[nanoPost] Amorphous/nanocomposite steels

Company USA

The Company utilizes an entirely analogous but different solid state processing route, called glass devitrification, to produce steels with bulk nanoscale microstructures. In this case, the supersaturated solid solution precursor material is a supercooled liquid, called a metallic glass. The glass structure itself is a very high strength structure since entire classes of defects including 1-d dislocations and 2-d grain and phase boundaries are eliminated. Unfortunately, the glass is not a defect free material, since it contains a large fraction of free volume defects so the full strength of the iron atomic bond is not realized. Upon subsequent heating, the metallic glass precursor transforms into multiple solid phases through devitrification. In glass forming steel systems, depending on the specific composition, the crystallization temperatures usually is in the range from 450°C to 750°C and the enthalpy of the glass to crystalline transformation varies typically from -75 J/g to -200 J/g. Since the glass forming steels commonly melt from 1000 to 1300°C, this means that the glass devitrification occurs at low fractions of the melting temperature (typically 0.4 to 0.7 Tm) where diffusion is limited and where the driving force, due to the metastable nature of the glass state, is extremely high. Thus, during devitrification a very high nucleation frequency occurs with limited time for grain growth before impingement between neighboring grains.

Analogous to the characteristic microstructures developed from the eutectoid transformation, the devitrified steels form specific characteristic microstructures which are nanoscale. By this route it is possible to develop very stable nanostructures which resist coarsening at elevated temperatures. It is this type of novel approach which has resulted in the development of amorphous/nanocomposite steels which exhibit strength levels which exceed the existing Paradox to a significant extent (45% of theoretical).