Volume 2, Issue 7 (July 2005)
Comparison of the Corrosion Fatigue Characteristics of CP Ti-Grade 4, Ti-6Al-4V ELI, Ti-6Al-7Nb, and Ti-15Mo
The purpose of this study was to evaluate and compare the corrosion fatigue (CF) characteristics of a series of titanium alloys including Grade 4 CP Ti (ASTM F 67), Ti-6Al-4V (ASTM F 136), Ti-6Al-7Nb (ASTM F 1295), and Ti-15 Mo (ASTM F 2066). Evaluation of alloy composition, microstructure, and Vicker's microhardness was performed to ensure that each material met the required specification. Smooth and notched CF tensile samples of each alloy were machined using low stress grind techniques and tested at 1Hz according to ASTM F 1801 in both Ringer's solution and distilled/deionized water at 37°C. Smooth CF samples had a 10 mm gauge length and a 2.5 mm gauge diameter, and notched CF samples had a 2.5 mm notch root diameter (Kt=3.2). A minimum of three samples was tested at five tension-tension sinusoidal load levels including a run-out level at 106 cycles. SEM analysis was performed on the fractured surfaces of representative samples of each alloy to characterize and compare the failure mechanisms.
Fatigue results revealed no differences between the smooth and notched samples of each alloy run in distilled water and those run in Ringer's solution. Results indicated corrosion fatigue mechanisms were not contributing to the fractures under these conditions. However, a significant reduction of fatigue strength was observed for the notched samples of each alloy compared to the smooth samples in identical solutions. These results suggest that even though there is no notch sensitivity in these alloys under static conditions, a notch under dynamic fatigue mechanisms may cause a substantial reduction in cycles to implant fracture. SEM analysis showed typical fatigue fracture morphologies on both the smooth and notched samples. Notch samples, however, did show more defined fluting in terracing due to the higher tri-axial stress state at the root of the notch. In conclusion, corrosion fatigue mechanisms were not contributing to the fracture of these alloys under the given conditions, but the presence of a notch significantly reduced the fatigue strength of all alloys tested.