Some trends observed in the elevated-temperature kinematic and isotropic hardening of type 304 stainless steel
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I.
INTRODUCTION
THISwork is part of a larger study to determine and quantify the microstructural origins of the two macroscopically measurable components of material strength or flow stress (cry), namely, the "isotropic" and "kinematic" components. In a previous investigation by two of the present authors,1 a root-mean-square (rms) equation was developed which related the elevated-temperature isotropic strength, or1 (that part of the strength that is independent of orientation or direction) of Type 304 stainless steel to its current subgrain size and the forest dislocation density. The equation was developed for various substructures tested under a single (standard) set of conditions (1023 K and a strain rate of 5.56 = 10_4 s-l). This strain-rate and temperature combination lies within the power-law-breakdown regime. In the second part of the investigation (reported here) behavior at 1138 and 1338 K, within the power-law regime, has been studied. Since it is commonly believed that the mechanism(s) of plastic deformation in pure metals and subgrainforming alloys within the power-law-breakdown regime is different from that in the power-law regime, it was not obvious without this study whether or not the predictions of the root-mean-square equation would be correct for behavior within the power-law regime. Furthermore, at 1023 K, it was found previously that forest dislocations dominate the strengthening; therefore an additional objective was to determine whether or not this trend was apparent within the power-law regime. The two objectives were accomplished by torsionally deforming annealed specimens (at a fixed strain rate) at the power-law temperatures to various transient or primary creep strains. (During primary or transient creep, the material undergoes hardening and the dislocation microstructure changes. Over secondary or steady-state creep the material
does not harden and the microstructure is generally regarded as fixed.) The specimens were then quenched and the dislocation microstructure was observed by transmission electron microscopy. The microstructure was then compared to the isotropic component of the measured flow stress for (a) consistency with the root-mean-square equation and (b) a determination whether forest dislocations still dominated the strength. Kinematic (or directional) hardening was also s t u d i e d , at 1023 and 1123 K. At each temperature, Bauschinger effect (strain-rate reversal) tests were performed at various primary creep strains. The "reverse" yield stress, o-r, was measured and the back stress, ~rB, was calculated [crB = (err- err)/2]. The back stresses determined from these tests were analyzed by considering the corresponding microstructures. Specifically, we were interested in whether or not the development of back stresses correlates with formation of a heterogeneous substructure.* *In their classic sense, "isotropic" strengthening refers to a uniJbrm expansion of the original yield locus about the origin while "kinematic" hardening refers to a uniform translation of the o
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