Five bearing Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) alloys were prepared from purely aluminum, zinc and tin (purity > 99.99%) Table 1. These alloys were put in a porcelain crucible and melted by an electric furnace at 700˚C. After 25 min from heating, the alloys became in a molten. The molten alloys are shooting on the rotating copper wheel which has a linear speed of 31.4 m/s giving a cooling rate of ~3.7 × 10⁵ K/s [19]. Increasing cooling rate was reported to increase solid solubility and hence decrease the micro segregation and therefore the second phase content [20] [21]. More alloying elements will remain in the primary α-Al dendrites that strengthen them by solid solution hardening. Increasing cooling rate also decreases the scale of microstructure, which in turn increases the strength of the alloy. Both these effects are beneficial because the application of the current alloy in bearing applications needs to be stronger.

Various techniques were used to investigate the structural properties of rapidly solidified alloys including x-ray diffraction using Cu Kα radiation at room temperature. The thermal properties of Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys studied by differential scanning calorimetry is carried out by [(DSC-16, Setaram, France) with a heating rate 10 K/min [22]. Scanning electron microscopy (SEM) of type (JEOL JSM-6510LV, Japan) operate at 30 KV with high resolution 3 nm has been used to study the microstructure analysis.

Figure 2 shows the XRD patterns for the melt-spun Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%,

0.5%, 1% and 1.5%) bearing alloys. The XRD pattern for all Al-20Zn-Sn alloys show that two phases α-Al of face centered cubic structure and AlZn intermetallic compound of anorthic structure exist together at the same 2𝜃 values. By addition of tin, tin combines with zinc forming SnZn phase of hexagonal structural. The XRD analysis is listed in Table 2. The crystal size (D) was calculated from the XRD pattern by using Scherer formula [26].

D = 0.9 λ β c o s θ (2)

where θ is the Bragg angle, λ is the x-ray wavelength; β is the broadening of the diffraction line measured at half its maximum intensity (FWHM). The crystal size of Al phase is decreased by increasing Sn content in the Al matrix which enhances the mechanical properties of Al bearing alloys.

After calculating volume of the unit cell (V) by through the lattice parameters, the number of atoms in the unit cell (n) is calculated using the following equations [27].

Σ A = p v 1.66020 (3)

Σ A = n A w (4)

where ΣA is the sum of the atomic weights of the atoms in the unit cell,ρ is the density (gm/cm³) listed in Table 1 and v is the volume of the unit cell (nm³), and Aw is the molecular mass. The number of the atoms per unit cell was calculated which is non-integral number of atoms per cell. Therefore, some of the atoms may be absent from some fraction of those lattice sites, which they would be expected to occupy.

The microstructures of the resulting melt spun Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) alloys of long ribbons of about 65 mm in thickness and 4 mm width, are shown in Figure 3 and the mean value and mean deviation of grain size calculated from SEM are listed in Table 3. The smaller value of grain size (5.838 µm) is observed in Al-20Zn-1.5Sn melt-spun bearing alloy. The microstructure of Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys are irregular grains with increasing the number of grains with increasing tin content and also Al phase refined with Sn addition up to 1.5% Sn which leading to improvement of mechanical properties.

The DSC curves obtained for Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) melt-spun bearing alloys are shown in Figure 4. A single endothermic peak from the DSC thermo-graphs showed the melting reaction. Melting temperature T_m, liquidus temperature T_L, solidus temperature T_s, enthalpy and entropy change of Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) melt-spun bearing alloys were calculated from DSC graphs and presented in Table 4. By increasing the tin content, the melting point, liquidus and solidus temperatures decreased as indicated from ternary Al-Sn-Zn phase diagram Figure 5 [28]. The melt spinning technique has an effect on the removal of the alloy from equilibrium and therefore the melting temperature will reach 930.7 K for Al-20Zn-1Sn and 931.1K for Al-20Zn-1.5Sn bearing alloys. This decrease is the result of rapid solidification, compared with other studies [29]. The higher value of enthalpy for Al-20Zn-1.5Sn may be due to decreasing crystal and grain size as indicated from x-ray and microstructure analysis respectively, that leading to enhancement of the mechanical properties. The specific heat (C_p) means the amount of heat per unit mass required to raise the temperature by one degree Celsius, where the specific heat capacity of alloys has been increased by increasing tin content. It is found that entropy change is lower value for Al-20Zn-1.5 Sn, this result is expected because it has a relatively ordered arrangement of lattice ions.

