Question

Describe how to increase strength of hypoeutectic alloys by

a)refining b) Solid solution c) more eutectic phase

Answer 1

a) Refinement:

Nowadays Casting is a process of choice for making components of any shape and size. We expect components to possess desired strength and stiffness. Failure against yield, fatigue, excess deflection and creep are the basic criterion in any conventional engineering design problem. Grain size is an important parameter that influences yield strength and toughness of materials. the ability of a metal to deform plastically is dependent on the ability of dislocations to move.

Grain size refinement is one of the processes of increasing the strength of materials. That is where we arrest the motion of dislocations causing dislocation pile up at grain boundaries leading to increasing the strength of the component. when dislocation stated owing to the application of the load, grain boundaries act as a barrier to stop dislocation motions. Smaller the grain size we get more grain boundaries, thus more resistant to dislocation motion, thus dislocation pileup occurs at grain boundaries, so the strength of the component increases. We can also control the cooling rate while casting in such a way that we get grain boundaries misorientation. This also increases the strength of the component. According to Hall-patch relation, the yield strength has an inverse square root relation with grain size (d).

hall-patch relation: $\sigma _{y}=\sigma _{i}+kd^{-1/2}$ .

thus the reduction in grain size increases the yield strength of the material.

b) Solid solution:

Solid solution strengthening mechanism is based on the principle that introducing an Impurity atom or foreign atom into the solid solution of an original material generates local stresses around the vicinity of the impurity atom has been introduced. Introducing of impurity or foreign atom induces distortion in the lattice and generates stresses. This stress can produce a barrier to dislocation motion, thus our material gets strengthened.

as we can see from above figure, Impurity atom causes shear stresses at A and B that opposes dislocation movement. This mechanism increases the strength of the material.

More eutectic phase:

the eutectic reaction is a three-phase reaction, where liquid transforms into two solid phases. Like in the cases of 30/70 eutectic case, this similar to the solid strengtening mechanism. By increasing the phase we could introduce impurity atoms and by controlling the rate of cooling strength of the component can be increased.