— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.

Effect of temperature on the fatigue‐crack propagation behaviour of inconel x‐750

The microstructural aspects that influenced the room temperature and elevated temperature fatigue-crack propagation response of annealed, conventional, and modified heat-treated Alloy 718 were studied Electron fractographic examination of Alloy 718 fatigue fracture surfaces revealed that operative crack growth mechanisms were dependent on heat treatment, heat-to-heat variations, temperature, and prevailing crack tip stress intensity level In the low temperature regime (below 538{sup 0}C), all fracture surfaces exhibited a faceted appearance at low {Delta} levels, which is indicative of crystallographic fracture along intense inhomogeneous slip bands The facets in the modified Alloy 718, however, were found to be rather poorly defined since the modified heat treatment tends to promote more homogeneous slip processes Under progressively higher stress intensity levels, the room temperature and elevated temperature fatigue fracture surfaces exhibited striations, followed by a combination of striations and dimple rupture at the highest {Delta} values Striation spacing measurements in all three heat-treated conditions were generally found to be in agreement with macroscopic growth rates at 24 and 538{sup 0}C Under high temperature conditions (above 538{sup 0}C), evidence of intergranular fracture was also detected on the fatigue fracture surfaces, particularly at low stress intensity levels This intergranular failure mechanism was found to be more extensive in the modified heat-treated Alloy 718 17 figures

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W.J. Mills

Papers

Effect of heat treatment and heat-to-heat variations in the fatigue-crack growth response of Alloy 718. Part 2. Microscopic observation