The effect of phase separation on subsequent crystallization in Al88Gd6La2Ni4

To understand the crystallization (devitrification) kinetics of Al-rich Al–RE–TM (RE=rare earth, TM=transition metal) amorphous alloys, the devitrification of Al₈₈Gd₆La₂Ni₄ was studied by in situ electrical resistivity and transmission electron microscopy (TEM) investigations. This glass composition was chosen because it shows a well-defined glass transition temperature (T_g) and transforms to α-Al on partial devitrification. Surprisingly, we show that crystallization appears to be preceded by a phase separation into Al-rich and solute-rich amorphous regions, having a typical dimension of 40 nm. TEM studies reveal a preferential rapid nucleation of α-Al at the interface of the phase-separated regions. A Johnson–Mehl–Avrami analysis of crystallization-induced changes in the electrical resistivity shows limited agreement with the theory, with an Avrami exponent n close to unity. This is explained by a simple numerical model that is consistent with the microstructure; i.e., rapid nucleation at the phase boundary followed by diffusion-limited growth.     

Local chemical and topological order in Al-Tb and its role in controlling nanocrystal formation

How the chemical and topological short- to medium-range order develops in Al-Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al₉₀Tb₁₀. Reverse Monte Carlo simulations and Voronoi tessellation analysis based on HEXRD experiments revealed a high coordination of Al around Tb atoms in both liquid and amorphous states. APT results show Al-rich and Al-depleted regions within the as-quenched alloy. A network structure of Tb-rich clusters divides the matrix into nanoscale regions where Al-rich clusters are isolated. It is this finely divided network which allows the amorphous structure to form. Al-rich regions are the locus for fcc-Al crystallization, which occurs before the intermetallic crystallization. FEM reveals medium-range ordered regions ∼2 nm in diameter, consistent with fcc-Al and trigonal-like Al₃Tb crystal structures. We propose that the high coordination of Al around Tb limits diffusion in the intermetallic network, allowing for the isolated Al-rich regions to form at high density. These regions are responsible for the extremely high density of Al nanocrystal nuclei.     

The evolution of the microstructure in amorphous Al85Ce8Ni5Co2 alloy during heat treatment and severe plastic deformation: A comparative study

The effect of three different devitrification routes, i.e. isothermal heat treatment, high pressure torsion and ball milling of amorphous Al₈₅Ce₈Ni₅Co₂ alloy have been studied systematically. The phase selection during the different crystallization sequences is explained by different thermodynamic barriers and growth rates. Low temperature heat treatment results in the formation of α-Al and a metastable phase (φ). Although, the nucleation of Al₁₁Ce₃ phase is also likely in this temperature range, the lack of its growth is explained by kinetic considerations. Ball milling results in the formation of the stable phase mixture (α-Al and Al₁₁Ce₃). This process can be considered as a high temperature heat treatment at intermediate pressures (<2 GPa). High pressure torsion at the applied pressure of 6 GPa results in the primary formation of α-Al nanocrystals.     

Microstructure and mechanical properties of bulk nanocrystalline Al–Fe alloy processed by mechanical alloying and spark plasma sintering

Nanocrystalline Al–5 at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000 MPa with a large plastic strain of 15% at room temperature and 500 MPa at 350 °C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al₆Fe nanocrystalline regions with 90 nm in diameter and a minor fraction of Al₁₃Fe₄ phase and coarsened 0.5–1 μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000 MPa at room temperature.     

Primary crystallization in Al-rich metallic glasses at unusually low temperatures

The initial stage of the primary crystallization reaction and the glass transition of the marginal metallic glass Al₈₉Y₆Fe₅ were investigated by conventional differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC), microcalorimetry, X-ray diffraction (XRD) and transmission electron microscopy. A sharp onset of the primary crystallization was found by microcalorimetry and XRD studies at temperatures which were 120 °C below the primary crystallization peak observed in conventional DSC. A systematic MDSC study of annealed samples revealed a wide spectrum of glass transition onsets, which show a strong dependence on the annealing conditions. In addition, the glass transition onsets can be linked to the initial stage of the primary crystallization. The spectrum of glass transition onsets observed is discussed with respect to the occurrence of phase separation preceding the nucleation and growth of dendritic aluminium nanocrystals.     

