Metastable Quasicrystalline Phase Research Articles

ICOSAHEDRAL PHASE OF Al65Cu25Fe15

The present work aimed to characterize the microstructure of the icosahedral phase (quasicrystalline phase-ϕ) of the system with stoichiometric composition of the quasicrystal Al65Cu25Fe15 . The ternary alloy with nominal composition of Al63Cu25Fe12 was processed by mechanical alloying (MA) as a viable solid state processing method for producing various metastable and stable quasicrystalline phases. The structural characterization of the obtained samples was performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the elemental composition was determined by dispersive energy spectroscopy (DES). The diffraction patterns of Al65Cu25Fe15 showed the presence of ω-Al7Cu2Fe , β-Al(Fe, Cu) and λ-Al13Fe4 phases that coexist with the thermodynamic quasicrystalline phase-ϕ. Finally, elemental analysis indicates that during alloy synthesis there is little variation of the ideal composition. The results indicate that alloys with high percentage of icosahedral phase can be obtained by casting in the air.

THE ICOSAHEDRAL PHASE OF Al63Cu25Fe12 SYSTEM

The present work aimed to characterize the microstructure of the icosahedral phase (quasicrystalline phase-ϕ) of the system with stoichiometric composition of the quasicrystal Al63Cu25Fe12. The ternary alloy with nominal composition of Al63Cu25Fe12 was processed by mechanical alloying (MA) as a viable solid state processing method for producing various metastable and stable quasicrystalline phases. The structural characterization of the obtained samples was performed by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM), while the elemental composition of the chemical elements Al, Fe and Cu were determined by X-ray spectroscopy technique of dispersive energy (EDS). According to the results of XRD, the diffraction patterns of Al63Cu25Fe12 showed the presence of β-Al(Fe, Cu) and λ-Al13Fe4 phases coexist with the thermodynamic ϕ-phase quasicrystalline. Finally, elemental analysis indicates that during alloy synthesis there is little variation of the ideal composition. The results indicate that alloys with high percentage of icosahedral phase can be obtained by casting in the air.

The Influence of Rapidly Solidified Ribbons Pre-treatment on Structure of Bulk AlFeMm Alloys Prepared by Powder Metallurgy

Rapidly solidified AlFe7Mm4 ribbons were prepared by melt spinning process. The ribbons were composed from super saturated solid solution of alloying elements in Al matrix, stable intermetallic phases and metastable quasicrystalline phases. Both metastable phases can potentially provide self-healing properties to the material - super saturated solid solituon by precititation and quasicrystals by decomposition to stable phases. Key problem of processing such materials is to preserve the microstructure of rapidly solidified material during its compaction. Spark plasma sintering enables powder solidification in very short time - in range of few minutes. This article describes the best way of raidly solidified ribbons before SPS compaction to obtain bulk material. The ribbons were solidified in the initial state and after cryo-milling. Ball-milling before of rapidly solidified ribbons was also tested. Vickers hardnes HV0.01 was also evaluated. Cryo-milling was chosen as an optimal pre-treatment before compaction by SPS.

Thermal stability, microstructure and crystallization kinetics of melt-spun Zr-Ti-Cu-Ni metallic glass

Melt-spun Zr 63.33Ti 8.89Cu 15.45Ni 12.33 glassy ribbons display a double-step devitrification behavior characterized by the precipitation of a metastable quasicrystalline phase in the first stage of the crystallization process, followed by the formation of crystalline phases in the following crystallization event. Investigation of the crystallization kinetics reveal that the initial part of the glass-to-quasicrystalline transformation ( x ≤ 55 vol.%) occurs by diffusion controlled growth with an increasing nucleation rate (Avrami exponent n ≥ 2.5), whereas the later stage of the transformation ( x > 55 vol.%) is dominated by the growth of the formed nuclei rather than by the generation of new nuclei (2.0 ≤ n ≤ 2.5). The activation energy for quasicrystallization is 360 kJ/mol, which is comparable to the values reported for other quasicrystal-forming Zr-based metallic glasses.

Limited quasicrystal formation in Zr–Ti–Cu–Ni–Al bulk metallic glasses

Bulk amorphous Zr 62− x Ti x Cu 20Ni 8Al 10 alloys with compositions of 0 ⩽ x ⩽ 7.5 were prepared by copper mold casting. Thermal stability and crystallization behavior of the specimens were characterized by differential scanning calorimetry. Structures were studied by X-ray diffraction and transmission electron microscopy. The mass density of the as-cast samples and of selected annealed states was determined. The difference in density changes, between as-cast glassy, relaxed and crystallized structures, depends on the Ti content of the alloys. The higher the packing density of the glassy state, the higher is the thermal stability of the bulk glass due to a decrease in atomic mobility. Small gradations of Ti in steps of 0.2 at.% reveal significant changes in the crystallization behavior of the alloys, concerning the size and the structure of the phases formed. Only compositions with 2 ⩽ x < 4 form a metastable quasicrystalline phase during the first transformation step.

