MORPHOLOGICAL AND STRUCTURAL STUDIES OF AMORPHOUS FE74CR18NI8 ALLOY PREPARED BY THE ROD-MILLING TECHNIQUE

The mechanical alloying process has been used for the formation of an amorphous austenitic stainless steel (Fe74Cr18Ni8) alloy by dry milling elemental powders of b.c.c. Fe, b.c.c. Cr and f.c.c. Ni in a rod mill at room temperature. The progress of the solid state amorphization reaction for the Fe75Cr18Ni8 ternary system has been followed as a function of the milling time by means of X-ray diffraction, an optical microscope, scanning and transmission electron microscopes and differential thermal analysis. The results show that a homogeneous amorphous phase of Fe74Cr18Ni8 alloy powder has been formed after 1080 ks of milling. The powder at the final stage of milling is fine in size (almost 1 mu m in diameter) and uniform in shape, with a sphere-like morphology. The thermal stability indexed by the crystallization temperature T-x of Fe74Cr18Ni8 amorphous alloy shows that the amorphous austenitic stainless steel is very stable up to temperatures as high as 1240 K. Above this temperature the amorphous phase is transformed to an ordered phase of f.c.c. Fe74Cr18Ni8 (the typical structure of austenitic stainless steel). The enthalpy change of crystallization, Delta H-x, is measured as -16.88 kJ mol(-1).