STUDY OF THERMO-PHYSICAL PROPERTIES OF SELECTED NICKEL-BASED SUPERALLOYS WITH USE OF DTA METHOD

摘要

The presented paper deals with study of thermo-physical properties of cast complex alloyed nickel based superalloys IN713LC, IN738LC and IN792-5A. In this work the technique of Differential Thermal Analysis was selected for acquisition and comparison of the phase transformation temperatures. The samples taken from superalloys in as received state were analysed at heating and cooling rates of 1, 5, 10, and 20 K/min using the experimental system Setaram SETSYS 18TM. Moreover, the transformation temperatures for zero heating/cooling rate were calculated. Based on a comparison of these temperatures it is possible to make the following conclusions: (ⅰ) The alloy IN792-5A has the highest temperature of solubility of the strengthening phase y' (1235°C); (ⅱ) the highest liquidus temperature (heating) obtained by extrapolation was found in the alloy IN713LC (1349°C), the lowest solidus temperature (heating) was found for the alloy IN738LC (1212°C); (ⅲ) At cooling an undercooling occurred in all alloys. In general it may be stated that the biggest under-cooling (TS, 47°C) was recorded in the alloy IN792 5A; (ⅳ) The width of the interval of the heat treatment window was the biggest in alloy IN713LC (44°C); (ⅴ) The alloy IN738LC is characterised by the widest interval of melting (124°C) and solidification (134°C), while the alloy IN792 5A has the narrowest interval of melting (82°C) and at the same time almost the same interval of solidification as the alloy IN738LC (129°C); (ⅵ) The obtained phase transformation temperatures were compared with the values of phase transformations temperatures calculated on the basis of established relationships. In order to obtain more precise description of the behaviour of Ni-based superalloys, during controlled heating/cooling of the initial material (as received state) during DTA analyses, all the samples of superalloys were subjected to a phase analysis using scanning electron microscopy. The course of phase transformations, in all the studied superalloys (IN713LC, IN738LC, IN792 5A) is likely to run according to the following reaction scheme (L = melt): L ←→γ, L ←→γ + MC, L←→γ/γ', L ←→γ + minority phases (such as M_3B_2, phase η),y←→γ'.