Thermal Stability of Rapidly Solidified Alloys of Aluminium with Transition Metals

Rapidly solidified (RS) Al-TM (TM = transition metal) alloys are perspective materials from scientific, as well as technological point of view. Generally, they are produced by the melt atomization or by the melt spinning. Subsequent compaction is commonly performed by the hot extrusion. Since transition metals, such as Cr, Fe, Ni, Zr, Ti, Mn and others, have low diffusion coefficients in solid aluminium (lower by several orders of magnitude than those of common alloying elements like Cu, Si, Mg, Zn etc.) the RS Al-TM alloys are characterized by a high thermal stability. In this paper, several RS Al-TM (TM = Cr, Fe, Ti, Mn, Ni) alloys prepared by the melt spinning and melt atomization are compared to commercially available 2xxx, 6xxx and 7xxx wrought alloys. The main structural features of both RS and wrought alloys are described. The RS alloys are characterized by the presence of micro and nano-scale crystalline and/or quasi-crystalline phases and supersaturated solid solutions. The elevated-temperature behaviour is compared for both groups of materials. The thermal stability of the investigated materials is determined by room temperature hardness measurements after various annealing regimes and a high thermal stability of the RS alloys is demonstrated. The microstructural changes and phase transformations occurring in the investigated materials upon heating are described. In the Al-TM alloys, very slow decomposition of the supersaturated solid solutions, precipitation and decomposition of the metastable quasi-crystalline phases occur.

Compositional dependence on phase selection during devitrification of amorphous Zr–Pt alloys

The devitrification pathways of alloys were determined using time-resolved synchrotron radiation. Amorphous Zr73Pt27 ribbons transformed directly to the stable Zr5Pt3 structure but ribbons with higher Zr contents first formed a metastable quasicrystalline phase before transforming to the Zr5Pt3 structure. Lower crystallization activation energies were observed for the higher Zr-content alloys.

Decomposition and metastable phase formation in the bulk metallic glass matrix composite Zr56Ti14Nb5Cu7Ni6Be12

Decomposition in the undercooled liquid phase and crystallization during isothermal annealing in the glassy matrix of the bulk Zr56Ti14Nb5Cu7Ni6Be12 composite were investigated by x-ray diffraction, small angle neutron scattering (SANS), scanning electron microscopy, and transmission electron microscopy (TEM). Upon isothermal annealing near the glass transition temperature TEM images show the presence of quasicrystals, while SANS data exhibit an interference peak indicating a quasiperiodic arrangement of these metastable crystals. Samples annealed in the solid state show a transformation into the more stable Zr2 TM-type (TM=Nb,Cu,Ni,Ti) phases. These results have been discussed on the basis of a phase separation process that determines the formation of the metastable quasicrystalline phase that is in good agreement with the behavior of other known bulk metallic glass-forming systems.

Quasicrystal formation in gas-atomized Zr80Pt20 powders

Gas-atomized powders with a nominal composition of Zr80Pt20 have been investigated as a function of powder size range. Atomized powders were screened to the following sizes for examination: <10 µm; +10, −20 µm; +20, −45 µm; +45, −75 µm; +75, −106 µm. For comparison of as-synthesized structure and crystallization behaviour, Zr80Pt20 melt-spun ribbons (MSRs) were also prepared. Results show that during atomization a metastable quasicrystalline (QC) phase is formed as the predominant phase independent of powder size; a small volume of amorphous phase and crystals were detected in finer and coarser sized powders, respectively. The QC grains formed during atomization are much larger than the nanosized quasicrystal obtained in the MSR, which is most likely attributed to the much slower cooling rates during gas atomization. Nucleation and growth occur during both rapid solidification routes. The formation of an icosahedral quasicrystal during gas atomization and melt spinning is consistent with previous suggestions that icosahedral local structure exists in liquid and amorphous phases and facilitates nucleation of quasicrystal formation in Zr80Pt20 alloys.

Influence of Pd on formation of amorphous and quasicrystal phases in rapidly quenched Zr2Cu(1−x)Pdx

The role of Pd in the transformation from an amorphous state to a metastable icosahedral quasicrystalline phase in Zr2Cu(1−x)Pdx (x = 0 to 1) metallic glasses was investigated using high-energy synchrotron X-rays and differential scanning calorimetry. The total scattering functions show an increasing development of the high-Q side of the second diffuse scattering peak at 5.09 Å−1 with increasing Pd content. The reduced radial distribution functions reveals that the bonding distance of the Zr–(Pd/Cu) pairs increases from 2.76 to 2.82 Å when x increases from 0.00 to 1.00, while the distance for the Zr–Zr pairs remains almost constant at 3.10 Å. Thermal analysis and X-ray diffraction together show that an amorphous-to-quasicrystal phase transition is not observed in the Zr2Cu alloy, but partial or total substitution of Cu by Pd in Zr2Cu(1−x)Pdx alloys does lead to quasicrystal formation.

