The separate roles of subgrains and forest dislocations in the isotropic hardening of type 304 stainless steel
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I.
INTRODUCTION
THE purpose
of this investigation is to correlate the microstructural parameters dislocation density and subgrain size with the elevated temperature flow stress in type 304 stainless steel. Such a correlation can be expressed in the equation, trlr,~ = f(A ,p)
[1]
where o-lr,~ refers to the elevated temperature flow stress at some reference strain rate and temperature, A refers to the subgrain size, and p to the dislocation density within the subgrains. There is an abundance of work described in the literature that relates the elevated temperature flow stress for various pure metals and alloys to these microstructural parameters. Such work can be grouped into two categories: (1) investigations relating the high temperature flow stress to the steady-state structure, and (2) investigations relating the high temperature flow stress to a transient structure. The equations appropriate to steady-state structure, by themselves, leave at least two important questions unanswered. First, since the steady-state subgrain size generally is directly related to the dislocation density in the subgrain interiors, these equations do not allow discrimination between the strengthening provided by each feature. Second, it is not clear whether these equations are relevant to transient structures. The results of different investigations involving transient structures 1-11 are conflicting, and the structures to which elevated temperature stress or strain rate are related are often not well defined, thus not easily facilitating a constitutive equation. A relationship such as Eq. [1] can, however, be formulated by elevated temperature tests of a variety of transient structure specimens, each having a characteristic subgrain size that is n o t directly related to its dislocation density. To produce specimens of this kind, special thermal and me-
M.E. KASSNER, formerly Research Assistant, Department of Materials Science and Engineering, Stanford University, is now with the Materials Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94550. A . K . MILLER and O.D. SHERBY are, respectively, Associate Professor and Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted August 3, 1981.
METALLURGICALTRANSACTIONS A
chanical processing has been utilized in the present investigation. First, samples were deformed (torsionally) to steady-state at a variety of temperatures and strain rates. This warm-working produced a variety of steady-state structures characterized by various subgrain sizes. After having reached steady-state the specimens were quenched. To provide dislocation densities not related to the subgrain size within individual specimens, the different steady-state structures were cold worked various amounts. The cold work (generally performed in torsion) increased the dislocation density while the other microstructural features (especially the subgrain size) remained fixed. The elevated temperature yield strengths of these structures
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