Thermal-Mechanical fatigue of Ti-48Al-2V alloy and its composite
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I.
INTRODUCTION
A S the technology for titanium aluminides and their composite matures, new applications keep arising, and these aluminides are likely to become one of the major structural materials used in demanding environments. One example is their use at elevated temperatures. In this application, they replace superalloys, which have high densities and impose penalties on weight-critical structures, tl,2j At elevated temperatures, even the hightemperature materials lose a portion of their roomtemperature properties. Furthermore, the loss becomes greater and results in material deterioration under changing thermal and mechanical conditions such as thermalmechanical fatigue (TMF). The TMF behavior is influenced mainly by range and frequency of fluctuating temperature and load, maximum temperature, hold time, specimen geometry, coefficient of thermal expansion, material property, metallurgical change, and environment.t3.4] The TMF has been a serious concern of aerospace, t5-8~ power-generation, I9'~~ ground-transportation,tH,L21 and ceramics industries, t~3~However, very few studies on the TMF of newly emerging titanium aluminides and their composites have been reported. The purposes of this study are to (1) characterize the TMF behavior of an alloy, consisting of titanium aluminides and its composite and (2) identify the TMF mechanism.
II.
EXPERIMENTAL PROCEDURE
A. Material and Specimen The employed specimen materials were 101.6 • 101.6 • 12.7 mm (4 • 4 • 1/2 in.) plates of a Ti-48A1-2V alloy and its composite, which was reinforced with EUN U. LEE, Materials Engineer, is with the Naval Air Warfare Center, Aircraft Division Warminster, Warminster, PA 19874-0591. Manuscript submitted May 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
7.5 vol. pct TiB2 particles. The plates were initially produced by vacuum hot-pressing powders of the matrix alloy Ti-48A1-2V and its (TiB2)p-reinforced composite at Martin Marietta Laboratories (Baltimore, MD). Subsequently, they were subjected to hot isostatic pressing at 1400 ~ under 207 MPa (30 ksi) for 4 hours to eliminate or minimize cavities. The plates were machined into rectangular sheet specimens, 98.4-mm (3-7/8-in.) long, 12.7-mm (1/2-in.) wide, and 3.2-mm (1/8-in.) thick.
B. Fatigue Test A rectangular specimen was gripped by water-cooled stainless-steel jaws in the vacuum (10 -s torr) chamber of a Gleeble 1500 thermal-mechanical test machine. The exposed length of the specimen between the grips was 30 mm. The specimen was heated by passing electric current through it and loaded hydraulically. A thermocouple was spot-welded on the midspan of the specimen, and it was used for monitoring and controlling temperature over the heated zone. The temperature was cycled between Tmin = 100 ~ and a Tmax, which ranged from 750 ~ to 1400 ~ Simultaneously, the specimen was also subjected to tensiontension loading with a load ratio 0.1. The applied cyclic stress ranges were 2.8 to 28 MPa and 4.2 to 42 MPa. In the TMF test, the fluctuating temperature and load had triangular wav
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