Materials Science Forum Vols. 783-786

Abstract: Positron Annihilation Lifetime Spectroscopy (PALS) is used to study the nanoporosity and fractional free volume in Ultra High Molecular Weight Polyethylene (UHMWPE) and composites with the addition of Martian Regolith (UHMWPE-MR) as-made and irradiated with ⁵⁶Fe heavy ions at an energy of 600 MeV/u to three different doses (10, 32, 64 Gy). The positron lifetime spectra were obtained using ²²Na positron source and the spectra were analyzed to two lifetime components using POSFIT program. First short lifetime component around 0.28 ns is related to positron annihilation in material including vacancy defects and the second long lived component around 1.7 ns is due to Positronium formation in free volume pores. UHMWPE-MR composites were shown to be less porous with much lower nanopores concentration compared to the UHMWPE polymer. The average size of the nanopores is around 0.5 nm (obtained from a simple model). Larger variations in positron lifetime parameters are observed with increasing irradiation dose for UHMWPE polymer compared to UHMWPE+MR composites. The 3-point bend test results also showed larger variations with increasing irradiation dose for the UHMWPE polymer. The variations in PALS parameters may indicate an increasing competition between two processes at higher irradiation doses: 1) vacancy defects aggregation and 2) escape of vacancy defects as the local temperature increases at higher doses resulting in increased vacancy defects mobility. Present results clearly indicate a qualitative inverse relationship between nanoscale porosity measured by positron life time and mechanical properties of UHMWPE and its composite with MR.

Abstract: In the present work selected results of TEM and XRD investigations of the new AA-6061 matrix composite materials reinforced with halloysite nanoparticles manufactured by mechanical alloying and hot extrusion are present. Halloysite nanotubes, being a clayey mineral of volcanic origin which is characterized by high porosity, large specific surface, high ion exchange and easy chemical and mechanical treatment, can be used as alternative reinforcement of metal matrix composite materials. The composite materials obtained as a result of mechanical alloying and hot extrusion are characterized with the structure of evenly distributed, disperse mineral phase particles in fine-grain matrix of AA-6061 alloy, facilitate the obtainment of higher values of strength properties, compared to the initial alloy. Thanks to HRTEM analysis, the occurrence of grains sized ca. 20÷30nm in the composite materials reinforced with halloysite nanotubes with 15% mass share has been confirmed. The crystallite size and lattice strain of the obtained composites were calculated from X-ray line broadening, applying different profiles of diffraction lines. The achieved values of crystallite size for composites powders after 6 hours of milling were in the range 65-150nm, depending on the calculation method.

Abstract: Aluminum matrix composites (AMCs) are characterized by improved mechanical properties in comparison to their unreinforced matrix alloys. But the knowledge about the fatigue behavior of AMCs in the HCF-and in the VHCF-regime is limited until now. Due to this AMC225xe and AMC xfine225 with an average SiC particle content of 25 vol.-% and particle sizes of 2.5 μm and 0.7 μm, respectively, as well as the base alloy AA2124 were fatigued up to 10¹⁰ cycles using the ultrasonic testing facility of the type "UltraFast-WKK-Kaiserslautern".To describe the fatigue behavior of the specimens several measuring devices were used to monitor and record the central process parameters. A very sensitive value to detect specimen failure at an early stage is the dissipated energy which can be determined as the integral of the generator power depending on the ultrasonic pulse time.In comparison to AA2124 the investigated AMCs have shown a considerably enhanced fatigue performance for stress amplitudes higher than 140 MPa. But below this stress amplitude for the matrix alloy run outs at 10¹⁰ cycles were realized whereas the AMCs failed at lower number of cycles still at lower stress amplitudes. Moreover, while crack initiation of the matrix alloy in all cases started at the surface for the AMCs the crack initiation point changes from surface to subsurface for more than 10⁷ cycles. The subsurface failures of the composites were caused by microstructural inhomogeneities which could be identified with EDX and micro-CT as particle clusters and copper-iron-rich inclusions.

