High-temperature low-cycle fatigue and lifetime prediction of Ti-24Al-11Nb alloy
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I.
INTRODUCTION
THE titanium aluminide, TiaAl-base Ti-24AI-1 INb alloy, has been the subject of considerable interest for potential high-temperature applications in advanced aircraft engines. Although a number of studies on the mechanical properties are reported, only a limited amount is known on fatigue behavior, I~j particularly regarding hold-time effects on high-temperature low-cycle fatigue (LCF). Incorporation of hold time at constant stress or strain is one of the methods of studying creep-fatigue interactions. Creep-fatigue experiments in air produce results that reflect the interactions of all three important failure modes, namely, fatigue, creep, and environment (oxidation). Such studies are o f importance in characterizing and understanding the fatigue behavior of a material for eventual high-temperature applications. This investigation was carried out to evaluate hold-time effects on the LCF of Ti-24AI-11Nb at 650 ~ in laboratory air atmosphere. A detailed analysis of the results reported elsewhere 12] and a life prediction methodology are presented here. II.
E X P E R I M E N T A L DETAILS
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The titanium aluminide alloy chosen for this study contains (at. pct)64.33Ti, 24.13A1, l l.26Nb, 0.05Fe, 0.21Cu, and 0.016Ta. A cylindrical casting of 200-mm diameter and 280-mm height was isothermally forged at 1065 ~ into a circular disc of about 50-mm thickness and 400-ram diameter, the details of which are given elsewhereJ 3~A segment of the forged disc was subjected to a heat-treatment cycle comprising 1150 ~ h, air cool + 815 ~ h, air cool + 593 ~ h, air cool that results in a basket-weave microstructure. Uniform gage specimens of 19.05-mm parallel length G. MALAKONDAIAH, National Research Council Associate, on leave from Defence Metallurgical Research Laboratory, Hyderabad, India 500-258, and T. NICHOLAS, Senior Scientist, are with WL/MLLN, Wright Laboratory, Wright-Patterson Air Force Base, OH 45433-6553. Manuscript submitted March 24, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
and 6.35-mm diameter with uniform end connections of 12.7-mm diameter were machined. The specimens were electropolished prior to testing. Fully reversed (R~ = - 1 ) , total axial strain-controlled LCF tests were performed at 650 ~ in laboratory air on a computer-controlled servohydraulic test system. A reflective quartz lamp heating system and water-cooled hydraulic grip assembly were employed. A triangular wave form of 0.167 Hz frequency was used. The chosen strain amplitudes for LCF testing were in the range -+0.4 to -+0.8 pct. The influence of hold time on the LCF behavior was studied at two strain amplitudes, namely, -+0.5 and -+0.6 pet. A hold period of 100, 500, or 1000 seconds was introduced into the triangular wave at peak tensile or compressive strain. The details of test conditions employed are given in Table I. During the course of each test, digitized stressstrain data were stored for a large number of cycles selected at closely spaced intervals. A 10 pct drop in peak tensile stress was considered as the fai
