Characteristics Study on Bi-Pb Based Alloys Quenched from Melt

Transcription

1 J. Mater. Sci. Technol., Vol.25 No.4, Characteristics Study on Bi-Pb Based Alloys Quenched from Melt Rizk Mostafa Shalaby Metal Physics Laboratory, Physics Department, Faculty of Science, Mansoura University, Mansoura, P.O. Box 35516, Egypt [Manuscript received October 20, 2008, in revised form December 1, 2008] Three different bismuth-lead systems namely, Wood s alloy (Bi 50 Pb 25 Sn 12.5 Cd 12.5 ), Newton s alloy (Bi 50 Pb 31.2 Sn 18.8 ) and Rose s alloy (Bi 50 Pb 28 Sn 22 ), with one used as fusible alloys were quenched from melt by melt spinning technique. Thermal analysis, structure and mechanical properties of all alloys have been studied and analyzed. From X-ray diffraction analysis, an intermetallic compound phase, designated Pb 7 Bi 3 is detected. The formation of an intermetallic compound phase causes a pronounced increase in the electrical resistivity. The Wood s alloy containing-cadmium exhibits mechanical properties superior to both the Newton s and Rose s alloys. The presence of cadmium in Wood s alloy decreases its melting point. Wood s alloy has better properties, which make it useful in various applications such as in protection shields for radiotherapy, locking of mechanical devices and welding at low temperature. KEY WORDS: Thermal properties; Electrical resistivity; Mechanical properties; Radiotherapy and fusible alloys 1. Introduction Fusible alloys are low melting temperature compositions containing bismuth alloys or fusible alloys, which were introduced to radiation thereby in 1970 to shape and block radiation fields [1]. Fusible alloys were employed in electric safety fuses [2 4], as especially low-melting solders and as a material for the construction of X-ray and electron radiotherapy shielding blocks [5,6]. This group usually includes alloys, whose melting temperature is relatively below the melting point of tin (230 C). They are a mixture of bismuth, lead, tin and cadmium to form a polyphase multicomponent eutectic. It has been established [7] that rapid solidification can produce high strength structural materials, tool and bearing materials, high temperature materials, corrosion-resistant materials, catalytic and storage material and finally 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 refined microstructure combined with increased alloying [8]. Rapid solidification 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, refinement of the as-cast grain size, modification of the segregation pattern and achievement of high point defect concentrations. When bismuth is alloyed with other metals such as lead, tin or cadmium, it forms low-melting alloys, which are extensively used for safety devices in fire detection and extinguishing systems [9,10]. Structure, hardness, internal friction, elastic and electrical properties of quenched Bi-Pb- Sn-Cd fusible alloys have been investigated [11 13]. The aim of the present work is to characterize study on bismuth-lead based alloys quenched from melt by melt spinning technique, and to compare structural, thermal, mechanical and electrical pro- Prof.; Tel.: ; address: rizk1969@yahoo.co.uk. perties with Wood s (Bi 50 Pb 25 Sn 12.5 Cd 12.2 ), Newton s (Bi 50 Pb 31.2 Sn 18.8 ) and Rose s (Bi 50 Pb 28 Sn 22 ) alloys. 