation step led to the formation of full-dense buttons with relative densities laid in the range between 99.23% to 99.76% without precipitations of any medium- or long-range ordered phase (s). Nanoindentation approach was employed to identify the nanohardness and Young’s modulus that were in the range between 7.74 to 9.32 GPa, and 135.26 to 151.15 GPa, respectively.

Glass-forming ability and I-phase formation in Y-doped Zr–Nb–Cu–Ni–Al glassy alloys

The effect of yttrium addition on the Glass-Forming Ability (GFA) and thermal stability of Zr58Nb3Cu16Ni13Al10 alloy has been investigated. (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)100− x Y x (x = 0–3 at.%) metallic glasses were formed by melt-spinning and copper-mould casting methods. By adding 1.5 at.% Y, the GFA of Zr58Nb3Cu16Ni13Al10 alloy is dramatically enhanced, and the critical diameter for glass formation increased from 7 mm to as large as 15 mm. The crystallization processes of the Y-free and Y-doped glassy alloys were studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. It was found that the initial precipitates are face-centred-cubic Zr2Ni and tetragonal Zr2Ni phases for the Y-free Zr58Nb3Cu16Ni13Al10 alloy, but a single icosahedral quasi-crystalline phase for the Y-doped (Zr0.58Nb0.03Cu0.16Ni0.13Al0.10)98.5Y1.5 alloy. This suggests that the yttrium greatly affects the local atomic configuration, which may be the origin for the enhanced GFA of the Y-doped alloys.

Formation of Zr 66.7Al 11.1Ni 22.2 noncrystalline alloys demonstrated by molecular dynamics simulations based on distorted plastic crystal model

Molecular dynamics simulations with a scheme to treat groups of atoms in an intermetallic compound as a hypothetical cluster were performed for Zr–Ni base alloys by allowing random orientations of the clusters that were imitated to plastic crystals in a class of soft matter. A binary Zr 66.7Ni 33.3 and ternary Zr 66.7Al 11.1Ni 22.2 and Zr 66.7Al 22.2Ni 11.1 alloys were analyzed for their local atomic arrangements and forming ability of a noncrystalline structure with total pair-distribution and interference functions and Voronoi polyhedra. The analysis revealed that a Zr 66.7Al 11.1Ni 22.2 alloy created from the tetragonal Zr 2Ni structure by replacing Ni with Al reproduces the features of the local atomic arrangements of the experimental data for a Zr 60Al 15Ni 25 bulk metallic glass (BMG) in a supercooled liquid state while the alloy from the Zr 6Al 2Ni structure by substituting Al and Ni atoms agrees to the BMG in an as-quenched state.

The microprocesses of the quasicrystalline transformation in Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass

The microprocesses of the amorphous-to-quasicrystalline transformation in Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass were studied in detail by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). It was found that the amorphous-to-quasicrystalline transformation in the present system undergoes a series of interprocesses and follows the sequence of the phase transformation of amorphous →FCCZr2Ni→Tetragonal Zr2Ni→Tetragonal Zr2Ni with a domain structure →Quasicrystals. HRTEM indicated that a domain structure with a high density of stacking faults was formed in the sample at the stage just prior to the formation of quasicrystals. This special structure seems to possess the structural symmetries similar to those of icosahedral quasicrystals. Nanobeam energy dispersive x-ray spectrum revealed that atomic diffusion was involved in all interprocesses of the phase transformations. This suggests that the amorphous-to-quasicrystalline transformation in the present bulk metallic glass is a nonpolymorphous reaction.

