Flow stress size effect in thin sheet metal with few grains across the thickness

Abstract

This report aims to identify the effects of heating on the microstructure of a 0.5mm thick 99.9% pure copper sample. On top of that, the effects of how grain size and grains across the thickness affects the mechanical properties of the copper will be addressed. Last but not least, the flow stress size effect of the material with different grains across the thickness would be explored in this report as well.

The copper samples were first machined into dog bone shapes with dimensions of gauge length 55mm and gauge width 10mm to facilitate future tensile testing and the machined specimens went through a range of annealing conditions to alter the microstructure of the specimen. Through a series of polishing and a chemical etching process, the microstructure of the specimens which underwent different annealing conditions was analyzed. A final tensile test was conducted to determine the mechanical properties of the varied annealed copper specimen.

Through the findings of the experiment undertaken, the author was able to conclude a few correlations. With an increased in annealing condition, the grain size of the copper specimen grew in size. Furthermore, the mechanical property of the thin copper specimen depends largely on the grain size and the thickness to grain size ratio. An inverse relationship was subsequently observed between the mechanical property and the grain size of the specimen. On the other hand, a direct relationship was observed between the mechanical property and the thickness to grain size ratio of the specimen. The author could thus conclude that the more grains there are across the thickness of the specimen, the better the flow stress size effect. The general Hall-Petch relationship was also observed to be valid, however a critical value of 0.22 for the inverse root of the grain diameter exist whereby the Hall-Petch relationship is being modified. The author thus conclude that the Hall-Petch effect governs the basic relationship between mechanical strength to grain size of the material but this general relationship changes for materials with a few grains across the thickness.