High Pressure Solidification Research Articles

Solidification of Al-xCu alloy under high pressures

In this study, Al-xCu alloy containing x=15, 33, and 40wt.% Cu was solidified under different pressures (atmospheric pressure, 1GPa, 2GPa, and 3GPa). The effects of the solidification pressure and aging treatment on the microstructural evolution and mechanical property of these alloys were investigated. The eutectic Al2Cu phase varied between dendritic to spherical morphology in Al-15Cu alloy with the application of pressure. Both the secondary arm spacing in Al-33Cu alloy and the size of primary Al2Cu in Al-40Cu alloy had decreased under the influence of high pressure during solidification. As a result of solid solution strengthening, the Vickers hardness increases significantly for the Al-xCu alloys with high-pressure solidification. The microstructure variation was further explained based on undercooling theory.

Microstructure and strengthening mechanism of Mg—5.88Zn—0.53Cu—0.16Zr alloy solidified under high pressure

Mg—5.88Zn—0.53Cu—0.16Zr (wt.%) alloy was solidified at 2—6 GPa using high-pressure solidification technology. The microstructure, strengthening mechanism and compressive properties at room temperature were studied using SEM and XRD. The results showed that the microstructure was refined and the secondary dendrite spacing changed from 35 μm at atmospheric pressure to 10 μm at 6 GPa gradually. Also, Mg(Zn,Cu)2 and MgZnCu eutectic phases were distributed in the shape of network, while under high pressures the second phases (Mg(Zn,Cu)2 and Mg7Zn3) were mainly granular or strip-like. The solid solubility of Zn and Cu in the matrix built up over increasing solidification pressure and reached 4.12% and 0.32% respectively at 6 GPa. The hardness value was HV 90 and the maximum compression resistance was 430 MPa. Therefore, the grain refinement strengthening, the second phase strengthening and the solid solution strengthening are the principal strengthening mechanisms.

Room temperature compressive properties and strengthening mechanism of Mg96.17Zn3.15Y0.50Zr0.18 alloy solidified under high pressure

The microstructures of Mg96.17Zn3.15Y0.50Zr0.18 alloys solidified under 2–6 GPa high pressure were investigated by employing SEM (EDS) and TEM. The strengthening mechanism of experimental alloy solidified under high pressure is also discussed by analyzing the compressive properties and compression fracture morphology. The results show that the microstructure of experimental alloy becomes significantly fine-grained with increasing GPa level high pressure during solidification process, and the secondary dendrite arm spacing reduces from 40 μm at atmospheric pressure to 10 μm at 6 GPa pressure. The morphology of the second phases changes from the net structure by the lamellar-type eutectic structure at atmospheric pressure to discontinuous thin rods or particles at 6 GPa pressure. Besides, the solid solubility of Zn in the Mg matrix is improved with the increase of the solidification pressure. Compared with atmospheric-pressure solidification, high-pressure solidification can improve the strength of the experimental alloy. The compressive strength is improved from 263 to 437 MPa at 6 GPa. The fracture mechanism of the experimental alloy changes from cleavage fracture at atmospheric pressure to quasi-cleavage fracture at high pressure. The main mechanism of the strength improvement of the experimental alloy includes the grain refinement strengthening caused by the refinement of the solidification microstructure, the second phase strengthening caused by the improvement of the morphology and distribution of the second phases, and solid solution strengthening caused by the increase of the solid solubility of Zn in the Mg matrix.

Structure of the Al90Y10 Alloy Formed upon Pressure Solidification

The effect of temperature and the melt cooling rate on the structure of an Al90Y10 alloy after high-pressure solidification is comparatively studied by X-ray diffraction analysis, optical microscopy, and electron microscopy. Only the crystalline α-Al and Al3Y phases form in the Al90Y10 alloy under all considered solidification conditions. The effect of the temperature, the cooling rate, and the pressure on the morphology, the size, and the microhardness of the structural alloy constituents, such as primary yttrium aluminide and eutectic crystals, is found.

Microstructure and mechanical properties of high-pressure-assisted solidification of in situ Al–Mg2Si composites

In this study, an Al–Mg2Si composite was solidified under a high pressure of 3 GPa and the effects on microstructural evolution, mechanical properties including hardness, friction, and wear behavior, and compression properties were investigated. The results indicate that the composite solidified under high pressure exhibited significantly enhanced hardness, compressive properties, wear, and friction resistance compared to the as-cast and solid solution–treated (ST) composites. The hardness of the composites under high-pressure solidification was 2.25 times and 1.37 times those of the as-cast and ST composites, respectively. The compressive yield strength, elastic modulus, and compressive strain were 121.5 MPa, 29.6 GPa, and 22.3% for the as-cast composite, 146.1 MPa, 44.5 GPa, and 35.8% for the ST composite, and 151.3 MPa, 49.3 GPa, and > 75% for the high-pressure-solidified composite, respectively. The friction coefficient of the composite under high-pressure solidification was only 44% of that of the as-cast composite and 67% of that of the ST composite. The wear modes were adhesive wear and abrasive wear for the as-cast composite, but only abrasive wear for the composite solidified under high pressure; the fracture mode transformed from cleavage fractures in the as-cast composite to quasi-cleavage fractures in the composite solidified under high pressure.