Influence of alloying on the formation of Zr-based quasicrystals

Glassy Zr–Cu–Ni–Al precursor alloys exhibit quasicrystal forming ability in a narrow concentration range via isothermal heating. The formation of quasicrystals has been explained by various arguments. Often, they are considered as Hume-Rothery phases, atomic size or bond strength dependent structures, or a hybrid structure built from Al 2Cu- and MoSi 2-type structural units. The aim of the present investigation is to find out the influence of alloying on quasicrystal formation using transmission electron microscopy and X-ray diffraction. Small addition of elements such as Sn or Mo was found to change the phase selection during the early stages of crystallization significantly. Substitution with Sn, having the same oxidation state, leads to the formation of the icosahedral phase. On the other hand, substitution with Mo, having a higher oxidation state, leads to the formation of the fcc big cube phase. However, in the present investigation substitution of atoms bigger than Zr such as Y or Ce leads to very easy formation of quasicrystals even at lower temperatures. Si, on the other hand, which is smaller than Zr leads to the formation of the tetragonal Zr 2Ni phase.

Quasicrystal Formation in a Zr-based Bulk Metallic Glass

ABSTRACTZr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass with a diameter of 2 mm and a length of 50 mm was prepared by copper-mould casting. The metallic glass obtained follows two-step crystallization reactions with the initial formation of metastable quasicrystals from amorphous phase, followed by the formation of a stable intermetallic Zr2Cu from the post-formed quasicrystals. In this paper, the microprocess of the amorphous-to-quasicrystalline transformation of Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass was studied in detail by TEM and high resolution TEM coupled with nanobeam EDX. It was found that the amorphous-to-quasicrystalline transformation in the present system does not follow the traditional nucleation/growth mechanism. Instead, it undergoes a series of inter-processes and follows the sequence of phase transformation of amorphous → FCC Zr2Ni → Tetragonal Zr2Ni → Quasicrystals. Nanobeam EDX revealed that atomic diffusion was involved in all inter-processes of the phase transformations, suggesting that the amorphous-to-quasicrystalline transformation in the present bulk metallic glass is a non-polymorphous reaction.

Quasicrystal Formation in a Zr-based Bulk Metallic Glass

ABSTRACTZr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass with a diameter of 2 mm and a length of 50 mm was prepared by copper-mould casting. The metallic glass obtained follows two-step crystallization reactions with the initial formation of metastable quasicrystals from amorphous phase, followed by the formation of a stable intermetallic Zr2Cu from the post-formed quasicrystals. In this paper, the microprocess of the amorphous-to-quasicrystalline transformation of Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass was studied in detail by TEM and high resolution TEM coupled with nanobeam EDX. It was found that the amorphous-to-quasicrystalline transformation in the present system does not follow the traditional nucleation/growth mechanism. Instead, it undergoes a series of inter-processes and follows the sequence of phase transformation of amorphous→FCC Zr2Ni →Tetragonal Zr2Ni → Quasicrystals. Nanobeam EDX revealed that atomic diffusion was involved in all inter-processes of the phase transformations, suggesting that the amorphous-to-quasicrystalline transformation in the present bulk metallic glass is a non-polymorphous reaction.

Stability of glassy state in Zr-based glassy alloys correlated with nano icosahedral phase formation

We investigated the stability of the glassy state by examination of transformation behaviour and structural analysis in Zr-based binary and ternary glassy alloys. It is well known that the melt-spun Zr-Cu binary alloys have a distinct glass transition prior to crystallization in a wide composition region of 20 to 70 at% Cu, while no supercooled liquid state appears in the melt-spun Zr-Ni binary alloys. It is expected that the local structure of the disordered state in the two alloy systems is different. The icosahedral quasicrystalline phase (I-phase) is formed as a primary phase in the Zr 70Cu 30 binary glassy alloy by substitution of Pd for Cu. The minimum Pd concentration for I-phase formation is 0.5 at%. In contrast, the primary phase is a tetragonal Zr 2Ni phase in a Zr 70Ni 30 amorphous alloy by substitution of Pd by less than 10 at% for the Ni element. The radial distribution function (RDF) of the Zr 70Cu 30 glassy alloy is different from that of the Zr 70Ni 30 amorphous alloy. The coordination numbers around Zr in the melt-spun Zr 70(Cu or Ni) 30 binary alloys are 12.4 and 10.7, respectively, indicating the possibility of existence of the icosahedral local atomic configuration in the Zr 70Cu 30 glassy alloy. It is also realized that a tetragonal Zr 2Ni-like local atomic configuration is formed in the Zr 70Cu 30 amorphous alloy. Considering that the I-phase is also formed in the Zr 60Ni 25Al 15 glassy alloy by addition of 3 at% Pd, where the distinct glass transition is observed, the appearance of the supercooled liquid region is attributed to the existence of the icosahedral local atomic configurations consisting of Zr-Cu or Zr-Ni-Al. These results show that the icosahedral short- and medium-range orders exist in the supercooled liquid as well as in the glassy phase and they stabilize the glassy state in the Zr-based alloys.

