Effect of Composition, Mechanical Alloying Temperature and Cooling Rate on Martensitic Transformation and Its Reversion
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Effect of Composition, Mechanical Alloying Temperature and Cooling Rate on Martensitic Transformation and Its Reversion in Mechanically Alloyed Stainless Steels Gökhan Polat1 · Hasan Kotan1 Received: 22 July 2020 / Accepted: 19 August 2020 © The Korean Institute of Metals and Materials 2020
Abstract Stainless steels with Fe/Cr/Ni ratios of 74/18/8, 71/17/12, and 55/20/25 were produced from elemental powders by high energy mechanical alloying at both room and cryogenic temperatures. The effect of mechanical alloying temperature on martensitic transformation, the reversion of deformation-induced martensite-to-austenite upon annealing, and the influence of cooling rate on the thermal stability of reversed austenite upon cooling to room temperature were investigated in detail by in-situ and ex-situ X-ray diffraction (XRD) experiments, transmission electron microscopy (TEM) and Thermo-Calc simulations. A relative comparison of stainless steels after room temperature mechanical alloying indicated that the low nickel-containing steel underwent an almost complete martensitic transformation. However, martensitic transformation by deformation through mechanical alloying at room temperature would not be possible with increasing nickel contents but was created partially at cryogenic temperature, the degree of which depended on the steel composition. The in-situ XRD studies exhibited that the deformation-induced martensite completely transformed to austenite at elevated temperatures. The complete reverse transformation temperature simulated by Thermo-Calc software was found to be lower than that of the experimentally determined ones. Additionally, the different cooling rates from the reversed austenite demonstrated that the slower cooling increased the thermal stability of reversed austenite at room temperature. Keywords Deformation-induced martensitic transformation · Cryogenic milling · Reverse transformation · Cooling rate · Austenite stability · Thermo-Calc simulation
1 Introduction There has been a growing interest in using severe plastic deformations (SPD) in the production of structural metallic materials for various engineering applications with a superior combination of ductility and strength [1–3]. In this regard, extensive investigations have been carried out on the deformation of austenitic stainless steels triggering austenite-to-martensite phase transformation, and subsequent annealing of deformation-induced martensite leading to reverse transformation back to austenite [4–6]. The amount of martensite formed after severe deformation and retained after thermal processing dramatically alters the mechanical * Hasan Kotan [email protected] 1
Department of Metallurgical and Materials Engineering, Necmettin Erbakan University, Konya 42090, Turkey
properties [7–10] as it enhances the strain hardening capability of the steels and assures a better combination of strength and elongation [11]. There are many studies focused on the martensitic transformation induced by deformation of the austenitic phase in stain
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