An investigation into the effects of hydrogen on the fracture and deformation behavior of Alloy X-750. Ph.D. Thesis

Thesis/Dissertation · Sat Jan 01 04:00:00 EST 1994

OSTI ID:236769

Symons, D M

This study investigated the hydrogen embrittlement of a nickel-base superalloy, Alloy X-750. The effect of hydrogen was examined through tensile and fracture toughness testing incorporating observations from scanning electron microscopy and light microscopy. The elongation for tensile specimens tested at 3.33 x 10(exp {minus}4) s(exp {minus}1) was reduced from 21% for noncharged specimens to 7.3% for 5.7 ppm hydrogen and was further reduced to 3.5% for 65 ppm hydrogen. The elongation further decreased as the strain rate was decreased for hydrogen-charged specimens while for noncharged specimens no strain rate effect was observed. Straining prior to hydrogen charging dramatically decreased elongation after charging, though in all cases the flow stress was achieved prior to failure. This showed plastic deformation was required for fracture. The intergranular failure mechanism was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence resulting in a fracture surface for all specimens consisting of intergranular ductile dimples. The effect of hydrogen was also investigated through fracture toughness testing. A model based on a critical strain criterion correlated the fracture behavior as a function of hydrogen. The fracture toughness of the noncharged specimen was 147 MPa-m(exp 1/2) and decreased to 52 MPa-m(exp 1/2) at a rapid loading rate and further decreased to 42 MPa-m(exp 1/2) for a slower loading rate. This was consistent with the need for hydrogen diffusion to enhance the embrittlement. The effect of hydrogen on the deformation behavior was studied to determine if there was a change in the deformation behavior, and how that would affect the fracture. The effect of hydrogen on the activation area was examined through stress relaxation testing and strain rate change tensile testing incorporating observations from transmission electron microscopy on the dislocation substructure.

• Research Organization: Carnegie-Mellon Univ., Pittsburgh, PA (United States)
• OSTI ID: 236769
• Report Number(s): N--96-21487; NIPS--96-33279
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CARBIDES
DEFORMATION
ELONGATION
FRACTURE PROPERTIES
GRAIN BOUNDARIES
HEAT RESISTING ALLOYS
HYDROGEN EMBRITTLEMENT
MICROSTRUCTURE
VOIDS