TABLE OF CONTENTS This chapter is concerned with the material changes that occur beneath the surface, ie the subsurface zones or internal features. The properties of these sub-surface zones are different from those of the bulk material and hence, being different, are referred to as altered material zones (AMZ) or layers. Figure 5.1 shows a schematic section through a surface section. The external features and how they can be specified have been dealt with in Chapter 4. In this chapter we are concerned with the internal features beneath the air/metal interface.
In the schematic of Figure 5.1, three regions can be seen. Firstly, an uppermost region, which has reacted with the air and consists of adsorbates and oxides as well as any deposits. Secondly, there is the region beneath this where the material that has been influenced and altered in some way by the manufacturing unit event (ie the AMZs). Thirdly, there is the bulk material that remains unchanged by the unit event. Usually the bulk material/AMZ transformation is gradual and often there is no clear demarcation line. For example, if the unit event involves plastic deformation, the AMZ will be work-hardened. The hardness will be a maximum at the surface and it will gradually decay to the bulk value. On photomicrographs, no clear transformation line can be seen between the AMZ and bulk. There are many types of alteration and, depending upon their nature, they can enhance or reduce service performance. The...
