Part VIII - Papers - The Effect of Unidirectional Solidification on the Properties of Cast Nickel-Base Superalloys

A study has been made of the effect of unidirectional solidification on the creep behavior, stress-rupture properties, and thermal shock resistance of four nickel-base superalloys. The alloys Mar-MZOO, B-1900, IN 100, and TRW 1900 show improved rupture ductility and thermal shock resistance when tested with the columnar grains parallel to the major stress axis during test. The relative improvement in creep-rupture properties depends on the intrinsic strength of the alloy, a property which depends on composition, heat-treatnient, and crystallographic orientation. The data clarifies some of the factors affecting the properlies of the cast nickel-base superalloy. COMPONENTS designed for high-temperature use have, in recent years, been fabricated by precision casting techniques using nickel-base alloys developed specifically for use in the conventionally cast condition. This development was a result of both the increasing complexity of the component and the recognition that high-temperature strength was incompatible with workability. The use of castings can be economically favorable, but, more important, the recent complex designs of air-cooled gas turbine blades and vanes in alloys which possess the necessary high-temperature strength cannot be forged. Common modes of failure of high-temperature components are excessive creep, creep rupture, and thermal fatigue. If rupture occurs then the mechanism is usually by inter crystalline cracking along those grain boundaries oriented transverse to the major stress axis. In the stronger alloys, rapid propagation of intercrystalline cracks result in apparent premature failure demonstrated by the absence of third stage creep in a creep-rupture test and low rupture ductility. Tensile ductility shows a similar trend, that is, decreasing ductility with increase in strength. It is not surprising, therefore, that increases in creep strength have only been obtained with a loss in resistance to thermal shock, a property which shows a dependence on tensile ductility. VerSnyder and ~uard' showed that the application of unidirectional solidification to a brittle Ni-Cr-A1 alloy both improved the rupture life of the alloy and increased rupture ductility. Since this casting method results in columnar grains, it appeared that the solution to the lack of ductility in nickel-base alloy components was the elimination of the source of failure, namely the transverse grain boundaries. This concept was recently developed2 to produce cast-to-size gas-turbine blades and vanes consisting entirely of columnar grains oriented parallel to the major stress axis of the component. Not only did the process impart increased rupture life and ductility to the components, but it also increased their resistance to thermal shock. During this development, the effect of unidirectional solidification on the properties of several alloys was investigated. The resulting data allows certain conclusions to be drawn regarding the factors affecting the creep and stress-rupture properties of the cast nickel-base superalloy and also its resistance to thermal shock. Detailed information was obtained on the alloys Mar-MZOO, B-1900, In 100, and TRW 1900. The composition of the alloys studied are shown in Table I. MATERIAL PREPARATION AND TESTING The conventionally cast alloys are cast as +-in.-diam bars in innoculated shell molds under conditions designed to maintain control of grain size. Unidirec-tionally solidified casting were produced in the form of 3-in.-diam ingots. Specimens from the latter were machined such that the axes of the columnar grains were parallel with the axis of the test bar, except in certain cases where the transverse properties of the material were being evaluated. The specimens were tested in an axial loading creep machine.' Temperatures were measured with thermocouples, attached just outside each end of the gage length, to assure uniform temperature along the entire specimen. Chromel/alumel thermocouples were used up to 1800"F and Pt/Pt-10 pct Rh thermocouples above 1800"F. The specimen temperature was recorded and maintained within i2"F throughout the test. Creep extensions were measured automatically with extenso-meters attached to ridges on the specimen and recorded continuously using linear variable differential transformers and multipoint recording equipment. MICROSTRUCTURE The microstructures of transverse sections of the four alloys studied are shown in Figs. 1 and 2. Fig. 1 shows optical micrographs of the alloys, both in the conventionally cast and unidirectionally solidified conditions. Each alloy displays a cored dendritic structure, a distribution of MC carbide, and the presence of the y — y' eutectic constituent.~ In the case of IN 100 and B-1900, Fig. 1 indicates that in these specimens the eutectic is degenerate. That is, it consists only of y' pools indicating that the y constituent has been solutioned during the cooling period after solidification. The major effect of unidirectional solidification on the microstructures of the alloys is a tendency to promote a more dendritic form of MC carbide and an increase in dendrite arm spacing, both effects being a result of the slower solidification rate in the unidirectional solidification technique. V, lec tron micrographs of transverse sections of the four alloys in the two cast conditions are shown in