Metastable phases in Zr-based bulk glass-forming alloys detected using a synchrotron beam in transmission

Nucleation and crystallisation in Zr-based bulk metallic glasses has been directly detected using high intensity high energy monochromatic synchrotron beam diffraction in transmission during in-situ heat treatment. Previously, it was established that crystallisation of the amorphous phase occurs via initial nucleation and growth of metastable phases that transform to equilibrium tetragonal Zr 2Cu before melting. However, no trace of nucleation of the metastable phase was obtained during cooling of the liquid alloy from above liquidus temperature T l and Zr 2Cu forms directly from the liquid. The metastable crystalline nanostructure obtained during rapid heating was found to depend sensitively on the alloy composition. Ti addition suppresses the previously detected metastable state and leads to a new phase with structure close to that of tetragonal Zr 2Ni phase but with a highly stressed nanostructure. On the other hand, increasing the Zr content to nearly 70 at.% allows easy preparation of the metastable state during rapid heating at rates of the order of 1 K/s as used in most DSC experiments.

Influence of hydrogen on formation and stability of Zr-based quasicrystals

Hydrogenation of quasicrystalline Zr69.5Cu12Ni11Al7.5 was performed electrochemically in a 2:1 glycerine–phosphoric acid electrolyte. Hydrogen absorption/desorption as well as the influence of hydrogen on the formation and stability was studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS). In comparison to amorphous Zr–Cu–Ni–Al the absorption kinetics in quasicrystalline Zr–Cu–Ni–Al were found to be faster. Desorption is hindered in both materials probably due to the formation of thin ZrO2 layers. Only partial desorption of hydrogen was observed by means of TDS to occur prior to the decomposition of the quasicrystalline phase. The observed increase in length during hydrogen charging indicates an interesting new result. Instead of the continuous increase of the specific volume per hydrogen atom at very low hydrogen concentration typical for the filling up of larger trapping sites, a decrease from a rather high value is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty ones.Above a hydrogen content of H/M=0.05 the formation of Zr–Cu–Ni–Al quasicrystals is replaced by tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2. At high hydrogen contents (about H/M=1.0) phase separation is assumed to take place followed by the formation of nanocrystalline ZrH2. Icosahedral Zr69.5Cu12Ni11Al7.5 was found to decompose through a discontinuous transformation by complex precipitation reactions; it transforms mainly into tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2. Hydrogenation of quasicrystals at low concentrations does not change the phases formed during decomposition. At hydrogen contents higher than H/M=1.1 decomposition of quasicrystals starts with the precipitation of tetragonal ZrH2−x.

