AN INVESTIGATION OF HIGH TEMPERATURE LOW CYCLE FATIGUE BEHAVIOR OF MATERIALS

An extensive test program was undertaken to understand the roles of microstructure and environment on the elevated temperature, strain controlled, low cycle fatigue (HTLCF) behavior of two classes of recently developed high temperature alloy systems. The first of these classes consisted of three near-alpha titanium alloys, Ti-5524S (Ti-5Al-5Sn-2Zr-4Mo-0.25Si), Ti-685 (Ti-6Al-5Zr-0.5Mo-0.25Si) and Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si). A wide variety of processing sequences were used to produce sixteen different microstructural conditions for both Ti-5524S and Ti-685. Two different microstructural conditions were examined for the Ti-6242S alloy. The different microstructural conditions for all of these alloys may be divided into two basic groups, all + processed microstructures which contain equiaxed primary alpha grains surrounded by a transformed beta matrix and processed microstructures which are the Widmanstatten or acicular alpha type of microstructure. The second class of alloys investigated in this study consisted of five ferritic steels, three of which contained 9% Cr and two containing 12% Cr.^ The results of this investigation are compared in terms of both macroscopic and microscopic aspects of the HTLCF process. Comparisons are made between the alloys examined in this research with results of other HTLCF studies in the literature.^ The cyclic stress-strain behaviors of different materials are compared and the suitability of various HTLCF lifetime prediction methods is examined. The influence of microstructures on the processes of fatigue crack initiation and growth are discussed in relation to HTLCF behavior. Creep damage and the potential for creep-fatigue interaction during HTLCF are examined. Environmental-fatigue interactions are also studied by comparing results of HTLCF tests in both air and vacuum. The complex oxidation behavior of titanium alloys at elevated temperature is investigated in detail in order to define the influence of environment on the fatigue behavior of these materials. Finally, the field of HTLCF is discussed in terms of a creep-fatigue-environment interaction.