JNTU B.Tech II MATERIAL SCIENCE AND METALLURGY Previous Question Paper (October - 2020 - R18)

Dislocation strengthening, also known as strain hardening or work hardening, is a mechanism where the strength and hardness of a metal are increased through plastic deformation. This process occurs because the density of dislocations within the crystal lattice increases significantly as the material is deformed.

As plastic deformation proceeds, new dislocations are generated and existing ones multiply through mechanisms such as the Frank-Read source. As the dislocation density increases, the average distance between dislocations decreases, leading to frequent interactions and entanglements. These interactions create a complex network of dislocation forests that act as obstacles to further dislocation motion.

The strengthening effect is primarily due to the following factors:

1. Dislocation-Dislocation Interactions: When dislocations move along their slip planes, they encounter other dislocations. The stress fields surrounding these dislocations interact, creating repulsive or attractive forces that impede further movement.
2. Dislocation Pinning: The intersection of dislocations creates jogs or sessile segments that act as pinning points, effectively locking the dislocations in place and requiring higher applied shear stresses to initiate further slip.
3. Increased Flow Stress: Because dislocations are the primary carriers of plastic deformation in metals, pinning them requires a higher external stress to force them through the tangled network. This manifests macroscopically as an increase in the yield strength and ultimate tensile strength of the material.

The relationship between the yield stress (σy) and the dislocation density (ρ) is empirically described by the Taylor relation:

Where σ0 is the initial yield stress, α is a material constant, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density. Consequently, as the material is cold-worked, the dislocation density increases, directly resulting in a higher yield stress, although this is accompanied by a reduction in ductility due to the exhaustion of the material's capacity for further plastic strain.

White cast iron and malleable cast iron differ significantly in their metallurgical structure and mechanical characteristics, primarily due to the heat treatment process applied to white cast iron to produce malleable iron.

The transformation from white cast iron to malleable cast iron is achieved through a process called malleabilization. This involves annealing white cast iron at temperatures between 800°C and 900°C for an extended period. During this heat treatment, the metastable cementite decomposes into temper carbon (graphite rosettes) and iron. If the cooling rate is slow, the matrix becomes ferritic, resulting in ferritic malleable iron; if cooled faster or alloyed, it results in pearlitic malleable iron, which offers higher strength but lower ductility.