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Phase transformations in the Cu-35Mn-25Al alloy have been
investigated by using transmission electron microscope (TEM) and
energy-dispersive X-ray spectrometer (EDS). In the as-quenched
condition, the microstructure of the alloy was a mixture of (
L21+B2+L-J ) phases. The B2 phase with a fine particle shape was
present within L21 domains. This feature has never been observed
by other workers.The L-J phase is a new phase having an
orthorhombic structure, which was found firstly by T. F. Liu and
S. C. Jeng in a Cu2.2Mn0.8Al alloy.[2]When the alloy was aged at
300℃, the fine B2 particles grew and no evidence of the L-J
phase could be detected. Therefore , the microstructure of the
alloy at 300℃is a mixture of ( L21+B2 ) phases. When the alloy
was aged at 500℃for short times, the shape of the B2 particles
changed from particle into needle-like. The microstructure is
still ( L21+B2 ) phases. However, when the aging time
wasincreased at this temperature, two kinds of precipitates,
namely γ- brass andβ- Mn, started to appear on the grain
boundary. After prolonged aging at thistemperature, the grain
boundary precipitation of(γ- brass + β- Mn) became predominant
. Therefore, the stable microstructure of the alloy at 500℃is
(γ- brass + β- Mn).The coexistence of the γ- brass and β- Mn
precipitates has never been observed by other workers in the Cu-
Mn-Al alloys. A further increase in the aging temperature up to
650℃ resulted in a rapid growth of theβ- Mn precipitates
within L21 matrix and no γ- brass precipitates could be
observed. Progressively higher temperature aging and quenching
experiments indicated that when the alloy was aged at 680℃or
above, the microstructure of the alloy was the same as that in
the as-quenched condition.
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