On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.     

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.     

Structure,phase composition,and strengthening of cast Al–Ca–Mg–Sc alloys

The structure and phase composition of Al–Ca–Mg–Sc alloys containing 0.3 wt % Sc, up to 10 wt % Ca, and up to 10 wt % Mg have been investigated in the cast state and state after heat treatment. It has been shown that only binary phases Al₄Ca, Al₃Sc, and Al₃Mg₂ can be in equilibrium with the aluminum solid solution. It has been found that the maximum strengthening effect caused by the precipitation of Al₃Sc nanoparticles for all investigated alloys is attained after annealing at 300–350°C.     

In situ observation of nucleation,fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract

This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.     

Stirred casting Al–Pb monotectic alloys with high damping capacity

In this study, the stirred casting with various processing parameters, such as stirring temperature and stirring speeds, was carried out on the Al–Pb monotectic alloys in order to make Pb particles distribute much more uniformly. More importantly, their damping capacities were systematically studied. The results show tha mechanical stirring can not only make Pb in the aluminum matrix uniformly distribute but also dynamically influence the damping capacity of this alloy system. The Al–Pb alloy was prepared under a slow speed at solid–liquid temperature region, wherein high volume fraction of Pb in alloy could be obtained. The high volume fraction of Pb gives high overall damping capacity. The dislocation damping and interface damping theories are mainly dominated to the alloys.     

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ′ (Al₂Cu) precipitates in Al–Cu–Ag alloys were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ′ (AlAg₂) precipitates were present. These dislocation loops were enriched in silver prior to θ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ′ precipitates were decorated by a layer of silver two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al–θ′ interfacial energy. The lateral growth of the θ′ precipitates was accompanied by the extension of this silver bilayer, resulting in the loss of silver from neighbouring γ′ precipitates and contributing to the deterioration of the γ′ precipitate assemblies.     

Microstructure and microsegregation in Al-rich Al–Cu–Mg alloys

Microstructure and microsegregation in two directionally solidified Al alloys, Al–3.9Cu–0.9Mg and Al–15Cu–1Mg (in wt%), were investigated for cooling rates between 0.78 and 0.039 K/s. Transverse and longitudinal sections were examined to exhibit dendritic microstructures. Fractions of solids formed were determined using quantitative image analysis and solute redistribution in the primary phase was determined using area scans. The model employed to calculate microsegregation is based on the Scheil model but including solid-state diffusion, dendrite arm coarsening and undercooling of the dendrite tip and the formation of eutectic. The model-calculated results were found to be in good agreement with the experimentally determined concentration distributions in the primary α phase and the amounts of phases formed. It was found that the dendrite morphology was best described by a cylindrical arm geometry and that the accuracy of the phase diagram could have a significant influence on the microsegregation predictions. For the alloy with low copper content, two types of embedded droplets were observed.     

In situ observation of phase selection in undercooled Ni–Al melts

Abstract

Raney-Ni type Ni–68·5 at-% Al alloys are used for catalytic applications in the chemical industry. In this work, melt undercooling experiments were performed by means of the electromagnetic levitation technique. High-energy X-ray diffraction was used to determine the solidification pathway and the origin of microstructure peculiarities in gas atomised powders. The direct observation of the phase selection in Ni–68·5 at-% Al has unambiguously revealed the primary phase formation of β-AlNi to be independent of the level of undercooling up to a maximum of 320 K. On cooling the β-AlNi phase undergoes a fast peritectic transformation L + AlNi → Al₃Ni₂ and is therefore not found in the as-solidified microstructure.     

The quasicrystalline phase formation in Al–Cu–Cr alloys produced by mechanical alloying

Almost single-phase decagonal quasicrystal with periodicity of 1.26 nm along 10-fold axis was produced in Al₆₉Cu₂₁Cr₁₀ and Al_72.5Cu_16.5Cr₁₁ alloys using combination of mechanical alloying (MA) and subsequent annealing. Phase transformations of as-milled powders depending on annealing temperature in the range of 200–800 °C are examined. Since the transformations can be explained based on kinetic and thermodynamic reasons it seems that applied technique (short preliminary MA followed by the annealing) permits to produce the equilibrium phases rather than metastable ones.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

HREM study and structure modeling of the η′ phase,the hardening precipitates in commercial Al–Zn–Mg alloys

The structure of the η′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η-MgZn₂ phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.     

Nanoquasicrystalline phase formation in binary Zr–Pd and Zr–Pt alloys

This paper reports nanoquasicrystalline phase formation in Zr_100−xPd_x (x=30 and 35) and Zr₈₀Pt₂₀ binary alloys and the kinetics of the nanoquasicrystallization process. While the icosahedral phase (i-phase) forms as a metastable phase in the transient stage during the crystallization of Zr–Pd amorphous alloy, it forms directly from the liquid during melt-spinning of Zr–Pt alloy. The isothermal kinetics studies show that i-phase forms from the Zr₇₀Pd₃₀ amorphous alloy by the primary crystallization process with the Avrami exponent in the range of 1.5–2.5. Three-dimensional atom probe analysis results suggest that the i-phase is slightly enriched with Zr with respect to the matrix and its composition is close to Zr₇₅Pd₂₅. The tendency of quasicrystallization of Zr-based alloys appears to have correlation with the enthalpy of mixing of the system.     

Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

The solute clusters and the metastable precipitates in aged Al–Mg–Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg–Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e. pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serve as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al–Mg₂Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al–Mg–Si alloys is discussed.     

GP-zones in Al–Zn–Mg alloys and their role in artificial aging

The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated <110> domains. The structural relation to the η′-precipitate is discussed.     

Solid-solution thermodynamics in Al–Li alloys

The relative equilibrium concentrations of lithium atoms distributed over different electron-structural states has been estimated. The possibility of the existence of various nonequilibrium electron-structural states of Li atoms in the solid solution in Al has been substantiated thermodynamically. Upon the decomposition of the supersaturated solid solution, the supersaturation on three electron-structural states of Li atoms that arises upon the quenching of the alloy can lead to the formation of lithium-containing phases in which the lithium atoms enter in one electron-structural state.     

Elemental and phase composition,morphology, and chemical features of the surface of Al–Ni Alloys in contact with liquid Ga–In eutectic

A set of aluminum–nickel alloys has been studied. The elemental composition of the samples has been determined by atomic emission and atomic absorption spectrometry. X-ray diffraction analysis has revealed that the alloying of the metals leads to the formation of Al₃Ni and Al₃Ni₂ intermetallic compounds, while a portion of Al remains in a metallic phase. The local chemical composition and surface morphology of the original alloys and the alloys activated with the liquid Ga–In eutectic have been studied by scanning electron microscopy and X-ray microanalysis. It has been shown that the original alloys are characterized by a pronounced morphological heterogeneity of interfacial regions in the near-surface layers. It has been found that the studied Al–Ni alloys are activated by the liquid Ga–In eutectic; however, one of the alloy components—the Al₃Ni intermetallic compound—does not undergo significant morphological and chemical changes in contact with the liquid eutectic.