J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 5 ( 2 0 0 4 ) 2 1 1 ± 2 1 7
The effect of rapid solidi®cation on the structure, decomposition behavior, electrical and mechanical properties of the Sn±Cd binary alloys MUSTAFA KAMAL, ABU BAKR EL-BEDIWI Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt E-mail:
[email protected];
[email protected] TAREK EL-ASHRAM Physics Department, Faculty of Education, Suez Canal University, Port Said, Egypt A group of rapidly solidi®ed Sn±Cd alloys has been prepared by a melt-spinning technique. X-ray diffraction, microstructure, and differential thermal analysis have been carried out. Young's modulus and internal friction have been measured, and the temperature dependence of resistivity has been evaluated. The results show a modi®cation in both the microstructure and decomposition behavior. Also, an interesting connection between Young's modulus and the axial ratio
c /a of the unit cell of the b-Sn was found in which Young's modulus increases by increasing the axial ratio
c /a. It was found also that the internal friction increases on increasing the Cd concentration. # 2004 Kluwer Academic Publishers
1. Introduction
It has been established [1] that rapid solidi®cation can produce high-strength structural materials, tool and bearing materials; high-temperature materials; corrosion-resistant materials; catalytic and storage material and ®nally electrical and magnetic materials. These properties actually depend on the structural changes produced in each particular case. For example, high mechanical strength is attainable in microcrystalline materials as a result of re®ned microstructure combined with increased alloying [2]. Rapid solidi®cation has several effects on the structure, the most important effects are; formation of metastable crystalline phases, formation of amorphous phases, extension of solid-state solubilities, re®nement of the as-cast grain size, modi®cation of the segregation pattern and achievement of high point defect concentrations. The structure of rapidly solidi®ed Sn±Cd binary system was studied in [3] and the results were that a new metastable phase called g is formed at 83 K and decomposes to a Sn-rich solid solution and Cd phase at 293 K. Also the solubility limit extends to 3 at %. Cadmium alloyed with tin makes excellent solders with a tensile strength two to three times greater than the most common solders. Also, the addition of cadmium to tin lowers the melting point to about 449 K as shown in the equilibrium phase diagram [4] (Fig. 1). The most important features of the Sn±Cd binary system are; the eutectic composition is at 33 wt % Cd [5] and the eutectic temperature is at 449 K [5±7]. The transformation at 406 K, which is due to the eutectoid decomposition of a Sn-rich intermediate b-phase, formed peritectically at 0957±4522
# 2004 Kluwer Academic Publishers
496 K [6, 8] is shown in Fig. 1. The solubility of Cd in Sn is small, about 1.5 wt % Cd, and it is almost constant with temperature up to 496 K [9]. The solubility of Sn in Cd, is very small and increases with increasing temperature until it reaches the maximum value 0.25 wt % Sn [10] at 449 K, and then it decreases with increasing temperature. So, the main purpose of the present work is to study the effect of rapid solidi®cation by a melt-spinning technique on the structure and decomposition behavior of the Sn± Cd binary alloys and hence on the resulting electrical and mechanical properties.
2. Experimental procedures
The materials used in the present work are Sn and Cd fragments and the starting purity was better than 99.99%. A group of Sn±Cdx (x 0, 2, 5, 10, 20, 25, 33, and 40 in weight per cent) has been produced by a single aluminum roller (200 mm in diameter) melt-spinning technique. The process parameters, the ejection temperature and the velocity of the wheel were ®xed at 773 K and 30:4 m s 1 , respectively. X-ray diffraction analysis was done on a Shimadzu X-ray diffractometer (DX30), CuKa radiation with Ni-®lter (l 0.154056 nm). The microstructure analysis was carried out on a scanning electron microscope (SEM) of type JEOL, SM-T20 operates at 19 kV. Differential thermal analysis (DTA) was carried out on a Shimadzu DT-50 with a heating rate 10 K min 1 . The temperature dependence of resistivity was carried out by the double-bridge method. The details of the double bridge-method have been described elsewhere [11]. The variation of temperature 211
Figure 1 Equilibrium phase diagram of Sn±Cd system [4].
during the resistance±temperature investigation was determined using a step-down transformer connected to a constructed temperature control. The heating rate was kept constant during all the investigations at 5 K min 1 . Young's modulus and internal friction have been measured by the dynamical resonance method [12±14].
