1. METALLURGY & MATERIALS SCIENCE
Course Objective:
To understand the basic fundamentals of Material science and
Physical metallurgy. The basic concepts to be taught will help for
the improvement, proper selection and effective utilization of
materials which is essential to satisfy the ever increasing demands
of the society.
unit - I
Structure of Metals and Constitution of
alloys
Learning Objective: To know the basic concepts of bonds in metals
and alloys. To understand the basic requirements for the formation
of solid solutions and other compounds.
Bonds in Solids – Metallic bond - crystallization of metals, grain and
grain boundaries, effect of grain boundaries on the properties of
metal / alloys – determination of grain size. Necessity of alloying,
types of solid solutions, Hume Rotherys rules, intermediate alloy
phases, and electron compounds.
1.1 Bonds in Solids – Metallic bond
Atomic bonding in solids
Primary Inter-atomic Bonds
2. Ionic Bonding
The bond when one of the atoms is negative (has an extra electron)
and another one is positive (has lost an electron). there exist a
strong, direct Coulomb attraction. An example of such bonding is
NaCl. In this molecule, there are more electrons around Cl, therby
forming Cl-
and less around Na, which results in the formation of
Na+
. Ionic bonds are the strongest bonds. In real solids, the ionic
bonding is usually combined with covalent bonding.
Covalent Bonding
In covalent bonding, the electrons are shared between the
molecules, to saturate the valency. The simplest example is
H2 molecule, where the electrons spend more time in between the
nuclei than the outside, thus producing bonding.
Metallic Bonding
In case of metals, the atoms are ionized, losing some of the
electrons from the valence band. Those electrons form an electron
sea, which binds the charged nuclei in place, in a similar way that
the electrons in between the H atoms in the H2 molecule bind the
protons.
Secondary Bonding (Van der Waals)
Fluctuating Induced Dipole Bonds
Since the electrons may be on one side of the atom or the other, a
dipole is formed: the + nucleus at the center, and the electron
outside. Since electron moves, the dipole fluctuates. This
fluctuation in the atom A produces a fluctuating electric field that is
3. felt by the electrons of an adjacent atom, B. Atom B then polarizes
so that its outer electrons are on side of the atom closest to the +
side (or opposite to the – side) of the dipole in A. This bond is called
van der Waals bonding.
Polar Molecule-Induced Dipole Bonds
A polar molecule like H2O (H atoms are partially +, O is partially -),
will induce a dipole in a nearby atom, leading to bonding.
Permanent Dipole Bonds
This is the case of hydrogen bond in ice. The H end of molecule is
positively charged and can bond to the negative side of another
dipolar molecule, like the O side of the H2O dipole.
1.1.1 Crystallization of metals
One of the superlative properties of all metals is crystallization, and
on the exact condition of the crystals in any metal often hangs
every degree of usefulness.
All metals have been found to crystallize on solidifying from the
molten state; even after the severest strains and deformations,
crystal nature persists. Deformed crystals give birth to a new
growth of crystals, if the temperature will allow the readjustment.
Maximum ductility usually accompanies well-grown crystals;
maximum strength accompanies the first incipient growths of a
newly disseminated structure from some previous formation,
probably through surface forces. Brittleness and the weakness
commonly are developed through the coalescence between large
crystals of the material by some substances of friable nature;
impurities and overheating thus form an aggravating combination.
4. An amorphous state is a plausible assumption to account for
cement between grains, the debris along slipped cleavage planes
and colloidal metal solidified by pressure instead of fusion. The
study of this condition is now assuming notice and promises
brilliant results for science and industry.
There are two general stages of phase transformation
(crystallization) process – nucleation and growth:
Nucleation
The process of formation of stable crystallization centers of a new
phase is known as Nucleation.
It can occur by either homogeneous or heterogeneous mechanism,
depending on the value of undercooling of the liquid phase.
Presence of foreign particles or other foreign substance in the
liquid alloy allows to initiate crystallization at minor value of
undercooling. This is known as heterogeneous nucleation.
If there is no solid substance present, undercooling of a hundred
degrees is required in order to form stable nuclei or “seeds”
crystals, providing following crystal growth (homogeneous
nucleation)
