How to produce a desired bimodal microstructure for optimized mechanical properties: Investigation of the mechanisms of abnormal grain growth in pulsed electro-deposited nickel

Abstract

Abstract Mechanical properties of metallic materials are often optimized by a specific heat treatment to adjust a required grain size. Thereby solute atoms, impurities or precipitates play an important role due to their retarding forces on the grain boundary movement. However, they not only stabilize small grain sizes during a heat treatment, it is also suggested that they introduce abnormal grain growth whereby for a small amount of grains the grain size increases tenfold and more. On the one hand abnormal grain growth impedes the adjustment of a required grain size; on the other hand it can be used to introduce bimodal grain structures which are known to combine opposing mechanical properties such as a high toughness and a high ductility. Therefore, the mechanisms of abnormal grain growth are investigated by varying the content of additives during the deposition process. Particle pinning is suggested as the mechanism responsible for abnormal grain growth at least in the initial stage, while a second phase may introduce extraordinary cubic grains growing abnormally in the late stage.