Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

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J. H. WEBER AND M. J. BOMFORD INCONEL* alloy MA 753 is a dispersion strengthened nickel-base superalloy made by *Trademarkof The InternationalNickelCompany,Inc. mechanical alloying which combines y ' precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750~ the fatigue strength of MA 753 is g r e a t e r than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is c o r r e lated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 a r e discussed as a function of temperature and m i c r o s t r u c t u r e and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the :e' p r e cipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides. T H E combination of oxide dispersion strengthening and precipitation hardening has recently been obtained in nickel-base superalloys using the mechanical alloying process. 1 The product, designated INCONEL alloy MA 753, exhibits tensile and rupture behavior similar to that of thoriated nicket at elevated temperature (>1090 K, 1500~ while at lower temperatures the strengths are typical of NIMONIC* alloy 80A, *TrademarkofTheInternationalNickelCompany,Inc. a wrought nickel-base superalloy having a composition similar to that of the MA 753 matrix. The mechanically alloyed product also exhibits good resistance to oxidation and stfifidation at elevated temperatures. Nickel-base superalloys and other high temperature alloys used in gas turbine engines a r e subjected to dynamic loading during operation. F o r alloy c h a r a c t e r i zation purposes, fatigue loadings simulating engine operation may be divided into two classes: 2 a) low f r e quency (less than 1 cycle/s) high strain fatigue and the associated problem of thermal fatigue; and b) high frequency (greater than about 2.5 • 103 cycles/min) low strain fatigue. High cycle fatigue of nickel base superalloys has been reviewed by Gell and Leverant 3 who conclude that the inherently low endurance limit at room temperature for these materials is due to the heterogeneous mode of deformation under cyclic loading. The planarity of slip in these superaltoys produces significant Stage I crack propagation. Stage H propagation also occurs in polycrystats at normal temperatures and strain rates in the absence of favorably oriented L H. WEBERis with The International Nickel Compa~ly,Inc., Paul D. Merica Research Laboratory,Sterling Forest, Suffern, New York. M. J. BOMFORDis with HenryWiggin& Company, L

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