Two things to explain Why does the transforma7on rate depend on temperature Further why isn t the trend monotonic Why does the microstructure depend on the temperature at which the transforma7on occurs To answer these ques7ons we need to consider the thermodynamics driving force for the transforma7on and the ability of the material to respond to the driving force via di usion 1 Solvus Temperature at which transforms to and vice versa 2 The driving force for the precipita7on of par7cles is the free energy di erence between the single phase a region and the two phase region The free energy di erence depends on temperature T is the solvus temperature and depends on alloy composi7on 3 H G T T T where G is the free energy di erence and H the enthalpy di erence The lower the temperature drop below T the greater the driving force for the precipita7on of par7cles and the greater the nuclea7on rate of the precipita7ng par7cles The phase change requires the movement of atoms by di usion Q D exp RT Higher temperature means higher di usivity and a greater growth rate for the par7cles 4 The transforma7on rate rate of phase change depends on the balance between the driving force and the di usivity Lower temperatures favor smaller par7cles high nuclea7on slow growth Higher temperatures favor larger par7cles low nuclea7on rapid growth 5 The transforma7on rate depends on where we are in the two phase region of the diagram 6 Informa7on about transforma7on rates at a given temperature are found in isothermal Time Temperature Transforma7on TTT diagrams In this context isothermal means that we assume any temperature changes occur instantaneously 7 One can build a TTT diagram by aggrega7ng data about when transforma7ons begin and end at many di erent temperatures 8 We can use the TTT diagram as a guide to the appropriate temperature at which to perform a heat treatment Prac7cally speaking it is di cult to cool something from 550 C to 350 C very quickly In prac7ce the material is cooled very quickly to room temperature then reheated to the desired temperature 9 The temperature at which nuclea7on of phase occurs has a large impact on the nal microstructure 450 C 10 Even though the reac7on 7mes are roughly the same holding at 350 C produces a much ner microstructure than 450 C Why is this 11 The temperature at which the precipita7ng q phase grows impacts the hardness and strength of the material Smaller par7cles are more e ec7ve at inhibi7ng disloca7on mo7on resul7ng in greater strength and hardness 12 Here is what real data looks like for 2014 aluminum a high strength alloy used in aerospace among other applica7ons Why does the tensile strength go down a er reaching a peak 13
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