Question

What are the basic characteristics of a metal you need to know while selecting different meal and alloys for medical implant? What is difference between single crystal and polycrystalline metal? Why do you think reducing grain size is one of the strategies to strengthen a metal alloy? (4+2+4)

Answer 1

Materials selected for the use as a medical implant should satisy two major criteria. Biofunctionality refers to the ability of the material to perform the required function. Biocompatibility is the how well the biomaterial adapts to the conditions in the human body. Orthopedic implants like the knee cap and prosthetic heart valves are a few examples of bio materials. In terms of biofunctionality the material should possess high yield strength, high ductility, high fatigue strength and a good fracture toughness. Now when we talk about the interactions of these materials in the human body, the material should be corrosion resistant, resistant to degradation by the bodily elements like fluids, proteins etc.

In a single crystalline material, there is only one grain or crystal and hence there are no grain boundaries whereas in a poly crystalline material there are multiple grains and thus the existence of grain boundaries. In a single crystal all the symmetry properties are in a single crystal. The properties change when the direction changes (anisotropic) Polycrystals are isotropic.

There are inherent flaws in materials called dislocations. These dislocations, upon the application of stress propagate in a crystal and eventually lead to the failure of the material. When we reduce the grain size, the movement of these dislocations is restricted by the change in the direction of the grains. The dislocations require more energy to change directions. Hence more applied stress is required delaying the failure of the material. Thus reducing the grain size creates more grain boundaries and higher requirement of applied stress thus strengthening the metal alloy.