- Email: [email protected]
A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy
Materials Chemistry and Physics 72 (2001) 401–404
A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy D. Bradai a , P. Zieba b,∗ , E. Bischoff c , W. Gust c a
Institut de Physique, Université des Sciences et de la Technologie Houari-Boumedienne, BP 32 El-Alia, Alger, Algeria Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta St. 25, 30-059 Cracow, Poland Max-Planck-Institut für Metallforschung and Institut für Metallkunde, Universität Stuttgart, Seestr. 92, 70174 Stuttgart, Germany b
c
Received 5 February 2001; received in revised form 26 February 2001; accepted 28 February 2001
Abstract The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10 wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mg–Al alloys; Discontinuous precipitation; Discontinuous dissolution; Grain boundary migration
1. Introduction Discontinuous dissolution (DD) is a solid-state phase transformation occurring by the backward migration of a former reaction front of a discontinuous precipitation (DP) colony [1–3]. The reaction starts at temperatures close to the solvus temperature and results in the formation of an inhomogeneous α ∼ solid solution according to the scheme: α + β → α∼ where α is the solute-depleted lamella and β is the solute-rich lamella. The DD reaction can occur at three different sites [4–6], namely the former reaction front of the DP colony, the impingement of two colonies of discontinuous precipitates and the original location of a grain boundary (Fig. 1). The Mg–Al alloys are a good example showing the common behaviour of the DD reaction [7,8]. In the case of a single seam of the DP, the reaction occurred only at the reaction front of a former DP colony. In the case of a double seam, the reaction started also at the original locations of the grain boundary ∗ Corresponding author. Present address: Institut für Metallkunde, Universität Stuttgart, Seestr. 75, D-70174 Stuttgart, Germany. Fax: +49-711-1211280. E-mail address: [email protected] (P. Zieba).
when the DD process at the reaction front of the former DP colony was in a very advanced stage. It will be shown in the present paper that after a special heat treatment the original locations of the grain boundaries are the most favourable sites for the start and development of the DD reaction in the case of a single seam morphology. Moreover, in the case of a double seam the reaction starts at the original locations of the grain boundary and is not observed at the reaction front of former DP colony. This has not been observed so far. 2. Experimental A Mg alloy containing 10 wt.% Al was prepared for the investigation by vacuum induction melting of the elements of high purity (3N5) and casting into 11 mm diameter rods. They were then homogenized at 703 K for 48 days in Duran glass capsules and water quenched. Samples of 6 mm thickness were cut off from the rods by spark erosion, and a final homogenization was applied at 703 K for 48 h, followed by a water quenching. The following procedure was applied in order to obtain the post-dissolution structures. The samples were aged at 495 K for 30 min and then the solution annealed at 695 K for 30 min, three times in succession. Then, the final ageing at 495 K for 30 min was performed, followed by annealing at 645 K for 7 min.
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 3 3 4 - 0
402
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
should note that in the present case, the applied annealing temperature of 695 K is far above the solvus one (630 K). Therefore, the dissolution of the single lamellae predominates while the reaction fronts of the former DP colonies remain essentially immobile. If such a treatment is repeated several times, then the initial grain can be refined by a factor of 3–7 [9]. However, in the present case, the final dissolu-
Fig. 1. Schematic representation of the DD at the reaction front of the discontinuous precipitates (RF/DP), the impingement of two colonies (IC), and the original location of a grain boundary (OGB). Grey areas denote α ∼ solid solution created due to the DD reaction, GB is a grain boundary, and α 0 is the supersaturated solid solution.
A standard metallographic technique was used for the sample preparation, including wet grinding, pre-polishing and “Minimet” polishing with 6 and 1 m diamond paste, using a “Nylon” polishing cloth. Prior to the optical and scanning electron microscopy the samples were etched with 3% nital. The microhardness measurements were carried out on a Fischer H100 apparatus. Five micro indentations were averaged for each hardness measurement.
3. Results and discussion Fig. 2 presents an optical micrograph of the Mg-10 wt.% Al alloy after the first ageing at 495 K for 30 min. The dark areas represent colonies of the discontinuous precipitates growing from the original location of a grain boundary. Application of dissolution annealing and then ageing for several times is usually made in order to refine the initial grain size without the need of a plastic deformation [9]. One
Fig. 2. Optical micrograph of a Mg-10 wt.% Al alloy aged at 495 K for 30 min. RF is the reaction front of the DP and OGB is the original location of a grain boundary.
Fig. 3. Optical micrographs (a–c) of a Mg-10 wt.% Al alloy annealed at 695 K for 30 min followed by 30 min at 495 K and then annealed at 645 K for 7 min. RF/DP and RF/DD are the reaction fronts of the DP and dissolution reactions, respectively. The arrows indicate the direction of the reaction front movement.
