INTRODUCTION

The modern machine building industry depends on the lifetime of some elements of loaded frictional units (bearings, bushing, inserts, etc.). In the majority of cases, their inability to work in the course of operation is related to the wear of elements comprising the units. Improving the reliability and safety of frictional units in the course of operation under high specific mechanical and thermal loadings is a serious problem, in particular, in the case of scanty lubrication or its absence, and abrasive wear, which both arise in the course of operation of machines and mechanisms under extreme conditions (cold, vacuum, aggressive medium, etc.). The solution of the problem requires a greater emphasis on the development and improvement of technologies for the preparation of antifrictional coatings and the restoration of highly loaded elements of frictional units, as well as designing antifrictional composite materials exhibiting a combination of high wear resistance and antifriction properties that are stable over a wide temperature range.

Owing to their high wear resistance and thermal conductivity and low coefficient of friction, metallic antifrictional materials comprising a soft matrix and solid inclusions are in high demand in the machine engineering. Copper-based antifrictional materials are the most relevant for steel machine components since the Fe–Cu pair has the positive enthalpy of mixing; as a result, an intermetallic compound cannot form at contact surfaces in the course of friction.

To increase the wear resistance of materials, intermetallics and solid solutions, whose element composition is similar to that of soft matrix, have been used in recent years along with traditional carbides, borides, oxides, which are characterized by weak wettability by metals in a melt.

The mechanochemical synthesis is an efficient method used for the preparation of intermetallic compounds (IMCs) and solid solutions [1–4]. It is known that the formation of single-phase product in the course of mechanical activation (MA) of metallic systems is unlikely since the heats of the formation of IMCs are not high and are close in magnitude to each other [5–7]. Previous studies showed the highest microhardness is typical of solid solutions and their incorporation into a soft matrix, whose element composition is identical to that of solid solution, ensures good wettability of the solid solution particles, which is significantly better than that of carbides, borides, etc. [8, 9].

The aim of the present study is to investigate the evolution of the phase composition of products of the mechanochemical synthesis in the Cu–10 wt % Al system using X-ray diffraction analysis.

EXPERIMENTAL

As the starting materials, we used a PMS 1 copper powder (State Standard GOST 4960–75) and a PA-4 aluminum powder (State Standard GOST 6058–73) with a particle size of ~45 µm. The mechanochemical synthesis was performed in an argon atmosphere using an AGO high-energy water-cooled planetary ball mill [10]. The volume of mill drum is 250 cm3; the loading of balls 5 mm in diameter and milled sample are 200 and 10 g, respectively. The rate of drum rotation about the common axis is ~1000 rpm.

X-ray diffraction (XRD) analysis was performed on an Empyrean Panalytical diffractometer equipped with a PIXcel3D position-sensitive detector using Cu Kα radiation, scanning step Δ2θ = 0.026°, and a time per point of 100 s. The phase composition and crystal structure of sample were determined using X‑ray diffraction data and the DIFFRACplus software, namely, EVA [11] and PDF4 [12] ICDD database. The lattice parameters of coexisting phases were calculated by least-squares procedure using Celref software [13]. The quantitative analysis was performed by the FullProf Rietveld method [14] using DIFFRACplus TOPAS software [15]. The microstructural parameters (the crystallite size L and microstrains ε) were determined by double Voigt method. To resolve the contributions of the crystallite size L and microstrains ε to the peak broadening, the Lorentzian and Gaussian functions were used, respectively.

RESULTS AND DISCUSSION

According to the equilibrium Cu–Al phase diagram (Fig. 1), the Cu-based solid solution region (α phase) is ranged to 9.4 wt % Al [16]. Several eutectoid and peritectoid solid-state transformations take place. The eutectoid point corresponds to an aluminum content of 15.4 wt %. The existence of the α2 phase explains the anomalous behavior of the temperature dependence of the specific heat capacity at ~300°С, which is observed for single-phase (α) and two-phase alloys. For a composition range of 9–16 wt % Al, the existence of one more stable phase, such as χ is assumed. It forms by eutectoid reaction at 363°С; the aluminum content corresponding to the eutectoid point is ~11.2 wt %. The CuAl2-based solid solution (θ phase) forms with the participation of liquid. The α phase (Cu-based solid solution) forms in a wide composition range and, as the temperature increases, the aluminum solubility in copper decreases. At 500, 700, 800, and 900°C, the solubility is 9.4, 8.8, 8.2, and 7.8 wt %, respectively. The α phase has a face-centered cubic lattice, similar to that of pure copper. The lattice parameter a increases as the aluminum content increases, namely, from 0.3615 nm (PDF4 #00-004-0836) for pure copper to 0.3624 nm for an aluminum content of 5.65 at % (PDF4 #01-074-5169), to 0.3662 nm for an aluminum content of 15 at % (PDF4 #04-004-5537), and to 0.3670 nm for an aluminum content of 22 at % (PDF4 #04-006-6355) [12] (Fig. 2).

