Otto Lohne

Microstructure of a pressure die cast magnesium—4wt.% aluminium alloy modified with rare earth additions

Abstract Addition of cerium-rich mixtures of rare earth (RE) elements to aluminium-containing magnesium pressure die cast alloys is known to improve the creep properties at elevated temperatures. In the present investigation, a detailed description of the microstructure of a magnesium-4 wt.% aluminium alloy containing 1.4 wt.% of a cerium-rich mixture of RE elements is presented. Particle types occurring and their distribution in the microstructure, as well as the distribution of elements in solid solution, are described. Heat treatment is carried out to verify if solid state precipitation occurs. Some qualitative arguments for the beneficial effect of RE elements on the creep properties are presented.

Spatially resolved modeling of the combined effect of dislocations and grain boundaries on minority carrier lifetime in multicrystalline silicon

A model for the combined effect of dislocations and grain boundaries on minority carrier lifetime has been developed. Lifetime varies with dislocation density, grain boundary misorientation, and the coincidence site lattice (CSL) nature of the boundaries. Minority carrier lifetime was measured with high spatial resolution (50 μm) using the carrier density imaging (CDI) technique on a silicon nitride passivated multicrystalline sample. Dislocation density was measured on the same sample by image recognition of optical microscope pictures of a Secco etched surface. Grain boundaries were then mapped and characterized by electron backscatter diffraction (EBSD). Lifetime was simulated based on the dislocation and grain boundary measurements. Parameters were chosen to match closely the simulated and measured maps. Very good two-dimensional (2D) correlation was obtained by assigning roughly equal importance to recombination at dislocations and grain boundaries. The value for the capture cross section, which give...

Grain size distribution in a complex AM60 magnesium alloy die casting

Presolidified equiaxed dendritic crystals are observed in magnesium cold chamber high pressure die castings. Depending on the rate at which new crystals are formed and to what extent they survive in the shot sleeve, a mixture of liquid and crystals is injected into the die cavity resulting in floating crystals in the casting. Box shaped die castings of the AM60 magnesium alloy have been made with a cold chamber high pressure die casting machine. The resulting microstructure is generally observed to consist of (a) a fine grained structure or (b) a mixture of fine grains and coarse grains which is either centred or dispersed in the through thickness cross-section. The prevalence of structures is observed to vary with position in the casting. Close to the gate a coarse grained microstructure dominates, while fine grains dominate further from the gate. The volume fraction of floating crystals in the casting is shown to depend on the initial superheat of the melt. IJCMR/492

Effects of grain refiner additions on the grain structures in HPDC A356 castings

Abstract In the cold chamber high pressure die casting process (HPDC) solidification begins when the metal is poured into the shot sleeve and impinges on the relatively cold shot sleeve wall and plunger. Therefore, a mixture of liquid and externally solidified crystals (ESCs) is injected into the die cavity. The mechanisms that control the formation of ESCs are not fully understood. In the work presented here, the microstructures of thin walled A356 aluminium alloy die castings have been investigated. The castings were produced by varying the melt superheat and constitutional conditions. It was found that the area fraction of ESCs (f sESC) increases when decreasing the melt superheat; a low superheat generates coarser, more globular ESCs, whilst a larger superheat results in branched, dendritic crystals; additions of Ti in solution increase the f sESC and additions of Al–5Ti–1B grain refiner increase the number of globular, coarse ESCs and generate a finer grain size in the casting. The results are discussed with special emphasis on the shot sleeve solidification conditions and the mechanisms that control the formation of the ESCs.

The microstructure of dislocation clusters in industrial directionally solidified multicrystalline silicon

The microstructure of commonly occurring dislocation patterns in industrial directionally solidified multicrystalline silicon has been systematically studied by light microscopy, electron backscatter diffraction, and transmission electron microscopy. The work has been focused on dislocation clusters on wafers near the top of cast blocks. In near {111} grain surface, dislocation arrays parallel to {110} plane traces are lying in parallel rows of {111} planes inclined to the surface, in mainly 〈112〉30∘ orientation. The dislocation configuration suggests that the microstructure may result from a recovery process. The dislocations formed during crystal growth and cooling have undergone transformations at high temperature in order to achieve low energy configurations for minimization of dislocation and crystal energy.

