Chemical and Microstructural Aspects of Creep Crack Growth in Inconel 718 Alloy

Abstract

Coordinated surface chemistry and microstructural studies were carried out, in conjunction with crack growth measurements, to better understand environmentally enhanced creep crack growth in Inconel 718 alloy. Creep crack growth response was determined as a function of stress intensity factor (K) and temperature in air, pure oxygen, moist and dry argon. Crack growth was found to be thermally activated, with apparent activation energies of about 287 f 46 kJ/mol in pure oxygen and 19 1 f 77 kJ/mol in moist argon. The growth rates were independent of oxygen pressure from 2.67 to 100 kPa at 973 K, and were about four orders of magnitude faster than those in high purity argon. Fractographic examinations showed predominantly intergranular cracking and the presence of copious amounts of niobium carbides. X-ray photoelectron spectroscopic (XPS) examinations showed that the oxides on the fracture surfaces were enriched in niobium oxide near the crack tip, and with iron and molybdenum away from the tip. Parallel XPS studies showed considerable segregation of niobium at the grain boundaries of polycrystals, and significant enrichment of single crystal alloy surfaces with niobium after heating for 1 h at temperatures above 775 K (with surface concentration in excess of 25 at% at 975 K). These results and observations suggest that niobium has a strong surface affinity in IN718 alloy and may be responsible for the environmental enhancement of creep crack growth at high temperatures. The oxidation and decomposition of niobium carbides as a source for segregated niobium and as the rate controlling process for crack growth are discussed. Niobium as a source for embrittment of other nickel-base superalloys is considered.