Materials Science Forum Vols. 704-705

Abstract: Al Foam Sandwiches (AFS) has been created by a process of hot pressing Al-TiH2 precursor (core) made by cold pressing together with steel sheet, and then foaming. The microstructure and composition of Al-Fe interface has been analyzed by SEM and Element Linescanning Electron Microscopy (ELEM). The experiment and analysis show that during hot pressing process, firm mechanical bonding is formed between steel sheet and core, during foaming, the intermetallic compound FeAl3 and eutectic structure (FeAl3+Al) were formed by diffuse of Fe and Al elements each other at the interface, which gave a rise to a good metallurgical bonding interface between sheet and core.

Abstract: The vacuum induction melting and control solidification is a new developed process for the manufacture of polycrystalline silicon with the advantage of quick heating rate compared to the traditional resistance heating method. The graphite susceptor of the induction furnace plays a key role in controlling the temperature of the melt of polycrystal silicon for solar cells. This paper investigates how different coatings painted on the susceptor would influence the heating efficiency and the temperature distribution of the silicon melt. A graphite susceptor is usually coated with a thin film of insulating material to reduce heat losses and to prevent vapors such as SiO to contaminate the susceptor. Numerical simulations show that the coating material at the outer surface of the susceptor should be the one with low thermal conductivity to prevent heat loss. On the other hand, the coating on the inner surface should be the material with high thermal conductivity to allow easy heat transfer.

Abstract: The magnetic and electrical properties of the La_0.67Ca_0.25Sr_0.08MnO₃/xAg composite system are systematically investigated as a function of Ag-added content. With increasing the Ag dopant amount, the magnetization decreases a little while Curie temperature (T_C) is almost independent of Ag content around 312 K. The resistivity reduces rapidly with Ag addition ( x < 0.25 ) due to the decomposed metal Ag, and then it increases slightly which is probably induced by the less Ag content related to the volatilization of Ag during calcinations. At low temperature, the ρ – T curves fit well by the expression of ρ =ρ₀ +ρ₂T² +ρ_4.5T^4.5 while all data for the above T_C can be fitted by using the adiabatic small-polaron-hopping model ρ=ρ₀T exp (E/k_BT ). The enhancement of MR effect ( 41% ) at room temperature is mainly related to the coexistence of intrinsic MR properties and the spin dependent scattering of conduction electrons at the interfaces. These results indicate that combining the doping effect with the composite method is an effective selection in enhancement of MR at room temperature, which is very meaningful for the application research.

Abstract: The indentation response of the closed-cell Al foam under the flat-end cylindrical indenter is examined by use the finite element method. The MSC/Marc finite element package is used to model the indentation response of the foam panels, and the Al foam was defined in MSC/Marc as the homogeneous, elasto-plastic material. The simulation result reveals that the deformation was found to be almost totally restricted to a spherical cap-shape compacted zone under the indenter, and the shape of the deformation zone is similar to the observed phenomenon from the indentation test.

Abstract: Using ring compression tests, the interfacial friction and flow stress of 3Y-TZP/Al₂O₃ composite at elevated temperatures were investigated. Theoretical calibration curves of the friction factor and the relative average pressure curves for the ring compression tests of 6:3:2 standard rings were drawn based on a velocity field capable of describing the bulge phenomena. The lubricant was the boron nitride (hexagonal). The tests were adopted at temperature range of 1400°C-1600°C. Results indicate that the interfacial friction factor has the value in the range of 0.34-0.49, so that boron nitride lubricant can be used effectively in present temperatures. As two extremely important parameters, the temperature and strain rate have no significant effect on the fraction factor. It is proved reliable that the ring-compression test at 1400°C and even higher is used to evaluate the performance of boron nitride lubricant.

