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Reagents for deep-etching Al-Si alloys: Scanning electron microscopy microstructures
213
METALLOGRAPH Y 20:213-222 (1987)
SHORT COMMUNICATION Reagents for Deep-Etching AI-Si Alloys: Scanning Electron Microscopy Microstructures
F. A. CALVO, A. J. CRIADO, J. M. GOMEZ de SALAZAR AND F. MOLLEDA
Departmento de Metalurgia, Facultad de Ciencias Quimicas, Universidad Complutense 28040 Madrid, Spain
Introduction The purpose of etches used in the metallographic examination of AISi alloys by scanning electron microscopy (SEM) is to dissolve one of the phases present, either the a (aluminum) solid solution or the silicon, so as to be able to observe the crystalline morphology and spatial distribution of the unetched phase. In these alloys, the mechanical properties are determined by the size, shape, and distribution of the silicon phase in the a matrix. Thus several authors have developed etches to reveal the morphology and distribution of silicon. Holmanova [I-3] used a mixture of HCI and HF in aqueous solution and long etching times (Table 1). However, at these long etching times the silicon also suffers attack, as it does with HF alone, thus making observations of the true microstructure difficult at high magnifications. A two-stage etching process has also been recommended by Holmanova for certain eutectic AI-Si alloys with a particular size and morphology of the silicon phase. This consists of a first stage, with a mixture of HCI and HF in aqueous solution as before, followed by etching with 25 wt.% HNO3 in water. This has been found by the present authors to damage the spatial distribution of the silicon phase because some of the phase can be detached by the turbulence caused by hydrogen evolution. Paul and Murrle [4] used 10% HC1 in aqueous solution and prolonged etching (Table 1). This etch is clean and effective for short periods of time. However, when the silicon is present as small and isolated particles © Elsevier Science Publishing Co., Inc., 1987 52 Vanderbilt Ave., New York, NY 10017
0026-0800/87/$3.50
F. A. Calvo et al.
214
(a)
(b) FIG. 1. Unmodified AI-20% Si alloy; sample taken from close to the surface of the casting. (a) Polyhedral primary silicon that is not perfectly shaped as a result of solidification conditions. (b) Detail of Fig. l(a) showing silicon free from a phase. (c) Detail at a higher magnification showing eutectic silicon. (d) Detail at high magnification showing a silicon dendrite growing from primary silicon. Etched 10 min in a saturated aqueous solution of HgCI2.
Deep-Etching AI-Si Alloys
215
(c)
(d) FIG. 1.
(Continued)
216
F. A. Calvo et al. TABLE l E t c h e s D e s c r i b e d in the Literature
Source [Req Holmanova [1-3]
Reagent
Alloy Modified eutectic
Holmanova [ I - 3 ]
15 ml HCI 35-38% 10 ml H F 38-40% 90 ml H20 HNO3 25% (aq.)
Paul & Murrie [4]
HCI 10% (aq.)
Unmodified eutectic
IBF [5]
HCI 25% (aq.)
Modified & unmodified eutectic
Unmodified eutectic
Time I (min) / T,°C
Time 2 (min) / T,°C
(30-60) / 20 (15-30) / 20 60 / 20
(2-4) / 20
15 / 20
Remarks Silicon damaged Only for low magnification H induces detachment of silicon Good for modified eutectic
IBF = The Institute of British Foundrymen.
in the aluminum matrix, this etching procedure is unreliable and contradictory results are obtained under identical conditions. The Institute of British Foundrymen [5] recommended 25% HCI in water for metallographic examination of these alloys by SEM (Table 1). Satisfactory results were obtained for unmodified chill-cast eutectic alloys using short etching times. For modified chill-cast eutectic alloys, the results were unsatisfactory due to the detachment of silicon particles by the turbulence caused by hydrogen evolution. Etching with aqueous HCI in the concentrations and for the times proposed by various authors led to some deterioration in the microstructure and to the presence of darkcolored reaction products that were difficult to remove by washing. The present authors propose as ideal reagents an aqueous solution of mercury halides (HgCI2) slightly acidified with HCI. These etches dissolve energetically the a phase, but leave the silicon phase unattacked, even when the etch is highly concentrated or after long etching times. Alternatively, etching with NaOH in aqueous solution for times up to 0.33 h reveals satisfactorily the microstructure of these alloys, whether chillcast, sand-cast, modified, or unmodified. Also, HCI in aqueous solution, catalyzed by a small amount of HgCI2, is a more controlled and effective etch than those proposed in the literature [4, 5]. An important point to bear in mind but little stressed in the literature is the effective stopping of the etching process when required and the
Deep-Etching Al-Si Alloys
217
elimination of reaction products that mar the microstructure. For maximum efficiency, an ultrasonic bath should thus be used at all stages.
