AlN-assisted internal oxidation behavior in Al-containing high Mn steels

Highlights

• The sequential oxidation behavior in Al-containing high Mn steels is clarified.

• The orientation relationship between AlN precipitates and γ-matrix is determined.

• Heterogeneous nucleation of MnAl₂O₄ on AlN is identified.

• HAADF-STEM analysis reveals the share of Al at the AlN/MnAl₂O₄ interface.

Abstract

The role of AlN on the internal oxidation behavior was investigated in Al-containing high Mn steels. Accelerated internal oxidation behavior was confirmed as Al contents increased. The sequential formation of internal oxidation and AlN + γ-matrix layers on the γ-matrix was observed in Al-containing steel after the heat-treatment in air. The former AlN + γ-matrix layer was replaced with the internal oxidation layer as the oxidation progressed. The heterogeneous nucleation of MnAl₂O₄ on the AlN was identified in between the internal oxidation and AlN + γ-matrix layers. MnAl₂O₄ shows the ${\left[10\overline{1}0\right]}_{\mathrm{AlN}}//{\left[112\right]}_{{\text{MnAl}}_2{O}_4}$ and ${\left(0001\right)}_{\mathrm{AlN}}//{\left(11\overline{1}\right)}_{{\text{MnAl}}_2{\mathrm{O}}_4}$ orientation relationship with the pre-existing AlN by sharing Al atoms. The coherency between MnAl₂O₄ and AlN and the evolution of oxides in Al-containing high Mn steels were further discussed.

Introduction

High Mn steels have received considerable attention owing to the increasing demand for high-strength and ductility in structural alloys. Twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) steels belong to this alloy system [1]. TRIP steels have high strength due to the transition of residual austenite to deformation-induced martensite during deformation [2]. The steel contains relatively low C and Mn contents, resulting in the formation of mechanically unstable austenite. Twinning, rather than phase transition, is the predominant deformation mechanism in TWIP steel [3]. The existence of austenite matrix and self-refining due to the formation of mechanical twin contributes to high strength and ductility in TWIP steel [4]. According to numerous investigations, stacking fault energy (SFE) is a critical component determining the deformation mechanism [[5], [6], [7]]. When SFE levels are in the 20–40 mJ/m² range [5], the twinning mechanism dominates; however, the deformation process shifts to phase transformation when it drops below 18 mJ/m² [8]. The SFE is related to the chemical composition of the steel. To balance strength and elongation in TWIP steels [9,10], many austenite stabilizers, such as C and Mn, are necessary. Moreover, the addition of Al in high Mn steel increases the SFE and yield strength through the solid solution hardening. The optimum TWIP effect is believed to lead to high flow stresses (600–1100 MPa) and exceptional elongations (60–95%) [[11], [12], [13]].

Previous studies have shown that austenitic steels with high manganese contents are susceptible to aqueous corrosion [[14], [15], [16]]. Their high corrosion rate is related to the poor passivation ability of Mn. Corrosion resistance characteristics and stress correction cracking (SCC) are closely related. Stress corrosion cracking (SCC) initiates and propagates crack-like corrosion on metal surfaces having high corrosion resistance when subjected to tensile loading in specific corrosive media [17,18]. When the Al is added to the high Mn steels, Al advances the corrosion resistance. The Al content increase yielded an increase of corrosion potential (E_corr) and a decrease in corrosion current density (i_corr) [19]. However, when the steel is exposed to a higher temperature, there is the possibility of AlN or (Al, Mn)-rich oxide particles, which degrade the corrosion resistance in the internal oxidation layer. Considering the corrosion resistance and mechanical properties, much attention has been given to Fe-Mn-Al-C alloys because of their potential to replace NiCr stainless, cryogenic, and non-magnetic steels [[20], [21], [22], [23], [24], [25]].

The large grain size acts relatively advantageous in oxidation or corrosion resistance [26,27]. The slab of TWIP steel shows a large grain size due to its single-phase character in all the temperatures. However, a thick oxidation layer is formed in the TWIP steel at a high temperature due to the amount of each oxidizing element, including Mn and Al. The accelerated oxidation kinetics in the steel brings the loss of steel during the manufacturing process and results in the increased production cost. Oxides formed on the steel surface also generate surface cracks during steel plate production. The generation of surface crack is generally found in processes containing straining or applying stress at the high-temperature regions, such as continuous casting processes [28] or hot rolling [29,30]. It has been reported that internal oxide promoted surface cracking in austenitic stainless steel [[28], [29], [30]]. As similar to austenitic stainless steel, it is conceivable that internal oxidation in high-Mn steel could affect hot ductility. Although even some research on the oxidation behavior was reported [31,32], the role of Al on the internal oxidation behavior and the oxidation mechanism was not revealed and identified yet.

Park et al. [33] investigated the oxidation behavior of a Fe-25Mn-1.5Al-0.5C alloy at 1250 °C using X-ray diffraction (XRD) and electron microscopy and found out that the oxide layers could be macroscopically classified into the external and internal oxidation layers. The external oxidation layer was confirmed in the Mn₂O₃, Mn₃O₄, MnFe₂O₄, (Mn, Fe)O, and MnAl₂O₄ phases from the surface into the matrix direction. In contrast, the oxide in the internal oxidation layer was identified as MnAl₂O₄ formed by the selective oxidation of Mn and Al. However, the detailed transition among various oxides during the oxidation process is still veiled. The formation of aluminum nitride (AlN) in austenite (γ)-matrix was also detected in XRD. AlN precipitate's morphology and crystal structure vary as the steel composition and matrix change. The crystal structure of AlN has been reported as cubic rock-salt, cubic B3, or HCP B4 phases [34]. Cheng et al. [35] studied AlN in the forms of a Widmanstätten side-plate precipitated in austenite (γ)-matrix. The crystal structure of AlN is HCP, which shows possible growing planes parallel to the close-packed planes of HCP and FCC structures; (0001)_AlN and ${\left(1\overline{1}1\right)}_{\mathrm{\gamma }}$. Li et al. [36] reported HCP AlN formation in α-matrix of a Fe-C-Mn-Al-Si-Cu alloy. The formed AlN in α-matrix showed ${\left[01\overline{1}0\right]}_{\mathrm{AlN}}//{\left[001\right]}_{\alpha }$ and (0001)_AlN//(110)_αorientation relationship (OR).

Although there were several reports on the oxidation behavior of Al-containing steels, the role of AlN and the evolution of the pre-existing oxide during the progress of oxidation were not clarified in detail. Therefore, in this study, we tried to describe the role of AlN on the internal oxidation behavior and the evolution of oxide in an Al-containing high Mn steel.