Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature
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9/27/03
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Fatigue Crack Growth Mechanisms in Ti6242 Lamellar Microstructures: Influence of Loading Frequency and Temperature F. SANSOZ and H. GHONEM Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low K , however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low K is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the a/b interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.
I. INTRODUCTION
THE a/b and near-a titanium alloys are widely used in commercial and military aeroengines.[1] In the past decades, a considerable amount of research has been conducted on these materials with the objective of fully exploiting their high-temperature potential. A number of investigations have succeeded in relating the details of the processing route to the mechanical properties at elevated temperatures.[2,3] A particular advance in that domain is the processing of fully lamellar microstructures, made of colonies of similarly aligned a/b platelets within large prior-b grains, which promote lower creep strain and better long fatigue crack growth behavior than any other titanium microstructures.[4,5,6] While several attempts have been carried out to understand the influence of the microstructure features on the mechanical properties, discrepancies exist concerning the nature of creepfatigue interactions in the crack growth behavior of lamellar microstructures. It is acknowledged that the fatigue crack growth (FCG) process at elevated temperature is a complex interaction between loading frequency, temperature, microstructure, and environment.[7,8] Earlier investigations have demonstrated th
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