Ion-irradiation-induced solid-phase epitaxial growth

Abstract

A model is proposed to explain ion-induced solid phase epitaxial growth (SPEG). The crystallization is by the adjustment of atomic positions in the vicinity of the crystalline/amorphous interface due to self-diffusion in the amorphous solid. The driving energy of SPEG is the difference in free energies between the amorphous and crystalline solids. Irradiation increases the self diffusivity of the amorphous solid by generating point defects in the amorphous solid and thus enhances the crystallization. An expression for the velocity of epitaxial growth is derived. The model explains numerous experimental facts and, at the limit of no irradiation, also gives a correct model of thermal solid-phase epitaxial growth.     

Multilayered phase formation after high-fluence implantation of nickel into aluminium

The formation of crystalline and amorphous layers has been observed in Al crystals after 5MeV Ni implantation up to a fluence of 7.6 10 17 Ni atomscm -2 . The layers are structured into sublayers perpendicular to the implantation direction with sharp interfaces between the phases and almost symmetrically arranged to both sides of the maximum of the implantation profile. The sublayers of thickness of the order of 100nm were analysed by transmission electron microscopy. They consist of polycrystalline Al 3 Ni, crystalline Al 3 Ni precipitates and small amorphous zones, depending on the local Ni implantation concentration.     

On the dislocation mechanism of amorphization of Si by indentation

The formation of an amorphous phase underneath a Vickers indentation produced on a Si(001) surface at room temperature has been observed by cross-sectional transmission electron microscopy. Two types of location are observed for the amorphous phase. One is formed just underneath the image of the indentation and the other is parallel to the slip planes of Si. It is concluded that the latter type, at least, is formed as a result of activation of dislocations which is induced by an external shear stress combined with a hydrostatic pressure.     

Pattern evolution of crystalline Ge aggregates during annealing of an Al/Ge bilayer film deposited on a SiO2 substrate

When an Al/Ge bilayer film deposited on a SiO2 substrate is annealed at 373- 398 K, Ge atoms diffuse out from the inner amorphous Ge layer and spread over the free surface of the outer Al layer to form crystalline Ge aggregates exhibiting complex substructures. Scanning electron microscopy observations indicate that the activation energy for the pattern evolution of Ge aggregates on the free surface because of annealing is 1.56 eV which is about half the activation energy for crystallization of amorphous Ge.     

Phase transitions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass at pressures up to 27 GPa

The phase transitions of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass have been investigated under high pressures and at room temperature. Direct resistance measurements in a diamond anvil cell provide evidence of a reversible transition between amorphous and crystalline phases; crystallization events occurred at 24 and 26.2 GPa on uploading, and crystalline-to-amorphous phase transitions were observed at 16 and 10.6 GPa on downloading. The phase transitions were confirmed by transmission electron microscopy observations.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

Interfacial reaction between silicon nitride and aluminium

Abstract

The microstructure of a silicon nitride/aluminium interface has been identified by analytical transmission electron microscopy. HIPed silicon nitride without any additive was bonded by pure aluminium at 800°C for 15 min. A thin (～ 5000 Å) reaction layer was recognized at the interface. It consisted of two regions. One an amorphous layer facing the aluminium; it contained aluminium, silicon and oxygen. The other was on the silicon nitride side and consisted of fine particles which were less than 100 Å in diameter. These particles were considered to have β'-sialon type structure.     

Adoption of near-coincident-site lattice orientations by contacting monolayer rafts of metallic nanoparticles with different superlattice periodicities

A hexagonal raft of monodisperse alkane-thiol-stabilized Au nanoparticles has been self-assembled from solution on to an amorphous C substrate and then subsequently a second layer of monodisperse but differently sized gold nanoparticles deposited on top of the first. Detailed analysis of electron micrographs obtained from various regions of this bilayer revealed the presence of several distinct epitaxial interface structures. A simple near-coincident-site lattice model is used to rationalize the existence of the observed characteristic nanoparticle interface structures.     

