Evidence of Martensitic Transformation in Fe-Mn-Al Steel Similar to Maraging Steel
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I.
INTRODUCTION
FE-MN-AL alloys exhibit a low density and some fall into the category of transformation-induced plasticity (TRIP) steels, while Fe-Mn-Al alloys that contain high concentrations of aluminum and manganese have the superior properties of low cost and low density. In addition, knowledge of the Fe-Mn-Al steel phase transformations is essential for future optimal alloy development and processing. Austenite and ferrite are two phases that are common in ferrous alloys, where austenite has a face-centered cubic (FCC) crystal structure and ferrite has a body-centered cubic (BCC) crystal structure. It is generally accepted that, in iron-based alloys, aluminum and manganese play a role as ferrite and austenite stabilizers, respectively, during phase transformations. Specifically, a higher manganese content in the alloy produces a higher proportion of austenite, unlike the Fe alloys at low temperature that are primarily ferrite.[1–4] Furthermore, in Fe-Mn-Al alloys possessing a low concentration of aluminum and a high concentration of manganese, a fully austenitic microstructure can be preserved even at low temperature. In this condition, the high-temperature
GURUMAYUM ROBERT KENEDY, YI-JYUN LIN and WEICHUN CHENG are with the Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106, Taiwan. Contact e-mails: D10303804@ mail.ntust.edu.tw; [email protected] Manuscript submitted March 19, 2020; accepted October 5, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
BCC (i.e., d) to FCC (i.e., c) phase transformation shifts to a lower temperature. This is unlike common ferrous alloys where this phase transformation is masked by the formation of another BCC (a) phase. By alloying ferrous alloys suitably, therefore, it is possible for the d (or a) to austenite transformation to occur at low temperature. The ferrite–austenite phase transformations in Fe-Mn-Al alloys after cooling from high temperature has been studied for several years. Austenite in the forms of grains, Widmansta¨tten side-plates,[5] massive grains,[6,7] and 18R type martensitic needles [8–10] can also be seen in the supersaturated ferrite matrix of the Fe-Mn-Al alloys under various cooling conditions. In Fe-Mn-Al alloys with high Al and low Mn concentrations, ferrite may exist as the only solid-state phase at low temperature. However, in the single-phase region of Fe-Mn-Al alloys, ferrite is brittle and most likely to be cracked during high-temperature quenching. The ferrite phase has little resistance to plastic deformation and cannot withstand significant thermal contraction upon quenching.[11] Ferrite micro-twins also appear in the single ferrite matrix of Fe-Mn-Al alloys after quenching from high temperature.[12] The twin planes of the BCC ferrite are (112) and the glide directions are ½11 1. The orientation relationship between the ferrite twin (t) and ferrite matrix (a) is given as follows: ½011ti½ 101a and (121)ti(211)a. When BCC twins are studied by TEM, extr
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