Formation and chemical leaching effects of nonequilibrium Al0.6(Fe25Cu75) alloy powder produced by rod milling

The formation and chemical leaching effects of a nonequilibrium Al_0.6(Fe₂₅Cu₇₅)_0.4 powder produced by rod milling is described. X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and vibrating sample magnetometry were used to characterize both the as-milled and leached specimens. After 400 h of milling, only the bcc AlFe phase with an amorphous phase was detected in the XRD patterns. The crystallite size for the bcc AlFe phase (110) after 400 h of milling was about 5.3 nm. The peak temperature and the crystallization temperature of the as-milled powders were 448.7 and 428.0 °C, respectively. Al atoms leaching from the as-milled bcc AlFe powders in the L1 condition did not alter the diffraction pattern significantly, even though Al atoms had been removed. After the L1 specimen was annealed at 500 °C for 1 h, the bcc AlFe phase transformed to the fcc Cu, Fe, and CuFe₂O₄ phases. The peak widths of L1 and L2 specimens were similar, but became broader than that of the as-milled powder. The saturation magnetization decreased with increasing milling time, and a value of 10.4 emu/g was reached after 400 h of milling. After cooling the specimen from 750 °C, the magnetization slowly increased at approximately 491.4 °C, indicating that the bcc AlFe phase had transformed to the fcc Cu and Fe phases.     

Chemical leaching and magnetic properties of non-equilibrium Al(Co–Cu) powders produced by rod milling

We report upon the chemical leaching and magnetic properties of nanoscale crystalline Al_0.6(Co₂₅Cu₇₅)_0.4 alloy powders produced by rod milling. X-Ray diffractometry (XRD), transmission electron microscopy, differential scanning calorimetry, vibrating sample magnetometry, and superconducting quantum interference device magnetometry were used to characterize the as-milled and leached specimens. After 400 h of milling, only the b.c.c. phase of the intermetallic compound γ-Al_3.892Cu_6.10808 was detected by XRD. After annealing the leached specimen at 600 °C for 1 h, the nanoscale crystalline phase was transformed into the f.c.c. Cu phase, and this was accompanied by a change in the magnetic properties. The peaks of the magnetization shifted towards lower temperature with increasing external field. The temperature behavior at T_f (45 K) for direct current (d.c.) magnetic susceptibility measurements was quite different for field cooling and zero-field cooling. After cooling the leached specimen from 800 °C, magnetization increased gradually.     

Effect of overall composition on thermally induced solid-state transformations in thick EB PVD Al/Ni multilayers

Multilayered Ni/Al samples 20–70 μm thick of different average stoichiometries were prepared by electron-beam physical vapour deposition technique. The periodicity in the multilayers varied between 0.2 μm and 0.8 μm. The course of the solid-state transformations initiated by heating the samples at a constant rate has been studied. Intermetallic Al₃Ni was found to be the first phase to form upon heating of all samples studied. Further reactions were dependent on the average composition of the sample so that Al–Ni compounds increasingly richer in Ni formed as the nickel content increased. In general, the phases Al₃Ni, Al₃Ni₂, AlNi, and AlNi₃ have been observed upon heating the samples to 600 °C. The phases were more likely to form in succession rather than to grow simultaneously.     

Infrared brazing of Ti50Ni50 shape memory alloy using pure Cu and Ti–15Cu–15Ni foils

Infrared brazing of Ti₅₀Ni₅₀ using two brazing filler metals was investigated in the study. Three phases, including Cu-rich, CuNiTi (Δ) and Ti(Ni,Cu), were observed in the Ti₅₀Ni₅₀/Cu/Ti₅₀Ni₅₀ joint after brazing at 1150 °C. The Cu-rich phase was rapidly consumed in the first 10 s of brazing, and the eutectic mixture of CuNiTi and Ti(Ni,Cu) phases were subsequently observed in the joint. Samples brazed for longer time resulted in less CuNiTi and more Ti(Ni,Cu) phases in the joint. The existence of CuNiTi phase deteriorated the shape memory effect of the joint, but Ti(Ni,Cu) could still preserve shape memory behavior even alloyed with a large number of Cu. Therefore, higher shape recovery ratio was observed for specimens brazed for a longer time period. Extensive presence of Ti₂(Ni,Cu) phase was observed in Ti₅₀Ni₅₀/Ticuni^®/Ti₅₀Ni₅₀ joint upon brazing the specimens up to 1150 °C. The bending test could not be performed due to the inherent brittleness of Ti₂(Ni,Cu) matrix. Moreover, the stable Ti₂(Ni,Cu) phase was difficult to be removed completely by increasing either brazing time and/or temperature.     

