Easy glass-forming alloy system usually has a “deep eutectic” feature. But for the ternary or multi-component alloys, phase diagrams of the systems are not always well-established in many cases. Therefore, it has to rely on the “trial and error” approach to develop easy glass formers. It is of interest to clarify the correlation between the BMG-forming composition and eutectic reaction, to understand the origin of the high glass-forming ability (GFA). Among the BMG families, the Cu-based BMGs have a potential application as new structural materials due to their high strength, assessable plasticity and excluding noble or toxic elements. Cu-ETM-M (ETM=Zr, Hf, M=Ti, Al) ternary alloys are the basic compositions of these Cu-based BMGs. In the present work, the Cu−Zr−Ti ternary system was selected to investigate the compositional dependence of BMG formation and the relation between GFA and eutectic reaction the alloy melt undergoes during solidification in two composition regions. The following conclusions are drawn:
1. Glass-forming ability of the alloys around the Cu60Zr30Ti10 composition in the Cu−Zr−Ti ternary system is strongly composition-dependent. The best glass former is Cu60Zr33Ti7 alloy with a Dc of 4 mm. This composition is near the univariant eutectic groove of (L→Cu8Zr3+Cu10Zr7). Compared with the CuZr binary BMGs related to the eutectic reaction of (L→Cu8Zr3+Cu10Zr7), the improvement of the glass-forming ability is caused by an effect that Ti significantly stabilizes the liquid.
2. Using the “3D pinpointing approach”, in the Cu-Ag-Zr-Ti quaternary system, Cu44.25Ag14.75Zr36Ti5 is discovered to have a Dc of 10 mm. Compared with the ternary alloy, the Ag addition plays a significant role in stabilizing the alloy melt. Such an effect is believed to be a main reason of improving GFA for the quaternary alloys.
3. The Cu44.25Ag14.75Zr36Ti5 alloy is a best glass former and solidification of its liquid upon cooling mainly undergoes a pseudo-binary eutectic reaction (L→Cu10Zr7+AgZr). The BMG-forming composition zones of the Cu−Zr−Ti and Cu-Zr-Ag ternary systems are linked with that of the Cu-Ag-Zr-Ti quaternary system in the 3D composition space. There exits a pseudo-binary eutectic reaction (L→Cu10Zr7+AgZr) in the Cu-Zr-Ag ternary system. Related to this eutectic reaction, the best glass former is Cu35.4Zr44.6Ag20 with Dc of 3 mm.
4. In the Cu-Zr-Ti ternary system, the Cu52Zr40Ti8 composition predicted using the CALPHAD calculation is not located at a ternary invariant eutectic point (L→Cu10Zr7+CuZr+Cu2ZrTi). Solidification of the Cu52Zr40Ti8 liquid upon cooling mainly undergoes a univariant eutectic reaction (L→Cu10Zr7+CuZr). Alloys around the Cu52Zr40Ti8 composition exhibit a GFA. Monolithic glassy rods of 4 mm in diameter can be fabricated under copper mould casting at least at five compositions, which are along the univariant eutectic groove of (L→Cu10Zr7+CuZr). The composition region is different from that previously reported by Inoue, indicating that more than two BMG-forming composition zones are present in this system.
5. In the Cu-Zr-Ti ternary system, the composition region bounded by the CuZr, Cu10Zr7 and Cu2ZrTi intermetallic, departure from the optimal composition range for glass formation, in situ composites comprised of metallic glassy matrix with Cu10Zr7 or CuZr as the primary phase can be fabricated. Under compression test at room temperature, the deformation and fracture of the monolithic glass and Cu10Zr7-containing composite is almost the same without any macroscopic plastic strain before failure. By contrast, the composites containing CuZr particles exhibit observed plastic strain around 4.2%. Such ductility results from the presence of the CuZr particles with martensitic structure.
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