358 Qin Shen et al. With regard to Δσcoherency in the two alloys, ɛ, ƒ, and R
are variables. The number density of the Fe–NiAl–Cu alloy is far larger than that of the Fe–NiAl alloy, so the volume fraction of the Fe–NiAl–Cu alloy is far larger than that of the Fe–NiAl alloy. The bcc Cu has a lattice constant that is larger than the lattice constant of the B2 NiAl phase (Wen et al., 2013), leading to a larger ɛ of the Fe–NiAl–Cu alloy. Therefore, the Δσcoherency of the Fe–NiAl–Cu alloy is larger. As a result, after aging for 4 h, the increment of strengthen- ing of the Fe–NiAl–Cu alloy is larger than that of the Fe–NiAl alloy.
CONCLUSIONS
In this paper, we presented results on the aging character- istics of precipitates and mechanical properties of the Fe–NiAl and Fe–NiAl–Cu alloys. The effects of Cu on the evolution of precipitation and the mechanical properties were discussed. The following conclusions are drawn:
1. The addition of Cu plays an important role in increasing the number density of NiAl nanoparticles, which is attributed to the fact that Cu increases the nucleation rate of NiAl nanoparticles by increasing the chemical driving force and decreasing the interfacial energy.
2. The growth and coarsening rate of core–shell precipitates in the Fe–NiAl–Cu alloy is lower than that of monolithic NiAl precipitates in the Fe–NiAl alloy, which is caused by the slower diffusion rate of Cu.
3. With increasing aging time to 128 h, the core–shell precipitates of the Fe–NiAl–Cu alloy decomposed into NiAl nanoparticles and Cu nanoparticles, side-by-side.
4. With the co-precipitation of Cu-rich and NiAl precipi- tates, the peak hardness of the Fe–NiAl–Cu alloy is higher than that of the Fe–NiAl alloy. By increasing the chemical strength and coherency strength, the addition of Cu improves the precipitation strength.
ACKNOWLEDGMENT
This work was supported by Steel Joint Funds of the National Natural Science Foundation of China (No. U1460103), China Academy of Engineering Physics Joint Funds of National Natural Science Foundation of China (No. U1530115), Key project of Shanghai Science and Technology Commission (12JC1404000), and National Key Research and Development Program of China (No. 2016YFB0700401).
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