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What is Design?
Engineering design is not an art or skill : it is a cognitive or intellectual process based on knowledge---- by John R. Dixon.

Engineering design is an iterative decision-making and problem solving activity to produce the plan to convert resoures optimally into systems or devices to fulfill a specified task. In case of machine design, the resources are material , machines and wquipments involved, and labour put into, and system is the machine product.

The activity is subjected to certain constraints. Those are (1) Problem-solving constraints, designer's problem-solving capabilities, time available, laboratory or computational facilities, and (2) the problem-solution constrains, cost of the product, availability of raw materials , equipments or manufacturing facilities.

Plan is a method, or scheme of actions , or a way proposed.

“Design is that area of human experience, skill and knowledge which is concerned with man’s ability to mould his environment to suit his material and spiritual needs.”

Design is essentially a rational, logical, sequential process intended to solve problems or “initiate change in man-made things”

For the term “design process,” we can also read “problem-solving process”, which in all but its abstract forms works by consultation and consensus.

The process begins with the identification and analysis of a problem or need and proceeds through a structured sequence in which information is researched and ideas explored and evaluated until the optimum solution to the problem or need is devised.

Design was not a total process. The work of participants in the process was often compartmentalised, each having little if any input in matters which fell outside the boundaries of their specific expertise. Thus, participants explored their ideas unilaterally, with one or another participant, through virtue of their “expertise”, imposing constraints upon all others.

ALLOYS
An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance usually has different properties (sometimes substantially different) from those of its components.
Contents
• 1 Properties
• 2 Classification
• 3 Terminology
• 4 See also

Properties:
Alloys are usually prepared to improve on the properties of their components. For instance, steel is stronger than iron, its primary component. The physical properties of an alloy, such as density, reactivity and electrical and thermal conductivity may not differ greatly from the alloy's elements, but engineering properties, such as tensile strength, shear strength and Young's modulus, can be substantially different from those of the constituent materials. This is sometimes due to the differing sizes of the atoms in the alloy—larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the alloy resist deformation, unlike a pure metal where the atoms move more freely. Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. However, for most pairs of elements, there is a particular ratio which has a single melting point; this is called the eutectic mixture.

Classification:

Alloys can be classified by the number of their constituents. An alloy with two components is called a binary alloy; one with three is a ternary alloy, and so forth. Alloys can be further classified as either substitution alloys or interstitial alloys, depending on their method of formation. In substitution alloys, the atoms of the components are approximately the same size and the various atoms are simply substituted for one another in the crystal structure. An example of a (binary) substitution alloy is brass, made up of copper and zinc. Interstitial alloys occur when the atoms of one component are substantially smaller than the other and the smaller atoms fit into the spaces (interstices) between the larger atoms.
Terminology:

In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminium used as a structural building material are also alloys. The term "alloy" is sometime used in everyday speech as a synonym for a particular alloy. For example, automobile wheels made of "aluminium alloy" are commonly referred to as simply "alloy wheels". The usage is obviously indefinite, since steels and most other metals in practical use are also alloys.

Crystalline and Non-crystalline materials:
Single Crystals: Crystals can be single crystals where the whole solid is one crystal. Then it has a regular geometric structure with flat faces.
Polycrystalline Materials: A solid can be composed of many crystalline grains, not aligned with each other. It is
called polycrystalline. The grains can be more or less aligned with respect to each other.
Where they meet is called a grain boundary.
Non-Crystalline Solids
In amorphous solids, there is no long-range order. But amorphous does not mean random,
since the distance between atoms cannot be smaller than the size of the hard spheres.
Also, in many cases there is some form of short-range order. For instance, the tetragonal
order of crystalline SiO2 (quartz) is still apparent in amorphous SiO2 (silica glass.)

4. Mechanical Properties of Metals
4.1 Elastic deformation:
When the stress is removed, the material returns to the dimension it had before the load
was applied. Valid for small strains (except the case of rubbers).
Deformation is reversible, non permanent
4.2 Plastic deformation:
When the stress is removed, the material does not return to its previous dimension but
there is a permanent, irreversible deformation.
In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke's
law:
σ = E ε
That is, E is the slope of the stress-strain curve. E is Young's modulus or modulus of
elasticity. In some cases, the relationship is not linear so that E can be defined
alternatively as the local slope:
E = dσ/dε
Shear stresses produce strains according to:
τ = G γ
where G is the shear modulus.Elastic moduli measure the stiffness of the material. They
are related to the second derivative of the interatomic potential, or the first derivative of
the force vs. internuclear distance (Fig. 6.6). By examining these curves we can tell
which material has a higher modulus. Due to thermal vibrations the elastic modulus
decreases with temperature. E is large for ceramics (stronger ionic bond) and small for
polymers (weak covalent bond). Since the interatomic distances depend on direction in
the crystal, E depends on direction (i.e., it is anisotropic) for single crystals. For randomly
oriented policrystals, E is isotropic.

2 Theoretical yield point:
Theoretical yield is the maximum quantity of a product that could be formed in a
chemical reaction if all the limiting reactant reacted to form products (distinguished from
actual yield).

Imperfections in Solids
1. Point Defects
Vacancies and Self-Interstitials
A vacancy is a lattice position that is vacant because the atom is missing. It is created
when the solid is formed. There are other ways of making a vacancy, but they also occur
naturally as a result of thermal vibrations.
An interstitial is an atom that occupies a place outside the normal lattice position. It may
be the same type of atom as the others (self interstitial) or an impurity atom.
In the case of vacancies and interstitials, there is a change in the coordination of atoms
around the defect. This means that the forces are not balanced in the same way as for
other atoms in the solid, which results in lattice distortion around the defect.
The number of vacancies formed by thermal agitation follows the law:
NV = NA × exp(-QV/kT)
where NA is the total number of atoms in the solid, QV is the energy required to form a
vacancy, k is Boltzmann constant, and T the temperature in Kelvin (note, not in oC or oF).
When QV is given in joules, k = 1.38 × 10-23 J/atom-K. When using eV as the unit of
energy, k = 8.62 × 10-5 eV/atom-K.
Note that kT(300 K) = 0.025 eV (room temperature) is much smaller than typical
vacancy formation energies. For instance, QV(Cu) = 0.9 eV/atom. This means that NV/NA
at room temperature is exp(-36) = 2.3 × 10-16, an insignificant number. Thus, a high
temperature is needed to have a high thermal concentration of vacancies. Even so, NV/NA
is typically only about 0.0001 at the melting point.