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The parallel tangent method widely applied to predict the composition and driving force to form a nucleus from an oversaturated solution is extended in this paper. The parallel tangent method is shown to (i) Over-estimates the composition difference between the first nucleus and the parent phase, (ii) Neglects the composition dependence of interfacial energies and (iii) Neglects the composition dependence of probability to form embryos prior to nucleation. New model equations are developed here for the composition dependence of the interfacial energies and probability to form the embryos as function of nucleus composition at given matrix composition. The most probable composition of the first nucleus is found at the maximum of the driving force of nucleation extended by the new model equations. The success of the extended method is demonstrated for an Al-Fe liquid alloy with 0.3 w% of Fe to predict the first nucleating intermetallic phases upon cooling after nucleation of the fcc phase. It is shown that although the prediction based on the parallel tangent method contradicts experimental observations, the prediction based on our extended method agrees with them. [ABSTRACT FROM AUTHOR]

The study deals with the element–phase composition, microstructure evolution, crystal-lattice parameter, and microdistortions as well as the size of the coherent scattering region in the Al–10.65Si–2.11Cu and Al–5.39Si–1.33Cu alloys irradiated with the high-intensity electron beam. As revealed by the methods of x-ray phase analysis, the principal phases in untreated alloys are the aluminium-based solid solution, silicon, intermetallics, and Fe2Al9Si2 phase. In addition, the Cu9Al4 phase is detected in Al–10.65Si–2.11Cu alloy. Processing alloys with the pulsed electron beam induces the transformation of lattice parameters of Al–10.65Si–2.11Cu (aluminium-based solid solution) and Al–5.39Si–1.33Cu (Al1 and Al2 phases). The reason for the crystal-lattice parameter change in the Al–10.65Si–2.11Cu and Al–5.39Si–1.33Cu alloys is suggested to be the changing concentration of alloying elements in the solid solution of these phases. As established, if a density of electron beam is of 30 and 50 J/cm2, the silicon and intermetallic compounds dissolve in the modified layer. The state-of-the-art methods of the physical materials science made possible to establish the formation of a layer with a nanocrystalline structure of the cell-type crystallization because of the material surface irradiation. The thickness of a modified layer depends on the parameters of the electron-beam treatment and reaches maximum of 90 µm at the energy density of 50 J/cm2. According to the transmission (TEM) and scanning (SEM) electron microscopy data, the silicon particles occupy the cell boundaries. Such changes in the structural and phase states of the materials response on their mechanical characteristics. To characterize the surface properties, the microhardness, wear parameter, and friction coefficient values are determined directly on the irradiated surface for all modification variants. As shown, the irradiation of the material surface with an intensive electron beam increases wear resistance and microhardness of the Al–10.65Si–2.11Cu and Al–5.39Si–1.33Cu alloys.

The study deals with the element-phase composition, microstructure evolution, crystal-lattice parameter, and microdistortions as well as the size of the coherent scattering region in the Al-10.65Si-2.11Cu and Al-5.39Si-1.33Cu alloys irradiated with the high-intensity electron beam. As revealed by the methods of x-ray phase analysis, the principal phases in untreated alloys are the aluminium-based solid solution, silicon, intermetallics, and Fe2Al9Si2 phase. In addition, the Cu9Al4 phase is detected in Al-10.65Si-2.11Cu alloy. Processing alloys with the pulsed electron beam induces the transformation of lattice parameters of Al-10.65Si-2.11Cu (aluminium-based solid solution) and Al-5.39Si-1.33Cu (Al1 and Al2 phases). The reason for the crystal-lattice parameter change in the Al-10.65Si-2.11Cu and Al-5.39Si-1.33Cu alloys is suggested to be the changing concentration of alloying elements in the solid solution of these phases. As established, if a density of electron beam is of 30 and 50 J/cm2, the silicon and intermetallic compounds dissolve in the modified layer. The state-of-the-art methods of the physical materials science made possible to establish the formation of a layer with a nanocrystalline structure of the cell-type crystallization because of the material surface irradiation. The thickness of a modified layer depends on the parameters of the electron-beam treatment and reaches maximum of 90 μm at the energy density of 50 J/cm2. According to the transmission (TE M) and scanning (SEM) electron microscopy data, the silicon particles occupy the cell boundaries. Such changes in the structural and phase states of the materials response on their mechanical characteristics. To characterize the surface properties, the microhardness, wear parameter, and friction coefficient values are determined directly on the irradiated surface for all modification variants. As shown, the irradiation of the material surface with an intensive electron beam increases wear resistance and microhardness of the Al-10.65Si-2.11Cu and Al-5.39Si-1.33Cu alloys. [ABSTRACT FROM AUTHOR]