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
403
Fig. 4. Schematic representation of the DD reaction at a single seam due to (a) the commonly observed behaviour of the DD and (b) the new mode of the DD. The arrows indicate the direction of the reaction front movement.
tion heat treatment was performed at 645 K. Such a temperature ensures the DD of the discontinuous precipitates. However, the obtained image of the post-dissolution structures is completely different from the results which have been reported so far (see e.g. Refs. [1,4–6,8]). This is presented in Fig. 3a–c. Here the lamellae are being dissolved, starting from the original position of a grain boundary towards the former reaction front of the DP colony (see arrows). This mechanism is clearly seen at a single seam (Fig. 3a), and at a double seam (Fig. 3b and c) created by the so-called S-mechanism in the former DP reaction. The schematic representation of the observed behaviour and a comparison with the commonly observed morphology of the DD reaction is shown in Figs. 4 and 5. One can see that in the present case, the DD reaction starts and proceeds at the original positions of the grain boundaries. In the present case, it is necessary to keep in mind the thermal history of the material. Ageing leads to the formation of a large number of new high-angle grain boundaries, being the reaction fronts of the discontinuous precipitates. As soon as the DP started, the high-angle character of the original grain boundaries disappears. Subsequent dissolution annealing is performed at high enough temperatures so that the locations of the reaction fronts are frozen and the continuous mode of dissolution prevails. Therefore, during second ageing the DP reaction starts at the former reaction front and
proceeds in the direction towards the original location of the grain boundary. In the present study, the cycle of ageing and dissolution was repeated for four times. Therefore, the reaction front movement occurred in such a sequence: away from (ageing I)/back to (ageing II) the original location of the grain boundary, and again away from (III ageing)/back to (final ageing) the original location of the grain boundary. Thus after final ageing the reaction front of the DP stopped at the original location of the grain boundary or its immediate vicinity, being the most favourable site for the start of the DD. One should note that the DP colony does not pass the original location of the grain boundaries due to presence of an enhanced content of impurities and/or undissolved remnants of grain boundary allotriomorphs. Moreover, one has also been observed [10] that the second ageing rather leads to the formation of single precipitates or even films of the solute-rich phase at the original locations of the grain boundaries, preventing discontinuous precipitates to be nucleated at this site. The microhardness values of the matrix, the lamellar mixture and the new solid solution formed after final annealing at 645 K are the following: 64 ± 3, 81 ± 7 and 69 ± 2 HV, respectively. This indicates that the new solid solution is much harder than the original matrix. This observation is in agreement with a theoretical calculation of the solute concentration redistribution in the α ∼ solid solution which predicted
Fig. 5. Schematic representation of the DD reaction at a double seam due to (a) the commonly observed behaviour of the DD and (b) the new mode of the DD. The arrows indicate the direction of the reaction front movement.
404
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
[11] a high degree of inhomogeneity that can be as large as 15 wt.% Al.
support during his stay in Stuttgart. One of the authors (P.Z.) also appreciates support from the Alexander von Humboldt Foundation and the Polish State Committee for Scientific Research.
4. Conclusions A special cyclic heat treatment allowed to observe an unusual behaviour of the DD reaction. In the case of a single seam morphology, the reaction starts at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction starts at the reaction fronts of the former DP colonies. In the case of a double seam morphology, the proposed explanation is attributed to the characteristic movement of the reaction front of the discontinuous precipitates away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgements D. Bradai is grateful to the Max-Planck-Institut für Metallforschung and Institut für Metallkunde for the financial
[10] [11]
I. Manna, Interf. Sci. 6 (1998) 113. A. Pawlowski, Arch. Metall. 34 (1989) 459. P. Zieba, W. Gust, Int. Mater. Rev. 43 (1998) 70. W. Scharfenberger, A. Schutrumpf, H. Borchers, Metall 23 (1969) 1126. T.H. Chuang, R.A. Fournelle, W. Gust, B. Predel, Trans. Jpn. Inst. Met. 27 (1986) 669. A. Pawlowski, P. Zieba, Mater. Sci. Eng. A 108 (1989) 9. D. Bradai, P. Zieba, W. Gust, M. Hadi-Kanifi, Pract. Metallogr. 35 (1998) 673. D. Bradai, M. Kadi-Hanifi, P. Zieba, W.M. Kuschke, W. Gust, J. Mater. Sci. 34 (1999) 5331. T.H. Chuang, W. Gust, R.A. Fournelle, in: A. Blass (Ed.), Engineering and Science of Materials, Universidade Federal, Florianapolis, Santa Catarina, 1986, p. 7. P. Zieba, Ph.D. Thesis, Polish Academy of Sciences, Cracow, 1987. D. Bradai, M. Hadi-Kanifi, P. Zieba, W. Gust, Prakt. Metallogr. 35 (1998) 673.
