Atom Probe Tomographic Characterization of Nanoscale Cu-Rich Precipitates 347
Table 1. The Matrix Compositions (at%) Measured by atom probe tomography in 17-4 Precipitate Hardened Stainless Steel at Different Aging Temperature.
Elements Fe
Cu Ni Al
Mn Si
Quenched 75.80±0.04
2.74±0.01 4.50±0.01
0.028±0.001 0.824±0.003 1.241±0.004
420°C
75.96±0.04 2.08±0.01 4.10±0.01
0.022±0.001 0.806±0.003 1.184±0.003
450°C
76.03±0.09 0.91±0.01 3.67±0.01
0.018±0.001 0.775±0.007 1.170±0.008
510°C
76.13±0.02 0.56±0.02 3.52±0.01
0.009±0.001 0.590±0.003 1.067±0.004
The error for each concentration c is σ = (c(1−c)/N)0.5, where N is the total number of atoms in volume.
indicated thatMncould occupy the sublattice of bothNi and Al in the B2-stuctured Ni–Al intermetallic phase and the segregation of Ni, Al, and Mn at the precipitates/matrix interfaces led to an interface energy increase and impeded coarsening of the precipitates (Isheim et al., 2006; Kolli et al., 2007; Zhou et al., 2011). Hence, segregation of Ni, Mn, and Al was observed around a core of pure copper (~100 at%). Moreover, Si was rejected from the core region to thematrix, compared with when lower tempering temperatures (420 and 450°C) were used. In addition, Mn and Al were significantly depleted in the matrix (Table 1), due to the formation of the Ni(Mn, Al) shell of CRPs. When the tempering temperature was increased to
570°C, the morphology of the precipitates became ellipsoidal rather than spherical, as in the sample tempered below 510°C, and similar Ni, Al, Mn, and Cu distributions were observed. However, Si was observed being enriched at the precipitates/matrix interfaces, with ~2.2 at% content and considerably depleted in the matrix, when tempering at
570°C (Table 1). A similar phenomenon was observed in reactor pressure vessel steel during long-term thermal aging and in low-carbon steel (Miller et al., 2006; Styman et al., 2015). It is well known that the atomic radius of Si (0.117 nm) is significantly smaller than the atomic radii of Mn (0.128 nm), Al (0.143nm), Ni (0.124 nm), and Cu (0.128 nm). The segregation of Si into the Ni(Mn, Al) phase would reduce the lattice misfit strain energy. Second, ele- ment partitioning in multicomponent alloys is controlled by the enthalpy of mixing between the constituent elements within solids (Takeuchi & Inoue, 2005). Si has a larger negative mixing enthalpy along with Mn (−45 kJ/mol) and Ni (−40 kJ/mol) among the elements of Mn, Fe, Al, Si, and Ni. The chemical interaction between Ni–Mn–Al–Si is significantly greater than the interaction between other elements, suggesting that Si has a larger tendency to partition into the Ni(Mn, Al) phase. Thus, ellipsoidal CRPs were observed with segregation of Ni, Mn, Al, and Si at the precipitates/matrix interfaces. Therefore, the tempering
570°C
76.84±0.02 0.25±0.01 3.17±0.01
0.006±0.001 0.530±0.002 1.017±0.01
Figure 11. Schematic diagrams of CRPs in cross section formed at different tempering temperatures. a: Solution treat- ment, (b) 420 oC, (c) 450 oC, (d) 510 oC and (e) 570 oC.
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