Question

a) Briefly discuss how plastic deformation occurs in crystalline materials. Discuss the important methods of strengthening used in (i) cold rolled 1018 steel, (ii) 2024Al alloy in T6 heat treated condition, (iii) brasses, and (iv) quenched and tempered 1045 steel.

Answer 1

As applied stress increases the HookesH Law of proportionality is no longer valid and deformation stars to become plastic causing permanent plastic strain.

In crystalline material plastic deformation generally takes place due to glide on slip planes along certain slip direction.

Slip is the prominent mode of plastic deformation.It involves sliding of blocks of crystals over each other along certain crystallographic plane called as slip plane.

Slip plane is the plane of greatest atomic density and slip direction is the closely packed direction along the slip plane.

At the time of slip each atom mormalln moves same integral number of atomic distance along a slip plane and hence produce a step. However,they don't effect the orientation of crystal.

Slip occurs in the case if applied shear stress exceeds a certain minimum allowable value called as critical resolved shear stress (cross).

In this way plastic deformation takes place in crystalline solids.

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Different methods of strengthening are..

A)grain size reduction

B) solid solution alloying

C)strain hardening

D) precipitation hardening

E) dispersion hardening

etc etc

..........

1) In cold rolled steel strain hardening method is used for strengthening. Increase in the percentage of filled work increases the mechanical tensile and yield strength but may reduces ductility to some extent.

Rolling or pressing(strengthening) is done at relatively low temperature.

2) for 2024 Aluminium alloy, method of strengthening used is PRECIPITATION HARDENING.

In precipitation hardening change in solid solubility with temperature is used to produce fine particle of impurity phase which reduce the movement of dislocation. As dislocations are major carriers of plasticity,hence hardness of material increases.

3) BRASS: SOLID SOLUTION ALLOYING is used for strengthening of brass. It consists of Copper(major portion is copper) and Zinc. It's properties and hardness depends on relative percentage of copper and zinc used.

4)In 1045 steel, transformation hardening method is used for strengthening.