Glass formation and crystallization in binary Zr–Pt systems

Previous rapid solidification studies of Zr–Pt alloys have focused primarily on compositions very close to the Zr80Pt20 eutectic. In the current work, a wide range of alloys, Zr x Pt100 − x (69 ≤ x ≤ 81), were synthesized by melt spinning to examine their starting structures and devitrification behaviours. Similar to the frequently reported formation of a metastable quasicrystalline phase during heating of near-eutectic compositions, ribbons with Zr contents as low as 75 at.% show comparable transformations, but at higher temperatures, before transforming to the stable Zr5Pt3 structure. With the quench rate used in this study, a further decrease below 75 at.% Zr precludes the formation of the metastable quasicrystalline phase and instead results in crystallization directly to the stable Zr5Pt3 structure. Preliminary analysis of the transformation kinetics reveals a lower activation energy for the transformation of an amorphous Zr–Pt structure to the metastable quasicrystal phase in comparison to the stable Zr5Pt3 phase.

Effects of liquid quenching and subsequent heating on the structure of Co-Al Alloys

The phase composition and structure of Co-Al alloys after liquid-quenching (LQ) and subsequent heating were determined by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry. After LQ, the Co-19.5 at % Al alloy consisted of two phases: CoAl (B2 structure, a=0.2852 nm) and an fcc Co1 − xAlx solid solution (a=0.3573 nm). The phase composition of the LQ alloy and the lattice parameters of the intermetallic phase CoAl and solid solution corresponded to a solidification temperature of 1100°, rather than to the eutectic temperature (1400°C). Heating to 1000°C brought the alloy to the equilibrium state. In the range 25–28 at % Al, the LQ alloys were single-phase and consisted of Co-enriched CoAl (B2). Decomposition of this intermetallic phase during heating in the calorimeter gave rise to an exothermic peak at 680°C and led to precipitation of hcp Co, which converted to an fcc Co1 − xAlx solid solution on heating to 1000°C. The activation energy of the decomposition process, evaluated by the Kissinger method, was Ea=125 ± 10 kJ/mol. The LQ Co-71.4 at % Al alloy consisted of three phases: metastable quasicrystalline phase (similar in structure to Al14Co3Ni3), Co2Al5, and CoAl. Heating brought the alloy to the equilibrium state (Co2Al5), with an exotherm near 690°C. The activation energy of this phase transformation was Ea=195 ± 15 kJ/mol. The LQ alloys containing 76.5 and 82 at % Al were single-phase and consisted of monoclinic Co4Al13 or Co2Al9 with reduced lattice parameters.

Meta-stable quasicrystal formation in vitrified and solid state synthesized amorphous Zr-based alloys

Amorphous Zr 70Pd 30, Zr 70Pd 20Cu 10, Zr 80Pt 20, Zr 70Pt 20Cu 10, and Zr 57Ti 8Nb 2.5Cu 3.9Ni 1.1Al 7.5 alloys were prepared by melt spinning and mechanical milling to compare their devitrification behaviors. The melt-spun ribbons transform from an amorphous structure to a meta-stable quasicrystalline phase. This amorphous-to-quasicrystalline transition is not observed in amorphous powders obtained by mechanical milling. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that in the absence of the short-range order obtained directly from the liquid, meta-stable quasicrystalline phase formation is not energetically favored in these Zr-based metallic glasses.

Devitrification studies of Zr–Pd and Zr–Pd–Cu metallic glasses

High temperature X-ray diffraction (HTXRD) is used to investigate the devitrification pathway for two related Zr-based metallic glasses, Zr 70Pd 30 and Zr 70Pd 20Cu 10. Both alloys have similar as-quenched structures and initially devitrify to form a meta-stable quasicrystalline phase. The HTXRD data for the Zr 70Pd 30 alloy shows the coexistence of the quasicrystalline and the Zr 2Pd (I4/mmm) crystalline phases over a range of 25 K. Conversely, the Zr 70Pd 20Cu 10 alloy shows an additional transformation of the quasicrystalline phase to a meta-stable Zr 2(PdCu) intermetallic (Zr 2Si type structure, I4/mcm) that polymorphically transforms to the Zr 2Ni type structure (I4/mmm) over a very narrow temperature range.

