Is steel a composite material

Answer 1

Steel is not usually considered a composite, as it is macroscopically homogeneous.

However, some steel types, including "classical" iron-carbon steel, can be considered as metal-matrix composites, as they contain a second phase... sometimes.

For simple iron-carbon steel, cooling after high-temperature forging or heat treatment will precipitate out iron carbide (cementite, Fe3C) particles and leave a carbon-depleted iron matrix. If cooling is slow, coarse bands of iron / cementite will form, a microstructure called pearlite, which is not very hard.

If the cooling speed is increased, the pearlite will become finer (finer bands), until another composite microstructure, with more acicular patterns forms, called bainite. This is also heterogeneous, i.e. a composite of carbon-poor iron and cementite.

Going to very fast cooling (quenching) will result in a single-phase (not composite) material called martensite. Here the carbon doesn't have time to "exit the iron", and this martensite phase is very hard, but also normally too brittle. Hence, it is normally re-heated to 200-400°C, a process called "tempering", where again some cementite precipitates out: it becomes a composite again, yielding a somewhat softer, but much tougher material.

Alloyed steels (i.e. with other elements than just iron and carbon) strongly vary in behaviour:

• Normal non-magnetic "austenitic" stainless steels are single-phase, not composites.
• Tool steels (high carbon + carbide-forming alloying elements) are definitely composites. They form a lot of hard particles, such as chromium carbides, that impart good resistance against wear.
• Magnetic Fe-Cr stainless steels may be essentially single-phase (very low carbon, better corrosion resistance) or also contain carbides (higher carbon, better strength)
• So-called "maraging" (martensite aging) steels may achieve both high strength and good corrosion resistance. Here, strength is imparted by an "aging" treatment around 500°C, to precipitate out so-called "intermetallic" particles, making it again a "composite material".

These considerations also apply to many other metallic alloys, based on metals such as aluminium, titanium or nickel. In most cases, the strongest variants are engineered to be "microcomposites" or "nanocomposites", i.e. they precipitate out intermetallic particles during heat treatment.

The reason behind such engineering is that the particles block dislocations, which are responsible for plastic deformation of metals. For each alloy, there is an optimum heat treatment to achieve the best "blocking ability" for dislocations, and thus the highest strength.