Second phase structure effect to the failure of an Al–Si casting

An Al–Si casting (blower pump) failed during pressure test. The material of the casting was aluminum–silicon alloy (BS-LM 9), in solution treated and aged condition. During pressure test the casting failed around 130 bar, while it was designed to sustain a pressure of 160 bar.The failed body was examined with the help of stereo microscope, optical microscope and scanning electron microscope. Fracture started from a sharp edge of a race present at the interior of the body. A granular dull gray fracture surface was observed during unaided visual examination. Fractographic studies showed that the mode of the fracture was transgranular and the material failed in brittle mode. Optical microscopic examination showed a dendritic structure having a continuous network of the secondary phase. A sample taken from the fracture region unveiled the crack propagation along the secondary phase network. It was recommend to use modification process during melting practice to avoid the stress raisers.     

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.     

Al–Ti–C phase diagram

Abstract

Equilibrium experiments have been performed at 1373, 1173, and 973 K, with alloys of compositions within the aluminium rich corner of the Al–Ti–C phase diagram. The samples have been metallographically investigated using light optical microscopy and a scanning electron microscope equipped with a system for energy dispersive spectrometry. Equilibrium phases, as well as effects of cooling, have been identified. Dynamic effects originating from cooling are discussed and a tentative phase diagram is proposed. It was predicted theoretically and confirmed experimentally that a class II reaction involving four phases occurs, i.e. Al(l) + TiC(s)?Al₃Ti(s) + Al₄C₃(s), below 1100 K.

MST/1807     

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α′ (bcc)- and ε (hcp)-martensite in the γ (fcc) matrix during plastic deformation was analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. Depending on the content of the alloying elements different phase transformations γ → ε, γ → α′ (TRIP effect), or the formation of deformation twins (TWIP effect) occurred. Additions of Al increased the stacking fault energy (γ_fcc) and suppressed the γ → ε transformation while Si decreased γ_fcc and sustained the γ → ε transformation. These steels with reduced densities of about 7.3 Mg m^?3 exhibit high tensile ductility up to 95% with true tensile strength of about 1100 MPa. The excellent plasticity induced by twinning or phase transformation up to extremely high strain rates of about <disp-formula><graphic href="splitsection2-m1.tif"/></disp-formula> results in an extraordinary shock resistance and allows for deep drawing and backward extrusion operations of parts with complex shapes.     

Very high cycle fatigue behaviors of Mn–Si–Cr series Bainite/Martensite dual phase steels

The present work studied the very high cycle fatigue (VHCF) behaviors of Mn–Si–Cr series Bainite/Martensite (B/M) dual phase high-strength low-alloy (HSLA) steels to ensure safe applications in railway and oil field. Four kinds of chemical compositions and seven types of B/M steels were designed and studied by ultrasonic fatigue test. The conception of apparent and intrinsic VHCF strength is addressed. Results show the effect of heat generation cannot be ignored in B/M steels in ultrasonic fatigue test, and higher is the intrinsic VHCF strength of materials, more remarkable is the negative effect of self-heating. More importantly, the VHCF property of B/M steels is less sensitive to the inclusion than that of tempered martensite steels, and the amount of retained austenite puts little effect on the VHCF property.     

Liquid–liquid phase equilibria,density difference,and interfacial tension in the Al–Bi–Si monotectic system

We have performed thermodynamic calculation of the phase equilibria in the ternary monotectic system Al–Bi–Si. The liquid–liquid miscibility gap in the Al–Bi–Si system extends over almost the entire concentration triangle. The thermal analysis data for (Al_0.345Bi_0.655)_100−x Si _x alloys (x = 2.5, 5, 7.5, and 10 wt%) excellently agree with the calculated phase diagram. The experimental density difference of the coexisting liquid phases shows a good agreement with the density difference calculated in the approximation of ideal solution using the densities of pure elements and the compositions of L′ and L′′ from the thermodynamic calculation. The liquid–liquid interfacial tension in the (Al_0.345Bi_0.655)_100−x Si _x liquid alloys increases with Si content. The experimental temperature dependence of the interfacial tension is well described by the power low in reduced temperature (T _C–T) at approach of the critical temperature with the exponent μ = 1.3, which is close to the value predicted by the renormalization group theory of critical behavior.     

