L Journal of Alloys and Compounds 346 (2002) 158–169 www.elsevier.com / locate / jallcom Microstructural characterization of stress-induced martensites evolved at low temperature in deformed powders of Fe–Mn–C alloys by the Rietveld method a a, b * P. Sahu , M. De , S. Kajiwara a Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700032, India b National Institute for Materials Science,1-2-1, Sengen, Tsukuba 305-0047, Japan Received 3 January 2002; received in revised form 20 March 2002; accepted 20 March 2002 Abstract The present study considers X-ray characterization of the microstructures of deformation-induced martensites of Fe–Mn–C alloy powders of grain size |50 mm (hand-filed) having compositions 5.6, 5.8 and 6.0 Mn and 1.0 C (mass%). The cold-worked powders were further subjected to transformation at low temperatures close to M and the evolved phases were again characterized microstructurally. s The methodology applied for characterization involves Rietveld’s whole X-ray profile fitting technique adopting the most recently developed software, MAUD (Materials Analysis Using Diffraction) which incorporates Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s.) and preferred orientation of the crystallites. The analysis also considers lattice defect-related features of the microstructure viz. stacking, twin, compound fault probabilities and dislocation density value. The cold-worked powders (hand-filed at room temperature) revealed the highest degree of transformation with 47, 43 and 42% volume fractions of martensites with increasing Mn concentration which for the bulk state of the same alloys transformed at low temperatures are 36, 40 and 47%. The same deformed alloy powders when subjected to low temperature transformation, evolved a maximum of 60, 68 and 62% volume fractions of martensites at 170, 175 and 190 K. The analysis reveals the occurrence of a high propensity of stacking faults in the deformed austenites 22 23 12 13 22 (10 –10 ) and high values of dislocation densities (10 –10 cm ) in the austenite and martensitic phases which assist in the formation of such a high concentration of martensites in the low temperature-treated deformed powder samples.  2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Dislocations; Phase transition; X-Ray diffraction 1. Introduction powder forms and the effects of dislocations, grain sizes, specimen sizes, and external stress etc. on both the It is well known that martensitic nucleation is heteroge- athermal and isothermal martensitic transformations were neous in nature [1] and the probability of finding a considered. From the results of these systematic studies, it nucleation site for martensitic transformation in powders has been concluded that the most important controlling decreases compared to that of its bulk specimen [1]. factor for martensitic nucleation is the plastic deformation Although studies on the kinetics of martensitic transforma- of austenite to accommodate the shape strain of the tion in the bulk state of such materials are too numerous to nucleating martensite. For powders (especially fine pow- be cited [2–11], few such studies in the powder state of ders), the relaxation of strains resulting from the trans- these materials seem to have been undertaken [1,12–16]. formation is easier making initiation of plastic deformation The important factors controlling nucleation events in difficult. This slows down the process of plastic accommo- martensitic transformation have been studied systematical- dation of strains associated with the transformation reduc- ly by Kajiwara et al. [17,18] employing X-ray and electron ing or even sometimes prohibiting the transformation microscopy in a number of alloy systems (Fe–Ni, Fe–Ni– itself. Most of these earlier studies of transformation with C, Fe–Mn–Ni, Fe–Ni–Mn–C) both in the bulk and powder samples considered powders of grain sizes ranging from 10 to 150 mm [1,12–15]. However recent studies of Kajiwara et al. [15] considered ultra fine particles having *Corresponding author. Tel.: 191-33-473-4971x153. E-mail address: msmd@mahendra.iacs.res.in (M. De). diameters 20–200 nm of Fe–Ni, Co and Co–Fe alloys 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(02)00495-4