Structural Constituent

a constituent of the micro-structure of an alloy characterized by a homogeneous average chemical composition and a uniform distribution and shape of the grains forming the constituent of the alloy’s phases. A structural constituent manifests itself during a phase transformation, and the microstructure can be made up of one or more such constituents. Thus, in eutectoid steel there is only one structural constituent after annealing, namely, pearlite, while hypereutectoid steel has two—pearlite and secondary cementite.

A structural constituent can be formed by crystallites (grains) of one or several phases. Thus, martensite in hardened steel and ferrite in pure iron are structural constituents consisting of the grains of a single phase; pearlite in steel and ledeburite in white iron are structural constituents formed by crystallites of different phases. The grains of the same phase in an alloy may be found in different structural constituents; for example, cementite in hypereutectoid steel enters into pearlite and is also present as secondary cementite. The properties of an alloy depend not only on the relative quantity of its phases but also on the presence of specific structural constituents, that is, on the shape, degree of dispersion, and arrangement of the grains of various phases.