PSI - Issue 16

Alexander Balitskii / Procedia Structural Integrity 16 (2019) 134–140 Alexander Balitskii / Structural Integrity Procedia 00 (2019) 000 – 000

138

5

This advantage gives the opportunity more adequate reflect the influence of operating environments on durability and life time of structural materials. Crack propagation rate was determined as a relation of increase of her length of a between two successive measuring of n and ( n+ 1):

1 da dN N N  a a    n n

.

(3)

n

n

1



By the 3D visualization of crack morphology it has been discover the structure of fatigue crack surface (full reconstruction of complete crack path after the fatigue tests on hydrogenated nickel-cobalt heat resistant alloys has shown on Fig. 1) and established the refer points on crack path, including the boundary between the matrix and intermetallic particles, crack opening, structural elements distributions on the surface for selection of next local areas for more precision fracture surface and TEM examinations.

Fig. 1. Reconstruction of complete crack path after the fatigue tests on hydrogenated nickel-cobalt heat resistant new generation super alloys.

Under the cyclic loading it has been established the Paris equation parameters ( c and n , see Table 5), which describe the linear part of da / dN curves (stable fatigue crack propagation range) at various hydrogen saturation parameters and hydrogen concentration in materials.

Table 5. Hydrogen influence on crack propagation parameters of Ni60Co15Cr8W8Al2Mo3 alloy. Hydrogen concentration, ppm , MPa m  K da / dN , mm/cycles С

n

R 2

min

max

min

max

0

27.12 24.74 23.11

68.55 55.72 49.40

9×10 -6

3.6×10 -2 1.7×10 -2 1.5×10 -2

6×10 -19 7×10 -19 1×10 -18

9.177 9.390 9.472

0.986 0.986 0.995

27.8 32.7

1.02×10 -5 1.09×10 -5

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 highest hydrogen saturation regimes of Ni51Co15Cr9W6Al5Mo4 alloy (800 °С, 35 MPa Н 2 , 36 hours, С Н = 32.7 ppm) number of cycles, which necessary for crack initiation is 3 times less in comparison with specimen in initial state. Durability of deformed Ni62Cr14Co10Mo4Al4Ti4 after hydrogenation (800 °С, 35 MPa Н 2 , 36 hours, С Н = 26.8 ppm) has decrease in 1.5 times. At crack initiation step in hydrogenated Ni51Co15Cr9W6Al5Mo4 Ni56Cr14Co15Mo5Al3Ti3 alloy it has been established that before intermetallic inclusion (400×200 μm ) (Fig. 2a, obtained by optical microscope) local stresses increased, after its passing – has decreased. By fracture surface investigation it has been found the micro cracks up to