The role of interface volume fractions in the nanocrystalline to amorphous transition in fully dense materials

Abstract

A modified tetrakaidecahedral grain shape model is proposed for a precise geometrical description and calculation of the volume fractions of grain interiors, grain boundaries and triple junctions, at very small grain sizes (less than 30 nm). The model predicts a smooth transition from crystalline to non-crystalline structure through intercrystalline components, especially triple junctions at the final stage. The prediction was examined by investigating the Young's modulus and hardness of Ni–P electrodeposits covering transitional structures (6.6 nm, 6.1 nm, 4.1 nm and amorphous). The values of both properties were found to decrease gradually with shrinking grain size towards the amorphous structure. This observation can be attributed to the intrinsically lower Young's modulus of, and those deformation mechanisms activated and facilitated by, the significantly increased amount of intercrystalline structure during the transition. These results are in agreement with the prediction of the model.