3.1. Phase Formation
Phase formation analysis was carried out. Results of the XRD patterns of AlCoCr
xCuFe HECs including different Cr content is shown in
Figure 2. As can be clearly seen, Cr05, Cr10, Cr15, and Cr20 HECs all showed FCC and BCC duplex phase structures. According to a previous report by [
26] of AlCoCrCuFe HEA, the disordered BCC phase and FCC phase was identified, which was consistent with the results of the current study. The strongest diffraction peak of BCC phase was at about 43°. With the addition of Cr content, the strongest diffraction peak of BCC decreased. As the alloy was composed of two phases and no intermetallic compound phase was obviously formed. It can be judged that with the increase in the Cr element, the content of the BCC phase decreased gradually. In addition, a significantly enhanced BCC peak was generated at 65° due to Cr aggregation between the dendrites at
x = 2.0 [
27].
The mixing entropy (
) of the AlCoCr
xCuFe HECs was positive and mixing enthalpy (
) was negative. The Gibbs free energy of HECs was negative, as shown by the Equation (1), which was beneficial to form the solid solution phase. Related research [
28,
29] has shown that the formation of the HEAs phase is influenced by properties such as
,
, atomic radius difference (δ), electronegativity difference (
), valence electron concentration (VEC), and Ω. These parameters can be expressed as Equations (2)–(7):
The meaning of the parameters in the equation is as follows:
—Gibbs free energy.
R = 8.314 J/(mol·K)—gas constant.
n—the number of element types.
—the mixing enthalpy of the i principal element and the j principal element in regular solution.
and —the atomic contents of the i principal element or the i principal element.
—the average radius of the alloy element atoms.
—the radius of the i element.
—the average electronegativity of the elements.
—the electronegativity of the i element.
—the melting point of the alloy.
The calculation results of the parameters of the laser cladding AlCoCr
xCuFe HECs in this study such as the
,
, δ,
, VEC, and Ω are shown in the
Table 3. The physical chemical/thermodynamic parameters of the alloy elements used in this study are shown in
Table 4 and
Table 5.
Zhang Yong et al. [
31] believed that the formation range of the solid solution phase was δ < 6.5%, −15 kJ/mol <
< 5 kJ/mol, 12 J/(K·mol) <
< 17.5 J/(K·mol). Guo et al. [
32] concluded that FCC solid solutions would be relatively stable when VEC ≥ 8.6, and BCC solid solutions will be relatively stable when VEC < 6.87. Moreover, Zhang Yong et al. [
33] indicated that the
and
T can both affect the
of the solid solution and the formation of the solid solution. Ω represents the effect of the interaction of
and
on the phase formation. They believe that when Ω ≥ 1.1,
≤ 6.6%, the solid solution phase of HEAs will be stable. It can be pointed out in
Table 3, that with the increase in Cr content, the
,
and VEC of the AlCoCr
xCuFe HECs decreased, while Ω increased. However, the
value was not in direct or inverse proportion with the Cr content. The parameters of
,
,
,
Ω for the Cr05, Cr10, Cr15, and Cr20 HECs were consistent with previous studies. The VEC for the AlCoCr
xCuFe HECs in this study were all between 6.87 and 8.6. The XRD results showed that AlCoCr
xCuFe HECs had a FCC and BCC two-phase solid solution, which was in line with the results of related research [
31]. The above-mentioned parameters were often used to predict the phase formation of fused and cast HEAs blocks. Laser cladding is characterized by concentrated heat, fast heating, fast cooling, and a small heat affected zone. The special heating and cooling process of AlCoCr
xCuFe HECs may force its phase formation to be different from that of the block HEAs. However, the relationship between the phase composition of AlCoCr
xCuFe HECs in this study and the parameters of
,
,
δ,
, VEC, and Ω were in good agreement with the phase formation range in existing research [
31,
32,
33].
3.3. Cavitation Erosion Performance and Mechanism
The curve in
Figure 4a exhibits the cumulative volume loss of test samples during the 20 h cavitation erosion test. Volume loss (VL) was used to characterize the cavitation resistance of HECs.
It can be clearly seen that the change in the curve can be divided into three different cavitation erosion periods [
7]: one is the incubation period with little material loss, the other is the period with severe material loss; and another stationary period with stable material loss. Cr05 HEC exhibited the lowest cumulative volume loss in all samples. With the increase in Cr content, the 20 h cumulative volume loss increased. This indicated that the cavitation erosion resistance of AlCoCr
xCuFe HECs was decreased with the addition of Cr content, reaching its worst at Cr20 HEC.
