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In-situ x-ray scattering study of the cubic to hexagonal transformation of AlN in Ti1-xAlxN
Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, The Institute of Technology. Seco Tools AB, Fagersta, Sweden.
Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany.
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2014 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 73, 205-214 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, we have studied the decomposition of arc evaporated Ti0.55Al0.45N and Ti0.36Al0.64N during heat treatment in vacuum by in-situ synchrotron wide angle x-ray scattering primarily to characterize the kinetics of the phase transformation of AlN from the cubic NaCl-structure to the hexagonal wurtzite-structure. In addition, in-situ small angle x-ray scattering measurements were conducted to explore details of the wavelength evolution of the spinodal decomposition, thus providing information about the critical size of the c-AlN rich domains prior to the onset of the h-AlN transformation. We report the fractional cubic to hexagonal transformation of AlN in Ti1-xAlxN as a function of time and extract activation energies between 320 and 350 kJ/mol dependent on alloy composition. The onset of the hexagonal transformation occurs at about 50 K lower temperature in Ti0.36Al0.64N compared to Ti0.55Al0.45N where the high Al content alloy also has a significantly higher transformation rate. A critical wavelength for the cubic domains of about 13 nm was observed for both alloys. Scanning transmission electron microscopy shows a c-TiN/h-AlN microstructure with a striking morphology resemblance to the c-TiN/c-AlN microstructure present prior to the hexagonal transformation.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 73, 205-214 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-106506DOI: 10.1016/j.actamat.2014.04.014ISI: 000337853100020OAI: oai:DiVA.org:liu-106506DiVA: diva2:716465
Funder
VINNOVA
Note

On the day of the defence date of the thesis the status of this article was Manuscript.

Available from: 2014-05-09 Created: 2014-05-09 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Microstructural evolution of TiAlN hard coatings at elevated pressures and temperatures
Open this publication in new window or tab >>Microstructural evolution of TiAlN hard coatings at elevated pressures and temperatures
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A typical hard coating on metal cutting inserts used in for example turning, milling or drilling operations is TiAlN. At elevated temperatures, TiAlN exhibits a well characterized spinodal decomposition into coherent cubic TiN and AlN rich domains, which is followed by a transformation from cubic to hexagonal AlN. Using in-situ synchrotron x-ray radiation, the kinetics of the second transformation was investigated in this thesis and the strong temperature dependence on the transformation rate indicated a diffusion based nucleation and growth mechanism. The results gave additional information regarding activation energy of the transformation and the critical wavelength of the cubic domains at the onset of hexagonal AlN. After nucleation and growth, the hexagonal domains showed a striking resemblance with the preexisting cubic AlN microstructure.

During metal cutting, the tool protecting coating is subjected to temperatures of ~900 ºC and pressure levels in the GPa range. The results in this thesis have shown a twofold effect of the pressure on the decomposition steps. Firstly, the spinodal decomposition was promoted by the applied pressure during metal cutting which was shown by comparisons with annealed samples at similar temperatures. Secondly, the detrimental transformation from cubic to hexagonal AlN was shown to be suppressed at elevated hydrostatic pressures. A theoretical pressure/temperature phase diagram, validated with experimental results, also showed suppression of hexagonal AlN by an increased temperature at elevated pressures.

The spinodal decomposition during annealing and metal cutting was in this work also shown to be strongly affected by the elastic anisotropy of TiAlN, where the phase separation was aligned along the elastically softer <100> directions in the crystal. The presence of the anisotropic microstructure enhanced the mechanical properties compared to the isotropic case, mainly due to a shorter distance between the c-AlN and c-TiN domains in the anisotropic case. Further improvement of the metal cutting behavior was realized by depositing individual layers with an alternating bias. The individual bias layers exhibited microstructural differences with different residual stress states. The results of the metal cutting tests showed an enhanced wear resistance in terms of both crater and flank wear compared to coatings deposited with a fixed bias.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 75 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1583
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-106507 (URN)10.3384/diss.diva-106507 (DOI)978-91-7519-372-4 (ISBN)
Public defence
2014-06-11, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
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Available from: 2014-05-09 Created: 2014-05-09 Last updated: 2014-05-09Bibliographically approved

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Norrby, NiklasRogström, LinaJohansson Jöesaar, Mats P.Odén, Magnus

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