3. Results and discussion
Fig. 2 shows the X-ray diffraction patterns for asquenched melt spun Sn±Cdx (x 0, 2, 5, 10, 20, 33, and 40 wt %) alloys. The peaks observed for Sn±Cd2 (Fig. 2(a)) are for a-Sn solid solution of Cd in Sn and there is no peak for Cd phase, which indicates a complete solid solubility at that concentration. The Cd phase appears in all alloys from 5 to 40 wt % Cd (Fig. 2(b)±(g)) and both the number and the intensity of the peaks of Cd phase increases with increasing Cd concentration, which indicates more precipitation of Cd phase in the Sn matrix. The variations of the axial ratio
c/a, the volume of the unit cell
v in nm3 and the measured density
d in g cm 3 with the variation of Cd concentration are shown in Fig. 3. In Fig. 3(a), the axial ratio decreases to a minimum value (0.539) at 5 wt % Cd then increases to a maximum value (0.5464) at 33 wt % Cd. Also, the variation of the volume of the unit cell (Fig. 3(b)) and the measured density have an opposite trend, in agreement with the following equation [15]; X
A
dn 1:6602
1
where SA is the sum of the atomic weights in the unit cell, d is the measured density in g cm 3 and v is the volume of the unit cell in nm3 . The value of the measured density for pure Sn rapidly solidi®ed is 6:45 g cm 3 , when this value is substituted in Equation 1, it gives 415.195 for SA. By dividing the value of SA by the atomic weight of Sn (118.69), it gives the number of the atoms per unit cell 3.498, which must be 4 for b-Sn. Therefore, some of the atoms may be missing from a 212
Figure 2 X-ray diffraction patterns of as-quenched melt-spun Sn±Cdx (x 0, 2, 5, 10, 25, 33, and 40 in weight per cent) alloys.
certain fraction of those lattice sites that they would be expected to occupy. The concentration of vacancies was calculated to be 7/1000, i.e., seven sites in 1000 sites should be vacant. Fig. 4 shows the SEM at a magni®cation of 3500 for as-quenched melt-spun Sn±Cdx (x 2, 5, 10, 15, 20, 25, 33, and 40 in weight per cent) alloys. Fig. 4(a) shows for Sn±Cd2 that the Cd phase does not appear, in agreement with the results of X-ray diffraction analysis, and the average diameter of the grain was measured to be 9.1 mm. Fig. 4(b) shows the SEM of Sn±Cd5 alloy, it shows that the g phase (dark phase) precipitates at the grain boundaries and the average diameter of the grain was found to be 6.39 mm. Fig. 4(c) shows for Sn±Cd10 that more re®nement of the grains occurs and the average diameter of the grain was found to be 3.6 mm. Fig. 4(d) shows for Sn±Cd15 that the g phase appears as discrete particles (dark phase) with irregular shapes. Similar microstructures were observed for Sn±Cd20 , Sn±Cd25 , Sn±Cd33 , and Sn±Cd40 alloys as shown in Fig. 5(e)±(h) respectively. The most important variations are the increase in the number of particles per mm2 and the appreciable increase in the particle length, while the width is slightly increased. The average values of grain diameter, the particle width, and the particle length are given in Table I. Fig. 5 shows the DTA for as-quenched melt-spun Sn± Cdx (x 2, 5, 10, 20, 25, 33, and 40 in weight per cent). alloys. There is a phase transition for all alloys at about 406 K, which is due to the transformation to the Sn-rich intermediate b-phase. The double peak, which is
T A B L E I The grain diameter and particle size of rapidly solidi®ed Sn±Cd alloys
Figure 3 Variation of (a) c/a ratio, (b) the volume of the unit cell
v and (c) the measured density
d with Cd concentration.