Figure 4b shows the erosion rate curves of the AlCoCr
xCuFe HECs. The incubation period of Cr05 HEC was about 4 h. However, Cr10, Cr15, and Cr20 HECs only experienced about 2 h of the incubation period. Unlike from the stable growth of Cr05 HEC and AISI 304 steel in the accumulation period, the growth of Cr10 and Cr15 was very rapid, and they only experienced about 1 h of rapid growth. With the increase in Cr content, the erosion rate of HECs during the cavitation stationary period increased, which was consistent with their 20 h cavitation cumulative volume loss results.
Figure 5 shows the nanoindentation characteristics of the AlCoCr
xCuFe HECs and AISI 304 steel. The Oliver-Pharr method was used [
36] for calculating the nanoindentation hardness and reduced elastic modulus of AlCoCr
xCuFe HECs. The equations used for calculation were (8)–(10).
where
,
,
,
,
,
,
,
,
represent the contact depth, displacement with maximum load, correction constant (
), unloading stiffness with maximum load, contact area, applied load in the process of nanoindentation, indentation depth, nanoindentation hardness, and reduced elastic modulus, respectively.
The research of H. Attar et al. [
37] indicated that the reduced elastic modulus of the nano indentation test was load dependent. In this study, the maximum load of the nano-indentation test was selected as 300 mN, aiming to make the indentation depth and cavitation erosion depth in the same order of magnitude. Hardness and modulus of elasticity are often regarded as mechanical parameters related to the cavitation resistance of a material [
2]. The micro-mechanical performance parameters, drawn from the nano-indentation tests, are shown in
Table 6.
Figure 6 shows the relationship between
,
,
,
/
and cumulative volume loss, respectively. The
and VL correlation coefficient was only
R2 = 0.19,
and the VL correlation coefficient was
R2 = 0.81. This indicates that
and
cannot affect the cavitation performance of HECs alone.
is described as the elastic strain to failure [
38] and is widely considered as a valuable measure to determine the elastic behavior limit of the contact surface. The
and VL correlation coefficient was
R2 = 0.88. This indicates that the proper combination of hardness and elasticity is an important reason for the improvement of cavitation erosion performance. However, an over high
will weaken the shear strength and reduce the wear resistance [
39]. Another parameter,
/
, was more related to VL (
R2 = 0.96). A higher
/
usually corresponds to a higher resistance to plastic deformation [
40]. This indicates that the appropriate plastic deformation resistance was sufficient to bear the cavitation impacts. Cr05 HEC obtained an excellent combination of hardness and elasticity, which made a great contribution to the improvement in cavitation erosion resistance.
Figure 7 exhibits the surfaces of the AlCoCr
xCuFe HECs with the 20 h cavitation erosion test. The DR structure can be clearly observed on the surface of Cr05 and Cr10, where it can be inferred that the DR structure is more difficult to damage in the cavitation erosion test. The BCC phase of the DR structure can make it work hardening when it is impacted by cavitation erosion, which can prevent further damage to the material caused by cavitation impact [
41]. FCC structures rich in copper tend to have a higher stacking fault energy [
42]. Higher stacking fault energy can result in poor cavitation resistance [
43], which makes the ID structure more vulnerable to cavitation impact. The relatively undamaged DR structure in Cr05 proves this point.
Figure 8 shows the XRD pattern of the AlCoCr
xCuFe HECs after the 20 h cavitation erosion test. Compared with the XRD patterns of HECs with different Cr contents before the cavitation erosion test, it can be found that the characteristic peak of the FCC phase decreased significantly near 50°, especially in Cr05 HEC. This also coincides with the surface morphology after the cavitation erosion test in
Figure 7. The exposed DR structure was caused by the damage of the ID structure by cavitation impact.
The parameters
, and δ influence the properties of high entropy alloys by influencing the phase formation process, so it is necessary to discuss the relationship between these parameters and the cavitation erosion resistance (expressed in VL). As shown in
Figure 9, there was no obvious correlation between
and VL (correlation coefficient
R2 = 0.241), while there was a strong correlation between δ and VL (correlation coefficient
R2 = 0.991). The parameter
can represent the driving force of the formation of the solid solution [
44]. High
will increase the degree of entropy chaos in the alloy system and significantly reduce the free energy of the alloy. The disordered distribution of different alloying elements in the crystal lattice promotes the formation of a solid solution. However, in
Figure 9a, VL and
did not show a strong correlation. At the same time, δ showed a strong correlation with VL. The collapse of cavitation in water can produce high temperatures of 2300 k–5100 k [
45]. The heat conduction in solid is realized by phonons and free electrons. With the decrease in Cr content, the difference in atomic radius increases. The serious lattice distortion of high entropy alloys leads to the scattering of phonons and lattice, which reduces the thermal conductivity [
46]. This also makes the high entropy alloy have certain advantages in the face of cavitation heat.