High Temperature Annealing of Dislocations in Multicrystalline Silicon for Solar Cells

Dislocation clusters have been shown to constitute the main efficiency reducing factor for multicrystalline silicon solar cells (Sopori et al. 2005). Multicrystalline silicon is made under less ideal conditions compared to monocrystalline silicon, in the sense that thermal fields and the lack of seeding create material with increased density of crystal defects, but also since the direct contact between crystal/melt and crucible/coating provides a rapid channel for impurity contamination. These two factors make multicrystalline silicon inferior compared to monocrystalline silicon in terms of solar cell efficiency (Green et al. 2009). It has been shown by Kveder et al. (Kveder et al. 2001) that the interaction between dislocation levels and impurity levels in the band gap may provide very efficient recombination channels, thus enhancing the efficiency reduction both of the dislocations and the impurities. Furthermore it has been shown that gettering of impurities is far less efficient in regions of high dislocation density (Bentzen et al. 2006).

Creep properties at 125 °C of an AM50 Mg alloy modified by Si additions

Abstract The creep behaviour of an AM50 alloy modified by the addition of different proportions of Si was investigated at 125 °C. Increases from 0.3 to 1.5 wt.% Si led to a significant reduction in secondary creep rate. The time to rupture under 100 MPa increased with Si content and peaked with Si = 0.5 – 0.8 wt.%; further increases in Si content did not induce significant variations in creep strength, since the reduction in creep rate was offset by a parallel reduction in ductility. Microstructural investigations led to the identification of the major strengthening phases: β-Mg17Al12 and Mg2Si. Results are discussed and analysed in the light of the more recent data on the creep response of other Mg–Al alloys.

Die Casting of Magnesium Alloys - The Importance of Controlling Die Filling and Solidification

High pressure die casting is characterised by rapid die filling and subsequent rapid cooling and solidification of the metal in the die. These characteristics are favourable for the mechanical properties of magnesium die casting alloys. Since the filling pattern and the cooling rate of the metal is highly dependent on both process parameters and geometry of the part, there is a natural variation in mechanical properties. Variations in filling pattern can be caused by differences in the filling conditions setup by the gating system, pre-solidification in the shot sleeve and during filling as well as variations in the timing of the pressure intensification. In the present work the effects of solidification during filling are discussed with emphasis on the resulting microstructures and the correlation with mechanical properties.

TEM Characterization of near Sub-Grain Boundary Dislocations in Directionally Solidified Multicrystalline Silicon

A crystal is known to achieve lower energy if lattice dislocations are re-arranged in arrays forming a sub-grain boundary through a recovery process. Interaction of boundary dislocations with glide dislocations is also expected to bring about local equilibrium. In this work, dislocations localised in the vicinity of a sub-grain boundary (mis-orientation ) are studied in detail by transmission electron microscopy in order to determine their source. Contrary to the processes described above, it appears that the sub-grain boundary is the source of these dislocations, which are emitted from some locally stressed parts of the boundary. Several slip systems have been activated along the boundary resulting in high density of dislocations. It appears, further, that dislocation propagation from one or more sources is disrupted by interaction with other dislocations or other defects. The dislocations from various sources will be piled up against the obstacles of the other, resulting in the localization of the dislocations close to the sub-grain boundary

The Effect of Grain Orientations on the Efficiency of Multicrystalline Solar Cells

The grain orientations of some areas on industrially cast multicr ystalline silicon wafers have been measured by using the EBSD technique in SEM. Wafers from the nearby material were processed to solar cells. The solar cell efficiency of this m aterial was measured at various positions across the cells by LBIC having a FWHM spot diameter of 12 μm. The results show that 1) The grain size and the distribution of orientations of the grain normals of the wafers vary from the bottom to the top of the ingot, 2) The efficiency of the solar cells varies with grain orientat ions and the energy loss depends on the surface texturing appearance which depends on the orientation of the grains. This information may be used to optimise the solidification paramet ers to improve solar cell efficiency.