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Description:
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The irradiation of metallic glasses to induce nanocrystallization was studied in
two metallic glass compositions , Cu50Zr45Ti5 and Zr55Cu30Al10Ni5 . Atomic mobility
was described using a model based on localized excess free volume due to displace -
ment cascades created by energetic particle irradiation . Due to the di erence in
cascade size among di erent masses of projectiles , a mass -dependent study was per -
formed . Metallic glass ribbon samples produced by melt -spinning were bombarded
with electron , He , Ar , and Cu particles . Electron irradiation and characterization
was performed "in -situ" by means of transmission electron microscopy . The di erent
metallic glasses showed dissimilar levels of radiation stability under electron irradi -
ation by Cu50Zr45Ti5 forming crystals 1 -10 nm in diameter embedded in the amor -
phous matrix after about 30 minutes of irradiation , while Zr55Cu30Al10Ni5 showed no
such crystallization . Increasing projectile mass caused an increase in the maximum
nanocrystal diameter up to approximately 100 nm in Cu irradiated Zr55Cu30Al10Ni5 .
Studies of di raction patterns of irradiated specimens showed nucleation of Cu10Zr7
phases in both specimens , as well as evidence of CuZr2 in Cu50Zr45Ti5 and both CuZr2
and NiZr2 in Zr55Cu30Al10Ni5 . Crystal sizes in irradiated Zr55Cu30Al10Ni5 specimens
showed bimodal distribution with many large (50 -100 nm ) crystals and many small
(1 -5 nm ) crystals . The small crystals in irradiated Zr55Cu30Al10Ni5 were determined
to be NiZr2 phase because of the low abundance of Ni . After exposure to 2 keV Ar ions , areas of composition roughly Cu10Zr7 were found by energy -dispersive X -ray
spectroscopy but no crystallization was found . Further crystallization was achieved in
decomposed specimens after electron irradiation . This shows that atomic segregation
is a necessary step before nucleation in metallic glasses . |