Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part II. Microstru
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I.
INTRODUCTION
IN Part 11 we presented the results of a study on the effects of strain, strain rate, and strain state on the transformation behavior of 304 stainless steel sheet. We are interested in how the microstructure evolves as a function of these variables. Although we find ample evidence of dislocation multiplication and interaction, faulting, twinning and y-,e (hcp) transformation, we concentrate primarily on the deformation-induced y-~a' (bcc) transformation. Specifically, in Part I we demonstrated that the Olson-Cohen analysis is consistent with our magnetic measurements of the amount of a ' martensite produced with strain. Also, we showed that the Olson-Cohen analysis gives a reasonable estimate for martensite formation under general strain (or stress) states if the von Mises effective strain, ~vM, is substituted for the uniaxial strain, e, in the relation f~' = 1 - e x p { - /311 - e x p ( - a e ) ] n} where f~' is the volume fraction of martensite and a ' , /3, and n are parameters defined in Part I. Moreover, we found that the amount of transformation at high rates (103 per second) was greater than at low rates (10 -3 per second) for small strains and significantly less for large strains. The mechanisms and microstructural aspects of the y - , a ' transformation with deformation have been studied extensively;2~15 the effects of strain state and strain rate on the microstructure of the transformation products have not. We are aware only of some rolling deformation and shock loading experiments in addition to the common uniaxial stress experiments. We examined the transformation products from uniaxial and biaxial experiments at both low and high L.E. MURR is Vice President for Academic Affairs and Research at Oregon Graduate Center, Beaverton, OR 97006. K.P. STAUDHAMMER, Staff Member, and S. S. HECKER, Deputy Division Leader, are both with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted September 15, 1980. METALLURGICAL TRANSACTIONS A
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strain rates to check how well the assumptions made in the Olson-Cohen analysis apply to our 304 stainless steel and to determine if the conclusions reached from the macroscopic observations in Part I can be substantiated microstructurally,
II.
MECHANISMS OF THE y-~a' TRANSFORMATION
The mechanisms of the transformation and the morphology of the transformation product depend greatly on alloy chemistry in Fe-Ni-Cr-C alloys. The 304 stainless steel used for the present investigation contained 18.12 pct Cr, 8.60 pct Ni, 0.06 pct C, 1.19 pct Mn, 0.20 pct Co, and 0.14 pct each of Cu, Mo, and W. Its stacking fault energy is very low (22 ergs/cm) and hence, one expects dissociated dislocations with extended stacking faults, deformation twins, and e (hcp) martensite during room temperature deformation. From the results reported in Part I we conclude that the Md temperatures for both a ' and e-martensite are above 50 ~ and the M, temperatures for a ' martensite below - 1 9 6 ~ We did not try to,determine the M, temperature for e martensite, In v
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