Formation And Mechanical Properties Of Zr-Al-Co-Cu Bulk Metallic Glasses

The motivation of this research is out of a continued sequence of simple questions:First, while Zr-Al-Ni-Cu has posed itself the most significant model system in bulk metallic glass (BMG) society, there exists hardly any extensive study on Zr-Al-Co-Cu, say, a system that is very close to the former in view of the phase diagram characteristics of their subsystems, the enthalpy of mixing between constituents, the crystal structures of alloy phases involved, and the closeness in the atomic size and melting point and so forth. Why are the BMG forming compositions so far known quite different from each other? Secondly, do their competing phases with glass formation share the same crystalline structure? Finally, is there any chance to discover a good combination of strength and plasticity in Zr-Al-Co-Cu BMG alloys? The present work is devoted to the answers for the three questions.Composition design for Zr-Al-Co-Cu quaternary alloys was based on four Zr-Al-Co basic ternary compositions, namely, Zr53Al23.5Co23.5,Zr55.8Al19.4Co24.8,Zr58.6Al15.4Co26 and Zr56.25Al18.75Co25 ([Zr9Co4]Al3), the last one being a composition derived from the atomic cluster plus glue atom model. BMG forming compositions would be expected by the alloying substitution of Cu. By means of copper mold casting, we have discovered three centimeter-scale BMG forming compositions:Zr55.gAl19.4Co17.4Cu7.4 (10 mm), Zr56.25Al18.75Co18.75Cu6.25 (10 mm) and Zr58.6Al15.4Co18.2Cu7.8 (10 mm), where the blanketed values stand for the critical diameter of BMG rod. The optimal BMG composition Zr55.8Al19.4Co17.4Cu7.4 exhibited characteristic thermal glass parameters of Tg= 744K, Tx= 809K, and Trg= 0.578.The crystallization behaviors of Zr-Al-Co-Cu BMGs have been examined in two different courses:the BMG reheating and the liquid solidification processes. The experiments were conducted by a combined use of differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. The crystallization counterparts of Zr-Al-Co-Cu BMGs were revealed to be the mixture of two phases, the known Al2CoZr6 (a=0.79 nm, c=0.6849 nm) and an unknown base-centered-orthorhombic (a=1.32 nm, b=0.81 nm, c=0.69 nm) phases. The two phases are structurally very complex. The BMG formation phenomenon in Zr-Al-Co-Cu is therefore attributed to the very sluggish crystallization kinetics of the two phases. Mechanical tests for Zr-Al-Co-Cu BMG alloys have exposed a good combination of strength and plasticity, which are superior than Zr-Al-Ni-Cu. In particular, the 3 mm diameter Zr56.25Al18.75Co12.5Cu12.5 BMG rods showed a Young's Modulus of 99 GPa, a yield strength of 1990 MPa, a fracture strength of 2040 MPa, and a remarkable plastic strain of 4%. The excellent mechanical properties of Zr-Al-Co-Cu BMGs give rise to their potential use as structural materials.