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Identification of a new tetragonal phase q in an Fe31.5 at. % Al alloy, using internal friction and X-ray diffraction
Abstract
Fe-Al alloys containing 25 and 31.5 at. % Al have been studied by internal friction, X-ray diffraction and dilatometric analyses. These techniques show for the first time that two internal friction peaks appear during the change in long range order type which occurs during the precipitation at 300degreesC of the metastable phase B2(FeAl). These peaks correspond to the formation of two phases : the equilibrium phase Fe3Al and a new intermediate tetragonal phase theta.
... Contrary to the above vacancy-and-metal atom-related explanations it was noticed that a similar peak (H = 1.6 eV, τ 0 = 1.7×10 -14 s) in Fe-32Al depends on the carbon content and the peak was explained as the "second" Snoek-type peak (i.e. jumps of carbon atoms) in the presence of an additional phase, other than equilibrium D0 3 [186,187,189]. At the same time the "ordinary" Snoek-type peak in the D0 3 structure was reported at lower temperatures. ...
eBook PDF / eBook PDF
Different anelastic phenomena are discussed in this book with respect to
iron-based binary and ternary alloys and intermetallic compounds of
Fe3Me type, where Me are α-stabilizing elements Al, Ga, or Ge. An
introduction into anelastic behavior of metallic materials is given, and
methods of mechanical spectroscopy and neutron diffraction are
introduced for the better understanding of structure-related relaxation and
hysteretic phenomena.
Keyword: Anelasticity, Damping Capacity, Magnetostricition, Structure
Transitions, Phase Transitions, Fe-Based Alloys, Intermetallic
Compounds, Mechanical Spectroscopy, In Situ Neutron Diffraction
The anelasticity of binary Fe –Al and ternary Fe –Al –Me (Me = Co, Cr, Ge, Mn, Nb, Si, Ta, Ti, Zr) alloys has been studied by mechanical spectroscopy in the Hz and kHz ranges. Relaxation peaks due to point defects – Snoek effect (S) with carbon interstitial jumps, carbon-vacancy complexes (X peak), Zener effect (Z), dislocations (D), and grain boundaries (GB) – are observed and their changes with addition of the third element are used to corroborate the respective proposed relaxation mechanisms, taking account of the changes of structure and order in the alloys which contain up to 5Co, 25Cr, 25Ge, 5Mn, 0.3Nb, 25Si, 6Ta, 4Ti, and 15Zr (all in at.%), respectively. In addition, some data of ordering temperatures, Curie temperatures, and hardness are provided for several alloys.
Legierungen aus Eisen und Aluminium finden in vielen industriellen Bereichen Verwendung, da sie ein niedriges Gewicht, eine höhere Festigkeit und eine bessere Korrosionsbeständigkeit gegenüber reinem Eisen oder vielen Stählen aufweisen. Das mechanische und thermische Verhalten solcher Legierungen zu kennen und zu verstehen ist entsprechend wichtig; hierfür bietet die Messung der inneren Reibung mittels Vibrating-Reed-Technik mit Biege- und Torsionsschwingungen eines dünnen Blättchens, einem Teilbereich der so genannten Mechanischen Spektroskopie, eine gute Möglichkeit, weil sie dynamische Prozesse bei atomaren Umordnungen bei zunehmender Temperatur zu detektieren und quantitativ zu analysieren gestattet.
In dieser Arbeit wird die Mechanische Spektroskopie an einer Reihe von verschiedenen Fe-Al Legierungen vorgenommen. Die untersuchten Konzentrationen variieren von reinem Eisen bis zu FeAl Legierungen mit 50 at.%Al Anteil, wobei die Variationsschritte bei Erhöhung des Al-Anteils unter 5 at%.Al liegen. Der untersuchte Temperaturbereich reicht von ca. 93 bis 923 K. In diesem Intervall lassen sich vier Maxima im Dämpfungsspektrum finden, der D-, S-, X- und Z-Peak. Den gefundenen Peaks werden mittels Variation unterschiedlicher Parameter, wie z.B. der thermischen und mechanischen Vorbehandlung oder des Kohlenstoffanteils, bestimmten Mechanismen zugeordnet. Der D-Peak wird mit Versetzungen (linienhaften Defekten) in Verbindung gebracht, während der S-Peak durch einen Snoek-artigen Mechanismus (Sprünge von Kohlenstoff-Atomen zwischen benachbarten Zwischengitterplätzen) gedeutet wird. Der X-Peak wird auf eine Relaxation kombinierter Kohlenstoff-Leerstellen-Paare zurückgeführt. Der Z-Peak kann dem Zener-Mechanismus (Reorientierung von Substitutionsatom-Paaren entlang der Spannungsrichtung) zugeordnet werden. Die gefunden Mechanismen werden mit Literaturangaben verglichen und verifiziert. Eine Gegenüberstellung der ermittelten Aktivierungsenergien illustriert die gefundenen Zusammenhänge. Eine eingehende quantitative Analyse des S-Peaks bei variierter Al-Konzentration erlaubt eine Abschätzung der Wechselwirkungsweite zwischen Al- und C-Atomen in der Legierung Fe-Al(-C).
