Thermal Rejuvenation of an Mg-Based Metallic Glass
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Bulk metallic glasses (BMGs) have attracted a lot of interest over the last 2 decades because of their superior mechanical properties, which originate from their unique long-range disordered microstructures.[1–3] The BMGs are always prepared by rapid quenching techniques; thus, they generally possess much higher configurational potential energy compared to their crystalline counterparts. For this reason, their microstructures tend to change to a more stable state with lower energy, and so-called ‘‘relaxation’’ occurs.[4] For some BMGs, however, their microstructures can be modified to a more metastable state with higher energy and the opposite way of relaxation, or so-called ‘‘rejuvenation,’’ occurs.[5] Generally, the relaxation degrades
WEI GUO is with the State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China, and also with the Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan. Contact email: [email protected] JUNJI SAIDA is with the Frontier Research Institute for Interdisciplinary Sciences, Tohoku University. MI ZHAO is with the School of Aerospace Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China. Contact email: [email protected] SHULIN LU¨ and SHUSEN WU are with the State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology. Manuscript submitted October 7, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the ductility of BMGs, while the rejuvenation can plasticize them.[4,5] Mg-based BMGs are a promising engineering material because of their high specific strength and low cost.[6,7] However, to date, the rejuvenation behavior of Mg-based BMGs has been rarely studied. In the present study, we have applied two thermal methods, recovery annealing[8] and the deep cryogenic cycling technique (DCT),[9] to rejuvenate an Mg59.5Cu22.9Ag6.6Gd11 (at. pct) BMG. The microstructures and mechanical properties are also investigated in detail. The composition was chosen as Mg59.5Cu22.9Ag6.6Gd11 (at. pct) to achieve good glass forming ability.[10] The purity of all raw materials (purchased from Sendai-Wako Co. Ltd.) was higher than 99.9 pct. First, Cu-Gd-Ag precursors were prepared by arc melting the raw materials together under a Ti-gettered Ar atmosphere. Then, the precursors were inductively melted to master alloys with suitable amounts of Mg pieces in a carbon crucible in a He atmosphere at 1073 K for 3 minutes. The impurity content of carbon in the master alloy was checked to be lower than 2 at. pct with EDS measurement. The master alloys were then remelted in a quartz tube and injection cast into a copper mold with an internal cavity of 2 mm in diameter and 35 mm in length to form the BMGs. The relaxed samples were prepared through heating the as-cast BMGs to 438 K (~ 1.04 times the glass transition temperature, Tg) for 1 minute and cooling at 20 K/min. Some of them were th
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