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Role of Nitrogen During Dry-Air Oxidation of TiAlNbCrSi Alloy Produced with Mould Casting (MC) and Electron Beam Melting (EBM)

Morgiel Jerzy, Dudziak Tomasz, Maj Łukasz, Kirchner A., Pomorska Małgorzata, Klöden B., Weissgärber T., Toboła Daniel

Archives of Metallurgy and Materials

2023

Vol. 68, iss. 3

1183--1189

The lack of room-temperature ductility of high-strength TiAl-based alloys called for complicated high temperature processing limiting their application areas. Introduction of additive manufacturing (AM) methods allowed to circumvent this disadvantage, but entailed microstructure refinement affecting, among the others, their oxidation resistance. The dry-air high temperature oxidation processing of TiAl-based alloys is relatively well covered for coarse grained materials, but to what extent the TiAl alloys are affected by the changes caused by the AM remains to be found out. Additionally, the role of nitrogen during these processes was to large extent omitted in previous works. Within the present experiment, the mould cast (MC) and the electron beam melted (EBM) Ti-48Al-2Nb-0.7Cr-0.3Si (at. %) RNT650 alloys were dry-air oxidized at 650°C for 1000 h. The TEM/EDS investigations allowed to confirm that the scale formed during such treatment consists of the layers occupied predominantly by TiO2+Al2O3/TiO2/Al2O3 sequence. Additionally, it was shown that N diffuses to the sub-scale and reacts with the substrate forming two distinct discontinuous sub-layers of α2-Ti3Al(N) and TiN. The scale over EBM was noticeably less porous and nitrogen penetration of the substrate was more extensive, while the MC showed higher susceptibility to local sub-scale oxidation.

Publisher Correction to: Characterization of the as-cast microstructure and selected properties of the X-40 Co-based superalloy produced via lost-wax casting

Rakoczy Łukasz, Grudzień-Rakoczy Małgorzata, Cygan Rafał, Rutkowski Bogdan, Kargul Tomasz, Dudziak Tomasz, Rząd Ewa, Milkovič Ondrej, Zielińska-Lipiec Anna

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 4

art. no. e170, 2022

The thermodynamical simulation predicts the phase transformation of M7C3 to M23C6, proven previously via electron microscopy. Some other reported experimental works suggest that this can also take place also during heating [22, 45, 46]. Considering this, the melting process of the primary M7C3 carbide can be that the M7C3 first undergoes a phase transformation into M23C6 and then melts, instead of directly melting. A similar conclusion was given by Gui et al. [47-49] based on experiments on the Co-based superalloy strengthened (in as-cast condition) by M7C3 and MC carbides. It was suggested that the creation of the liquid phase follows the reaction M23C6 + α→L. The reaction was initiated on the M23C6/α interface and proceeded towards the center in the range of 1280 - 1348 ˚C. Before melting, the MC eutectic carbide degenerated, and its morphology changes to a well-rounded shape. Exceeding 1400 ˚C leads to the melting reaction of MC + α→L in the X-40 Co-based superalloy.

Characterization of the as‑cast microstructure and selected properties of the X‑40 Co‑based superalloy produced via lost‑wax casting

Rakoczy Łukasz, Grudzień-Rakoczy Małgorzata, Cygan Rafał, Rutkowski Bogdan, Kargul Tomasz, Dudziak Tomasz, Rząd Ewa, Milkovič Ondrej, Zielińska-Lipiec Anna

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 3

art. no. e143

The X-40 Co-based superalloy is often used in the aerospace industry directly in as-cast condition and its analysis in this state is essential to understand further possible phase transformations during service. With this in mind, this work focuses on characterizing the material’s as-cast microstructure, phase transformation temperatures and oxidation resistance. Observations and analyses were performed via thermodynamic simulations, X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), scanning-transmission electron microscopy (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDX), dilatometry (DIL) and differential scanning calorimetry (DSC). The microstructure of the dendritic regions consisted of the α matrix, with MC, M7C3 and M23C6 carbides being present in the interdendritic spaces. Based on DIL, it was found that precipitation of the Cr-rich carbides from the saturated α matrix may occur in the range 650-750 °C. DSC determined the incipient melting and liquidus temperatures of the X-40 superalloy during heating to be 1405 °C and 1421 °C, respectively. Based on oxidation resistance tests carried out at 860 °C, it was found that the mass gain after 500 h exposure was 3 times higher in the air than in steam.