Phase stability in a powder-processed Al–Mn–Ce alloy

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al₂₀Mn₂Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al₂₀Mn₂Ce, and Al₆Mn, with a small amount of Al₁₂Mn and Al₁₁Ce₃. Upon heat-treatment at up to 450 °C, the Al₂₀Mn₂Ce and Al₆Mn phases decompose to give a hard stable phase mixture with 72–73 % Al₁₂Mn plus 13–14 % each of Al₁₁Ce₃ and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al₃(Mn,Ce) phase, 9 % Al₁₂Mn, 8 % Al₆Mn, and 1 % Al₁₁Ce₃. The formation of large volume fractions of Al₁₂Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.     

Production and mechanical properties of metallic glass-reinforced Al-based metal matrix composites

Al-based metal matrix composites were synthesized through powder metallurgy methods by hot extrusion of elemental Al powder blended with different amounts of metallic glass reinforcements. The glass reinforcement was produced by controlled milling of melt-spun Al₈₅Y₈Ni₅Co₂ glassy ribbons. The composite powders were consolidated into highly dense bulk specimens at temperatures within the supercooled liquid region. The mechanical properties of pure Al are improved by the addition of the glass reinforcements. The maximum stress increases from 155 MPa for pure Al to 255 and 295 MPa for the samples with 30 and 50 vol.% of glassy phase, respectively. The composites display appreciable ductility with a strain at maximum stress ranging between 7% and 10%. The mechanical properties of the glass-reinforced composites can be modeled by using the iso-stress Reuss model, which allows the prediction of the mechanical properties of a composite from the volume-weighted averages of the components properties.     

Modification of the reactive synthesis of porous FeAl with addition of Si

Porous Fe25Al–xSi intermetallics with various contents of Si were fabricated via reactive synthesis using Fe + Al + Si powder mixtures, in which the elemental Si was added to control the self-propagating High-temperature Synthesis (SHS) occurred during sintering process. The Si element added in Fe25Al could make the exothermic reactions in the synthesis process become mild and the total heat release from the system decrease. With increasing Si content from 0 to 5 wt-%, the open porosity, the maximum pore size and permeability increased from 38·14 to 50·12%, 27·9 to 35·7 μm and 5·13 × 10^?12 m² to 11·9 × 10^?12 m², respectively. Si element could affect the microstructure and phase evolution during the middle temperature sintering. Scattered τ₁ ((AlSi)₅Fe₃) phase surrounded Fe₂Al₅ layer transformed into loop-shape with increasing Si content. Fe(Al_1?x Si _x )₃ and Al₃FeSi phases substitute Fe₂Al₅ phase as the main intermediary phase when the Si content is beyond 3 wt-%.     

Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al–Ni–Co–Zr–Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al₁₉(Ni,Co)₅Y₃ plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al₃Y and Al₃Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al₁₉(Ni,Co)₅Y₃ plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al₁₉(Ni,Co)₅Y₃ plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al₁₉(Ni,Co)₅Y₃ inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.     

Structure and mechanical strength of Al-V-Fe melt-spun ribbons containing high volume fraction of nanoscale amorphous particles

A new nonequilibrium structure consisting of nanoscale amorphous particles surrounded by fcc-Al phase was found to form in an Al₉₄V₄Fe₂ alloy rapidly solidified at the condition of circumferential velocity of 40 m/s and ejection temperature of molten alloy between 1273 and 1423 K. Deviations of the alloy component and solidification condition cause the formation of the nanoscale mixed structure of Al+icosahedral(I) phases or Al+I+amorphous phases. The sizes of the amorphous and Al phase regions are about 10 and 7 nm, respectively, and the volume fraction of the amorphous phase region is about 60%. The formation of the nanoscale amorphous particles in coexistence with Al phase is presumably due to the suppression of the transition from super-cooled liquid to I-phase resulting from the retardation of the diffusivity of the solute elements. The tensile strength is as high as 1400 MPa for the mixed amorphous+Al phases and decreases significantly by the transition to I+Al phases. The first success in fabricating a nano-amorphous structure is particularly important for the subsequent development of nanophase materials.     

