Accession Number:

AD0600515

Title:

MICROMECHANISMS OF CLEAVAGE FRACTURE IN POLYCRYSTALLINE IRON.

Descriptive Note:

Progress rept. no. 6,

Corporate Author:

MASSACHUSETTS INST OF TECH CAMBRIDGE

Personal Author(s):

Report Date:

1964-05-01

Pagination or Media Count:

62.0

Abstract:

The initiation and propagation of cleavage microcracks in coarse-grained vacuum-melted ferrite, containing 0.035 and 0.007 per cent carbon, were studied by means of tensile tests carried out between room temperature and -196 C, and by special metallographic procedures. Cleavage microcracks develop in ferrite during the strain-hardening portion of the stress-strain curve at low temperatures, and are initiated mainly by cracks which form in the carbides. Twinning does not play an important role in crack initiation over the entire temperature range studied. Carbide cracks during plastic deformation at all temperatures investigated, but they lead to microcracks in the ferrite only when the applied stress is high enough to permit the carbide cracks to act as Griffith cracks. Carbide cracks also lead to the formation of large voids during the necking of specimens tested in the ductile and transition temperature regions. Pre-existing twins provide strong barriers to microcrack propagation. Twinning also causes the disappearance of the discontinuous-yield phenomenon at low temperatures. A model for microcrack initiation by carbide cracking is proposed, and the conditions leading to brittle fracture are discussed. Author

Subject Categories:

Distribution Statement:

APPROVED FOR PUBLIC RELEASE