A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy D. Bradai a , P. Zieba b,∗ , E. Bischoff c , W. Gust c a
Institut de Physique, Université des Sciences et de la Technologie Houari-Boumedienne, BP 32 El-Alia, Alger, Algeria Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta St. 25, 30-059 Cracow, Poland Max-Planck-Institut für Metallforschung and Institut für Metallkunde, Universität Stuttgart, Seestr. 92, 70174 Stuttgart, Germany b
c
Received 5 February 2001; received in revised form 26 February 2001; accepted 28 February 2001
Abstract The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10 wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mg–Al alloys; Discontinuous precipitation; Discontinuous dissolution; Grain boundary migration
1. Introduction Discontinuous dissolution (DD) is a solid-state phase transformation occurring by the backward migration of a former reaction front of a discontinuous precipitation (DP) colony [1–3]. The reaction starts at temperatures close to the solvus temperature and results in the formation of an inhomogeneous α ∼ solid solution according to the scheme: α + β → α∼ where α is the solute-depleted lamella and β is the solute-rich lamella. The DD reaction can occur at three different sites [4–6], namely the former reaction front of the DP colony, the impingement of two colonies of discontinuous precipitates and the original location of a grain boundary (Fig. 1). The Mg–Al alloys are a good example showing the common behaviour of the DD reaction [7,8]. In the case of a single seam of the DP, the reaction occurred only at the reaction front of a former DP colony. In the case of a double seam, the reaction started also at the original locations of the grain boundary ∗ Corresponding author. Present address: Institut für Metallkunde, Universität Stuttgart, Seestr. 75, D-70174 Stuttgart, Germany. Fax: +49-711-1211280. E-mail address: [email protected] (P. Zieba).
when the DD process at the reaction front of the former DP colony was in a very advanced stage. It will be shown in the present paper that after a special heat treatment the original locations of the grain boundaries are the most favourable sites for the start and development of the DD reaction in the case of a single seam morphology. Moreover, in the case of a double seam the reaction starts at the original locations of the grain boundary and is not observed at the reaction front of former DP colony. This has not been observed so far. 2. Experimental A Mg alloy containing 10 wt.% Al was prepared for the investigation by vacuum induction melting of the elements of high purity (3N5) and casting into 11 mm diameter rods. They were then homogenized at 703 K for 48 days in Duran glass capsules and water quenched. Samples of 6 mm thickness were cut off from the rods by spark erosion, and a final homogenization was applied at 703 K for 48 h, followed by a water quenching. The following procedure was applied in order to obtain the post-dissolution structures. The samples were aged at 495 K for 30 min and then the solution annealed at 695 K for 30 min, three times in succession. Then, the final ageing at 495 K for 30 min was performed, followed by annealing at 645 K for 7 min.
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 3 3 4 - 0
402
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
should note that in the present case, the applied annealing temperature of 695 K is far above the solvus one (630 K). Therefore, the dissolution of the single lamellae predominates while the reaction fronts of the former DP colonies remain essentially immobile. If such a treatment is repeated several times, then the initial grain can be refined by a factor of 3–7 [9]. However, in the present case, the final dissolu-
Fig. 1. Schematic representation of the DD at the reaction front of the discontinuous precipitates (RF/DP), the impingement of two colonies (IC), and the original location of a grain boundary (OGB). Grey areas denote α ∼ solid solution created due to the DD reaction, GB is a grain boundary, and α 0 is the supersaturated solid solution.
A standard metallographic technique was used for the sample preparation, including wet grinding, pre-polishing and “Minimet” polishing with 6 and 1 m diamond paste, using a “Nylon” polishing cloth. Prior to the optical and scanning electron microscopy the samples were etched with 3% nital. The microhardness measurements were carried out on a Fischer H100 apparatus. Five micro indentations were averaged for each hardness measurement.
3. Results and discussion Fig. 2 presents an optical micrograph of the Mg-10 wt.% Al alloy after the first ageing at 495 K for 30 min. The dark areas represent colonies of the discontinuous precipitates growing from the original location of a grain boundary. Application of dissolution annealing and then ageing for several times is usually made in order to refine the initial grain size without the need of a plastic deformation [9]. One
Fig. 2. Optical micrograph of a Mg-10 wt.% Al alloy aged at 495 K for 30 min. RF is the reaction front of the DP and OGB is the original location of a grain boundary.
Fig. 3. Optical micrographs (a–c) of a Mg-10 wt.% Al alloy annealed at 695 K for 30 min followed by 30 min at 495 K and then annealed at 645 K for 7 min. RF/DP and RF/DD are the reaction fronts of the DP and dissolution reactions, respectively. The arrows indicate the direction of the reaction front movement.
