A hierarchy of metallic engineering would reflect general-use, non ferrous metals as the first rung of the ladder. These materials, predominantly copper and aluminum and their alloys (e.g., brass and bronze), are easily formed and highly serviceable in many natural environments. Next up would be steels and cast irons. These ferrous, iron-carbon-based alloys are the most used metals for general construction. While the first two levels of the hierarchy constitute the bulk of metals usage, they have very little high-temperature capability and corrode in many environments. A broad step over the first two levels is the category of stainless steels. These iron-based metals have major alloying additions of chromium and nickel with other elements being important in some grades. These premium alloys span a wide range of chemistries and properties, serving extensive applications in both consumer durables and capital goods markets.
At the top of the metals hierarchy are the high-alloy materials that are technologically sophisticated in their make-up and manufacture and are specialized in their properties and applications.
These materials are often referred to, either collectively or in subgroups, as high-performance alloys, high-technology alloys, corrosion-resistant alloys, heat-resistant (or high-temperature) alloys and super alloys. They are the elite of metallic engineering materials. They deliver the needed service performance in the most demanding of applications, where other metals fall short. This group of metals can be found at the forefront of mankind’s unending progression in such critical areas as energy production and conservation, aerospace transportation and exploration, efficiency of industrial processes, and protection of the environment.
These high-performance alloys span a range of chemical compositions. They are often nickel- or cobalt-based and may contain exotic elements such as molybdenum, tungsten, and niobium. Some are as simple as the two-thirds nickel/one-third copper in the original alloy (alloy 400/UNS N04400*) in the category, and some are so some complex that over a dozen elements have specified limits (e.g., alloy 718/UNS NO7718).