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Abstract
The transition and melting temperatures for two compositions Pb-10wt.%Sn and Pb-10Sn-1.5wt.%Zn were identified by Differential Thermal Analysis (DTA). The transition temperature and melting temperature were found to be 423 K and 583 K for the first composition and 403 K and 555 K for the second composition. This indicates that, in ternary composition, the transition and melting temperatures reduce by 20 K and 28 K, respectively, compared with binary composition. In addition to this changing, the addition of 1.5wt.%Zn also appeared to refine and stabilize the grain size of the alloy.
The Mechanical and electrical properties of Pb-10wt.%Sn and Pb-10Sn-1.5wt.%Zn alloys were studied. In addition, the relation between these properties and microstructures of these alloys were investigated. In the mechanical properties, this thesis presents an investigation of stress-strain characterization of Pb-10wt.%Sn and Pb-10Sn-1.5wt.%Zn alloys at the temperature range from 373 to 443K and different strain rate range from 5.0x10-5 to 2.5x10-3s-1. The relationships between flow stress, strain rate, temperature, and grain size were evaluated. The results showed two deformation modes: (i) continuous work softening and fracture following a sharp stress peak in Pb-10wt.%Sn, (ii) work hardening with steady state flow and fracture just after yielding in Pb-10Sn-1.5wt.%Zn. In the work softening type, relatively coarse grains from 30 to 60?m were produced by Dynamic Recrystallization (DRX). In the work hardening with steady state flow type, fine grains less than 10?m were developed by effective alloying with zinc. The flow stress and strain rate sensitivity parameter, m =d(log?)/d(log ), are strong functions of strain rate, temperature and Zn-content. This study pointed that the log?- log curve is s-shaped with a region of maximum m (m=0.5) at an intermediate strain rate, which increases with increasing temperature. In this region, the optimum superplasticity was observed. Metallographic observations have been confirmed that grain boundary sliding (GBS) accommodated by dislocation motion is the important mode of deformation.
The transient creep was investigated for Pb-10wt.%Sn and Pb-10Sn-1.5wt.%Zn alloys over a temperature range 373-443 K at stresses ranging from 8.33 to 14.58 MPa. The transient creep data were fitted by the power-law equation: ?tr=?tn, where ? is the creep coefficient and n is the creep exponent. Both alloys showed enhancement in the creep parameters ? and n during transformation. In the low of transition point, n increases in a low rate while increases in a high rate at above of transition point. We found that n ranges from 0.3 to 0.43 for the first composition and from 0.28 to 0.42 for the second composition. Also, the parameter ? changed with working temperature as the same behaviour. We showed that ? ranges from 0.11x10-3 to 1.154x10-3 for the first composition and from 0.362x10-3 to 9.61x10-3 for the second composition.
Also, under the previously mentioned, the steady state creep of two alloys was investigated. The results of creep characteristics showed two deformation regions (below and above 423 K and 403 K for binary and ternary alloys, respectively). The strain rate sensitivity parameter (m) has been found to increase by increasing the working temperature up to about 0.48 and 0.57 for the binary and ternary alloys, respectively. The activation energies of steady state creep of the binary and ternary alloys have been found to be 54.3 - 63 and 51.5 - 62 kJ mol-1, respectively. Creep data, Metallographic observations and X-ray analysis confirmed that grain boundary sliding accommodated by diffusion is the rate controlling mechanism at high temperatures where as, dislocation motion at low temperatures. It was established that Pb-10Sn-1.5wt.%Zn alloy shows superior superplastic behaviour compared with Pb-10wt.%Sn alloy as a result of its lower melting temperature (Tm), finer grain size, and the multiplicity of types of interphase boundaries. It was also established experimentally that the grain boundary sliding (GBS) takes place by sliding of group of grains on the plane of sliding for the first time.
Moreover, the change of electrical resistivity of Pb-10wt.%Sn and Pb-10Sn-1.5wt.%Zn alloys was measured at different ageing temperatures and times. It was found that, generally, the electrical resistivity (?) for Pb-10Sn-1.5wt.%Zn alloy is less than that for Pb-10wt.%Sn alloy. It was found that the electrical resistivity ranges from 3.1x10-5 ?.cm to 4.2x10-5 ?.cm for the first composition and from 2.9x10-5 ?.cm to 3.75x10-5 ?.cm for the second composition at all temperature range. This difference is attributed to the presence of the third phase of zinc in the ternary alloy. The activation energies were found to be from 29 to 31.2 kJ mol-1 for Pb-10wt.%Sn and from 27 to 28.5 kJ mol-1 for Pb-10Sn-1.5wt.%Zn alloy