Synthesis and characterization of molybdenum aluminide nanoparticles reinforced aluminium matrix composites

Aluminium matrix composites reinforced with molybdenum aluminide nanoparticles were synthesized by ball milling and reactive sintering of the mixture of aluminium and 10 wt% hydrated molybdenum oxide powders. Sintering the as milled powder in air below 750 °C produced MoAl₁₂ intermetallic compound nanoparticles, at 750 °C produced a mixture of MoAl₅ and MoAl₄ nanoparticles and at 800 °C under Argon atmosphere produced predominantly MoAl₄ intermetallic nano-particles in the Al matrix. The powder compacts sintered in air below 750 °C produced MoAl₁₂ whereas at 750 °C or above formed the Al matrix composite reinforced with the MoAl₅ nanoparticles. These nanoparticles become agglomerated to take up some irregular shaped flakes in the metal matrix. The reaction between Al and hydrated Mo oxide powders was found to be a favorable way to produce predominantly a particular Mo–Al intermetallic compound at a particular temperature. The Al₂O₃ particles formed as another reaction product, in all the above reactions, remain distributed in these composites. The composites thus formed were characterized by SEM-EDS, DTA, XRD and TEM analysis.     

Preparation and luminescent properties of europium-activated YInGe2O7 phosphors

Effects of precursor milling on phase evolution and morphology of mullite (3Al₂O₃·2SiO₂) processed by solid-state reaction have been investigated. Alumina and silica powders were used as starting materials and milling was taken place in a medium energy conventional ball mill and a high-energy planetary ball mill. Milling in a conventional ball mill although decreases mullite formation temperature by 200 °C, but does not considerably change mullite phase morphology. Use of a planetary ball mill after 40 h of milling showed to be much more effective in activating the oxide precursors, and mullitization temperature was reduced to below 900 °C. Whisker like mullite was formed after sintering at 1450 °C for 2 h and volume fraction of this structure was increased by increasing the milling time. XRD results showed that samples mechanically activated for 20 h in the planetary ball mill were fully transformed to mullite after sintering at 1450 °C, whereas Al₂O₃ and SiO₂ phases were still detected in the samples milled in the conventional ball mill for 20 h and then sintered at the same conditions.     

The ternary system Al–Ni–Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1x_Al0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, τ₁ to τ₄, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ₃-Al₃NiTi₂ with the MgZn₂-type structure ranging from Al₃₀Ni₂₈Ti₄₂ (composition lowest in Al) to Al₅₀Ni₁₆Ti₃₄ (composition richest in Al) and for τ₂-Al₂NiTi. The complex atom site substitution mechanism in τ₃ changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al_xTi_1-x}₂+τ₃, we observed two closely adjoining three-phase equilibria: ₂-AlTi₃+Ni{Al_xTi_1-x}₂+ τ₄-AlNi₂Ti and ₂-AlTi₃+τ₃-Al₂NiTi₂+τ₄-AlNi₂Ti. The earlier reported “homogeneous phase at Al₂₃Ni₂₆Ti₅₁′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.     

Analyzing the Effects of Milling and Sintering Parameters on Crystalline Phase Evolution and Mechanical Properties of Al86Ni8Y6 and Al86Ni6Y4.5Co2La1.5 Amorphous Ribbons

In the present study, Al₈₆Ni₈Y₆ and Al₈₆Ni₆Y_4.5Co₂La_1.5 bulk amorphous nanocomposites were synthesized by spark plasma sintering of milled melt spun ribbon particles. The as-cast ribbons were of near amorphous nature with minute amount of FCC Al embedded in the amorphous matrix. Milling of the ribbons resulted in partial devitrification due to mechanical crystallization. The milled ribbon particles were sintered in the temperature and pressure range of 300-500 °C and 500-700 MPa, respectively. It was observed that nominal amount of amorphous phase was retained at 500 °C and 500 MPa. With increase in sintering pressure and decrease in sintering temperature, the amount of crystalline phase evolution decreased, and maximum amount of amorphous phase was retained at 300 °C and 700 MPa. The microstructure consisting of amorphous phase embedded with hard intermetallic phases led to increase in the nanohardness of Al₈₆Ni₈Y₆ and Al₈₆Ni₆Y_4.5Co₂La_1.5 as-cast ribbons from 3.26 ± 0.59 GPa and 3.81 ± 0.58 GPa to 6.06 ± 0.70 GPa and 6.14 ± 0.82 GPa, respectively, for the corresponding consolidated amorphous nanocomposite. Microhardness of the three and five component system bulk samples was 4.19 ± 0.13 GPa and 3.6 ± 0.13 GPa, respectively.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Sintering of alumina-niobium carbide composite

Several studies have been focused on particulate-dispersed Al₂O₃ composites in order to improve both room and high temperature mechanical properties and wear resistance. In the present work Al₂O₃-NbC composites have been pressureless sintered and their microstructures analysed as a function of NbC and Y₂O₃ concentration, the latter added as sintering aid. The compositions used in this study were Al₂O₃-xNbC and (Al₂O₃ 3%Y₂O₃)-xNbC, (x = 10, 20 and 40 wt%) and the sintering was performed at 1650 °C/30 min and 1750 °C/15 min. A density greater than 96% of the theoretical density was reached even for those materials sintered at 1650 °C. The observed microstructure was more homogeneous for the samples with Y₂O₃ addition and the Y₃Al₅O₁₂ phase was detected. The Al₂O₃ grain growth restraining due to the NbC concentration was more pronouncedly in samples sintered at 1750 °C.     

