keV Xenon Ion Research Articles

Interface assisted formation of a metastable hcp phase by ion mixing inan immiscible Ag-Ni system

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in theAg80Ni20 multilayered films, in which the excess interfacial freeenergy provided partial driving force for alloying in an equilibrium immiscibleAg-Ni system characterized by a large positive heat of formation. Themetastable phase was identified to be of D019 hcp structure with astoichiometry of Ag3Ni and can therefore be considered as a Hume-Rothery 7/4electron compound. A thermodynamic calculation showed that when the multilayersconsisted of 8 or more bilayers, the interfacial free energy could elevate theirinitial energy levels up to a state higher than that of the hcp phase, and thatwhile the multilayers composed of 6 or less bilayers, it was thermodynamicallynot favoured to form the hcp phase. Kinetically, the metastable hcp phase wasgrown from the fcc Ag lattice through a fast sliding mechanism. Furthermore, anab initio calculation showed that a minimum total energy of the Ag3Niphase did correspond to the above observed metastable state, indicating that thestability of the metastable Ag-Ni hcp phase was correlated with its electronicstructure.

Metastable phases induced by ion irradiation in the equilibrium immiscible Fe–Cu system

Anomalous structural evolution was induced by room-temperature 200 keV xenon ion irradiation and it results in the formation of various new metastable phases in the equilibrium immiscible Fe–Cu system. First, nanosized quasicrystals were formed in an amorphous matrix through a two-step transition of crystal to amorphous to quasicrystal in Fe70Cu30 multilayered films. The real compositions of the amorphous matrix and quasicrystals were determined to be close to Fe70Cu30 and Fe50Cu50, respectively. Moreover, the same icosahedral phase was also obtained in another similarly designed Fe50Cu50 multilayered sample upon 850 °C thermal annealing, confirming the existence of such a metastable state. Second, amorphous alloys were formed in a composition range of 30–50 at. % of Cu. Third, a Cu-based face-centered-cubic (fcc) solid solution was formed at an alloy composition of about Fe30Cu70 and, interestingly, another fcc structured metastable crystalline phase was obtained at a composition very close to that of Fe50Cu50. A Gibbs free energy diagram of the Fe–Cu system was calculated, with special consideration of excess interfacial free energy stored in the multilayered films, within the framework of Miedema’s model and it gives a reasonable thermodynamic explanation to the metastable phase formation observed. The amorphous-to-quasicrystal transition is also discussed in terms of the similarity in the atomic configuration between the icosahedral and amorphous short-range orders. Furthermore, molecular dynamics study showed that Fe and Cu could be mixed on an atomic scale in the metastable phases observed.

Sequential formation of quasicrystalline and amorphous phases in the Fe–Nb and Fe–Ta multilayers upon ion irradiation

An unusual phase formation sequence of first quasicrystalline and then amorphous phases was observed in the Fe–Nb and Fe–Ta multilayers upon room temperature 200 keV xenon ion irradiation. The sequential structural evolution was attributed to the gradually destroying of order in the original crystalline lattices with increasing ion dose.

Dissociation study of tellurium cluster ions, Ten+ (n = 25-85) using secondary ion mass spectrometry

Dissociation pathways of tellurium clusters, Te(n)(+) (n = 25-85), were investigated by secondary ion mass spectrometry. Positively charged ions were generated from a tellurium sheet by bombardment with 10 keV xenon ion beam. Mass analyses of cluster ions were performed using a grand-scale sector mass spectrometer. In the first field-free region, Te(n)(+) (n = 25-80) had a large dissociation probability with five- and six-atom emission and Te(n)(+) (n = 50-85) had a slightly large dissociation probability with 10- and 11-atom emission. Five- and six-atom dissociation in the second field-free region could be also observed. These results were most likely due to the cluster emission processes for Te(n)(+), although sequential atom emission and cluster emission could not be distinguished by this type of experiments.

Formation of Quasicrystalline Phases in the Equilibrium Immiscible Co-Cu and Fe-Cu Systems by Ion Mixing/Solid-State Reaction

AbstractAnomalous structural evolution was induced in the equilibrium immiscible Co-Cu and Fe-Cu systems by 100 or 200 keV xenon ion irradiation at 77 K or room temperature. In the Co15Cu85 and Fe70Cu30 multilayered films, nanosized quasicrystals were formed in an amorphous matrix, through a two-step transition of crystal-to-amorphous-to-quasicrystal. The obtained quasicrystals are Co-Cu dodecagonal and Fe-Cu icosahedral phases with twelve-fold and five-fold rotational symmetries, respectively. The real compositions of the amorphous matrix were determined to be close to Co10Cu90 and Fe70Cu30, while those for quasicrystals are nearly Co20Cu80 and Fe50Cu50, respectively. Moreover, the same dodecagonal and icosahedral phases were also obtained in the specifically designed Co50Cu50 and Fe50Cu50 multilayered samples upon thermal annealing at 500°C and 850°C, respectively, confirming the existence of these new metastable states in the respective systems. Besides, our molecular dynamics study showed that either Co or Fe could be mixed with Cu at an atomic scale in forming some metastable alloy phases. The amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the quasicrystal and amorphous short-range orders.

