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Quantification of grain boundary precipitation and the influence of quench rate in 6XXX aluminum alloys
Materials Characterization 58 (2007) 40 – 45
Quantification of grain boundary precipitation and the influence of quench rate in 6XXX aluminum alloys D. Steele ⁎, D. Evans, P. Nolan, D.J. Lloyd Novelis Global Technology Centre, 945 Princess Street, Kingston, Ont. Canada K7L 5L9 Received 11 July 2005; received in revised form 22 March 2006; accepted 22 March 2006
Abstract The extent of grain boundary precipitation has a strong influence on the ductility, fracture toughness and formability of Al alloys. An experimental method for quantifying the level of grain boundary precipitation is described and applied to the Al alloy AA6111 subjected to a range of quenching rates. A threshold quenching rate can be identified above which the degree of grain boundary precipitate is insensitive to the quenching rate. © 2006 Elsevier Inc. All rights reserved. Keywords: Grain boundary; Precipitation; Quenching rate; Al alloys
1. Introduction The formability and fracture behaviour of polycrystalline materials is influenced by the presence of second phase precipitation at the grain boundaries. Studies have shown that as the level of precipitation at the grain boundaries increases, its fracture toughness and formability decreases [1,2]. Intergranular fracture during forming, particularly in bending operations, can be linked to a decrease in boundary strength resulting from the presence of intergranular precipitates. In the case of 6XXX series heat treatable age hardening alloys, thermal processing involves solution heat treatment followed by quenching to place soluble phases in solid solution. The quench rate following solution heat treatment determines the extent to which solutes diffuse to the boundaries, and the degree of precipitation at the ⁎ Corresponding author. E-mail address: [email protected] (D. Steele). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.03.007
boundaries, and is therefore a critical parameter in the processing route. For commercial, continuous processes in particular, a method of evaluating the quench efficiency is desirable. Optimisation of the parameters involved in the quench practice has ramifications for the process cost, as well as for the mechanical properties of the material being produced. Where the process specifics are unknown, a measure of the level of precipitation on grain boundaries can also give an indication of the quench history. This paper discusses a boundary phase quantification method that involves the imaging of electrochemically polished material via scanning electron microscopy (SEM), and subsequent image analysis to generate a normalised value describing the amount of precipitate per unit length of boundary. The method is applied to the heat treatable Al alloy AA6111, whose composition ranges are given in Table 1, but it can be modified for other alloys. The method is shown to be sufficiently sensitive to differentiate between specimens that have
D. Steele et al. / Materials Characterization 58 (2007) 40–45
41
Table 1 Typical composition for AA6111 (wt.%)
6111
Mg
Si
Cu
Fe
Mn
Cr
Ti
0.5–1.0
0.6–1.1
0.5–0.9
≤0.4
≤0.4
≤0.1
≤0.1
been quenched at slightly different rates, and provides a correlation between the quench rate and the extent of boundary precipitation. 2. Experimental methods 2.1. Modified Jominy quench The susceptibility of 6XXX alloys to grain boundary precipitation during quenching from solution heat treatment has been studied using 3 mm thick extruded strips of the same compositions as the sheet materials of interest. Grain structures in these test specimens differ from those present in rolled sheet, but the precipitation mechanisms are equivalent. For each specimen, a series of thermocouples is embedded at 2 mm intervals, as illustrated in Fig. 1. The samples were solution heat treated for 10 min at 560 °C, and then each specimen was partially immersed in water for quenching from one end. The thermocouples sample the resulting temperature change from just above to 40 mm above the waterline, measuring a quench rate gradient with increasing distance from the water line. A sampling rate of 200 to 300 Hz is used to produce the thermal quench profiles from different positions in the sample. Following the quench, the thermocouples are removed, and the specimen is electropolished and analysed using the conditions and procedure described in the following sections.
Fig. 1. Arrangement of thermocouples in a specimen for the modified Jominy quench experiments.
