Microstructural investigation on deformation behavior of high purity Fe–Cr–Ni austenitic alloys during tensile testing at different temperatures P. Behjati n , A. Kermanpur, A. Najafizadeh, H. Samaei Baghbadorani Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 26 June 2014 Received in revised form 26 August 2014 Accepted 28 August 2014 Available online 9 September 2014 Keywords: Austenitic alloys Deformation Martensite Stacking fault energy Driving force abstract In this paper, optical and transmission electron microscopy techniques and tensile tests were used to investigate the deformation behavior of high purity Fe–Cr–Ni austenitic alloys during low temperature deformation. Microstructural investigations revealed that deformation-induced martensitic transforma- tions occurs through the steps γ-ε-α 0 . Remarkably, it was found that the contributions of both stacking fault energy (SFE) and driving force should be considered in explaining the occurrence of deformation-induced α 0 -martensite, which significantly affects the mechanical behavior of the alloy. Further, the findings confirmed the previous works suggesting that the low SFE of the alloy does not necessarily lead to dislocations planarity. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Fe–Cr–Ni austenitic stainless steels are the most widely used stainless steels in many important industries such as chemical, medicine, petrochemical, machinery, automobile, nuclear and shipyard, due to their excellent corrosion resistance, good weld- ability, excellent formability and high tensile strength [1]. These alloys receive their strength mainly through solid solution strengthening, strain hardening or grain refining technique. Fe–Cr–Ni austenitic stainless steels usually exhibit low yield strength, but relatively high tensile strength and elongation, i.e., their strain hardening capability is good and extends to relatively high strain levels. This motivates the search for strengthening mechanisms affecting the strain hardening rate to increase the strength of these steels without sacrificing other properties, such as ductility and toughness [2]. The strain induced phenomena such as mechanical twinning (Twinning Induced Plasticity, or TWIP) and/or martensitic reac- tions (Transformation Induced Plasticity, or TRIP), are known to enhance the strain hardening of these steels [3–6]. Plastic defor- mation of the metastable austenite phase slightly above the M s temperature (martensite start temperature) can give birth to the formation of ε and α 0 phases with hexagonal-close-packed (HCP) and body-centered-cubic (BCC) crystal structures, respectively. It has been shown that the deformation-induced martensitic transformations in the Fe–Cr–Ni austenitic alloys occur most probably through the γ-ε-α 0 steps [7]. A single stacking fault (SF) can be considered as an elementary ε -martensite embryo. The SFE can be calculated using free energy difference between perfect FCC austenite and the ε-martensite embryo [8]: SFE ¼ 2ρ ΔG γ-ε þ E str À Á þ 2σðnÞ ð1Þ where ρ is the density of atoms in a close packed plane in moles per unit area, ΔG γ-ε is the chemical free-energy difference between the austenite and ε-martensite phases, σ(n) is the surface energy and E str is strain energy, which in the case of γ-ε transformation is small. When the SFE is less than 2σ(n) term, the formation of ε-martensite becomes energetically favorable. ε bands of finite thickness can be formed from such embryos, either by a regular creation of SFs on alternate (111) γ planes or more probably, by an irregular overlapping process [9]. In the latter case, faults are created randomly at first on parallel planes and then the process changes gradually to the regular HCP stacking sequence. By contrast, when the SFE is higher than the 2σ(n) term, the overlapping on successive {111} planes should be more favorable, forming a deformation twin embryo. It is well established that deformation temperature [10–12] and chemical composition [13,14] are the most effective factors in controlling the austenite stability which is mainly attributed to their influence on SFE of the alloy [15–17]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.08.077 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ98 3113915742; fax: þ98 3113912752. E-mail address: p.behjatipournaki@ma.iut.ac.ir (P. Behjati). Materials Science & Engineering A 618 (2014) 16–21