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Microstructural characterisation and thermal stability of an Mg-Al-Sr alloy prepared by rheo-diecasting
A commercial Mg-6Al-2Sr (AJ62) alloy has been prepared by a semisolid rheo-diecasting (RDC) process. The microstructure of the RDC alloy exhibits typical semisolid solidification features, i.e., 8.4 vol% primary α-Mg globules (23 μm in diameter), formed in the slurry maker at the primary solidification stage, uniformly distributed in the matrix of fine α-Mg grain size (8.2 μm) and intergranular eutectic Al4Sr lamellae, which resulted from secondary solidification inside the die. A ternary Mg-Al-Sr phase was also observed. Heat treatment revealed the extreme thermal stability of the RDC AJ62 alloy. The hardness showed little change up to 12 hours at 450°C, whilst the Al4Sr eutectic lamellae were broken up, spheroidised and coarsened during the annealing. The RDC alloy offers superior mechanical properties, especially ductility, over the same alloy produced by high pressure die-casting
An experimental study on a motion sensing system for sports training
In sports science, motion data collected from athletes is
used to derive key performance characteristics, such as stride length
and stride frequency, that are vital coaching support information. The
sensors for use must be more accurate, must capture more vigorous
events, and have strict weight and size requirements, since they must
not themselves affect performance. These requirements mean each
wireless sensor device is necessarily resource poor and yet must be
capable of communicating a considerable amount of data, contending
for the bandwidth with other sensors on the body. This paper analyses
the results of a set of network traffic experiments that were designed
to investigate the suitability of conventional wireless motion sensing
system design � which generally assumes in-network processing - as
an efficient and scalable design for use in sports training
An energetic blast wave from the December 27 giant flare of the soft gamma-ray repeater 1806-20
Recent follow-up observations of the December 27 giant flare of SGR 1806-20
have detected a multiple-frequency radio afterglow from 240 MHz to 8.46 GHz,
extending in time from a week to about a month after the flare. The angular
size of the source was also measured for the first time. Here we show that this
radio afterglow gives the first piece of clear evidence that an energetic blast
wave sweeps up its surrounding medium and produces a synchrotron afterglow, the
same mechanism as established for gamma-ray burst afterglows. The optical
afterglow is expected to be intrinsically as bright as at t\la
0.1 days after the flare, but very heavy extinction makes the detection
difficult because of the low galactic latitude of the source. Rapid infrared
follow-up observations to giant flares are therefore crucial for the
low-latitude SGRs, while for high-latitude SGRs (e.g. SGR 0526-66), rapid
follow-ups should result in identification of their possible optical
afterglows. Rapid multi-wavelength follow-ups will also provide more detailed
information of the early evolution of a fireball as well as its composition.Comment: Updated version, accepted for publication in ApJ Letter
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