While the polishing apparatus is designed for the preparation of 2 and 3 mm diameter TEM (transmission electron microscope) specimens, a larger specimen holder is available for the creation of 3 mm discs from a masked 15 mm diameter coupon by dissolution of the unmasked matrix. This holder, in conjunction with specialised large aperture cathodes produces a 10 mm diameter electropolished circle, which is a sufficient area for the required analysis. One side of the coupon is masked with electrolytic tape to provide an appropriate current density at the surface of the side to be polished. The electrolyte used is 30% nitric acid in methanol, and the polishing conditions are 9 V at − 20 °C, for 60 s. Magnesium containing precipitates intersecting with the resulting surface react with the electrolyte; magnesium is leached out and the precipitate undergoes a composition and volume change. When viewed in the SEM in
2.2. Specimen preparation In order to minimise the number of variables influencing the observed microstructure, the specimens are prepared using standardised conditions in a commercial apparatus1. The oxidation reaction of the Mg containing precipitates [3], (i.e. Mg2Si and the quaternary Al4Cu2Mg8Si7 “Q” phase), in the electrolyte results in SEM images easily thresholded to separate the precipitate fraction. The SEM instrument conditions are set to maximise the backscattered contribution in the secondary electron image, thereby generating sufficient channeling contrast to facilitate locating the grain boundaries.
1
Struer's Tenupol jet electropolishing device.
Fig. 2. Electropolished surface of a 6111 specimen forced air quenched from solution heat treatment.
42
D. Steele et al. / Materials Characterization 58 (2007) 40–45
Fig. 3. Screen captures from the analysis sequence of the area in Fig. 1. An overlay is created tracing the boundaries (a), which is transformed into a binary image (b). Precipitates are discriminated by grey level thresholding (c), and a logical AND is performed with image (b) to produce an image of boundary precipitates (d).
secondary electron mode, these precipitates are easily discerned, appearing bright with respect to the background [Fig. 2].
700× has been selected as a compromise between resolution in a digitally acquired image comprised of 1024 × 832 pixels, and a sufficiently large field of view to include multiple grains per field. At this magnification
2.3. Imaging The precipitates of interest here are those that lie on the grain boundaries. The location of the boundaries must therefore be apparent when analysing the images, but polishing conditions have been selected to avoid etching of the boundaries, and to produce a surface as free of topography as possible, with the exception of the precipitates. In some cases, depending on the process route and alloy composition, the decoration of the boundaries by reacted precipitates is enough to mark their locations. In others, where the number of precipitates is small, or so large in the matrix as to obfuscate the location of the boundaries, electron channeling contrast is used to demarcate the grains. The magnification required is determined by the grain and precipitate size. For commercial 6111 sheet, where the grain size is on average 30 μm, a magnification of
Fig. 4. A typical set of thermocouple quench traces for a Jominy experiment after solution heat treatment. The distances are from the waterline.
D. Steele et al. / Materials Characterization 58 (2007) 40–45
43
Fig. 5. Precipitation levels in a 6111 Jominy quench specimen. The region in [a] underwent a quench rate from solution heat treatment of 7 C°/s, while the region in [b] was quenched at 140 C°/s.
the instrument spot size can be set fairly high, and a large objective aperture can be used, both at the expense of resolution but with a gain in contrast. The backscattered electron contribution to the secondary image is increased, and channeling contrast is available to demarcate the grains. The SEM used was a JEOL 5800, at an acceleration potential of 10 kV, and a working distance of 10 mm. Brightness and contrast conditions are maintained from image to image to facilitate thresholding during image analysis. 2.4. Image analysis After spatially calibrating an image of interest, a binary image consisting of the boundary locations is produced [Fig. 3a,b].2 This is done simply by tracing over the boundaries to create an image overlay for subsequent transformation into a binary image. The boundary is traced with a wide “pen” to minimise the influence of operator precision on the final boundary length. The pen width used will influence the final precipitate per unit boundary value obtained, and must be maintained constant if comparisons between materials are to be made. Typically we have used an 11 pixel pen diameter, which for the conditions stated above results in a sampled grain boundary width of roughly 2 μm. As stated earlier this is 2
Image analyses were carried out using Media Cybernetics ImagePro Plus.
