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.     

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.     

Decagonal quasicrystal with ordered body-centred (CsCl-type) hypercubic lattice

An ordered structure, which was found in a decagonal quasicrystal with 0.4 nm periodicity in a conventionally solidified Al₇₀Ni₂₀Ru₁₀ alloy, has been examined theoretically by the projection of an ordered body-centred (CsCl-type) hypercubic lattice. The ordered structure can be characterized as an ordered arrangement of two kinds of atom columnar cluster with different directions of pentagonal symmetry. Two lattices, which are constructed by connecting the atom clusters in the same directions, can be interpreted as ordered sublattices formed by each of vertex and body-centred positions of the CsCl-type hypercubic lattice. Diffraction patterns calculated from the ordered and disordered body-centred hypercubic lattices replicate well the electron diffraction patterns observed from the Al-Ni-Ru and other decagonal quasicrystals.     

Changes in electrical resistivity during isothermal annealing of thermally disordered Cu-15at.%Pd alloys

The changes in the electrical resistivity of thermally disordered Cu-15at.%Pd alloys during isothermal annealing at various temperatures has been investigated. The results obtained are compared with the changes in electrical resistivity calculated under the assumption that ordering proceeds only during isothermal annealing. The electrical resistivity in this alloy first increases and then decreases during isothermal annealing, and the magnitude of the increase decreases as the annealing temperature is lowered. A comparison between the results of measurement and calculation shows that the increase in electrical resistivity is considerably larger than that deduced at the early stage of ordering and occurs after the start of the ordering. An electron micrograph of a sample annealed at 643K for 42000s (11.7h) revealed that not only swirl-like antiphase domains but also fine domains with narrow stripes are present. Furthermore, it is shown that the electron diffraction pattern from the fine domains includes somewhat diffuse extra spots, which have never been observed in the L1 2 -type ordered structure, and that the fine domains disappear after a long anneal. These results indicate that the formation and disappearance of any quasistable phase influence the large increase and subsequent decrease in electrical resistivity during isothermal annealing.     

A new high-pressure phase transition in a zirconium-niobium alloy

It is reported for the first time that a g M y transformation can be induced in a g -stabilized zirconium alloy subjected to shock pressure. The y phase formed in the g matrix has been found to have a plate shape akin to martensitic plates. The lattice correspondence between the g and y structures has been found to be the same as that produced by thermal treatment. The formation of the plate-shaped y phase is explained in terms of a mechanism involving shear on <112> planes of the bcc lattice and the mechanical instability of the g phase.     

Icosahedral-phase formation and stability in Ti Zr Co alloys

We present the first report of icosahedral phase i-phase formation in rapidly quenched alloys of Ti Zr Co . Electron diffraction patterns of i TiZrCo 53 27 20 contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti 3d transition metal Si O and Ti Zr Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti Zr Ni i-phases, showing none of these features. The quasilattice constant for a i TiZrCo, 0 51 nm, is close to that of i TiZrNi . The i-phase in rapidly q quenched Ti Zr Co alloys is deeply metastable and transforms exothermically o to the hexagonal Laves phase about 500 C. The Laves phase transforms to the o T bcc solid solution phase b-Ti and the Ti Ni-type fcc structure for 630 C. A 2 reversible transformation between the b-Ti and the hexagonal solid solution phase a-Ti is observed on temperature cycling; the Ti Ni-type fcc phase is 2 stable over the entire temperature range.     

A new stable icosahedral quasicrystal in the Cd-Mg-Dy system

A new stable icosahedral quasicrystal has been found in annealed Cd-Mg-Dy alloys. The composition of the icosahedral phase was determined to be approximately Cd₆₆Mg₂₁Dy₁₃. Powder X-ray and electron diffraction patterns revealed that the phase has a primitive icosahedral lattice with a quasilattice parameter a_R = 0.5634 nm. The electron diffraction study confirmed that the phase has a well ordered primitive icosahedral structure.     

Stress anomaly in Al-rich Ti-Al single crystals deformed by the motion of 1/2«110] ordinary dislocations

The microstructure and plastic deformation behaviour of Al-rich Ti-Al single crystals containing 54.7 and 58.0 at.% Al have been examined, focusing on the effect of chemical ordering of a Al5Ti3 superstructure on anomalous strengthening. Fine precipitates with the Al5Ti3 superstructure were developed in the L10 matrix of the Ti-58.0 at.% Al alloy. The size and volume fraction of the precipitates varied depending on temperature. An anomalous increase in the yield stress of the two alloys appeared at around 800oC. This strengthening mechanism is discussed on the basis of the difference in antiphase-boundary energies on (111), (110) and (001) planes in the Al5Ti3 phase.     