Influence of Al on Quasicrystal Formation in Zr-Ti-Nb-Cu-Ni-Al Metallic Glasses

ABSTRACTThe effect of Al on the crystallization behavior of (Zr0.616Ti0.087 Nb0.027Cu0.15Ni0.12)100−xAlx melt-spun glassy ribbons with x = 7.5, 5, 2.5 and 0 was investigated by differential scanning calorimetry, x-ray diffraction and transmission electron microscopy. The devitrification of the ribbons is characterized by the formation of a metastable quasicrystalline phase during the first stage of the crystallization process even for the alloy with x = 0. Therefore, Al is not essential for quasicrystal formation in the present alloys. However, it affects the properties of the amorphous as well as of the quasicrystalline phase. With decreasing Al content, the temperature range of stability of the quasicrystalline phase increases whereas the thermal stability of the amorphous phase decreases together with a slight decrease of the extension of the supercooled liquid region. Thus, it is concluded that although the addition of Al improves the stability of the glassy phase, it has no beneficial effect on the formation of quasicrystals.

Influence of Al on Quasicrystal Formation in Zr-Ti-Nb-Cu-Ni-Al Metallic Glasses

ABSTRACTThe effect of Al on the crystallization behavior of (Zr0.616Ti0.087Nb0.027Cu0.15Ni0.12)100–xAlx melt-spun glassy ribbons with x = 7.5, 5, 2.5 and 0 was investigated by differential scanning calorimetry, x-ray diffraction and transmission electron microscopy. The devitrification of the ribbons is characterized by the formation of a metastable quasicrystalline phase during the first stage of the crystallization process even for the alloy with x = 0. Therefore, Al is not essential for quasicrystal formation in the present alloys. However, it affects the properties of the amorphous as well as of the quasicrystalline phase. With decreasing Al content, the temperature range of stability of the quasicrystalline phase increases whereas the thermal stability of the amorphous phase decreases together with a slight decrease of the extension of the supercooled liquid region. Thus, it is concluded that although the addition of Al improves the stability of the glassy phase, it has no beneficial effect on the formation of quasicrystals.

Formation of quasicrystals in Zr–Pd–(Cu) melt spun ribbons and mechanically milled powders

Amorphous Zr70Pd30 and Zr70Pd20Cu10 alloys were prepared by mechanical milling and melt spinnng to compare their devitrification behaviors. The devitrification of mechanically milled Zr70Pd30 and Zr70Pd20Cu10 powders occurs via a single-step, first-order transformation to a stable Zr2Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary meta-stable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon. During mechanical milling of an amorphous melt spun ribbon, crystallization of the quasicrystalline phase appears to precede disordering into an amorphous structure having an different short range order. Deformation of an amorphous melt spun ribbon by repetitive rolling at ambient temperature crystallizes the meta-stable quasicrystalline phase.

Crystallization of ZrTiCuNiBe bulk metallic glasses

The crystallization behaviour, and microstructural changes of ZrTiCuNiBe bulk metallic glass (BMG) were investigated in the as-prepared, preannealed, doped, and high pressure annealed conditions by means of differential scanning calorimetery (DSC)_X-ray diffraction (XRD), neutron scattering, density and acoustic measurements. Significant structural relaxation induced by preannealing and high pressure annealing were observed in the BMG. The effects of relaxation, phase separation, additions, and high pressure on the crystallization of the BMG were investigated. A detectable precipitation of metastable quasicrystalline phase at the first crystallization step, and further transition to stable crystalline phases at high temperature, accompanied by induced microstructural and property changes were observed. It was found that the crystallization behaviour as well as the apparent activation energy of the crystallization was sensitive to the external conditions.

Synthesis route-dependent formation of quasicrystals in Zr70Pd30 and Zr70Pd20Cu10 amorphous alloys

The devitrification of mechanically milled amorphous Zr70Pd30 and Zr70Pd20Cu10 powders occurs via a single-step, first-order transformation to a stable Zr2Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary metastable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon.

Effect of hydrogen on Zr 65Cu 17.5Al 7.5Ni 10 metallic glass

Glassy Zr 65Cu 17.5Al 7.5Ni 10 alloy samples with 0.13 and 0.46 at.% oxygen were prepared by single-roller melt-spinning. Subsequently, the ribbons were electrolytically charged to different hydrogen-to-metal ratios 0.035≤H/M≤1.6. The effect of hydrogen on the thermal stability and on the crystallisation behaviour of the alloy samples was studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). With increasing hydrogen content, a reduction of the undercooled liquid region and a significant change in the crystallisation process was observed. Hydrogen was found to suppress the oxygen-triggered formation of the metastable quasicrystalline phase, whereas the metastable f.c.c.-phase formation seems to be less affected. Furthermore, hydrogen-induced copper segregation was detected after long-term charging to high H/M ratios at room temperature and already for samples with low hydrogen contents upon heating. At very high H/M ratios, the thermal behaviour of the investigated Zr 65Cu 17.5Al 7.5Ni 10 alloy samples is determined by preferential zirconium hydride formation and successive decomposition causing the formation of new stable intermetallic phases with lower zirconium content.