The stress corrosion cracking (SCC) was investigated for AerMet 100 and 300M steels in four aqueous NaCl solutions of different concentrations (0.035-3.5%), three of which had an identical electrical conductivity (12.44 x 10" 4 m 2 S mol" 1 ). Especially, the variation of threshold stress intensity for SCC, Kiscc, with cathodic potential was evaluated, employing rising step load test method. The K IS cc increased, peaked at around the potential of-0.7 V S CE, and then decreased with increasing potential for AerMet 100 steel. On the other hand, the Kiscc did not change much with potential for 300M steel. The open circuit potential and the corresponding Kiscc were greater for AerMet 100 steel than for 300M steel, indicating the former nobler and more SCC resistant. The SEM fractographs showed mixed cleavage and intergranular cracking, more cleavage for AerMet 100 steel but more intergranular for 300M steel, at all potentials employed.

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Public reporting burden for this collection of infornation is estimated to average I hour per response, including the time for reviewing instrnctions, searching existing data sources, dthering and umaumsinig the data need and conVtng and reviewing this collectton of infornattiot. Send conmnents regarding this burden estimate or any other aspect of this collection of informati•n. including suggestiots for reducing this burden, to Departient of Defetise. Washington Headquarters Servicet, Directorate for Informiation Operations and Reports (0704-0198), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondenus should be aware that notwithustandig any other provision of law, no person shall be subject to any penalty for railing to comply with a collection of information if it does ot display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRES_

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, SUPPLEMENTARY NOTES ABSTRACTThis study was conducted to characterize the fatigue behavior of underaged and overaged 7075 aluminum alloy in vacuum of 4x10"" torr. An emphasis was placed on clarifying the effects of different microstructures, arising from the underaging and overaging treatments, on the threshold fatigue crack growth (FCG). The FCG was measured under constant amplitude loading of stress ratios, ranging from 0.1 to 0.85, in vacuum of 4x10'" torr. The result was analyzed in terms of threshold stress intensity range (AKJ for FCG and maximum stress intensity (K" IT ). It was found that the underaging resulted in lower electrical conductivity, higher hardness, and greater resistance to threshold FCG than the overaging. This is attributable to the difference in microstructure. SUMMARYIn order to establish the role of microstructure on fatigue in the absence of corrosion, the fatigue crack growth (FCG) behavior of underaged and overaged 7075 aluminum alloy was investigated in vacuum of 4xl0" 8 torr. The specimens were subjected to fatigue loading of constant amplitude with various stress ratios, ranging from 0.1 to 0.85, in vacuum of 4xl0" 8 torr. The measured FCG was analyzed in terms of two driving force parameters, threshold stress intensity range (AKn,) and maximum stress intensity (Kmax)-The result indicates that the underaged specimen has lower electrical conductivity, higher hardness, and greater resistance to threshold FCG than the overaged one. This is attributed to the different microstructures, produced by the different aging treatments, and the resultant different slip modes, planar and wavy. n NAWCADPAX/TR-2001/177 CONTENTS

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