Onset Of Amorphization Research Articles

Glass formation in a multicomponent Zr-based alloy by mechanical attrition and liquid undercooling

The synthesis of a multicomponent Zr60Al10Ni9Cu18Co3 glass by mechanical alloying has been investigated using thermal and structural analysis and compared with a metallic glass produced by liquid undercooling. The solid-state amorphization reaction is preceded by rapid solution of smaller solute atoms in the Zr matrix with a concomitant reduction in grain size to 10 nm at the amorphization onset. A fully amorphous mechanically alloyed sample shows relaxation compared to a sample synthesized by liquid cooling probably due to removal of residual inhomogeneities on the scale of the nanocrystal size at amorphization. While the kinetic pathways for the two synthesis methods differ, the relaxed amorphous phases from each method exhibit identical thermodynamic properties.

Length scale dependence of high-pressure amorphization: the static amorphization of anorthite

AbstractHigh-pressure amorphization of anorthite has been observed by energy-dispersive X-ray diffraction of powdered samples held under static pressure in a diamond anvil cell. The onset of amorphization is accompanied by a significant reduction in the intensity of Bragg reflections at pressures between 10 and 14 GPa, and anorthite becomes completely X-ray amorphous between 14 and 20 GPa. These pressures are significantly lower than those suggested by earlier birefringence studies. The discrepancy can be reconciled in terms of a model of high-pressure amorphization in which partially amorphized anorthite can be regarded as a spatially heterogeneous anti-glass, with long-range order maintained but translational disorder dominating at shorter correlation lengths.

High-pressure behaviour of germanate olivines studied by X-ray diffraction and X-ray absorption spectroscopy

Germanate olivines Mg2GeO4, Ca2GeO4 and CaMgGeO4 have been studied by high-pressure X-ray Diffraction and high-pressure X-ray Absorption Spectroscopy. The three compounds were compressed, in the 0–30 GPa pressure range, at room temperature in a diamond-anvil cell, silicon oil being used as the pressure transmitting medium. Values of K0 are 166 ± 15, 117 ± 15 and 152 ± 14 GPa for Mg2GeO4, Ca2GeO4 and CaMgGeO4 respectively. These olivines all exhibit compression anisotropy, the a axis being the least compressible. Crystal to crystal phase transitions have been observed in Mg2GeO4 and Ca2GeO4 above 12 GPa and 6 Gpa respectively. The nature of these structural changes remains unclear yet. The onset of amorphization has been observed in Mg2GeO4 and Ca2GeO4 at pressures above about 22 and 11 GPa respectively. These phase transitions and amorphization processes do not involve any detectable increase in the coordination number of germanium atoms. At higher pressure (P >23 GPa), we report the onset of a transition from a phase with fourfold coordinated germanium to a phase with higher germanium coordination number in CaMgGeO4.

Energy-dependent amorphization of Ge by Ne, Ar or Kr ion irradiation

Abstract Thin Ge specimens have been irradiated at room temperature with Ne, Ar or Kr ions of different energies, and the doses required for complete amorphization determined by in situ transmission electron microscopy and electron diffraction. Onset of amorphization was detected after the lowest ion doses, reflecting amorphization by individual ions. The ion dose required for complete amorphization increased nearly linearly with ion energy over the range 0·5-3·5 MeV for all ions. Amorphization cross-sections have been determined for all the ions and energies used. The amount of damage, expressed as the number of displacements per atom, required for complete amorphization decreased with increasing ion energy or ion mass. The decrease with increasing ion energy or ion mass is due to a decrease in irradiation-enhanced annealing of amorphous volumes as a result of a decrease in the fraction of low-energy atomic displacements to Ge. Increasing the fraction of low-energy atomic displacements to Ge atoms during...

Atomic structural evolution of pressure-induced amorphization processes in β-cristobalite

The changes in the atomic structure of β -cristobalite before and after the onset of the amorphization are examined in detail by analysing the Molecular Dynamics trajectories of Si and O ions. It is found that long SiO bonds with bond lengths of 1.9-2.5 Å emerge after the onset of amorphization. Most of oxygen ions bonded to Si with long SiO bonds come from neighbouring SiO 4 tetrahedrals. The breakage of SiO bonds is found to occur frequently during the amorphization process, with the majority of the broken bonds being long SiO bonds. Our results show that bond breaking and relatively large atomic displacements play important roles in the pressure-induced amorphization of β -cristobalite and that the structural changes in the material during amorphization involve both short and inter-medium range orders.

