Glass-Forming Ability and Early Crystallization Kinetics of Novel Cu-Zr-Al-Co Bulk Metallic Glasses

Xiaoliang Han, Yusheng Qin, Kai Qin, Xuelian Li, Shenghai Wang, Jun Mi, Kaikai Song, Li Wang
2016 Metals  
In recent years, CuZr-based bulk metallic glass (BMG) composites ductilized by a shape memory B2 CuZr phase have attracted great attention owing to their outstanding mechanical properties. However, the B2 CuZr phase for most CuZr-based glass-forming compositions is only stable at very high temperatures, leading to the uncontrollable formation of B2 crystals during quenching. In this work, by introducing Co (i.e., 4, 5, and 6 at. %) and 10 at. % Al into CuZr-based alloys, the relatively good
more » ... relatively good glass-forming ability (GFA) of CuZr-based alloys still can be achieved. Meanwhile, the B2 phase can be successfully stabilized to lower temperatures than the final temperatures of crystallization upon heating CuZr-based BMGs. Unlike previous reported CuZr-based BMGs, the primary crystallization products upon heating are mainly B2 CuZr crystals but not CuZr 2 and Cu 10 Zr 7 crystals. Furthermore, the primary precipitates during solidification are still dominated by B2 crystals, whose percolation threshold is detected to lie between 10 ± 2 vol. % and 31 ± 2 vol. %. The crystallization kinetics underlying the precipitation of B2 crystals was also investigated. Our results show that the present glass-forming composites are promising candidates for the fabrication of ductile CuZr-based BMG composites. Metals 2016, 6, 225 2 of 12 to the unique transformation-mediated deformation mechanism [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] . As a result, such BMG composites show not only good tensile plastic deformation ability but also work-hardening capacity, which further promotes the development of BMGs to be used as structural materials in future [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] . Until now, many CuZr-based glass-forming compositions have explored to fabricate ductile BMG composites with obvious work hardening [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] . Usually, during rapid solidification, the formation of B2 CuZr crystals in the CuZr-based glassy matrix, including their distributions, volume fractions, and gran sizes, strongly depends on the cooling rates, casting temperatures, and melting frequency of master alloys. By carefully controlling the casting parameters mentioned above during solidification, BMG composites with good mechanical properties could be prepared [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] . However, the optimum microstructures of CuZr-based BMG composites processing homogenously distributed and appropriate volume fractions of relatively fine B2 crystals only can be simultaneously obtained by considering two key factors: (1) inhibiting the easy decomposition of the high-temperature metastable B2 CuZr phase into the low-temperature equilibrium phases (i.e., CuZr 2 , Cu 10 Zr 7 or other equilibrium crystals) via a eutectoid reaction during solidification by controlling compositions of CuZr-based alloys [20-32]; (2) avoiding the interpenetration of B2 spherical crystals during solidification when their volume fractions reach a critical crystalline volume fraction between 30 and 50 vol. % [29, 30] . In other words, in order to control the formation of the B2 CuZr phase during quenching, it should be considered to stabilize the B2 CuZr phase with minor element additions instead of only adjusting the casting process. Both of these factors raise a big challenge to manipulate the formation of the B2 phase during rapid quenching so as to explore CuZr-based shape memory BMG composites with optimum mechanical properties. Previous results have demonstrated that more than 4 at. % Co addition into near-equiatomic Cu-Zr binary alloys can significantly enhance the thermal stability of the B2 CuZr phase so that the B2 CuZr phase can be easily preserved at low temperatures [34] [35] [36] . However, the glass-forming ability (GFA) of CuZr-based alloys with the addition of Co can be gravely deteriorated, and only amorphous ribbons can be produced. Under these circumstances, other micro-alloying elements should be also introduced into CuZr-based alloys in an attempt to maintain the GFA or reduce the deterioration of GFA induced by the Co addition [34] [35] [36] . It has been found that the addition of Al, Ag, Ti, or rare earth metals can enhance the GFA of CuZr-based alloys even though the thermal stability of the B2 CuZr phase cannot be effectively improved at the same time [37] [38] [39] [40] [41] . Therefore, systematic investigations have been devoted to fabricate Cu-Zr-Al-Co BMGs and their composites by carefully adjusting the contents of Al and Co [31, [42] [43] [44] . However, so far, a balance between the thermal stability of the B2 CuZr phase and the GFA of the CuZr-based alloy has not yet been achieved [31, [42] [43] [44] . On the one other hand, another method, i.e., partially crystallizing a glass in an extremely short time, was proposed to fabricate ductile 46] . However, the major crystallization products of CuZr-based BMGs during annealing are Cu 10 Zr 7 , and/or CuZr 2 and others, but not the B2 CuZr phase [33] . Meanwhile, the B2 CuZr phase only exists at higher temperatures than the final temperature of crystallization [33] . Hence, both the heating and cooling rates and the annealing temperature during partial crystallizing must be very high, which severely restricts the widespread use of this preparation method. Therefore, it is desirable to explore new CuZr-based BMGs with good GFA, whose major crystallization products become the B2 CuZr phase, in order to easily fabricate CuZr-based BMG composites by controlling the rapid solidification of melts or fast and partial-crystallizing BMGs. In this paper, (Cu 0.5 Zr 0.5 ) 90−x Al 10 Co x (x = 4, 5, and 6 at. %) BMGs with good GFA and high thermal stability were successfully prepared, and their major crystallization products are the B2 CuZr phase. Their crystallization kinetics were also investigated. Materials and Methods The (Cu 0.5 Zr 0.5 ) 90−x Al 10 Co x (x = 4, 5, and 6 at. %) master alloys were prepared by arc melting appropriate amounts of the constituting elements (>99.9% purity) under a Ti-gettered argon Metals 2016, 6, 225 3 of 12
doi:10.3390/met6090225 fatcat:7mgfbv7555egzavw477fkbttqu