153,926 research outputs found
Silver segregation to \theta' (Al2Cu)-Al interfaces in Al-Cu-Ag alloys
\theta' (Al2Cu) precipitates in Al-Cu-Ag alloys were examined using high
angle annular dark field scanning transmission electron microscopy
(HAADF-STEM). The precipitates nucleated on dislocation loops on which
assemblies of {\gamma}' (AlAg2) precipitates were present. These dislocation
loops were enriched in silver prior to \theta' precipitation. Coherent, planar
interfaces between the aluminium matrix and \theta' precipitates were decorated
by a layer of silver of two atomic layers in thickness. It is proposed that
this layer lowers the chemical component of the Al-\theta' interfacial energy.
The lateral growth of the \theta' precipitates was accompanied by the extension
of this silver bi-layer, resulting in the loss of silver from neighbouring
\gamma' precipitates and contributing to the deterioration of the \gamma'
precipitate assemblies.Comment: Pre-print. 12 pages, 7 figure
Kinetics of coherent order-disorder transition in
Within a phase field approach which takes the strain-induced elasticity into
account, the kinetics of the coherent order-disorder transition is investigated
for the specific case of alloy. It is shown that a microstructure
with cubic precipitates appears as a transient state during the
decomposition of a homogeneous disordered solid solution into a microstructure
with tetragonal precipitates embedded into a disordered matrix. At
low enough temperature, favored by a weak internal stress, only
precipitates grow in the transient microstructure preceding nucleation of the
precipitates that occurs exclusively at the interface of the solid
solution with the precipitates. Analysis of microstructures at
nanoscopic scale shows a characteristic rod shape for the
precipitates due to the combination of their tetragonal symmetry and their
large internal stress.Comment: 2 postscript figures and 1 JPG pag
Deformation and Prefered Orientation of Precipitates in Cold Worked Al-Zn Alloys
Fiber textures of drawn wires of Al-Zn alloys containing precipitates were studied mainly by X-ray methods. In supersaturated solid solution the wire texture was a double fiber texture with [100]and [111]. It was considered that a phase precipitates were rotated with matrix, while they were deformed. And then the matrix containing stable precipitates (Zn) had [100] and [111] textures. The intense spots, corresponding to [111] fiber texture of matrix, in the Debye rings of (002)p and (101)p planes of precipitates were clearly observed. But, on the other hand, spots, corresponding to [100] fiber texture of matrix, shown the prefered orientation of precipitates in (002)p Debye ring were very weak and brodening
Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets
We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05
films to their magnetic properties. The formation of ferromagnetic nanometer
sized inclusions in a defect free Ge matrix fabricated by low temperature
molecular beam epitaxy is observed down to substrate temperatures T_S as low as
70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron
energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions
as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount
of these precipitates decreases with decreasing T_S. Magnetometry of the films
containing precipitates reveals distinct temperature ranges: Between the
characteristic ferromagnetic transition temperature of Mn_5Ge_3 at
approximately room temperature and a lower, T_S dependent blocking temperature
T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3
precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates
with blocked magnetic moments is observed. At the lowest temperatures, the
films show features characteristic for a metastable state.Comment: accepted for publication in Phys. Rev. B 74 (01.12.2006). High
resolution images ibide
Minority carrier lifetime in silicon photovoltaics : the effect of oxygen precipitation
Single-crystal Czochralski silicon used for photovoltaics is typically supersaturated with interstitial oxygen at temperatures just below the melting point. Oxide precipitates therefore can form during ingot cooling and cell processing, and nucleation sites are typically vacancy-rich regions. Oxygen precipitation gives rise to recombination centres, which can reduce cell efficiencies by as much as 4% (absolute). We have studied the recombination behaviour in p-type and n-type monocrystalline silicon with a range of doping levels intentionally processed to contain oxide precipitates with a range of densities, sizes and morphologies. We analyse injection-dependent minority carrier lifetime measurements to give a full parameterisation of the recombination activity in terms of ShockleyâReadâHall statistics. We intentionally contaminate specimens with iron, and show recombination activity arises from iron segregated to oxide precipitates and surrounding defects. We find that phosphorus diffusion gettering reduces the recombination activity of the precipitates to some extent. We also find that bulk iron is preferentially gettered to the phosphorus diffused layer rather than to oxide precipitates
Nanocharacterisation of precipitates in austenite high manganese steels with advanced techniques: HRSTEM and DualEELS mapping
To achieve optimal mechanical properties in high manganese steels, the precipitation of nanoprecipitates of vanadium and niobium carbides is under investigation. It is shown that under controlled heat treatments between 850°C and 950°C following hot deformation, few-nanometre precipitates of either carbide can be produced in test steels with suitable contents of vanadium or niobium. The structure and chemistry of these precipitates are examined in detail with a spatial resolution down to better than 1 nm using a newly commissioned scanning transmission electron microscope. In particular, it is shown that the nucleation of vanadium carbide precipitates often occurs at pre-existing titanium carbide precipitates which formed from titanium impurities in the bulk steel. This work will also highlight the links between the nanocharacterisation and changes in the bulk properties on annealing
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