Structural phase transformations of rare-earth modified transition alumina to corundum

The effect of rare-earth dopant on transformations of the γ→θ→ phases in fine alumina powders under vacuum was investigated by in situ neutron diffraction from 500 to 1300 °C. La-doped (1 mol%) Al₂O₃ powders (surface area 170 m²/g) were prepared by an impregnation technique. Below 800 °C both samples contain a dominant γ-phase. Above 1000 °C, transformation of the γ-phase to the intermediate θ-phase, and then completely to the -phase (corundum) was observed. Addition of 1 mol% of La in Al₂O₃ effectively shifts the -phase formation temperature from 1125 °C for pure alumina to 1250 °C, probably due to the larger size of La compared to Al ions, which hinders ionic diffusion in the processes of sintering and transformation. Consequently, doping La in alumina improves the surface-area and thermal stability at high temperatures, which is important for catalytic applications.     

Sintering characteristic of Al2O3-reinforced 2xxx series Al composite powders

Sintering characteristic of Al₂O₃-reinforced 2xxx series Al composite powders was investigated in order to obtain enhanced densification. In order to confirm the effect of the ceramic phase, Al composite powder, AMB 2905 (Al–3.2Cu–1.0Mg–5.0Al₂O₃), was used as the starting powder. Al blended powder, AMB 2712 (Al–3.8Cu–1.0Mg), was also used for comparison. The sintered density of the blended powder was about 93% of the theoretical value at 620 °C. The sintered density of the composite powder was about 95% at 630 °C. A small decrease in the density of each powder caused by swelling was observed after holding time of 10 min at the sintering temperature. After 20 min, the density slightly increased. The diffusion of the liquid phase was faster in the composite powder sintered specimen than in the blended powder sintered specimen. The liquid phase is thought to have infiltrated into the spaces between ceramic agglomerates. The results show that a greater amount of liquid phase is needed to enhance the sinterability of 2xxx series Al composite materials.     

Effects of replacing Ni by Co on the crystallization behaviors of Al–Ni–La amorphous alloys

Effects of replacing Ni by Co on the crystallization behaviors of three Al–Ni–La amorphous alloys, i.e. Al₈₅Ni₉La₆, Al₈₆Ni₉La₅ and Al₈₇Ni₈La₅ were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the glass-forming ability decreases when Ni is replaced by excessive Co. Meanwhile replacing Ni by Co improves the thermal stability, enlarges the supercooled liquid region ΔT_x and promotes the precipitation of the metastable phase(s) as the primary phase. The apparent activation energy E_a1 of the first reaction changes complicatedly during the replacement and is strongly dependent on the type of the primary phase, i.e. diffusion of atoms.     

Magnetic properties of Al–Gd–Ni orthorhombic compounds

The magnetic properties of the Al₄GdNi, Al₂GdNi and AlGd₂Ni₂ compounds have been investigated using magnetic measurements in the temperature range 4–800 K and magnetic fields up to 9 T and X-ray photoelectron spectroscopy at room temperature. The compounds Al₄GdNi and AlGd₂Ni₂ order antiferromagnetically at T_N = 20 and 25 K, respectively, and Al₂GdNi orders ferromagnetically at T_C = 39 K. The results are discussed in terms of the Ruderman–Kittel–Kasuya–Yosida theory, the polarization of 5d electrons by the local exchange interaction 4f–5d and the spin fluctuations on Ni sites.     

Solid-phase equilibria in the Ti-rich part of the Ti---Al system

The solid-state-phase equilibria in the Ti-rich portion of the Ti---Al system (0–50%Al) have been investigated at temperatures between 800°C and 1415°C by electron probe microanalysis (EPMA) and by transmission electron microscopy (TEM) on diffusion couples as well as on annealed alloys. The TEM observations on the diffusion couples show that the phase field ₂, (ordered hexagonal DO₁₉) extends up to the β (bcc A2) phase field, leading to two peritectoid transformations, β + → ₂ at T = 1210 ± 10°C and β + ₂→ at T = 1160 ± 10°C (: hexagonal A3). The diffusion-couple experiments yield the tie lines of the observed phase equilibria. The present results confirm the existence of a eutectoid transition → ₂ + λ (λ: ordered fcc Ll₀) at about T = 1120 ± 10°C. An updated phase diagram based on these data is presented.     

