A metallographic technique for high temperature creep damage assessment insingle crystal alloys

The use of single crystal (SX) nickel-base superalloys will increase in the future with the introduction of sx blades into large gas turbines far base -load electricity production. Prolonged periods of use at high temperatures may cause creep deformation and the assessment of damage can give large fi nancial savings. A number of techniques can be applied for life assessment, e.g., calculations based an operational data, nondestructive testing or ma terial interrogation, but because of the uncertainties involved the techniq ues are often used in combination. This paper describes a material interrog ation (metallographic) technique far creep strain assessment in SX alloys. Creep tests have been performed at 950 degrees C on the SX alloy CMSX-4 and quantitative microstructural studies performed on specimens interrupted at various levels of strain. It was found that the strengthening gamma'-parti cles, initially cuboidal in shape, coalesced to form large plates or rafts normal to the applied stress. The gamma-matrix phase also formed plates. CM SX-4 contains similar to 70 vol % gamma'-particles and after creep deformat ion the microstructure turned itself inside out, i.e., the gamma "matrix" b ecame the isolated phase surrounded by the gamma'-"particles." This can cau se problems for computerized image analysis, which in this case, were overc ome with the choice of a suitable measurement parameter. The rafts reached their maximum length before 2 percent strain, but continued to thicken with increasing strain. Although of different dimensions, the aspect ratios (le ngth/thickness ratio) of the gamma-prime rafts and the gamma plates were si milar at similar levels of strain, increasing from similar to 1 at zero str ain to a maximum of similar to 3 at about 1-2 percent strain. Analysis of m icrostructural measurements from rafting studies on SX alloys presented in the literature showed that the aspect ratios of the gamma and gamma'-phases were similar and that at a temperature of 950-1000 degrees C a maximum len gth/thickness ratio of about 2.5-3.5 is reached at 1 to 2 percent creep str ain. Measurement of gamma-prime raft or (or gamma plate) dimensions on long itudinal sections of blades is thus a suitable method for high temperature creep damage assessment of SX alloys. This gives a considerable advantage o ver conventional Ni-base superalloys whose microstructures are usually very stable with respect to increasing creep strain.