The stress-strain characteristic curve at room temperature is plotted by using tensile test machine as shown in Figure 6 and elastic moduli (E is the Young’s modulus, G is the shear modulus and B is the bulk modulus), tensile strength and yield strength can be calculated then listed in Table 5. The strength of the material is confined by bonding forces, which are reflected by the elastic constants. Tensile strength is the maximum on the stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension [30]. Yield strength is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible [30]. The tensile strength increased in Sn addition from 0 to 1.5% as in good agreement with that of typical Al alloys [31]. Adding content of Sn depends on the Sn softening and the cracking of the porosity produced which effect hardness and tensile properties of the alloys [32].

From microstructure analysis, it is indicated that incresing Sn content leading to increase the number of grains and also Al phase refined with Sn addition. So that tensile strength, yield strength and elastic moduli increased. The Al-20Zn-1.5Sn melt-spun bearing alloy shows a maximum increase of the Young’s modulus value of 125 ± 11 GPa, the tensile strength of 680 ± 15 MPa and yield strength of 573 ± 9 MPa. In the Al-20Zn-0.5Sn and Al-20Zn-1.5Sn alloys graphs, the stress profiles do not decrease after the maximum stress value like the rest of the alloys. Because, these alloys are high ductile and strength compared with other alloys. In order to be tough, a material must be both hard and ductile as indicated from stress-strain diagram of Al-20Zn-1.5 Sn alloy. When the shear stress reaches the value of the critical resolved shear stress, the plastic deformation begins as indicated in Figure 5. The critical shear stress CSS shows the maximum value of 10 ± 1 GPa for Al-20Zn-1.5Sn melt-spun bearing alloy. The ability of a material to absorb energy then deform plastically without fracturing is the toughness of the material [33]. By integrating the stress–strain curve in Figure 5, toughness can be calculated and listed in Table 5. It shows maximum toughness value of (23 ± 2.1) × 10⁶ J/m³ for Al-20Zn-1.5Sn bearing alloy.

The indentation length versus tin composition of the melt spun Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys at constant load 10 gf and indentation time 5 seconds are shown in Figure 7, where the indentation length shows the smallest value at 1.5 wt% Sn. The variation of Vickers hardness with tin composition at constant load 10 gf and indentation time 5 seconds is showed in Figure 8. The average hardness values of Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys are listed in Table 6. It is noted that average hardness values increase by increasing Sn content up to 1.5% Sn from 240 ± 11 to 275 ± 11 MPa. The stress exponent calculated from Equation (5) and listed in Table 6 according to [34] and [35]:

n = [ ∂ ln d ˙ ∂ ln H v ] d (5)

The stress exponent that is calculated from Equation (5) is used to determine deformation mechanisms, where H_V is the number of Vickers hardness, d is the length of indentation diagonal, and d ˙ is variation rate of diagonal indentation length. The stress exponent n is calculated from the slope of straight line obtained by plotting d ˙ against H_V on double logarithmic scale as indicated in Figure 9 [36]. The stress exponent is indication of mechanism of deformation

during room temperature. The stress exponent (n) values in the range 2.4 - 4.2 as shown in Table 6, indicate that the grain boundary sliding is the possible mechanism during room temperature creep deformation of melt spun Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys in good agreement with that of typical Al alloys [37].

The indentation creep behavior is shown in Figure 10 by plotting strain against indentation time (indentation creep curve) of Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys. The first stage shows faster increase of strain with indentation time which starts from beginning up to 10 sec of indentation time. Second stage indicates a slow increase region for all alloys where the strain increased slowly. Fracture of the specimen does not occur because hardness

test is actually a compression test therefore the third stage cannot be recorded as happened in an ordinary creep test. According to the power law creep, the creep rate decreased when the stress exponent for Al-20Zn-1.5Sn bearing alloy increased due to an increase in yield strength [38]. Therefore, the higher value of stress exponent (n) and yield strength of Al-20Zn-1.5Sn bearing alloy is more resistant to creep of indentation compared to Al_80−x-Zn₂₀-Sn_X (X = 0, 0.2%, 0.5%, 1% and 1.5%) bearing alloys.