Thermomagnetic study of devitrification in Fe-Si-B-Cu-Nb(-X) alloys

Thermomagnetic measurements have been used to study the magnetic and structural changes occurring at the two steps of the crystallization process of Fe_73.5Si_13.5B₉Cu₁Nb₁X₂ (X = Zr, Nb, Mo and V) alloys. Alloying raises the thermal stability of the amorphous phase against nanocrystallization in the order V < Mo < Nb < Zr and some differences in the final crystalline phases are found. The Curie temperature of the amorphous phase increases (about 15K) during structural relaxation. In the course of nanocrystallization a further increase of about 30K in the Curie temperature of the amorphous intergranular is observed for samples with X = Zr, Nb and Mo, but only of about 15K for samples containing V. The observed increase in the Curie temperature of the Fe-Si phase between the end of the first crystallization process and the end of the second crystallization process is associated with a reduction in the Si content, in agreement with X-ray diffraction results.     

Internal stress effect on grain-boundary diffusion in submicrocrystalline metals

Grain-boundary diffusion in the regime affected by internal stresses of triple-line disclinations is analysed. The concentration profiles in a stressed solid averaged over a distribution of hydrostatic stress gradients are calculated. It is shown that the stresses from triple-line disclinations in as-prepared submicrocrystalline metals can significantly increase the effective grain-boundary diffusion coefficient, which is obtained by fitting the concentration profiles to the solution of a diffusion equation for an unstressed polycrystal.     

Formation and analysis of amorphous and nanocrystalline phases in Al–Cu–Mg alloy under friction stir processing

Homogeneous amorphous and nanocrystalline phases formed in the nugget zone of a friction stir-processed Al–Cu–Mg alloy have been studied. X-ray diffraction analysis indicated a diffuse scattering peak with characteristics of an amorphous phase existed in the range 15°–29°. Further, TEM analysis proved the existence of an amorphous structure. Friction stir processing provides special physical conditions, such as high temperature, high hydrostatic pressure and large shear stress, which could induce the amorphization of the alloy.     

Identification of crystal nucleation and growth in an amorphous FeZr2 alloy by means of electrical resistance measurements

A polymorphous crystallization process in an amorphous FeZr₂ alloy has been investigated by means of accurate electrical resistance measurements (ERMs) at elevated temperatures. It was found that, upon crystallization of the amorphous alloy, the electrical resistance increased with increasing temperature, exhibiting three distinct stages. Quantitative microscopy observations revealed that the three stages originated from crystal nucleation, from subsequent growth of crystal nuclei and from coarsening of the crystallites respectively. The activation energies for the crystal nucleation and growth determined from the ERM data agree satisfactorily with the data in the literature. The success in identification of the crystal nucleation and growth processes by means of ERMs may originate from differences in electrical resistance changes due to the crystal formation and the crystalline-amorphous interface formation processes from the amorphous phase.     

Reactive solid-state dewetting

High-temperature annealing in air of thick crystalline silver films deposited on high-purity nickel foils promotes solid-state dewetting, whereas no hole through the film is produced when annealing under high purity argon. Scanning electron microscopy observations of the film surface and of cross-sections reveal that dewetting occurs only if a nickel oxide layer forms at the silver-nickel interface, as a result of oxygen diffusion through the silver film. The main dewetting mechanism over short times (1h 1120K in air) is observed to be the condensation, at the silver-nickel oxide interface, of vacancies into voids which grow towards the free surface of the silver film.     

Pressure-Induced transitions in amorphous TlxSe100−x alloys

Abstract

The electrical resistivity of bulk semiconducting amorphous Tl_xSe_100?x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Å and c = 4·95 Å and TlSe having a tetragonal structure with a = 8·0 Å and c = 7·0 Å     

Microstructural characterization and high-temperature strength of hot-pressed silicon nitride ceramics with Lu2O3 additives

The microstructures of two hot-pressed Si₃N₄ ceramics, with 3.33 and 12.51 wt% Lu₂O₃ additive, have been characterized using transmission electron microscopy. The microstructures of both samples consisted of elongated β-Si₃N₄ grains and a secondary phase, contained in pockets surrounded by the grains, with a crystalline or amorphous form. In the 3.33 wt% Lu₂O₃-containing Si₃N₄ ceramic, all the multiple-grain junctions were completely crystalline while, in the 12.51 wt% Lu₂O₃-containing Si₃N₄ ceramic, approximately half the junctions were devitrified. A thin intergranular amorphous film present between the two-grain boundary was common; however, a film-free grain boundary was observed in the 12.51 wt% Lu₂O₃ sample. The film-free grain boundary was determined to be approximately 35%. Both ceramics fractured in four-point flexure between 1200 and 1600°C. Their high-temperature strength is closely associated with the nature of the grain-boundary phase formed during the sintering process.     

