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On the formation of λ-MnAl4

Comment on “A study of phase equilibria in the Al-rich part of the Al–Mn system” by B. Grushko and S. Balanetskyy (Int. J. Mater. Res. 99 (2008) 1319–1323)
Published/Copyright: June 11, 2013
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International Journal of Materials Research
From the journal International Journal of Materials Research Volume 101 Issue 5

F. Weitzera, Yong Dub, Julius C. SchusteraaInnovative Materials Group, Universität Wien, Wien, AustriabState Key Lab. for Powder Metallurgy, Central South University, Changsha, Hunan, ChinaOn the formation ofk-MnAl4Comment on “A study of phase equilibria in the Al-rich part of the Al–Mnsystem” by B. Grushko and S. Balanetskyy(Int. J. Mater. Res. 99 (2008) 1319–1323)Grushko and Balanetskyy conclude from an elaborate dif-ferential thermal analysis (DTA) study of Al–Mn alloys,that under equilibrium conditionsk-MnAl4forms in an allsolid state reaction and does not coexist with the liquidphase at any temperature [1]. This is the same result asfound in the two previous DTA studies of this system byMurray et al. [2] and Du et al. [3]. However, annealing +quenching of alloys in the relevant composition and tem-perature range yieldedk-MnAl4to be formed from liquid(+l-MnAl4). This contradiction was resolved by the find-ing that the decomposition of MnAl6into L +k-MnAl4istoo slow to be detectable by DTA [3]. As a consequencethe DTA data do not yield the true equilibrium diagram.Ignoring these results Grushko and Balanetskyy arguethat the formation ofk-MnAl4in the equilibrated +quenched alloys did not occur at the heat treating tempera-ture, but rather metastably during cooling. Perhaps it wasstated in a too concise manner, but the facts do not supportthis hypothesis [3]:Assuming the DTA data derived phase diagram to be cor-rect, the alloys prepared in [3] containing between 10 and13 at.% Mn could only be quenched from the L +l-MnAl4phase field or the L + MnAl6phase field. In the first casek-MnAl4could be thought to be formed during coolingdue to a possible nucleation delay of MnAl6. Then, the al-loys must still containl-MnAl4after cooling in addition tok-MnAl4formed. This is not the case with our alloys asreported in [3]. Assuming the second case, i.e. that the cho-sen annealing conditions would have been within theL+MnAl6phase field, nok-MnAl4could have formed atall.Our DTA traces obtained and referred to in [3] and thedata published by [1] are almost identical:All alloys investigated (containing between 5 and17.5 at.% Mn, equilibrated at 6208C) showed, at a heatingrate of 5 K min–1, an endothermic peak with an onset tem-perature between 730 and 7208C. The temperatures de-creased with increasing Mn content. Up to the compositionof MnAl6these onset temperatures were unusually stronglyheating rate dependent. At Mn-contents above the composi-tion of MnAl6this behavior was not different from the heat-ing rate dependency of the eutectic temperature. Reducingthe heating rate and extrapolating these onset temperaturesto a heating rate of zero yielded a temperature of721 ± 28C (Fig. 1). The relative peak sizes (area) of thesesignals increased with the Mn-content up to MnAl6and de-creased at higher Mn-contents.Alloys consisting after equilibration of MnAl6+k-MnAl4exhibited an additional small endothermic peakat 704 ± 28C (Fig. 1). The onset temperatures of this peakwere not unusualy heating rate sensitive and the peak sizeat 17.5 at.% Mn was larger than at 15 at.% Mn, apparentlyincreasing with the amount ofk-MnAl4present in the alloy.To associate the temperatures of the DTA data with thecorresponding phase reactions we made the followingexperiments: Alloy Al-13 at.% Mn was equilibrated at6908C and quenched. X-ray diffraction (XRD) and energydispersive spectroscopy (EDS) analysis proved it to consistof MnAl6+ (solidified) liquid (Fig. 2a; MnAl6peaks are in-dexed based on the crystal structure given by Kontio andKoppens [4]). In a next step this alloy was equilibrated at7158C and quenched again. This yieldedk-MnAl4+ liquid(solidified to Al) butnol-MnAl4(Fig. 2b;k-MnAl4peaksare indexed based on the crystal structure reported by Krei-ner and Franzen [5]). After reannealing at 6908C MnAl6+(solidified) liquid were again present (Fig. 2c). Thus the re-F. Weitzer et al.: On the formation ofk-MnAl4Int. J. Mat. Res. (formerly Z. Metallkd.) 101 (2010) 5611BBasicFig. 1. DTA heating curves (5 K min–1) of alloys specially preparedto rule out even traces of Si contamination (equilibration at 6908CinZrO2crucibles sealed in Ta capsules, which in turn were sealed inquartz ampoules).
Received: 2009-1-26
Accepted: 2009-6-4
Published Online: 2013-06-11
Published in Print: 2010-05-01

© 2010, Carl Hanser Verlag, München

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https://doi.org/10.3139/146.110326

Articles in the same Issue

  1. Contents
  2. Contents
  3. Editorial
  4. Prof. Dr. Monika Feller-Kniepmeier 80th birthday
  5. Basic
  6. Model calculation of inoculation and experimental verification in two alloy-systems
  7. Enrichment of boron at grain boundaries of platinum-based alloys determined by electron energy loss spectroscopy in a transmission electron microscope
  8. Measurement of the lattice misfit of Pt–Al–Cr superalloy by convergent beam electron diffraction (CBED)
  9. Microstructural and micromechanical characterisation of a Pt–Al–Cr–Ni–Re alloy by means of transmission electron microscopy and nanoindentation
  10. X-ray reflections from the γ/γ′-microstructure of nickel-base superalloys: effect of the plane tilting
  11. Physicochemical properties and creep strength of a single crystal of nickel-base superalloy containing rhenium and ruthenium
  12. Interdependence between glass stability and phase formation sequence during crystallization of Zr46.8Ti8.2Cu7.5Ni10Be27.5 bulk glass
  13. On the formation of λ-MnAl4
  14. Applied
  15. Development of Ni–Mn-based alloys for the fast epitaxial braze-repair of single-crystalline nickel-based superalloys
  16. Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy
  17. Microstructural stability of an Ni–Mo based Hastelloy after 10 MeV electron irradiation at high temperature
  18. Oxidation behaviour of TiAl-based intermetallic coatings on γ-TiAl alloys
  19. Oxidation and fatigue behaviour of γ-TiAl coated with HIPIMS CrAlYN/CrN nanoscale multilayer coatings and EB-PVD thermal barrier coatings
  20. On the stress corrosion cracking behaviour of 6XXX series aluminium alloys
  21. Investigation of surface film nanostructure and assessment of its impact on friction force stabilization during automotive braking
  22. Creep strength of a binary Al62Ti38 alloy
  23. Bowing out of a dislocation from wall of persistent slip bands (PSB)
  24. Effect of martensite volume fraction on mechanical properties of dual-phase treated AISI-4012 sheet steels
  25. The joint properties of dissimilar aluminum plates joined by friction stir welding
  26. Notification
  27. DGM News
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