Experimental and Theoretical Studies on Composition Limits of Metallic Glass Formation in the Ni-Mo System

The Ni-Mo system was selected as a model system to study the composition limits of metallic glass formation by ion mixing experiments as well as by molecular dynamics simulation. The 200 keV xenon ion mixing was conducted for ten Ni-Mo mutilayered samples with various concentrations and the results showed that the glass-forming composition range was 39.8-80.4 at.% Mo, which is in good agreement with the assessment from an empirical model. Through comparing the stability of the Ni-rich and Mo-rich solid solutions with various concentrations versus their amorphous counterparts, two critical supersaturated solid solubilities were found by molecular dynamics simulation to be 21 at.% Mo in Ni and 25 at.% Ni in Mo, from which the glass forming range was deduced to be within 21-75 at.% Mo. The correlation of the simulated glass-forming range with those obtained by ion irradiation and solid-state reaction is also discussed.

Irradiation-induced growth of nanoquasicrystals from amorphous matrix in the equilibrium immiscible Fe–Cu system

A microstructure of nanosized quasicrystals embedded in an amorphous matrix was formed in the Fe70Cu30 multilayered films upon room temperature 200 keV xenon ion irradiation. The initial Fe and Cu crystalline phases in the as-deposited multilayers transformed into a unique amorphous phase at a dose of 8×1014 Xe+ cm−2 and further irradiation, i.e., up to a dose of 5×1015 Xe+ cm−2, induced the growth of the quasicrystals in some local areas in the amorphous matrix. High-resolution electron microscopy examination revealed that the compositions of the quasicrystals and amorphous matrix were close to Fe50Cu50 and Fe70Cu30, respectively. Apparently, the above microstructure was formed through a two-step phase transition along the increase of ion dose and the amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the icosahedral and amorphous short-range orders.

Ion‐Induced Amorphization and Regrowth of C49 and C54 TiSi2

C49 and C54 films were implanted with 200 keV xenon ions at −185°C, and the regrowth of C49 and C54 grains was studied following thermal annealing. The low‐temperature implant resulted in partial amorphization of the C54 layer, as evidenced by transmission electron microscopy. Upon annealing at temperatures below ∼600°C, a double‐layer structure was obtained consisting of a C49 layer on top of C54 . For an equivalent implant in C49 , the C49–C54 transition temperature was delayed as compared with the phase transition for a nonimplanted C49 film. Following room‐temperature implantation of C49 with 100 keV germanium ions at various doses or low doses of 70 keV , no reduction in the C49‐C54 transition temperature was observed upon thermal annealing. According to our results, defects caused by or ion implantation of C49 do not promote formation of C54 . © 1999 The Electrochemical Society. All rights reserved.

Interface-driven alloying and metallic glass formation in nano-multilayers in an immiscible Y–Nb system

Several new amorphous alloys were obtained in an immiscible Y–Nb system by room temperature 190 keV xenon ion mixing of Y–Nb multilayered films, which were designed to include a sufficient fraction of interfacial atoms and thus possessed a high free energy comparable to that of the amorphous state. In addition, two metastable f.c.c. phases were formed in Y-rich and Nb-rich multilayered films, respectively. The growth kinetics of the f.c.c. phases and their effect on the composition range of amorphization are also discussed. To understand the observed unusual alloying behaviors, a Gibbs free-energy diagram was constructed based on Miedema's model. The diagram included the free-energy curves of all the involved phases as well as that of the Y–Nb multilayered films including the extra interfacial free energy and thus gave a qualitative and reasonable interpretation to the formation of metastable alloys upon ion mixing. Furthermore, some multilayered films with a sufficient fraction of interfacial atoms were subjected to steady-state thermal annealing, which also resulted in the formation of the Y–Nb amorphous phases, suggesting that alloying in this immiscible system was actually driven by the interfacial free energy.

