Kornorientierungsabhängiges Nitrierverhalten kubischer und hexagonaler Substrate
Beschichtungs- und Oberflächentechnik
Zusammenfassung der Projektergebnisse
Previous low temperature nitriding treatments have been investigated the nitriding behavior of Co-based alloys with mainly initial face-centered cubic (fcc) crystal structure. The development of nitrogen-supersaturated expanded fcc phase, so-called expanded austenite (fccexp), was observed and studied. In general, formation of nitrogen-supersaturated expanded solid-solution improves the hardness as well as wear resistance of materials without deteriorating the corrosion resistance. First part of this research was allocated to investigate the gaseous nitriding behavior of the Co-Cr alloys with an initial hexagonal close packed (hcp) crystal structure, which is the stable phase at the applied nitriding temperature. Comparing to the previous researches, additional structural changes due to the diffusion of nitrogen into the hcp lattice were observed and thus, more comprehensive view on the formation and decomposition of the nitrided layer upon low temperature nitriding of the Co-based alloys was presented. These additional sequences include: Formation of the nitrogen-supersaturated expanded hcp phase (hcpexp), arising from a chemical origin, and its gradual transformation into the expanded austenite (fcc exp) at lower temperature. Besides, the reverse transformation from fccexp to hcpexp was observed by applying higher nitriding temperature as a result of layer decomposition. In addition, occurrence of aforementioned transformation from expanded hcp to expanded fcc phase depends on the crystallographic orientation of the surface grains, which can be discussed in a view of development of the residual macrostresses on a sample surface. Several data points collected on surface of nitrided specimen reveal that the transformation is slower in hcp orientations with the highest Schmid factor of the basal slip mode with respect to the nitriding direction. Also, unlike most previous studies on accommodation of interstitials atoms in the hcp-based metals, in this research, as a result of nitrogen dissolution into the Co-Cr lattice at low nitriding temperature, a relatively large unit volume expansion of hcp exp is accompanied by only minor change of the c/a ratio. Second part of this research was allocated to investigate gaseous nitriding behavior of Mobased alloys. Mo-based alloys are applied as a constructive material in high temperature applications. To improve the surface tribological properties of these alloys, the thermochemical treatments such as nitriding or carburizing can be applied. In the course of this research, internal nitriding behavior of binary Mo-Ti and Mo-V alloys were compared in a temperature range from 700 °C to 1100 °C using Forming gas. The outcomes of this research clarify the distinct difference between the nitriding behavior of two aforementioned alloys with regards to the nitride precipitation and coarsening. The stronger interaction between V and N was realized in comparison to Ti and N to develop alloying element nitride in Mo matrix. Besides, during nitriding of Mo-V alloy, the crack-free nitrided layer at moderate temperature of 900 °C and the higher inclination to precipitation coarsening, so-called discontinuous coarsening, at 1100 °C, hint on easier accommodation of developed macrstresses by the Mo-matrix surrounded by dispersed VN nitrides. On the other hand, the obvious contradictory behavior of Mo-Ti alloy including formation of severally cracked nitrided layer at 900 °C and the limited progress of precipitation coarsening at 1100 °C indicates the higher strength of TiN precipitated-hardened Mo matrix. The experimental observations were linked to the thermodynamic understanding of nitride precipitation by calculating interaction parameter for formation of VN and TiN nitrides in Mo-matrix. In general, this research proposed the application of Mo-based alloys containing another alloying element such as vanadium rather than group IV elements (Ti, Zr, ...) for gaseous nitriding at the moderate temperature of 900 °C to inhibit the embrittlement. The Mo-Ti alloys, however, can still be applied at the higher nitriding temperatures such as 1100 °C, in which the stress-relieving is slightly activated by the progress of precipitation coarsening.
Projektbezogene Publikationen (Auswahl)
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Anisotropic nitriding behavior upon formation of expanded hcp in Co-Cr alloys. Scripta Materialia, 203, 114041.
Akhlaghi, Maryam; Martin, Stefan; Dallmann, Johannes; Hock, Rainer; Körner, Carolin & Leineweber, Andreas
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Gaseous nitriding of Co-10 at% and -15 at% Cr alloys at 400 °C and 450 °C. Journal of Alloys and Compounds, 907, 164535.
Akhlaghi, Maryam; Hock, Rainer; Dallmann, Johannes; Krapf, Anna; Merle, Benoit; Hofsäss, Hans; Körner, Carolin & Leineweber, Andreas
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“Phase transformation upon low temperature nitriding of Co-Cr alloys”, Conference TMS 2022, Anaheim, USA, 2022.
M. Akhlaghi, S. Martin, J. Dallmann, R. Hock, C. Körner & A. Leineweber
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“Response of Mo-15 at.% V to gaseous nitriding”, Proceedings-International Conference on Solid-Solid Phase Transformation in Inorganic Materials, pp. 171-173, Xián, China, 2022.
M. Akhlaghi, C. Körner & A. Leineweber
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Comparison between nitriding behavior of Mo-Ti and Mo-V alloys. International Journal of Refractory Metals and Hard Materials, 112, 106126.
Akhlaghi, Maryam & Körner, Carolin
