Abstract
Abstract Nanocrystalline nickel-iron layers are produced electrochemically on copper discs by varying the current density and then annealed in a vacuum furnace at a temperature range between 200 and 800 °C. Grain size, iron content, texture and microstrain of the microstructure are primarily characterized by X-ray diffraction (XRD). Instrumented indentation tests and microbending tests for mechanical characterization are carried out. The iron contents of the investigated layers are 5.7, 8.8, 13.5 and 17.7 wt.-%. By varying the annealing temperature, the reduction of the microstrains is initiated at 200 °C and ends at a temperature of about 280 °C. Primary recrystallization starts slightly higher at 220 °C and is completed at 300 °C. With higher iron content, the indicated temperatures shift to slightly higher values. Indentation modulus, Young's modulus, indentation hardness and strength change considerably after the annealing treatment. Fracture strain at the edge, as a measure of ductility, decreases immediately after annealing at 200 °C to 0 %. Low annealing temperatures occurring before the beginning of primary recrystallization lead to an increase in indentation hardness and 0.01-% offset bending yield strength Rp0.01∗ as compared to the electrochemically deposited initial state. After annealing at high temperatures, the mechanical parameters are mostly below the initial values for electrochemical deposition. Hall-Petch (HP) behavior is observed for Rp0.01∗, both for the electrochemically deposited specimens down to almost 6 nm and for the specimens annealed at high temperatures. Specimens annealed at low temperatures deviate from the HP straight line to higher values. In this case, an increase in strength is assumed to be due to the very small nanocrystalline (nc) grain sizes, segregation at the grain boundaries and a decrease in dislocation density. Indentation hardness measurements show almost no dependence on D−0.5 for the electrochemically deposited specimens and also for annealed specimens below 30nm grain size. Above 30nm, the indentation hardness values are considerably higher than for the HP straight line. Overall, the hardness and strength values of the nc specimens, electrochemically deposited or additionally annealed, are significantly higher than those of the microcrystalline (mc) specimens.
Highlights
EIT rises up to 300 °C to a plateau of about 250 GPa and drops significantly beyond 400 °C down to 145 GPa for the specimens annealed at 800 °C
Indentation hardness HIT rises before measuring the beginning of recovery
The maximum values of Rp0.01* for all three iron contents occur before the beginning of the recrystallization
Summary
The nickel-iron layers are electrodeposited on 100 mm copper discs provided by a manufacturer of microdrives. An industrial electrolyte (pH = 3) especially developed for microsystem technology is used for the electrochemical deposition [3]. Efforts are made to generate highly uniform microstructures on larger surface areas. The direct current (DC) density is varied from 0.5 to 5 A × dm-2 [3, 10]. The iron content of the finished nickel-iron layers measured by EDX is 5.7 to 17.7 wt.-% Fe (see Table 1). To take 5 × 5 mm[2] square specimens and strips, respectively, microbending specimens from the deposited layers, a precision cutting machine is used. The small bending specimens are referred to as “microbending specimens”
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