The substructure of (252) f martensite formed in an Fe- 8Cr- 1 C alloy
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I.
INTRODUCTION
FERROUS plate martensite exhibiting a habit plane close to (252)f is formed within certain composition limits in alloy systems such as Fe-C, 1 Fe-Cr-C, 2 Fe-Mn-(Cr)-C, 3'4 Fe-Ni(Cr)-C, 5 and Fe-Ni-Mn. 6 For example, in Fe-C alloys the (252)f transformation is associated with intermediate carbon contents (0.8 to 1.4 pct C). At low carbon content lath martensite exhibiting a habit plane close to {111}fis formed, while at high carbon content, i.e., 1.8 pct C, the martensite is plate shaped and exhibits the (3 15 10)f habit plane. The transformation crystallography of (252)f martensite has not yet been successfully explained by using the phenomenological crystallographic theories, 7'8 which appear to apply to most other martensite transformations, including the (3 15 10)f transformation in ferrous alloys. Thus, the (252)f transformation has been classified as a "nonbehaving" transformation and considerable efforts have been made in order to establish the reason for this unusual behavior. The crystallography of the (252)f transformation has been extensively investigated and will be described in the following paper. Several investigations have also been concerned with the substructure of (252)f martensite. Shimizu et al. 9 observed very thin martensite plates, obviously representing an early stage of the transformation, in an Fe-8Cr-IC alloy. Similar thin plates were subsequently observed also in other alloys exhibiting the (.252)f transformation,6'10'~l although they are not observed in alloys exhibiting the (3 15 10)f transformation. Twins on the (l12)b plane, which is the complementary shear plane in (3 15 10)f martensite, ~2have been
B. P. J. SANDVIK, formerly with the University of Illinois, is now at the Laboratory of Physical Metallurgy, Helsinki University of Technology, Espoo 15, Finland. C. M. WAYMAN is with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted February ll, 1983. METALLURGICALTRANSACTIONSA
observed even in the thin plates. 6'9 The twins are believed to be a result of the complementary shear of the crystallographic theories. 7'8 Stacking faults or dislocations on the (lll)f plane are usually observed in association with the edges of thin plates. 6'9 Only those three {252}f habit plane variants which make an angle of about 25 deg with the faulting plane are observed in association with a certain {lll}f plane. 9 It has not been established if the martensite is nucleated from the faults or if the faults are posttransformational defects which formed because of stress interaction of the austenite with the martensite shape transformation strain. Thick martensite plates typically contain a variable density of twins on the (l12)b plane, m3 The twins are usually concentrated to only one side of the plate, but sometimes they may exist inside the plates and form a "mid-rib", along which the twin density is highest. 13,14Considerable bending of the twins has also been observed. 13Untwinned regions of thick
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