Crystallization in Zr-Cu-Ni-Al Metallic Glasses

AbstractZr-Cu-Ni-Al exhibits one of the best glass forming alloys known. In a narrow concentration range, however, annealing leads to the formation of a metastable icosahedral phase. The aim of this paper is to investigate in detail the influence of alloying with Hf or Si, Sn, Y, Cr and Mo on the quasicrystal formation in Zr69.5Cu12Ni11Al7.5 glasses.Whereas Y addition leads to loss in thermal stability and pronounced formation of quasicrystals, the addition of Si, Sn or Mo does not deteriorate thermal stability. Sn and Si were observed even to improve thermal stability significantly, but also to change phase selection during the early stages of crystallization. Si addition results in the formation of the tetragonal Zr2Ni phase and Mo as well as Cr to the formation of the fcc, so-called big-cube phase.Nucleation rates for the quasicrystal formation were measured by means of crystallization statistics. By modeling the obtained nucleation rates in the framework of diffusion controlled classical nucleation the interface energies (between quasicrystals and amorphous matrix) as well as metastable melting temperatures for the quasicrystalline phase could be derived. In all cases nucleation could be described best with a homogeneous model with transient rates.In order to understand the formation of the icosahedral structure also data on the influence of oxygen and hydrogen contamination as well as recent results of other authors on the influence of noble metals such as Ag, Pd, Au and Pt will be discussed in detail. There is some evidence that the quasicrystals are a hybrid of two structural elements, i.e. the tetragonal Zr2Ni (Al2Cu) and the tetragonal Zr2Cu (MoSi2) structure. It is further assumed that an icosahedral short-range order of the melt is stabilized by the addition of elements like Hf, probably Y or by contamination with very small amounts of oxygen, thus leading to a reduced interfacial energy between the quasicrystals and the amorphous matrix and therefore to a lower nucleation barrier.

Oxidation of Zr-based metallic glasses in air

Oxidation of Zr-based metallic glasses in air below the glass transition temperature was studied in the temperature range between 330°C and 370°C by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Oxidation kinetics were observed to follow a parabolic law except for Zr65Cu25Al10. Whereas Zr70Ni20Al10 is oxidation resistant, the oxidation rate increases from Zr75.6Cu12.2Ni12.2toZr65Cu25Al10andZr69.5Cu12Ni11Al7.5;Zr65Cu27.5Al7.5 has the fastest oxidation of the glasses investigated. Oxidation resistance was found to deteriorate by addition of Cu, but improved by Al additions. The scales formed during the oxidation have a nanocrystalline microstructure consisting mainly of tetragonal and monoclinic ZrO2 probably incorporating the other metals; no Zr-depleted zone could be observed in the remaining glassy or crystallized matrix. Crystallization was found to compete with the oxidation process of most glasses; only Zr65Cu25Al10 was still nearly amorphous after oxidation annealing for 4 h at 360°C. Depending on the glass tetragonal Zr2Ni and Zr2Cu as well as hexagonal Zr6NiAl2 were identified in the matrix.

Influence Of Hydrogen On The Thermal Stability Of Zr-Based Quasicrystals

AbstractThe high number of tetrahedrally coordinated sites for interstitial hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline Zr-Cu-Ni-Al alloys a candidate for hydrogen storage applications. The icosahedral phase in Zr69.5Cu12Ni11Al7.5 has been shown to absorb electrochemically hydrogen up to H/M = 1.6. Only partially desorption of hydrogen was observed by TDA at about 500°C. Since hydrogen desorption seems to be hindered by a surface barrier, the hydrogen remains in the quasicrystal thus influencing their thermal stability. Such effects of hydrogen were studied by DSC and microstructural investigations. With increasing hydrogen content the decomposition of icosahedral Zr69.5. Cu12Ni11A17.5 is shifted to lower temperatures. Quasicrystals decompose by a discontinous transformation into the tetragonal Zr2Cu phase, the tetragonal Zr2Ni and the hexagonal Zr6NiAl2. Hydrogenation does not influence the phases formed during decomposition, but leds to the formation of a finer microstructure. We assume that the defects formed during the hydrogenation accelerate the nucleation of the stable crystalline phase.

Single-crystal elastic constants of Zr2Ni

The ultrasonic pulse-echo-overlap technique has been used to determine the six independent elastic constants of tetragonal Zr2Ni over the temperature range 4.2–300 K. The temperature dependence of the shear constants, particularly C66, are atypical and indicate a significant degree of mode softening at low temperatures. The results correlate with the recent calculation of Sinha and Harmon in that Zr2Ni satisfies their conditions for the development of charge density waves with attendant mode softening and support the view that mode softening and superconductivity are correlated.