Hydrogen assisted crack initiation and propagation in nickel-cobalt heat resistant superalloys

Abstract

It has been investigated the Ni-Co alloys (obtained from powder 0.1...0.3 mm under hot gaseous (in argon) isostatic pressure (up to 300 MPa) (Ni60Co15Cr8W8Al2Mo3) (Firth Rixon Metal Ltd, Sheffield) and deformed (obtained by vacuum induced remealting) materials (Ni62Cr14Co10Mo5Nb3Al3Ti3) for gaseous turbine discs. Investigation has performed in the range of temperature 25…800°С and hydrogen pressure up to 70 MPa. By the 3D visualization of crack morphology it has been discovered the structure of fatigue crack surface and established the refer points on crack path, including the boundary between the matrix and intermetallic particles (400×200 μm), crack opening, structural elements distributions on the surface for selection of next local areas for more precision fracture surface and TEM examinations. Hydrogen influence on cyclic crack resistance parameters appears in the decreasing of loading cycles number (with amplitudes 15 MPa) in hydrogenated specimens of both alloys and increase with hydrogen concentration. At the highest hydrogen saturation regimes of Ni60Co15Cr8W8Al2Mo3 alloy (800°С, 35 MPa Н2, 36 hours, СН = 32.7 ppm) number of cycles, which necessary for crack initiation is 3 times less in comparison with specimen in initial state. At crack initiation step in hydrogenated Ni56Cr14Co15Mo5Al3Ti3 alloy it has been established that before intermetallic inclusion (400×200 μm) local stresses increased, after its passing – has decreased. By fracture surface investigation it has been found the micro cracks up to 40 μm. Thin structure of heat resistant superalloys has characterises by disperse phase agglomeration with dimensions from 5 to 30 nm and crack propagation has a jumping character with no less then 50…70 nm steps.