to minimise the reliance on analyst precision in tracing the boundary, but also to accommodate the somewhat expanded volume of the oxidised portion of the precipitates extending from the specimen surface. Maintaining a 2 μm sampled width by altering the number of pixels defining the “pen”, allows images collected at differing magnifications to be compared. The contrast of the resultant boundary image is inverted, if necessary, such that the boundaries form the image region (i.e. black or white) to be measured by the analysis program used. The original image is thresholded to generate a binary image of the precipitate [Fig. 3c], and then a logical “AND” operation is performed using the boundary trace image and the precipitate image. The result is a binary image containing those precipitates which lie on the grain boundaries [Fig. 3d]. The total area of the boundary precipitates is measured, and this is divided by the total length of sampled boundary, which is obtained by measuring the area of traced boundary and dividing that value by the width of the trace. The calculation can be expressed as APLB ¼ APPT =ðAB =SP NPB Þ; where APLB is the area of precipitates in μm2 per μm of boundary, APPT is the total combined area of the precipitates at the boundaries in μm2, AB is the area of the dilated boundaries in μm2, SP is the size of a single pixel in μm, and NPB is the number of pixels used to
44
D. Steele et al. / Materials Characterization 58 (2007) 40–45
Fig. 6. The influence of cooling rate on grain boundary precipitate content in AA6111.
define the boundary (or pen) width during the tracing operation. A value for the area of particles per boundary length is calculated rather than a value for number of particles per boundary length due to the lack of spacing between particles in many cases. Multiple images are captured and analysed for each specimen, and a cumulative average plot is produced to ensure that a meaningful average has been derived from the heterogeneous data.
susceptibility by measuring the precipitation at a given quench rate. Measuring the quench rate in different regions of the Jominy sample, together with the extent of grain boundary, enables the susceptibility to grain boundary precipitation in different alloys to be assessed. Further, a threshold cooling rate, above which precipitation can be avoided, can be identified for a given alloy composition. Fig. 4. shows a typical set of thermocouple traces from a quench experiment. If the initial portion of each trace, i.e. from the start of the quench down to about 250 °C, is considered and treated as linear, then a cooling rate may be derived. The decrease in slope for the traces corresponding to sample regions further from the waterline is apparent. Fig. 5 shows SEM secondary electron images from two extremes in the cooling rate regions analysed. The micrographs are collected at the same distance from the waterline as the embedded thermocouples. Fig. 5a corresponds to a quench rate of 7 C°/s, while Fig. 5b corresponds to a quench rate of 140 C°/s. The level of grain boundary precipitation can then be related to the quenching rate, as in Fig. 6. This figure shows that for quenching rates above 140 C°/s there is essentially no effect of quenching rate on the extent of grain boundary precipitation, and this identifies a threshold quenching rate for this alloy. 3.2. Commercial quench rate evaluation
3. Results and discussion 3.1. Jominy quench In the 6XXX series of alloys, the composition will determine how susceptible a given alloy is to boundary precipitation when the alloy is cooled following solution heat treatment. It is possible to determine an alloy’s
Commercially, heat treatable Al sheet is solutionised and quenched on a continuous heat treat line that involves various means of heating the sheet to the solutionising temperature, followed by sprays to quench the material prior to coiling. The example in Fig. 3 is from a study of the quench efficiency of various conditions on a commercial continuous solution heat treatment line. In
Fig. 7. Relative efficiency of several configurations of the quench section in a continuous solution heat treatment line.
D. Steele et al. / Materials Characterization 58 (2007) 40–45
45
order to quantify quench efficiency and optimise conditions, the level of boundary precipitation was measured and compared for different commercial conditions on the line. Fig. 7 illustrates the resulting ranking of conditions in the trial, and these levels of grain boundary precipitation can be compared with those in Fig. 6 to obtain an estimate of the quenching rate. While a cursory scanning electron microscope examination of polished sections of the material produced under these various conditions does not reveal which condition is best, the optimum condition can be determined by quantifying the boundary precipitation. The method is sufficiently sensitive that it has been adopted as a routine indicator and characterisation tool. It should be noted that only boundary precipitation by soluble phases is being considered here, due to its potentially deleterious effect on material formability. However, matrix precipitation may also occur at slower cooling rates, and these matrix features can be differentiated from grain boundary effects.
field images also complicate image analysis and the quantifying of precipitates.
