Materials technology

Diversity is the most noteworthy characteristic of materials. It seems sensible, therefore, to begin this section with a taxonomy. The following is taken from the Table of Contents of a standard reference work (Lynch 1975):

Ferrous Alloys
Light Metals
Aluminum-Base Alloys
Nickel-Base Alloys
Other Metals
Glasses and Glass Ceramics
Alumina and other Refractories
Composites
Polymers
Electronic Materials
Nuclear Materials
Biomedical Materials
Graphitic Materials

The list can be further expanded. For instance, composites can be subdivided into metal matrix composites, ceramic matrix composites, and polymer matrix composites. Even a cursory summary of all these technologies in a short chapter is bound to be unbalanced and, in many ways, unsatisfactory. One can scarcely hope to do more than pick out a few salient topics.

A selection principle is urgently needed. It is therefore probably useful to start with the observation that the present economic importance of the material categories is virtually inverse to their present day interest from a research perspective. Natural materials such as wood, paper, rubber, leather, cotton, wool, stone, and clay are still enormously important in the world economy, but they are declining in importance, if only because newer alternatives are increasingly available (Larson et al. 1986). The same is true of the "old" metals copper (bronze, brass), lead, zinc (pewter), tin, silver, and gold.

From the research perspective, iron and steel, too, have largely had their day in the sun. The technology of iron smelting was essentially fully developed by the 1830s. The steel industry burst into prominence after 1860, after a long accumulation of incremental improvements in furnace design and metallurgy culminated in the great innovations of Bessemer, Kelley, Mushet, Siemens, Martin, and Thomas.

The metallurgy of steel and ferro-alloys progressed rapidly for the next half century or so. However, although significant process improvements have continued since World War II, illustrated in figure 4.2, the potential of iron based alloys technology has been largely (albeit not entirely) exhausted.⁴ Newer technologies in this area include direct casting of strip (which cuts energy consumption) and wider use of high-strength low-alloy (HSLA) steels, which cuts the weight of structures such as auto and truck chassis.

Aluminium was a curiosity metal until the simultaneous invention in the 1880s (by Hall in the United States and Heroult in France) of a practical electrolytic reduction process. This process is still universal in the industry, though improved processes have been developed to the pilot stage. The problem for aluminium is that - aside for aircraft and structural components that can be produced by rolling, bending, or extrusion without machining (such as roof panels, window frames, and cans) - it is currently too expensive to substitute for steel for most uses. However, this limitation is now gradually being overcome in the auto sector, where aluminium is beginning to substitute for steel and even cast-iron (for engines). This substitution would likely accelerate in the event that light-weight battery-powered electric cars become more popular. The fact that aluminium is relatively easy to recycle (for example, cans) is a positive indicator for this development.

Since World War II, research emphasis in materials science has shifted to polymers, ultra-light composites, and special materials for limited applications such as semiconductors, supermagnets, superconductors, hard surfaces, and nickel- or cobalt-based "super-alloys." Most of these developments have been driven by military or aerospace requirements (for electronic equipment, airframes, and jet engines, for instance). In any case, copper, steel, and aluminium metallurgy - whether moribund or not - will not be considered further in this chapter. Nor will we discuss the properties or production processes of other old materials, including concrete, glass, wood, or paper.

To be sure, any of the "old" materials may enjoy a revival, in terms of research interest, because of either a new method of processing (e.g. plywood or fiber board) or a new use (e.g. the superconducting properties of certain tin alloys). But, given the necessary brevity of this chapter and the enormous scope of its coverage, it seems justified to start by eliminating this group from further consideration.

Fig. 4.2 Process improvements in metallurgy, 1953-1974 (Source: NAS/NRC 1989)