Archive for January, 2011

Dislocation Phenomena

The Dislocation defect in crystals

In materials science, a dislocation is a crystallographic defect or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. The theory was originally developed by Vito Volterra in 1905. Some types of dislocations can be visualized as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side.

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Types of Materials

 

Metals:

Metals are elements that generally have good electrical and thermal conductivity.Many metals have high strength, high stiffness, and have good ductility. Some metals, such as iron, cobalt and nickel are magnetic.At extremely low temperatures, some metals and intermetallic compounds become superconductors.

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Time-Temperature-Transformation (TTT ) Diagram

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower.

Figure 1a. TTT Diagram

In Figure 1a the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.)

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The Casting Process Pictures

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Martensite Formation

 

Martensite is a hard, brittle form of steel with a tetragonal crystalline structure, created by a process called martensitic transformation. It is named after metallurgist Adolf Martens (1850-1914), who discovered its structure under his microscope during his metallographic research and explained how the physical properties of different types of steel were affected by their microscopic crystalline structures. Martensite commonly is found in tools such as hammers and chisels and in swords.

Figure 1 - The martensite is formed by rapid cooling

The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure. This martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms) and the parent austenite becomes mechanically unstable. At a constant temperature below Ms, a fraction of the parent austenite transforms rapidly, then no further transformation will occur. When the temperature is decreased, more of the austenite transforms to martensite. Finally, when the martensite finish temperature (Mf) is reached, the transformation is complete.

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Fe-Fe3C T-T-T Diagram

 

Fe-Fe3C T-T-T Diagram, Adapted from Callister pg. 295, Fig. 10.6

The time-temperature transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite. Above 550 C, austenite transforms completely to pearlite. Below 550 C, both pearlite and bainite are formed and below 450 C, only bainite is formed.

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Metallurgy Glossary

Metallurgy Glossary

Activity: A function of the chemical potential of a system.
Alloy: A metallic substance that is composed of two or more elements.
Austenite: Face-centered cubic iron or an iron alloy based on this structure.
Bainite: The product of the final transformation of austenite decomposition.

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Phase Diagram of Steel

Fe-Fe3C Phase Diagram, Materials Science and Metallurgy, 4th ed., Pollack, Prentice-Hall, 1988

Figure above  shows the equilibrium diagram for combinations of carbon in a solid solution of iron. The diagram shows iron and carbons combined to form Fe-Fe3C at the 6.67%C end of the diagram. The left side of the diagram is pure iron combined with carbon, resulting in steel alloys. Three significant regions can be made relative to the steel portion of the diagram. They are the eutectoid E, the hypoeutectoid A, and the hypereutectoid B. The right side of the pure iron line is carbon in combination with various forms of iron called alpha iron (ferrite), gamma iron (austenite), and delta iron. The black dots mark clickable sections of the diagram.

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