3.3. Alternative methods
References
As stated, the precipitation of specific interest here is that which occurs on the material grain boundaries. Any method used to quantify the precipitate must therefore differentiate between boundary and matrix precipitation. Techniques such as conductivity measurement or DSC (Differential Scanning Calorimetry) do not provide information specific to the boundaries, and studies benefit from the inclusion of microstructural imaging [4,5]. Because the aim is to provide a method which may be used for routine evaluation of commercially produced material, the method must also provide statistically relevant information in a reasonable time frame. TEM is one other technique that offers the required spatial resolution, visual identification of boundaries, and subjective differentiation of matrix and boundary precipitates by the analyst. It has been used in numerous studies of precipitation in aluminum alloys, including boundary phase studies (e.g. [6–8]), but suffers from the necessity for small samples with a limited viewable area. The multiple contrast mechanisms typifying TEM bright
[1] Briant CL. Grain boundary structure, chemistry and failure. Mater Sci Technol 2001;17:1317–3123. [2] Li BQ, Reynolds AP. Correlation of grain boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure. J Mater Sci 1998;33:5849–53. [3] Jensen CL. Transformation of Mg2Si during TEM foil preparation. Metallography 1987;20:335–46. [4] Esmaeili S, Poole WJ, Lloyd DJ. Electrical resistivity studies on the precipitation behaviour of AA6111. Mater Sci Forum 2000;331–337:995–1000. [5] Esmaeili S, Wang X, Lloyd DJ, Poole WJ. On the precipitation hardening behaviour of the Al–Mg–Si–Cu Alloy 6111. Metall Mater Trans A 2003;34A:751–63. [6] Li BQ. Quantitative analysis of grain boundary precipitates in an Al–Cu–Mg–Ag alloy exposed to long term thermal exposure. Microstruct Sci 1997;25:379–85. [7] Weatherly GC, Perovic A, Mukhopadhyay NK, Lloyd DJ, Perovic DD. The precipitation of the Q phase in an AA6111 alloy. Metall Mater Trans A 2001;32A:213–8. [8] Dumont D, Deschamps A, Bréchet Y, Sigli C, Ehrström JC. Characterisation, of precipitation microstructures in aluminium alloys 7040 and 7050 and their relationship to mechanical behaviour. Mater Sci Technol 2004;20:567–76.
4. Summary An analytical method for quantifying the extent of grain boundary precipitation has been developed and used to compare the influence of quenching rate after solutionising on the extent of precipitation. Because the method relies on the interaction between electro-polishing solution and the precipitates, it does not give an absolute value of grain boundary precipitation. However, the method described is a very simple procedure that provides a good comparative quantification of the grain boundary precipitate levels for different quenching rates. Acknowledgements The authors are grateful to Novelis Inc. for permission to publish this work.
Quantification of grain boundary precipitation and the influence of quench rate in 6XXX aluminum alloys D. Steele ⁎, D. Evans, P. Nolan, D.J. Lloyd Novelis Global Technology Centre, 945 Princess Street, Kingston, Ont. Canada K7L 5L9 Received 11 July 2005; received in revised form 22 March 2006; accepted 22 March 2006
Abstract The extent of grain boundary precipitation has a strong influence on the ductility, fracture toughness and formability of Al alloys. An experimental method for quantifying the level of grain boundary precipitation is described and applied to the Al alloy AA6111 subjected to a range of quenching rates. A threshold quenching rate can be identified above which the degree of grain boundary precipitate is insensitive to the quenching rate. © 2006 Elsevier Inc. All rights reserved. Keywords: Grain boundary; Precipitation; Quenching rate; Al alloys
1. Introduction The formability and fracture behaviour of polycrystalline materials is influenced by the presence of second phase precipitation at the grain boundaries. Studies have shown that as the level of precipitation at the grain boundaries increases, its fracture toughness and formability decreases [1,2]. Intergranular fracture during forming, particularly in bending operations, can be linked to a decrease in boundary strength resulting from the presence of intergranular precipitates. In the case of 6XXX series heat treatable age hardening alloys, thermal processing involves solution heat treatment followed by quenching to place soluble phases in solid solution. The quench rate following solution heat treatment determines the extent to which solutes diffuse to the boundaries, and the degree of precipitation at the ⁎ Corresponding author. E-mail address: [email protected] (D. Steele). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.03.007
boundaries, and is therefore a critical parameter in the processing route. For commercial, continuous processes in particular, a method of evaluating the quench efficiency is desirable. Optimisation of the parameters involved in the quench practice has ramifications for the process cost, as well as for the mechanical properties of the material being produced. Where the process specifics are unknown, a measure of the level of precipitation on grain boundaries can also give an indication of the quench history. This paper discusses a boundary phase quantification method that involves the imaging of electrochemically polished material via scanning electron microscopy (SEM), and subsequent image analysis to generate a normalised value describing the amount of precipitate per unit length of boundary. The method is applied to the heat treatable Al alloy AA6111, whose composition ranges are given in Table 1, but it can be modified for other alloys. The method is shown to be sufficiently sensitive to differentiate between specimens that have