Experimental Procedure Samples containing between 1.5 and 20% silicon were chill-cast from a melt made from aluminum and a master AI-20% Si alloy (Fe 0.6%). A chill-cast, sodium-modified eutectic alloy was also produced. Chemical etching, stopping of this process, and washing were carried out in an ultrasonic bath. The experimental details are given in Table 2. The various microstructures revealed on etching were examined using SEM.
Results and Discussion Etches consisting of mercury halides in an aqueous solution slightly acidified with HCI gave excellent results for hypereutectic sand- or chillcast unmodified AI-Si alloys that normally possess polyhedral primary silicon (Figs. 1[a] and [b]). These etches were very energetic but revealed,
F16. 2. Small polyhedral primary silicon particle formed during solidification in a weld test. Etched 10 rain in a saturated aqueous solution of HgCI2.
F. A. Calvo et al.
218
(a)
(b) FIG. 3. (a) Microstructure of a chill-cast eutectic alloy modified with sodium. (b) Detail at higher magnification. Etched l0 min in a 30% NaOH aqueous solution.
Deep-Etching Al-Si Alloys
219
(a)
(b) FIG. 4. (a) Microstructure of a chill-cast unmodified eutectic alloy exhibiting various silicon morphologies. (b) Detail at higher magnification of dendritic eutectic silicon. Etched 20 min in a 30% NaOH aqueous solution.
220
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Deep-Etching AI-Si Alloys
221
FxG. 5. Siliconparticle in a 20% Si chill-cast alloy after annealing for 24 h at 550°C. Etched 5 rain in a 30% NaOH aqueous solution.
without damage, all the details of the silicon phase (Figs. l[c] and [d]), even for the small primary silicon formed during the solidification of a weld pool produced during the discharge of an electric arc in a welding test (Fig. 2)[6]. When acidified HgBr2 aqueous solutions were used, the attack was not as deep as that obtained with HgCI2. For sand- or chillcast eutectic alloys (unmodified or modified), the etching times required were not as long as those necessary for the hypereutectic alloys. An aqueous solution of NaOH gave very satisfactory results for alloys with 1.5 to 13% silicon containing various silicon morphologies. The conventional microstructures of sand- or chill-cast AI-Si eutectic alloys in their modified (Figs. 3[a] and [b]) and unmodified (Figs. 4[a] and [b]) states were clearly revealed when these alloys were etched according to the conditions given in Table 2. Even the small spherical silicon particles formed after long annealing treatments [7] were clearly shown by this etching treatment (Fig. 5). Etching with aqueous solutions of HC1 catalyzed by HgCI2 yielded no better results than those obtained with mercury halide or NaOH etches.
Conclusions 1. Mercury halides deeply etch the et phase in hypereutectic sand- or chill-cast unmodified AI-Si alloys, with no damage to the silicon phase.
222
F. A. Calvo et al.
2. A concentrated aqueous solution of NaOH is good for etching eutectic or hypereutectic, sand- or chill-cast, unmodified or modified AI-Si alloys with very fine silicon microstructures. 3. Etching with mercury halides and NaOH requires the use of a stop bath. Final washing in a slightly acidified solution is also necessary after using a mercury halide etch. 4. The use of an ultrasonic bath during etching, stopping, and washing improves the efficiency and cleanliness, and does not damage the original silicon microstructure. References 1. M. Holmanova, the spatialdistributionofeutecticinmodifiedandunmodified siluminium, Praktische Metallographie 11:155-159 (1974). 2. M. Holmanova and J. Klaban, New aspect of silumin crystallization, Skrarentz 11:312315 (1974). 3. M. Holmanova, The influence of the solidification rate on the spatial distribution of eutectic silicon in technical siluminius, Praktische Metallographie 12:59-68 (1975). 4. J. Paul and U. Murrle, Assessing the changes in shape of structural components in deep etched specimens, Praktische Metallographie 18:413-423 (1981). 5. The Institute of British Foundrymen, Typical Microstructures of Cast Metals, (E. F. Boultbee and G. A. Schofield, Eds.) Birmingham (1981). 6. F. A. Calvo, B. G. Mellor, and A. J. Criado: Report to the European Research Office Group U.S.A., C. No. DAJA 37-80-C-0298 (1981). 7. F. A. Calvo, A. J. Criado, J. M. Gomez de Salazar, and F. Molleda, Empleo del HgCI2 como reactivo de ataque. Estudio del Eutectico AI-Si obtenido mediante ciclos de enfriamiento ne convencionales, Revista de Metalurgia 21:249-253 (1985).
