Materials Science Forum Vols. 706-709

Abstract: Silica-nylon6 composites were fabricated using two types of silica nanoparticle fillers with different surface modifications. Type A particles (treated with hexamethyldisilazane) were uniformly dispersed but only displayed weak long-range interaction with the nylon6 matrix; in contrast, type B particles (modified with 3-aminopropyltriethoxysilane) formed covalent bonds with the nylon6 chains but their dispersion is not good. The silica-nylon6 composites synthesized were subjected to quasi-static and dynamic tension to study the effects of strain rate and nanoparticle fraction. Results show that compared to quasi-static loading, both pure nylon6 and the composites exhibit a higher strength but lower ductility under dynamic loading. With respect to the influence of the nanoparticles, both particle types cause an increase in the elastic modulus and tensile strength. The effect of the two particles on ductility differs – particle A reduces ductility, while particle B decreases ductility under quasi-static loading but enhances it noticeably for dynamic loading. Particle B enhances the mechanical properties more significantly, especially in terms of ductility. These results suggest that ensuring strong particle-matrix bonding is more crucial than good particle dispersion.

Abstract: Hot dynamic densification method was developed by combining self-propagating high temperature synthesis (SHS) with explosively shock powder compaction technique. This method is extremely short time processing. The main purpose in this study is to perform from synthesis to densification of TiB₂-TiN system high temperature ceramic composites and TiB₂-TiNi-Cu system functionally graded materials (FGMs) in one step. In TiN-TiB₂ ceramic composites, they showed up to 95% of relative density. It was appeared by TEM observations that both the two phases joined tightly each other. The FGMs also were produced by the same technique. They indicated no interlayer exfoliation and no macro cracks after thermal shock tests from 973 K to room temperature. It was shown that thermoelastic property of intermetallic TiNi phase as intermediate layer between ceramics and metal layers operated effectively.

Abstract: The impact compressive failure behavior of a unidirectional T700/2521 carbon/epoxy laminated composite in three principal material directions or fiber (1-), in-plane transverse (2-) and through-thickness (3-) directions is investigated on the conventional split Hopkinson pressure bar (SHPB). Cubic and rectangular block specimens with identical square cross section are machined from an about 10 mm thick composite laminate. The uniaxial compressive stress-strain curves up to failure at quasi-static and intermediate strain rates are measured on an Instron testing machine. It is shown that the ultimate compressive strength and strain exhibit no strain-rate effect in the 1-direction, but a slight strain-rate effect in the 2-and 3-direction over a range of strain rates from10^-3 to 10³/s.

Abstract: The dynamic crushing behavior of cellular metals is closely related to their microstructure. Two types of random defects by randomly thickening/removing cell walls are investigated in this paper. Their influences on the deformation modes and plateau stresses of honeycombs are studied by finite element simulation using ABAQUS/Explicit code. Three deformation modes, i.e. the Homogeneous Mode, the Transitional Mode and the Shock Mode, are used to distinguish the deformation patterns of honeycombs under different impact velocities. The critical impact velocity for mode transition between the Homogeneous and Transitional modes is quantitatively determined by evaluating a stress uniformity index, defined as the ratio between the plateau stresses on the support and impact surfaces. It is found that the critical impact velocity decreases with increasing thickening ratio but increases with increasing removing ratio. The plateau stress on the impact surface heavily depends on the impact velocity due to the inertia effect. The random defects lead to a weakening effect on the plateau stress. For the honeycombs with randomly removing cell walls, the weakening effect is especially obvious at a moderate impact velocity. For the honeycombs with randomly thickening cell walls, the weakening effect is particularly severe at a low impact velocity, but this effect almost disappears when the impact velocity is high enough.

Abstract: Sintered lamellar soft magnetic composite (SL-SMC) is a promising material for power frequency applications and for all types of electric motors, including induction motors. Alumina coated Fe-3%Si particles cut from steel sheets offer both the advantages of standard laminated steel products, and of soft magnetic composites (SMC). As SMC’s, this material can be shaped using standard near-net shape powder metallurgy (P/M) techniques, which represents a big advantage over coated laminated steel assemblies. Indeed, the P/M process, as compared to laminated steel assemblies, gives access to better design and improved motor assembling techniques, at lower production costs, by limiting machining and stamping. The unique structure of the SL-SMC offers better DC magnetic properties than standard SMC’s, allowing them to cover a larger range of applications. However, there still remains work to be done in developing this new technology to its full potential, using low cost industrial techniques. This paper is divided into two main parts. The first is dedicated towards assessing the possibility of producing SL-SMC parts using an automatic industrial compaction press. The impact of two different lubrication systems, admixed solid lubricants and die wall lubrication, on the density and green strength of the motor parts, will be determined. Also, the process capability will be evaluated on the basis of “part to part” stability. The second section of this study will determine the impact of different processing parameters on the mechanical and magnetic properties of the Fe-3%Si alloy. Results show that by carefully controlling heat treatment, very high maximum induction, low magnetic loss and good mechanical property, can all be reached in fabricated parts.

