The Description of the Isothermal Decomposition Processes in AlSc Alloys by means of Electric Resistivity Measurements and TEM Investigations

Abstract

The decomposition behaviour of two AlSc alloys (csc = 0.18; 0.36 at.%) after direct quench to room temperature and subsequent ageing at temperatures in the range 200 "C 5 T, 5 550 "C was investigated by means of isothermal resistivity measurements and TEM. In the temperature range 325 "C 5 T, 5 400 "C both alloys show independent of the Sc content an "inversion" of the decomposition kinetics, obviously caused by the transition of the coherent into the incoherent A1,Sc phase.Das Entmischungsgeschehen zweier AlSc-Legierungen (csc = 0.18; 0.36 at.%) nach direktem Abschrecken auf RT und nachfolgender Endauslagerung bei 200 "C 5 T, 550 "C wurde mittels isothermer Widerstandsmessungen und TEM untersucht. Im Temperaturintervall325 "C 5 T, 6 400 "C ist unabhangig von der jeweiligen Sc-Konzentration eine ,,Inversion" der Entmischungskinetik, hervorgerufen durch den Phaseniibergang koharent/teilkoharent, zu beobachten.

Kinetics and mechanisms of precipitation in an Al–0.2wt.% Sc alloy

“…Here, two transformation time minima are very clearly observed. Both [19] and [28] link the differences in kinetics between the lowand the high-temperature regime to observations of coherent Al 3 Sc particles at low temperatures and incoherent Al 3 Sc particles at high temperatures. Once again it must be stressed that all these investigations are based on specimens that were water-quenched prior to isothermal annealing, and that a direct comparison with the present result may not be valid.…”

“…The shift in γ values between 350 and 370 • C could be related to coherency loss at the particle/matrix interface at the higher temperature regime. Such loss of coherency is reported to occur when the Al 3 Sc particles grow beyond a critical size [16,23,27,28,54], and an increase in the γ value due to the introduction of interfacial dislocations in continuously precipitated, spherical Al 3 Sc particles has been reported [54]. It is also possible that the interfaces of discontinuously precipitated Al 3 Sc particles have some properties that modify the γ value.…”

“…Fig. 7 of reference [28] contains essentially the same type of information as a TTT-diagram. Here, two transformation time minima are very clearly observed.…”

“…In order to minimise the Sc-addition that is needed to give the desired effects in an alloy, it is necessary to be able to predict the modus and kinetics of the formation of Al 3 Sc particles from a supersaturated solid solution. Several investigations of the precipitation kinetics and precipitation mechanisms of Al 3 Sc have been published in the technical literature over the past 10-20 years [16][17][18][19][20][22][23][24][25][26][27][28][29][30][31][32][33]. The published material does, however, give only a limited overview of the regimes in which either of the two precipitation mechanisms occur, and the differences in precipitation kinetics between the two mechanisms are not accounted for.…”