Coarsening of Al3Sc precipitates in an Al-0.28 wt pct Sc alloy

Abstract

The Ostwald ripening of Al 3 Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al 3 Sc precipitates obey the t 1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t Ϫ1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al 3 Sc interface energy, ␥, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of ␥ using coarsening data obtained from TEM and electrical resistivity measurements. The value of ␥ estimated from the LSW theory is 218 mJ m Ϫ2 , which is nearly identical to 230 mJ m Ϫ2 from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2 Ϯ 6.0) ϫ 10 Ϫ4 m 2 s Ϫ1 and 176 Ϯ 9 kJ mol Ϫ1 , respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.dependence of the coarsening behavior of precipitates in an Al-0.3 wt pct Sc alloy aged at 573 to 723 K. In addition, the diffusivity of Sc in Al is obtained from coarsening experiments. Iwamura and Miura [10] have examined the coarsening behavior of Al 3 Sc precipitates in an Al-0.2 wt pct Sc alloy at 673 to 763 K on the basis of TEM observations with the numerical model. The radius for coherent/semicoherent transition of the precipitates is determined from TEM images as 15 to 40 nm. The average radius, r, of the Al 3 Sc precipitates obeys the r 3 growth law both in the coherent stage (r Ͻ 15 nm) and in the semicoherent stage (r Ͼ 40 nm). However, in the intermediate stage, where coherent and semicoherent precipitates coexist (15 Ͻ r Ͻ 40 nm), coarsening is delayed.By measuring independently both the growth rate of precipitates and the rate of depletion of the matrix supersaturation during coarsening of the precipitates, independent reliable values of the matrix/precipitate interface energy and the diffusion coefficient of solute in the matrix can be determined from coarsening data alone. [6] This approach has already been applied in several binary systems. [11][12][13][14][15] The interface energy can also be obtained from measurements of the matrix solute concentration during coarsening, together with the mean precipitate size, using the Gibbs-Thompson equation. This direct application of the Gibbs-Thompson equation has been directed at Al-Li [16] and Cu-Ni-P systems. [17] In this work, the coarsening kinetics of the Al 3 Sc precipitates in an Al-0.28 wt pct Sc alloy aged at 673, 698, and 723 K have been investigated. The Al/Al 3 Sc interface energy, ␥, and the diffusivity of Sc in Al have been independently derived from measurements o...

Developing Sc and Zr containing Al-B C metal matrix composites for high temperature applications /

“…This slope is also reported to be an indication of the coarsening behavior of precipitates, often called the time exponent of coarsening. For example, the value of 1/n was predicted to be 1/3 according to the classic LSW theory, which well met the experimental value in the Al-Sc binary alloys [67,71]. (1) Ability to form nano-size intermetallic precipitates.…”

“…The LSW theory predicted a mean precipitate radius coarsening from an initial value r 0 , to r t with time t, expressed as [65][66][67]:…”

“…where, k is a coarsening rate constant related to the temperature and the composition of the material [67][68][69], and n is the inverse time exponent [70]. Usually, T o " is much smaller than 7", and is therefore neglected [71].…”

Using cluster dynamics to model electrical resistivity measurements in precipitating AlSc alloys

“…Except for Jo and Fujikawa's data [6] for which only a poor agreement could be obtained, cluster dynamics manage to reproduce reasonably well experimental measurements of electrical resistivity [8][9][10]. For the highest annealing temperatures corresponding to the lowest supersaturations, a time shift could appear between the simulated and the experimental resistivity, the time evolution predicted by cluster dynamics being too fast.…”

“…Indeed Watanabe et al [9] manages to observe a linear variation of the resistivity with t −1/3 . This behavior was obtained once the precipitates became incoherent.…”