D570
Journal of The Electrochemical Society, 160 (11) D570-D577 (2013) 0013-4651/2013/160(11)/D570/8/$31.00 © The Electrochemical Society
Structure and Mechanical Properties of Electrodeposited Nanocrystalline Ni-Fe Alloys Antonello Vicenzo∗,z Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, 20131 Milano, Italy Nickel-iron alloy coatings were produced by electrodeposition from an additive free electrolyte, at room temperature and current density in the range of 1 to 5 A dm−2 , with Fe content up to 75 wt%. The structure and mechanical properties of the electrodeposited alloys are reported in the present work and analyzed focusing on structure-property relationships. In particular, the influence of the hydrogen evolution reaction is highlighted as a process factor affecting alloy phase structure, notably the composition limit of the γ-phase field. The variations of the mechanical properties with alloy composition are analyzed in the light of the concurrent modifications in phase structure and crystal size of the alloys. In particular, an assessment of the different factors influencing the hardness of γ phase alloys is proposed. Solid solution effects contribute significantly to the strength of γ phase alloys over a wide composition range, approximately from 5 to 25%, though a complex interplay between solid solution and Hall-Petch strengthening needs to be envisaged to account for the variations in hardness with composition over this range. Moreover, it is emphasized that with decreasing grain size, the increasing level of internal stresses and decreasing stiffness engender significant softening in nanocrystalline γ phase alloys with Fe content exceeding about 25%. © 2013 The Electrochemical Society. [DOI: 10.1149/2.109311jes] All rights reserved. Manuscript submitted August 28, 2013; revised manuscript received October 9, 2013. Published October 18, 2013.
The electrodeposition of Ni-Fe alloys has been extensively studied for application ranging from thin film head technology1,2 and magnetic devices3 to electroforming of engineering parts and microsystems.4–8 In particular, soft magnetic Ni-Fe alloy films are used in magnetic force micro-sensors and actuators and are exploitable for magnetic micro power applications.9,10 The range of the latter applications for ECD Ni-Fe alloy has been recently reviewed.11 Dimensional stable Ni-Fe alloys were proposed for micro-electromechanical systems manufacture12 and considered for electroforming of alloy 42 lead-frames used in IC packaging and shadow masks for CRT monitors.13 More generally, high strength Ni alloys with good tensile ductility are sought as a replacement for electroformed nickel which is especially penalized by a limited thermal stability.14 Research activity has also extended to electroplating of ternary and even quaternary alloys for improving key physical properties and material performance in terms of mechanical endurance, corrosion resistance or electrocatalytic behavior.1,2,15–19 More recently, this long-standing research interest has been renewed in the prospect of improvement of key material properties by nanostructuring.20–22 Not surprisingly, electrodeposited (ECD) Ni-Fe alloys have been the subject of several studies over a long span of time, which however have yet to result in a detailed and comprehensive picture of material properties over the entire compositional range (possibly with the exception of the magnetic behavior, which is, however, particularly sensitive to microstructure variations). Research interest has spanned from Ni rich alloy,20 with composition in the range of Permalloy materials,21,23,24 to Fe-rich alloys having either the Invar composition25–27 or with Ni content in the range 10–15%.28–30 Detailed information about the influence of process parameters on compositional and structural properties of Ni-Fe coatings in the whole composition range of the binary system were reported by Grimmet et al.,31 Myung et al.,32 and Leith et al.,33 depositing from either mixed sulfate-chloride or sulphamate-chloride electrolytes, respectively. More recently, physical properties of ECD Ni-Fe alloy thin films were reported by Tabakovic et al.,34 in a wide composition range, and for nanowire arrays by Rousse.35 Properties of nanocrystalline Ni-Fe electrodeposits in the composition range of fcc and two-phase alloys were also studied by Li and Ebrahimi36 and McCrea et al.,13 using simple and complex electrolytes, respectively, showing strong differences in microstructure and mechanical properties of the electrodeposits. The present work is a study of the structure and mechanical behavior of nanocrystalline Ni-Fe alloys electrodeposited at room temperature from a simple electrolyte, without addition of either a grain ∗ z
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refiner or a complexing agent. Standard additives used as grain refiner and leveler in Ni-Fe electrodeposition (e.g. saccharine) are known to be responsible for high temperature embrittlement37,38 and reduced corrosion resistance.39–41 Low temperature operation would be a distinct advantage for through mask electrodeposition, particularly in the filling of patterns of small feature size, and in template-assisted fabrication of nanostructures, e.g.,35 allowing for the use of heat sensitive masking or template materials. Moreover, low temperature processing in Ni-Fe electrodeposition would be beneficial to the service life of the bath. Eventually, Ni-Fe alloy electrodeposits produced at room temperature have not yet been characterized in much detail. In the present work, mechanical properties, derived from indentation measurements, are reported for relatively thick deposits (typically in the range 20–25 μm) with composition ranging from pure Ni to Fe content up to 75%, along with results of in-situ measured residual stress for 5 μm thick deposits with composition in the range up to about 50% Fe. The mechanical characterization of Ni-Fe electrodeposits produced at room temperature is expected to provide a coherent set of data, which may be a useful source of information for the selection and the optimization of process conditions as well as for the development of ECD ternary alloys of potential interest for engineering application of thick coatings.
Experimental Ni-Fe alloys were deposited from nickel sulphamate and iron sulfate electrolytes at pH 3.2(±0.1). The nickel sulphamate, Ni(H2 NSO3 )2 , concentration was maintained at 0.95 M. Nickel bromide, NiBr2 (0.05 M), was used as anodic depolariser instead of the chloride, which notoriously induces a high level of internal stress in deposits.42 The Fe(II) sulfate salt, FeSO4 · 7H2 O, was also preferred to the chloride salt since the latter was found to promote morphological degradation during growth of Fe rich thick deposits.43 Ferrous sulfate concentration was varied in the range 0.005 to 0.30 M in order to obtain coatings with different Fe content, keeping unchanged the concentration of all other chemicals. In addition to the metal salts, the electrolyte formulation included boric acid, H3 BO3 (0.4 M), sodium lauryl sulfate, C12 H25 OSO3 Na (∼10−4 M), and ascorbic acid, C6 H8 O6 (7 × 10−3 M), the latter two chemicals being used as anti–pitting and reducing agent, respectively. The electrolytes were pre-treated with hydrogen peroxide (2 mL l−1 ) and by active charcoal (1 h, ∼60◦ C), before addition of ferrous salt and organic components. Finally the solutions were pre-electrolysed at low current and with large cathodic area (
