Fatigue and fracture behavior of an aluminum-lithium alloy 8090-T6 at ambient and cryogenic temperature
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I.
INTRODUCTION
O V E R the past few years, much attention has been paid to the development and application of aluminum-lithium alloys because of their high elastic modulus, low density, good resistance to propagation of fatigue cracks, as well as high strength. Unfortunately, the addition of lithium into the alloy can lead to decrease of both toughness and ductility that restricts further development and application. Recent investigations indicate that aluminumlithium alloys display enhanced toughness and an improved strength-toughness relationship at low temperature, t~-6] It would be expected that aluminum-lithium alloys are attractive candidate materials for application in advanced aerospace structures at low temperature, such as liquid-hydrogen, liquid-oxygen, and natural gas fuel tanks, especially for existing and future transatmospheric and hypersonic aircraft applications. Therefore, the investigation on the fatigue behavior of alloy at low temperature is considered to be very significant and necessary but has not been reported so far. The purpose of the present study is to examine fatigue and fracture behavior of an aluminum-lithium alloy 8090-T6 at ambient and liquid nitrogen temperatures.
II.
MATERIALS AND P R O C E D U R E S
An alloy, of composition similar to 8090, (in weight percent) 2.56 pet Li, 1.44 pct Cu, 1.18 pct Mg, 0.15 pet
Zr, 0.009 pct Fe, 0.05 pet Si, balance AI, was selected for the present study. An ingot of this alloy was prepared in 10 Kg heats by inductive smelting under high-purity argon at about 0.7 atm pressure and casting into a graphite mold. The ingots were scalped off the surface layer and homogenized at 400 ~ for 16 hours and then 515 ~ for 10 hours. After preheating at 450 ~ for 2 hours, the ingots were extruded into bars 18 mm in diameter which were then hot rolled into 4.5-mm-thick plates. Both fatigue and tensile specimens were taken from the plate in L-T orientation. Specimens of the alloy were solutionized at 525 ~ for 1 hour. Following the solution heat treatment, the alloy was quenched in iced water and then aged at 190 ~ for 16 hours to develop a peak precipitation hardening (T6). An hour-glass type of fatigue specimen (Figure 1) was machined. In order to observe the deformation characteristics on the surface, the specimens were carefully polished with diamond paste before testing. Fatigue tests under load control were conducted on a Schenck Servohydraulic machine at room (300 K) and liquid nitrogen (77 K) temperatures, with sinusoidal waveform at a frequency of 50 Hz. The stress ratio (R) is zero~ The conventional S-N curves of the alloy were determined. The slip patterns on the surface and fractograph were observed in JEOL T-200 and $360 scanning electron microscopes. Investigations of the microstructure by transmission electron microscopy (TEM) were performed on a PHILIPS* EM420 analytical *PHILIPS is a trademark of Philips Electronic Instruments Corporation, Mahwah, NJ.
Y.B. XU, Associate Professor, Y. ZHANG, Associate Professor, Z.G. WANG, Professor, and Q.Z
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