D. Steele et al. / Materials Characterization 58 (2007) 40–45
41
Table 1 Typical composition for AA6111 (wt.%)
6111
Mg
Si
Cu
Fe
Mn
Cr
Ti
0.5–1.0
0.6–1.1
0.5–0.9
≤0.4
≤0.4
≤0.1
≤0.1
been quenched at slightly different rates, and provides a correlation between the quench rate and the extent of boundary precipitation. 2. Experimental methods 2.1. Modified Jominy quench The susceptibility of 6XXX alloys to grain boundary precipitation during quenching from solution heat treatment has been studied using 3 mm thick extruded strips of the same compositions as the sheet materials of interest. Grain structures in these test specimens differ from those present in rolled sheet, but the precipitation mechanisms are equivalent. For each specimen, a series of thermocouples is embedded at 2 mm intervals, as illustrated in Fig. 1. The samples were solution heat treated for 10 min at 560 °C, and then each specimen was partially immersed in water for quenching from one end. The thermocouples sample the resulting temperature change from just above to 40 mm above the waterline, measuring a quench rate gradient with increasing distance from the water line. A sampling rate of 200 to 300 Hz is used to produce the thermal quench profiles from different positions in the sample. Following the quench, the thermocouples are removed, and the specimen is electropolished and analysed using the conditions and procedure described in the following sections.
Fig. 1. Arrangement of thermocouples in a specimen for the modified Jominy quench experiments.
While the polishing apparatus is designed for the preparation of 2 and 3 mm diameter TEM (transmission electron microscope) specimens, a larger specimen holder is available for the creation of 3 mm discs from a masked 15 mm diameter coupon by dissolution of the unmasked matrix. This holder, in conjunction with specialised large aperture cathodes produces a 10 mm diameter electropolished circle, which is a sufficient area for the required analysis. One side of the coupon is masked with electrolytic tape to provide an appropriate current density at the surface of the side to be polished. The electrolyte used is 30% nitric acid in methanol, and the polishing conditions are 9 V at − 20 °C, for 60 s. Magnesium containing precipitates intersecting with the resulting surface react with the electrolyte; magnesium is leached out and the precipitate undergoes a composition and volume change. When viewed in the SEM in
2.2. Specimen preparation In order to minimise the number of variables influencing the observed microstructure, the specimens are prepared using standardised conditions in a commercial apparatus1. The oxidation reaction of the Mg containing precipitates [3], (i.e. Mg2Si and the quaternary Al4Cu2Mg8Si7 “Q” phase), in the electrolyte results in SEM images easily thresholded to separate the precipitate fraction. The SEM instrument conditions are set to maximise the backscattered contribution in the secondary electron image, thereby generating sufficient channeling contrast to facilitate locating the grain boundaries.
1
Struer's Tenupol jet electropolishing device.
Fig. 2. Electropolished surface of a 6111 specimen forced air quenched from solution heat treatment.
42
D. Steele et al. / Materials Characterization 58 (2007) 40–45
Fig. 3. Screen captures from the analysis sequence of the area in Fig. 1. An overlay is created tracing the boundaries (a), which is transformed into a binary image (b). Precipitates are discriminated by grey level thresholding (c), and a logical AND is performed with image (b) to produce an image of boundary precipitates (d).
secondary electron mode, these precipitates are easily discerned, appearing bright with respect to the background [Fig. 2].
700× has been selected as a compromise between resolution in a digitally acquired image comprised of 1024 × 832 pixels, and a sufficiently large field of view to include multiple grains per field. At this magnification
2.3. Imaging The precipitates of interest here are those that lie on the grain boundaries. The location of the boundaries must therefore be apparent when analysing the images, but polishing conditions have been selected to avoid etching of the boundaries, and to produce a surface as free of topography as possible, with the exception of the precipitates. In some cases, depending on the process route and alloy composition, the decoration of the boundaries by reacted precipitates is enough to mark their locations. In others, where the number of precipitates is small, or so large in the matrix as to obfuscate the location of the boundaries, electron channeling contrast is used to demarcate the grains. The magnification required is determined by the grain and precipitate size. For commercial 6111 sheet, where the grain size is on average 30 μm, a magnification of
Fig. 4. A typical set of thermocouple quench traces for a Jominy experiment after solution heat treatment. The distances are from the waterline.
