When describing the structure of a material, we make a clear distinction between its crystalstructure and its microstructure. The term crystal structure is used to describe the averagepositions of atoms within the unit cell, and is completely specified by the lattice type andthe fractional coordinates of the atoms (as determined, for example, by X-ray diffraction).In other words, the crystal structure describes the appearance of the material on an atomic(or ) length scale. The term microstructure is used to describe the appearance of thematerial on the nm-m length scaleThemicrostructural features of a given material may vary greatly when observed at differentlength scales. For this reason, it is crucial to consider the length scale of the observationsyou are making when describing the microstructure of a material--------Example : yield strength measures the resistance to plastic flow, which is controlled by the mechanism of dislocation motion. Dislocations are line defects whose motion is more sensitive to precipitates, grain boundaries etc. than to the lattice. The latter constitutes microstructure, as previously discussed.------Flake (lamellar)graphite is characteristic of gray iron, andcomponents such as aluminum, carbon, and siliconpromote its formation. When the coolingrate increases, the flakes get finer, and their distributiontends to be interdendritic.Type A graphite in a hypereutecticgray iron is shown in Fig. 60. It has the mostdesired shape and distribution, and it is verycharacteristic of gray iron casts with high machinabilitylength of flakes -- -- types of graphiteNodular Graphite in Ductile Iron. The additionof magnesium in the inoculation processdesulfurizes the iron and makes graphite precipitateas nodules rather than flakesbulls-eye structureNodule size and shape perfectioncan vary, depending on composition and coolingrate.Temper Graphite in Malleable Iron. Tempergraphite is formed by annealing white castiron castings to convert carbon in the form ofcementite to graphite, called temper-carbon nodules.mallebleThe matrix of gray, nodular, compacted, andmalleable cast irons can be pearlitic, pearliticferritic,ferritic-pearlitic, or ferritic. The samematrix constituents can be present in white castiron, but cementite precipitates from the melt,rather than graphite, due to crystallization in ametastable system.The matrix microstructure depends on chemicalcomposition as well as on the temperatureof the eutectoidal transformation