Relatively dense submicrometer grain-sized MMC Ni–P alloys have attracted much scientific and technical attention because of their excellent functional properties compared with conventional polycrystalline materials [18
]. The mechanical properties, owing to their fine grain size, are characterized by high strength and relatively high ductility [21
]. Their corrosion resistance, high hardness, and wear resistance as well as electrical, magnetic, and catalytic properties have made these alloys excellent candidates for use in electric motors, chemical applications, and several others areas [20
These alloys can be prepared by two principal processes: electroless or electrodeposition methods. Both methods require the use of a complex bath composed of several salts, each having a specific role (reducing agent (generally, the hypophosphite anion), complexing agent, pH buffers, accelerators, stabilizers, and wetting agents) [25
]. The alloys elaborated by the deposition methods commonly come in film form. Thus, an additional heat treatment is needed to improve the crystallinity and mechanical properties of these materials. This step confers to these films a very high hardness and can be used as wear-resistant coatings. However, these materials are subject to severe brittleness, which makes their workability not an easy task.
In the present work, along with our previous works [7
], we proposed a novel method to elaborate Ni–P alloys in the form of nanoparticles instead of films. This method requires a simplified bath only constituted of polyol, in which the hypophosphite is added in order to enhance the reducing power of the medium. The obtained Ni–P nanoparticles are spherical with well-controlled size and polycrystalline structure. Unlike the Ni–P alloys prepared by electroless or electrodeposition [22
], nanoparticles prepared by the polyol process show high crystalline quality. While electroless and electrodeposition processes enable the elaboration of films, our bottom-up strategy using the R-SPS process and starting from nanopowders of Ni–P alloys leads to bulk submicrometer grain-sized MMCs, which are characterized by an Ni matrix and Ni3
P inclusions as a reinforcement material (wt % of 5–10%). This is coherent with our previous results obtained for 100 nm Ni–P alloys [8
In the work of [8
], by varying the heat treatment conditions (temperature, time, and pressure), various MMC materials were obtained with a tunable grain size, varying in the ranges of 179–560 and 101–223 nm for Ni and Ni3
In the present work, the R-SPS conditions were fixed (temperature of 600 °C, holding time of 10 min, and pressure of 53 MPa), while the starting nanoparticle size varied from 50 to 220 nm. This resulted in MMCs that also had a tunable grain size between 247 and 638 nm for Ni and between 97 and 187 nm for Ni3P. A relationship exists between the starting nanoparticle size of Ni–P alloys and the grain size of the Ni in the as-obtained MMCs; the larger the size of the starting Ni–P alloy particles, the larger the size of Ni grains in MMCs.
As can be seen, the two approaches led to almost identical grain size ranges for both components.
The grain sizes inferred from TEM images were significantly higher than the crystallite sizes inferred from XRD analyses. Crystallites are in fact the smallest domain inside the grain, which scatter X-rays coherently. This coherency breaks even if the crystallites have very small misorientation (1°–2°). Thus, the crystallites inside of the grain can be considered as subgrains [29
]. More interesting to note, the grain size varied linearly with the crystallite size; it increased when the crystallite size increased. Furthermore, for Ni grains, the grain size/crystallite size ratio varied in a narrow range: 6.5–8, similar to that observed in the work of [8
]. Accordingly, the mechanical and magnetic properties of these materials can either be correlated to the submicrometer microstructure scale (grain size) and/or to the nanometer scale (crystallite size).
To summarize, the different investigations conducted (TEM, XRD, density, chemical analysis) show that the MMC materials consist of a matrix of nickel polygonal grains with submicron size and Ni3
P as reinforcement in the form of spherical particles with size in the sub-micrometer range, but significantly lower than that of Ni grains. Furthermore, all obtained composites have a residual porosity, which decreases when the grain size increases. All together, these parameters likely control the mechanical and magnetic properties of the as-obtained MMCs. However, our recent work [8
] has shown that Ni grain size plays the key role to tune the as-properties.
Thus, all mechanical and magnetic properties of the as-obtained MMCs are discussed and tentatively correlated with the grain size of the matrix, namely, Ni. Furthermore, the influence of Ni3P or porosity on this general behavior is specified in each case.
Yield strength, maximum stress, and plastic strain are mainly driven by the grain size of Ni, which constitutes the matrix. Figure 8
reports the yield strength (maximum stress is not reported, as it behaves similarly) and the plastic strain as a function of grain size. Similar behaviors were observed for MMCs published in the work of [8
]. As shown in Figure 8
, yield stress increased when the grain size decreased. Conversely, the plastic strain increased when the grain size increased. An important crossover of the mechanical response of the MMCs appeared around 350 nm of Ni grain, which corresponded to a crystallite size of around 50 nm. It is interesting to note that this threshold value was also observed for MMC samples studied in the work of [8
] (Figure S2
While the mechanical characteristics deduced from the compression tests do not depend on the porosity of the materials, Vickers hardness has a subtle behavior; it depends on the porosity in addition to the Ni grain size. It increases with the grain size to reach a maximum value for a grain size close to 380 nm, and then it decreases. Balancing porosity and Ni grain size makes it possible to obtain the highest hardness (p% 2.2, grain size 383 nm). The samples with a smaller grain size correspond to high porosities, which are harmful for obtaining high hardness. Beyond the critical size of 380 nm, the sample has a very low porosity, but it corresponds to an exaggerated growth of all nickel grains and Ni3P particles. This abnormal grain growth outweighed the increase in relative density, and hence explains the microhardness decrease.
