No abstract

No abstract

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 ЊC using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K max increasingload-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ⌬K th , when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.

Sustained-load creep tests using smooth, round bar specimens of Renè 80 were conducted at 900°C (1652°F) in two environments, namely, laboratory air and a 90% Na2SO4/10% NaCl molten salt film. Results show approximately a factor-of-two reduction in the rupture life of specimens tested in the molten salt environment. Metallographic examination of the failed specimens showed greater depths of environmental attack in the cross section as stress level increased from 207 to 310 MPa (30 to 45 ksi). Below a “critical” stress level, the depth of environmental attack remains nearly constant. Failure initiated by cracking along oxide-metal interfaces resulting from oxygen penetration and the formation of internal oxide fingers in the substrate material. A periodic unloading/reloading fatigue cycle superimposed on sustained-load creep tests had no effect on rupture lives for specimens tested in laboratory air,and the depth of environmental attack remained constant. However, rupture lives obtained in the molten salt environment were significantly reduced by the periodic fatigue cycle where spallation of the surface oxide layer during cycling was extensive and the depths of hot corrosion attack were greater.

The fatigue crack growth rate of an orthorhombic + beta titanium aluminide, nominally Ti-25Al-25Nb, was studied as a function of temperature (25 to 750°C), environment (air and vacuum), frequency (0.001 to 1.0 Hz), and superimposed hold times (1 to 1000 s) under computer-controlled constant Kmax testing conditions. In addition, fatigue crack growth rates from the near-threshold region to rates greater than approximately 10-7 m/cycle were determined at room and elevated temperatures. Results show that the fatigue crack growth rate exhibits a combination of cycle- and time-dependent behavior and is sensitive to environment over the entire temperature range. At elevated temperature, crack growth per cycle is found to increase with decreasing frequency in both laboratory air and vacuum, suggesting a contribution from environmentally assisted crack growth. Growth rates in vacuum are as much as an order of magnitude lower than those obtained in air. Further, hold times of increasing duration are found to slightly decrease and then increase the crack growth rate at elevated temperature. At elevated temperatures, crack growth behavior appears to be a complex interaction of environmental degradation at the crack tip, crack-tip blunting due to creep, and cyclic fatigue (resharpening of the crack tip). An attempt was made to correlate the observed fatigue crack growth rates with the mechanism, or mechanisms, of fracture. The crack growth characteristics were compared with those of the alpha-2 titanium aluminide, Ti-24Al-11 Nb, and a conventional high-temperature titanium alloy, Ti-1100.