The mechanical properties at ambient and elevated temperatures of two Al-15 vol pct B 4 C composites, S40 with 0.4 wt pct Sc and SZ40 with 0.4 wt pct Sc and 0.24 wt pct Zr, are investigated during long-term thermal annealing. The presence of large B 4 C particles in the microscale has a moderate but stable strengthening effect on Al-B 4 C composites at ambient and elevated temperatures, while the precipitation of nanoscale Al 3 Sc and Al 3 (Sc, Zr) in the composite matrix provides a predominate contribution to the composite strength, which is varied by tested temperatures. The Al 3 Sc precipitates in S40 remain coarsening resistant at 523 K (250°C), whereas the Al 3 (Sc, Zr) precipitates in SZ40 are thermally stable at 573 K (300°C) over 2000 hours of annealing. At higher annealing temperatures (573 K (300°C) for S40 and 623 K (350°C) for SZ40), both Al 3 Sc and Al 3 (Sc, Zr) precipitates become coarsening with prolonged annealing time. The yield strength of S40 and SZ40 at ambient temperature decreases with the increasing precipitate size, which can be explained by the classical precipitate shearing and Orowan bypass mechanisms. At elevated temperatures [523 K to 623 K (250°C to 350°C)], considerably lower yield stresses are observed compared to those at ambient temperature, which invokes a dislocation climb mechanism. The predicted yield strengths at elevated temperatures by the combination of dislocation climb and Orowan models are in good agreement with the experimental data.
SYNOPSISIn this paper the swelling and melting behavior of several polymers, particularly acrylic fibers, is studied by thermomicroscopy and differential scanning calorimetry in detail. It is confirmed that the protrusion peak on the tail part of the DSC thermogram of swelling polymers is closely associated with the macromolecular entanglements. The proposed swelling DSC method is proved to be very effective for studying polymer entanglements. It is also shown that the melting point, whether of the paracrystalline region or of entanglement structural unit, can be well fitted to Flory's melting point depression theory.
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