Temperature and Orientation Dependence of Creep Damage of Two Ni-base Superalloys

Both polycrystalline (PC) and directionally-solidified (DS) Ni-base superalloys are commonly applied as turbine materials to primarily withstand creep conditions manifested in either marine-, airor land-based gas turbines components. The thrust for increased efficiency of these systems, however, translates into the need for these materials to exhibit considerable strength and temperature resistance. This is critical for engine parts that are subjected to high temperature and stress conditions sustained for long periods of time, such as blades, vanes, and combustion pieces. Accurate estimates of stress and deformation histories at notches, curves, and other critical locations of such components are crucial for life prediction and calculation of service intervals. In the current study, the classical Kachanov-Rabotnov model for tertiary creep damage is implemented in a general-purpose finite element analysis (FEA) software. Creep deformation and rupture experiments on samples from two representative Nibase superalloys (PC and DS) tested at temperatures between 649 and 982°C and two orientations (longitudinallyand transversely-oriented for the DS case only) are applied to extend this damage formulation. The damage model coefficients corresponding to secondary and tertiary creep constants are characterized for temperature and orientation dependence. This updated formulation can be implemented for modeling full-scale parts containing temperature distributions. INTRODUCTION Both polycrystalline (PC) nickel-chromium-cobaltmolybdenum alloys (similar to wrought HAYNES® 230) and directionally-solidified (DS) Ni3Al superalloys (similar to cast DS René 80) are commonly applied in turbine components designed to transport super-heated, high pressure gases through ponding Author. Tel.: +1-407-823-4986; Fax: +1-407-823-0208; [email protected]. om: http://proceedings.asmedigitalcollection.asme.org/ on 08/05/2015 Te power generation equipment. Each class of materials is solidsolution strengthened with chemical compositions imparting exceptional combinations of high-temperature strength and oxidation resistance. One characteristic of both of these materials is the high strength levels they maintain at elevated temperatures. The resistance of the alloys to high-temperature corrosion enhances the usefulness of its strength. Although they are suitable below 1000°C, they both display high levels of creep-rupture strength, even at temperatures of 982°C (1800°F). These traits, combined with good resistance to oxidizing and carburizing atmospheres, make the alloys especially suitable for long-term, high-stress use at elevated temperatures. For example, wrought NiCr alloys have good combinations of metallurgical stability, strength, and oxidation resistance at high temperatures. These alloys are also readily formed and welded by conventional techniques for use in transition lines, exhaust ducting, etc. The DS materials have dual-phase γ-γ′ microstructures that have been optimized to maximize creep and fatigue resistance. The material contains matrix and precipitate phases, as shown in Fig. 1.a. Grains are columnar and aligned along the primary stress axis of the component such as a turbine blade axis. Dissimilar to the PC materials discussed above, DS superalloys can be assumed as transversely isotropic. Two orientations commonly considered for characterizing the uniaxial behavior of DS materials are those in which grains are longitudinallyoriented (L-oriented) and transversely-oriented (T-oriented), as shown in Fig. 1.b. It should be noted that due to fact that more grains are expected to be contained in the cross-section of a Toriented sample as opposed to the L-oriented specimen, the Toriented case is expected to have behavior similar to a PC material Ni-base superalloy. The grains along the Land Torientations of the DS material are shown in Fig. 2. 1 Copyright © 2007 by ASME rms of Use: http://www.asme.org/about-asme/terms-of-use Downloaded Figure 1: (a) Image of the matrix and (course and fine) precipitate phases, and (b) sketch of grain boundaries and sectioning convention for a DS Ni-base superalloy. 3 μm (a) DS axis Grain boundary (GB) (b) L-oriented T-oriented Service conditions for these components are designed using practical experience along with finite element modeling. Despite the strengths of these materials, the combinations of temperature and stress gradients, corrosive conditions, and complex duty cycling facilitate the onset of a myriad of damage mechanisms which lead to crack initiation and subsequent rupture. Calculations of remaining creep rupture life estimates are also developed using experience and secondary creep modeling. Because this strategy lacks consideration of the temperature-dependent, strain-softening behavior (i.e., tertiary creep), both strain history and life estimates have limited accuracy. By incorporating creep damage, a substantial improvement in life prediction is achievable. Creep deformation and rupture data are used to implement a constitutive model in a general purpose finite element analysis code. Experimental results are then compared with numerical calculations. TERTIARY CREEP DAMAGE A method has been established for modeling tertiary creep at high temperatures. This method is based on the Norton power law for secondary creep, i.e.,