observed for Sn±Cd2 (Fig. 5(a)) is due to the decomposition of the Sn-rich intermediate b-phase. The alloy of composition Sn±Cd25 gives a single endothermic peak for melting at 449 K (Fig. 5(e)). This single peak with no pasty range indicates that the Sn±Cd25 alloy melts at one temperature. This behavior is similar to the melting of a eutectic alloy, since heating a mixture of the eutectic composition leads to a single sharp endotherm
Alloy
Grain diameter (mm)
No. of Cd particles per mm2
Cd particle width (mm)
Cd particle length (mm)
Sn±Cd2 Sn±Cd5 Sn±Cd10 Sn±Cd15 Sn±Cd20 Sn±Cd25 Sn±Cd33 Sn±Cd40
9.1 6.39 4.8 Ð Ð Ð Ð Ð
Ð 12 160 18 170 19 600 20 050 21 150 24 750 26 071
Ð 1.6 1.3 1.83 1.88 1.92 1.95 2
Ð 1.8 1.5 3.66 4.82 5.74 6.52 7.5
[16]. However, the decomposition behavior of the eutectic alloy (Sn±Cd33 ) indicates that there is a pasty range as shown in Fig. 5(f ) in the small peak after the eutectic arrest. The variation of the solidus Ts and liquidus Tl temperatures with Cd concentration is shown in Fig. 6(a). The solidus temperature decreases by increasing the Cd concentration, from 496 K at 2 wt % Cd until it reaches the arrest at 449 K (the eutectic temperature). The liquidus temperature decreases by increasing the Cd concentration, from 503 K at 2 wt % Cd until it coincides with the solidus temperature at 25 wt % Cd, then it increases on increasing the Cd concentration. The variation in the decomposition behavior of rapidly solidi®ed Sn±Cd alloys shown in Fig. 6(a) and the equilibrium phase diagram of the Sn±Cd system shown in Fig. 1 may be due to the structural variations that are caused by rapid solidi®cation such as the extension of solid solubility, the high concentration of vacancies, and the modi®cation in the microstructure. Fig. 6(b) and (c) show the variation of enthalpy DH and entropy DS of fusion with Cd concentration, respectively. It is clear from Fig. 6(b) that, DH decreases with increasing Cd concentration until it reaches the minimum value 19 kJ kg 1 at 20 wt % Cd, then it increases with increasing Cd concentration. The same behavior was observed for the entropy of fusion DS as shown in Fig. 6(c), the minimum value of DS was found to be 40 J kg 1 K 1 for Sn±Cd20 . It is clear that the DS depends on the Cd concentration and does not obey Richard's rule. Fig. 7 shows the temperature dependence of resistivity for as-quenched melt-spun Sn±Cdx ribbons (x 2, 10, 20, 25, 33, and 40 in weight per cent) in the temperature range from 300 to 505 K. This shows that the linear behavior for metals from 300 K to about 406 K for all alloys, then a change in the slopes of the straight lines occurs for all alloys. This is due to the transformation to the Sn-rich intermediate b-phase as found from the results of the DTA. For the Sn±Cd2 alloy there is a decrease in the resistivity in temperature range from 470 to 495 K. This decrease is due to the decomposition of the Sn-rich intermediate b-phase. This is illustrated in the double peak in the DTA curve, which was obtained for this alloy (Fig. 5(a)). Finally, the rapid increase in the resistivity that is due to the melting of the alloys, in agreement with the results of the DTA. Fig. 8(a) shows the variation of the resistivity at 300 K with Cd concentration. It shows that a high value of resistivity
29:6610 8 O m was obtained for Sn±Cd2 alloy, then 213
Figure 4 Scanning electron micrograph (SEM) at a magni®cation of 3500 of as-quenched melt-spun Sn±Cdx (x 2, 5, 10, 20, 25, 33, and 40 in weight per cent) alloys.
214
Figure 5 Differential thermal analysis of as-quenched melt-spun Sn± Cdx (x 2, 5, 10, 20, 25, 33, and 40 in weight per cent) alloys.