Ion-beam induced disordering and onset of amorphization in spinel by defect accumulation

Ion-irradiation induces amorphization in many intermetallics and ceramics, but spinel (MgAl2O4) is considered a “radiation resistant” ceramic. Spinel was irradiated with 1.5 MeV Kr+ at 20 K and observed in situ by transmission electron microscopy (TEM). The spinel remained crystalline to a high dose of 1 × 1016 ions/cm2, without any evidence of amorphization. Another spinel was preimplanted with Ne (400 keV and 50 keV). The microstructure revealed a still crystalline material with 8 nm interstitial loops. After irradiation with 1.5 MeV Kr+ (20 K), amorphization, a result of cation disordering, initiated at a dose of 1.7 × 1015 ions/cm2. At a dose of 1 × 1016 ions/cm2, the spinel was partially amorphous and the remaining crystalline domains disordered. These results show that spinel can be disordered and that amorphization can be triggered by the introduction of stable defects, followed by ion irradiation at low temperature.

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2.

A Unified Approach to Solid-State Amorphization and Melting

The crystalline-to-amorphous (c-a) phase transformation can be induced by a variet of solid-state processes ranging from energetic particle irradiation, interface inter diffusion reactions, hydrogen charging and mechanical deformation to the application of high pressures. During the past decade, such transformations have become the focus of considerable research not only because of their potential technological applications, but also because of strong scientific interest in the relationship between the c-a transition and the melting process.A common feature underlies all solid state amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacement can induce volume change and elastic softening of the lattice. A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zr3Al. The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ion at room temperature in a high-voltage electron microscope interfaced to a tandem ion accelerator. The rapid decrease in the intensities of both fundamental and superlattice reflections show that irradiation introduces antisite defects (chemical disorder) as well as static atomic displacements. The disordering of the long-range ordered structure, which occurs prior to the onset of amorphization, is accompanied by a volume expansion of about 2.5% and a ~25% decrease in the average velocity of sound. This decrease in sound velocity corresponds to a ~50% decrease in the average shear modulus, which is comparable to that observed for many metals during heating to melting. The volume dependence of this disorder-induced elastic softening is also similar to that associated with heating. In both cases, the shear modulus is a linearly decreasing function of volume expansion. However, for a given amount of expansion, the softening associated with static atomic displacements is nearly twice as large as that associated with increasing anharmonic lattice vibrations.

Compression and pressure-induced amorphization of Co(OH)2 characterized by infrared vibrational spectroscopy.

The infrared-active (${\mathit{A}}_{2\mathit{u}}$) O-H vibration of Co(OH${)}_{2}$ decreases in frequency under hydrostatic compression to 51 GPa at 290 K. Similarly, the bond anharmonicity, determined from the ${\ensuremath{\nu}}_{1}$\ensuremath{\rightarrow}${\ensuremath{\nu}}_{2}$ absorption-band difference, increases by more than a factor of 2 between 0 and 20 GPa. Both changes are attributed to an increase in the O-H bond length due to enhanced hydrogen bonding under pressure. The full width at half maximum (FWHM) of the fundamental absorption band increases abruptly by \ensuremath{\sim}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 11.2 (\ifmmode\pm\else\textpm\fi{}0.3) GPa, and continues to increase at a rate of \ensuremath{\sim}3.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/GPa up to 36 GPa. Above 36 (\ifmmode\pm\else\textpm\fi{}2) GPa and below the onset of amorphization, the FWHM changes at a slower rate, 0.8 (\ifmmode\pm\else\textpm\fi{}0.1) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/GPa. The abrupt change in FWHM is reversible on decompression, and is interpreted in terms of a pressure-induced crystal-to-glass transition exhibiting a small hysteresis compared to similar compounds. The rapid variation in FWHM above the transition pressure suggests that the amorphous structure is continuously modified between 11.3 and 36 GPa.

Ion-Beam and Electron-Beam Induced Amorphization of Berlinite (AIPO4)

AbstractBerlinite (AIPO4) is isostructural with α-quartz. Like α-quartz, berlinite undergoes a pressure-induced amorphization at 15 ±3 GPa; however, upon release of the pressure, unlike α-quartz which remains amorphous, berlinite returns to the original crystalline structure of the single crystal. Berlinite was irradiated with 1.5 MeV Kr+ at temperatures ranging from 20 to 600K. The onset of amorphization was examined by monitoring the electron diffraction pattern by in situ transmission electron microscopy (TEM) at the HVEM-Tandem Facility at Argonne National Laboratory. The berlinite was easily amorphized at 20K at a relatively low dose of 4x1013 ions/cm2 or 0.05 dpa (displacements per atom). The critical amorphization dose increases with the sample temperature. These experiments also showed that the focused electron beam can locally amorphize the berlinite. After these irradiations, berlinite remained amorphous. At 500 °C, berlinite began to recrystallize: small areas of crystalline material appear in the aperiodic matrix. These results suggest that pressure-induced amorphization and ion-beam induced amorphization, in the case of berlinite, are different processes that result in two different aperiodic structural states.

