Types of solid solution and Hume-Rothery rules

1. Solid Solution alloys
C.Karthika
AP/Physics
Kongu Engineering College

2. Alloy
❖ Metals in their pure form are seldom used in
engineering applications. Most of the useful metallic
materials are combinations of metals known as alloys.
❖ An alloy is any combination of two or more elements
that results in a substance possessing metallic
properties.
❖ Elements may combine in different ways to form
alloys. The components of the alloys are usually
completely soluble in the liquid state.
❖ In solid state the elements may form mechanical
mixtures, solid solutions or intermediate phases.

3. ❖Solvent represents the element
or compound that is present in
the greatest amount
❖Solute is used to denote an
element or compound present in
a minor concentration.
❖In Alloys: The element which is
present in the largest proportion
is called the base metal
(solvent), and all the other
elements present are called
alloying elements (solute).
Solution

4. mechanical mixtures

5. Mechanical mixtures
❖Mechanical mixtures are formed when the
two elements are completely insoluble in
the solid state.
❖For example, lead is insoluble in iron. The
alloy of lead and iron is an intimate
mechanical mixture of the components
where each component retains its own
identity, properties and crystal structure.

6. Intermetallic compounds
❖ Intermediate structures formed between two metals are
neither the parent metals nor like an alloy
❖ If two elements have high difference in electronegativity,
they tend to from a system called intermetallic compound.
❖ Intermetallic compounds like MgSe, PbSe, Mg2Si, Cu2S are
cubic
❖ whereas NiAs, MnSe, CuSn are hexagonal

7. Solid solution alloy
❖ A solid solution is a solid-state solution of one or
more solutes in a solvent
❖ A solid solution forms when, as the solute (impurity) atoms
are added to the host material, the crystal structure is
maintained and no new structures (phases) are formed.
❖ This is analogy with a liquid produced as the molecules
intermix, and its composition is homogeneous throughout.
❖ A solid solution is also compositionally homogeneous; the
impurity atoms are randomly and uniformly dispersed within
the solid.

8. Types of Solid solution
Depending upon the the places occupied by the
solute atoms, solid solutions could be either
substitutional or interstitial -Point defects

9. ➢ Interstitial solid solution
Interstitial solid solutions form for relatively small
impurity atoms that occupy interstitial sites (voids)
among the host atoms.
⚫ For metallic materials that have relatively high atomic
packing factors, these interstitial positions are relatively
small.
⚫ Consequently, the atomic diameter of an interstitial
impurity must be substantially smaller than that of the
host atoms. Normally, the maximum allowable
concentration of interstitial impurity atoms is low (less
than 10%). Even very small impurity atoms are
ordinarily larger than the interstitial sites, and as a
consequence they introduce some lattice strains on the
adjacent host atoms.
⚫ Carbon forms an interstitial solid solution when added
to iron (steel); the maximum concentration of carbon is
about 2%. The atomic radius of the carbon atom is
much less than that for iron: 0.071 nm versus 0.124 nm

10. Hume-Rothery rules
(Interstitial solid solution)
1. Atomic radius: Solute atoms should have a
smaller radius than 40% of the radius of
solvent atoms.
2. Electronegativity: The solute and solvent
should have similar electronegativity.
3. Valence factor: two elements should have the
same valence. The greater the difference in
valence between solute and solvent atoms, the
lower the solubility.

11. Substitutional solid solution
Solute or impurity atoms replace or substitute
for the host atoms.

12. Hume-Rothery rules (Substitutional solid solution)
Example : copper & nickel
Atomic radii for Cu and Ni are 0.128 and 0.125 nm, respectively;
both have the FCC crystal structure; and their electronegativities are 1.9 and 1.8. Finally,
the most common valences are 1 for Cu (although it sometimes can be 2) and 2 for Ni.
1. Atomic size factor - Appreciable quantities of a solute may be accommodated in
this type of solid solution only when the difference in atomic radii between the
solute and solvent atom types is less than 15% Otherwise the solute atoms
will create substantial lattice distortions and a new phase will form.
2. Crystal structure – The crystal structures of solute and solvent must be similar.
3. Electronegativity - The solute and solvent should have similar electronegativity. If
the electronegativity difference is too great, the metals tend to form
intermetallic compounds instead of solid solutions.
4. Valences – Complete solubility occurs when the solvent and solute have the
same valency. The solubility of a metal with higher valence in a solvent of
lower valence is more compare to the reverse. Zn is more soluble in Cu than
Cu soluble in Zn.

13. Ordered Solid solution -atoms of solute occupy certain
preferred sites in the lattice of the solvent.
In Cu-Au systems, Cu atoms occupying face Centred
sites and Au atoms occupying the corner sites of the fcc
unit cell.
Ordered & disordered Substitutional solid solution
Disordered Solid solution - the atoms of the solute are
present randomly in the lattice of the solute

14. Is alloy a compound?
What is the difference between solid
solution and mechanical mixture?
No. In alloys metals and non metals do not chemically
react with each other. Alloy is just a physical mixture. Alloy retains
the properties of the constituent elements
Solid solution: Completely dissolved - homogeneous mixture
Mechanical mixture: Just mixed not completely dissolved -
heterogeneous mixture
Questions

15. Why steel is so strong?
Usually materials deforms by the movement of
dislocations. The carbon interstitials make Steel Stronger by
fully or partially blocking the movement of dislocations
What are the advantages of solid solution ?
Formation of solid solution usually cause increase of electrical
resistance and mechanical strength and decreases
Brass, Bronze and Steel - Classify them
based on solid solution type.
⮚ Brass, Bronze- substitutional solid solution
⮚ Steel- interstitial solid solution

16. Thank you………..