SEARCHING FOR STRUCTURAL AMORPHOUS STEELS

During the last four decades amorphous metallic alloys (with neither crystalline structure nor grains and grain boundaries) with thickness in the order of micrometers have been produced. These thicknesses are required in order to reach cooling rates necessary to avoid crystallisation. Increasing the thickness of these amorphous systems (with the corresponding reduction in cooling rates) with the propose of using them as structural materials has been a constant challenge to science and materials engineering. Commonly known as Bulk Metallic Glasses (BMGs), various alloys have been studied in Pd, Au, La, Zr, Mg based systems, mostly since 1989. As a result of these studies, amorphous materials between 0.5 and 72 mm thick have been obtained and are being used for commercial products since 1992. Fe-based BMGs, first developed in 1995, have regained special interest because of their unusual mechanical and physical properties such as high strength and hardness (3000-5500 MPa and 850-1370 Hv respectively), excellent magnetic properties, strong corrosion resistance, high thermal stability and, in same cases, low material cost. These systems are also called Structural Amorphous Steels (SASs) and a thickness of 16 mm can be obtained nowadays when using a composition of Fe41Co7Cr15Mo14C15B6Y2 (at.%). In the present work we make a brief description of state of the art in BMGs and SASs; their mechanical properties and technological employments are commented. We also analyze the theoretical models and empirical rules used to obtain BMG. Finally, we present our preliminary results when using the FeCoSiBNb amorphous system with a thickness of 2.5 mm. X-ray diffraction, differential thermal analysis and Vickers microhardness were employed as characterization methods.