CHAPTER 7. MECHANICAL PROPERTIES OF MATTER 7.4
Introduction of Impurities ESCDX
Most pure metals are relatively weak because dislocations can move easily within them.
However, if impurities are added to a metal (e.g., carbon is added to aniron sample),
they can disturb the regular structure of the metal and so prevent dislocations from
spreading. This makes the metal stronger.
Alloying ESCDY
An alloy is a mixture of ametal with other substances. In other words, alloying involves
adding impurities to a metal sample. The other substances can be metalor non-metal.
An alloy often has properties that are very different to the properties ofthe substances
from which it is made.The added substances strengthen the metal bypreventing dis-
locations from spreading. Ordinary steel is analloy of iron and carbon. The carbon
impurities trap dislocations. There are many types of steel that also include other met-
als with iron and carbon. Brass is an alloy of copper and Zinc. Bronzeis an alloy of
copper and tin. Gold and silver that is used in coins or jewellery are alsoalloyed.
Tempering ESCDZ
Tempering is a processin which a metal is melted then quickly cooled. The rapid
cooling is called quenching. Usually tempering is done a number oftimes before a
metal has the correct properties that are neededfor a particular application.
Sintering ESCEA
Sintering is used for making ceramic objects, among other things. Inthis process a
substance with a very high melting point is heated so that its particlesstick together.
The resulting product isoften very pure and canbe formed into a desiredshape during
the process. Unfortunately, sintered products arebrittle.