Thermodynamic Properties of Si–Ge and Si–Sn Melts

The mixing enthalpies of Si–Ge and Si–Sn liquid alloys were measured in an isoperibolic calorimeter. The results demonstrate that the formation of Si–Ge melts is accompanied by a small heat release, while the formation of Si–Sn melts is an endothermic process. Calculations of the Si activity in Si–Sn melts by Schroeder's equation indicate large positive deviations from Raoult's law.     

High-temperature phase transition behavior and magnetocaloric effect in a sub-rapidly solidified La–Fe–Si plate produced by centrifugal casting

A sub-rapidly solidified La Fe_(11.6)Si_(1.4)plate was fabricated directly from liquid by centrifugal casting method.The phase constitution,microstructure and magnetocaloric effect were investigated using backscatter scanning electron microscopy,X-ray diffraction,differential scanning calorimetry and physical property measurement system.When the plate was annealed at 1373 K,1 phase was formed by a solid-state peritectoid reaction.A first-order magnetic phase transition occurred in the vicinity of188 K,and the effective refrigeration capacities reached 203.5 J/kg and 209.7 J/kg in plates annealed for1 h and 3 h,respectively,under a magnetic field change of 3 T.It is suggested that centrifugal casting may become a new approach to prepare high-performance La–Fe–Si magnetocaloric plates for practical applications,which could largely accelerate the formation of 1 phase during high-temperature heat-treatment process due to refined and homogeneous honeycombed microstructure.     

Nanocrystalline Si3N4 with Si–C–N shell structure

Nanocrystalline Si₃N₄ with an amorphous Si–C–N shell structure was synthesized by mechanically activating Si₃N₄ and graphite powder in argon atmosphere at room temperature. Twenty hours of mechanical activation resulted in occurrence of CN bond, which can be identified using Fourier transform infrared spectrometry (FT-IR). When the mechanical activation period was extended to 67 h and then to 90 h, the CN bond was further established. The formation of CN bond under the mechanical activation for 90 h was further confirmed using X-ray photoelectron spectroscopy (XPS). The thickness of Si–C–N shell is 5–7 nm as observed using high-resolution transmission electron microscope.     

Effect of initial composition on phase selection in Ni–Si powder blends processed by mechanical alloying

The effect of initial powder blend composition on the synthesis and formation mechanism of nickel silicide phases was investigated by mechanical alloying in Ni-60 and Ni-66.7?at.% Si powder blends. It was noted that the equilibrium NiSi phase started to form in the early stages of milling and that the amount of the NiSi phase in the milled powder increased with increasing milling time. Even though, under equilibrium conditions, a mixture of both the NiSi and NiSi₂ phases was expected to be present in the Ni-60?at.% Si composition and the stoichiometric NiSi₂ phase in the Ni-66.7?at.% Si composition, the NiSi phase was present in both the compositions investigated. However, while only the NiSi phase was present homogeneously in the Ni-60?at.% Si powder blend, both the NiSi phase and a very small amount of unreacted Si were present in the powder blend of Ni-66.7?at.% Si composition. This unexpected phase constitution in the milled powders was attributed to a partial loss of Si during mechanical alloying of the powder blends, confirmed by energy dispersive X-ray spectrometer analyses, and explained on a thermodynamic basis.     

