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A study was undertaken to develop an understanding of the fatigue response of superalloy B-1900+Hf under combined thermal and mechanical strain cycling in air. Comparative evaluations were made with existing thermal-mechanical data of B-1900+Hf and with results of a comprehensive study of the fatigue behavior of the same alloy under isothermal conditions. The thermal-mechanical fatigue (TMF) response was investigated for constant amplitude, fully reversed, mechanically strained cycling of uniaxially loaded specimens in the temperature range from 400 to 925°C. Experiments were conducted both with maximum strain in-phase with maximum temperature and out-of-phase with maximum temperature.
The TMF cycling was observed to cause more cyclic hardening than in isothermal fatigue experiments at the maximum and minimum temperatures. In terms of mean stress or plastic strain range, out-of-phase cycling was shown to be more deleterious than in-phase or isothermal cycling. However, few differences were observed in terms of the stabilized stress ranges. The asymmetric cyclic hardening/softening behavior is explained in terms of coarsening of the γ′ and associated strain field. For TMF cycling, the high temperature flow stress depends on the density of the misfit dislocations, whereas the low temperature flow stress is controlled by the magnitude and sign of the applied stress. The TMF cracking modes are discussed. The results show that the fracture criterion under TMF cycling is stress based.
thermal-mechanical fatigue, superalloy, cyclic hardening, damage mechanisms, directional coarsening
Research Assistant, Massachusetts Institute of Technology, Cambridge, MA
NSERC Research Fellow, Ecole Polytechnique, Montreal, Quebec
Professor, Massachusetts Institute of Technology, Cambridge, MA