Abstract:
Initiative of development and characterization of Pb free high temperature solders was taken.
Two systems such as Zn-xMo (x= 0.4, 0.6 and 0.8 wt% Mo) and Zn-xCr (x= 0.2, 0.4 and 0.6
wt% Cr) were chosen for this study. Samples of the selected compositions were fabricated
through casting in a permanent metal mold. Their microstructural, thermal, mechanical and
electrical characteristics were determined using several analytical techniques such as
Scanning Electron Microscope (SEM), Differential Thermal Analyzer (DTA), X-Ray
Diffractometer (XRD), Thermomechanical Analyzer (TMA), Universal Testing Machine
(UTM), Hardness tester, Conductivity Meter etc.
Experimental findings revealed that with the addition of Mo and Cr particles grain refinement
occurred in both Zn-xMo and Zn-xCr systems which was evident in the scanning electron
micrographic images. Energy-dispersive X-ray spectroscopy (EDS) spots in several zones of
the microstructure confirmed the presence of Mo and Cr particles throughout the respective
structures. XRD analysis showed a very little or no shift in the peak pattern compared to
standard Zn pattern indicating no new phase formation. However, crystallite size calculation
using Scherrer equation showed that, average crystallite size decreases with increasing Mo
content and it ranged from around 34.0-29.4 nm in case of Zn-xMo system, while the average
crystallite size of Zn-xCr system ranged between 29.7 and 28.8 nm. Brinell hardness showed
an incremental trend with increasing Mo content. Similar results were obtained in Zn-xCr
system. Brinell hardness number of Zn-xCr increased from 33.98 in Zn-0.2 Cr to 42.74 in
case of Zn-0.6 Cr alloy. Similarly, hardness increased from 29.71 to 41.28 in case of Zn-0.4
Mo to Zn-0.8 Mo respectively. Tensile strength of unalloyed zinc increased from 31.09 MPa
to 33.04 MPa when alloyed by 0.8% Mo. In a similar manner, strength of Zn-Cr solders
increased from 40.53 to 47.07 MPa due to an increase in 0.4% Cr than that of the former.
Fractographic analysis of the tensile fracture surfaces with SEM indicated mixed (neither
completely ductile nor completely brittle) fracture mode of both Zn-xMo and Zn-xCr
systems. Coefficient of thermal expansion (CTE) of Zn-Cr system was found to vary from
4.5-5×10-5 (1/ºC) while in Zn-Mo system it was 4.14-2.64×10-5 (1/ºC). Electrical conductivity
of both Zn-xMo and Zn-xCr decreased with increasing Mo and Cr contents respectively. The
relative conductivity of Zn-xMo and Zn-xCr varied from 27.35 to 26.55 and 25.3 to 22.7
%IACS (International Annealed Copper Standard) respectively in the selected composition
range. DTA results suggested that onset of melting started at temperature below the melting
temperature of pure Zn. It was found that, the onset temperature of melting of all three
compositions of the Zn-xMo system was around 416ºC whereas, the finishing temperature of
melting was around 423ºC indicating a melting range of around 7ºC. On the other hand, DTA
graphs exhibited that, the onset of melting of the Zn-xCr alloys started at around 412ºC and it
finished near about at 418ºC showing a melting/solidification range of about 6ºC.
From the experimental findings it can be said that Zn-xMo as well as Zn-xCr systems might
be promising candidate for high temperature solders in replacement of Pb based solders.