Short-range order in Al-rich 𝛄-TiAl alloys studied by high-resolution transmission electron microscopy with image processing

The atomic arrangements of short-range order (SRO) in two Al-rich γ-TiAl alloys with 62.5 and 60.0at.%Al respectively have been investigated by high-resolution transmission electron microscopy with image processing. For both alloys, SRO structures are formed in Ti-rich (002) layers of the L1₀ ordered γ-TiAl matrix, in the course of the phase transformation from the Al₅ Ti₃ long-period superstructure phase at lower temperatures to the h-Al₂ Ti phase at higher temperatures. The SRO structures are composed of three types of ordered cluster: Ti₄ Al type (square shaped), Ti₃ Al type (fat rhombus shaped) and Ti₂ Al type (lean rhombus shaped). The ordered clusters form local microdomains of the Al₅ Ti₃ and Al₃ Ti₂ superstructures in addition to the Al₅ Ti₃ and h-Al₂ Ti superstructures, by different methods of tiling the ordered clusters. Such ordered clusters or microdomains tend to be in contact with each other through common {310) boundaries. This results in peculiar diffuse streaks in diffraction connecting intensity maxima arising from the long-period superstructures.     

A study of the fcc (FeCo) 23 B 6 phase in fully crystallized Fe-Co-Nb-B-Cu alloys

Fe-Co-Nb-B-Cu alloys lose their nanocrystalline microstructure at a second crystallization process in which (FeCo) 23 B 6 crystals appear as the main boride phase. In this work the structural characteristics and composition of this phase are studied. The amount and grain size of the (FeCo) 23 B 6 phase increase as the Co content in the alloy increases. After recrystallization, f -FeCo crystals remain at a nanometric size. The lattice parameter and Curie temperature of the (FeCo) 23 B 6 phase are reported.     

ω-phase formation in a rapidly solidified Cr-40 at.% Al alloy

The diffuse ω structure has been identified by electron diffraction for the first time in a C11_b matrix of the melt-spun Cr-40 at.% Al alloy ribbon. The C11_b matrix consists of nanometre-scale ordered domains produced by a long-period chemical ordering in the precursor A2-B2 structure. Each C11 _b unit cell in the Cr-40 at.% Al alloy is based on three cubic B2 cells along the c axis and contains two antiphase boundaries. The structure of the diffuse ω is consistent with that observed by De Fontaine. The relative stability of the crystalline ω structure has been predicted with respect to bcc-type precursor phases as a function of the displacement parameter z from calculation of the cohesive energy of the ω phase using the full-potential linear muffin-tin orbital method. The results show that the three-dimensional crystalline ω structure is stable with respect to bcc-type precursor phases in the Cr-Al system and reveals the physical background as to why the ω structure in Cr-40 at% Al is diffuse in nature.     

Synthesis,transformation and superconductivity of dual phase In–Sn alloy nanoparticles embedded in an Al matrix

We report the synthesis of nanoembedded biphasic alloy particles of In–Sn near eutectic alloy compositions embedded in an aluminium matrix by rapid solidification processing. Detailed transmission electron microscopy indicates that the two phases present at room temperature in as-synthesized samples are β and γ phases with tetragonal and hexagonal crystal structures, respectively. These co-exist with a small amount of single phase In or Sn particles with sizes less than 10?nm. Low temperature magnetization measurements indicate a superconducting transition temperature of 5?K, suggesting complete decomposition of the β-phase at small sizes and at low temperature. The small particles show type II behavior with a critical field Hc₁≈44?G and two values for Hc₂ of 250 and 1000?G, respectively. These values are considerably lower than those observed in bulk In–Sn alloys.     

Transmission electron microscopy characterization of the high-Tc superconductor HgBa2Ca3Cu4O 10+ delta

The structures of HgBa3Ca4Cu4O10+ delta(Hg-1234) samples have been characterized by means of high-resolution electron microscopy, electron diffraction, X-ray energy-dispersive spectroscopy (XEDS) and electron-energy-loss spectroscopy (EELS) techniques. A modulated structure with a modulation vector of q = (0.036-0.048) b* + 0.5c* was found for Hg-1234. High-resolution electron microscopy on the modulated structure revealed slight displacements of atomic layers along the c direction. The anomalous contrast induced by the modulation was observed to be distributed asymmetrically between the HgOdelta layers, leading to the breakdown of the (001) mirror symmetry of the basic structure. XEDS compositional analyses showed a depletion of mercury for the modulated Hg1234 compared with the unmodulated Hg-1234, while EELS analysis suggested the presence of carbon in the modulated Hg-1234 phase. In addition, a sidecentred ordered (Cu, Hg)-1223 phase with low mercury content was identified as one of the major impurity phases in the Hg-1234 samples.     

