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.     

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.     

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.     

Nucleation of misfit dislocations in strained-layer InGaAs on GaAs

Abstract

Austenitic 316L stainless steel alloys annealed at 550°C for 100 h present a few ferrite precipitates surrounded by a new interfacial phase, here called the I-phase, that develops at the level of the austenite/ferrite interface. The I-phase presents the typical patterns of an icosahedral quasicrystal with a primitive hypercubic lattice of parameter A = 0.63 nm. The marked orientation relationships between the I-phase and the ferrite precipitates strongly suggest that this phase results from a decomposition of the ferrite and not of the austenite. The I-phase is metastable and transforms eventually after annealing at 700°C to the stable crystalline σ-phase.     

Formation of ductile Al-based metallic glasses without rare-earth elements

Al₇₅Cu₁₇Mg₈ is a eutectic composition according to the ternary phase diagram, which can be quenched into a fully amorphous phase by adding 2-8at.% Ni, but the addition of a similar percentage of Gd failed to form the amorphous phase. The amorphous alloys obtained exhibit two broad diffuse peaks in the X-ray diffraction curves and, correspondingly, two halo rings in the electron diffraction patterns, implying that two types of local atom configuration exist. Thermal analysis of the amorphous alloys indicates that the primary crystallization peak shifts to higher temperatures with increasing Ni content. The occurrence of a nucleation and crystal growth peak during isothermal crystallization reveals the amorphous nature of the quenched ribbon alloys. The quenched amorphous ribbons do not break after bending by 180°. Mechanical testing yielded a tensile strength of 810MPa for (Al₇₅Cu₁₇Mg₈)₉₅Ni₅, and a vein structure, characteristic of amorphous fracture, is apparent in scanning electron micrographs. The different effects of Ni and Gd on the glass formation indicate that the large atomic size of Gd is not critical to the glass formation.     

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.     

Hydrogenation of Ti50Zr25Co25 amorphous ribbons and its effect on their structural and mechanical properties

Changes in the structure and mechanical properties of Ti₅₀Zr₂₅Co₂₅ melt-spun ribbons upon electrolytic hydrogenation have been investigated. Structural analyses of the as-spun ribbons revealed the sporadic presence of icosahedral (i) quasicrystalline short-range order (SRO) in the amorphous structure. Upon cathodic charging with hydrogen, the i-SRO vanished for a hydrogen content of about 21 at.% (hydrogen-to-metal atom ratio, H/M: 0.26), resulting in a fully amorphous structure. Simultaneously, the fracture strength was found to increase while the ribbons preserved their bending ductility. However, a significant reduction in the fracture strength and ductility of the ribbons was observed for hydrogen concentrations larger than 26 at.%. Variations in the mechanical stability are discussed based on a structure-property correlation.     

Formation of Al3Ni2-type nanocrystalline τ 3 phases in the Al–Cu–Ni system by mechanical alloying

An elemental powder mixture of Al (70 at.%), Ni (15 at.%) and Cu (15 at.%) was milled in a high-energy ball mill for various times ranging from 10 to 100?h to form ternary intermetallic alloys. X-ray diffraction and transmission electron microscopy techniques were employed for characterization of the samples. The dissolution of the individual elements into an alloy led to the formation of a τ₃ vacancy-ordered phase after 100?h of milling. This phase was found to be quite stable against milling, and no other crystalline and amorphous phases could be detected. The powder after 100?h of milling was found to contain mostly τ₃ nanophases with partial ordering, and with crystallite sizes in the range 10–20?nm along with a lattice strain of ～0.675%. The milled powder, after annealing at 700°C for 20, 40 and 60?h, revealed the formation of a strain-free and ordered τ₃ phase with a crystallite size of 80?nm, indicating grain coarsening. It is interesting to note that the mechanical energy imparted during milling could not completely destroy the vacancy ordering in the τ₃ phase, unlike other stoichiometric Al–Cu–transition metal (TM) systems, where the disordered B2 (bcc) phase is commonly observed instead of any vacancy-ordered phases.     