A new approach to establish both stable and metastable phase equilibria for fcc ordered/disordered phase transition: application to the Al–Ni and Ni–Si systems

Both two-sublattice (2SL) and four-sublattice (4SL) models in the framework of the compound energy formalism can be used to describe the fcc ordered/disordered transitions. When transferring the parameters of 2SL disregarding the metastable ordered states into those of 4SL, inconsistence in either stable or metastable phase diagrams could appear, as detected in both Al–Ni and Ni–Si systems. To avoid such a kind of drawback, this behavior was analyzed and investigated in the Ni–Si and Al–Ni systems with the aid of first–principle calculations. Furthermore, a new approach considering both the stable and metastable fcc ordered phase equilibria deduced from the first–principles calculations was proposed to perform a reliable thermodynamic modeling for the fcc ordered/disordered transition. The Ni–Si system was then thermodynamically assessed using the presently proposed approach. The good agreement between the calculation and experiments demonstrates the reliability of the proposed approach. It is expected that the approach is valid for other systems showing complex ordered/disordered transitions.     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

High–temperature phase chemistry of the system Gd–Pd–O

Anisothermal sectionof the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibrationof samples and phase identification after quenching by optical and scanning electron microscopy, X–ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides Gd₄PdO₇,Gd₂PdO₄ and Gd₂Pd₂O₅ were identified. Liquid alloys, the four inter–metallic compounds and Pd–rich solid solutionwere found to be inequilibrium with Gd₂O₃.

Based on the phase relations, four solid–state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 920 to 1320 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to cross check the data. An advanced version of the solid–state cell incorporating a buffer electrode with yttria–stabilized zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high–temperature thermodynamic measurements. The standard Gibbs energy of formation of the inter–oxide compounds from their component binary oxides can be represented by the following equations:

Gd₄PdO₇(s) : Δ_f(ox)G⁰/J mol^–1 = –25,030 + 0.33T (±140), Gd₂PdO₄(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –25,350 + 0.84T (±135), Gd₂Pd₂O₅(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –48,700 + 0.38T (±270)

Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Gd–Pd–O are developed.     

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.     

Growth direction and Si alloying affecting directionally solidified structures of Al–Cu–Si alloys

The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-%Cu–4 wt-%Si and Al–6 wt-%Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.     

Accurate phase–height mapping algorithm for PMP

In phase measurement profilometry (PMP), the projector can be regarded as another camera according to the reversibility of the light path principle. The relationship of projecting spatial points to image plane of camera and projector is studied, and the phase–height mapping equation without projector distortion is obtained. The equation is then expanded to a polynomial for the convenience of calibration. Furthermore, the relation between the distortion value and the phase is investigated. Finally the phase–height mapping algorithm considering projector distortion and its polynomial expression are acquired. The accuracy of approximation is studied and compared with another two existing algorithms by computer simulation. It is revealed that the absolute error of the new algorithm expressed with quartic polynomial reaches 5.380× 10^?3 mm and its standard deviation reaches 3.354× 10^?4 mm under general lens distortion. The accuracy of the new algorithm is the highest among the three algorithms. In experiment, the standard deviation of the measurement reaches 0.04 mm even though the result is affected by measurement error.     

Structural and chemical stability of Ta–Si–N thin film between Si and Cu

Thermal stability and barrier performance of reactively sputter deposited Ta–Si–N thin films between Si and Cu were investigated. RF powers of Ta and Si targets were fixed and various N₂/Ar flow ratios were adopted to change the amount of nitrogen in Ta–Si–N thin films. The structure of the films are amorphous and the resistivity increases with nitrogen content. After annealing of Si/Ta–Si–N(300 Å)/Cu(1000 Å) structures in Ar–H₂ (10%) ambient, sheet resistance measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) were employed to characterize barrier performance. Cu₃Si and tantalum silicide phase are formed at the same temperature, and the interdiffusion of Si and Cu occurs through the local defect sites. In all characterization techniques, nitrogen in the film appears to play an important role in thermal stability and resistance against Cu diffusion. A 300 Å thick Ta₄₃Si₄N₅₃ barrier shows the excellent barrier property to suppress the formation of Cu₃Si phase up to 800°C.