Superalloys have been widely used as gas turbine engine components and other high-temperature applicatiion at elevated temperature service. High temperature low cycle fatigue (HTKCF) has been one of the most important material properties to be considered. Therefore, continued efforts have been made to explain the mechanisms that govern the high temperature low cycle fatigue in the r' precipitation strengthened alloys.
A286 alloy is an iron-base superalloy, which contains 25wt.% Ni and 15wt.%Cr with Ti,Al and other minor alloying additives. It is mainly strengthened by an aging treatment which precipitates the ordered fcc r' phase, ＄Ni_3＄(Ti,Al). However variation in the minor alloying additives significantly influences the precipitation sequence. The specific compositional modification of the Fe-base superalloy containing higher Ti and lower Al investigated in this study was designed to increase the amount and stability of r' phase by adding Nb element. Based on this new desigh, "Nb-modified A286 alloy" was developed to improve the strength, fatigue and creep-rupture resistances compared to the commercial A286 alloy. However, the effects of creep-fatigue interaction on the high temperature low cycle fatigue (HTLCF) behavior in this newly developed Nb-A286 alloy and its damage mechanism under these complicated loading conditions are not clearly understood.
In this study the effects of test temperature, applied strain range and various superimposed hold times on HTLCF behavior of this material were investigated and the damage process under these conditions was discussed. In the continuous low cycle fatigue of Nb-A286 alloy, the decreasing value of the slope of the Coffin-Manson plot for the higher temperature of 650℃ in comparison with the case of less than 550℃ is occurred. This is not affected by oxidation and grain boundary segregation, but the change in the fatigue cracking mode with the test temperature. The precipitation of the n phase at the grain boundary during the low cycle fatigue is thought to provide the site for the grain boundary cavitation which promotes the intergranular failures in the higher temperature low cycle fatigue. This can be confirmed by the results of low cycle fatigue at 25℃ after heat treatment which formed the n phase at the grain boundary. Also, the discontinuity of the slope in the Coffin-Manson plot at the temperature ranges of 550∼575℃ arises from the change in the mode of the fatigue cracking from the transgranular to the predominantly intergranular cracking modes with the applied strain range. The reason for this change of the fatigue cracking is attributed to a combination of mutually competing factors involving the grain boundary precipitate, the density of slip band, the concentration of stress at points where the slip bands impinge upon the grain boundaries withh the applied strain range.
The n precipitate at grain boundary is one of the important factors controlling the intergranular cracking under HTLCF, introducing the reduction of fatigue life. The value of the activation energy of n phase formation with total strain range was analyzed by using the in-situ XRD and DTA techniques. Thea applied strain during the low cycle fatigue decreases the activation energy of n phase formation at the grain boundary. The trend of decreasing the temperature of n phase formation with the applies strain was found to be due to the accommodation of deformation near grain boundary.
The creep-fatigue characteristics of Nb-A286 alloy with hold time and wave form were also investigated. Fatigue life of Nb-A286 alloy with hold time is shorter than that of Nb-A286 alloy with no hold time. As the hold time is increased, the fatigue life is decreased further because of a larger amount of stress relaxation during hold time to give the additional creep damage. Also, the fatigue life under tensile hold time is much shorter than that under compressive hold time. It is found that this is not explained by the value of mean stress, but by the change of microstructure with wave form.
As the results of low cycle fatigue tests of Nb-A286 alloy, the major damage under HTLCF is evaluated to be the combination of grain boundary precipitates formed during the fatigue and the stress concentration by planar slip band near grain boundary. Also, the suggestion of reducing the damage under HTLCF of Nb-A286 alloy was made in this study.
This evaluation of fatigue damage and suggestion of better alloy design can be helpful to qualify this alloy gas turbine engine application and to maximize the component reliability and performance of benefits. These results can also be the basis of developing new alloys with good mechanical properties at high temperature.