Hot compressive deformation behavior and electron backscattering diffraction analysis of Mg95.50Zn3.71Y0.79 fine-grained alloy solidified under high pressure

An Mg95.50Zn3.71Y0.79 fine-grained solidified alloy with a grain size of 16 μm was prepared by high-pressure solidification. The microstructure characteristics and hot compressive deformation behavior of the alloy solidified under high pressure were compared with the atmospheric-pressure solidified alloy by carrying out the unilateral compression tests under a strain rate in the range of 0.001–1.0 s−1 and at a deformation temperature in the range of 523–573 K. The true stress-true strain curve of the high-pressure solidified alloy shows the typical dynamic recrystallization rheological curve. EBSD results show that when the deformation was carried out at 573 K, nearly 90% dynamic recrystallization occurred in the high-pressure solidified alloy, and the newly formed grains were distortionless and had low dislocation density. The high-pressure solidified alloy showed a double-peak basal texture at a strain rate of 1.0 s−1. The two peak points showed a maximum pole density of 9.88 and 7.91, less than that in atmospheric-pressure alloy. When the deformation was carried out at the following conditions: deformation temperature = 573 K, strain rate = 0.001, and true strain = 0.9, the average Schmid factor (SF) for basal slip of the grains in the high-pressure solidified alloy was 0.419, and SF value for basal slip in 91% grains was greater than 0.3.

Dynamic recrystallization kinetic of fine grained Mg-Zn-Y-Zr alloy solidified under high pressure

Fine grained Mg96.17Zn3.15Y0.79Zr0.18 alloy with an average grain size of 20 μm was prepared by high pressure solidification. The dynamic recrystallization (DRX) behavior of the fine grained Mg alloy solidified under the pressure of 4 GPa was studied via isothermal compression experiments. The tests were performed under the strain rate of 0.001–1.0 s−1 and at a deformation temperature of 523–623 K on a Gleeble-3500D thermal-mechanical simulation machine. The DRX kinetic of the fine grained Mg alloy solidified under high pressure was established, and the microstructures of the alloy under different hot compression conditions were analyzed by electron back-scattering diffraction (EBSD). According to the experimental results, the DRX kinetic model of the fine grain Mg alloy solidified under high pressure was XDRX=1-exp[-0.75445(ɛ-ɛcɛ*)1.066208]. The Avrami exponents of n and k were 1.066208 and 0.75445 respectively, higher than those in the conventional casting alloy. The DRX volume fraction of the fine grain Mg alloy solidified under the pressure had a tendency to increase obviously with the strain rate decreasing and the deformation temperature increasing, which is different from the one in the conventional casting alloy. When compressed at 523 K, the DRX volume fraction of the fine grained Mg alloy solidified under high pressure was 85% under the strain rate of 1.0 s−1 and could be up to 95% under the strain rate of 0.001 s−1. The DRX volume fraction of the conventional casting alloy was only 67% although under the condition of 623–0.001 s−1. It was shown that the fine grained Mg alloy solidified under high pressure had a strong DRX capacity.

Crystal structure and mechanical properties of a new ternary phase in Mg-Zn-Y alloy solidified under high pressure

A new ternary phase, Mg64.093±0.004Zn15.355±0.002Y20.552±0.005 (at. %) was found in a Mg-Zn-Y alloy prepared by high pressure solidification. A full set of crystal structure parameters including the atomic coordinates of this new phase were determined using X-ray diffraction with Rietveld refinement and electron probe micro-analysis. Furthermore, the mechanical properties of this phase, including the Young's modulus and hardness were reported by nanoindentation test. Theoretical elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio of this phase were calculated by first-principles. Results indicate that this phase is a ductile materials phase.

Mechanism of formation of fibrous eutectic Si and thermal conductivity of SiC p /Al-20Si composites solidified under high pressure

Abstract As an advanced electronic packaging material, SiC p /Al-20Si composites have been fabricated by high pressure solidification. The mechanism of formation of fibrous eutectic Si and the thermal conductivity (TC) behavior of the composites have been systematically investigated. High pressure decreases diffusion of Si phase significantly; moreover, SiC addition hinders the diffusion and provides nucleation sites for the Si phase. Both high pressure and SiC contribute to the formation of fibrous eutectic Si. The SiC p /Al-20Si composite solidified at 3 GPa exhibits lowest TC due to the formation of a supersaturated Al(Si) solid solution, whereas it is highest after ageing treatment. The optimal TC of the 45% SiC p /Al-20Si composite is 166.8 W/m·K at room temperature. The TC of the composites can be well predicted by the Hasselman-Johnson (H-J) theoretical model.

