348 Zemin Wang et al.
temperature is expected to give differences in the interface compositions, as the schematics display in Figure 11. The structure of CRPs would probably transform from BCC to the intermediate 9R, 3 R, or FCC (Monzen et al., 2000), which requires further investigation with a combination of APT and high-resolution TEM techniques.
CONCLUSIONS
The APT method was utilized to investigate CRPs in 17-4 PH SS tempering at different temperatures for 4 h. The important conclusions concerning the chemistry and the strengthening effects of CRPs are portrayed as the following:
1. Only Cu is clustering in the sample tempered at 420°C, whereas Ni and Mn are homogenously distributed in the matrix. In the sample tempered at 450°C, Ni and Mn segregate to the Cu-rich clusters, wherein they are homogenously distributed. This corresponds to the strongest hardening effect, with maximum number density and a moderate radius of the precipitates.
2. The elements Ni, Al, and Mn segregated at the CRPs/ matrix interface resulting in a core-shell structure in the samples tempered at temperatures above 510°C. In the sample tempered at 570°C, a Ni(Mn, Al) phase encapsu- lating the core of Cu precipitates was formed, which was proved to be beneficial for the precipitate coarsening, due to the strain energy reduction and to the larger negative mixing enthalpy with Mn and Ni. Also, Si was obviously enriched in Ni(Mn, Al) phase.
ACKNOWLEDGMENTS
This work was supported by the National Key Research and Development Program of China (No. 2016YFB0700401), NSAF (No.U1530115), Steel Joint Funds of the National Natural Science Foundation of China (No.U1460103) and State Key Lab of Advanced Metals and Materials (No. 2014-Z08).
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