Sequential disordering during ion-induced amorphization in the Mo–Fe multilayered films

Amorphous alloys were formed within an extended broad composition range from 26 to 67 at.% of Mo by room temperature 200 keV xenon ion mixing of multilayered films in the Mo–Fe system with a small negative heat of formation of −3 kJ/mol, provided the multilayered films were properly designed to include sufficient fraction of interfacial atoms, thus possessing enough excess interfacial free energy. Moreover, a sequential disordering of Mo and Fe lattices was observed, for the first time, in the process of ion-induced amorphization in the Fe–Mo multilayered films. The sequential behavior can be attributed to the fact that Fe acted as a fast diffuser into Mo lattice, leading to an earlier disordering of the Mo crystalline structure and might be also associated with a difference in Debye temperature of the two metals.

Formation of non-equilibrium solid phases in Fe-Nb multilayered films by ion irradiation

In the Fe-Nb system, amorphous alloy, metastable crystalline fcc and hexagonal phases were formed in Nb-rich multilayered films by room temperature 200 keV xenon ion mixing. A heat of formation diagram of the system was constructed based on Miedema's model and Alonso's calculation method, and the diagram gave a relevant interpretation for the observed phase formation. The formation of fcc metastable phase can be interpreted by a reverse martensitic transition of bcc-fcc. The amorphous and hexagonal phases are thought to form through a traditional nucleation and growth mechanism with an additional consideration of the irradiation effect, which kinetically hindered the formation of a complicated intermediate compound.

Thermal stability of ion-beam mixed and laser-irradiated FeNiCr multilayers

A multilayer film having Fe 50Ni 25Cr 25 overall composition has been deposited by e-gun and submitted to two non-equilibrium treatments: ion-beam mixing with 100 keV xenon ions to a dose of 2×10 16 ions cm −2, and pulsed laser irradiation with a beam of 1.1 J cm −2 (FWHM=8 ns and λ=694 nm). The iron layers consisted of the 57Fe isotope, in order to perform a good structural characterisation by means of Mössbauer spectroscopy. Additional structural information was achieved by glancing angle X-ray diffraction analysis. It is found that both treatments give rise to a complete mixing of the layers and that a disordered solid state solution is formed. Structurally, the solid solution obtained is mainly bcc after ion irradiation and is a bcc/fcc mixture after laser irradiation. The metastability of these structures has been studied by heat treatments in vacuum at 450, 640 and 800 °C. Heating at 450 °C induces a solid-state diffusion among the parent elements, which reduces the co-ordination of iron atoms with the alloying elements, as observed in other metastable FeNiCr film or bulk alloys. At higher temperatures a bcc to fcc phase transformation gradually takes place until, at 800 °C, the samples display the thermodynamically stable fcc structure. The thermal evolution of the irradiation multilayers has been compared with that of unirradiated ones.

Metastable alloys synthesized by ion mixing in immiscible Ti–Nb and Ti–Ta systems

Room temperature 200 keV xenon-ion irradiation of multilayers, designed to include sufficient fraction of interfacial atoms, resulted in the formation of amorphous alloys in the immiscible Ti–Nb and Ti–Ta systems, which have previously been recognized as non-glass forming ones because of their positive heats of formation and some other factors. In addition, some metastable crystalline f.c.c. phases were also obtained. To explain the unusual alloying behaviors, Gibbs-free-energy diagrams of the systems were constructed based on Miedema's model and showed that the interfacial free energy originating from chemical and elastic contributions elevated the multilayered films to an energy level higher than those of the observed amorphous and metastable crystalline phases, thus generating a driving force for alloying in these immiscible systems. Meanwhile, the possible growth kinetics of the metastable crystalline phases are also discussed.

Different driving force for metallic glass formation NbNi and NbY systems by ion mixing and solid-state reaction

Room temperature 190 keV xenon ion mixing was conducted for the NbNi (heat of formation = −45 kJ/mol) and NbY (heat of formation = +44 kJ/mol multilayered films, which were designed on the basis of free energy calculation. Metallic glasses were obtained not only in the NbNi system but also the NbY system. To discuss the singularity of the observed metallic glass transition, Miedema's model was employed to construct the corresponding Gibbs-free-energy diagrams, including the free energy curves of amorphous phase and the multilayered films with various fractions of interfacial atoms. The constructed diagram can give a qualitative and reasonable interpretation to the ion mixing results. More interestingly, the interface concept was also applicable to solid-state reaction and thermal annealing of the multilayered films designed under similar considerations and indeed resulted in the formation of glasses in the Nb-Y system, which has long been recognized as a non-glass system.

