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How to Read Time Temperature Transformation Diagrams

Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson defines the terms phase and microstructure, introduces time temperature transformation diagrams, and explains how these diagrams can be used to predict the phases and microstructures present in a material.

Chocolate Fudge

If you have ever made chocolate fudge, you know that you must heat all the ingredients and let the mixture cool very slowly. After the mixture cools, you stir the mixture rapidly to form tons of tiny sugar crystals. This makes the fudge smooth and delicious.

If you instead cool the mixture quickly, the crystals will form sooner and grow much larger, making the fudge grainy. Like fudge, many of the materials we use every day are affected by how fast they are cooled.

It is important to control the cooling rate to make good fudge
fudge

The Importance of Controlled Cooling

The metals in cars and bikes, the silicon in computer chips, and the glass in windows are usually heated to very high temperatures and then cooled in a controlled manner. Materials can be cooled very quickly in a cold water bath, cooled very slowly in an oven, or even cooled to an intermediate temperature and then held for a long period of time before being completely cooled. The exact heating and controlled cooling process affects many important material properties such as strength, conductivity, and even color.

Phases and Microstructures

The same material can have very different properties depending on how it was cooled, because the cooling affects the phase and microstructure of the material.

A phase is a region in a material which is uniform at the atomic scale. One important characteristic of a phase is the way the atoms arrange in the phase. Since the atoms in a material can arrange in many different ways, one material can have many different phases.

The arrangement of atoms in two different phases
phases

The microstructure of a material is the arrangement of the phases on the microscopic scale. You can use a microscope to observe the microstructure of a material.

Two different microstructures of the same material as viewed in a microscope
microstructure

Both the phase and the microstructure of a material affect its properties. For example, some steel is magnetic whereas other steel is not, because only some phases of steel are magnetic. Likewise, the strength of steel will depend on the phases and microstructure of the steel.

Blacksmiths

We have all seen or heard of a blacksmith heating metal until it glows. Many people think that blacksmiths only heat metal to make it easier to bend. While heating does make it easier to bend the metal, heating and then cooling a metal in a controlled manner can also affect the metal's strength and ductility by changing its phases and microstructure.

The cooling rate of metal will determine the metal
hot metal

Let's consider a type of steel called eutectoid steel. When eutectoid steel is cooled very quickly from a very high temperature, it forms steel that is very hard and very brittle (meaning that it tends to shatter instead of bending).

However, if this same steel is allowed to cool very slowly, it will form a softer steel that is ductile (meaning that it tends to bend instead of shattering). This type of steel can be easily formed into metal wires.

Thus, the same steel can be hard and brittle or soft and ductile depending on how fast the steel is cooled. This difference occurs because the cooling of the steel affects the phases and the microstructure of the steel.

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