THE DUCTILE-BRITTLE TRANSITION IN BODY-CENTRED CUBIC TRANSITION METALS

Journal Article · Thu Feb 01 04:00:00 EST 1962 · Phil. Mag.

DOI:https://doi.org/10.1080/14786436208211853· OSTI ID:4828450

Johnson, A A

The significance of the parameters sigma /sub i/, and k/sub y/ in Petch's empirical relation sigma /sub y/ = sigma /sub i/ + k/sub y/d /sup -1/2/ between yield stress and grain size is discussed. It is concluded that sigma / sub i/ is made up of two terms sigma /sub i/- (L) and sigma /sub i/, (I). The first, sigma /sub i/(L), repr esents the stress required to propagate a Luders band through a single crystal having no lattice friction stress. It is a function of the stress, sigma /sub d/, required to unpin a dislocation from its impurity atmosphere and of the work-hardening Properties of the metal. The second, sigma /sub i/ (I), is a friction stress probably resulting from the presence of impurity atoms. The grain size parameter, k, measures the extra work- hardening which occurs as a result of forced slip during the propagation of yielding through a polycrystal. It is a function of the work-hardening rate of the metal and of the unpinning stress. A criterion for brittle fracture is derived which is based on the observation that, at and above the ductile-brittle transition temperature, a growing microcrack is unable to penetrate the region of forced slip near the boundary of the grain in which it is nucleated. This criterion is used to calculate values of gamma /sub e/, the effective surface energy for fracture, for a number of b.c.c. transition metals. The interpretation of these values of gamma /sub e/ is discussed and it is suggested that for a metal to be brittle it must have both strongly locked dislocations and a high work-hardening rate. A new interpretation of existing experimental results on the hardening and embrittlement of mild steel and molybdenum by neutron irradiation is developed. The deviation from Petch's relation shown by irradiated molybdenum is shown to result from the combination of a particularly high unpinning stress and a high work-hardening rate. (auth)

• Research Organization: Imperial Coll. of Science and Tech., London
• Sponsoring Organization: USDOE
• NSA Number: NSA-16-021068
• OSTI ID: 4828450
• Journal Information: Phil. Mag., Journal Name: Phil. Mag. Vol. Vol: (8), 7
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

ATOMS
BRITTLENESS
CARBON STEELS
CRACKS
CRYSTALLIZATION
CRYSTALS
DEFECTS
DEFORMATION
DISLOCATIONS
DUCTILITY
EQUATIONS
FAILURES
FRICTION
GRAIN SIZE
IMPURITIES
LATTICES
METALS, CERAMICS, AND OTHER MATERIALS
MOLYBDENUM
MONOCRYSTALS
NEUTRON BEAMS
NUCLEATE BOILING
SLIP
STRESSES
SURFACE TENSION
TEMPERATURE
TEMPERING
TESTING
TRANSITION METALS