High temperature fatigue of a polycrystalline nickel base superalloy

A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-(14-19)Co-4.75Mo-3Al-3.75Ti-1.75Ta-0 .7Hf-0.06Zr-0.02C-0.0175B (wt-%) has been undertaken. Comparison of constan t load fatigue crack growth tests conducted in air and a vacuum at 725 degr eesC and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied st ress intensity range. In particular it contributes to enhanced embrittlemen t of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in freq uency at 725 degreesC in both air and a vacuum caused an increase in the cr ack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725 degreesC rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergranular crac k growth. At lower R ratios the influence of time dependent processes is le ss pronounced, especially in a vacuum. For the purposes of prediction a str aightforward linear summation model using sustained load crack growth resul ts combined with high frequency growth rates was found to model reasonably the influence of frequency on crack growth rates for the air tests at 725 d egreesC. This can be rationalised by the observation that in air intergranu lar failure proceeds by linking of prior cracked or embrittled grains some distance ahead of the crack tip. At lower values of stress intensity range a slight under prediction of growth rates was evident, but improvements cou ld be made through the use of triangular waveform data. For the vacuum test s, the linear summation model was consistently found to over predict growth rates due to the transitory nature of the sustained load crack growth rate s under repeated loading and was not generally deemed suitable. Under these conditions damage occurs close to the crack tip and direct interaction bet ween time dependent and time independent mechanisms will occur. This is not taken into account by a summation approach and more accurate modelling of damage formation in the varying strain fields ahead of the crack tip is req uired to predict this.