2. Experimental Wood s, Newton s and Rose s alloys were used, which were melted in an electric furnace using bismuth, lead, tin and cadmium with purities of 99.99%. Rapidly solidified ribbons about µm in thickness and about 5 mm in width were prepared using a single roller method in air with surface velocity 31 m/s. The X-ray diffraction (XRD) was carried out of using CuKα radiation at room temperature. The mechanical properties were examined in an air atmosphere by a dynamic resonance method. The hardness ribbon samples were measured on a Vickers microhardness tester (Model FM-7-Tech Group, Japan). In situ resistivity measurements were carried out by so-called double bridge method. The heating rate was kept constant during all the investigations at 5 K/s. Differential thermal analysis (DTA) was carried out on a Shimazu DT-50 with a heating rate of 10 K/min in central Lab., Menofia University, Faculty of Science, Egypt. 3. Results and Discussion 3.1 XRD analysis Figure 1 shows the XRD patterns for asquenched melt-spun ribbons of compositions Wood s (Bi 50 Pb 25 Sn 12.5 Cd 12.5 ), Newton s (Bi 50 Pb 31.2 Sn 18.8 ) and Rose s (Bi 50 Pb 28 Sn 22 ) alloys (in wt pct). The structure of Bi 50 Pb 25 Sn 12.5 Cd 12.5 alloy (Fig. 1(a)) contains a number of peaks and intensity as Pb 7 Bi 3 increases, which indicates the formation of more of it, at 2θ=29.35, 30.35, 33.27, 43.20, 52.19, 56.05, 56.46, 63.39, and and crystalline phases α-bi, β-sn, meaning a complete solubility of Pb in the Bi matrix. For Bi 50 Pb 31.2 Sn 18.8 alloy (Fig. 1(b)), compound Pb 7 Bi 3 is formed as indicated by the peaks at 2θ=29.41, 30.79, 33.31, 43.15, 52.22, 56.48, 63.41, and For Bi 50 Pb 28 Sn 22

2 450 J. Mater. Sci. Technol., Vol.25 No.4, 2009 Fig. 1 XRD patterns for melt spun-ribbons of: (a) Wood s alloy, (b) Newton s alloy, (c) Rose s alloy alloy (Fig. 1(c)), the compound Pb 7 Bi 3 at 2θ=29.38, 30.53, 30.80, 33.27, 43.10, 52.15, 56.42, 63.32, and The formation of the intermetallic compound Pb 7 Bi 3 was not normally obtained under equilibrium conditions. The crystalline and intermetallic compound phases were found after all prepared alloys were rapidly solidified using melt spinning technique (Table 1). The average values of crystallite size of all alloys are given in Table 2. It is Table 1(a) X-ray analysis for quenched Wood s alloy Bi Bi Pb 7 Bi Pb 7 Bi Sn Pb 7Bi Cd Bi Cd Bi Pb 7 Bi Sn Bi Bi Pb 7 Bi Pb 7 Bi Pb 7Bi Bi Bi Pb 7 Bi Sn Bi Bi Sn Sn Bi Bi Bi Pb 7Bi Pb 7 Bi Table 1(b) X-ray analysis for quenched Newton s alloy Bi Bi Pb 7 Bi Pb 7 Bi Sn Pb 7Bi Bi Bi Pb 7Bi Sn Bi Sn Bi Bi Pb 7 Bi Bi Bi Pb 7Bi Bi Bi Pb 7Bi Sn Bi Pb 7 Bi Sn Bi Bi Sn Table 1(c) X-ray analysis for quenched Rose s alloy Bi Bi Pb 7Bi Pb 7 Bi Sn Sn Sn Pb 7 Bi Bi Bi Pb 7Bi Sn Sn Bi Bi Pb 7 Bi Sn Pb 7Bi Bi Bi Bi Pb 7Bi Sn Bi Pb 7 Bi Bi Sn Bi Bi Sn Sn Bi

3 J. Mater. Sci. Technol., Vol.25 No.4, Table 2 Phase designation, crystal system and crystallite size after rapid solidification using melt spinning technique Phase designation Crystal system Crystal size/nm Wood s alloy α-bi Rhombhedral hexagonal cell 28 Pb 7Bi β-sn Tetragonal 41.8 Cd 40.3 Newton s alloy α-bi Rhombhedral hexagonal cell 93.9 Pb 7Bi β-sn Tetragonal 107 Rose s alloy α-bi Rhombhedral hexagonal cell 50 Pb 7Bi β-sn Tetragonal 80 It is clear that the electrical resistivity of Wood s alloy is higher than that of Newton s and Rose s alloys. This is due to the presence of transition metal Cd in Wood s alloy. Such an electrical resistivity is considered to have been attained by the combination of several effects; the uniform distribution of fine precipitates; the introduction of internal defects such as dislocations and grain boundaries; and the formation of the intermetallic compound phase Pb 7 Bi 3. Table 3 lists the electrical resistivity, electrical conductivity for Wood s, Newton s and Rose s alloys obtained by the melt spinning technique. Fig. 2 Electrical resistivity vs temperature for: (a) Wood s alloy (Bi 50Pb 25Sn 12.5Cd 12.5), (b) Newton s alloy (Bi 50Pb 31.2Sn 18.8), (c) Rose s alloy (Bi50Pb28Sn22) rapidly solidified from melt clear that the crystallite size of Wood s alloy has the lowest value than that of Newton s and Rose s alloys. The role of rapid solidification and Cd addition is clear. 