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I.

INTRODUCTION

D U R I N G the past decade, increased emphasis has been placed on the fracture mechanics characteristics of metallic materials for use in high performance aerospace systems. While fracture toughness is a requirement which has been demanded for applications such as the B 1 bomber, an even more critical parameter in assessing the useful life of a part is the fatigue crack growth rate (FCGR). As such, this mechanical property has received extensive attention and many data on various materials have been developed. However, in many cases, it is not clear from these data exactly how various material characteristics, such as strength level, microstructure, and oxygen level separately influence the FCGR. It was the goal of the present work to separate out the effect of these various factors for a new fracture resistant titanium alloy, Ti-4.5A1-5.0Mo-I.5Cr (CORONA 5*). 1-6 *This designation was developed from the three organizations involved in developing the alloy, which was the fifth of a series of compositions: Co It Industries, Rockwell International, and Naval Air Systems Command.

To separate out these effects, a matrix of conditions was evaluated (Table I). The effect of oxygen level was studied at two levels (0.100 and 0.174 wt pct) while keeping the microstructure constant. Constant microstructure was attained by solution treating at - 3 0 ~ (55 ~ and - 9 0 ~ (160 ~ below the beta transus temperature for both oxygen levels. This was then followed by aging treatments at lower temperatures designed to give the desired strength levels. The solution treatment temperature was considered critical G.R. YODER, Metallurgist, is with Material Science and Technology Division, Naval Research Laboratory, Code 6384, Washington, DC 20375. F.H. FROES, Technical Area Manager, Titanium Programs, is with Air Force Wright Aeronautical Laboratories, MLLS, Wright-Patterson Air Force Base, OH 45433. D. EYLON, Principal Investigator, is with MetcutMaterials Research Group, P.O. Box 33511, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted March 17, 1983.

METALLURGICAL TRANSACTIONS A

Table I.

Heat-Treat Conditions

Material Oxygen Strengtha Condition Level Goal L1 Low LO L2 Low LO L3 Low INT L4 Low INT H1 High INT H2 High INT H3 High HI H4 High HI

Solution Treatment,b ~176 885 (1625)/1 830 (1525)/4 885 (1625)/1 830 (1525)/4 915 (1675)/1 845 (1550)/4 915 (1675)/1 845 (1550)/4

Aging Treatment,b ~176 790 (1450)/8 730 (1350)/8 540 (1000)/16 495 (925)/24 790 (1450)/8 730 (1350)/8 595 (1100)/8 540 (1000)/16

~LO = Low: 825 MPa (120 ksi) UTS INT = Intermediate: 960 MPa (140 ksi) UTS HI = High: 1100 MPa (160 ksi) UTS bFollowed by air-cool (AC)

as previous work had indicated that the coarse alpha formed at this temperature controlled the fracture path rather than the fine alpha resulting from subsequent aging, v The goal strength levels were 825 MPa (120 ksi) and 960 MPa (140 ksi) UTS for the low oxygen material and 960 MPa (140 ksi)and 1100 MPa (160 ksi)for the high oxygen material. The two materials at the 96