Erbium and ytterbium solubilities and diffusivities in aluminum as determined by nanoscale characterization of precipitates

Binary aluminum alloys with 0.03–0.06 at.% RE (RE = Yb or Er) were aged to produce coherent, nanosize Al₃RE precipitates in an α-Al matrix. The temporal evolution of precipitate radii and matrix concentrations at 300 °C were measured by transmission electron microscopy and local-electrode atom-probe tomography, respectively. The temporal dependence of the matrix concentration of each RE was utilized to determine its solubility in Al. The solubility and the coarsening rate constants were used to determine the diffusivity of each RE in α-Al and the α-Al/Al₃RE interfacial free energies at 300 °C. When compared to Sc, both Yb and Er exhibited smaller solubilities but larger diffusivities in α-Al and larger α-Al/Al₃RE interfacial energies.     

Quasi-peritectic solidification reactions in 6xxx series wrought Al alloys

Secondary intermetallic phase formation during directional solidification of two 6xxx series wrought Al alloys at low growth velocities of 5–30 mm/min has been investigated using differential scanning calorimetry, transmission electron microscopy and scanning transmission electron microscopy. Thermodynamic calculations predict that a quasi-peritectic reaction, L+Al₁₃Fe₄ → α-Al+α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al₁₃Fe₄/α-AlFeSi particles, but composite Al₁₃Fe₄/β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L+Al₁₃Fe₄ → α-Al+β-AlFeSi. More detailed analysis suggests that the metastable β-AlFeSi quasi-peritectic reaction is more favourable both at nucleation and during growth. No unique orientation relationship was found between primary Al₁₃Fe₄ and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al₁₃Fe₄ and β-AlFeSi, are discussed.     

Topological instability and glass forming ability of Al–Ni–Sm alloys

The thermal crystallization of Al-based metallic glasses can be described in association with the topological instability λ criterion. In the present work, we report on the crystallization behavior and glass forming ability of Al-rich, Al–Ni–Sm alloys, designed with compositions corresponding to the same topological instability condition of λ ≈ 0.1. Amorphous melt-spun alloys were prepared with the following compositions, varying the ratio of Ni and Sm elements: Al_87.5Ni₄Sm_8.5, Al_83.5Ni₁₀Sm_6.5, Al_80.5Ni_14.5Sm₅ and Al_76.5Ni_20.5Sm₃. The glass forming ability of each alloy composition was evaluated based on the thermal parameters obtained from DSC runs and on X-ray diffraction patterns. Better glass forming ability was observed in compositions whose Sm content was increased and Ni content reduced. Thermal crystallization of the alloys with low Sm content showed only one crystallization peak and no glass transition event. In alloys with higher rare-earth content, a glass transition event was clearly detected before the crystallization event. The results are interpreted considering the different types and proportions of Sm–Al and Ni–Al clusters that can be formed in the alloys along the λ ≈ 0.1 line. They also emphasize the relevance of these different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallization.     

Al NMR measurement of fcc Al configurations in as-quenched Al85Ni11Y4 metallic glass and crystallization kinetics of Al nanocrystals

The crystallization of melt spun Al₈₅Ni₁₁Y₄ ribbon has been studied through systematic heat treatment in the 200-550 °C temperature range using the techniques of nuclear magnetic resonance (NMR) spectroscopy, electrical resistivity, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). ²⁷Al NMR spectroscopy provides a direct and sensitive method for determining the atomic fraction of α-Al and analysis of the α-Al crystallization kinetics. NMR results revealed low concentrations (3.9 at.%) of α-Al configuration in the as-quenched material and confirm that they have α-Al short-range order. The ability of NMR to directly detect the α-Al configuration in the as-quenched state is compared and contrasted with XRD. Volume fractions of α-Al phase calculated from NMR are also compared and contrasted with that calculated from XRD for samples subjected to isothermal annealing. Qualitative and quantitative agreement between the two techniques is found after the XRD fractions are corrected according to literature methods. Heat treatment at moderate temperatures (200 °C) is shown to result in transformation kinetics dominated by the growth of α-Al consistent with heterogeneous nucleation, while higher temperatures (300 °C) lead to kinetics consistent with growth from homogeneous nucleation sites.     

Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys

The effects of adding 0.02 or 0.06 at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al₃(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300 °C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al₃(Sc,Zr) (L1₂) precipitates in the high-Si alloy subjected to double aging (8 h/300 °C for Sc precipitation and 32 days/400 °C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al₃(Sc,Zr) (L1₂) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al₃Sc; and (iii) Si partitions preferentially to Al₃Sc (L1₂) rather than Al₃Zr (L1₂).     

Glass forming ability and microstructure of hard magnetic Nd60Al20Fe20 glass forming alloy

Glass forming ability (GFA), magnetic properties and microstructure of Nd₆₀Al₂₀Fe₂₀ as-cast rod were investigated and further compared with Nd₆₀Al₁₀Fe₃₀ glass forming alloy. The rod prepared by suction casting with a diameter of 3 mm exhibits the typical amorphous nature in XRD pattern, distinct glass transition in DSC traces and hard magnetic properties. It is found that the diameter of cast Nd₆₀Al₂₀Fe₂₀ glassy rod is much larger than the critical section thickness (Z_c) of bulk metallic glass (BMG) predicted from DSC measurements. A few nano-crystalline particles with the structure and composition similar to A_x phase in Nd–Fe alloys were found embedded randomly in amorphous matrix and could be the origin of hard magnetic properties of the as-cast rods. The GFA of the alloy appears to be enhanced by the precipitation of metastable nano-particles with small positive forming enthalpy and the real Z_c of the alloy could be less than 1 mm predicted by parameter γ.     

A novel ternary eutectic in the Nb–Al–Ni system

A novel ternary eutectic reaction L → Nb₂Al + Al₃Nb + AlNbNi occurs at the composition of Al–40.4Nb–2.42Ni and the temperature of 1553.6 °C. The third phase is an Al-rich AlNbNi phase with the composition Al–33.3Nb–12.3Ni. Ternary Nb–Al–Ni eutectic exhibits regular microstructure with the predominantly fibrous morphology.     

Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel

In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8?kN, obtained from the weld produced with a 1600?rev min^?1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe₂Al₅ and Fe₄Al₁₃ intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength.     

Influence of a high magnetic field on columnar dendrite growth during directional solidification

The influence of a high magnetic field on the growth of MnBi, α-Al and Al₃Ni dendrites in directionally solidified Bi–Mn, Al–Cu and Al–Ni alloys have been investigated. Results indicate that the magnetic field changes the dendrite growth significantly. Indeed, the magnetic field aligns the primary dendrite arm and the effect is different for different dendrites. For the MnBi dendrite, an axial high magnetic field enhanced the growth of the primary dendrite arm along the solidification direction; however, for the α-Al and Al₃Ni dendrites, the magnetic field caused the primary dendrite arm to deviate from the solidification direction. At a lower growth speed, a high magnetic field is capable of causing the occurrence of the columnar-to-equiaxed transition (CET). Moreover, it has also been observed that a high magnetic field affects the growth of the high-order (i.e., secondary and tertiary) dendrite arms of the α-Al dendrite at a higher growth speed; as a consequence, the field enhances the branching of the dendrite and the formation of the (1 1 1)-twin planes. The above results may be attributed to the alignment of the primary dendrite arm under a high magnetic field and the effect of a high magnetic field on crystalline anisotropy during directional solidification.     