On the orientation relationship between a2 precipitates and the B2 phase in a Ti-47at.%Al-2at.%W-0.5at.%Si alloy

An k -phase Mn-Al-C single crystal was grown and annealed to obtain an intermediate stage of the k M transformation. The crystal was analysed by transmission electron microscopy, and direct evidence for the operation of the shear mode was obtained. The transformation occurs in two steps from the hcp structure ( k ) over the orthorhombic structure ( k ' phase) to the tetragonal phase. The study reveals the morphologies of k ' and , the necessity for reordering during the second transformation step ( k ' M ) which is not a purely martensitic transformation, and the selection of variants which is controlled by stresses through the selection of one k ' variant.     

Thermomagnetic gravimetry study of crystallization behaviour in amorphous Fe-Si-B-Nb alloys

The effect of the Nb content on the crystallization process of amorphous Fe84- xSi6B10Nbx alloys has been investigated using thermomagnetic gravimetry (TMG). In the TMG measurement, the sample weight was electronically zerobalanced, and then an apparent weight (AW) applied by placing a magnet beneath the sample pan. The heating treatment was carried out over temperature range between 373 and 1093 K, in order to detect the formation of a ferromagnetic crystallization-induced phase. The form of the TMG curve changed markedly in the vicinity of 2-3 at.% Nb. In the alloys containing less than 2 at.% Nb, a two-stage increase in AW was observed with increasing temperature, but only a single-stage increase occurred for the alloys containing more than 3 at.% Nb. On the basis of these results, it is inferred that the Nb content plays a key role in the structure of the amorphous phase, supporting our recent claim for a mechanism of nanocrystallization.     

Amorphization under fracture surface in hydrogen-charged and low- temperature tensile-tested austenitic stainless steel

ABSTRACT

The microstructure just below the fracture surface in hydrogen-charged stable austenitic SUS 316L stainless steel, which was subjected to a low strain rate tensile test at ?70°C, was studied by a combination of the focused-ion-beam method and transmission electron microscopy. An amorphous region with a chemical composition almost identical to that of the polycrystalline region was found under the lath-like structure on the fracture surface, although no deterioration of tensile properties by hydrogen appeared. In the amorphous region, band-like regions with wavy contrasts were observed, which were often accompanied by cracks at the boundaries. The presence of the amorphous region with band-like regions implies that amorphization occurred due to high-density vacancies accompanied by agglomerations of excess vacancies in the hydrogen-charged SUS 316L stainless steel that was tensile-tested at low temperatures.     

Electropulsing-induced strengthening of steel at high temperature

Electropulsing usually promotes a metastable phase to evolve towards its equilibrium state. This work reports an alternative case, where electropulsing promotes the decomposition of the stable δ-phase in duplex stainless steel at high temperature. This decomposition enables both the γ-phase and the σ-phase in the steel to survive and hence to strengthen it at high temperature. The hardness of the quenched sample with electropulsing treatment is 49.4% higher than that without electropulsing treatment. A fundamental understanding of the observation is developed.     

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.     

Hydrostatic pressure-enhanced solid-phase epitaxy

Abstract

A large enhancement of solid-phase epitaxial growth (SPEG) due to hydrostatic pressure is explained by stress-enhanced self-diffusivity in the amorphous solid. The crystallization is by the adjustment of atomic positions in the vicinity of the crystalline/amorphous (c-a) interface due to self-diffusion in the amorphous phase, assisted by a free-energy decrease equal to the difference in free energies between the amorphous and crystalline phases. Owing to a mismatch in the bulk moduli between the amorphous and crystalline phases, non-hydrostatic stresses are developed near the c-a interface under hydrostatic pressure. Non-hydrostatic stresses in the amorphous layer enhance the mobility of point defects in the amorphous layer. This leads to an increased self-diffusivity in the amorphous layer and, therefore, an enhancement of the SPEG rate.     