3.2 Electrical properties The electrical resistivity of Wood s (Bi 50 Pb 25 - Sn 12.5 Cd 12.5 ), Newton s (Bi 50 Pb 31.2 Sn 18.8 ) and Rose s (Bi 50 Pb 28 Sn 22 ) fusible alloys on heating to 390 K is shown in Fig. 2. The resistivity is linear with temperature for all the melt spun alloys as seen in Fig. 2. It is shown that Wood s alloy has a high electrical resistivity when compared with the Rose s and Newton s alloys. That is because Cd causes a change in the internal structures, such as a change of the atomic positions in Bi, Sn and Pb 7 Sn 3 as seen in the X-ray analysis (change in peak intensity) and also the function of a Cd phase in the matrix. Also, the differences in atomic size, valence, crystal structure, and electronegativity make the Cd atoms more effective scatters of conduction electrons, thereby raising the resistivity. An experimental fact, known for a long time is that the contributions to resistivity ρ are from several sources, such as types of phonon scattering by impurities or by mechanical deformation of additive. This rule, known as Mathiessen s rule may be written ρ total = ρ temperature + ρ impurities + ρ deformation Table 3 Electrical resistivity ρ, and electrical conductivity σ of quenched alloys ρ/10 8 Ω m σ/ Ω 1 m Wood s alloy 81.2± Newton s alloy 75.0± Rose s alloy 65.0± Mechanical properties The average values of Vickers hardness (H v ) and all mechanical properties of Wood s, Newton s and Rose s alloys are listed in Table 4. It is seen that the Vickers hardness and all mechanical properties of Wood s alloy are higher than those of Newton s and Rose s alloys. From these results, it is obvious that Wood s (Bi 50 Pb 25 Sn 12.5 Cd 12.5 ) alloy has a higher elastic modulus and minimum internal friction compared with Newton s and Rose s alloys. This is because Cd causes a change in the internal structure, such as of atomic positions as seen in the XRD patterns and their analysis. In addition, a Cd phase is formed in the matrix (Cd improves the mechanical properties), because interatomic bonding in the matrix increases the elastic modulus and presence of intermetallic compound Pb 7 Sn 3 phase. The maximum shear stress τ max, which is produced by a locally applied pressure, occurs on the central axis below the pressurized region [14,15]. This will be given by the following equation: τ max = 1/2H v {1/2(1 2ν) + 2/9(1 + ν)[2(1 + ν) 1/2 } where ν is Poisson s ratio and its value is 0.4, this gives τ max = 0.31H v The Young s modulus (E), shear modulus (G), bulk modulus (B), Hardness (H v ), maximum shear stress (τ m ), and internal friction (Q 1 ) of all prepared alloys

4 452 J. Mater. Sci. Technol., Vol.25 No.4, 2009 Table 4 Mechanical properties of all melt spun alloys Q 1 /10 3 E/GPa B/GPa G/GPa τ m/mpa H v/mpa Wood s alloy Newton s alloy Rose s alloy Table 5 Solidus temperature (T s ), liquidus temperature (T l ) and melting point of all melt spun alloys Solidus temperature, T s/ C Liqidus temperature, T l / C Melting point/ C Wood s alloy Newton s alloy Rose s alloy Fig. 3 Variation of Vicker hardness with applied load for all prepared alloys: (a) Wood s alloy, (b) Newton s alloy, (c) Rose s alloy are listed in Table 4. Values of B and G were calculated using the standard equations: B = E/3(1 2ν) and G = E/2(1 + ν) After first calculating E using the dynamic resonance method, resistance to plastic indentation was determined by an indentation hardness test, in which a small hard indenter was pressed into the surface by a standard load and the size of indentation was then measured. The experimental determinations involved the measurements of the applied load at constant time dependence of hardness in addition to the effect of Cd addition to Bi-Pb-Sn ternary elements. The change in microhardness H v with load indentation at constant time is shown in Fig. 3. It is found that the hardness decreased with increasing applied load [16]. 