Microstructure Evolution and Mechanical Stability of Supersaturated Solid Solution Co-Rich Nanocrystalline Co-Cu Produced by Pulsed Electrodeposition

Abstract

Thick films of supersaturated solid solution nanocrystalline Co-Cu (28 at.% Cu) were synthesized through the pulsed electrodeposition technique. Microstructural changes of nanocrystalline Co-Cu were intensively studied at various annealing temperatures. Annealing at 300 °C results in a spinodal decomposition within the individual grains, with no grain coarsening. On the other hand, distinct phase separation of Co-Cu is detected at annealing temperatures beyond 400 °C. Static micro-bending tests show that the nanocrystalline Co-Cu alloy exhibits a very high yield strength and ductile behavior, with no crack formation. Static micro-bending tests also reported that a large plastic deformation is observed, but no microstructure change is detected. On the other hand, observation on the fatigue resistance of nanocrystalline Co-Cu shows that grain coarsening is observed after conducting the cyclic micro-bending test.

Strategies to Achieve High Strength and Ductility of Pulsed Electrodeposited Nanocrystalline Co-Cu by Tuning the Deposition Parameters

“…Of course, a more valid grain size number can be obtained through transmission electron microscopy (TEM), but it is not conducted here. According to previous work [6], the grain size number from TEM images (~23 nm) is between the grain size number from Scherer (~14 nm) and Williamson-Hall (~31 nm) approaches. According to the previous measurement characteristic, S4 may exhibit grain size in the range of 9-72 nm, which is the biggest among S1-S5.…”

“…Extensive studies and investigations were carried out to find specific ranges of deposition parameters of nanocrystalline Co-Cu. According to previous work [6,22], deposition at a pulse current density of 500 A/m 2 leads to the formation of deposits with inhomogeneous distribution of microstructure and chemical composition, while porous and unstable (powdery and with high internal stresses) deposits were produced at a pulse current density of 1500 A/m 2 . Compact and homogeneous nanocrystalline Co-Cu deposits could be obtained at pulse current densities of 750-1000 A/m 2 ; thus, this range was selected for the deposition of bulk nanocrystalline Co-Cu.…”

“…On the other hand, a relatively compact deposit up to~300 µm of thicknesses is obtained in the sample deposited at a duty cycle of 20% (see Figure 1c). However, homogeneous and stable deposits of nanocrystalline Co-Cu are only visible up to~400 µm of thicknesses at a pulse-on time of 2 ms [6]. The inhomogeneity is strongly affected by significant concentration decrease of electrolyte components (e.g., Co and Cu ions and Saccharin), which leads to the formation of coarser grains with higher Cu concentration at the area near the surface of deposits [22].…”

“…However, previous work on bulk high pressure torsion (HPT)-processed Cu-rich nanocrystalline Co-Cu [5] showed that an elongation at fracture of less than 10% was still reported. In addition, observation of micro-mechanical properties of the pulsed electrodeposited (PED) Co-rich nanocrystalline Co-Cu still reported unclear information [6]. Therefore, a study on mechanical properties and improvement in ductility of nanocrystalline Co-Cu needs further investigation.…”

“…In the PED-processed nanocrystalline materials, adjusting deposition parameters (for instance: current density, pulse-on time, and duty cycle) has a significant influence on the resulting microstructure as well as on the porosity, which has a direct impact on their mechanical properties [11,12,[17][18][19]. It is also possible to improve the mechanical properties of nanocrystalline materials, particularly nanocrystalline Co-Cu, through specific annealing treatment that leads to microstructural changes (e.g., spinodal and phase decomposition) and recovery processes (e.g., relaxation of grain boundaries and internal stresses) [6,20,21]. Improvement in strength and ductility of PED-processed nanocrystalline Co-Cu is expected by tuning deposition parameters as well as annealing treatment.…”