h. r. shahverdi

Achieving extreme hardness in the newly synthetic steel formed by converting from initial amorphous state to subse-quent crystalline structure –named as devitrification process- was studied in this research work. Results of TEM observa-tions and XRD tests showed that crystallized microstructure were made up four different nano-scale phases i.e., α-Fe, Fe 36 Cr12 Mo10, Fe 3 C and Fe3 B. More, Vickers hardness testing revealed a maximum hardness of 18.6 GPa which is signifi-cantly harder than existing hardmetals. Detailed kinetic and structural studies have been proof that two key factors were contributed to achieve this extreme hardness; supersaturation of transition metal alloying elements (especially Nb) and also reduction in the structure to the nano-size crystals.

The term NanoSteels is called to some special steels consisting of nanosize phases (i.e. ferrite, cementite, and austenite), grains, and carbides (e.g. vanadium and M2C) produced by nanotechnology. It is proof that exotic physical, mechanical, and magnetic properties can be obtained from nanostructured steels. Fabrication methods of nanostructure steels can be divided into two main categories, SPD (severe plastic deformation) and melt base (crystallization from amorphous state) methods. Among all of the severe plastic deformation techniques (i.e. ECAP, HPT, and ARB), equal channel angular pressing (ECAP) is especially attractive because it can economically produce bulk of ultra- fine grain (UFG) materials. On the other hand, crystallization from amorphous state in bulk metallic glasses is a unique approach toward the mass production of nanostructure ferrous alloys. In the experimental process, crystallization of α – Fe phase during annealing process of Fe55Cr18Mo7B16C4 bulk amorphous alloy has been evaluated by X- ray diffraction and TEM observations. It is known from the TEM observations that crystalline α – Fe phase nucleated in the structure of the alloy in an average size of 10 nm and completely mottled morphology.