The magnetic properties were also found to be closely related to the Ni grain size (Figure 9
). The coercive field (Hc) increased when the grain size decreased, similar to yield strength, while saturation magnetization values (Ms) increased with the grain size in a manner similar to the plastic strain (curves corresponding to previous work [8
] are shown in Figure S3
). Like the mechanical behavior, a crossover behavior was observed around 350 nm of Ni grain size for the magnetic behavior of both sample series elaborated in the work of [8
] and the present work. It is interesting to notice that this grain size corresponds to a crystallite size of around 50 nm, which is the critical crystallite size of the Ni magnetic mono-domain (Table 4
All these results show that R-SPS is a useful sintering process to control the microstructure and physical properties of MMCs based on nickel. This can be achieved by following two different methods. The first one, developed in the work of [8
], consists of varying the SPS conditions and starting from a given particle size. The second method (present work) consists of keeping identical SPS conditions and varying the size of the starting particles. Both processes led to MMCs with grain sizes varying in a wide range from 200 to 680 nm for Ni. This enables tuning the mechanical and magnetic properties on a large scale and opens the possibility of a crossover of these properties within the same composition only by controlling the growth of Ni grains.
In this work, together with the work of [8
], we have shown that decreasing the grain size led to greater strength and a less ductile submicrometer grain-sized material in accordance with the Hall–Petch law [32
]. This may be explained on the basis of dislocation density in the grains and subgrains (crystallites). Indeed, small grains contain few dislocations in comparison with large grains, inducing weaker stress at the grain boundary. Furthermore, decreasing the grain size leads to an increase of grain boundaries, which impedes the dislocation motion. In this case, the dislocation propagation in the adjacent grain needs a more important applied stress [34
Finally, compared with the results published in the literature on pure nickel materials with comparable average sizes, we note that the yield strength, maximum stress, and microhardness are significantly higher [13
] in our case. This result is mainly caused by the following:
The presence of a secondary phase (Ni3
P). Indeed, compared with single-phase samples with the same particle size, the presence of a secondary phase strengthens the microstructure and leads to higher values of microhardness and yield strengths [36
The difference in microstructure of these materials induced by the elaboration process. In the present study, the R-SPS process led to MMCs based on nickel with Ni3
P as a reinforcement starting from nanoparticles of Ni–P metastable alloys. The formation of Ni3
P occurred in situ by diffusion of P from the inside of the nanoparticles to their boundaries. This mechanism leads to strong chemical bonding between matrix and reinforcement components [38
Decreasing the grain, and thus the crystallite size of the nickel, also affects the magnetic behavior. It leads to a higher coercive field (Hc) and lower saturation magnetization (Ms).
Because, as shown before, the grains of the as-obtained MMCs consisted of crystallites with sizes varying in the nanometer range (30–80 nm), it clearly appears that the magnetic behavior can be tentatively explained on the basis of interactions between these crystallites considered as magnetic nano-domains. The increase in Hc may be because of the change of the magnetization reversal mechanism in these nano-domains. When their size decreases, the crystallites are more magnetically close to mono-domains, and thus the magnetization reversal proceeds via a coherent rotation of the magnetic moment in each domain. This coherent mechanism needs a higher applied field [31
] compared with the percolation process in a multi-domain magnetic state. Also, as soon as the crystallite size of the nickel approached the critical size of nickel (50 nm) (see Figure 8
, blue numbers in parentheses), the coercive field of the MMC materials went abruptly to zero at RT (i.e., crossover-like behavior of the magnetic properties). This result allows us to speculate on the competition between the thermal instability of the magnetization moment into the mono-domain nanoparticles [31
], causing the superparamagnetic behavior with the magnetic interactions between domains favoring the ferromagnetic properties. Our results show that approaching the critical size of the Ni crystallites drifts the thermal instability effect at higher temperatures than the RT one. Indeed, as shown in the inset of Figure 6
b, when the crystallite size of the Ni matrix decreased below the critical size of Ni nanoparticles (i.e., Dc = 50 nm), the so-called blocking effect appeared at higher temperatures than the RT one. At RT, in fact, the hysteresis cycle went from a typical multi-domain magnetic behavior (NiP220SPS) to a soft ferromagnetic one (NiP50SPS). This allows the observation of soft ferromagnetic behavior for MMC systems presenting smaller Ni crystallite sizes than the critical one.
Unlike Hc, Ms also depends on the presence of Ni3
P (a paramagnetic compound), which explains the slight decrease of Ms of all compounds of the series in comparison with bulk Ni. However, inside this series, as for Hc, a regular variation was observed in the function of grain and crystallite sizes (i.e., when the grain and crystallite sizes decreased, the Ms decreased). Indeed, when the crystallite size decreased, the number of atoms at the surface increased. This led to an important number of disoriented moments in comparison with well-oriented moments inside of the crystallite. Such division into two magnetic sub-domains is responsible for the Ms decrease in comparison with the Ms of large particles, with the outside domain (surface domain) being considered a magnetic dead surface [39