3.4 Thermal Properties The melting behavior of Wood s, Newton s and Rose s alloys rapidly solidified from melt during heating was investigated by Shimadzu DTA-50. Differential thermal analysis (DTA) was carried out at a heating rate of 10 K/min and rate of flow 20 ml/min under pure nitrogen protective atmosphere. The heating cell was loaded with two platinum crucibles: one contained the reference material, while the other contained the specimen under test. Figure 4 shows Fig. 4 DTA of all melt spun alloys: (a) Wood s alloy, (b) Newton alloy, (c) Rose s alloy the DTA thermograms from the melt spun Wood s, Newton s and Rose s alloys during heating. There are only endothermic peak for quaternary alloy (Wood s alloy) while there are two endothermic peaks for ternary compositions (Newton s and Rose s) alloys. The first peak at low temperature corresponds to low melting point of Bi and the second one at high temperature when the alloy was completely melted. This means the increase in pasty range for Newton s and Rose s alloys. The variation of liquids temperature T l and solidus temperature T s for all fusible alloys are listed in Table 5. The melting points of the quenched Wood s, Newton s and Rose s alloys are also listed in Table 5. It is clear that the melting point of Wood s is lower than that of Newton s and Rose s alloys, because the presence of Cd in Wood s alloy decreases its melting point. 4. Conclusions (1) Rapid solidification by melt spinning technique leads to formation of the intermetallic com-

5 J. Mater. Sci. Technol., Vol.25 No.4, pound Pb 7 Sn 3. (2) The addition of Cd in Wood s alloy refines the crystallite size. (3) The alloy containing-cd exhibits better mechanical properties than the Newton s and Rose s alloys. (4) The presence of Cd in Wood s alloy decreases its melting point of Wood s alloy. (5) The Vicker microhardness of Wood s alloys strong depends on the applied load. REFERENCES [1 ] T.J. Marshall, G.T. Mott and M.H. Grieverson: Brit. J. Radiol., 1975, 48, 924. [2 ] M.T. McCormack, Y. Degani, H.S. Chen and W.R. Gesick: JOM, 1996, 48, 54. [3 ] I. Manna and S.K. Pabi: Phys. Status Solidi A, 1991, 123, 393. [4 ] J. Perkins and G.R. Edwards: J. Mater. Sci., 1975, 10, 136. [5 ] G.P. Glasgow: Med. Dosim., 1991, 16, 13. [6 ] C.R. Blackwell and K.D. Amunason: ibid, 1991, 15, 127. [7 ] H. Jones: J. Mater. Sci., 1984, 19, [8 ] E.W. Collings, C.E. Mobley, R.E. Maringer and H.L. Gegel: Rapidly Quenched Metals III, Vol.1, ed. B. Cantor, The Metals Society, London, 1978, 188. [9 ] M. Kamal, M. Mazen, A.B. El-Bediwi and E. Kashita: Radiat. Eff. Defetts Solids, 2005, 160(8), 369. [10] M. Kamal, A.B. El-Bediwi and M.B. Karman: J. Mater. Sci. Mater. Electr., 1998, 9, 425. [11] M. Kamal, M. Mazen, A.B. El-Bediwi and E. Kashita: Radiat. Eff. Deffects Solids, 2005, 160(8), 369. [12] M. Kamal, A.B. El-Bediwi and M.B. Karman: J. Mater. Sci.-Mater. Electron., 1998, 9, 425. [13] M. Kamal and A.B. El-bediwi: J. Mater. Sci.-Mater. Eletron., 2000, 11, 519. [14] S. Timosshenko and S. Goodier: Theory of Elasticity, 3rd edn, McGraw-Hill, New York, 1970, 407. [15] J.J. Gilman: J. Appl. Phys., 1975, 46, [16] R.M. Shalaby: J. Mater. Sci.-Mater. Eletron., 2004, 15, 205.