Crack propagation and mechanical properties of electrodeposited nickel with bimodal microstructures in the nanocrystalli
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he article focuses on the fatigue performance after a moderate heat treatment of nanocrystalline (nc) nickel, which leads to the formation of a bimodal microstructure in the nc to ultrafine grained (ufg) regime. Electrodeposition was used to produce nc macro nickel samples with grain sizes of about 40 nm for mechanical testing. The thermal stability of the material as well as the influence on the mechanical properties and the fatigue crack propagation behavior was investigated. The results of tensile and fatigue tests are discussed in respect to the chosen production method and boundary conditions. In this context, the influence of the bath additives used during the plating process was investigated and rated as the major challenge for a further improvement of the thermal stability and mechanical properties of the material. Finally, a co-deposition of nickel and metal oxides with enhanced thermal stability is presented.
I. INTRODUCTION
In the last decades, many articles about mechanical properties and fatigue behavior of nanocrystalline and ufg materials have been published.1–6 Most researchers argue that special grain size distributions and textures improve the mechanical behavior of the material, but they neglect the synthesis method and boundary conditions under which the material was produced.3,6 The microstructure of materials is thereby not sufficiently described by the grain size distribution and texture analysis. Even more crucial for the mechanical properties or the fatigue performance of the material are other strong influencing factors like dislocation densities or amount and type of impurity atoms and especially where they are located in the material. The keyword to this topic is segregation at grain boundaries, which has been known for many years to have a strong impact on the strength of the boundaries and, therefore, on the macroscopic material properties.7,8 There are articles, of course, which describe the influence of the production process on the microstructure, respectively, grain size and texture9 and sometimes also on mechanical properties as in the case of high pressure torsion experiments,10 but often, the correlation between grain size, dislocation densities, production technique, and contained impurity atoms is not mentioned. This is especially problematic for production techniques which are very sensitive to a slight modification of any process Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.353
parameter or which are strongly dependent on the use of additives during the processing11 to obtain nc grain sizes like the electrodeposition process. Unfortunately, this leads to an incorporation of impurities in the sample material, although the amount of the incorporated impurities is often well below 0.04 wt%.12 The problem is that without such impurities, it is nearly impossible to reduce the grain size down to the nc regime and prevent grain growth, even at room temperature. Electrodeposition is thereby one o
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