SOLIDIFICATION OF UNDERCOOLED Pb-Sb ALLOYS
J. C. M. Neto1, W. B. de Castro2
1
Unidade Acadêmica de Engenharia Mecânica - Universidade do Estado do Amazonas
– Escola Superior de Tecnologia
2
Unidade Acadêmica de Engenharia Mecânica - Universidade Federal de Campina
Grande – Centro de Ciências e Tecnologia
69050-001 – Manaus – AM
[email protected]
ABSTRACT
Rapid Solidification Processing (RSP), of metals and alloys, is establish by increasing of
the undercooling applying high cooling rates (102 - 106 K/s) or by reduce nucleation sites
using low cooling rates (1 K/s). Melt undercooling opens new solidification pathways for
new non-equilibrium phases and unusual microstructures. Several techniques have
been developed to reduce nucleation sites and produce increased undercooling in
metals and alloys including the fluxing technique. The aim of this paper is to study the
influence of the undercooling level on microstructures of Pb-7,6%Sb alloy by using the
fluxing technique. Samples without flux and with flux 30% P2O5 + 20% SnO + 50% SnF2
(%mol) were used. The increasing undercooling occurred in sample that used flux and
the refinement primary dendrites and eutectic was observed when the undercooling
increases. Increasing the undercooling led to a higher growth rate, hence morphological
refinement occurs.
Keywords: Undercooling, Sn-Bi alloys, microstructure, rapid solidification.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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INTRODUCTION
Rapid Solidification Process, for metals and alloys, can be established by applying
high cooling rates (102 – 106 K/s) or by inducing high undercooling conditions under low
cooling rates, as low as 1 K/s. Melt undercooling opens new solidification pathways for
new non-equilibrium phases and non-conventional microstructures as structural
refinement, novel crystalline or amorphous phases and solid solubility extension(1).
Several techniques(2) have been developed in order to reduce nucleation sites and
produce high undercoolings; one example is the fluxing technique. In this technique the
liquid is immersed in a material that isolates it from contact with the crucible walls and
atmosphere, it dissolves impurities or changes their structure to make them less active,
and provides heterogeneous nucleation sites(3). The first application of fluxing technique
dates back to 1941(4) when undercooled 150 g of the Fe by as much as 258 K using
Soda Lime Glass flux. Kui et. al.(5) (1984) studied bulk glass formation in Pd40Ni40P20
alloy, using the B2O3 flux. They demonstrated glass formation in a 4-g sample by
cooling rate at only 1 K/s. Recently, Han et al.(6) observed critical undercooling of 421 K,
349 K, 380 K and 381 K for Ni–25%Cu, Ni–50%Cu, Ni–67%Cu and Ni–75%Cu alloys
respectively with glass fluxing method. The solidified microstructure was mainly
characterized by a morphological transition from coarse dendrites to equiaxial grains.
The rapid crystal nucleation, growth, and the marked solute trapping effect under high
undercooling conditions are responsible for this morphology transition. The ability to
attain a variety of measurable undercooling is prerequisite for detailed examination and
understanding of the phase selection kinetics involved in the development of nucleation
controlled product structures during rapid solidification(7). This paper presented results of
an investigation into the influence of the flux on the undercooling level and into the
influence of the undercooling level on the microstructures of Pb-Sb alloys.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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EXPERIMENTAL METHODS
The samples, 4g of 99,99% purity hypoeutectic composition alloy (Pb-7,6wt%Sb)
without denucleating agent (flux) and with the denucleating agent (flux) were loaded in a
70mm long and 20mm diameter quartz crucible, purged with high purity argon and
evacuated up to 10-3 Torr. The flux was P2O5-SnO-SnF2. It is inorganic glass with
softening (347 K). To avoid sample vibration, a special furnace configuration was used
(figure 1). In this system, the sample was independently attached from the heating unit.
By using such an experimental set-up, the heating unit was able to move easily along
the quartz crucible, which allowed one to obtain rapid heating and cooling of the sample.
The temperature measurements were performed by using a mineral insulation 1.5 mm in
diameter J type thermocouple. This thermocouple encapsulated was immersed into the
melted sample to warrant accuracy. The nucleation temperature was detected by finding
the inflection point in the temperature versus the time cooling curve. Cooling curves
were recorded by using a computerized date acquisition system. Microstructures were
analyzed by optical microscopy (OM).