Characterization of Si Implantation and Annealing of InP by Raman Spectroscopy

AbstractRaman scattering was used to assess the lattice damage caused by Si+implantation in InP, as well as the lattice recovery achieved after rapid thermal annealing (RTA). Semi-insulating InP was implanted with Si+with doses in the range of 1012to 5xl014cm”2. Raman scattering measurements show a progressive intensity reduction of the characteristic first- and second-order InP Raman peaks and an enhancement of the disorder activated modes with increasing dose. The onset of amorphization was found to be at about 1014cm”2. RTA of the implanted samples at 875 °C for 10s results in a very good recovery of the InP lattice even for the highest dose, as confirmed by Raman scattering measurements.

Ion Damage Production in GaAs/Al0.6Ga0.4As Heterostructures

AbstractThe non-uniform distribution of damage with depth and the onset of amorphization produced in Al0.6Ga0.4As/GaAs heterostructures by 1.5 MeV Kr+ ion implantation at 77K have been investigated by using a combination of RBS channeling and cross-sectional high-resolution TEM techniques. The extent of damage increases with increasing depth in the Al0.6Ga0.4As layer. This depth dependence of the damage can be correlated with an increase in the number of recoil events with an energy between 30 and 50 keV. We propose that the amorphization of Al0.6Ga0.4As requires the superposition of cascade-type damage on a background population of point defects which is created by implantation with high energy ions.

In situ TEM study of ion induced amorphization at low temperature in Al 3Ti

Phase modifications induced at low temperature ( T ≤ 40 K) in Al 3Ti by ion irradiation were investigated by in situ transmission electron microscopy (TEM). Irradiation with heavy ions induces a complete amorphization at a dose of approximately 1 dpa. After partial chemical disordering, a crystalline phase change from the DO 22 structure into the L1 2 structure is observed only after the onset of amorphization. These results suggest that a lattice strain release is produced by the amorphization process allowing the transition towards a higher symmetry phase. On the other hand, light ion irradiation leads to partial chemical disordering and to strong lattice distortions, but the transition c-a is not completely achieved at doses higher than 2 dpa.

The Mechanism of Excimer Laser-Induced Amorphization of Ultra-Thin Si Films

ABSTRACTTo better understand the involved phase transformation Mechanism, we are studying the excimer laser-induced amorphization (ELA) of ultra-thin Si films on oxidized Si substrates. In this paper, we show that the onset of amorphization of hydrogen-free Si films on SiO2 substrates upon increases in the energy density is associated with the onset of complete melting of the film. Once complete melting occurs, further increases in the incident energy density and/or increases in the substrate temperature can lead to incomplete amorphization of the film. Planar view TEM analysis of nearly-amorphized Si films reveals a heterogeneous microstructure, which consists of a mixture of densely dispersed amorphous-like annular regions (∼20 to 40 μm−2), embedded within and typically separated by a region containing finegrained small crystals. Such a cellular microstructure strongly suggests that amorphization occurred not via a homogeneous but via a heterogeneous transformation. In particular, the microstructure paints a scenario in which amorphization proceeded via nucleation of solids, which is then followed by interfacial amorphization. The experimental results unambiguously reveal (1) that the previously proposed criteria of the melt duration and the vertical temperature gradient are irrelevant in determining amorphization of supercooled liquid Si films and (2) that the quenching rate, not surprisingly, is the important parameter.

Energy Dependence of Amorphization of Ge by Kr Ions

ABSTRACTThin Ge specimens have been irradiated with Kr ions of different energies, and the dose required for complete amorphization determined by in situ transmission electron microscopy. Because Ge is directly amorphized by a single energetic Kr ion, onset of amorphization was detected after the lowesi ion doses. The Kr dose required for complete amorphization was found to increase linearly with ion energy over the range 0.5 MeV to 3.5 MeV. With the assumption that the defect density required for amorphization is independent of ion energy, the number of defects produced in a thin specimen by each ion decreases with increasing energy as the reciprocal of the incident ion energy. TRIM calculations indicate that there is a slight decrease in the amount of damage required with increasingion energy.

Amorphization in Intermetallic Compounds FeTi and CoTi Under 1-MeV Electron Irradiation

Electron irradiation induced amorphization of FeTi and CoTi was studied by high voltage electron microscopy (HVEM) from 10 to 160°K. The complete amorphization was observed in both compounds, with the critical dose at 10°K and the critical temperature being about 1.7 dpa and 110°K for FeTi, and about 1.3 dpa and 90°K for CoTi. The onset of amorphization occurred in both compounds after substantial chemical disordering when irradiated below Tc, while the point defect clusters formed above Tc. In addition, the pseudo ten fold symmetry (PTEFS) diffraction spots were observed in selected area diffraction (SAD) pattern of both compounds prior to complete chemical disordering and thus complete amorphization.

