Fatigue Behavior in Nickel.Based Superalioys: A Literature Review L. Garimella, P.K. Liaw, and D.L. Klarstrom Authors' Note: [nconeL [ncoloy, and Nicalon are regzstered trademarks. In this literature review, tl,e present un- derstanding regarding the effects of micro- structure, loading conditions, and environ- ments on the fatigue behavior of nickel-based superalloys is reviewed. INTRODUCTION Superalloys are alloys developed for elevated-temperature service; the term was first used shortly after World War II to describe a group of alloys developed for use in turbosuperchargers and air- craft-turbine engines that required high- temperature performance. In addition to good high-temperature strength, su- peralloys also exhibit oxidation and cor- rosion resistance. Superalloys usually consist of various formulations made from elements such as nickel, cobalt, iron, and chromium as well as lesser amounts of W, Mo, Ta, Nb, Ti, and A1. Manv types of alloys come under the broad coverage of superal- loys, including iron-based alloys con- taining chromium and nickel, complex iron-nickel-cobalt compositions, carbide- strengthened cobalt-based alloys, solid- solution strengthened cobalt-based al- loys, and precipitation- or dispersion- strengthened nickel-based alloys. Super- alloys are used both in wrought and cast form. The high-temperature applications of superalloys are extensive, includingcom- ponents for aircrafts, chemical-plant equipment, and petrochemical equip- ment. Superalloys are used as disks, bolts, shafts, cases, blades, vanes, burner cans, afterburn- ers, and thrust reversers in air- craft and industrial gas turbines; stock gas reheaters in steam-tur- ~ bine power plants; turbocharg- ers, exhaust valves, hot plugs, precombustion cups, and valve- ~. seat inserts in reciprocating en- gines; hot-work tools and dies g and cast dies in metal process- ing; prosthetic devices and den- tistry; aerodynamically heated skins and rocket-engine parts in space vehicles; trays, conveyors, and fixtures in heat-treating equipment; and control-rod drive mechanisms, valve stems, springs, and ducts in nuclear power plants. Nickel-based superalloys are the most widely used of all the classes of superal- loys. (Descriptions of some of the super- alloys beyond nickel-based alloys are readily available in the literature, t-25) They have a complex composition and good high-temperature properties, and their use extends to the highest homolo- gous temperature of any common alloy system.' Fatigue properties in nickel-based alloys are affected by different parameters relating to microstructural and testing conditions. A great deal of attention is currently being given to the low-cycle fatigue per- formance of superatloys, particularly in the field of turbine-engine designs, where there is every indication that it is likely to become the deciding factor in the selec- tion of an alloy. Besides providing fun- damental information relating to the performance of individual alloys, low- cycle fatigue data may be applied to pre- dict the life of engineering components when subjected to a similar straining cycle as that simulated in a laboratory. Fatigue properties in nickel-based al- loys are affected by different parameters 1100 ~-i~.., o, , , , ~ .... , .... , .... , , , 1000 i~ ~a ~a~%'~"+~ ae(t)/2 = 2.6~ a a ~,.~-%.= ao ~ 9 a ,,+a 900 I T = 25~ 9 Under-Aged + Peak-Aged 800 a Over-Aged 700 600 z~e(t)/2 = 0.57% 5 0 0 ~ , , , ~,,,I , t , ! , , , , I ~ , , , , , , , 0 1 2 3 4 5 Cumulative Plastic Strain relating to microstructural and testing conditions. FATIGUE BEHAVIOR For several decades, fatigue has been known as one of the major causes of failure in engineering structures. Sev- eral parameters have now been identi- fied as playing significant roles in affect- ing the growth of defects. 26-37 These are intrinsic parameters like alloying, heat treatment, microstructure, and elasto- plastic behavior; the mechanical factors such as the crack geometry, load ampli- tude, and stress load ratio* and the physico-chemical parameters, including the nature, composition, and tempera- ture of the environment surrounding the crack tip. Most materials for engineering appli- cations are subjected to fluctuating stresses; this accounts for a large num- ber of failures. These failures are due to fatigue and must be distinguished from static failures. Most failures are due to a combination of steady-state and cyclic stresses. The contribution of each of these stresses would depend on a host of pa- rameters, which would include the rela- tive stress levels for each and the tem- perature of operation. Superalloys must constantly combat the result of the su- perposition of these stresses at elevated temperatures. Hence, failure in most cases is neither pure fatigue nor pure creep. The study of these mechanisms is important since it gives an insight into the contribution of each. Fatigue testing encompasses all mechanical testing in which fluctuating stresses are consid- ered. Hence, a wide range of dif- ferent conditions are involved. A high proportion of fatigue tests are conducted with specific en- gineering applications in view. This makes it difficult to com- pare the results of different al- loys. The available data are some- times of limited applicability, and the comparison of proper- ties is not straightforward. Fatigue can be classified in many ways based on different 6 parameters. Based on the num- ber of cycles for failure, it can be Figure 1. Cyclic response curves for the underaged, peak- aged, and overaged materials as a function of cumulative plastic strain tested at ke(t)/2 = 0.60 and 2.60%. 30 Stress ratio, otherwise called load ratio or R-ratio, is defined as c,,,,,/c,~,,, where ~ and c are the minimum and maxi- mum stresses, respectively. 1997 July | JOM 67