Investigation on the microstructure and mechanical properties of a cast Mg–6Zn–5Al–4RE alloy

Mg–6Zn–5Al–4RE (RE = Mischmetal, mass%) alloy was prepared by metal mould casting method. The microstructure and mechanical properties of the as-cast and heat-treated alloys were investigated. The results show that the phase compositions of the as-cast state alloy are supersaturated solid solution -Mg, lamellar β-Al₁₂Mg₁₇, polygonal Al₃RE and cluster Al₂REZn₂ phases. The mechanical properties, especially the ultimate tensile strength and elongation of the alloy were significantly improved by the heat treatment. Fracture surface of tensile specimens was analyzed by optical microscope and scanning electron microscope.     

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.%.     

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.     

Electrochemical behaviour assessment of novel Mg-rich Mg–Al–RE alloys (RE = Ce, Er)

The electrochemical behaviour of new Mg–Al–RE (RE = Ce, Er) alloys AE91 was investigated in 0.01 M NaCl electrolyte (pH = 12) and compared with that of the most commonly used Mg alloy in the automotive field, the AZ91D. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples prior to the electrochemical tests. AE91 alloys showed very similar microstructures characterized by a three-phase appearance: a Mg-based solid solution containing only Al and two intermetallic phases γ(Mg₁₇Al₁₂) and (Al_1 − xMg_x)₃Ce or (Al_1 − xMg_x)₂Er. Free corrosion potential measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy revealed improved passivity behaviour compared to AZ91D alloy. The apparent presence of trace amounts of rare earth oxides in the passive film is presumed to be the reason for the enhanced corrosion resistance of AE91 alloys in the aggressive environment considered.     

Influence of strong static magnetic field on intermediate phase growth in Mg–Al diffusion couple

Effects of strong magnetic field on intermediate phase growth in Mg–Al diffusion couples is addressed. It was found that both Al₃Mg₂ (β) and Al₁₂Mg₁₇ (γ) formed after annealing at different temperatures with or without the field. Systematic measurements showed that thickness of intermediate phases was reduced due to the application of the 10 T static magnetic field. Data analysis show that reduction of intermediate layer thickness is attributed to the decreasing of frequency factor (k₀) under the magnetic field, however the activation energy (Q) for layer growth is almost not changed irrespective of the application of the field. Layer thickness decreasing is suggested to be related with the retardation of atom diffusion resulting from the magnetic field and a possible theory based on ambipolar diffusion is discussed to explain this effect.     

Phase diagram of the Nb–Al–Si ternary system

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb₅Si₃, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb₃Si₅Al₂ was observed. The solubility data of Al in Nb₅Si₃ and NbSi₂ as well as Si solubility in Nb₃Al, Nb₂Al and NbAl₃ were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

Preparation and magnetic properties of amorphous Co–Zr–B alloy nano-powders

The magnetic Co–Zr–B amorphous alloy powders, having a nearly spherical morphology with diameters <50 nm, were obtained successfully by the reduction of an aqueous solution of zirconium sulphate and cobalt chloride with an aqueous solution of sodium borohydride. XRD, selected-area electron diffraction (SAED) and differential scanning calorimetry (DSC) studies showed that the resultant were partially amorphous together with a tiny volume fraction of crystalline phases, and the main amorphous phase consisted of the Zr-based amorphous particles and the Co-based Zr-containing amorphous particles. It is found that the Co/Zr ratio in the powders was indistinguishably equal to the Co²⁺/Zr⁴⁺ ratio in the original mixed solution and the boron content of the samples increased along with the addition rate of NaBH₄ solution. The crystallization temperatures of the resultant powders were in the range of 765.1–771.3 K. The thermal stability of amorphous Co–Zr–B powder increased with increasing the zirconium content. When the Co/Zr ratio in the samples increased from 1.94 to 5.14, the saturation magnetization increased monotonously from 4.76 to 8.87 emu/g, but the coercivity increased irregularly from 15.98 to 26.81 Oe.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: Part I – Chemical compositions of Al3(Sc1−xZrx) precipitates

Atom-probe tomography (APT) and high-resolution transmission electron microscopy are used to study the chemical composition and nanostructural temporal evolution of Al₃(Sc_1−xZr_x) precipitates in an Al–0.09 Sc–0.047 Zr at.% alloy aged at 300 °C. Concentration profiles, via APT, reveal that Sc and Zr partition to Al₃(Sc_1−xZr_x) precipitates and Zr segregates concomitantly to the -Al/Al₃(Sc_1−xZr_x) interface. The Zr concentration in the precipitates increases with increasing aging time, reaching a maximum value of 1.5 at.% at 576 h. The relative Gibbsian interfacial excess of Zr, with respect to Al and Sc, reaches a maximum value of 1.24 ± 0.62 atoms nm⁻² after 2412 h. The temporal evolution of Al₃(Sc_1−xZr_x) precipitates is determined by measuring the time dependence of the depletion of the matrix supersaturation of Sc and Zr. The time dependency of the supersaturation of Zr does not follow the asymptotic t^−1/3 law while that of Sc does, indicating that a quasi-stationary state is not achieved for both Sc and Zr.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

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.     

Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation

Compound ceramic coatings prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO₂ solution were oxidized under different temperature in air. The phase composition and surface morphology of the coatings before and after oxidation were investigated by X-ray diffractometry and scanning electron microscopy, respectively. Meantime, the weight gains and the high temperature oxidation characteristics of the coated samples were investigated. The results show that the coatings prepared by MPO were composed of a large amount of Al₂TiO₅ and a little -Al₂O₃ and rutile TiO₂. And the oxidation process of the coated samples included the decomposition of the Al₂TiO₅ in the coating, the oxidation of the substrate and the changes of the coating structure. After high temperature oxidation, the increase of -Al₂O₃ in the coating was due to the decomposition of Al₂TiO₅, whereas the increase of rutile TiO₂ in the coating was attributable to both the decomposition of Al₂TiO₅ and the oxidation of the Ti substrate. The main crystalline of the coatings became rutile TiO₂ after the oxidation of 1000 °C for 1 h. The decomposition of Al₂TiO₅ in the coating occurred at 900 and 1000 °C, and its half decomposition time was less than 1 h at 1000 °C. Increasing oxidation temperature or extending oxidation time, the weight gains of coated samples was increased to different extent. However, the weigh gains of the coated samples was much lower than that of the substrate, so the ceramic coatings improved the oxidation resistance of Ti alloy greatly under the experimental conditions.     

Single-crystal growth of the complex metallic alloy phase β-Al–Mg

The development of a single-crystal growth route for the complex metallic alloy phase β-Al₃Mg₂ is presented. After initial probing of the phase diagram in the vicinity of the existence range of the β-phase, we performed single-crystal growth experiments employing various techniques. The Czochralski and self-flux growth turned out to be the most suitable for this phase, and with both we reproducibly achieved single crystals of several cubic centimeters in volume. While the Czochralski technique allows for the production of deliberately oriented single crystals, the self-flux technique is capable of producing very large but unoriented single grains.     

La0.9Sr0.1Ga0.8Mg0.2O3−δ sintered by spark plasma sintering (SPS) for intermediate temperature SOFC electrolyte

The La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) powders for intermediate temperature SOFC electrolyte have been synthesized by glycine-nitrate combustion process. The as-synthesized powders show almost pure perovskite phase. And then, the as-synthesized powders were sintered by SPS at 1300 °C to prepare electrolyte. The SEM, XRD and AC impedance were employed to characterize the microstructure, phase and electrical conductivities. Results show that the grain size is very fine, less than 1 μm, and the relative density of the pellet after sintering by SPS is about 94.7%. There is very little amount of secondary phases after SPS and the grain boundary and secondary phase resistance is very small. The electrolyte sintered by SPS shows higher conductivities than that sintered by conventional method at the same temperature. The activation energy at lower temperatures (400–700 °C) and higher temperatures (700–800 °C) is about 0.94 and 0.49 eV, respectively. Spark plasma sintering is a promising and effective method to sinter the LSGM electrolyte.     

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.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: Part II-coarsening of Al3(Sc1−xZrx) precipitates

The coarsening behavior of four Al(Sc,Zr) alloys containing small volume fractions (<0.01) of Al₃(Sc_1−xZr_x) (L1₂) precipitates was investigated employing conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The activation energies for diffusion-limited coarsening were obtained employing the Umantsev–Olson–Kuehmann–Voorhees (UOKV) model for multi-component alloys. The addition of Zr is shown to retard significantly the coarsening rate and stabilize precipitate morphologies. HREM of Al(Sc,Zr) alloys aged at 300 °C reveals Al₃(Sc_1−xZr_x) precipitates with sharp facets parallel to {1 0 0} and {1 1 0} planes. Coarsening of Al-0.07 Sc-0.019 Zr at.%, Al-0.06 Sc-0.005 Zr at.% and Al-0.09 Sc-0.047 Zr at.% alloys is shown to be controlled by volume diffusion of Zr atoms, while coarsening of Al-0.14 Sc-0.012 Zr at.% is controlled by volume diffusion of Sc atoms.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.