HUME-ROTHERY RULES, Study notes of Metallurgy

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HUME-ROTHERY RULES

Presented by :- Er. Saurabh Malpotra

Hume-Rothery rules

• Hume-Rothery (1899-1968) was a metallurgist who studied the alloying of metals. His research was conducted at Oxford University where in 1958, he was appointed to the first chair in metallurgy.
• The Hume-Rothery rules are a set of basic rules describing the conditions under which an element could dissolve in a metal, forming a solid solution. There are two sets of rules, one which refers to substitutional solid solutions, and another which refers to interstitial solid solutions.

Interstitial Solid Solution Rules

1. Solute atoms must be smaller than the pores in the

solvent

lattice.

2. The solute and solvent should have similar

electronegativity.

In contrast to intermetallic and compounds, solid

solution in general are

• Easier to separate,
• Melt over a rang in temperature,
• have properties that are influenced by those of solvent and

solute,

• Usually show a wide range of composition so that they are

not expressed by a chemical formula

Hume-Rothery Rule 1:Atomic Size

Factor (the 15%) Rule.

• Extensive substitutional solid solution occurs only if the relative difference between the atomic diameters (radii) of the two species is less than 15%. If the difference > 15%, the solubility is limited. Comparing the atomic radii of solids that form solid solutions, the empirical rule given by Hume- Rothery is given as:

Thermal Equilibrium Diagrams

• Definition:- A diagram which shows variations of phases of

a metal or ally with respect to changes in temperature is

called as phase equilibrium diagram.

• A system is at equilibrium if its free energy is at a minimum,

given a specified combination of temperature, pressure and

composition.

• The (macroscopic) characteristics of the system do not

change with time — the system is stable.

Phase Equilibrium: Solubility Limit

• Solution – solid, liquid, or gas solutions, single phase
• Mixture – more than one phase
• Solubility Limit:

Maximum concentration for which only a single

phase

solution exists.