Als zusätzliche Ergänzung werden bestimmte binäre Legierungskonzentrationen um ein drittes Element erweitert, um so Aussagen, die bei der Fe-Al Legierung gefunden wurden, zu verifizieren, aber auch um sie auf die ternären Legierungen zu übertragen.
Anelastic relaxations by point defects and dislocations in Fe-Al alloys
Iron-Aluminium alloys are applied in different industrial areas owing to their superior properties, like lower weight, higher stiffness and better corrosion resistance than for pure iron or many kinds of steels. Accordingly it is important to know and to understand the mechanical and thermal behaviour of such alloys. Measurements of internal friction using the vibrating-reed-technique with flexural and torsional vibrations, which is part of the so called mechanical spectroscopy, provide a good choice with the possibility to detect and to analyse qualitatively the dynamical processes during atomic reorientation when the temperature is increased.
In this work mechanical spectroscopy is conduced with a series of different Fe-Al-alloys. The Al concentrations vary from pure iron up to FeAl with 50 at.%Al, with concentration steps less than 5 at.%. The measured temperature range varies from approximately 93 K to 923 K. In this interval four maxima in the damping spectra are detected, the D-, S-, X- and Z-Peak. Via variation of different parameters, like the thermal and mechanical pre-treatments or the carbon ratio, it is possible to assign each peak to a specific mechanism. The D-peak is connected to dislocations (line defects), while the S-peak is interpreted as a Snoek-like mechanism (jumps of carbon interstitial atoms). The X-peak is ascribed to a relaxation of combined carbon and vacancy pairs. The Z-peak is related to a Zener-mechanism (reorientation of substitional atom pairs according to the direction of strain). The proposed mechanisms are compared and verified with data from literature. A comparison of the determined activation energies illustrates the found coherences. A detailed quantitative analysis of the S-peak with varying Al concentrations permits to estimate the interaction range between Al- and C-atoms in the alloy Fe-Al-(C).
For an additional development certain binary alloy concentrations are extended by a third element, in order to verify the achieved statements, as well as to transfer them to the ternary alloys.
Two types of physical approaches for simulation of the Snoek-type relaxation in low and high alloyed iron are examined to explain the experimental results obtained for Fe–Al–C and Fe–C–Cr alloys. The first approach developed by Smirnov–Tomilin is to calculate all octahedral positions available for interstitial atoms with different amount of substitute atoms in the first coordination shell and to simulate the loss maximum as a sum of all partial peaks according to the above mentioned interstice positions. The second approach takes into account the all pairwise interatomic interaction between solute atoms in a few coordination shells due to their interatomic elastic and ‘chemical’ interaction according to Khachaturyan–Blanter theory. The change of activation energy of ‘diffusion under the stress’ for interstitial atoms in that case is not a linear function of substitutional concentration in solution. Both physical models (short- and long-range interatomic interaction) for the Snoek-type relaxation in quenched ternary alloys (Fe–C–Me) are examined from the viewpoint of a distance of interatomic interaction taken into account and checked using experiments. It is shown that contrary to the second approach, the first type of calculations is reasonable for relatively low alloyed solid solution only. Decomposition (Fe–Cr) and ordering (Fe–Al) change the parameters of atomic distribution in bcc solid solution and lead to the corresponding change in the Snoek relaxation parameters. The use of an adequate physical model and structure parameters allows to explain corresponding effects and, vice versa, the internal friction spectrum allows to estimate quantitatively atom redistribution in alloyed ferrite.
In order to investigate effects of ordering on the Snoek peak of carbon in Fe-Al alloys, internal friction measurements have been carried out on alloys containing 0 to 30.0 at.% Al and about 0.01 wt.% C. A relaxation peak was found to occur at 130°C at the frequency of about 1 cps in the region between 19.3 and 24.5 at.% Al after appropriate heat-treatments, and it was attributed to stress-induced diffusion of carbon in ordered Fe3Al lattice. Another peak occured at 160°C in the region between 25.1 and 30.0 at.% Al, but its origin was not so well clarified. A brief discussion is made concerning the relaxation mechanism of the 130°C-peak.