The glass transition phenomenon in these alloys is preceded by
a slight exothermic relaxation, often observed in glasses with a
high enthalpy state upon casting [46]. This is attributed to excessive
structural free volume, whereby the reduction in free volume
during heating is proportional to the exothermic heat of this structural
relaxation prior to the glass transition [47]. Compared with
the glass transition temperatures of reported Ca-based BMG with
higher Ca contents [2] ranging from 102 to 107 C, the glass transition
temperatures of these alloys are slightly higher, ranging
from 119 to 129 C, thereby indicating an increased thermal stability
of the SCL, which is probably due to the increased number of
Ca–Zn, Ca–Mg and Mg–Zn bonds in the structure. It can be seen
that the addition of Zn has the greatest effect on increasing the
glass transition temperature, which is probably due to the higher
binding potential of the Ca–Zn atomic pair (22 kJ mol1
) compared
with Ca–Mg (6 kJ mol1
) and Mg–Zn (4 kJ mol1
) [48].
Hence, CaBMG4 has both the highest Zn content and the highest
glass transition temperature. However, owing to these higher glass
transition temperatures, the size of the SCL region in these alloys is
reduced (16–24 C) compared with those with higher Ca content
(35–50 C), as the temperature of the primary crystallization reaction
in all these Ca-based BMG appears to be less affected by alloy
chemistry, occurring at similar temperatures in each case.
4.2. Bio