On the substructure of athermal and isothermal martensites formed in an Fe-21 Ni-4mn alloy
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I.
INTRODUCTION
M A R T E N S I T E plates with a (252)I habit plane have been observed in various ferrous 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 In general, these martensites contain more than one type of internal defects which have been suggested to implement the inhomogeneous shear(s) of the phenomenological crystallographic theory. Twins on (112)b* and stacking faults on *Subscripts b and f refer to the bcc martensite and fcc austenite, respectively.
(01 l)b a r e the most generally observed internal defects found in isolated (252),, plates. 7'8 However, there is considerable variability in substructure from plate to plate. Thus, the substructure of (252)f martensite is usually described as "complex", and a precise characterization of the (252)t substructure has not yet been made. 9 Relevant to the results presented herein, several investigations have been concerned with the substructure of (252)i martensite. 4'6'9 Plates forming a macroscopic (252)s habit plane in an Fe-Ni-Mn alloy have been found to consist of many smaller plates. 6 That is, a plate which appears to be a large single plate in surface relief actually consists of many smaller fragmentary plates with the same orientation which meet along a coalescence plane, (01 l)b I[ ( 111 )s.9 Apparently the growth of the subplates is limited by ( l l l ) j stacking faults in the austenite, presumably resulting from the accommodation of the martensite shape strain. In turn, deformation of the martensite at the coalescence sites, mainly on the twinning system (112)b [ 1 11 ]b, leads to a rather complex substructure in the overall macroscopic plates.I~ D-Z. YANG, formerly with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, is now with Dalian Institute of Technology, Dalian, China; B. P. J. SANDVIK, formerly with the Department of Metallurgy and Mining Engineering, University of Illinois, is now with the Research Center, Rautaruukki Oy, SF-92170, Raahensalo, Finland; C.M. WAYMAN is Professor in the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted October 17, 1983. METALLURGICALTRANSACTIONS A
Kajiwara 6 using an Fe-22.9Ni-3.8Mn alloy observed very thin martensite plates, corresponding to an early stage of transformation. These thin plates contained twins, but the observed interface of the twins was reported to be some 5 deg from the actual {112}b twin plane. Most of the thicker plates he observed were preferentially twinned at only one side of the plate. In that the (252)i transformation in ferrous alloys has not yet been adequately explained 9 and that there is no concensus of view concerning the crystallography and morphology of such plates, the present work is presented in the spirit of extending previous work dealing with the (252)f transformation. In the Fe-21Ni-4Mn alloy presently described, both athermal and isothermal formation of (252)s martensite occurs.
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