Effect of high pressure on microstructure of cast Mg-8Zn-0.5Zr-0.5Gd alloy

Mg-8Zn-0.5Zr-0.5Gd alloy was prepared by high pressure solidification. Effect of high pressure on microstructure, micro-hardness and corrosion behavior in Hank's solution of the Mg-8Zn-0.5Zr-0.5Gd alloy were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffractometer (XRD). The results showed that, compared with the conventional solidification, high pressure solidification obviously refined the grain size of Mg-8Zn-0.5Zr-0.5Gd alloy. The grain size was refined from 200-300 μm to 100-200 μm and the secondary dendrite arm spacing reduced from 30-50 μm to 10-30 μm. Moreover, the solubility of Zn in the alloy increased and the amount and size of Mg-Zn-Gd phases significantly decreased. The micro-hardness of the alloy solidified under high pressure was improved significantly from 56.17 HV to 63.14 HV. The polarization resistance (R p) of the alloy had a substantial increase in simulated body fluid, thus the corrosion rate was significantly reduced from 4.0 to 2.7 mm·year-1.

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.

Constitutive model and deformation microstructure of fine-grain Mg-Zn-Y alloy solidified under high pressure

Fine-grain Mg95.50Zn3.71Y0.79 alloy was prepared by high pressure solidification. By comparison with the conventional casting alloy, the true stress-strain curve characteristic and deformation microstructure of Mg95.50Zn3.71Y0.79 alloy solidified under high pressure were studied via unilateralism compress tests under the strain rate of 0.001–1 s−1 and deformation temperature of 523–623 K. Constitutive equations were constructed. According to the experimental results, compared to the conventional casting alloy, the true stress-strain curve of the fine-grain alloy solidified under high pressure not only had the high strain hardening characteristic but the dynamic recrystallization softening after the peak stress was more than the working hardening and would soon reach a stable flow stress – strain state. The deformation activation energy of the alloy solidified under high pressure was 151 kJ/mol, around 49 kJ/mol lower than that of the conventional casting alloy. The fine-grain Mg-Zn-Y alloy solidified under high pressure could obtain 95 percent of dynamic recrystallization grain at 573 K during hot deformation process.

Effect of high pressure solidification on tensile properties and strengthening mechanisms of Al-20Si

Al-20Si alloy specimens were manufactured using high pressure solidification. The microstructure, the mechanical properties as well as the strengthening mechanisms of the alloy were investigated. The results show that the primary Si phase disappears, the supersaturated α-Al prevails, and the eutectic Si phase is well refined after high pressure solidification. The ultimate tensile strength (UTS), the yield strength (YS) and the elongation increase gradually with increasing solidification pressure and the values of the Al-20Si alloy solidified at 3 GPa are 365 MPa, 237 MPa and 2.98%, respectively, corresponding to an increase of 83%, 57%, and 412% compared with the alloy solidified at atmospheric pressure. The alloy solidified at atmospheric pressure shows typical cleavage fracture, whereas the alloy solidified at 3 GPa exhibits quasi-cleavage fracture. Solid solution strengthening is the dominating strengthening mechanism after high pressure solidification.

Modeling of yield strength in binary hypoeutectic alloy under high pressure solidification

A new mathematical model was developed to predict the yield strength under high pressure solidification. This model can be used in the binary hypoeutectic alloy system which contains solid solution and intermetallic compound. Following this model, the total change of yield strength under high pressure is affected by solid solution, eutectic spacing and grain size. This effect can be quantitatively measured by the change of a pressure-solid solubility function f(P). This model reveals that along with the variation of f(P), change in the solubility and eutectic spacing lead to opposite influences on yield stress. Application of the model to the binary Mg-20.3 wt%Al solidified under different pressure was conducted. It was found that this model can perfectly explain the change in the yield strength of Mg-20.3 wt%Al alloy solidified under high pressure. This model can provide theoretical guidelines for choosing reasonable solidification pressure to optimize microstructure and properties of binary hypoeutectic alloy in high pressure solidification.