Mechanically induced crystalline–glassy phase transformations of mechanically alloyed Ta55Zr10Al10Ni10Cu15 multicomponent alloy powders

New multicomponent Ta-based glassy alloy powder was synthesized by mechanical alloying (MA) the elemental powders of Ta₅₅Zr₁₀Ni₁₀Al₁₀Cu₁₅ at room temperature, using a low-energy ball milling technique. During the early stage of milling the agglomerated crystalline powders are mechanically crushed and fresh surfaces are rapidly created. Kneading of such ground powders enhances the atomic diffusion and leads to local alloying. As the MA time increases, the number of vacancies in the Ta lattice (base material) increases so that the atoms of the alloying elements for Zr, Al, Ni and Cu tend to migrate to the open defected lattice of metallic Ta. The number of atoms of the alloying elements that migrate to the bcc lattice of the base material are increasing with increasing MA time and this leads to a monotonic expansion of the Ta lattice. Further milling time (86–130 ks) plays an important role in increasing the rate of diffusion and this leads to an increase in the number of migrated atoms of the alloying elements that pass into the Ta lattice. The a₀ of the yielded solid solution at this stage does not change anymore with increasing MA time and a homogeneous supersaturated bcc-solid solution is obtained after 130 ks of MA time. This solid solution, which is subjected to continuous imperfections, is gradually transformed into a glassy phase upon increasing the MA time. The glassy powders of the final-product (1080 ks) in which its glass transition temperature (T_g) lies at a high temperature (834 K), crystallize through a single sharp exothermic peak at 1004 K (T_x). The total enthalpy change of crystallization (ΔH_x) is −10.32 kJ/mol. The width of the supercooled liquid region before crystallization (ΔT_x) of the synthesized glassy powder shows the largest value (170 K) of any reported metallic glassy system.     

Mechanical properties and oxidation behaviour of two-phase iron aluminium alloys with Zr(Fe,Al)2 Laves phase or Zr(Fe,Al)12 τ1 phase

Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al)₂ or τ₁ phase Zr(Fe,Al)₁₂. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ₁ phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0₃ Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0₃ Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%.     

The ternary system Al–Ni–Ti Part II: thermodynamic assessment and experimental investigation of polythermal phase equilibria

The Al–Ni–Ti phase diagram has been thermodynamically assessed and a consistent set of thermodynamic functions has been developed. The thermodynamic modeling is based on an experimental investigation of the phase equilibria in the composition range of 0.1x_Al0.7. Alloys were prepared by argon-arc or vacuum-electron beam melting of elemental powder blends. X-ray powder diffraction, metallography, SEM and EMPA-techniques were employed to analyze the samples in the as-cast state as well as after annealing at 800, 900 and 1000°C. The existence of the four ternary compounds, τ₁ to τ₄, has been confirmed, although homogeneity regions differ significantly from reports in the literature. The homogeneous phase, previously claimed at “Al₂₃Ni₂₆Ti₅₁”, is shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic structure. The congruent melting behavior of τ₄=AlNi₂Ti is confirmed, but, in contrast to earlier reports, primary crystallization and congruent melting have been observed for τ₁=Al₁₃Ni₂Ti₅ and τ₃=Al₃NiTi₂. In contrast to earlier assessments, τ₁,τ₂ and τ₃ are experimentally found to be stable at 800, 900 and 1000°C. The thermodynamic modeling of the ternary phases τ₂ and τ₃ is done with simplified sublattice models, considering their crystal structure and homogeneity ranges. The sublattice model for τ₄ is taken from an earlier asessment of the nickel-rich ternary phase equilibria. The present assessment covers the entire composition range. An application to the solidification behavior of ternary alloys is also exemplified.     

Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

The phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ alloys were studied. The pseudobinary C36-type Laves phase Sr(Al,Mg)₂ was found as a structural intermediate between the Zintl phase and the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase, were determined to be x=0–0.10, 0.45–0.68 and 0.80–1, respectively. The Mg-substituted Zintl phase Sr(Al_0.95Mg_0.05)₂ can be hydrogenated to Sr(Al,Mg)₂H₂ at about 473 K. However, the Sr(Al,Mg)₂H₂ directly decomposes into SrH₂ and Sr(Al,Mg)₄ starting at 513 K. When the temperature is 573 K, the C36 Laves phase Sr(Al_0.5Mg_0.5)₂ can be hydrogenated into SrMgH₄ and Al, while the C14 Laves phase Sr(Al_0.1Mg_0.9)₂ is hydrogenated into SrMgH₄, Mg₁₇Al₁₂ and Mg.     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.