Creep-rupture kinetics of heat-resistant alloys

Conclusions

• 1. By using vacuum metallography it was shown that, in Ni−Cr base heat-resistant alloys, grain boundary microcracks substantially normal to the tensile stress appeared after 30–50% of the expected life of the specimen under creep conditions. Their number and size increased with time and eventually led to complete rupture.

• 2. Failure during creep proceeded in two stages: first stage, gradual development of cracks along grain boundaries; second, short-time fracture. The loss of strength with time was associated with grain boundary crack formation.

• 3. Intragranular slip was not observed by vacuum metallography during high-temperature testing. This was at least true of magnifications under 200 dias. Rupture occurred by growth of grain boundary cracks. This is why Ni−Cr alloys showing a 10–15% elongation in short-time rupture tests failed most brittlely under creep conditions at the same temperature, with only 1–3% elongation.

• 4. In tests of relatively short duration, the total elongation at fracture was considerably higher due to extensive intragranular slip.

• 5. In cast ZhS3 alloy, grain boundary cracks appeared at later creep stages than after preceding working since their development in the cast alloy was hampered by carbide precipitates.

• 6. Removal of the surface layer by electropolishing considerably increased the pre-crack formation period thereby lengthening the useful life of the material.