Enhanced thermal stability of the devitrified nanoscale icosahedral phase in novel multicomponent amorphous alloys

In this paper, details are given for the structural evolution of (Ti33Zr33Hf33)(70)(Ni50Cu50)(20)Al-10, (Ti25Zr25Hf25Nb25)(70)(Ni50Cu50)(20)Al-20, and (Ti33Zr33Hf33)(70)(Ni33Cu33Ag33)(20)Al-10 amorphous alloys, part of wider program of alloy development by equiatomic substitution. All three alloys initially crystallize by forming a nanoscale icosahedral phase. However, at higher temperatures, their decomposition sequences differ significantly. The nanoscale icosahedral phase in the (Ti33Zr33Hf33)(70)(Ni50Cu50)(20)Al-10 alloy decomposes into a mixture of Zr2Cu-type and icosahedral phases. This icosahedral phase still exists after heating up to 970 K, indicating a high thermal stability of this phase. The nanoscale icosahedral phase in the (Ti33Zr33Hf33)(70)(Ni33Cu33Ag33)(20)Al-10 alloy also transforms into a mixture of Zr2Cu-type and icosahedral phase during the second exothermic reaction but then transforms into a mixture of Zr2Cu-type and Ti2Ni-type phases. The nanoscale icosahedral phase in the (Ti25Zr25Hf25Nb25)(70)(Ni50Cu50)(20)Al-10 alloy decomposes into a mixture of Ti2Ni-type and MgZn2-type phases during the second exothermic reaction. It is concluded that the formation of the Zr2Cu-type phase retards the decomposition of the nanoscale icosahedral phase, which increases the thermal stability. In contrast, formation of Ti2Ni-type and MgZn2-type phases accelerates the decomposition of the nanoscale icosahedral phase, which decreases its thermal stability.