Abstract: A numerical test code is used to study the matrix-inclusion interfacial debonding for particulate reinforced composites. In our numerical model, It is assumed to be a three-phase composite composed of matrix, particulate and the interfaces between them. The finite element program is employed as the basic stress analysis tool when the elastic damage mechanics is used to describe the constitutive law of meso-level element and the maximum tensile strain criterion and Mohr-Coulomb criterion are utilized as damage threshold. A single inclusion of gradually interfacial debonding and a complex structure with 20 inclusions of the interfacial damage were studied under plane stress conditions. Results of stress distribution and interface debonding type obtained by numerical method agree well with the MARK and ABAQUS. The influence of heterogeneity of the matrix materials on the resulting process and the stress distribution of the failure process are also studied in the paper. It is found that the numerical test code can help to understand the failure mechanism of the model and it is an effective way to investigate the interfacial damage of composite materials. Keywords: Numerical test, interface, particulate reinforced composite, crack

Abstract: Concrete is a heterogeneous composite material. The heterogeneity consists of the distribution and shape of aggregate, interfacial transition zone (ITZ) and the inhomogeneity of each component materials. The key in numerical models for simulating the fracture behaviors is how to describe the heterogeneity actually. In this paper, at meso-scale level general-purposed digital image processing technologies are utilized to characterize the heterogeneity resulting from the shape and distribution of aggregates and ITZ, and at micro-scale level, a statistical method (e.g. Weibull distribution) is used to describe the heterogeneity of each phase. And then a multi-scale numerical model based on digital image is proposed to simulate fracturing process of concrete under loading condition. The proposed model can take the actual distribution and shape of aggregate into account. The fracturing process of concrete in uniaxial compressive tests is simulated by using the model. The results show that the shape of aggregates plays an important role in stress distributions to influence the damage evolution during loading. The proposed model is capable of capturing the complete failure process of concrete materials that includes the initiation, propagation and coalescence of microcracks as well as cracking pattern associated with different loading stages, which is a new tool to study the fracturing behaviors of concrete in more detail. Key words: digital image; heterogeneity characterization; fracturing process; concrete

Abstract: nanoscale precipitates in the Al-Zn-In-Mg-Ti-Mn alloy were investigated by regular and high resolution transmission electron microscopy (TEM and HRTEM) and by selective area electron diffraction (SAED). The cast alloy contains the hexagonal structure MgZn₂ and the orthorhombic structure Al₆Mn precipitates, but after aged the precipitates of the cubic structure Mg₂Zn₁₁ were found. The Mg₂Zn₁₁ precipitates can act as activation centre, make the alloy pitting. But the activation of Al₆Mn precipitates is very little.

Abstract: first of all, this paper intensive studies the hydraulic fracturing mechanism of heterogeneous material resultant from the seepage – damage coupling, based on which, the coupled seepage and damage effects during hydraulic fracturing is investigated, and the difficulties of hydraulic fracturing study are how to determine failure modes and breakdown pressure. By using numerically testing method, the mechanical mechanism of hydraulic fracturing under asymmetric distribution of pore pressure with three holes, including fracture patterns, stress flied distribution, initial fracturing pressure and breakdown pressure, was studied by using the FSD model. According to the results of this research, crack extension direction of hydraulic fracturing is influenced not only by the local pore pressure around crack tip, but also by the gradient distribution of macro pore water pressure. The fracturing direction always propagates towards the regions of higher local pore pressure and breakdown pressure decreases with the increasing of local pore pressure. These results are well aggress with the results in the experiments. This research has important theoretical and engineering value. In engineering practice, we can use the porous asymmetric hydraulic fracturing technology to control the direction of crack propagation. Keywords: hydraulic fracturing; numerical simulation; heterogeneous material; asymmetrical

Abstract: The dead or scrap WC_P/Fe-C composites parts were remelted via medium frequency induction furnace. Two recycled composites rings were made of the remelted mixture by centrifugal casting method at 720rpm and 920rpm, respectively. The effect of rotating speed of centrifugal machine on microstructures and properties of the recycled composites rings were investigated by mechanical property tester, SEM, EDS, and XRD. The results show that the recycled rings are a composite structure consisting of outer recycled composites region reinforced with undissoved WC_P and core Fe-C alloy region unreinforced. The undissoved WC_P-distribution in the recycled composites region is even. At the rotating speed of 720 rpm, the particle volume fraction in the recycled composites region reaches 54 vol.%, the hardness and impact toughness attain to HRC55.8 and 3.5J/cm², respectively. As the rotating speed is raised to 920 rpm, the particle volume fraction rises to about 70 vol.%, the hardness increases to HRC63.3 and yet the impact reduces to 2.8 J/cm². The microstructure in the inner Fe-C alloy region consists of bainitic, precipitated carbides with short rod like shape and graphite phase. Finally, the high-speed sliding wear behaviour of the recycled composites rings was investigated in the paper. Keywords: The dead or scrap composites parts; Recycled composites; rotating speed of centrifugal machine; Microstructures and properties.