Superalloys with Low Segregation

Abstract

Segregationdue to solidification of highly alloyed superalloy is a very complex process.

A New Way to Improve the Superalloys

“…A high pure K38 alloy with <O.O005P, cO.O5Si, no B and Zr was melted, and the solidification process measurement indicated that its final solidification temperature was raised to 1280°C. The metallographic observation of the as-cast microstructures revealed that a lot of (y + y') eutectic existed in the conventional K38 alloy was eliminated after purification [4]. In superalloys, (y + y') eutectic is resulted from the non-equilibrium solidification, and its amount indirectly indicates the segregation degree.…”

“…The compositions of the tested alloys are listed in Table I. The experimental technique of the solidification process determination has been discussed elsewhere [4]. The specimens of about 10mmx10mmx1Omm were put in graphite boats, heated in a silicon carbide furnace to 1420°C for 5 min, cooled slowly to different temperatures, kept for 10 min, and then, quenched in water.…”

“…By this so-called low segregation technique, the segregation of the current cast superalloys can be greatly decreased and their alloying levels are permitted to increase without damaging the structure stability. Based on INlOO, IN792 and IN738 alloys which are the advanced cast blade superalloys respectively for aeroplane, marine and industrial engines, a series new alloys with improved mechanical properties or hot corrosion resistance have been developed by raising the contents of Al,Ti,Ta or Cr [4,14]. These alloys were proved without harmful phase precipitation after prolonged exposure.…”

“…It is well proved that due to much narrower solidification temperature ranges, low segregation DS super-alloys are much easier to adopt DS technology. DZ38G is a directionally solidified M38G alloy with 50°C higher service temperature than IN738 alloy [4,14], several hundreds of blades made by this alloy have been produced without the appearance of any cracking and passed 250h engine trial.…”

Determination of transition temperatures during freezing and melting of interdendritic phases in Ni based superalloys

“…In the quenching directional solidification method, the temperatures along the length of the mush is calculated from measurement of the dendrite tip temperature and the average gradient within the mush by the insertion of thermocouples within the melt and this is correlated with the quenched microstructure. [3][4][5][6] However, there are concerns that the thermal gradient within the mush is not linear, as this approximation is more valid in the vicinity of the dendrite tip and therefore temperatures deduced in this manner are not very accurate. Besides, there is an increasing difficulty in delineating the solid existing at the moment of quench from the actual quenched microstructure with increasing fraction solid (.80%), i.e.…”