Radiation-induced amorphization of ordered intermetallic compounds CuTi, CuTi2, and Cu4Ti3: A molecular-dynamics study.

Solid-state amorphization resulting from the introduction of chemical disorder and point defects in the ordered intermetallic compounds CuTi, ${\mathrm{CuTi}}_{2}$, and ${\mathrm{Cu}}_{4}$${\mathrm{Ti}}_{3}$ was investigated, with use of the isobaric-isothermal molecular-dynamics method in conjunction with embedded-atom potentials. Antisite defects were produced by randomly exchanging Cu and Ti atoms, and vacancies and interstitials were created by removing atoms at random from their normal sites and inserting atoms at random positions in the lattice, respectively. The potential energy, volume expansion, and pair-correlation function were calculated as functions of the numbers of atom exchanges and point defects. The results indicated that, although both chemical disordering and point-defect introduction increased the system energy and volume, the presence of point defects was essential to trigger the crystalline-to-amorphous transition. By comparing the pair-correlation function calculated after the introduction of point defects with that of the quenched liquid alloy, the critical damage dose (in dpa, displacements per atom) for amorphization was estimated for each compound: \ensuremath{\sim}0.7 dpa for CuTi, \ensuremath{\sim}0.5 dpa for ${\mathrm{CuTi}}_{2}$, and \ensuremath{\sim}0.6 dpa for ${\mathrm{Cu}}_{4}$${\mathrm{Ti}}_{3}$. At the onset of amorphization, the volume expansions were found to be \ensuremath{\sim}1.9%, \ensuremath{\sim}3.7%, and \ensuremath{\sim}1.7% for these respective compounds. In general, the results obtained in the present work are in good agreement with experimental observations.

Amorphization in Zr3Ai irradiated with 1-mev e- and Kr+

In situ electron microscopy is used to study irradiation-induced amorphization in Zr3Al from 10 to 295 K by 1-MeV electrons and at 295 K with 1-MeV Kr+. The onset of amorphization is observed when the long-range order parameter decreases substantially with both electron and Kr+. Diffuse streaks are observed in the diffraction pattern prior to amorphization. This is attributed to a softening of the shear elastic constant,C′ = (C 11 −C 12)/2, due to static displacement of atoms. This observation is consistent with a large softening of shear modulus reported in Zr3Al irradiated with 1-MeV Kr+, for which a shear elastic instability has been identified prior to the onset of amorphization. In order to complete amorphization with electrons, a large dose, ≳26 dpa, is required at 10 K, while with Kr+, amorphization is completed by a dose of only 0.8 dpa. After the same dose of 26 dpa with electrons at 57 and 295 K, only partial amorphization and rhombohedral distortion are observed. Defect aggregation is also observed during irradiation at all three temperatures. The anomalously large dose required for complete amorphization with electrons is ascribed to point defect migration, as evidenced by the formation of defect aggregates. The partial amorphization at higher temperatures is explained by two factors, faster defect migration and rhombohedral distortion, which both provide alternative paths to amorphization for reducing the accumulated strain.

Brillouin Scattering: Its Application to the Study of Damage and Amorphization

A brief review of Brillouin scattering is made emphasizing its advantages and disadvantages over conventional ultrasonic techniques. One of its advantages is that the elastic properties of very thin films (~1 μ) can be investigated. We have used this technique to study changes in shear elastic constants of materials irradiated with high energy ions. These particles only penetrate ~1 μ into the material but produce very high defect concentrations which, in certain cases, can lead to amorphization. In Nb3Ir and Zr3Al very large changes are observed before the onset of amorphization, while in Si the changes occur concomitantly with amorphization. The significance of the hardening observed in Zr3Al when it becomes amorphous will be discussed.

Amorphization mechanisms in ion implanted metal films and single crystals

The mechanisms leading to amorphization due to high dose ion implantation into elemental thin films and single crystals are discussed. X-ray diffraction, Rutherford backscattering and channelling experiments show that the amorphization process in both substitutional and interstitial alloy systems exhibits similar features. At low impurity concentrations an accumulation of strains is observed due to the forced introduction of impurity atoms into the lattice of the target material. The strains are governed by the lattice site occupation of the impurity atoms, the atomic size mismatch between host and impurity and the local environment of the impurities. At a certain strain level and impurity concentration depending on the alloy system a sudden strain release coinciding with the onset of amorphization is observed. This behaviour is consistent with a theoretical amorphization model that treats the strains created by the introduction of foreign atoms into a lattice as the driving force for amorphization. The evolution of the amorphous phase over an extended region of impurity concentration is a result of the statistical distribution of impurities and local strains that lead to the formation of small amorphous clusters wherever the local impurity concentration and thus the local strains exceed a threshold value.