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FATIGUE CRACK GROWTH IN A TiAI ALLOY WITH LAMELLAR MICROSTRUCTURE

DAVID L. DAVIDSON Southwest Research Institute, P.O. Box 28510, San Antonio, TX 78228 ABSTRACT The mechanisms of fatigue crack advance are examined for a lamellar a 2 + •'yalloy. Crack growth rates are highly dependent on the orientation of the loading axis to the lamellae direction. Thus, the material has some of the characteristics of a composite. For crack growth perpendicular to the lamellae, the mechanisms of crack advance are similar to those of other titanium alloys, while crack growth parallel to lamellae has other characteristics. INTRODUCTION The mechanisms and micromechanics of fatigue crack growth through the x+p3 titanium alloys, Ti-6AI-4V (RA) [1] and CORONA-5, [2] and the aC 2 +10 titanium aluminide alloys, Super Alpha 2, [3] and 2411, have been examined and compared [4]. The growth rates for large fatigue cracks through these alloys is different, but the mechanisms of growth were similar. For all these alloys, crack growth near threshold required a large number of cycles (AN) before crack advance (Aa). The sequence of events accompanying crack extension was observed to be similar to that found for aluminum alloys; a sharp crack blunted as the number of cycles increased, followed by crack extension and resharpening. For the a+p alloys, slip lines were observed to form at the crack tip during the blunting process, and crack advance occurred by breakdown of these slip lines. For the a 2 +0 alloy 2411, crack blunting was observed also, but often an a 2 particle near the crack tip broke, and crack advance occurred by linking of this broken particle with the main crack tip. This paper reports on fatigue crack growth through a titanium base alloy with a microstructure fundamentally different than any previously examined. The alloy is based on the intermetallic compound TiAI (y) mixed with a substantial volume of a 2 phase. The composition tested was Ti-47AI-0.9Cr-0.8V-2.6Nb (in at.%). Processing resulted in a microstructure of a 2 + y lamellae with some regions of equiaxed Y. Dissolved oxygen was measured as 700 ppm (weight). This alloy composition and microstructure has demonstrated higher ductility and Mat. Res. Soc. Symp. Proc. Vol. 273. @1992 Materials Research Society

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toughness than other gamma-based alloys [5,6]. MATERIAL AND MICROSTRUCTURE The microstructure of this alloy, supplied by Dr. Kim of the Metcut Materials Research Group, is complex. The large, approximately equiaxed regions seen in Fig. 1(a) are colonies of lamellae. Average size of these colonies was 1.2 mm. When a mixed acid etch was used [7], some of these platelets etched and some did not. Transmission electron microscopy (TEM), Fig. 1(b) revealed a very complex structure consisting of many wide and narrow lamellae.

Fig. 1. Microstructure of the material showing (a) SEM (secondary image) of the colony structure, and (b) TEM of lamellae. From the alloy composition and the phase diagram [8], the microstructure should consist of a mixture of Ti 3 AI (a 2 , DO 19 )