XPS study of the initial stages of oxidation of α2-Ti3Al and γ-TiAl intermetallic alloys

The initial stages of oxidation of α₂-Ti₃Al and γ-TiAl alloys at 650 °C under low oxygen pressure are shown to be characterized by three stages using XPS. Stage I is a pre-oxidation stage characterized by the adsorption and absorption of oxygen species. When the subsurface is saturated with dissolved oxygen (16 and 2 at.% on α₂-Ti₃Al and γ-TiAl, respectively), the selective oxidation of the alloy leads to the nucleation and growth of ultrathin (∼0.5 and ∼1.2 nm on α₂-Ti₃Al and γ-TiAl, respectively) alumina layers (stage II). The growth kinetics of the alumina layers is limited by the transport of Al in the alloy, leading to Al-depletion in the metallic phase underneath the oxide. When a critical concentration is reached (Ti₈₂Al₁₈ and Ti₇₅Al₂₅ on α₂-Ti₃Al and γ-TiAl, respectively), titanium oxidation occurs (stage III). Ti(III) and Ti(IV) oxide particles are formed at the surface of the still growing alumina layer.     

Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti_0.52Al_0.48N, c/w-Ti_0.34Al_0.66N and c-Cr_0.32Al_0.68N) widely used in industry, has been performed in detail. The hardness of Ti_0.52Al_0.48N and Ti_0.34Al_0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~ 34.1 and 38.7 GPa with T_a up to 900 °C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated T_a, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr₂N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr_0.32Al_0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti_0.34Al_0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr_0.32Al_0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al₂O₃-rich and Cr₂O₃-rich layers, with only ~ 1.4 μm oxide scale thickness, after thermal exposure for 10 h at 1050 °C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950 °C.     

Precipitation evolution in Al–Zr and Al–Zr–Ti alloys during aging at 450–600 °C

The transformation of Al₃Zr (L1₂) and Al₃(Zr_1−xTi_x) (L1₂) precipitates to their respective equilibrium D0₂₃ structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L1₂ precipitates nor their transformation to the D0₂₃ structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L1₂ precipitates transform to disk-shaped D0₂₃ precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the α-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D0₂₃ phase with α-Al. The transformation is very sluggish and even at 575 °C coherent L1₂ precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.     

Core–shell nanoscale precipitates in Al–0.06 at.% Sc microalloyed with Tb,Ho, Tm or Lu

The age-hardening response at 300 °C of Al–0.06Sc–0.02RE (at.%, with RE = Tb, Ho, Tm or Lu) is found to be similar to that of binary Al–0.08Sc (at.%), except that a shorter incubation period for hardening is observed, which is associated with nanoscale RE-rich Al₃(RE_1?xSc_x) precipitates. In addition, Al–0.06Sc–0.02Tb (at.%) has a much lower peak microhardness than that of Al–0.08Sc (at.%) due to the small solubility of Tb in α-Al(Sc). Peak-age hardening occurs after 24 h, and is associated with a high number density of nanoscale Sc-rich Al₃(Sc_1?xRE_x) precipitates. Analysis by three-dimensional local-electrode atom-probe tomography shows that x increases with increasing atomic number, and that the REs partition to the core of the precipitates.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

Local structure variations in Al89La6Ni5 metallic glass

Reduced density functions (RDFs) from electron diffraction measurement of an Al₈₉La₆Ni₅ metallic glass were obtained and analysed using Reverse Monte Carlo (RMC) refinement accompanied by density functional theory calculations. Three different RDFs were observed from regions in close proximity (within microns) of each other, with the first/second peaks respectively located at 2.76/3.32, 2.90/3.46 and 2.96/3.55 Å (all ±0.02 Å). The first type of RDF was found to be the more prevalent. X-ray dispersive spectroscopy and electron energy loss spectroscopy showed no measurable composition variations on this scale. According to the RMC refinements, the variations in the peaks are due to slight lengthening in the mean Al–Al and larger lengthening of the Al–La first-neighbour contacts, while the mean Al–Ni contact shortens. These local structure variations in a nominally homogeneous Al₈₉La₆Ni₅ metallic glass are likely to be important to the understanding of metallic glass properties.