Importance of chamber gas pressure on processing of Al-based metallic glasses during melt spinning

The effect of chamber gas pressure on the amorphicity of Al₈₅Ni₅Y₁₀ alloy was studied for the melt-spinning process. The amorphicity of as-quenched ribbons was characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chamber atmosphere pressure is crucial to the cooling rate of melt spinning. At high vacuum, at pressure less than 0.001?atm, fully crystalline fragments are obtained. Monolithic amorphous ribbons were only obtained at a gas pressure of 0.1?atm, 0.2?atm or higher. The extended contact length between ribbons and the copper wheel contributes to the high cooling rate of melt spinning in Al-based glass forming alloys; that is supported by images recorded by a high-speed camera. Higher chamber pressure increases contact length between ribbons and the wheel, which is qualitatively elucidated by Bernoulli's equation.     

Misfit disclinations and dislocations at crystal-glass interfaces

A theoretical model is suggested which describes misfit defect structures at crystal-glass interfaces. In the framework of the model the two basic types of misfit defect are distinguished: misfit disclinations (generated as extensions of parent disclinations present in the adjacent glassy phase) and 'dilatation' misfit dislocations (contributing to accommodation of the dilatation misfit associated with difference between the characteristic interatomic distances in the adjacent crystalline and glassy phases). The elastic energy density of crystal-glass interfaces is estimated.     

Onset of plasticity in a Σ = 5 GaAs compliant structure

Compliant structures have been fabricated in which a thin GaAs layer (thickness between 10 and 20nm) was bonded on top of a GaAs substrate with a large twist angle (about 37). This twist angle value was chosen so that the energy of the boundary (coincident boundary of type =5 (001)) was minimized. The structure of the interface was characterized and the onset of plasticity in such a compliant substructure was investigated using nanoindentation that allowed the low-load deformation regime to be observed. The results are compared with those obtained under the same conditions on a GaAs bulk substrate alone. No plastic zone was observed by transmission electron microscopy in the compliant structure under loads below 0.25mN while, under the same loads, plastic deformation was observed in the bulk substrate. For higher loads (2mN), plastic-flow enhancement was observed in the compliant structure. The results are discussed in the light of the arrangement of dislocations observed in the plastic zones.     

Correlation between heat- and deformation-induced crystallization of amorphous Al alloys

Deformation-induced crystallization is correlated with thermal-induced crystallization in alloys with different compositions in a single amorphous alloy system. In Al₈₇Y₆Ni₅Co₂ and Al₈₅Y₈Ni₅Co₂ alloys, which undergo primary crystallization during heating, deformation-induced crystallization of fcc-Al has been observed. In Al₈₃Y₁₀Ni₅Co₂ alloy, which undergoes eutectic-like crystallization, no deformation-induced crystallization was observed. These observations can be explained by the presence or absence of quenched-in nuclei and the work required for the creation of an amorphous/crystalline interface under compressive stress.     

Cooling-rate-dependent microstructure and mechanical properties of a CuZrAlAg alloy

A Cu₃₆Zr₄₈Al₈Ag₈ alloy rod with a diameter of 10 mm was fabricated using a copper-mould suction casting method. Structural characterization revealed that the rod has different microstructures along the casting direction, including a complete amorphous structure and an amorphous/crystalline composite structure with different amount of B2 CuZr phase. Nanoindentation tests showed that the hardness and the elastic modulus of the crystalline phase are lower than those of the amorphous matrix. The hardness and the elastic modulus of the amorphous matrix decrease with decreasing crystalline proportion of the alloy, while the Vickers hardness of the alloy increases with a reduction in the crystalline proportion.