D. Steele et al. / Materials Characterization 58 (2007) 40–45
43
Fig. 5. Precipitation levels in a 6111 Jominy quench specimen. The region in [a] underwent a quench rate from solution heat treatment of 7 C°/s, while the region in [b] was quenched at 140 C°/s.
the instrument spot size can be set fairly high, and a large objective aperture can be used, both at the expense of resolution but with a gain in contrast. The backscattered electron contribution to the secondary image is increased, and channeling contrast is available to demarcate the grains. The SEM used was a JEOL 5800, at an acceleration potential of 10 kV, and a working distance of 10 mm. Brightness and contrast conditions are maintained from image to image to facilitate thresholding during image analysis. 2.4. Image analysis After spatially calibrating an image of interest, a binary image consisting of the boundary locations is produced [Fig. 3a,b].2 This is done simply by tracing over the boundaries to create an image overlay for subsequent transformation into a binary image. The boundary is traced with a wide “pen” to minimise the influence of operator precision on the final boundary length. The pen width used will influence the final precipitate per unit boundary value obtained, and must be maintained constant if comparisons between materials are to be made. Typically we have used an 11 pixel pen diameter, which for the conditions stated above results in a sampled grain boundary width of roughly 2 μm. As stated earlier this is 2
Image analyses were carried out using Media Cybernetics ImagePro Plus.
to minimise the reliance on analyst precision in tracing the boundary, but also to accommodate the somewhat expanded volume of the oxidised portion of the precipitates extending from the specimen surface. Maintaining a 2 μm sampled width by altering the number of pixels defining the “pen”, allows images collected at differing magnifications to be compared. The contrast of the resultant boundary image is inverted, if necessary, such that the boundaries form the image region (i.e. black or white) to be measured by the analysis program used. The original image is thresholded to generate a binary image of the precipitate [Fig. 3c], and then a logical “AND” operation is performed using the boundary trace image and the precipitate image. The result is a binary image containing those precipitates which lie on the grain boundaries [Fig. 3d]. The total area of the boundary precipitates is measured, and this is divided by the total length of sampled boundary, which is obtained by measuring the area of traced boundary and dividing that value by the width of the trace. The calculation can be expressed as APLB ¼ APPT =ðAB =SP NPB Þ; where APLB is the area of precipitates in μm2 per μm of boundary, APPT is the total combined area of the precipitates at the boundaries in μm2, AB is the area of the dilated boundaries in μm2, SP is the size of a single pixel in μm, and NPB is the number of pixels used to
44
D. Steele et al. / Materials Characterization 58 (2007) 40–45
Fig. 6. The influence of cooling rate on grain boundary precipitate content in AA6111.
define the boundary (or pen) width during the tracing operation. A value for the area of particles per boundary length is calculated rather than a value for number of particles per boundary length due to the lack of spacing between particles in many cases. Multiple images are captured and analysed for each specimen, and a cumulative average plot is produced to ensure that a meaningful average has been derived from the heterogeneous data.
susceptibility by measuring the precipitation at a given quench rate. Measuring the quench rate in different regions of the Jominy sample, together with the extent of grain boundary, enables the susceptibility to grain boundary precipitation in different alloys to be assessed. Further, a threshold cooling rate, above which precipitation can be avoided, can be identified for a given alloy composition. Fig. 4. shows a typical set of thermocouple traces from a quench experiment. If the initial portion of each trace, i.e. from the start of the quench down to about 250 °C, is considered and treated as linear, then a cooling rate may be derived. The decrease in slope for the traces corresponding to sample regions further from the waterline is apparent. Fig. 5 shows SEM secondary electron images from two extremes in the cooling rate regions analysed. The micrographs are collected at the same distance from the waterline as the embedded thermocouples. Fig. 5a corresponds to a quench rate of 7 C°/s, while Fig. 5b corresponds to a quench rate of 140 C°/s. The level of grain boundary precipitation can then be related to the quenching rate, as in Fig. 6. This figure shows that for quenching rates above 140 C°/s there is essentially no effect of quenching rate on the extent of grain boundary precipitation, and this identifies a threshold quenching rate for this alloy. 3.2. Commercial quench rate evaluation
3. Results and discussion 3.1. Jominy quench In the 6XXX series of alloys, the composition will determine how susceptible a given alloy is to boundary precipitation when the alloy is cooled following solution heat treatment. It is possible to determine an alloy’s
Commercially, heat treatable Al sheet is solutionised and quenched on a continuous heat treat line that involves various means of heating the sheet to the solutionising temperature, followed by sprays to quench the material prior to coiling. The example in Fig. 3 is from a study of the quench efficiency of various conditions on a commercial continuous solution heat treatment line. In
Fig. 7. Relative efficiency of several configurations of the quench section in a continuous solution heat treatment line.
