Received Date: November 24, 2012; Accepted Date: December 17, 2012; Published Date: December 23, 2012
Citation: Markovic BR, Živkovic DT, Manasijevic DM, Talijan NM, Sokic MD, et al. (2012) Phase Equilibria Calculation and Investigation of Hardness and Electrical Conductivity for Alloys in Selected Sections of Bi-Cu-Ni System. J Powder Metall Min 2:104. doi: 10.4172/2168-9806.1000104
Copyright: © 2012 Markovic BR, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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The results of phase equilibria calculation and the experimental investigation of alloys in selected sections of Bi-Cu-Ni system with bismuth constant molar content of 0.6; 0.7; 0.8; and 0.9 are presented in this paper. Thermodynamic calculation was done according to the CALPHAD method using PANDAT software, while chosen alloys in the selected sections were experimentally investigated using hardness and electrical conductivity measurements.
Lead-free solders; Bi-Cu-Ni system; Phase equilibria calculation; Alloys characterization
Alloys of Pb-Sn system are commonly used solder materials in electrical and electronics industry due to their low cost and unique combination of physical, chemical and mechanical properties, as well as their reliability. According to recent EU legislation - DIRECTIVE WEEE (Waste from Electrical and Electronic Equipment) and DIRECTIVE 2002/95/EC (Restriction of the use of certain Hazardous Substances (RoHS) in electrical and electronic equipment), the use of lead containing solders is prohibited in many industries from July 1st, 2006. Therefore, a great effort has been made on the development of new Pb-free soldering and brazing materials [1-6].
The Bi-Cu-Ni system belongs to the group of potential candidates for lead-free solder materials in the frame of Cu-Ni-based lead-free systems for high temperature application. This system represents the environmental friendly alternative to Sn-based solders which has been examined recently in the frame of COST MP0602 “HISOLD” project  and has been described in two recent references [8-10]. Gao et al. , investigated the phase equilibria of the Bi–Cu–Ni system at 300, 400, and 500°C using metallography and electron probe microanalysis on equilibrated alloys and diffusion couples, while Markovic et al. , performed a thermodynamic modeling of the Bi–Cu–Ni system using the CALPHAD method and the investigation of different alloys characteristics in .
In order to contribute to the knowledge on the Bi–Cu–Ni system and a better understanding of the processes occurring during soldering and during operation of the soldered devices, the results of phase equilibria calculation and the experimental investigation of alloys in the selected sections of the Bi-Cu-Ni system are presented in this paper.
The investigated samples taken from four sections of the Bi-Cu- Ni system with constant bismuth mole fraction of 0.6; 0.7; 0.8; and 0.9 were prepared in an induction furnace using metals of p.a. purity in protected atmosphere and homogenized at 800°C for 2 h under an argon atmosphere and then furnace cooled to room temperature using cooling rate of 2°C /min.
The composition of the investigated samples is presented in table 1.
|F section - with constant xBi=0.6|
|G section - with constant xBi=0.7|
|H section - with constant xBi=0.8|
|J section - with constant xBi=0.9|
Table 1: Composition of the investigated samples
The hardness and electrical conductivity measurements of the samples were investigated. The electrical conductivity of investigated samples was measured using the standard apparatus SIGMATEST 2.069 (Foerster) eddy current instrument for measurements of electrical conductivity of non-ferromagnetic metals, based on complex impedance of the measuring probe with diameter of 8mm.
The hardness measurements were done using standard procedure according to Vickers.
The phase equilibria in chosen Bi-Cu-Ni sections were calculated according to the well known CALPHAD methodology , using PANDAT thermodynamic software Vs. 8.0 [12,13]. The calculation was based on known thermodynamic data for the pure elements, socalled SGTE (Scientific Group Thermodata Europe) values , and optimized binary and ternary thermodynamic parameters given in .
For experimental confirmation of the calculation results, the thermal analysis data of the samples in the selected sections of the Bi- Cu-Ni system, obtained for heating measurements, including liquidus temperatures and other phase transition temperatures, were used from our previous work .
The calculated phase diagrams of the investigated sections are presented in figure 1, together with experimentally determined DTA/ DSC points .
As can be noticed, there is a reasonable agreement between the thermodynamic prediction and the experiment, while the only slight deviation between calculation and experiments can be seen at liquidus temperature lines. More, there are two invariant reactions in this system - the ternary quasi-peritectic reaction and the ternary eutectic reaction :
(U1): LIQUID+BINI ↔ FCC_A1+Bi3Ni (at 430°C)
(E1): LIQUID ↔ FCC_A1+BI3NI+RHOMBO_A7 (at 269°C)
The obtained calculation results are also in agreement with the experimental structural analysis from , confirming that the Bi–Cu– Ni system consists of five phases (liquid, RHOMBO_A7 (Bi), FCC_A1 (Cu,Ni), BiNi and Bi3Ni), which is in accordance to its crystallographic data .
In the second part of the paper, the results of experimental investigation of some Bi-Cu-Ni alloys are given. So, the results obtained by hardness measurements are shown in table 2 and figure 2. There were three sets of hardness measurements at the cross-section of each sample. As can be seen, the hardness decreases rapidly with bismuth concentration increase for selected alloys in the investigated sections. Noticed behaviour can be explained as the consequence of dominant phases present in the alloys structure, mostly Bi-based phases - BiNi, Bi3Ni and RHOMBO_A7, due to the fact that bismuth posess the lowest hardness value among the three constitutive metals in ternary system Bi-Cu-Ni.
|Sample||Mean value (HV5)||Standard deviation|
Table 2: The results of hardness measurements.
The results of electrical conductivity measurements are given in table 3 (three measuring series), where the electrical conductivity dependence on the composition is showed in figure 3. It can be noticed that increase of bismuth content influences the decrease in electrical conductivity for all samples in the investigated section, which is in accordance with the electrical conductivity of pure bismuth (≈ 0.867 MS/m [16,17]). The electrical conductivity decrease with bismuth content increase is the consequence of prevailing phases presence in the structure, i.e. the bismuth based phases BiNi, Bi3Ni and RHOMBO_A7 influencing the electrical conductivity decrease.
|Sample||Mean value (MS/m)||Standard deviation|
* The elecrical conductivity values for H2, J1 and J3 samples were reported in our previous paper 
Table 3: The results of electrical conductivity measurements.
The selected sections in the Bi-Cu-Ni system with constant bismuth molar content of 60; 70; 80; and 90 at% have been investigated in this paper. The phase diagrams of these sections were calculated by CALPHAD method using PANDAT thermodynamic software, while some alloys in the mentioned sections were subjected to hardness and electrical conductivity measurements. The obtained phase diagrams were proven by literature experimental results and the main invariant reactions and phases were confirmed. The measured values of hardness and electrical conductivity decreased with bismuth concentration increase for all the investigated alloys. Here presented results are further contribution to the better knowledge of phase equilibria, structural, mechanical and electrical properties of Bi-Cu-Ni alloys as a new potential lead-free solder material for the high temperature application.
This work presents the part of realization in the frame of projects N°172037 and N°34023, supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, and in the frame of European COST MP0602 Action.
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