Dynamic Strain Aging of Various Steels
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I.
INTRODUCTION
THE elastic
interaction between interstitials and dislocations in steels results in strong dislocation pinning and leads to the important phenomenon of strain aging. Two types of strain aging can be characterized--static strain aging where the aging process takes place after prestraining, and dynamic strain aging where aging is sufficiently rapid to occur during straining. Static strain aging results in the reappearance of the upper yield point and yield point elongation, and dynamic strain aging leads to inhomogeneous deformation characterized by serrated flow (the Portevinle Chatelier effect). In both cases, however, there is an increase in flow stress and work hardening rates and a decrease in ductility. 1 Using the terminology of Baird, 1 the steels first undergo "jerky" flow and at higher temperatures exhibit the more pronounced "serrated" flow. During jerky flow, the mobile dislocations are weakly pinned by the interstitials, and there is no discrete sharp drop in load at regular intervals as is during serrated flow. Dynamic strain aging will occur when the rate of straining is such that the interstitials can diffuse and pin the mobile dislocations, and serrations occur because of a rapid generation of new dislocations which are needed to sustain flow. The probability of remobilizing immobile dislocations is essentially zero in this regime. 4 In the process of generating new dislocations the stress increases, but once the dislocations are released, the stress drops to sustain their movement till the interstitials diffuse and repin these dislocations at which point the stress increases again to generate new dislocations. Static strain aging has been more thoroughly investigated than dynamic strain aging. In general, the mechanical property changes produced by strain aging appear to be directly related to interstitial content. Rashid 2'3 has shown that in high strength low alloy steels (HSLA), the strength increases due to static strain aging are lower and the kinetics slower than in 1008 steels. The lower increase was attributed to the lower amount of free interstitials in the HSLA steels, most of the carbon and nitrogen existing in combination with the microalloying elements, and the slower ANIL K. SACHDEV is Staff Research Engineer, Metallurgy Department, General Motors Research Laboratories, Warren, M1 48090. Manuscript submitted March 23, 1981. METALLURGICALTRANSACTIONS A
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kinetics to interactions between interstitials and the microa~loy carbonitrides. 2 Published data on the dynamic strain aging of 1008 steels have shown that ductility is severely reduced at temperatures between 100~ and 250 ~ 4 and that the onset of serrations has an activation energy equal to that of interstitial diffusion in ferrite. 5 A systematic comparison of the dynamic strain aging characteristics of dual-phase, HSLA, and 1008 steels is available only to the extent of room temperature properties subsequent to dynamic strain aging (between 20 ~ to 600 ~ and for prestrains only up to 5 pct. 6'7 The purpose of thi
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