Question

Would you expect an amorphous piece of solid polyethylene (PE) to be isotropic or anisotropic? Briefly explain............(2 points) Briefly explain why, upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases. What is this structure called? What kind of effect does this structure have on the melting point of the alloy and why? (5 points)

Answer 1

1) An Amorphous solid polythene would be Isotropic because

In the case of amorphous solids, the value of physical properties does not change with the change of directions. The property remains the same in all the directions. It is called isotropy.

2) upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases due to eutectic microstructure forms as a result of the diffusional partitioning of elements. According to the diffusional ledge mechanism, the partitioning of elements occurs through the shortest path of diffusion. The alternating layers structure fulfills the shortest diffusion path.

And the structure is called is lamellar structure notall eutectic systems produce lamellar structures. Only if the interfacial energy between the two solids (S1/S2; S1 and S2 denote Solid 1 and Solid 2) is lower than those of S1/Liquid and S2/Liquid a lamellar eutectic structure is formed.

The effect of intensive shear on the solidification behavior and microstructural evolution of binary Zn-Al alloys is presented at hypoeutectic, eutectic, and hypereutectic compositions. It is found that the intensive shear, applied on the eutectic melt prior to solidification at a temperature above but close the eutectic temperature, can significantly reduce the size of eutectic cells, but the solidified microstructure still remains the lamellar morphology.