Effect of Cu on Nanoscale Precipitation Evolution 357


Table 4. The Matrix Composition of Fe–NiAl and Fe–NiAl–Cu Alloys, Which is Determined From the Plateau Section of the Proxigram Concentration Profile Outside the NiAl Precipitates for 4, 32, and 128 h.


4 h


at% Ni


Al


Cu Fe


Fe–NiAl 3.7±0.65


3.1±0.57 —


93.2±0.89


Fe–NiAl–Cu 2.6±0.02


2.1±0.04 0.4±0.01 94.5±0.05


Fe–NiAl 2.7±0.15


2.1±0.26 —


95.0±0.19 32 h


Fe–NiAl–Cu 2.2±0.09


1.2±0.08 0.3±0.05 96.1±0.2


Fe–NiAl 2.5±0.07


1.4±0.06 —


96.0±0.2 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.


perfect core–shell structures. This is attributed to that after longer aging, the Cu precipitates begin to transform to 9R and face-centered cubic (fcc) Cu (Othen et al., 1994). This trans- formation is accompanied by a change in morphology from equiaxed to elongated. In order to minimize the total interfacial energy, the NiAl precipitates nucleate with a spheroidal morphology, rather than with a shell covering the entire Cu precipitate in a core–shell structure, which would involve a higher interfacial area to volume ratio (Kapoor et al., 2014).


Effect of Cu on Mechanical Properties


With the addition of Cu, the microhardness of the Fe–NiAl– Cu alloy shows significant differences compared with that of the Fe–NiAl alloy, as shown in Figure 1. During the early aging period, the Fe–NiAl alloy exhibits a slight decrease in hardness, which is due to the partial removal of dislocations and defects. However, the hardness of the Fe–NiAl–Cu alloy increases from ~218 to ~288HV after the first 0.5-h-aging period, indicating that the hardening effect of the Ni, Al, and Cu clusters is much larger than the softening due to removal of dislocations and defects. Then with the co-precipitation of NiAl particles and Cu-rich particles, the Fe–NiAl–Cu alloy reaches higher peak hardness than the Fe–NiAl alloy. With increasing aging time, the peak hardness of the Fe–NiAl–Cu alloy remains for a period of time, which is attributed to the difference in nucleation and growth rates of Cu-rich nano- particles and NiAl nanoparticles (Gagliano & Fine, 2004), whereas the hardness of the Fe–NiAl alloy decreases quickly after reaching the peak value. The decrease in hardness of both alloys at the late stage of aging is due to the coarsening of precipitates. The tensile properties reveal that Cu additions sig-


nificantly enhance the precipitation strengthening response of Fe–NiAl alloys, which is important for the design of low-cost high-strength alloys. So it is necessary to study the strengthening mechanism of the nanoparticles with and without Cu. Using the experimental data determined by APT at 4 h aging, the strengthening contributions were analyzed. The interaction mechanism between the dislocations


and the particles can be either Orowan looping or particle shearing (Jiao et al., 2015). In the current work, the pre- cipitates are very small, so we expect the precipitation strength to follow the cutting mechanism.


First, when a nanoparticle is sheared by a dislocation,


chemical strength occurs due to the production of new particle–matrix interfacial areas, as given by (Jiao et al., 2015)


Δσchemical = 2M bLT1


2ðÞ γinterfacialb ;


3 2


(4)


where M = 3 is the Taylor factor, b = 0.25 nm the Burgers vector of the matrix, T the line tension of the dislocation, usually taken to be Gb2


calculated by L= 0:866 RN


nanoparticle radius and number density, respectively (Jiao et al., 2015). For the two alloys, γinterfacial and L are the variables of


ðÞ


1 2


Δσchemical. By using the maximum separation method, the R and N of the nanoparticles can be obtained, as shown in Figure 6. The value of N of the Fe–NiAl–Cu alloy is far greater than that of the Fe–NiAl alloy, resulting in a smaller L of the Fe–NiAl–Cu alloy than that of the Fe–NiAl alloy. In addition, the NiAl nanoparticles in the Fe–NiAl alloy are stable B2 structure (Kumar et al., 1992; Bei et al., 2008), which are coherent with the matrix. However, there are NiAl precipitates and Cu precipitates in the Fe–NiAl–Cu alloy As aging continues, the Cu precipitates in the Fe–NiAl–Cu alloy become incoherent with the matrix owing to the structural transformation (Kolli & Seidman, 2008), so γinterfacial of the Fe–NiAl–Cu alloy is larger. In conclusion, the value of Δσchemical of the Fe–NiAl–Cu alloy is expected to be larger than that of the Fe–NiAl alloy. Second, an elastic stress field appears in the matrix


surrounding the nanoparticles, owing to the lattice coher- ency of the ordered nanoparticles with the ferrite matrix and volumetricmisfit strains. The coherency strength is given by (Jiao et al., 2015)


Δσcoherency =4:1MGϵ3 where ϵ=1:5 Δa


3 πR3N, where R and N are the average nanoparticle radius and number density, respectively.


mismatch, with Δa/a as the lattice parameter mismatch at room temperature (Taillard & Pineau, 1982), and ƒ the volume fraction of nanoparticles, which is estimated by f = 4


a 2f 1 2


1 2


R b


; (5)  is the constrained lattice parameter


shear modulus of the matrix and γinterfacial is the interfacial energy. The mean particle spacing, L, in the slip plane is , where R and N are the average


2 (Jiao et al., 2015), G = 80GPa is the 128 h


Fe–NiAl–Cu 2.1±0.04


1.0±0.08 0.2±0.01 96.4±0.09


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