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
403
Fig. 4. Schematic representation of the DD reaction at a single seam due to (a) the commonly observed behaviour of the DD and (b) the new mode of the DD. The arrows indicate the direction of the reaction front movement.
tion heat treatment was performed at 645 K. Such a temperature ensures the DD of the discontinuous precipitates. However, the obtained image of the post-dissolution structures is completely different from the results which have been reported so far (see e.g. Refs. [1,4–6,8]). This is presented in Fig. 3a–c. Here the lamellae are being dissolved, starting from the original position of a grain boundary towards the former reaction front of the DP colony (see arrows). This mechanism is clearly seen at a single seam (Fig. 3a), and at a double seam (Fig. 3b and c) created by the so-called S-mechanism in the former DP reaction. The schematic representation of the observed behaviour and a comparison with the commonly observed morphology of the DD reaction is shown in Figs. 4 and 5. One can see that in the present case, the DD reaction starts and proceeds at the original positions of the grain boundaries. In the present case, it is necessary to keep in mind the thermal history of the material. Ageing leads to the formation of a large number of new high-angle grain boundaries, being the reaction fronts of the discontinuous precipitates. As soon as the DP started, the high-angle character of the original grain boundaries disappears. Subsequent dissolution annealing is performed at high enough temperatures so that the locations of the reaction fronts are frozen and the continuous mode of dissolution prevails. Therefore, during second ageing the DP reaction starts at the former reaction front and
proceeds in the direction towards the original location of the grain boundary. In the present study, the cycle of ageing and dissolution was repeated for four times. Therefore, the reaction front movement occurred in such a sequence: away from (ageing I)/back to (ageing II) the original location of the grain boundary, and again away from (III ageing)/back to (final ageing) the original location of the grain boundary. Thus after final ageing the reaction front of the DP stopped at the original location of the grain boundary or its immediate vicinity, being the most favourable site for the start of the DD. One should note that the DP colony does not pass the original location of the grain boundaries due to presence of an enhanced content of impurities and/or undissolved remnants of grain boundary allotriomorphs. Moreover, one has also been observed [10] that the second ageing rather leads to the formation of single precipitates or even films of the solute-rich phase at the original locations of the grain boundaries, preventing discontinuous precipitates to be nucleated at this site. The microhardness values of the matrix, the lamellar mixture and the new solid solution formed after final annealing at 645 K are the following: 64 ± 3, 81 ± 7 and 69 ± 2 HV, respectively. This indicates that the new solid solution is much harder than the original matrix. This observation is in agreement with a theoretical calculation of the solute concentration redistribution in the α ∼ solid solution which predicted
Fig. 5. Schematic representation of the DD reaction at a double seam due to (a) the commonly observed behaviour of the DD and (b) the new mode of the DD. The arrows indicate the direction of the reaction front movement.
404
D. Bradai et al. / Materials Chemistry and Physics 72 (2001) 401–404
[11] a high degree of inhomogeneity that can be as large as 15 wt.% Al.
support during his stay in Stuttgart. One of the authors (P.Z.) also appreciates support from the Alexander von Humboldt Foundation and the Polish State Committee for Scientific Research.
4. Conclusions A special cyclic heat treatment allowed to observe an unusual behaviour of the DD reaction. In the case of a single seam morphology, the reaction starts at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction starts at the reaction fronts of the former DP colonies. In the case of a double seam morphology, the proposed explanation is attributed to the characteristic movement of the reaction front of the discontinuous precipitates away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgements D. Bradai is grateful to the Max-Planck-Institut für Metallforschung and Institut für Metallkunde for the financial
[10] [11]
I. Manna, Interf. Sci. 6 (1998) 113. A. Pawlowski, Arch. Metall. 34 (1989) 459. P. Zieba, W. Gust, Int. Mater. Rev. 43 (1998) 70. W. Scharfenberger, A. Schutrumpf, H. Borchers, Metall 23 (1969) 1126. T.H. Chuang, R.A. Fournelle, W. Gust, B. Predel, Trans. Jpn. Inst. Met. 27 (1986) 669. A. Pawlowski, P. Zieba, Mater. Sci. Eng. A 108 (1989) 9. D. Bradai, P. Zieba, W. Gust, M. Hadi-Kanifi, Pract. Metallogr. 35 (1998) 673. D. Bradai, M. Kadi-Hanifi, P. Zieba, W.M. Kuschke, W. Gust, J. Mater. Sci. 34 (1999) 5331. T.H. Chuang, W. Gust, R.A. Fournelle, in: A. Blass (Ed.), Engineering and Science of Materials, Universidade Federal, Florianapolis, Santa Catarina, 1986, p. 7. P. Zieba, Ph.D. Thesis, Polish Academy of Sciences, Cracow, 1987. D. Bradai, M. Hadi-Kanifi, P. Zieba, W. Gust, Prakt. Metallogr. 35 (1998) 673.