Metallurgical Transactions

, Volume 1, Issue 11, pp 3203–3213 | Cite as

Unstable flow in martensite and ferrite

  • M. J. Roberts
  • W. S. Owen
Mechanical Behavior


It is proposed that unstable plastic flow of iron alloys containing carbon or nitrogen occurs when (∂ lnν/∂τ *) T becomes negative (v is the average velocity of the dislocations andτ * the effective stress acting upon them) at the maximum in the force-velocity curve deduced from either the Snoek or the Cottrell steady-state drag model. In addition, a nucleating event is necessary. A model is developed which predicts relationships between the applied stress, the strain rate and the temperature at the onset of unstable flow, and also the activation energy associated with the event. Experiments have been carried out on iron-carbon and iron-nickel-carbon alloys covering a range of carbon concentration in solution. The alloys were either annealed ferrite or martensite and, thus, the extremes of the range of possible dislocation density were examined. Two distinct types of plastic instability were identified: jerky flow, the result of Snoek interaction, and serrated flow due to Cottrell drag. All the qualitative and quantitative features of the phenomenon which were examined were found to be in complete accord with the model. The average velocity of the dislocations as a function of the temperature at the start of unstable flow has been deduced from the model and an estimate of the density of dislocations moving at the time has been made using measured values of the critical strain rate. The strain rate and temperature at which unstable flow disappears were measured as a function of the carbon concentration and the two types of substructure. The data show that an explanation based on the assumption that the phenomenon involved is the same as that producing the Köster internal-friction peak is untenable. An alternative suggestion, based on the relative stability of a Cottrell cloud and a carbide precipitate, is discussed briefly and qualitatively.


Ferrite Martensite Carbon Concentration Serrate Flow Unstable Flow 
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Copyright information

© The Minerals, Metals & Materials Society - ASM International - The Materials Information Society 1970

Authors and Affiliations

  • M. J. Roberts
    • 1
  • W. S. Owen
    • 2
  1. 1.Homer Research LaboratoriesBethlehem Steel Corp.Bethlehem
  2. 2.The Technological InstituteNorthwestern UniversityEvanston

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