Materials Science and Engineering A273 – 275 (1999) 507 – 511 Effects of thermal cycling and plastic deformation upon the Gibbs energy barriers to martensitic transformation in Fe-Mn and Fe-Mn-Co alloys A. Baruj a,b, *, A. Ferna´ndez Guillermet a,b , M. Sade a a Centro Ato´mico Bariloche, CNEA, (8400) S. C. Bariloche, Argentina b CONICET, S. C. Bariloche, Argentina Abstract In a previous study of the effect of thermal cycling (TC) upon the fcc/hcp martensitic transformation (MT) in the binary system Fe-Mn, we evaluated the so-called resistance-to-start-the-transformation energy (RSTE) by applying models for the Gibbs energy of the fcc and hcp phases, and found a well defined pattern of variation as a function of the number of phase transformation cycles. In the first few cycles, the so-called promotion effects upon the MT were observed, but in the following cycles inhibition effects were predominant. In the presented work we further explore the applicability of our previous generalizations, by focusing on a ternary system: the Fe-Mn-Co system. We show that Co additions do not modify the pattern of variation of the RSTEs with the number of cycles. The effect of inducing plastic deformation (PD) of the fcc phase on the MT temperatures is also studied and compared with the effect of TC. The inhibition effect caused by PD is qualitatively the same as that of TC, as hypothesized in our previous work. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Martensitic transformations; Fe-Mn; Fe-Mn-Co; Thermal cycling; Plastic deformation www.elsevier.com/locate/msea 1. Introduction In a previous paper [1], a study of the effect of the thermal cycling (TC) upon the fcc/hcp martensitic transformation temperatures (MTTs) in the Fe-Mn sys- tem was presented. One of the key ideas of that work was to use Gibbs energy functions (G m ) to express the effects of TC in terms of the so-called resistance-to- start-the-transformation energy (RSTE) for each trans- formation, a quantity which is defined later on in this paper (Section 4). The variation of the RSTEs with the number N of martensitic transformations (MTs) in- duced by TC in the material, revealed a systematic pattern of behavior, namely, a promotion of the MT during the first few thermal cycles, followed by an inhibition of the transformation during the rest of the TC process. On the basis of such information, and transmission electron microscopy (TEM) observations by other authors [1], a microstructural picture was developed, which was aimed at explaining the evolution of the nucleation and growth of the nuclei during the TC process. The promotion step was associated with the formation of nucleation centers for the hcp phase (i.e. stacking faults and twins) in a matrix with an initially low density of crystalline defects. Since the density of dislocations and other possible obstacles to the transformation increases with N, TC was expected to cause a progressive inhibition of the transformation, in complete accord with the observations [1]. More recently, two different approaches were adopted in an attempt to refine this picture. The first approach con- sists of TEM observations in Fe-Mn samples at various steps of the TC process, which are aimed at identifying the type of crystalline defects and their interactions. The second approach focuses on the effects upon the MTTs of gradually modifying the microstructure through TC and plastic deformation (PD) in ternary alloys. This paper deals with the results of applying the latter to Fe-Mn-Co alloys. There are two main reasons for choosing Co as a ternary alloying element. First, the Fe-Mn-Co system forms the basis of higher-order sys- * Corresponding author. Tel.: +54-2944-445290; fax: +54-2944- 445299. E-mail address: baruj@cab.cnea.gov.ar (A. Baruj) 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0921-5093(99)00389-5