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Abstract
Abstract
The research concerned with the study of superplasticity in (Pb-65.5Sn-3.4Zn) and (Pb-65Sn-1Zn) ternary alloys. These alloys chosen in the present study because of they are a candidate materials to be useful for various microelectronic connection in computer industry and other technical and economic indices of production. The effect of the presence of a third phase (Zn) on superplastic behavior examined. The study concerned also the microstructure dependence of the electrical and mechanical properties of these alloys. The effect of temperature and applied stresses on creep has been studied.
In the first section, the transient creep is studied at different applied stresses ranging from 3.123 to 6.83 MPa and at different test temperatures near the transformation temperatures in the temperature range from 353 to 413 K. The results of creep characteristics showed (one transition point at 383 K) two deformation regions (below and above 383 K). The transient creep is described by (?tr = ?tn), where ?tr and t are the transient creep strain and time, the parameters n and ? were calculated. The exponent n was found to have values ranging from 0.7 to 1.08 for Pb-65.5Sn-3.4Zn alloy, and from 0.664 to 1.07 for Pb-65Sn-1Zn alloy. The parameter ? was found to exhibit values ranging from 0.6x10-4 to 19x10-4 for Pb-65.5Sn-3.4 Zn alloy and from 2.5x10-4 to 21x10-4 for Pb-65Sn-1Zn alloy. Both alloys showed enhancement in the creep parameters n and ?, a small deviation about (20K) of the transition point towards low temperature is obtained in comparative to those of binary eutectic Pb-Sn alloy.
The parameter ? was related to steady state creep rate (?st) through the equation {?=?0(?st)*}. This equation was found to be valid with the exponent ? ranging from 0.76 to 0.96 and from 0.63 to 0.93 for Pb-65.5Sn-3.4Zn, and Pb-65Sn-1.0Zn alloys, respectively.
The activation enthalpies of the transient creep below and above the transformation temperature have been found to be 20.53 and 55 kJmol-1 for Pb-65.5Sn-3.4Zn, and 22.66 and 58.23 kJmol-1 for Pb-65Sn-1Zn alloy, in low and high temperature regions, respectively characterizing grain boundary sliding (GBS).
The stress dependence of n and ? shows a threshold value, which decreases as Zn content increase. The enhanced values of n and ? for the eutectic alloy is started at ? ?4.75 MPa while for the ternary alloy at ? ?5.2 MPa depending on the Zn content in both alloys. Below the threshold stresses, the two parameters increase slightly while above it a rapid increase were observed, this means that the ternary eutectic alloy is more superplastic than the ternary alloy.
The steady state creep of the alloys were investigated in the same temperature range and stresses previously mentioned. The results of creep characteristics show (one transition point at 383 K) two deformation regions (below and above 383 K). The strain rate sensitivity parameter m exhibits low value (0.31 ? 0.04) up to about 383 K for both compositions. After that, m, increases with temperature until 413 K reaching the value of 0.64 and 0.53 for the eutectic and ternary composition, respectively. The activation energies of steady state creep have been found to be 40 and 58 kJmol-1 for ternary eutectic and 41 and 59.7 kJmol-1 for ternary alloys in low and high temperature respectively, characterizing grain boundary diffusion. Also, the grain diameter has been determined, it has been found to be (3.05 ±0.15) and (3.35±0.25) ?m for Pb-65.5Sn-3.4Zn, and Pb-65Sn-1.0Zn alloys, respectively.
The steady state strain rate ?st at any particular temperature was found to increase monotonically by increasing the applied stresses and the transformation temperature. The steady state strain rate ?st was found to exhibit values ranging from 4.7x10-3 to 155.5x10-3 for Pb-65.5Sn-3.4 Zn ternary eutectic alloy, and from 2.8x10-3 to 140x10-3 for Pb-65Sn-1Zn ternary alloy. The average particle size and X-ray half-line width (?2?) were found to change with increasing working temperature. Also, the temperature dependence of the precise value of lattice constant a¬o of Pb-rich phase and co/ao of Zn and Sn-rich phases of the two alloys are studied.
Moreover, the change in electrical resistivity of Pb-65.5Sn-3.4Zn eutectic and Pb-65Sn-1.0Zn ternary alloys were measured at various ageing temperatures for various ageing times. Both alloys showed enhancement in the reaction rate in comparison to those of binary eutectic Pb-Sn alloy. It was found that the Pb-Sn-Zn ternary alloys show progressive reduction in the electrical resistivity (up to 65%) compared with the Pb-Sn binary eutectic composition.
The values of activation energies of formation and dissolution of precipitates are of 9.3 kJmol-1 and 12.1 kJmol-1 for Pb-65.5Sn-3.4Zn ternary eutectic alloys, and 9.9 kJmol-1 and 13.2 kJmol-1 for Pb-65 Sn-1.0Zn ternary alloys, in low and high temperature regions, respectively.
The variation in the microstructure of both alloys during creep processes as a function of working temperatures have been investigated using, Optical Microscope (OM), Scanning Electron Microscope (SEM), and Transmission Electron Microscope (TEM). Our results is agree for the first time with Ball and Hütchison model.
The hardness also, is studied for the two alloys. They have the same behavior. The addition of Zn content leads to reduce the hardness i.e. show superior superplastic behavior.
Differential Thermal Analysis (DTA) for Pb-65.5Sn-3.4Zn ternary eutectic and Pb-65Sn-1.0Zn ternary alloys were measured. It is clear that, as Zn-content increases up to 3.4% Tm decreases from 455 K for Pb-65Sn-1.0Zn ternary alloys to 477.9 to Pb-65.5Sn-3.4Zn ternary eutectic i.e. the higher the Zn content in the alloy, the lower the melting temperature is, and the larger the endothermal effect, too.