Supercooled liquid foaming of a Zr-Al-Cu-Ag bulk metallic glass containing pressurized helium pores

Foaming of a Zr-based metallic glass in the supercooled liquid is successfully performed by introducing pressurized pores and subsequent isochronal annealing. Melting of a Zr₄₈Cu₃₆Al₈Ag₈ powder under 12 MPa pressurized helium atmosphere followed by water quenching introduces spherical helium pores, whose average diameter and volume fraction are estimated respectively to be 30 μm and 7%, into a fully glassy bulk Zr₄₈Cu₃₆Al₈Ag₈ alloy. The isochronal annealing of the porous alloy below the crystallization temperature under atmospheric pressure of argon enables the expansion of pores by viscous flow deformation of the supercooled liquid, resulting in a high porosity structure up to 70% with a uniform cell size and cell distribution.     

Devitrification of nano-scale icosahedral phase in (Ti0.33Zr0.33Hf0.33)50(Ni0.5Cu0.5)40Al10 alloy with enlarging supercooled liquid region

This letter reports devitrification of novel multicomponent (Ti_0.33Zr_0.33Hf_0.33)₅₀(Ni_0.5Cu_0.5)₄₀Al₁₀ alloy, developed by equiatomic substitution. The supercooled liquid region of the (Ti_0.33Zr_0.33Hf_0.33)₅₀(Ni_0.5Cu_0.5)₄₀Al₁₀ alloy increased with progress of the first exothermic reaction. The supercooled liquid region of 57 K in the as-quenched state was observed to increase to 108 K after heat treatment for 180 min at 743 K. The corresponding microstructure after heat treatment for 180 min at 743 K consisted of a homogeneous distribution of the nano-scale icosahedral phase embedded in the remaining amorphous matrix. Therefore, it is feasible to understand that the devitrification of the nano-scale icosahedral phase occurs in the (Ti_0.33Zr_0.33Hf_0.33)₅₀(Ni_0.5Cu_0.5)₄₀Al₁₀ alloy by the primary crystallization.     

Phase transformations in Al–Mg–Zn alloys during high pressure torsion and subsequent heating

The structure, phase composition, and their thermal evolution were studied in case of ternary Al–Zn–Mg alloys before and after high-pressure torsion (HPT) in Bridgman anvils. The as-cast non-deformed alloys contained the fine particles of Mg₃₂(Al,Zn)₄₉ (τ phase), MgZn₂ (η phase), AlMg₄Zn₁₁ (η′ phase), and Mg₇Zn₃ phases embedded in the matrix of Al-based solid solution. During heating in differential scanning calorimeter (DSC), all these phases dissolved around 148 °C. The τ nanoparticles coherent with (Al) matrix-formed instead around 222 °C. HPT of the as-cast alloys strongly refined the grains of (Al) solid solution from 500 μm to 120–150 nm. The particles of τ, η, η′, and Mg₇Zn₃ phases fully dissolved in the (Al) matrix. During the following DSC-heating, particles of η phase appeared and grew. Their amount became maximal around 166 °C. The growth of η phase in the fine-grained HPT-treated alloys instead of τ phase in the coarse-grained ones is explained by the shift of the (Al) + η/(Al) + η + τ/(Al) + τ lines in the Al–Zn–Mg ternary phase diagram due to the grain boundary (GB) adsorption. At 166 °C the η phase formed the continuous flat layers in numerous (Al)/(Al) GBs. This corresponds to the complete GB wetting by the η phase. Other (Al)/(Al) GBs contain separated lenticular η particles (incomplete GB wetting). Increasing the temperature from 166 to 320 °C led to the disappearance of the completely wetted (Al)/(Al) GBs. In other words, the transition from complete to the incomplete wetting of (Al)/(Al) GBs by the η phase proceeds between 166 °C and 320 °C.     

