Principles of Optical and Electron Microscopy

Abstract

The aim of this lecture is to provide participants of ETIM-95 with a guide to the techniques and procedures which enable the microstructures of the metals and alloys to completely characterized. In general metals are opaque, lustrous and relatively heavy, easily fabri-cated and shaped, have good mechanical strength and high thermal and electricty conductivity conductivity. All these properties are a consequence of the metallic bond; metal atoms have only a fiw electron in the outer shell which are shared between atoms forming and electron cloud and by coulomb attraction.Changes in the strength of this bond cause differences in optical, electrical, mechanical and thermal properties of various metals and alloys. The simple regular crystalline structures of metals result from this non-specific and nondirectional bond which holds atoms in close packed arrangements so that the pure metals, generally, have one of the face centered cubic (fcc), body centered cubic (bcc) or hexagonal close packed (hcp) structures of the fourteen crystal systems shown in Fig. 1. These crystallographic arrangements give rise to mate-rials of relatively high ductility since they are resis-tant to tensile stresses and less resistant to shearing forces. However the overall mechanical properties of met-als are controlled by "defect" structure within the cryst-allographic arrangement of atoms such as dislocations, point defects, etc. Mechanical and chemical properties can be modified by the addition of alloying elements which are used to advance in a range of commercial materials. In many alloy systems, compositions and heat treatments are selected that produces complex distributions of phases to give the required properties. To understand the response of metals and alloys to static, dynamic and cyclic stresses, various environments and temperatures, it is essential to be able to describe the "total micro-structure". For this it may be necessary to combine a knowledge of the chemical composition, crystal and defect structure, and the proportion and distribution of various phases present. This branch of science dealing with the microstructure of metals and alloys are called metall-ography.