Crystal instability in nanocrystalline materials

As the grain size in nanocrystalline materials is made ever smaller, the questions of what the smallest grain size could be and what factors influence it become highly relevant to material synthesis and application. Using extensive atomistic simulation and theoretical analysis, this paper shows that the crystalline phase instability sets the ultimate limit for grain size reduction below which amorphization occurs. The instability is caused by the combined effect of structural disorder present at grain boundaries and the internal inhomogeneous strain fields associated with solutes or impurities. A phase diagram describing the instability or crystal-to-glass transition is constructed from a Ginzburg-Landau theory based on the effects of the two types of disorders and their interactions. The mean critical grain size is shown to range from several nanometers to tens or hundreds of nanometers, depending on the impurity or solute concentration.