• Fatigue performace of cast components is mainly affected by porosity. • An alternative fatigue procedure that accounts the effect of porosity was developed. • The model presented an average logarithmic error below 2% and absolute below 16%. • The required porosity parameters can be obtained by computed tomography. • In high series sectors, a statistical store with fracture surface data could be used. Porosity is one of the main parameters in the fatigue life of cast alloys as it affects both crack initiation and propagation phases. However, most fatigue assessments present a lack of accuracy as they only consider the effect of the initial pore on the fatigue limit. Therefore, in the present study, first, the effect of the main porosity characteristics on the crack initiation and propagation phases is evaluated. Then, an alternative fatigue assessment model is proposed and implemented for Low Pressure Die Casting (LPDC) A356-T6 aluminium alloy. The model accounts for the effect of the initial pore size and distance to surface on the initiation phase as well as fractographic porosity and average pore size on the fracture surface on the propagation phase. Finally, the model was validated by means of experimental tests. Results have shown that the proposed model is able to qualitatively predict the scatter trend of samples tested at the same load. In addition, the model presents high quantitative accuracy in life estimation with an average logarithmic error below 2% and an absolute average error below 16%. [ABSTRACT FROM AUTHOR]

Transient horizontal directional solidification (THDS) experiments have been carried out with Al-7wt.%Si-0.15Fe, Al-7wt.%Si-3wt.%Cu-0.15wt.%Fe and Al-7wt.%Si-0.3wt.%Mg-0.15wt.%Fe alloys, to identify experimental relationships between growth rates (GR), cooling rates (CR), tertiary dendrite arm spacings (λ3) and microhardness (HV). Optical microscopy and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) were used to perform a comprehensive microstructural characterisation of the β-Al5FeSi, ω-Al7Cu2Fe, θ-Al2Cu, π-Al8Mg3FeSi6 and α-Mg2Si intermetallic phases. The addition of Cu and Mg to the Al-7wt.%Si-0.15wt.%Fe alloy led to the precipitation of ω and π phases from the β phase. It has been found for all analysed alloys that power experimental functions given by λ3 = constant.(GR)-1.1 and λ3 = constant.(CR)-0.55 best describe the variation of λ3 with corresponding thermal and microstructural parameters. [ABSTRACT FROM AUTHOR]

The fatigue strength of cast aluminium alloys is known to be greatly affected by different defect types related to the manufacturing process, particularly microshrinkage pores created during the solidification phase of the casting process. Even if certain classification procedures are given in the standard ASTM E155-15 [1], the presence of defects is not readily related to the capacity of a component or a structure to meet the requirements of the mechanical technical specifications. The present study aims at establishing a clear link between certain microstructural features and the average fatigue strength. This is possible by looking for the average size of critical defects and using a relevant statistical analysis. More exactly, the Murakami approach based on the statistics of extremes is employed. The main originality of this work lies in the application of this approach to the case of a real structure submitted to high cycle fatigue damage: engine cylinder heads, used in the automotive industry. Indeed, both fatigue tests and microstructural characterizations are carried out on cylindrical specimens and real structures. The specimens are subjected to uniaxial and multiaxial loading conditions [2]. Original fatigue tests, developed by PSA to load in-service critical regions, are carried out on cylinder heads. Systematic analyses of fatigue failure surface are conducted to obtain the statistics of critical defects at the origin of the failures for both specimens and structures. In parallel, critical regions and the associated local loading mode in the structure are characterized by an appropriate high cycle fatigue analysis. The latter, combined with the fatigue test data and the statistical analysis of the critical defects, leads to a discussion about the size effect and an approach is proposed for a relevant fatigue design procedure. [ABSTRACT FROM AUTHOR]

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Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al 3 Zr x V 1- x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour. [ABSTRACT FROM AUTHOR]