Irradiation-induced alloying in immiscible Mo–Cu system through multilayer technique

Room temperature 200 keV xenon-ion irradiation of multilayers, designed under interfacial free energy concern, did result in the formation of Mo–Cu metallic glasses only within a narrow Mo-enriched composition range. In addition, some Cu-based alloys of metastable fcc solid solutions were also formed at different irradiation stages. To obtain a possible thermodynamic interpretation, a Gibbs-free-energy diagram was calculated based on the semiquantitative Miedema’s model and the related methods proposed by Alonso. The free energy of the Mo–Cu multilayers including excess interfacial free energy was also estimated and was compared to that of the amorphous state. It turned out that the interfacial free energy stored in the multilayered films could raise the energy level of the multilayers to a state higher than that of the metastable phases and thus played an important part, in comparison to the energy deposited by irradiation, in inducing alloying in this immiscible system.

Alloy phases formed in immiscible CuMo and CuW systems by multilayer-technique

By adding an estimated interfacial free energy contribution, CuMo and CuW multilayered films were properly designed to possess an abnormally high free energy. For the CuMo system, room temperature 200 keV xenon ion mixing of the multilayers resulted in the formation of Mo-enriched amorphous alloys and some Cu-based metastable fee solid solutions. While for the CuW system, thermal annealing of the multilayers resulted in the formation of some metastable alloys. To elucidate this unusual alloying behavior, free energy curves of the respective phases and of the multilayered films were calculated based on Miedema's model. It turned out that the interfacial free energy in the multilayered films elevated the initial energy level of the multilayers to a state higher than that of the observed metastable phases and therefore served as a driving force for alloying in these immiscible systems.

Observation of ion-induced alloy phase formation in an immiscible YTa system

Amorphization was achieved by room temperature 200 keV xenon-ion mixing in an immiscible YTa system, which has a positive heat of formation exceeding +40 kJ/mol. In addition, three new metastable crystalline phases, i.e., an fee phase (fee-I) in Y-rich films, an hep and another fee phase (fee-II) in Ta-rich films, were observed at different ion mixing stages. To give a possible explanation to these anomalous alloying behaviors, a Gibbs-free-energy diagram of the system was calculated based on Miedema's model of heat of formation and a method suggested by Alonso. The free energy of the YTa multilayered films containing excess interfacial free energy was also estimated and added into the diagram. It was found that the excess interfacial free energy stored in the multilayered films could increase the initial energetic state of the multilayers and therefore played an important role in ion-induced alloying in this immiscible system.

Amorphization in aluminum oxide induced by ion irradiation

Amorphization in sapphire induced by irradiation has been observed for years but it is still in discussion whether displacements or implanted ions have an important role for the amorphization. In this work, we perform irradiation with high-energy heavy ions in thin film alpha-alumina below room temperature, so that the effects of atomic displacements, especially of damage cascades, can be revealed. Amorphization was detected at temperatures from 90 K to 230 K under irradiation with 600–900 keV xenon and krypton ions, while no amorphization was detected under irradiation with argon nor oxygen ions. The critical energy density to induce amorphization is estimated. Four stages on the crystalline-to-amorphous transformation was observed from the temperature dependence of the dose-to-amorphization. The effect of damage cascades on the irradiation-induced amorphization and defect kinetics resulting in an increase of the dose-to-amorphization will be discussed.

In situ study of ion-beam induced lattice damage in calcium fluoride crystals

Lattice damage in calcium fluoride induced by 360 keV xenon ion irradiation was measured in situ using Rutherford backscattering and channeling (RBS/C) techniques. Calcium fluoride single crystals of (100) orientation were irradiated at − 115 and 35°C by Xe ion beams along a random direction 15° off the (100) axis. A 2 MeV He ion-beam from a 3 MV tandem accelerator was incident on the CaF 2 crystal along the (110) axis to determine the radiation damage. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 × 10 16 Xe/cm 2. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. Analysis of cross-sectional transmission electron microscopy (XTEM) revealed a damaged but single-crystalline CaF 2 layer. This study demonstrates the radiation tolerance of CaF 2 under displacive irradiation at low to ambient temperatures.

Irradiation-induced alloying in an immiscible TaTi system

In the properly designed multilayered films according to an interfacial free energy estimation, amorphous alloys were formed upon room temperature 200 keV xenon-ion irradiation in the TaTi system, although it was previously considered as a non-glass forming one because of its positive heat of formation as well as the unfavored atomic size and electronegativity. In addition, two metastable Ta-rich and Ti-rich fcc phases of different lattice constants were also obtained. To explain the unusual alloying behaviors, a Gibbs-free-energy diagram of the system was constructed based on Miedema's model. The calculated diagram showed that the interfacial free energy resulting from chemical and elastic contributions in the films could raise the initial energetic state of the multilayered films to a state higher than those of the observed amorphous and metastable crystalline phases, thus generating a driving force for alloying in this immiscible TaTi system. The possible growth kinetics of the metastable crystalline phases is also discussed.