Influence of high pressure during solidification on the microstructure and strength of Mg-Zn-Y alloys

The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction (XRD) and Vickers-hardness testing. Under atmospheric-pressure solidification, Mg-6Zn-1Y consisted of α-Mg, Mg7Zn3 and Mg3YZn6; whilst Mg-6Zn-3Y consisted of α-Mg, Mg3Y2Zn3 and Mg3YZn6. Under 6 GPa high-pressure solidification, both alloy consisted of α-Mg, MgZn and Mg12YZn. The shape of the main second phase changed from a lamellar structure formed for atmospheric-pressure solidification to small particles formed for solidification at 6 GPa pressure. The dendrite microstructure was refined and was more regular, and the length of the primary dendrite arm increased under 6 GPa high-pressure solidification, which was attributed to increasing thermal undercooling, compositional undercooling and kinetics undercooling. After solidification at 6 GPa pressure, the solid solubility of Y in the second phase and the Vickers-hardness increased from 15 wt.% and 69 MPa for Mg-6Zn-1Y to 49 wt.% and 97 MPa; and from 19 wt.% and 71 MPa for Mg-6Zn-3Y alloy to 20 wt.% and 92 MPa, respectively.

Modeling of eutectic spacing in binary alloy under high pressure solidification

A new mathematical model was developed to predict the average eutectic spacing under high pressure solidification. The model can be used in the binary alloy system which contains solid solution and intermetallic compound. Following this model, the change of eutectic spacing under high pressure can be predicted if the volumetric conditions of the phases are known. Application of the model to the binary Mg-20.3wt.%Al solidified under different pressure was conducted. It was found that the results predicted by this model show an excellent agreement with the eutectic spacing of Mg-20.3wt.%Al alloy solidified under high pressure. And during experiment, an ultimate pressure was found, the microstructure tends to grow as divorced eutectic when the solidified pressure is higher than this ultimate pressure, the formula is not applicable.

Could RME biodiesel be potentially harmful to modern engine? solidification process in RME

A sample of Fatty Acid Methyl Ester (FAME) has been examined in high pressure conditions. Solidification process in the FAME sample has been observed. The aim of the measurements was to pressurize the sample with different rates of compression. During the pressurization the changes of the transmitted and scattered light intense induced by pressure were recorded. The measurements done at various speed of compression as well as decompression have shown creation of scattering centers, what is typical for high pressure solidification of fats and fatty acids. The character of those changes was expected for the pressure induced phase transitions, observed by authors for various lipids. The experiments were conducted at room temperature. Phase transition was appearing at the pressure above 230MPa. The phase transition pressure was changing from 230MPa to 300MPa, depending on the compression rate. The presented experimental methods have wide applicability to determine rheological properties of those kind of liquids.

The thermal expansion behaviour of SiCp/Al–20Si composites solidified under high pressures

Al–20Si matrix composites reinforced with SiC particles have been fabricated by high pressure solidification and the effects of solidification pressure and SiC volume fraction on the thermal expansion behaviour have been systematically investigated. A peak of the coefficient of thermal expansion (CTE) is observed during the first heating process due to the precipitation of Si from the supersaturated Al(Si) solid solution, whereas the CTE increases monotonically during the second heating process. The Al–20Si alloy solidified at 3GPa shows the lowest CTE value during the second heating as a result of the modification of the eutectic silicon and of the small size and high volume fraction of the precipitated Si particles. The CTE of the composite decreases with increasing the SiC content in both heating processes. The theoretical model agrees well with the experimental results and our study indicates that high pressure solidification is a promising route for the development of SiCp/Al–Si composites for packaging applications.

Structure of GP zones in Al–Si matrix composites solidified under high pressure

The structure of the Guinier–Preston (GP) zones precipitated in the Al–Si matrix composites after natural aging is investigated. The results show that, after natural aging, spherical GP zones with a diamond crystal structure form in the supersaturated α-Al matrix produced by high pressure solidification. The lattice parameter of the GP zones is 1.62nm and their structure consists of Si3 atomic clusters. The GP zones disappear and Si crystals are formed in the α-Al matrix after heating to 673K. The precipitation sequences in the supersaturated α-Al matrix can be summarized as: α-Al (SSSS)→GP zones→equilibrium Si phase.

Research on High Pressure Solidification Microstructure and the Microhardness of Al-Mg-Zn Alloy

The solidification microstructure and the microhardness of Al-11Mg-4.5Zn alloy under high pressure were researched by optical microscope, scanning electron microscope (SEM), X-ray diffraction(XRD) and microhardness tester. The results revealed that high pressure solidification refined the dendrite structure, and made the primary dendrite arms grow long. The alloy all consisted of α-Al phase and Mg32(Al, Zn)49 phase whether under normal pressure or high pressure solidification conditions. However, the morphology of the Mg32(Al, Zn)49 phases changed from fork-like of normal pressure to the graininess of high pressure, the size of them reduced, and that of the amount decreased. Furthermore, the solid solubility of Mg and Zn in α-Al phase increased comparing with those of normal pressure, and they were the largest at 4GPa high pressure, reaching 11.37% and 4.15%, respectively. The microhardness of Al-Mg-Zn alloy had an increment with the pressure increasing.