PSI - Issue 16

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

139

Alexander Balitski / StructuralIntegrity Procedia 00 (2019) 000 – 000

2

40 μm (Fig. 2b, obtained by SEM). Thin structure of heat resistant superalloys has characterise by disperse phase agglomeration with dimensions from 5 to 30 nm (Fig. 2 c – f, obtained by TEM) and crack propagation has a jumping character with no less then 50…70 nm steps (Fig. 2f). a c e

b

d

f

Fig. 2. Hydrogen assisted crack initiation under cyclic loading in nickel-cobalt heat resistant new generation super alloys on macro (a), micro (b) and nano (c – f) level.

As shown in Ming (2009) under high stress take place the phenomenon of grain boundaries twisted and displacement of the grain boundaries absorb most mechanical stress applied to the specimens and generate potential wells for hydrogen migrating and settling. So, the twisted grain boundaries and the slipping plains surrounded them trapped hydrogen (Balitskii et al. (2018a), Ming (2009)). On the other hand due to unique tritium autoradiography investigation (Ming (2009)) on the nano level (when hydrogen distribution visually appear in the microstructure) it has been established, that hydrogen concentrated around the grain boundaries, inclusion and carbides. After start if the mechanical stress applied, the deformation and displacement of grain boundaries create local wells of potential energy of hydrogen migration. After that twisted and rotated grain boundaries and slipping planes surrounded them trapped significant hydrogen (Ming (2009)). Thus deformed and displaced grain boundaries transfer to the most favourable area for hydrogen assisted crack initiation and propagation in nickel-cobalt heat resistant superalloys. On the initiation stage near crack tip of Ni60Co15Cr8W8Al2Mo3 it has been established that when crack has approach to intermetallic including (400×200 μm ) the local tensions increase, after it rounding – has decreased. Hydrogen influence on cyclic crack resistance parameters appears in the decreasing of loading cycles number in hydrogenated specimens of both alloys and increase with hydrogen concentration. At highest hydrogen saturation regimes of Ni51Co15Cr9W6Al5Mo4 alloy 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 Ni51Co15Cr9W6Al5Mo4 Ni56Cr14Co15Mo5Al3Ti3 alloy it has been established that before intermetallic inclusion local stresses increased, after its passing – has decreased (which correspond with decreasing of crack opening). By fracture surface investigation it has been found the micro cracks up to 40 μm. Thin structure of heat resistant superalloys has characterised 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. 4. Conclusions