Experimental Simulation Study On Solid Solution Strengthening In Nanocrystalline Alloys Using Multilayered Films

For coarse-grained materials,there have been many deep studies and mature theories to explain the solid solution effect,but the existing theories are not necessarily suitable for nanocrystalline alloys.There are still controversies over whether solid solution strengthening exists in nanocrystalline alloys and how large the effect is.The main origin of these controversies is when the solute content in nanocrystalline alloys changes,other microstructural factors such as grain size,grain boundary composition,and grain boundary width will also change.The intercorrelation of these microstructural factors make it hard to reveal the individual solid solution effect in nanocrystalline alloys.This thesis designs and prepares two series of Cu-Ti alloy nano-multilayered films consisting of different low-alloy-content crystalline layers and same high-alloy-content amorphous layers.In such multilayered films,due to the fact that the grain size is insensitive to composition when crystalline alloys nucleate and grow on amorphous layers,two crystalline layers with different solute content grow as columnar crystals with the same diameters;meanwhile,due to the interruption of amorphous layers,the heights of columnar crystals in crystalline layers were limited and equaled to the layer thicknesses.Through the independent change of composition and grain size of columnar crystals in crystalline layers,the individual solid solution effect in nanocrystalline alloys was revealed.The results show that the increase of Ti content from 0.3 to 5.3 at.% results in a constant hardness enhancement of 1.5±0.2 GPa,as the height of the 10-nm-diameter columnar crystals decreases from 188 nm to 6 nm.This indicates that the solid solution effect is independent of grain size whether the deformation behavior is dominated by intragranular dislocation mechanism or dominated by grain boundaries.