XRD patterns of the as-deposited Ni-Co-Fe and Ni-Co-Fe-P coatings are shown in Figure 1. The structures of

coatings are face-centered cubic (fcc) solid solutions, as revealed by both XRD and TEM observations. Line broadening in the diffraction patterns, which indicate considerable grain size reduction (Table 2), is observed with increasing P content in the as-deposited coatings. The grain size of the as-deposited coatings calculated by using the Scherrer formula is shown in Table 2. The grain size of the as-deposited coating is less than 10 nm. The coatings are typical nanocrystalline coatings. X-ray diffraction patterns of nanocrystalline coatings in Figure 1 show strong (111) and weak (200) reflection peaks, which indicates that the coatings have a texture. It can be seen that the (200) peak intensity relative to the (111) peak increases with increasing P in the coatings.

Figure 2 presents bright field TEM images and SAED patterns of the as-deposited Ni-45.05%Co-14.48%Fe and Ni-40.41%Co-6.16%Fe-1.63%P alloy coatings. The samples show uniform nanocrystalline microstructure.

Figures 3(a)-(c) show X-ray diffraction patterns of nanocrystalline coatings annealed at various temperatures for 1 h. In the samples annealed for 1 h up to 375˚C, the diffraction patterns are similar to that of the as-deposited sa- mple with regard to XRD peak intensity and broadening. However, in the samples annealed above 450˚C for 1 h, the peak intensity drastically increased and the full width at half maximum of the peak significantly decreased. The latter indicates that the occurrence of grain growth in the annealing temperature range above 450˚C.

It can be seen that in the case of Ni-45.05%Co-14.48%Fe coating, the as-deposited sample and samples annealed below 375˚C show a (111) texture, while the samples annealed above 450˚C show a strong (111) texture. Therefore, it is evident that during annealing, the <111> oriented grains grew faster than <100> and other oriented grains. According to Li’s calculation [12] , the surface energy is the lowest for the {111} plane and increases in the order of {100}, {110}, {311}, {210}, etc.

During annealing the texture of nanocrystalline Ni-40.41%Co-6.16%Fe-1.63%P deposit changed from <111> to <100>. The texture change was attributed to preferential growth of <100> grains at the expense of <111>

grain to decrease the thermal strain energy of the deposit [13] .

In the annealed specimens, the reflections of b.c.t. Ni₃P phase are observed after annealing at 450˚C or above for 1 h (Figure 3(c)). During annealing a transformation take place that follows the sequence: structural relaxation → P segregation → Ni₃P phase formation.

The grain size of the as-deposited and annealed coatings is shown in Table 2. No obvious grain growth of all three coatings was observed after annealing below 375˚C. The rapid grain growth takes place when annealing temperature rises up to 450˚C, however, the grain size is still less than 100nm. It indicates that all three nanocrystalline coatings have excellent thermal stability, and the Ni-40.41%Co-6.16%Fe-1.63%P is the best. The grain size of three coatings grows to be sub-micron grade after annealing above 600˚C.

The thermal stability of nanocrystalline coatings was characterized by DSC. Figure 4 presents calorimetry scans for each coating at a heating rate of 20˚C∙min⁻¹. Take the Ni-45.05%Co-14.48%Fe coating for example, three different stages of grain growth were found: 1) grain boundary relaxation (120˚C - 430˚C). Grain boundary relaxation and a little grain growth take place at this stage; this relaxation leads to positions of “low energy” for the grain boundaries creating first grown grains “accidently” by removing low angle grain boundaries; 2) abnormal grain growth (430˚C - 500˚C). The rapid grain growth occurs at this stage; 3) growth towards equili-

brium (above 500˚C). Grain growth that comes about at this stage is dominated by lattice diffusion.

Ts presents the onset temperature of rapid grain growth. The peak temperature (Tp) is the point of maximal heat release on the DSC scan. They can be used as a relative indicator of thermal stability. The total enthalpy released during grain growth can be obtained by integrating the area under the curves and the values are summarized in Table 3. It can be found that the onset temperature and peak temperature of nanocrystalline Ni- 45.05%Co-14.48%Fe deposit is much higher than that of nanocrystalline Fe-Ni [8] and Ni-Co alloys [9] . The effect of multi-component alloying can be attributed to the change in the mobility of the grain boundaries by the solution drag mechanism [14] .

A kinetic analysis of grain growth was obtained by scanning samples from each of the three coatings at multiple heating rates (B = 10, 20, 40˚C∙min⁻¹). Ni-40.41%Co-6.16%Fe-1.63%P exhibited the highest Tp values at each scanning rate. Values of Ts and Tp are summarized in Table 3. The increase in Tp with scanning rate can be used to obtain activation energy of grain growth. Figure 5 presents a modified Kissinger plot; the activation energy is obtained from the slope of ln(B/Tp) as a function of 1/Tp. Activation energies of 229.59, 369.3 and 198.44 kJ∙mol⁻¹ are obtained for Ni-45.05%Co-14.48%Fe, Ni-40.41%Co-6.16%Fe-1.63%P, and Ni-30.1%Co- 2.15%Fe-3.38%P, respectively. It indicates that during annealing, P-atoms segregate to the grain boundaries lead to a higher thermal stability. While saturation of P and precipitation occurs earlier leading to s slightly lower stability.

Mori et al. [15] calculated the relaxation time for the particle bearing boundary and is expressed as

Here λ is the distance of particles, r is the average radius of particles, 2d presents the grain size, and is relaxation time of the particle-free boundary.

The grain growth of Ni₃P phase takes place during annealing, leading to the decrescence of λ and the augmentation of r and d. According to the Equation (2), the relaxation time (τ) of nanocrystalline Ni-30.1%Co- 2.15%Fe-3.38%P coating decreases because of the formation of Ni₃P.