Received July 1985; accepted October 1986.
METALLOGRAPH Y 20:213-222 (1987)
SHORT COMMUNICATION Reagents for Deep-Etching AI-Si Alloys: Scanning Electron Microscopy Microstructures
F. A. CALVO, A. J. CRIADO, J. M. GOMEZ de SALAZAR AND F. MOLLEDA
Departmento de Metalurgia, Facultad de Ciencias Quimicas, Universidad Complutense 28040 Madrid, Spain
Introduction The purpose of etches used in the metallographic examination of AISi alloys by scanning electron microscopy (SEM) is to dissolve one of the phases present, either the a (aluminum) solid solution or the silicon, so as to be able to observe the crystalline morphology and spatial distribution of the unetched phase. In these alloys, the mechanical properties are determined by the size, shape, and distribution of the silicon phase in the a matrix. Thus several authors have developed etches to reveal the morphology and distribution of silicon. Holmanova [I-3] used a mixture of HCI and HF in aqueous solution and long etching times (Table 1). However, at these long etching times the silicon also suffers attack, as it does with HF alone, thus making observations of the true microstructure difficult at high magnifications. A two-stage etching process has also been recommended by Holmanova for certain eutectic AI-Si alloys with a particular size and morphology of the silicon phase. This consists of a first stage, with a mixture of HCI and HF in aqueous solution as before, followed by etching with 25 wt.% HNO3 in water. This has been found by the present authors to damage the spatial distribution of the silicon phase because some of the phase can be detached by the turbulence caused by hydrogen evolution. Paul and Murrle [4] used 10% HC1 in aqueous solution and prolonged etching (Table 1). This etch is clean and effective for short periods of time. However, when the silicon is present as small and isolated particles © Elsevier Science Publishing Co., Inc., 1987 52 Vanderbilt Ave., New York, NY 10017
0026-0800/87/$3.50
F. A. Calvo et al.
214
(a)
(b) FIG. 1. Unmodified AI-20% Si alloy; sample taken from close to the surface of the casting. (a) Polyhedral primary silicon that is not perfectly shaped as a result of solidification conditions. (b) Detail of Fig. l(a) showing silicon free from a phase. (c) Detail at a higher magnification showing eutectic silicon. (d) Detail at high magnification showing a silicon dendrite growing from primary silicon. Etched 10 min in a saturated aqueous solution of HgCI2.
Deep-Etching AI-Si Alloys
215
(c)
(d) FIG. 1.
(Continued)
216
F. A. Calvo et al. TABLE l E t c h e s D e s c r i b e d in the Literature
Source [Req Holmanova [1-3]
Reagent
Alloy Modified eutectic
Holmanova [ I - 3 ]
15 ml HCI 35-38% 10 ml H F 38-40% 90 ml H20 HNO3 25% (aq.)
Paul & Murrie [4]
HCI 10% (aq.)
Unmodified eutectic
IBF [5]
HCI 25% (aq.)
Modified & unmodified eutectic
Unmodified eutectic
Time I (min) / T,°C
Time 2 (min) / T,°C
(30-60) / 20 (15-30) / 20 60 / 20
(2-4) / 20
15 / 20
Remarks Silicon damaged Only for low magnification H induces detachment of silicon Good for modified eutectic
IBF = The Institute of British Foundrymen.
in the aluminum matrix, this etching procedure is unreliable and contradictory results are obtained under identical conditions. The Institute of British Foundrymen [5] recommended 25% HCI in water for metallographic examination of these alloys by SEM (Table 1). Satisfactory results were obtained for unmodified chill-cast eutectic alloys using short etching times. For modified chill-cast eutectic alloys, the results were unsatisfactory due to the detachment of silicon particles by the turbulence caused by hydrogen evolution. Etching with aqueous HCI in the concentrations and for the times proposed by various authors led to some deterioration in the microstructure and to the presence of darkcolored reaction products that were difficult to remove by washing. The present authors propose as ideal reagents an aqueous solution of mercury halides (HgCI2) slightly acidified with HCI. These etches dissolve energetically the a phase, but leave the silicon phase unattacked, even when the etch is highly concentrated or after long etching times. Alternatively, etching with NaOH in aqueous solution for times up to 0.33 h reveals satisfactorily the microstructure of these alloys, whether chillcast, sand-cast, modified, or unmodified. Also, HCI in aqueous solution, catalyzed by a small amount of HgCI2, is a more controlled and effective etch than those proposed in the literature [4, 5]. An important point to bear in mind but little stressed in the literature is the effective stopping of the etching process when required and the
Deep-Etching Al-Si Alloys
217
elimination of reaction products that mar the microstructure. For maximum efficiency, an ultrasonic bath should thus be used at all stages.
