Glass forming ability and bending plasticity evolutions in Zr-Co-Al bulk metallic glasses and their structural origin

The effect of composition tuning on glass forming ability (GFA), mechanical properties and atomic structure in ZrxCo84−xAl16 bulk metallic glasses (BMGs) with x = 51-58 has been investigated by using copper-mold suction casting, nanoindentation, high-energy X-ray diffraction, extended X-ray absorption fine structure experiments and ab initio molecular dynamics simulations, respectively. It is found that GFA firstly increases then decreases with increasing Zr content, exhibiting a maximum diameter of 7 mm with the Zr content of 55 and 56, while an abrupt reduction of bending plasticity occurs as the Zr content drops below 55. Both experimental and theoretical results find that GFA is remarkably enhanced, due to the abnormal reduction in the fractions of the Co-centered ⟨0, 2, 8, 1⟩, ⟨0, 3, 6, 2⟩ and ⟨0, 3, 6, 3⟩ polyhedra, and the denser atomic packing of Zr atoms around center Co atom, as the Zr content increases from 51 to 56. Meanwhile, the simultaneously improved plasticity is associated with the increased Co-centered ⟨0, 3, 6, 0⟩, and Al-centered ⟨0, 2, 8, 1⟩ and ⟨0, 3, 6, 2⟩ polyhedra, which provide more fertile sites for the nucleation of shear transformations. Further Zr addition to 58 increases remarkably the fractions of Zr- and Al-centered ⟨0, 1, 10, 2⟩ and ⟨0, 3, 6, 4⟩ polyhedra, and causes looser atomic packing of Co atoms around center Zr atom, leading to the reduction in GFA. However, the plasticity is not enhanced further but remains nearly constant, which may be related to the deteriorated effect on plasticity of the decreased fractions of Zr-centered ⟨0, 1, 10, 5⟩ and Co-centered ⟨0, 3, 6, 0⟩ polyhedra, and the increased fractions of Zr-centered ⟨0, 1, 10, 3⟩ and ⟨0, 2, 8, 4⟩ and Co-centered ⟨0, 2, 8, 0⟩ polyhedra. Compositional tuning, with internal structure tuned to favor both GFA and plastic flow, offers new possibilities in developing ductile BMG materials.