Chemistry (Metallic glasses) - Term Paper Example

Chemistry Metallic Glasses In the pre-industrial era, 11 metals were extensively used; Au, Ag, Cu, Fe and Hg were some of the common elements used. The progress of refinement and metallurgical techniques have contributed to the increase in utilized metals to about 75. Metallic materials in their bulk form are limited to their crystalline structure due to the principles of phase transformation and solidification (Inoue, Wang and Zhang, 1). The suppression of the transformation of the super cooled liquid to crystalline phase due to the addition of novel alloy components alloys for the production of glass (disordered) metallic alloys with unique and superior characteristics.

In particular multicomponent metals, the phase transformation from super cooled liquid to crystalline phase is retarded by orders of 8 to 9. The high glass forming ability, castability, printability and unique characteristics of glassy alloys extend applications when compared to the conventional crystalline alloys. Table 1 gives examples of glassy alloys and when they were first reported.The highest glass forming ability is achieved at compositions near the multicomponent eutectic point with the lowest melting point and is evaluated based on thermal stability parameters of reduced glass transition interval of super cooled liquid region (Inoue, Wang and Zhang, 3).

It is reported that the tensile strength of bulk glassy alloys is higher than that of crystalline alloys of the same young’s modulus whereas their young’s modulus is a third as high as crystalline alloys of the same tensile strength. Alloy systems are classified as ferrous and nonferrous (Table 3 and Table 1). Table 1 Critical diameter and preparation method for bulk glassy alloy systems with critical diameters of over 1 cm.The discovery and development of bulk glassy alloys were based on research on alloys that bore the three component rules for the stabilization of super cooled liquid.

The first rule is the need for a multicomponent that consists of more than three elements. The second is components having atomic mismatches greater than 12% of the three elements, and the third is the negative heat of mixing among the three major components.An excellent linear relationship satisfying Hooks law has also been observed between tensile strength and young’s modulus (figure 3). The twist angle and shear stress has been found to be 3 times higher than for crystalline alloys. The bulk glassy alloys have high strength reflected by fatigue endurance limits after 107cycles ranging between 0.15 to 0.38 and dynamic durability (Inoue, Wang and Zhang, 4).

Figure 1 Relationship between tensile strength and Young’s modulus for typical transition metal base bulk glassy alloys. The data of conventional crystalline alloys are also shown for comparisonThe endurance of glassy alloys when used as machinery parts has been found to be longer compared to convention tool steel. From figure 1, it is observed that Ni-based alloy gears are more durable and maintain shape after 2500 hours. Wear resistance is attributed to the smoothness at the surface without grain boundary, the highly homogenized structure and corrosion resistance (Inoue, Wang and Zhang, 5).

Figure 2 Wear and shear of conventional steel gear and glassy alloy gears (Inoue, Wang and Zhang, 5)Alloys that contain Nano-crystalline, Nano quasi-crystalline or micro scale dendrites have been shown to exhibit elongation under uniaxial compressive deformation. They also have improved yield strength and ductility. However, dendritic crystalline phase glassy alloys have reduced yield strength. Iron based composite bulk glassy alloys exhibit improved soft magnetic properties as in the case of Nanoscale bcc Fe phase dispersion state and hard magnetic properties as observed in Nano scale bct-FePt phase dispersion (Inoue,6).

Porous glassy alloys show a decrease in specific weight but have an increased homogenous dispersion of spherical pores within the glassy matrix.Table 2 Bulk glassy alloy systems and year reported (Inoue, Wang and Zhang, 2)ReferencesInoue, A., X. M. Wang, and W. Zhang. "Developments and applications of bulk metallic glasses." Rev. Adv. Mater. Sci 18 (2008): 1-9.