Structural evolution of B2-NiAl synthesized by high-energy ball milling

Structural evolution during the synthesis of B2–NiAl intermetallic compound by mechanical alloying of equiatomic elemental mixtures was studied by Rietveld analysis, DSC and HTXRD. The lattice parameter, crystallite size, microstrain, amount of phase and ordering of the B2 phase were monitored as a function of milling time. Formation of the B2–NiAl phase shows a sigmoidal behavior, which suggests that Johnson–Mehl–Avrami nucleation and interface-controlled growth are the responsible mechanisms in the transformation. Almost complete transformation (~ 97 mol%) was obtained after 25 h of milling. A specific phase transformation sequence during milling was not absolutely determined, however, the sequence Ni + Al → NiAl₃ → Ni₂Al₃ → B2–NiAl was identified by HTXRD. This sequence was confirmed by DSC. The transformation temperature of the B2–NiAl phase and the presence of additional intermetallic compounds show a direct dependence on the Ni–Al layer spacing. Using a production-scale Simoloyer horizontal Attritor Mill, the presence of Ni₂Al₃ phase was observed prior to the full synthesis of B2-NiAl.     

Effect of thermo-mechanical histories on the microstructure and properties of Zr<Subscript>65</Subscript>Al<Subscript>10</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>15</Subscript> metallic glassy plates

Effect of thermo-mechanical histories during hot rolling in the supercooled liquid region on the microstructure and properties of Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glassy plates was investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC), microhardness and electrical resistivity measurements. It was found that some nano-scale clusters and a few crystalline phases were dispersed in the amorphous matrix, which may depress the crystallization onset temperature (T_x). The microhardness increased while the electrical resistivity first increased and then decreased with hot rolling times. So, it is important for the working and forming of bulk metallic glasses in the supercooled liquid region to take the thermo-mechanical histories into account.     

Effect of upset forging on microstructure and tensile properties in a devitrified Al–Ni–Co–Y Alloy

Devitrified Al—transition metal—rare earth alloys offer routes to obtain higher volume fractions of dispersed strengthening phases than conventional precipitation routes. Here, we report a study of the microstructure–property relationships of an Al–Ni–Co–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. Microstructural characterization by electron microscopy and serial section FIB tomography show that the alloy comprises an FCC Al matrix and 44 % by volume of elongated Al₁₉(Ni,Co)₅Y₃ plates with the Al₁₉Ni₅Gd₃ structure. The plates are aligned with the extrusion direction in the as-extruded alloy, and tensile data show a distinct anisotropy in yield strength and strain to failure. These data are consistent with the alloy acting more like a unidirectional short-fiber-reinforced metal–matrix composite than a conventional precipitation-hardened alloy. During axial upset forging, the ternary plates do not break up, but instead they rotate, until at large upset strains they lie perpendicular to their original orientation with corresponding changes in the tensile properties. The materials exhibit yield strengths of up to 713 MPa and tensile elongations of up to 5 %. Thus, such systems could form the basis for truly deformable high-strength low-density metal–matrix composites.     

Investigation of mechanical properties and devitrification of Cu-based bulk glass formers alloyed with noble metals

The mechanical properties, glass-forming ability, supercooled liquid region and devitrification behaviour of the Cu–Zr–Ti–(Pd, Ag, Pt and Au) bulk glass formers were studied by using a mechanical testing machine, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. The bulk glassy alloys of diameter 2 mm were formed in the Cu₅₅Zr₃₀Ti₁₀Pd₅ and Cu₅₅Zr₃₀Ti₁₀Ag₅ alloys while Cu₅₅Zr₃₀Ti₁₀Au₅ bulk alloy showed mixed glassy and crystalline structure. No glassy phase was formed in the Cu₅₅Zr₃₀Ti₁₀Pt₅ bulk alloy whereas the glassy phase was formed in all of the ribbon samples prepared by rapid solidification. The studied alloys except for the Pt-bearing one have slightly increased compressive fracture or yield strength values compared to ternary Cu₆₀Zr₃₀Ti₁₀ glassy alloy. At the same time Pd and Au addition significantly expand the supercooled liquid region of Cu–Zr–Ti glassy alloy and increase Young's modulus. A nanoicosahedral phase is primarily formed in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au)₅ glassy alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion-controlled growth. Nearly the same quasilattice constant obtained in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au)₅ alloys illustrates the same type of the icosahedral phase in these alloys. However, no icosahedral phase was found in the Cu₅₅Zr₃₀Ti₁₀(Ag,Pt)₅ alloys.     