D. Steele et al. / Materials Characterization 58 (2007) 40–45
45
order to quantify quench efficiency and optimise conditions, the level of boundary precipitation was measured and compared for different commercial conditions on the line. Fig. 7 illustrates the resulting ranking of conditions in the trial, and these levels of grain boundary precipitation can be compared with those in Fig. 6 to obtain an estimate of the quenching rate. While a cursory scanning electron microscope examination of polished sections of the material produced under these various conditions does not reveal which condition is best, the optimum condition can be determined by quantifying the boundary precipitation. The method is sufficiently sensitive that it has been adopted as a routine indicator and characterisation tool. It should be noted that only boundary precipitation by soluble phases is being considered here, due to its potentially deleterious effect on material formability. However, matrix precipitation may also occur at slower cooling rates, and these matrix features can be differentiated from grain boundary effects.
field images also complicate image analysis and the quantifying of precipitates.
3.3. Alternative methods
References
As stated, the precipitation of specific interest here is that which occurs on the material grain boundaries. Any method used to quantify the precipitate must therefore differentiate between boundary and matrix precipitation. Techniques such as conductivity measurement or DSC (Differential Scanning Calorimetry) do not provide information specific to the boundaries, and studies benefit from the inclusion of microstructural imaging [4,5]. Because the aim is to provide a method which may be used for routine evaluation of commercially produced material, the method must also provide statistically relevant information in a reasonable time frame. TEM is one other technique that offers the required spatial resolution, visual identification of boundaries, and subjective differentiation of matrix and boundary precipitates by the analyst. It has been used in numerous studies of precipitation in aluminum alloys, including boundary phase studies (e.g. [6–8]), but suffers from the necessity for small samples with a limited viewable area. The multiple contrast mechanisms typifying TEM bright
[1] Briant CL. Grain boundary structure, chemistry and failure. Mater Sci Technol 2001;17:1317–3123. [2] Li BQ, Reynolds AP. Correlation of grain boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure. J Mater Sci 1998;33:5849–53. [3] Jensen CL. Transformation of Mg2Si during TEM foil preparation. Metallography 1987;20:335–46. [4] Esmaeili S, Poole WJ, Lloyd DJ. Electrical resistivity studies on the precipitation behaviour of AA6111. Mater Sci Forum 2000;331–337:995–1000. [5] Esmaeili S, Wang X, Lloyd DJ, Poole WJ. On the precipitation hardening behaviour of the Al–Mg–Si–Cu Alloy 6111. Metall Mater Trans A 2003;34A:751–63. [6] Li BQ. Quantitative analysis of grain boundary precipitates in an Al–Cu–Mg–Ag alloy exposed to long term thermal exposure. Microstruct Sci 1997;25:379–85. [7] Weatherly GC, Perovic A, Mukhopadhyay NK, Lloyd DJ, Perovic DD. The precipitation of the Q phase in an AA6111 alloy. Metall Mater Trans A 2001;32A:213–8. [8] Dumont D, Deschamps A, Bréchet Y, Sigli C, Ehrström JC. Characterisation, of precipitation microstructures in aluminium alloys 7040 and 7050 and their relationship to mechanical behaviour. Mater Sci Technol 2004;20:567–76.
4. Summary An analytical method for quantifying the extent of grain boundary precipitation has been developed and used to compare the influence of quenching rate after solutionising on the extent of precipitation. Because the method relies on the interaction between electro-polishing solution and the precipitates, it does not give an absolute value of grain boundary precipitation. However, the method described is a very simple procedure that provides a good comparative quantification of the grain boundary precipitate levels for different quenching rates. Acknowledgements The authors are grateful to Novelis Inc. for permission to publish this work.