Crystallization behaviour of Al-based amorphous alloy and nanocomposites by rapid quenching

The microstructure and crystallization behaviour of melt-spun Al₈₈Ni₉Ce₂Fe₁ amorphous alloy and nanophase composites have been studied by means of X-ray diffraction, transmission electron microscopy and scanning and isothermal calorimetry. The diffraction patterns from Al₈₈Ni₉Ce₂Fe₁ amorphous alloys are diffuse, indicating a basically amorphous structure but contain two rings presumed to be associated with quenched-in nuclei. In the cases of Al₈₈Ni₉Ce₂Fe₁ nanophase composites, nanoscale precipitated particles are homogeneously dispersed in an amorphous matrix, and the crystallite diameter and volume fraction are sensitive to quenching conditions. During thermal crystallization, a two-step phase transformation occurs in the amorphous alloy and nanocomposites, which is characterized by a diffusion-controlled precipitation of nanoscale Al particles and the growth of a Al₃(Ni, Fe) nanophase prior to a Al₁₁Ce₃ nanophase. This study gives insight into structure-control for obtaining nanophases dispersed in an amorphous matrix by rapid quenching.     

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.     

Toward an understanding of aging in plutonium from direct measurements of stored energy

ABSTRACT

We present here the first direct measurement of the radiation-damage-induced energy stored in δ-phase plutonium. The primary mode of radioactive decay of ²³⁹Pu occurs with a time constant of τ=1.1?×?10¹²?s. Each decay imparts about 85?keV of recoil energy to the uranium byproduct, 5.2?Mev to the alpha particle, and a spectrum of mostly low energy gamma rays with the most probable at 51?keV [1]. Most of the decay energy is converted immediately to heat, releasing about 1.9?mW/g. However, some thermally-recoverable energy remains trapped. Reported here are measurements of that stored energy using differential scanning calorimetry (DSC) applied to ²³⁹Pu-2.0 at.%Ga δ-phase alloy. Retained energy of ～2 J/g saturates at about 5 months and is unchanged after 30 years. The magnitude of the stored energy agrees with a short-bond defect model that that we present here. This model treats radiation damage as a Pu impurity with shortened bond lengths. It explains the change in known properties with age and predicts that density increases with age, contrary to current thinking. The short-bond impurities proposed are expected to act like other impurities, affecting strength, phase transitions, grain boundaries and other metallurgical properties.     

Formation of the icosahedral quasicrystalline phase in Zr70Pd30 binary glassy alloy

It is found that a single icosahedral quasicrystalline phase is formed as a primary precipitation phase in the melt-spun Zr₇₀Pd₃₀ binary glassy alloy with a two-stage crystallization process. The onset temperature of the transformation from the amorphous to the icosahedral phase is 701 K at the heating rate of 0.67 K s-1. The size of the icosahedral particles lies in the diameter range below 10 nm and the particles are distributed homogeneously. The second-stage crystallization reaction results in the formation of a Zr₂Pd phase through a single exothermic reaction. The formation of the nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the Zr-Pd binary glassy state. Comparison with the thermal stability of an icosahedral phase in the Zr-Ni-Pd system shows that the icosahedral phase is stabilized by the addition of Ni. The stabilization is due to the restraint of the long-range rearrangement of the constitutional elements resulting from the strong chemical affinity between Zr and Ni.     

Unexpected nanoscale phase separation in sputterdeposited Ni-25 at.%Al thin films

Thin films of nominal composition Ni-25at.%Al, sputter deposited from a target of the intermetallic compound Ni 3 Al on unheated substrates, exhibit an unexpected phase separation, in contrast with other intermetallic-forming systems such as Ti-Al which are deposited as compositionally homogeneous amorphous films under similar conditions. Precipitates of a novel tetragonal phase, a few nanometres in size, were formed in the matrix consisting of a fcc Ni-rich Ni(Al) phase and a hcp Ni-rich Ni(Al) phase. Ni-Al films of the same composition deposited on heated substrates exhibited the formation of a single, chemically long-range-ordered Ni 3 Al phase with the L1 2 structure, the thermodynamically stable phase for this composition.     

Structure of a cubic phase related to Cd?Mg?rare-earth quasicrystals

The structure of a Cd₆₈Mg₁₂Dy₂₀ crystalline phase denoted as the φ-phase, which has a composition close to that of the Cd₆₆Mg₂₁Dy₁₃ icosahedral quasicrystalline phase, has been investigated by electron diffraction and scanning transmission electron microscopy (STEM). The φ phase has a fcc lattice with a = 21.6Å. High-angle annular dark-field STEM with Z contrast confirms that the phase has the Cd₄₅Sm₁₁-type structure. The atomic cluster in the structure is shown to be characterized by a Friauf polyhedron with tetrahedral symmetry.