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 Ga–Pd–Sc icosahedral quasicrystal and approximant phases

ABSTRACT

We report the formation of an icosahedral quasicrystal and its approximant in the Ga–Pd–Sc alloy. The primitive-type quasicrystal with a six-dimensional lattice constant of 0.713?nm formed in the melt-quenched Ga₅₃Pd₃₀Sc₁₇ alloy, with a similar composition to that of the Ga₅₅Pd₃₀Sc₁₅ 1/1 approximant. The atomic-scale observation and chemical analysis of the 1/1 approximant by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that the approximant consisted of Tsai-type clusters with a characteristic chemical ordering. Furthermore, a series of approximants were observed in the Ga₅₅Pd₃₀RE₁₅ alloys by replacing Sc with other rare-earth elements (REs) (RE?= Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). These approximants, which have relatively small lattice constants and consequently smaller RE–RE distances compared with those for other RE-containing Tsai-type approximants, may be candidates for materials with novel electronic and magnetic properties similar to those observed in Au-based quasicrystals and approximants.     

A new counter-example to Kelvin's conjecture on minimal surfaces

A survey of the published literature on undercooled metallic and oxide melts suggests that phase selection during solidification can be categorized as nucleation controlled or growth controlled. Common characteristics governing the phase-selection pathway have been identified for various alloy systems. It is recognized that when competing stable and metastable phases share the same crystalline characteristics and have comparable interface kinetic coefficients, the principle of nucleation control applies for primary phase formation in a deeply undercooled melt. However, there can be a difference of two or three orders of magnitude in the interface kinetic coefficients for competing phases, either between an ordered intermetallic compound and a disordered solid solution, or between a crystalline phase with a high level of complexity and a simple crystal. In such cases, the principle of growth control will apply; more specifically, the phase with the faster growth kinetics should be favoured and the competing counterpart with sluggish interface kinetics should be suppressed at high undercoolings. Some simple predictions are suggested on the basis of this principle when considering stable and metastable phase diagrams. The specific conditions under which the present categorization is applicable are outlined. Future work is required to elucidate phase competition under conditions of very rapid solidification.     

Lattice-based similarity measures between ordered trees

A clustering algorithm has recently been developed by Reitman and Rueter to express both the structure of chunking in multi-trial free recall and the order of chunk production. The resulting ordered trees differ from ordinary rooted trees in that the elements of a chunk, at any level, may be restricted to a specific ordering. In order to make comparisons of long-term memory structures between subjects, a measure of the similarity between trees is needed. Previously developed similarity measures are shown to be inadequate for ordered trees. Lattice theory is used to generate new similarity measures suited to these richer structures. First, ordered trees are shown to form a nonmodular, graded lattice. Then, moves through this lattice are defined and used to produce several distance measures. These new measures are compared both to each other, and to existing measures, by examining the properties of each measure, and through application to hypothetical trees. The lattice-based measures prove to be theoretically superior, but lack computational ease. The general problem of describing paths in a nonmodular lattice is discussed.     

Coexistence of adjacent vacancy-ordered and eutectic phases in Al–Cu–Ni alloys

ABSTRACT

Copper-mould-cast Al–Cu–Ni alloys show adjacent coexistence of in situ grown ordered and eutectic phases. A bimodal microstructure of α–Al and eutectic α-Al+θ-Al₂Cu phases with length-scale hierarchy evolves during solidification. Microstructural analysis through Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) shows the presence of Vacancy-ordered phases (VOPs) with different morphologies in two different compositions.     

Vacancy trapping on lattices with different coordination numbers

Abstract

The kinetics of disorder→order transformations in alloys have been studied by Monte Carlo simulations. These simulations employed a vacancy diffusion mechanism, and were set on lattices having different coordination numbers, z. At low temperatures, the vacancy mobility is dominated by processes in which the vacancy becomes confined to local atomic arrangements that serve as vacancy traps. The scaling with z of the ordering kinetics can be interpreted in terms of the strengths and probabilities of vacancy traps on the different lattices.     

Cross-sectional transmission electron microscopy of electrodeposited Ni-P alloys

Abstract

Ultramicrotoming has been employed to prepare thin cross-sections of Ni-P alloys electrodeposited on to a copper substrate. The sections, as thin as 10nm or even less, can be prepared readily with negligible damage to the alloys. Subsequent examination of the ultramicrotomed sections in an analytical transmission electron microscope has revealed that Ni-P alloys of low P content consist of fine layers of different composition and crystallinity. High-resolution transmission electron microscopy of ultramicrotomed sections has also revealed the presence of ordered regions, a few nanometres in size, in the Ni₇₇P₂₃ alloy which has been thought previously to be amorphous.