Review Problems/Short Answers (5pts each) 1. How do grain boundaries contribute

Question

Review Problems/Short Answers (5pts each) 1. How do grain boundaries contribute to the hardening/strengthening in metals? What type of hardening is it called? 2. Two parts have been made of the same material (e.g. carbon steel), one.was.formed by.cold working and the other formed by hot working. Demonstrate the differences in mechanical properties (yield strength, modulus, ductility) and microstructure you might observe between the two. Link the discussions with the knowledge about microstructure of the materials. 4. What are some of the advantages of amorphous materials compared to crystalline materials in applications? 5. What are some of the unique challenges of amorphous materials compared to crystalline materials in manufacturing? 6. Most polymer exhibits pseudoplastic behavior when melted. Give an example of manufacturing practice that take advantage of such characteristic to facilitate the process. 7. The 7000 series aluminum alloys are considered to be a high-strength high-performance type of aluminum alloys. Describe how the mechanical strength of this grade of Al-alloy is enhanced and link the discussions to their microstructures. Also, what are the challenges of the manufacturing of these alloys? 8. Do metal-matrix composites have advantages over fiber-reinforced polymer composites? Explain.

Explanation / Answer

1.Grain boundaries are basically an interface between two types of grains or crystals.It is an arrest for the dislocations or we can say there is hinderance for the dislocations movement because of grain boundaries.Since the lattice structue of adjacent grains differ in orientations.It requires more energy for a grain to travel from one part to other.Grain boundaries make it more harder for dislocations to make their way.Thus more energy is required to break a materials as impurities are trapped in grain boundaries.So strength of the material increases.This is also called Hall-Petch Strenghtening.The lesser the diameter of grains the more force required to break it.But there is a limit to this small size as well.

2.Cold working is employed within the range of recrystalline temperature of materials.So more force is needed to make a component and the material formed is work hardened .It has more strength.It has good surface finish and dimensional accuracy .But ductility for such materials is less.On the other hand,Hot working because of higher temperature(Higher than recystalline temperature) increases the ductility component of the material and less force is needed to make the material.Scaling is there and dimensional accuracy is not as good as colde worked material.Strength is also less comparitively.

4. Amorphous materials are of short range order and softened over a range of temperatures.They do not have sharp melting point but beacuse of this they have the ability to get into different shapes easily.Examples can be Garden lights,Solar wristwatches ,Solar calculators in all these cases silicon is better used in amorphous form.They are isotropic as their propeerties are independent of directions.They have good solubility also thats why they are prefferd in some drugs over crytalline forms.

5.Amorphous materials do not have a sharp melting point temperature as they are softened over a range of temperatures.So the basic problem lies in their cooling rate,Rapid solidification is needed to produce amorphos materials .This adds to the cost of manufacturing.Additive manufacturing is a favourable technology.More over its difficult to main their shape since they elastically rebound to their initial stage because of their softness.Hence,more enhance environment is needed to manufacture them.They can be made by Mechanical alloying,Ion iraadiation and rapid solidification techniques.

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