The tensile test machine is showed in Figure 1. The specimen is fitted in the laws of the testing machine and subjected to a tensile load. The applied load (stress) and the resulting elongation (strain) of the specimen are measured computerized. The process is repeated with increased load until the specimen breaks. Using these data a stress-strain diagram is plotted using computer set up contains software program to find out tensile properties.

The standard technique used for measuring the hardness of metals is Vickers hardness test. Vickers hardness is a measure of the hardness of a material, calculated from the size of an impression produced under load by a pyramid-shaped diamond indenter [13].

The tensile test machine is most frequently performed to determine mechanical properties. Information on various tensile properties like proportional limit, elastic limit, yield point, yield strength, ultimate strength, fracture strength, toughness and modulus of elasticity is given in our article. Using tensile test machine for drawing stress-strain characteristic curve as shown in Figure 6 to calculate tensile strength, yield strength and elastic moduli and then listed in Table 3. The alloys after increasing tin and bismuth to 0.5wt% Sn-0.54wt% Bi transform from elastic-plastic behavior to plastic behavior as showed in Figures 5(b)-(d). The critical shear stress (the minimum shear stress required to begin plastic deformation or slip) which is acting on the slip plane at which the lattice

becomes unstable. That is, the plastic deformation begins when the shear stress attains the value of the critical resolved shear stress as shown in Figure 6. It is cleared that a presence of large amount α-Al rich phase in Al-0.1Zn-0.5Sn-0.54Bi causes the strengths resistant deformation increases. Consequently, the increase of resistant of deformation means the increase of strength. The elastic constants of the metallic alloys which were fundamental physical properties especially for the mechanical properties such as strength, plastic deformation and fracture were reported previously using single crystals [17]. From Table 3, the free machining Al-0.1Zn-0.5Sn-0.54Bi and Al-0.1Zn-1Sn-1.08 alloys have higher values of Young’s modulus, shear modulus, bulk modulus and critical shear stress. The highest values of tensile strength (431.5 MPa), yield strength (393.9 MPa), fracture strength (431.6 Mpa), toughness (15.8 × 10⁶ J/m³) generated from Al-0.1Zn-0.5Sn-0.54Bi alloy to meet the needs of free machining aluminum alloy applications.

Having computed the elastic moduli, one can calculate the Debye temperature, which is an important fundamental parameter closely related to many physical properties such as elastic stiffness, specific heat and melting temperature. The expression for the Debye temperature θ_D in terms of the sound velocities for an isotropic body is given by the following Equation (5) [18]. Values of the Debye temperature of Al-Zn-Sn-Bi alloys are shown in Table 5.

θ D = h k B ( 3 4 π V a ) 1 / 3 V m (5)

where h is the plank’s constant, k_B is the Boltzmann constant, N_A is Avogadro’s number, V_a is the molar volume calculated from the effective molecular weight

and density (i.e. M ρ ) and V_m is the mean ultrasonic velocity defined by the relation

V m = 1 3 ( 1 V l 3 + 2 V t 3 ) − 1 / 3 (6)

where V_t and V_l are the transverse and longitudinal wave velocities in the solid defined by the relations

V t = G ρ and V l = 3 B + 4 G 3 ρ (7)

where B is bulk modulus and G is the shear modulus.

Spalling generally produces cracks normal to surface whenever the stresses arise from thermal contraction. Such cracking most commonly occurs in brittle materials as a result of thermal contractions. In certain cases either a phase transformation is an important causative factor. The spalling behavior of a material is related to several properties other than the thermal expansion coefficient, these properties include strength, modulus of elasticity, and thermal diffusivity [18]. It is convenient; to assign an index to spalling resistance of quenched ribbon, using the following equation

Spalling   Resistance   Index ( SRI ) = D t h T s α E (8)

where D_th is the thermal diffusivity, E is the young’s modulus, T_s is the tensile strength and α is the thermal expansion. Table 5 shows that the spalling resistance Index values are very small and negligible.

The creep behavior of the free machining Al-Zn-Sn-Bi alloys is presented in Figure 7, where the indentation diagonal length is plotted against the indentation time applying a constant load of 10 gf. It is observed that by increasing loading time, the indentation length increased. Hardness values listed in Table 6. It is cleared that Values of hardness increased by increasing Sn-Bi concentration due to Sn and Bi precipitates in Al-matrix. Figure 8 shows the variation of Vickers hardness number versus indentation time for Al-Zn-Sn-Bi alloys. Figure 7 generally shows a slow decrease in hardness with time. Figure 7 noted that the hardness of Al-Zn-Sn-Bi alloys at starting point 5 sec gradually increases with increasing tin and bismuth content reaches its maximum value of 418.4 MPa for Al-0.1Zn-1.5Sn-1.63Bi. The stress exponent can be obtained by the following equation and listed in Table 3 according to [19] and [20] :

n = [ ∂ ln d ˙ ∂ ln H v ] d (9)

This equation is used to determine the mechanisms of deformation, where the type of mechanism is determined by the value of the stress exponent, where d is the length of indentation diagonal, H_V is the number of Vickers hardness and d ˙ is the rate of variation in the diagonal length of indentation. A straight line would be obtained by plotting d ˙ against H_V on double logarithmic scale as shown in Figure 9, whose slope gives the stress exponent [21]. The values of stress exponent used to determine the mechanisms controlling the deformation process. The n values around 1 are related to diffusion creep, n values close to 2 related to grain boundary sliding and mechanisms attributed to dislocation movement such as slip creep are related to n values in the range of 5 - 7 which moves up to higher values than 8 when particles reinforcement take place [22]. The grain boundary sliding mechanism is shown in Al-0.1Zn-Sn-Bi alloys.

Indentation creep Resistance

Figure 10 plots the strain and indentation time relationships of Al-Zn-Sn-Bi alloys. We obtain typical indentation creep curve, as shown in Figure 10. The first stage records a fast increase of strain with time of the indentation starts from beginning up to 10 s of indentation time. The second stage represents a slow increase region in which the strain increases by lower rates for all alloys. In general, Al-0.1Zn-0.2Sn-0.22Bi alloy have lower creep resistance.