- Introduction- Diffusion - Diffusion mechanisms - Steady state diffusion - Non steady state diffusion - Factors that influence diffusion
Refer chapter 5 of Material Science and Engineering: An introduction by William Callister.
Refer course website page U.Self study material for NPTEL videos related to above topics.
The iron-carbon phase diagram is important in engineering as it provides the basis for understanding all cast irons and carbon steels and their heat treatment. For structural and mechanical applications, steels and other alloys based on iron (the ferrous alloys) are the dominant engineering alloys. They are intrinsically stiff, strong and tough, and mostly low cost. High density is a drawback for transport applications, allowing competition from light alloys,wood and composites. The wide range of applications reflects the many classes of ferrous alloy – different chemistries combined with different process histories. Here, we focus on ferrous alloys which are predominantly iron and carbon alone.
Examples of typical applications of iron-carbon alloys are:
- Cast iron brake discs. Brake discs use sliding friction on the brake pads to decelerate a moving vehicle,
generating heat in the process.The key material properties are therefore hardness (strength) for wear resistance, good toughness and a high maximum service temperature. Casting is the cheapest way to make the moderately complicated shape of the disc. Cast irons contain typically 2-4 wt % carbon, giving a lot of the hard compound iron carbide, giving excellent precipitation hardening.
- Plain carbon steel: I-beams, cars and cans. If we had to single out one universally dominant material, we might well choose mild steel – iron containing 0.1-0.2 wt % carbon. Almost all structural steels, automotive alloys and steel packaging (beer and food cans) are made of mild steel (or a variant enhanced with a few other alloying additions). All of these applications are wrought – the alloy is deformed extensively to shape. The excellent ductility of plain carbon steel enables this, while the deformation process exploits their work hardening to give the required product strength. Ductility, toughness and decent strength are vital in a structural material.
Many of the engineering materials possess mixtures of phases, e.g. steel, paints, and composites. The mixture of two or more phases may permit interaction between different phases, and results in properties usually are different from the properties of individual phases. Different components can be combined into a single material by means of solutions or mixtures. A solution (liquid or solid) is phase with more than one component; a mixture is a material with more than one phase. Solute does not change the structural pattern of the solvent, and the composition of any solution can be varied. In mixtures, there are different phases, each with its own atomic arrangement. It is possible to have a mixture of two different solutions!
A pure substance, under equilibrium conditions, may exist as either of a phase namely vapor, liquid or solid, depending upon the conditions of temperature and pressure. A phase can be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics i.e. it is a physically distinct from other phases, chemically homogeneous and mechanically separable portion of a system. In other words, a phase is a structurally homogeneous portion of matter. When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties; a disparity in one or the other set of properties is sufficient.
There is only one vapor phase no matter how many constituents make it up. For pure substance there is only one liquid phase, however there may be more than one solid phase because of differences in crystal structure. A liquid solution is also a single phase, even as a liquid mixture (e.g. oil and water) forms two phases as there is no mixing at the molecular level. In the solid state, different chemical compositions and/or crystal structures are possible so a solid may consist of several phases. For the same composition, different crystal structures represent different phases. A solid solution has atoms mixed at atomic level thus it represents a single phase. A single-phase system is termed as homogeneous, and systems composed of two or more phases are termed as mixtures or heterogeneous. Most of the alloy systems and composites are heterogeneous.
It is important to understand the existence of phases under various practical conditions which may dictate the microstructure of an alloy, thus the mechanical properties and
usefulness of it. Phase diagrams provide a convenient way of representing which state of aggregation (phase or phases) is stable for a particular set of conditions. In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena.
oCrystallography:
It is that branch of science in which the internal structures of crystals,
their properties, external or internal symmetries of crystals are studied.
oVarious terms
associated with Crystallography.
(i)Crystal: It is a solid which constituent
atoms or molecules are arranged in a systematic geometric pattern.
(ii)Structure:
The structure implies the arrangement of the atoms within a crystal.
Other important terms – Space
lattice, Unit cell, Atomic planes, lattice parameters, Miller indices etc.
oClassification of
solids:
1.Crystalline: If the atoms or
molecules in a solid are arranged in
regular fashion, then it is known as
Crystalline solid.
2.Amorphous: When the atoms or molecules in a solid are arranged in
an irregular fashion, then it is
known as amorphous solid.
oPolycrystalline
Materials: When in a
solid material is build up of a large number of small interlocking crystals or
grains it is called polycrystalline.
oSpace lattice
& Crystal lattice:
The atoms arrange themselves in distinct pattern in space called a space
lattice. It can be considered as an infinite array of points in space, so
arranged that it divides space into equal volumes with no space excluded. In a
space lattice every point has identical surroundings.
oUnit cell &
its lattice parameters:
It is the smallest group of atoms possessing the symmetry of the crystal.