The phase composition of the Al–Ni–Mn–Fe–Si–Zr system is analysed as applied to heat resistant nikalines (aluminum alloys of a new generation based on Ni containing eutectic), which are strengthened by the Al3Zr (L12) nanoparticles. It is shown that the presence of iron and silicon considerably complicates the phase analysis when compared with the AN4Mts2 base alloy. Silicon strongly widens the crystallization range, which increases the tendency of the alloy to form hot cracks during casting. It is shown that economically doped nikaline AN2ZhMts substantially exceeds the most heat resistant cast aluminum alloys of the AM5 grade in the totality of its main characteristics (heat resistance and mechanical and production properties). [ABSTRACT FROM AUTHOR]

ABSTRACT The low-cycle fatigue behaviour of a cast Al-12Si-CuNiMg alloy, with a high content of Si, is investigated at 200, 350 and 400 °C. The fatigue test results show that the alloy exhibits symmetrical hysteresis loops, moderate cyclic softening and higher fatigue resistance at higher temperature. The fracture surface analysis reveals that more tear ridges are formed at higher temperature, which strongly affect the fatigue resistance. Furthermore, evaluation of the material fatigue resistance using an energy-based Halford-Marrow model indicates that the material's ability to absorb and dissipate plastic strain energy is enhanced as temperature increases. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the complex modification process of P and Sr for hypereutectic Al–30Si alloys was investigated. After the addition of 2.0% Al–3P and 0.7% Al–10Sr master alloys continuously added during the melt inoculation and solidification process, the primary Si can be significantly refined from more than 200.0μm decreasing to 32.8μm with homogeneous distribution and eutectic Si was obviously modified as fibrous simultaneously. The modification mechanism was also studied. It is found that the melt temperature has influence in the interaction between P and Sr. At about 1000°C, AlP is surrounded by Al4Sr phase to form encapsulation structure, which would react with Sr to form Sr3P2 at approximate 1300°C. However, P and Sr take the respective form of AlP and Al4Sr at 770°C, which just be the reason for the excellent complex modification of Si phase using continuous refining method. [Copyright &y& Elsevier]

The role of various intermetallic precipitates (IMPs), or secondary phase particles, in governing the wear and corrosion performance characteristics of cast aluminium alloys is outlined in this brief review. Such alloys are especially important in transport applications where their low weight, low cost and recyclability make them very attractive. However, alloy wear and/or corrosion behaviour often limit their industrial application, and more work needs to be carried out to extend their use into other areas. Careful control of IMP nucleation and growth rates may be beneficial, especially in alloys exposed to corrosive environments. Silicon, copper and magnesium are all important elements for enhanced mechanical strength and tribological performance but often to the detriment of alloy corrosion resistance. Other elements such as iron may also play a significant role in deleterious IMP formation. Use of dispersoids based on novel (quasicrystals) seed alloys with similar lattice characteristics to the α-Al matrix may result in further exploitation of these alloys. [ABSTRACT FROM AUTHOR]

Abstract: The solute nanostructures formed in the primary α-Al grains of a semi-solid metal cast Al–7Si–0.6Mg alloy (F357) during ageing at 180°C, and the age-hardening response of the alloy, have been systematically investigated by transmission electron microscopy, atom probe tomography and hardness testing. A 120h natural pre-ageing led to the formation of solute clusters and Guinier–Preston (GP) zones. The natural pre-ageing slowed down the precipitation kinetics 6-fold during 1h ageing at 180°C, but this effect diminished after 4h when the sample reached the same hardness as that without the pre-ageing treatment. It reduced the number density of β″ needles to approximately half of that formed in samples without the treatment, and postponed the peak hardness occurrence to 4h, four times that of the as-quenched sample. A hardness plateau developed in the as-quenched sample between 1 and 4h ageing corresponds to the growth of the β″ precipitates and a significant concurrent decrease of solute clusters and GP zones. The average Mg:Si ratio of early solute clusters is <0.7 while that of GP zones changes from 0.8 to 0.9 with increasing size, and that of β″ needles increases from 0.9 to 1.2. β″ needles, GP zones and solute clusters are important strengthening solute nanostructures of the alloy. The partitioning of solutes and precipitation kinetics of the alloy are discussed in detail. [Copyright &y& Elsevier]