the resistivity decreases by increasing the Cd concentration to a minimum value
15:5610 8 O m for Sn±Cd20 alloy. Finally, a slight increase in the resistivity occurs beyond 20 wt % Cd. The reason for the high resistivity of Sn±Cd2 is that this alloy is a single-phase alloy in which the Cd atoms are dissolved into the matrix. The differences in atomic size, valence, crystal structure, and electronegativity make the Cd dissolved atoms more effective scatterers of conduction electrons, thereby raising the resistivity. The observed decrease in the resistivity beyond 2 wt % Cd is due to the precipitation of Cd in the Sn matrix. Since the resistivity of an alloy that has two phases follows the mixture law given by: ralloy Va ra VCd rCd
2
where Va and VCd are the volume fractions of a-phase and Cd-phase, respectively, and ra and rCd are the resistivities of a-phase, and Cd-phase, respectively. The straight line in Fig. 8(b) represents the mixture law given by Equation 2. It is clear that the produced alloys show a positive deviation from the mixture law. This positive deviation is due to the high concentration of vacancies obtained by rapid solidi®cation. Fig. 9(a) shows the variation of the dynamic Young's modulus E with the variation of Cd concentration. At ®rst the value of E for pure tin rapidly solidi®ed was found to be 35 GPa compared with 45 GPa for conventional tin, i.e., the value of E decreases due to rapid solidi®cation. On the addition of cadmium the value of E decreases to a
Figure 6 (a) Variation of solidus
Ts and liquidus
Tl temperatures; (b) the enthalpy
DH and (c) the entropy of fusion
DS with Cd concentration.
minimum value 18 GPa at 5 wt % Cd. Then the value of E increases to 35 GPa at 10 wt % Cd and then decreases to 27 GPa at 20 wt % Cd. The maximum value of E is 52 GPa obtained at 33 wt % Cd and then it decreases to 20 GPa at 40 wt % Cd. This observed behavior of Young's modulus with Cd concentration can be explained in terms of the variation in the c/a ratio of the unit cell of Sn matrix. By plotting the variation of E with the c/a ratio we obtain a curve as shown in Fig. 9(b). It is evident from this curve that Young's modulus increases by increasing the c/a ratio. Also, the two alloys Sn±Cd2 and Sn±Cd40 , which have the same c/a ratio, 215
Figure 7 Temperature dependence of resistivity of as-quenched meltspun Sn±Cdx (x 2, 10, 20, 25, 33, and 40 in weight per cent) alloys.
Figure 8 Variation of the resistivity at 300 K with Cd concentration.
have the same Young's modulus
E. The maximum value of E, 52 GPa at 33 wt % Cd, corresponds to the maximum value of the c/a ratio, which is 0.5464 obtained at that concentration. The increase in the c/a ratio means the stretching of the unit cell along the c-axis, this modi®cation in the shape of the unit cell of the Sn matrix may result in an increase in the bond strength, which results in an increase in the Young's modulus. Fig. 9(c) shows the variation of the internal friction Q 1 with the variation of Cd concentration. It shows that the value of Q 1 for rapidly solidi®ed tin was found to be 0.042. Internal friction of Sn increases with cadmium content, reaching the maximum value at 33 wt % Cd. The increase in the internal friction is due to the motion of substitutional Cd atoms in Sn crystals, since the presence of vacancies facilitates the motion of the Cd atoms.
4. Conclusions
Several conclusions can be drawn from the above, the most important are; 1. An interesting connection exists between Young's modulus and the axial ratio
c/a of the unit cell of the bSn, since it has been found that, by increasing the axial ratio, Young's modulus increases and vice versa. 2. High concentration of vacancies can be obtained by 216
Figure 9 (a) Variation of the dynamic Young's modulus
E with Cd concentration; (b) variation of E with the c/a ratio of the b-Sn and (c) variation of the internal friction
Q 1 with Cd concentration.
rapid solidi®cation using the melt-spinning technique, which cannot normally obtained even at high temperatures. 3. The increase in Cd concentration leads to the re®nement of the average grain diameter and increases the Cd particle length greatly, to a value of 20 mm at 40 wt % Cd, while the Cd particle width is kept unchanged at a nearly constant value of about 1.8 mm. 4. The decomposition behavior of the Sn±Cd binary
system is affected by rapid solidi®cation using the meltspinning technique. This is indicated in the DTA heating curves obtained for the Sn±Cd25 and Sn±Cd33 alloys. 5. The electrical resistivity shows a positive deviation from the mixture law due to the high concentrations of vacancies, which are caused by rapid solidi®cation.
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Received 24 September 2002 and accepted 10 July 2003
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