Optical and electronic metallography

Generally speaking, the wrought and heat-treated processing of metal alloys determines their properties of use. Controlling microstructures before and after thermal, thermochemical or thermomechanical treatments is essential to mastering their effects and controlling the process.

These processes are based on two types of data, the most common methods of measurement being :

• analysis of physico-chemical transformation points (thermal, dilatometric and magnetic analyses) [M 90] ;

• analysis of microstructures (or constituent topography: shape, phase dispersion) by optical or electron microscopy, and associated techniques, detailed in this article.

Optical metallography is the most widely used technique for observing microstructures. As metals do not transmit visible light at common thicknesses, special preparation is required. This involves polishing and then etching the surface to be observed.

Structural analysis is topographic, but micrography is not the only way to map the shape of structural components. Any systematic, point-by-point analytical exploration of a physical or mechanical property can map the properties of the structure's constituents. A common example is analysis by scanning microprobe or scanning electron microscope. In all cases, a particular property modulates the signal reflected or transmitted by the metal.

The almost universal use of a digital camera coupled to a microscope enables images to be processed by software and, to a certain extent, the semi-automatic characterization of microstructures. By motorizing the vertical axis of an optical microscope table, 3D reconstruction of microstructures is made possible by image analysis coupled with controlled table movement.

The application of these techniques leads to metallographic knowledge that can be used to optimize the choice of materials, as well as their treatment, which is decisive in maximizing the efficiency of the product-process-material triptych, and, ultimately, the suitability of the product, from both a technical-economic and environmental point of view.