Where (?) is the Bragg angle, K is the Scherrer constant (0.9), (?) is the peak width at half the maximum intensity of peak (FWHM) in radian, (?) is the Bragg angle and ? is the X-ray wavelength (?Cu = 0.15406 nm). A straight line was obtained with the slope of (2?) and the intercept as (0.9?/D) when ?cos? was plotted against Sin? and the results of linear fitting were listed in Table 4. The D of ?-Sn phase was calculated 65.7 and 115.71 nm for the plain and composite alloys, respectively.
Microstructural characteristics of the plain and composite solders are discussed in terms of; (i) grain morphology, homogenous distribution and their size (ii) presence of the second phase, and (iii) percentage of eutectic zone. Fig. 3(a-b) is a SEM images exhibiting the as-solidified microstructure (cooling rate = 5 oC/s) of the plain and composite solder with different magnification. It revealed a eutectic structure involving of a dendrite ?-Sn solid solution, an irregular polygon of Zn8Cu5 as well as small round particle and/or needle-like of ?-Zn and a eutectic phase. The eutectic zone of SZC-505 solder contained very high volume ratio of the dendrite ??Sn phase, and its average grain size was relatively small (6.8 ?m). The eutectic phase had both ?-Sn and the needle-like of ?-Zn phase. The needle-like of ?-Zn crystals were 4.2 ?m long and 0.89 ?m wide. The average spacing (?) between IMC particles was about 2.78 ?m. The eutectic area was about 11.8 ± 1.5%. However, large black polygon and /or scallop morphology phase of Zn8Cu5 were observed in both solders.
Addition of 0.5 wt % of nano-Al2O3 particles to the SZC-505 hypoeutectic solder was observed to change the as-solidified SEM microstructure, as shown in Fig. 4a–b. Generally, the constituent element of IMCs composition interior solder matrix and the eutectic zone were detected using energy dispersion X-ray spectroscopy (EDS) analyses. Fig. 5a-c represents EDS analysis of eutectic region and IMCs scattered in solder matrix. The manifestations of EDS analysis revealed that, the eutectic zones were contained Al, O, Cu, Sn and Zn. This evidence for demonstrating the Al2O3 nanoparticles were incorporated into the solder matrix and Zn8Cu5 and Cu6Sn5 IMC were precipitated phases. In order to precisely determine the influence of nano-structured nano-Al2O3 particles on the morphology of Zn8Cu5, the size of the IMCS and the spacing between them in the synthesized solder samples were detected in 20 randomly chosen sites and the average values estimated based on these data. These data are summarized in Table 3. Moreover, Fig. 6 shows comparison of Zn8Cu5 average sizes and the spacing sizes between Zn8Cu5 in the plain and composite solder. More results revealed significant improvements in the refinement of dendrite ?-Sn grains. Additionally, the eutectic phase further increased and the fine spherical of ?-Zn was observed and there were no large block-like Zn8Cu5 or Cu6Sn5 particles in the structure. The refinement of ?-Sn grains of composite solder can be assigned to the pinning effect of Al2O3 nanoparticles on grain boundaries and resulting in a limited grain growth . Generally, Addition of nanoparticles had great impact on the needles morphology of ?-Zn phase and transformed into fine spheroid that scattered throughout Sn matrix. On another hand, the existence of Al2O3 particles may be contribute to replacement the Sn atoms by Zn atoms and preferentially formed Cu5Zn8 IMCs rather than Cu6Sn5 interior the matrix. This can interpret based on thermodynamically calculation, because Gibbs free of Cu5Zn8 IMC is smaller than those of Cu6Sn5 IMC as reported by —- . Therefore, Cu5Zn8 IMC can serve as additional nucleation sites to activate the nucleation process during solidification. As a result, nearly all the large Cu6Sn5 precipitates vanished due to insufficient time to grow up, thereby benefiting the mechanical properties of solder. It is worth to mention that, some authors correlated the microstructure evolution with the adsorption theory of surface-active materials .Tsao et al. studied the microstructural evolution of near equilibrium solidified Sn-Ag-Cu solders doped with nano Al2O3 particles . They interpreted the refinement of IMCs based on the surface activity of nanoparticles that have high adsorption coefficient and/or smaller surface tension energy. However, addition of nano-metric Al2O3 particles could decrease the surface energy and/or decrease the growth of IMCs size . Because, IMCS size (?m) are larger than size of Al2O3 nanoparticles. Therefore, the merged of active surface Al2O3 nano-metric particles delay the ripening rate, and the refinement effect is accomplished. Moreover, based on the digital photographic image analysis program and Zener equation, (Eq.1) the pinning stress was evaluated which restricted the growth of ?-Sn grains.
where f is the volume friction of nano-metric Al2O3 particles, d is the diameter of reinforced particles and ?GB is the surface energy per unit area of ?-Sn grain boundary (?GB=0.425 J/m2) . The pinning stress ?p act on grain boundary of ?-Sn phase for SSC-505 and SZC-Al2O3 solder was estimated as 20.2 and 18.5 kPa, respectively. Clearly, the pinning stress of composite solder is higher than plain solder alloy due to finer IMCs and ?-Sn grain.
3.4 Stress–Strain Tensile Characteristics
Fig. 7 shows the tensile stress–strain (???) graphs of SZC-505 and SZC-Al2O3 solders strained with constant strain rate of 3.5×10-3 s-1 at tested temperature of 30 oC. Both ??? curves presented nearly plateau shaped with approximately stable flow stress (?flow) at 20.8 and 35.6 MPa for SZC-505 and SZC-Al2O3 solders, respectively. It was observed the both curves intensive dependent on alloy composition. Furthermore, ?flow of SZC-Al2O3 composite solder was much higher than SZC-505 solder by ~71.2%. However, the nearly stable flow stress was presented the resultant of opposite effects of dynamic recovery and work hardening during plastic deformation. Additionally, the motion of dislocations was restricted as a reason of embedding of Al2O3 nanoparticles as well as the uniformly dispersion of finer IMCs in solder matrix. Hence, the freedom of dislocations (i.e its ability to pass through climb and cross slip planes) in composite solder was further less than plain solder which reflect the increase in ?flow .
The main values of Young modulus (YM), ultimate tensile strength ?UTS), yield stress ?YS), fracture stress ?FS) and ductility (Elongation %) of the plain and composite solders were listed in Table 4. Incorporation of 0.5 wt% nano-metric Al2O3 particles was found to have a significant impact on the tensile parameters. Notably increases were recorded in YM by ~39%, UTS by ~76.2% and in YS by ~56 % with reduction in the ductility (~ 32 %) of SZC-Al2O3 composite solder.
Enhancement in the tensile parameters was accomplished as reason of adding nano-metric Al2O3 particles that act as reinforced substance. They are scattered uniformly and homogeneously distributed in ?-Sn phase which help to hump the grain boundary and provide high potential barrier that impeding the motion of dislocation. Therefore, they were strengthening the solder matrix and implement pile-ups of intensive dislocation at grain boundaries . Moreover, addition of nano-metric Al2O3 particles pinning grain boundaries and limited their growth as well as formed the new smaller sub-grains (see Fig. x). Simultaneously, they might be increased the friction force between sub-grain that help to prevent the sliding of ?-Sn grains. Since, the ductility of solder was decreased after Al2O3 addition.