Experimental Procedure Samples containing between 1.5 and 20% silicon were chill-cast from a melt made from aluminum and a master AI-20% Si alloy (Fe 0.6%). A chill-cast, sodium-modified eutectic alloy was also produced. Chemical etching, stopping of this process, and washing were carried out in an ultrasonic bath. The experimental details are given in Table 2. The various microstructures revealed on etching were examined using SEM.
Results and Discussion Etches consisting of mercury halides in an aqueous solution slightly acidified with HCI gave excellent results for hypereutectic sand- or chillcast unmodified AI-Si alloys that normally possess polyhedral primary silicon (Figs. 1[a] and [b]). These etches were very energetic but revealed,
F16. 2. Small polyhedral primary silicon particle formed during solidification in a weld test. Etched 10 rain in a saturated aqueous solution of HgCI2.
F. A. Calvo et al.
218
(a)
(b) FIG. 3. (a) Microstructure of a chill-cast eutectic alloy modified with sodium. (b) Detail at higher magnification. Etched l0 min in a 30% NaOH aqueous solution.
Deep-Etching Al-Si Alloys
219
(a)
(b) FIG. 4. (a) Microstructure of a chill-cast unmodified eutectic alloy exhibiting various silicon morphologies. (b) Detail at higher magnification of dendritic eutectic silicon. Etched 20 min in a 30% NaOH aqueous solution.
220
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Deep-Etching AI-Si Alloys
221
FxG. 5. Siliconparticle in a 20% Si chill-cast alloy after annealing for 24 h at 550°C. Etched 5 rain in a 30% NaOH aqueous solution.
without damage, all the details of the silicon phase (Figs. l[c] and [d]), even for the small primary silicon formed during the solidification of a weld pool produced during the discharge of an electric arc in a welding test (Fig. 2)[6]. When acidified HgBr2 aqueous solutions were used, the attack was not as deep as that obtained with HgCI2. For sand- or chillcast eutectic alloys (unmodified or modified), the etching times required were not as long as those necessary for the hypereutectic alloys. An aqueous solution of NaOH gave very satisfactory results for alloys with 1.5 to 13% silicon containing various silicon morphologies. The conventional microstructures of sand- or chill-cast AI-Si eutectic alloys in their modified (Figs. 3[a] and [b]) and unmodified (Figs. 4[a] and [b]) states were clearly revealed when these alloys were etched according to the conditions given in Table 2. Even the small spherical silicon particles formed after long annealing treatments [7] were clearly shown by this etching treatment (Fig. 5). Etching with aqueous solutions of HC1 catalyzed by HgCI2 yielded no better results than those obtained with mercury halide or NaOH etches.
Conclusions 1. Mercury halides deeply etch the et phase in hypereutectic sand- or chill-cast unmodified AI-Si alloys, with no damage to the silicon phase.
222
F. A. Calvo et al.
2. A concentrated aqueous solution of NaOH is good for etching eutectic or hypereutectic, sand- or chill-cast, unmodified or modified AI-Si alloys with very fine silicon microstructures. 3. Etching with mercury halides and NaOH requires the use of a stop bath. Final washing in a slightly acidified solution is also necessary after using a mercury halide etch. 4. The use of an ultrasonic bath during etching, stopping, and washing improves the efficiency and cleanliness, and does not damage the original silicon microstructure. References 1. M. Holmanova, the spatialdistributionofeutecticinmodifiedandunmodified siluminium, Praktische Metallographie 11:155-159 (1974). 2. M. Holmanova and J. Klaban, New aspect of silumin crystallization, Skrarentz 11:312315 (1974). 3. M. Holmanova, The influence of the solidification rate on the spatial distribution of eutectic silicon in technical siluminius, Praktische Metallographie 12:59-68 (1975). 4. J. Paul and U. Murrle, Assessing the changes in shape of structural components in deep etched specimens, Praktische Metallographie 18:413-423 (1981). 5. The Institute of British Foundrymen, Typical Microstructures of Cast Metals, (E. F. Boultbee and G. A. Schofield, Eds.) Birmingham (1981). 6. F. A. Calvo, B. G. Mellor, and A. J. Criado: Report to the European Research Office Group U.S.A., C. No. DAJA 37-80-C-0298 (1981). 7. F. A. Calvo, A. J. Criado, J. M. Gomez de Salazar, and F. Molleda, Empleo del HgCI2 como reactivo de ataque. Estudio del Eutectico AI-Si obtenido mediante ciclos de enfriamiento ne convencionales, Revista de Metalurgia 21:249-253 (1985).
Received July 1985; accepted October 1986.