Fig. 1.
figure 1

Equilibrium Cu–Al phase diagram [16].

Full size image
Fig. 2.
figure 2

Lattice parameter a vs the aluminum content in the Cu(Al) solid solution [12].

Full size image

It is known that in the case of interacting solid—liquid metal systems, a phase with the highest content of low-melting component is primarily formed [17, 18]. It was found that the mechanochemical interaction in such systems also starts from the formation of the phase with the highest content of low-melting component [19–21]. In the case of systems characterized by higher melting temperatures, the enthalpy of formation of intermetallic compounds, along with the aggregate state of a substance, begins to play an important role upon mechanochemical interaction. It was shown that a phase characterized by the highest enthalpy of formation is the primarily formed phase [22–24]. The experimentally determined enthalpies of formation of the CuAl2 and Cu9Al4 compounds are ‒10 and –16 kJ/mol, respectively [25, 26]. The calculated enthalpy of mixing of the solid solution of aluminum in copper for the Cu–10 wt % Al system is ~5.5 kJ/mol [5–7]. It may therefore be expected that in the course of mechanochemical formation of the Cu(Al) solid solution, the CuAl2 and Cu9Al4 IMCs should be primarily formed.

Results of X-ray diffraction study (Fig. 3) of the phase composition of MA products formed in the course of the synthesis of solid solution in the Cu-10 wt % Al system indicate that the formation of solid solution of aluminum in copper occurs via the formation of intermediate IMCs of this system. After mechanical activation for 40 s, the Cu9Al4 (~9 wt %) and CuAl2 (~1 wt %) IMCs appear in the system. The microstrains of residual copper (~80%) are not high and are 0.3%. The lattice parameter retains almost unchanged and is equal to that of pure copper.

Fig. 3.
figure 3

X-ray diffraction patterns of the Cu–10 wt % Al system taken after MA for (a) 40 s, (b) 2, (c) 4, and (d) 8 min.

Full size image

The increase in the MA time to 2 min leads to a substantial decrease in the copper content, an increase in the amount of both IMCs, and the appearance of solid solution of aluminum in copper (Table 1). The resolution of (111) diffraction reflection with allowance for copper, Cu9Al4 intermetallic, and solid solution of aluminum in copper (Figs. 4a–4c), which was measured after MA for different times, confirms the high amounts of the IMC and solid solution in the products of mechanochemical synthesis already after 2-min MA. It is necessary to note that microstrains of copper increase in the course of activation, whereas the formed Cu(Al) solid solution is initially characterized by high microstrains.

Table 1.   Phase composition and X-ray diffraction parameters for the products of mechanochemical synthesis of the Cu–10 wt % Al system mechanically activated for 2 min
Full size table
Fig. 4.
figure 4

The (111) reflection for the Cu-Al system mechanically activated for (a) 2, (b) 4, and (c) 8 min: (1) Cu; (2) Cu9Al4; (3) Cu(Al), and (4) approximated curve.

Full size image

As the MA time increases to 4 min, the solid solution content substantially increases (Fig. 4b). The data given in Table 2 indicate the complete transformation of CuAl2, most likely, into Cu9Al4 through mechanical alloying with copper.

Table 2.   Phase composition and X-ray diffraction parameters of the products of mechanochemical synthesis of the Cu–10 wt % Al system mechanically activated for 4 min
Full size table

The Cu9Al4 also reacts with copper; as a result, the Cu(Al) solid solution forms and the copper content decreases substantially. It should be noted that the microstrains of the formed solid solution decrease, whereas the microstrains of residual copper continue to increase.

After 8-min MA, the amount of Cu9Al4 intermetallic in the system decreases to ~10 wt %; the solid solution becomes the main phase (~90 wt %) in the synthesized products (Fig. 4c). The microstrains of the solid solution are ~1%; the crystallite size is 35–40 nm. The further increase in the MA time does not lead to substantial changes in the solid solution content.

The aluminum content in the solid solution (XAl), which was determined using the concentration dependence of the lattice parameters (Fig. 2) and X-ray diffraction data (Table 3), is 15.7 at % (or 7.4 wt %), i.e., the ultimate aluminum solubility (9.4 wt %) is not reached.

Table 3.   Parameters (determined by XRD) of the products of mechanochemical synthesis in the course of MA of the Cu–10 wt % Al (20.7 at %) system
Full size table

Thus, the mechanochemical formation of the Cu(Al) solid solution is realized for a short time and occurs via the formation of the Cu9Al4 and CuAl2 intermetallic compounds, which react with copper under the conditions of high-energy activation in the planetary ball mill. The final product of the mechanochemical synthesis is the mixture of two phases, namely, 90 wt % solid solution and 10 wt % Cu9Al4. The aluminum concentration in copper reaches 7.4 wt %.