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| Dobrovská J., Dočekalová S., Smetana B. |
| STUDY OF COOLING RATE EFFECT ON TRANSFORMATION TEMPERATURES IN THE COURSE OF IN 738LC SOLIDIFICATION |
Abstract
The paper deals with the effect of cooling rate on the values of transformation temperatures of IN 738LC nickel based superalloy. Particular attention was given to the determination of phase transformation temperatures in the course of solidification and cooling and to the effect of varying cooling rate on these temperatures. Samples taken from as-received state of superalloy were heated with controlled ramp rates (1, 5, 10 and 20°C/min) and immediately after melting they were cooled with the same controlled ramp rate with the help of the laboratory experimental system SETARAM SETSYS 18TM TG/DTA/TMA. The technique of Differential Thermal Analysis (DTA) was selected for the detailed study of nickel based superalloys. Then the microanalysis of minority phases was conducted with use of X-ray spectroscopy and microstructure of the individual samples was documented by scanning electron microscopy.
There were found out these main results: i) The phase transformation temperatures were determined from DTA curves (the solid formation temperature from the liquid, the MC carbide formation temperature, the ?/?' eutectic formation temperature, the final solidification temperature, the ?' precipitation temperature from the ? matrix). Moreover, the equilibrium transformation temperatures were calculated by extrapolating of measured temperatures to zero cooling rates. It was found out that the effect of the undercooling (supercooling) on the cooling curves is significant, mainly in case of the beginning of MC carbide formation and the ?/?' eutectic formation temperature; ii) It was verified that as the solidification rate decreased, the precipitations and particles are more coarsed. Dimensions and distribution of MC carbides in assessed samples corresponded to applied cooling rates – as increasing solidification rate the carbide morphology changed from the blocky and script type to fine script type and spotty type.
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| Keywords: transformation temperatures|DTA-method|X-ray spectroscopy|IN 738LC| |
| No 4 (2007), p. 539-545 |
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