Abstract: Mechanical properties of thick electrodeposits of Ni and Ni–Co alloys

Abstract

The possibility of using electrodeposited materials in massive form, i.e., as thick deposits, is of interest when the properties are attractive. We have investigated the mechanical behavior of nickel and nickel-cobalt alloy electrodeposits, which are known to attain high strength even in the absence of additives.1-4 (Plating techniques have been described elsewhere.4,5) High-purity deposits of Ni and alloys of up to 43% Co were produced in 8-mm thickness, and the mechanical properties examined as functions of both Co content and history (as-plated compared to material annealed 1 h at 575 K). Yield and ultimate tensile strengths increase with Co additions, as shown in Fig. 1. The strengths shown are high, attained in part by fine grain size (in the range 1000-2000 Å). Ductility was somewhat more complex in its behavior, but was ≳10% reduction in area in all cases.4 It appears in Fig 1 that the strengths at ∠20% Co are above the trend which is implied by the remaining data. It is tempting to attribute this increased strength to formation of the ordered phase Ni3Co,6 but there is a considerable body of evidence, recently reviewed,7 which shows that no significant order occurs in Ni-25% Co alloys. Thus, order is not responsible for the shape of the strength-composition curves in Fig 1.7,8 It is often found that as a material’s strength increases, its resistance to unstable fast fracture in the presence of a crack is reduced. The fracture toughness KQ was therefore studied, and it decreased fairly steeply with increasing Co.4 A valid KIc was obtained at the highest alloy content studied (43% Co); the value was 38 MN/m2 (∠34RSI-in.1/2) which is a reasonable toughness at the strength level shown in Fig. 1. Annealing the pure Ni at a variety of temperatures increased the grain size from an initial value of 0.12 to as much as 20 μm. These data, combined with data for cast and wrought Ni,9 are shown in Fig 2. It will be noted that the normal Hall-Petch behavior of Ni9 only extends down to about 1-μm grain size, whether the material is of wrought or plated origin. Below about 1-μm grain size, the yield strength becomes virtually independent of grain size.4 This behavior has been observed before in steels,10 and it has been made part of a general analysis of microstructural size behavior.11 The explanation for the behavior shown in Fig. 2 is not clear; it has been attributed to a change in the character of dislocation sources with decreasing grain size, from the sources in grain interiors which predominate at large grain sizes, to grain boundary sources1216 in small grains.9.10 Electron microscopy on deformed thin foils is consistent with this explanation,9 although other interpretations are no doubt possible. The behavior shown in Fig. 2 is also consistent with earlier reports on hardness changes with annealing in electroplates17 and with further work on Ni-Co alloys.18 Behavior of this kind may, therefore, be general in fine-grained materials, and it places a limit on the strengthening attainable by grain-size refinement. It is also worth pointing out that the toughness of the Ni-43% Co alloy (38 MN/m2 in the as-plated condition) may be attributed to the fine grain size of this material, 0.18 μm; it has often been suggested that fracture toughness can be increased by grain-size refinement.19 This material, which exhibited non-brittle fracture in the form of ductile rupture by microvoid coalescence,4 is therefore of considerable interest as a structural material, particularly in circumstances which take advantage of electrodeposition processes, auch as joining.20 It should be emphasized that the materials investigated here were thick electrodeposits. Therefore, we can recommend their evaluation for use in structures with confidence, since the behavior is far removed from that of ’’thin’’ films and should be typical of material in structurally useful dimensions.