Investigation of mechanical properties and devitrification of Cu-based bulk glass formers alloyed with noble metals

The mechanical properties, glass-forming ability, supercooled liquid region and devitrification behaviour of the Cu–Zr–Ti–(Pd, Ag, Pt and Au) bulk glass formers were studied by using a mechanical testing machine, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. The bulk glassy alloys of diameter 2 mm were formed in the Cu₅₅Zr₃₀Ti₁₀Pd₅ and Cu₅₅Zr₃₀Ti₁₀Ag₅ alloys while Cu₅₅Zr₃₀Ti₁₀Au₅ bulk alloy showed mixed glassy and crystalline structure. No glassy phase was formed in the Cu₅₅Zr₃₀Ti₁₀Pt₅ bulk alloy whereas the glassy phase was formed in all of the ribbon samples prepared by rapid solidification. The studied alloys except for the Pt-bearing one have slightly increased compressive fracture or yield strength values compared to ternary Cu₆₀Zr₃₀Ti₁₀ glassy alloy. At the same time Pd and Au addition significantly expand the supercooled liquid region of Cu–Zr–Ti glassy alloy and increase Young’s modulus. A nanoicosahedral phase is primarily formed in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au)₅ glassy alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion-controlled growth. Nearly the same quasilattice constant obtained in the Cu₅₅Zr₃₀Ti₁₀(Pd,Au)₅ alloys illustrates the same type of the icosahedral phase in these alloys. However, no icosahedral phase was found in the Cu₅₅Zr₃₀Ti₁₀(Ag,Pt)₅ alloys.     

Crystallization kinetics of Zr60Al15Ni25 bulk glassy alloy

The crystallization kinetics of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isochronal and isothermal conditions has been investigated by differential scanning calorimetry (DSC). The microstructure of as-cast Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy is observed by high-resolution electron microscopy (HREM). It is found that there exist nanocrystals with a size of about 7 nm in the glassy matrix, which are not observed in the XRD image. The results of Kissinger analysis show that the effective activation energies for glass transition (457 kJ/mol) and crystallization (345 kJ/mol) are high, indicating that it has large thermal stability against crystallization. The crystallization of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isothermal annealing can be modeled by the Johnson-Mehl-Avami equation. The crystallization kinetics parameters show that the isothermal crystallization starts from the growth of the pre-existing nanocrystals and the crystallization process is diffusion-controlled.     

Structure and microstructure of leached Raney-type Al–Ni powders

The structure and microstructure of some leached Raney-type Al–Ni alloys of different compositions have been investigated by neutron diffraction and by small-angle neutron scattering. It was found that all alloys contain a crystalline face-centred cubic (fcc) Ni phase as well as an Al₃Ni₂ phase, the amount of which is decreasing with increasing Al content of the initial alloy. Both the Ni and the Al₃Ni₂ phases are conjectured to be non-stoichiometric. There is no indication of any other crystalline phase. The size of the Ni crystallites in all leached alloys has been found to be of the order of 30 Å, whereas the size of the Al₃Ni₂ ones varies with initial alloy composition and is found to be in the range of 100–250 Å. The change in structure by doping the initial alloys with small amounts of Ti and Cr is after leaching marginal.     