Figure 1. Schematic illustration of the experimental apparatus.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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RESULTS AND DISCUSSION
Influence of the flux on the undercooling level
The table 1 shows the undercoolings degree obtained for the alloys. The sample A
were melted and cooled without flux enveloping the sample and the sample B were
melted and cooled with P2O5-SnO-SnF2 flux enveloping the sample; these last
procedures leaded to improvement of the undercooling level. Generally, there are two
reasons why flux is used as a denucleating agent of the samples. First, it can separate
the sample from the crucible, thus, averting heterogeneous nucleation caused by
crucible wall. Second, it can trap impurities attached on the surface and inside the alloys
melt and purify the melt to get a deep undercooling. But, the physical mechanism of the
flux removing the impurities is an unsolved problem and needs to be discussed. Sun et
al.(8) has presented the physical mechanism of denucleating agent trapping impurities. If
the sample and denucleating agent are taken as a system under study, then after the
sample to be headed to molten state, impurities inside the melt would move to the
interface between denucleating agent and the alloy, where the impurities could be
presumable dissolved or deactivated by the molten denucleating agent, by the
convection aroused by gravity-induced segregation and temperature gradient. The
mechanism of denucleating agent trapping impurities (figure 2), which are on the
interface between the molten alloy and denucleating agent, is that the interface tension
σi
– f
(between the impurities and the flux) is lower than σm - i (between melt and
impurities) and σm - f (between melt and flux). This fact leads to the decrease of system
free energy, ∆G < 0, and the process of denucleating agent trapping impurities can be
carried out spontaneously. This case has probably occurred with our samples because
∆Tn1 of the undercooled samples flux were higher than ∆Tn1 of the undercooled sample
without flux. Besides, the P2O5-SnO-SnF2 flux was efficient as a denucleating agent
because obtained a larger undercooling (∆T1), as show the table 1.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Table 1- Undercooling values of the Pb-Sb alloys.
Pb-Sb
Alloy
TL
Tn1
∆T1 = TL – Tn1
(K)
(K)
(K)
A
547
530
13
NO
B
548
527
20
P2O5-SnO-SnF2
SAMPLES
(wt%)
7,6
Flux
Figure 2- A schematic illustration for the process of flux trapping impurity.
Influence of the undercooling level on microstructure
The microstructural analysis for Pb-7,6wt%Sb hypoeutectic alloys presented
microstructures consisting of β-Sn dendrites primary and eutectic, as shown in figure 3.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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A microstructure refinement of the β-Pb dendrites size occurs with the enhancement of
undercooling.
Figure 3 - Microstructure of the Pb-7,6wt%Sb alloy. a) Sample A undercooled of 13
degree; b) Sample B undercooled of 20 degree.
The increasing of the undercooling of 13 to 20 K for Pb-7,6wt%Sb resulted too in
reduced dendrite arm spacing of 75µm to 37µm, as shown figure 3. The increasing of
the undercooling led to a high growth rate, hence morphological refinement occurred(9).
Some works(10,11,12,13,14) have reported dendrite refinement in undercooled samples. On
this mechanism a considerably high amount of interfacial energy is stored comparison to
the gain of volume energy. The reduction of the interfacial energy acts as a driving force
for the on going morphology change mechanism leading to a microstructural refinement.
It means that, rapid growing dendrites or eutectic become morphologically unstable and
decay with a reduction of the interface area, as well as the driving force for such a
process. These conclusions can to be used for explanation of the results obtained of the
sample in this work.
CONCLUSION
Undercooling processing was carried out using flux technique. The Pb-7,6wt%Sb
hypoeutectic alloy that were melted and cooled without flux enveloping the sample and
melted and cooled with P2O5-SnO-SnF2 flux enveloping the sample. The samples with
flux enveloped obtained larger undercooling levels. Probably the physical mechanism of
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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trapping impurities was that the interface σ between the impurities and the P2O5-SnOSnF2 flux was lower than the interface σ and this leads to the decrease of the system
free energy ∆G and the process of trapping impurities was carried out spontaneously.
Refinement dendritic occurred with undercooling increasing. Rapid growing dendrites
and eutectic become morphologically unstable and decay with a reduction of the
interface area, as well as the driving force for such a process.
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