Abstract: Active cooling techniques are often required to achieve high rates of heat dissipation in thermal management applications. Open-cell porous metals are good candidates for use as heat exchangers. This paper studies the fluid transport and thermal properties of porous copper samples with different pore structures manufactured using the LCS method. The results showed that the permeability increases with porosity but decreases with pore size. The thermal conductivity increases with relative density according to the power law. The effects of porosity and pore size on the heat transfer performance of the porous copper samples are significant, due to their effects on the permeability and thermal conductivity. For the porous copper samples with double-layer structures, the permeability follows the rule of mixture and the heat transfer coefficient can be predicted by a recently developed segment model.

Abstract: Threshold pressure is a very important parameter for melt alloy successfully infiltrating into the porous preform. However, the precise measurement for threshold pressure is very difficult for the reason that infiltration process is undertaken very fast under extreme elevated temperature and high pressure without effective measuring devices to monitor it. A totally new measuring device was proposed and fabricated, which can be used to monitor the infiltration process “visually” and measure the threshold pressure directly at the same time. The infiltration speed can be controlled by adjusting the gas flow speed. The infiltration behavior of melt AZ91D alloy in Al2O3sf preform was researched at temperature of 800°C and pressure of 0.6 MPa. The optimized gas velocity was controlled at 25L/min. The degree of vacuum of the infiltration cavity was set 30kPa in experiments. The volume fraction of Al2O3sf was 10%. Under these conditions, the threshold pressure of melt AZ91D alloy into porous Al2O3sf preform was found to be related with vacuum degree in infiltration chamber, and it was about 30 kPa

Abstract: MN and CrMN type nitride precipitates in 12%Cr steels have been investigated using atomic resolution microscopy. The MN type nitrides were observed to transform into CrMN both by composition and crystallography as Cr diffuses from the matrix into the MN precipitates. Thus a change from one precipitate type to another does not necessarily involve nucleation of the new precipitate type followed by dissolution of the old precipitates. By studying the interface between these nitrides and the matrix, it could be observed that the MN and CrMN type precipitates had a few nanometers thick amorphous layer between the crystalline nitride and ferrite matrix. Usually precipitates are described as having (semi) coherent or incoherent interfaces, but in this case it is more energetically favourable to create an amorphous layer instead of the incoherent interface.

Abstract: Novel methods of switching magnetism with electric fields and vice versa, and aiming at magnetoelectric (ME) data processing are reported. First, the patented MERAM^@ uses the electric field control of exchange bias via an epitaxial Cr₂O₃ layer and exchange coupling to a Pt/Co/Pt trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic field in a 3-1 composite of a vertically poled BaTiO₃ thick film embedding CoFe₂O₄ nanopillars produces a regular surface polarization pattern with rectangular symmetry. Its possible use for data processing is discussed.

Abstract: We report on the mechanical response of semiconductor substructures formed by InP membranes bonded to silicon. The bonded surfaces are of the order of ~ 1 cm2 and were bonded using oxide-free direct wafer bonding. Both the plastic response of the InP membrane and the InP/Si interface strength have been investigated using instrumented nanoindentation.

Abstract: The faceting and migration behavior of low angle <100> grain boundaries in high purity aluminum bicrystals was investigated. In-situ technique based on orientation contrast imaging was applied. In contrast to the pure tilt boundaries, which remained straight/flat and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry – the energy of pure tilt low angle <100> boundaries is anisotropic, whereas that of mixed tilt-twist boundaries isotropic with respect to boundary inclination.

Abstract: The effect of the Ti and Zr transition metals on the D03-Fe3Al intermetallic compounds has been investigated by means of ab initio Pseudo Potentials numerical simulations based on Density Functional Theory. Two main issues will be addressed the understanding of the role of these two transition metals in terms of stability of the bulk at the light of their site preference in the D03-Fe3Al structure the behaviour of Ti and Zr transition metals in the sigma 5 (310) [001] grain boundary and their effect on the structural stability of this interface. An important issue when studying these aspects is to take into accounts the effect of temperature. This requires a molecular dynamics treatment of the atoms in the supercell. The technique known as ab initio molecular dynamics (AIMD) solves these problems by combining ‘on the fly’ electronic structure calculations with finite temperature dynamics. Thus, our study was conducted both using the conventional static ab initio calculations (0K) as well as by taking into account the effect of temperature (Ab Initio Molecular Dynamics).