Question

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4. a) Strain energy component is not considered while dealing with the n during solidification. Why? (2) while dealing with the nucleation kinetics event b) What would be the effect of higher undercooling on the interian or higher undercooling on the interlamellar spacing of pearlite and explain why? (2) c) Coring is a big headache in alloys having a wide freezing range. Explain why. (2) d) State the difference between natural aging and artificial aging or p between natural aging and artificial aging of precipitation-strengthened Al alloys. (4)

Answer 1

SHOT ON POCO F1 a in the case of homogeneous nuclection of a solid com its undercoded meit, in this case also we are o interested in calculating the critical size of the machen for homogeneous nuclection of the associated free energy changes. In this case of a solid-solid phase transformation, in laddition to the changes in bulk free energy interfeial energy, typically, there is also a strain energy contribution which needs to be accounted for; this is because, typically the volume of original å resultant unlike fluids, solids can support sheer strenes This strain energy is related to misfit energy as ito is imperfect fitting of the transformed phase into the can be shown to the be proportion. original volume, which al to the volume of the transformed phase. It is in election of precipitation this respect that the homogeneous differs from that of the seled from its pure melt-

SHOT ON POCO F1 POCO ń coring happens when a heated alloy, such as a cau. No 8 sm, cools in non-equilibrium conditions. This causes the i exterior of the maternal to solidify before the interior coring causes the centers of the grains to retain more of the highes metting temperature element. Alloys must be manufactured in proper equilibrium temperature conditions to give them strength a durability coring results when more of the higher melting temp. element is refained at the centre gains in an alloy, when this occurs, the dendrite aurs formed from the exterior have a different composition of them alloy in the interior regions resulting in a local Compositional difference that reduces the alloy's quality þesformance. It can be removed by subsequent anneling hot working processes.

So As the undervooling increases, so does the doining force & the transformation occurs more rapidly. : Redistribution of the alloying element at the austenite- i peerlite interface is a there erface is a thermodynamic requirement for growth at low Undercoating when ferrite & Gerrantite with compositions close to equilibrium must be formed The Undercoding degree is increased through a temporary Undercoding treetment at bainite transformation Hop. before the beartite is othermal transformation at 500'c to 620°C. The transformation kinetics reveals an increased nose temperature & a rightwerd shift of TTT mere under high undercoding conditions. In addition, the under cooling breatment leads to an increased driving form force during pearlite transformation, which is refined hiearchical structures of responsible for the pearlite. As a result size reduction is achieved for pearlite too colonies & lamellere attributed to an increase in reduction in area,

4d) Nature Artificial Aging is done at elevated temperetures Aging is done at room tempereture. * A solution heat treatment raises the temperature . of the alloy to a region where the materials forms a single phase solid solutions. This makes the alloy in the material. uniform & helps evenly distribute the alloying elements when it is quended down to room temperature the alloying elements become super saturated & having a driving force to precipitates farm & different phases Arti final aging elevates the temperature of the alloy in order to accederete that process. Once the desired amant of precipitates has formed the makenal is requenched to freeze" the precipitation process at a useful paints Since overeging often leeds to drops in properties. In some Al alloys the be organization of the supersaturated alloying atoms can take place at room temperature, albert over a moderately long time period. This nature aging may be good because it allows for an softer & caner to machine alloy which then initially POCOns without extra energy. SHOT ON POCO F1