Abstract: Various metallurgical refining processes of SOG (Solar Grade)-Si are being developed aiming its cost reduction for the PV (Photovoltaic) market. High purity silicon can also be obtained from Al-Si alloys using solidification theory. Since the solubility of impurity elements in silicon decreases rapidly with temperature decrease much above the Al-Si eutectic temperature, the removal of impurities from silicon at low temperature is expected to be effective. Most of impurity elements with small segregation coefficient can be removed during solidification. Key technology in this process is to separate the primary solid silicon from the liquid eutectic matrix during solidification. Some methods by an electromagnetic force or by combining flocculation and filtration techniques have been proposed to separate the solid silicon. The present study describes a new way to extract the primary silicon crystal from Al-Si alloys using centrifugal force during solidification for a silicon solar feedstock. Primary silicon was separated in the shape of foam during solidification, and pure Si flakes after acid leaching could be obtained.

Abstract: In development of 12%Cr high temperature steels used for fossil fired power plants, the precipitation of large Z-phase particles, CrMN, has been identified as a major problem since they replace small and finely distributed MN particles. This causes a premature breakdown in the long-term creep strength of the steel. The Cr content promotes Z-phase precipitation, making MN strengthening of these materials unfeasible, since 12%Cr is necessary for oxidation resistance. The authors have suggested an acceleration of Z-phase precipitation to obtain a fine and stable distribution of CrMN instead of MN, thus preserving long-term creep strength. This can be done by alloying with Ta instead of Nb and V. Recent investigations have indicated a direct transformation of MN into CrMN to take place, not the traditional nucleation/dissolution process. In this paper atomic resolution microscopy shows how Cr atoms diffuse from the steel matrix into TaN precipitates and physically transform them into CrTaN. The crystal structure of the precipitates changes from that of a typical MN NaCl type crystal structure to a Z-phase crystal structure with alternating double layers of Cr and TaN. Since there is a large contrast between heavy Ta atoms and light Cr atoms, the ordering of the Cr layers inside the TaN particles can clearly be observed.

Abstract: This paper presents technology of multicrystalline silicon solar cells with laser texturisation step. The texturing of polycrystalline silicon surface using Nd:YAG laser makes it possible to increase absorption of the incident solar radiation. Moreover, the additional technological operation consisting in etching in 20 % KOH solution at temperature of 80°C was introduced into technology of the photovoltaic cells manufactured from laser textured wafers allows to remove laser induced defects but cause the texture to flatten out reducing it optical effectiveness. This paper demonstrates, that laser processing is very promising technique for texturing multicrystaline silicon independent on grains crystallographic orientation compared to conventional texturing methods in technology of solar cells.

Abstract: Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors. Two materials with different density and purity have been examined by optical microscopy, X-ray diffraction (XRD), instrumented indentation tests (FIMEC) and mechanical spectroscopy. For both the materials yield stress and elastic modulus strictly depend on the residual porosity. Moreover, the material with higher porosity (≈ 9%) is not stable and remarkable modulus variations are observed during heating. The IF spectrum exhibits a relaxation Q^-1 peak superimposed to an exponentially increasing background. The peak is a single Debye peak with activation energy H = 74.86 kJ mol^-1 and pre-exponential factor τ₀ = 1.76 x 10^-9 s that has been ascribed to dislocation interaction with intrinsic point defects (autointerstitial and substitutional).

Abstract: Effect of tempering temperature ranged from 400 to 720°C on mechanical properties and microstructure of a P92-type creep resistant steel was investigated. The hardness value of 400 HB, which was obtained after solution treatment, increased to 430 HB with increasing the tempering temperature to 525°С. Further increase in the tempering temperature resulted in gradual decrease in hardness, which approached a level of about 220 HB after tempering at 720°С. The equiaxed particles of MX-type carbonitrides with a size of about 30 nm were precipitated randomly after tempering under all conditions. At temperatures below 525°C, the tempered martensite lath structure (TMLS) was characterized by a random distribution of fine M₃C-type carbides and MX-type carbonitrides. The precipitation of M₂₃C₆ was observed after tempering at T ≥ 525°C. At 525°C, the M₂₃C₆ carbides appeared as thin films on high-angle boundaries (HAB), while M₂₃C₆ particles having almost equiaxed shape and located on various boundaries including low-angle lath boundaries precipitate at higher temperatures.