Master of Science in Mechanical Engineering (MSME)


Mechanical Engineering

Document Type



IN738LC, a nickel based superalloy has wide applications in the gas turbine engines because of its ability to retain its strength at high temperatures involved. The stability of the γ' Ni3Al (Ti, Nb) precipitate microstructure which gives the alloy its strength is very important. After an initial annealing at 1200°C for 4 hours and water quenching, a uniform distribution of single size (70nm) γ' fine precipitates is obtained. Growth of the γ' particles, their size and distribution are studied after subsequent annealing heat treatments, conducted at different temperatures for different time periods. The average precipitate size data of the nearly single size precipitates was obtained from the SEM photomicrographs of the samples annealed at temperatures 1040, 1080, and 1100°C for different holding times in the range 3-100 hours. Activation energy for precipitate growth of the single size precipitates was then calculated from the data. The activation energy obtained for growth was found to increase with the increase in size of precipitates. This result is in conformity with the similarly increasing activation energy values with precipitate sizes obtained after 25 hours of annealing time at different lower temperatures [14]. Additional samples also were heated up to 200 hours at the above mentioned temperatures in order to determine any critical maximum size to which the particles would grow before starting to split or dissolve. The results showed anomalous growth of the precipitates into large islands along γ matrix grain boundaries. Three different types of dissolution of the particles after reaching a maximum critical size of ~9-10μm at 1100°C were observed. Creep tests were conducted on the as-received specimens with duplex size precipitate microstructure at temperatures 1040, 1080, and 1100°C, with an applied stress of 31 MPa. The tests were interrupted after 40 hours of testing to study the behavior of growth of precipitates, and define initial stages of rafting when alloys are subjected to continuous application of low stress at high temperatures. The precipitates grew larger in size, but no rafting was observed, perhaps due to application of very low stress and short time involved, even though the tests were conducted at real high temperatures.



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Committee Chair

Aravamudhan Raman