Question

Define hardness, and explain the variation of hardness with respect to the slip systems in the material.

Answer 1

Hardness is resistance to the scratch or abrasion. It is the property to resist deformation. Material with high yield are hard as they resist penetration. Hardness can be improved by certain heat treatment, case hardening and work hardening processes. There are two method of hardness measurements

1. Scratch hardness- commonly measured by Mohr's test

2. Indentation hardness measured by -

- Brinell hardness method

- Rockwell hardness

- Vickers hardness

- Knoop hardness

Number of slip system is defined as the product of " number of closed pack plane and number of closed pack direction".

Closed pack plane are the planes with highest value of planar density. For example in BCC [ 1 1 0] and in FCC [111] is the closed pack plane.

Slip system is basically an index of ductility of material & more is the ductility less is the hardness.

Comapring between BCC & FCC material, BCC material will have more number of slip system, however material doesn't exhibit ductility. Instead BCC is hard and fails in a brittle manner. The reason for hardness and brittleness of BCC structure is the random orientation of neighbouring closed pack plane.

Though FCC materials have relatively less number of slip system than BCC, yet FCC materials are less harder than BCC because the available slip system in FCC materials are systematically aligned to facilitate the movement of dislocation.

Due to the random orientation of Slip system in BCC the dislocations movement is restricted so more stresses are required to move the dislocation, which makes BCC harder.