What is the Difference Between Hypoeutectic and Hypereutectic

Hypoeutectic and hypereutectic are two terms that describe the composition of alloys in relation to a special point on a phase diagram called the eutectic point. This point represents the specific composition and temperature at which a liquid alloy solidifies into a uniform mixture of two solid phases.

What is the difference between hypoeutectic and hypereutectic? Hypoeutectic refers to an alloy with a composition that falls to the left of the eutectic point on the phase diagram, whereas hypereutectic alloy has a composition that falls to the right of the eutectic point on the phase diagram.

Key Areas Covered

1. What is Hypoeutectic 
      – Definition, Features
2. What is Hypereutectic
      – Definition, Features, Structure 
3. Similarities Between Hypoeutectic and Hypereutectic
      – Outline of Common Features
4. Difference Between Hypoeutectic and Hypereutectic
      – Comparison of Key Differences
5. FAQ: Hypoeutectic and Hypereutectic
      – Answers to Frequently Asked Questions

Key Terms

Alloy, Eutectic Point, Hypoeutectic, Hypereutectic

What is Hypoeutectic

Hypoeutectic alloys are formed when the composition falls to the left of a special point on a phase diagram called the eutectic point. This simply means they contain a lower proportion of the second element compared to the ideal eutectic ratio. These alloys have good castability and weldability. The presence of a lower amount of the second element allows for a smoother flow during casting and reduces the risk of cracking during welding. Depending on the specific alloy system, hypoeutectic compositions can offer good corrosion resistance due to the presence of the primary phase.

Hypoeutectic alloys are widely used in many industries due to their specific properties. A common example is the Al-Si system, where hypoeutectic compositions are used for engine blocks, cylinder heads, and other automotive components due to their good castability, machinability, and decent strength. Some hypoeutectic copper alloys are used in making electrical applications due to their strength and conductivity.

What is Hypereutectic

Hypereutectic alloys are formed when the composition falls to the right of the eutectic point on a phase diagram.  In other words, they contain a higher proportion of the second element compared to the ideal eutectic ratio. The presence of a large volume of the hard and abrasive eutectic phase makes hypereutectic alloys excellent for applications requiring high wear resistance. Depending on the specific system, the eutectic mixture can contribute to increased strength compared to hypoeutectic alloys.

Certain hypereutectic tool steels use the hard eutectic phase to create wear-resistant cutting edges for tools. Hypereutectic aluminum-silicon alloys, which have exceptional wear resistance, are used in pistons and cylinder liners due to their ability to withstand friction and high temperatures.

Similarities Between Hypoeutectic and Hypereutectic

1. Both rely on phase diagrams to define their composition relative to the eutectic point.
2. Both hypoeutectic and hypereutectic alloys find applications in various industries.

Difference Between Hypoeutectic and Hypereutectic

Definition

• An alloy is hypoeutectic if it contains a lower proportion of the second element compared to the eutectic composition. In contrast, an alloy is hypereutectic if it has a higher proportion of the second element compared to the eutectic composition.  

Microstructure

• The microstructure of a hypoeutectic alloy consists of a matrix of the primary phase (the element that solidifies first) with embedded regions of the eutectic mixture. The amount of eutectic mixture increases as the composition moves further away from the eutectic point on the phase diagram. Meanwhile, a hypereutectic alloy has a matrix of the eutectic mixture with embedded particles of the primary phase. As the composition moves further to the right on the phase diagram, the amount of primary phase present decreases.

Properties

• Due to the presence of a lower amount of the second element, hypoeutectic alloys generally exhibit good castability, weldability, and machinability. However, hypereutectic alloys offer excellent wear resistance and potentially improved strength due to the dominance of the hard and abrasive eutectic mixture.

Conclusion

Hypoeutectic and hypereutectic refer to alloy compositions relative to a special point on a phase diagram. Hypoeutectic alloys have less of the second element and offer good castability and weldability, while hypereutectic alloys have more and excel in wear resistance.

FAQ: Hypoeutectic and Hypereutectic

1. What is the difference between hypoeutectic and hypereutectic steel?

The key difference between hypoeutectic and hypereutectic steel lies in their carbon content relative to the eutectoid point. Hypoeutectic Steel contains less carbon than the eutectoid point, while hypereutectic Steel contains more carbon than the eutectoid point.

2. What does hypoeutectic mean?

The term hypoeutectic refers to an alloy that has a composition containing less of the minor component compared to the eutectic mixture.

3. What is the difference between hypoeutectic and hypereutectic cast irons?

The difference between hypoeutectic and hypereutectic cast irons lies in their carbon content relative to the eutectic point in the iron-carbon phase diagram. Hypoeutectic cast iron contains less carbon compared to the eutectic composition, while hypereutectic cast iron contains more carbon than the eutectic composition.

4. Why use hypereutectic?

We use hypereutectic because of its properties like enhanced strength and wear resistance, better performance at high temperatures, and its dimensional stability.

5. What is the limitation of hypereutectic pistons?

The main limitation of hypereutectic pistons is their brittle nature compared to other types like forged pistons. This means they can crack or shatter under very high stress.

Reference:

1. “Hypoeutectic Alloy.” Science Direct.
2. “Hypereutectic Piston.” Wikipedia.

Image Courtesy:

1. “Eutectic system phase diagram” By Eutektikum_new.svg: *Eutektikum.gif: Dr. Báder Imrederivative work: Michbich (talk)derivative work: Wizard191 (talk) – Eutektikum_new.svg (CC BY-SA 3.0) via Commons Wikimedia