Tensile behavior of Cu-coated Pd40Cu30Ni10P20 metallic glassy wire

Abstract

Catastrophic brittle fracture of monolithic metallic glass (MG) hinders engineering application of MGs. Although many techniques has been tried to enhance tensile ductility of metallic glasses, the enhancement is quite limited. Here, we show the effect of electrodeposited Cu coating on tensile plasticity enhancement of Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires, with different volume fractions of copper coatings (R), from 0% to 97%. With increasing R, tensile elongation is enhanced to 7.1%. The plasticity enhancement is due to confinement of the Cu coatings, which lead to multiple and secondary shear bands, according to SEM investigations. In addition, the SEM images also show that the patterns on the fracture surface of the Cu-coated MG wires vary with volume fraction of the Cu coatings. The size of shear offset decreases with increasing R. The viscous fingerings on the fracture surface of monolithic MG wire changes into dimples on the fracture surface of Cu coated MG wires with R of 90% and 97%. The electrodeposition technique used in this work provides a useful way to enhance plasticity of monolithic MGs under tensile loading at room temperature.

Figure 1

Surface and cross-sectional morphology of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires: (a) optical image of the cross section of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wire; (b) SEM image of the surface of homogeneous Cu coating without detected defects.

Figure 2

Engineering tensile stress-strain curves of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires with coating volume fraction R of 0%, 45%, 70%, 90%, and 97%, and electrodeposited pure Cu. The straight dash lines are eyesight guide of the tensile stress-strain curves of the as-cast Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wire and the wire with R of 45% for showing the nonlinearity of the curve. The inset indicates five stages of tensile deformation process of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires. In stage I, both the coating and the wire core deform linearly. In stage II, the coating deforms plastically while the wire core elastically. In stage III, both the coating and the wire core deform plastically. In stage IV, the wire core fractures. In stage V, the coating necks and then fractures.

Figure 3

Catastrophic shear fracture to necking transition of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires. (a) Variation of the reduction of area $(A_0-A_f)/A_0$ with volume fraction of Cu coating R. (b,c and d) are fracture morphology of Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires with R of 45%, 70% and 90%, respectively. (e) Morphology of the MG core of the Cu-coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wires with R of 97%.

Figure 4

Fracture patterns on the fracture surface of Pd₄₀Cu₃₀Ni₁₀P₂₀ MG core: (a) viscous fingering on the fracture surface of as-cast Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wire; (b) viscous fingerings and dimples on the fracture surface of the MG wire core of the wire with a R value of 45%; (c,d) dimples on the fracture surface of the MG wire cores in the wires with R values of 90% and 97%.

Figure 5

Electrodepositing Cu onto Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wire for tensile testing. (a) Schematic illustration of electrodeposition setup. The difference between our setup and conventional one for Cu electrodeposition is that a motor is connected to the cathode to rotate the MG wire to make the coating thickness homogeneous. (b) Cu coated Pd₄₀Cu₃₀Ni₁₀P₂₀ MG wire for tensile testing. The fillet between the grip section and the reduced section of the prepared tensile testing sample forms during electrodeposition because of the meniscus electrolyte around the MG wire at the position where the wire meets the surface of the electrolyte.