Effect of transition metal on the hydrogen storage properties of Mg–Al alloy

Mg–Al alloys were prepared via sintering combined with ball milling, and the effect of a transition metal (TM = Ti, V, Ni) on the hydrogen storage properties of these alloys was investigated; the alloys were characterized via X-ray diffraction, pressure composition isotherms, and differential scanning calorimetry. The results showed that the alloys were mainly composed of Mg and the Mg₁₇Al₁₂ phase, and the cell volume of these phases decreased after the addition of TM (TM = Ti, V, Ni), which is attributed to the improved hydrogenation kinetics of Mg–Al alloy. Moreover, the hydrogenation/dehydrogenation temperature of the Mg–Al alloy decreased with the addition of TM (TM = Ti, V, Ni). Ti, Ni, and V acted as a catalyst, thereby lowering the reaction barrier for dehydrogenation and promoting the reversible hydrogenation reaction of the Mg–Al alloy. The onset temperature of dehydrogenation of the Mg–Al–V alloy was ~244 °C, which was 66 °C lower than that of the Mg–Al alloy (~310 °C). And the apparent activation energy of the Mg–Al–V alloy was 80.1 kJ mol^?1, where it was 34.6 kJ mol^?1 lower than that of Mg–Al alloy.     

Ti-based amorphous alloys with a wide supercooled liquid region

Ti-Cu-Ni-Co quaternary amorphous alloys produce by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in Ti-Cu binary amorphous alloys. The largest temperature interval of the supercooled liquid region (ΔT_x) is as large as 90 K for Ti₅₀Cu₂₅Ni₂₀Co₅.There is a tendency for ΔT_x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in ΔT_x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.     

Preparation and crystallization of Ti53Cu27Ni12Zr3Al7Si3B1 bulk metallic glass with wide supercooled liquid region

Ti-based bulk metallic glass (BMG) alloy with the composition of Ti₅₃Cu₂₇Ni₁₂Zr₃Al₇Si₃B₁ was prepared by copper molder casting method and ribbon sample was prepared by melt spinning to compare. The thermal instability of this glass phase was examined by using differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The results revealed that the supercooled liquid region (ΔT_x), glass transition temperature (T_g) and reduced glass transition temperature (T_g/T_m) of the glassy alloy are detected to be 69, 685 and 0.62 K, respectively. The crystallization behavior of the Ti-based glass phase was also investigated by annealing the glass phase at series temperatures above T_g. The annealed microstructures were examined by means of X-ray diffraction experiments. The crystallization process of the BMG can be characterized by metastable crystalline phases at the first crystallization step and further transition to stable crystalline phases at high temperature through metastable crystalline phase.     

Barothermal analysis of phase transformations of an Al-15 at % Ni alloy and its structure and magnetic properties

Phase transformations of the 15Ni-85Al alloy have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ? 100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al₃Ni in the Al matrix. Melting and crystallization in compressed argon have been shown to increase the micropore concentration in the material by about one order of magnitude relative to the as-prepared alloy. Crystallization of the alloy at 100 MPa has a significant effect on its microstructure and the particle size and morphology of the intermetallic phase in the aluminum matrix and increases the unit-cell volumes of the Al and Al₃Ni. We have compared the magnetic properties of the alloy before and after barothermal analysis.     

Dynamics and microstructural evolution of bulk amorphous Cu12.5Ni10Zr41Ti14Be22.5 during phase separations: a small-angle neutron scattering approach

Small-angle neutron scattering has been applied to investigate in detail the structural relaxation of bulk amorphous Cu_12.5Ni₁₀Zr₄₁Ti₁₄Be_22.5 alloy in the supercooled liquid range (620–673 K) and its effect on subsequent crystallization around the crystallization point (673 K). The interference events from typical phase separation were recorded as the alloy was annealed in the supercooled liquid range. It was revealed that the crystallization in the alloy which was previously relaxed in the supercooled liquid range was significantly prohibited, and further phase separation was observed. Considerable temperature dependence of these phase separations was observed. It has been demonstrated that the phase separation developed via the spinodal mode and the achieved microstructure consisted of one droplet-like supercooled liquid phase embedded in the similarly disordered matrix. The droplets showed a bar-like pattern and distributed in a relatively regular form in the matrix. The phase separations exhibited sluggish coarsening kinetics with much smaller exponent than the Lifshitz–Slyozov–Wagner value. However, the dynamic scaling property of the phase separations at different temperatures has been approved by our scaling analysis. This revised version was published online in November 2006 with corrections to the Cover Date.