Effect of Cu on Nanoscale Precipitation Evolution 355


Table 3. Core Composition (at%) of the NiAl Nanoparticles Obtained by Proxigrams After Aging at 500°C for 4, 32, and 128 h.


4 h


at% Ni


Al


Cu Fe


Fe–NiAl


33.3±13.6 8.3±8.0 —


58.3±14.2


Fe–NiAl–Cu 12.3±2.8


20.3±3.4 39.9±4.2 27.5±3.8


Fe–NiAl 37.6±4.2


36.5±4.2 —


24.2±3.7 32 h


Fe–NiAl–Cu 24.2±3.8


24.4±3.8 38.3±4.3 13.3±3.0


Fe–NiAl 51.5±2.6


40.9±2.6 —


6.4±1.3 128 h


Fe–NiAl–Cu 28.8±4.4


29.8±4.5 36.5±4.7 4.8±2.1


The uncertainty for each concentration c is (c(1−c)/N)0.5, where N is the total number of atoms used for calculating the concentration value.


500°C, for the Fe–NiAl alloy, the equivalent radius of the NiAl nanoparticles increases continuously from ~1.6nm at 4 h to ~4.1nm at 128 h, whereas that of the Fe–NiAl–Cu alloy increases from ~1.2nm at 0.5 h to ~3.5nm at 128 h. Moreover, in the samples aged for 128 h, there is a certain number of NiAl nanoparticles with a size of 5.5–6.0nm in the Fe–NiAl alloy, whereas the NiAl nanoparticles in the Fe–NiAl–Cu alloy are all <5.5nm. It indicates that the NiAl nanoparticles of the Fe–NiAl–Cu alloy grow at a relatively slow rate, though they form earlier than those of the Fe–NiAl alloy. The number density of the NiAl nanoparticles in the Fe–NiAl alloy increases from ~0.02 ×1023m−3 at 4 h to ~1.7×1023m−3 at 32 h and then decreases to ~0.3×1023m−3 at 128 h. In comparison, the variation in precipitate number density of the Fe–NiAl–Cu sample is rather simple with aging time; the number density decreases from 15.4×1023m−3 to 1.1×1023m−3 as the aging time is increased from0.5 to 128h. In general, with increasing aging time, the nanoparticles of the two alloys grow continuously in size, whereas the number density decreases, which is coincident with the atom maps shown in Figure 4. In addition, the number densities of the NiAl nanoparticles in the Fe–NiAl–Cu alloy are always higher than those in the Fe–NiAl alloy. The compositions of NiAl nanoparticles in the Fe–NiAl


and Fe–NiAl–Cu alloys after aging for 4, 32, and 128 h are shown in Table 3. The NiAl precipitates are identified utilizing an isoconcentration surface methodology (Hellman et al., 2003) with the threshold concentration set at 20 at% (Ni + Al). The grid size is 0.2nm and the delocalization dis- tance is 3 nm. The concentrations presented in Table 3 were obtained utilizing the proximity histogram (proxigram for short) (Hellman et al., 2000). The uncertainty for each con- centration c is (c(1−c)/N)0.5, where N is the total number of atoms used for calculating the concentration value. These NiAl nanoparticles do not only consist of Ni and Al, but also contain significant amounts of Fe and Cu. With increasing aging time, the Ni and Al contents of the NiAl nanoparticles increase, whereas the Fe content gradually decreases, and the Cu content (of the Fe–NiAl–Cu alloy) remains almost con- stant. The solubility of Cu in the NiAl nanoparticles, found in the present study, is in good agreement with the obser- vations of Horing et al. (2009). In the same condition, the Ni and Al concentrations of the NiAl nanoparticles in the


Fe–NiAl alloy are higher than those in the Fe–NiAl–Cu alloy. Moreover, as the aging time increases from 32 to 128 h, the Ni and Al contents of the NiAl nanoparticles of the Fe–NiAl alloy increase from 37.6±4.2 at% and 36.5±4.2 at% to 51.5±2.6 at% and 40.9±2.6 at%, respectively, whereas the NiAl nanoparticles in the Fe–NiAl–Cu alloy show only a slight increase in Ni and Al contents. The 1-nm-thick atom maps through the centers of the


precipitates in the Fe–NiAl–Cu alloy are shown in Figure 7. The circles around the Cu segregation zone correspond to 20% Cu isoconcentration surfaces, whereas the circles around the Ni and Al segregation zone correspond to 15% (Ni +Al) isoconcentration surfaces. At the aging time of 0.5 h, Ni, Al, and Cu atoms segregate from the matrix, as shown in Figure 7a.Ni and Al are found enriched next to the Cu-rich precipitate. Similar segregation has been observed previously (Kapoor et al., 2014). With an increased aging time to 4 h, the precipitate presents a core–shell structure, with Cu particles in the core and NiAl particles in the shell, as shown in Figure 7b. Kolli & Seidman (2008) also found similar core–shell structures in the overaged stage. Figure 7c shows that at the prolonged aging of 128 h, the NiAl nanoparticle has formed on only one side of a Cu nanoparticle, which is similar to the observation of Zhang et al. (2013) when studying a Fe–Cu–Ni–Al–Mn alloy aged for 2,000 h.


DISCUSSION


Effect of Cu on the Nucleation of NiAl Nanoparticles


With the addition of Cu, the number density of NiAl nanoparticles increases dramatically, as shown in Figure 6. It can be deduced that Cu promotes the nucleation of NiAl nanoparticles. According to classical nucleation theory, the nucleation rate, giving the number of nanoparticles per unit time per volume, is expressed by (Aaronson & Legoues, 1992)


dN dt nucleation / exp -ΔG*


; kBT


(2)


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