High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939
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I.
INTRODUCTION
THE basic understanding
of life determining factors under high temperature loading is difficult not only in the case of creep-fatigue interaction but also under pure creep or fatigue loading. One of the major problems is the separation of the stages of crack initiation and crack propagation. Life predictions become clearer as soon as crack propagation from existing defects is the predominant failure mechanism and fracture mechanics methods can be applied. Besides environmentally induced surface defects, such as pits, mainly inclusions or pores are preferred starting points for fatigue cracks. The effect of environment on fatigue life in terms of fracture mechanics was theoretically discussed in a recent investigation.~ It was possible to determine quantitatively the corrosion fatigue life for three different types of corrosion behavior. The problem of critical defect size and fatigue crack growth in P/M-alloys was also recently discussed. 2 The effect of inclusions on the stress controlled fatigue behavior was investigated on the P/M-alloy Udimet 7 0 0 . 3 The inclusions were treated as sharp cracks. The calculations were performed for surface cracks as well as for internal cracks and always the calculated curves gave a conservative bound of the measured values. Small differences between measured and calculated values were explained with an initiation period. For spheroidal graphite cast irons, fatigue life was found to be dominated by crack growth from micropores. 4 For the cast nickel base superalloy IN 939, metallographic evidence exists 5 that casting pores play the dominant role in the HCF-life. To find a quantitative relation between fatigue life and casting porosity in this alloy and in the very similar alloy IN 738 LC is one aim of the present investigation. An essential requirement for this purpose is a knowledge of the laws of fatigue crack growth under several different conditions. For high temperature materials, fatigue crack growth is usually studied in terms of the linear elastic AK-concept, even at high temperatures. 6ql It has been demonstrated 12 that this linear elastic parameter can be satW. HOFFELNER is Research Scientist with Brown Boveri Research Center, CH-5405 Baden, Switzerland. Manuscript submitted August 26, 1981. METALLURGICAL TRANSACTIONS A
isfactorily used for a nickel-base alloy (Udimet 700) up to 850 ~ For the cast Ni-base superalloy IN 738 LC some fatigue crack growth measurements were performed within the COST-50 program, a European collaborative program on materials for gas turbines. These results were summarized by Speidel and Pineau. 13 At RT and at 850 ~ a strong influence of mean load on the fatigue threshold was found in air and in the Paris region a high amount of scatter between different laboratories could be seen. With the same alloy the influence of microstructure on fatigue crack growth rates at different temperatures was investigated by Scarlin. ~4 The small differences detected between the normal heat treated condition and additionally aged samples
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