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Peculiarities of the phase relations in Mg-rich alloys of the Mg–Nd–Y system
Journal of Alloys and Compounds 367 (2004) 17–19
Peculiarities of the phase relations in Mg-rich alloys of the Mg–Nd–Y system L.L. Rokhlin∗ , T.V. Dobatkina, I.E. Tarytina, V.N. Timofeev, E.E. Balakhchi Baikov Institute of Metallurgy and Material Science, Russian Academy of Science, Leninsky prospect 49, 119991 GSP-1, Moscow, Russia
Abstract The decomposition at 250 ◦ C of the Mg supersaturated solid solution in the ternary Mg–Nd–Y system and in binary Mg–Nd and Mg–Y alloys was studied. The kinetics of precipitation in ternary Mg–Nd–Y alloys differs from the kinetics of precipitation in binary Mg–Nd and Mg–Y alloys. Precipitates with the same crystal structure form in a shorter ageing time in ternary Mg–Nd–Y alloys than in binary Mg–Y alloys. The change of volume and the kinetics of solid solution decomposition in the ternary Mg–Nd–Y alloys as compared with the binary ones suggest that there is a certain solubility of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Rare-earth metals; Solid solution decomposition; Phase relations
1. Introduction Magnesium alloys containing both neodymium and yttrium are interesting as light structural materials with high strength properties at near room and elevated temperatures [1,2]. The high strength properties of these alloys are related to particular characteristics of the Mg–Nd–Y phase diagram. The Mg–Nd–Y phase diagram was studied in the Mg-rich region [3]. It is characterized by the existence of binary compounds of the systems Mg–Nd and Mg–Y in equilibrium with the Mg solid solution (Fig. 1). No other phase was observed in equilibrium with the Mg solid solution. The existence of solubility of Nd in the equilibrium phase Mg24 Y5 and solubility of Y in the equilibrium phase1 Mg41 Nd5 was assumed in [3]. This is in agreement with the chemical similarity of the two rare-earth metals and their ability to form continuous solid solutions with each other, and is supported by the trends of the tie-lines in the Mg corner. In this work, this was confirmed from the investigation of the Mg solid solution decomposition in ternary alloys. Such decomposition is possible, since the solubility of both neodymium and yttrium in solid Mg decreases with decreasing temperature. ∗
Corresponding author. E-mail address: [email protected] (L.L. Rokhlin). 1 Formula of the Mg-rich compound in the Mg–Nd system is assumed according to [4]. 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.08.004
The results of this investigation are interesting, moreover, because the highest strength properties of the alloys are reached when this process occurs.
2. Materials and methods Two ternary alloys, Mg–3% Nd–4.5% Y, Mg–3% Nd–8.5% Y, and three binary alloys, Mg–3% Nd, Mg–4.5% Y, Mg–8.5% Y were chosen for investigation. The alloys were prepared by melting in an electrical resistance furnace in steel crucibles under flux in order to prevent burning of the melts. The flux consisted of chlorides and fluorides of alkali and alkali–earth metals in appropriate proportions (38–46% MgCl2 , 32–40% KCl, 3–5% CaF2 , 5–8% BaCl2 , 1.5% MgO, <8% (NaCl + CaCl2 )). Metals of high purity were used as starting materials, Mg 99.96%, Nd 99.85% and Y 99.87%. During melting, neodymium and yttrium were added as master alloys, Mg–33.4% Nd and Mg–35.1% Y, prepared beforehand. In equilibrium conditions, the alloys prepared were within the limits of the Mg solid solution area at high temperatures and outside this area at low temperatures. The alloys were cast into steel moulds. The ingots obtained were about 15 mm in diameter and 90 mm long. They were cut into pieces for the investigation. The specimens were solution-treated by heating at 510 ◦ C for 6 h, followed by quenching in cold water. They were then isothermally aged at 250 ◦ C for up to 128 h.
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L.L. Rokhlin et al. / Journal of Alloys and Compounds 367 (2004) 17–19
Fig. 1. Isothermal section of the Mg–Nd–Y phase diagram at 500 ◦ C, from [3].
The process of the Mg solid solution decomposition during ageing was checked by hardness and electrical resistivity measurements. Hardness was measured using the Brinel method with a 250 kg load and an indenting steel ball of 5 mm in diameter. The hardness values are presented in kg/mm2 . Electrical resistivity was measured by a compensation method using equipment that made it possible to obtain a voltage sensitivity of ±0.05 mV. Taking into account the errors in the determination of the specimen dimensions and instrumental errors, the overall error of the electrical resistivity determination was estimated to ±0.7%. The electrical resistivity measurement enabled us to check depletion of the supersaturated solid solution. As the electrical resistivity decreases during ageing, the solid solution is depleted. The hardness changes during the decomposition of the supersaturated solid solution, usually following a curve with a maximum. The height and place of the hardness maximum during ageing enables us to estimate the kinetics of the solid solution decomposition and formation of precipitates. The structure of the alloys was studied using a transmission electron microscopy with JEM-1000 equipment from Jeol (Japan). The thin foils for investigation were prepared by thinning small plates cut from the specimens. Thinning was performed in a 20% solution of nitric acid in alcohol. The structure of the alloys and the electron diffraction patterns were observed using an accelerating voltage of 500 kV.
Fig. 2. Change of hardness during ageing at 250 ◦ C.
relative changes of these properties with respect to the low hardness and electrical resistivity of Mg–3% Nd as cast, the observed effects appear to be quite significant. This is not the case for the alloys Mg–4.5% Y and Mg–8.5% Y. In these
3. Results and discussion The results of the hardness and electrical resistivity measurements at 250 ◦ C are presented in Figs. 2 and 3. They show typical resistivity and hardness curves for the decomposition of the supersaturated solid solution, but with some peculiarities for every alloy. The alloy Mg–3% Nd shows the lowest electrical resistivity and hardness after quenching. In contrast, the alloy with 3% Nd + 8.5% Y showed the highest hardness and electrical resistivity values after quenching. Other alloys showed intermediate values of hardness and electrical resistivity in correlation with the contents of the alloying elements. All binary alloys showed small absolute changes of the properties during ageing. However, in the case of the alloy Mg–3% Nd these small effects should be compared with the initially low values. Considering the
Fig. 3. Change of electrical resistivity during ageing at 250 ◦ C.
L.L. Rokhlin et al. / Journal of Alloys and Compounds 367 (2004) 17–19
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Fig. 4. Microstructure and diffraction pattern of a Mg–3% Nd–8.5% Y alloy (after ageing at 250 ◦ C, 8 h).
alloys, the relative changes of the properties are quite weak. This is particularly apparent if the hardness of the alloys Mg–3% Nd and Mg–4.5% Y is compared. The behavior of the ternary alloys during ageing is quite unusual. The effects differ from those observed in the binary ones. A greater quantity of alloying elements is precipitated from the solid solution during ageing in the ternary alloys than in the binary ones. Another remarkable difference between the ternary and binary alloys is the different kinetics of the solid solution decomposition. In the ternary alloys the solid solution decomposition runs more rapidly than in the corresponding binary Mg–Y alloys, but slower than in the corresponding binary Mg–Nd alloy. In this respect, the hardness maximum of the alloy Mg–3% Nd–8.5% Y is the most impressive. The maximum value is significantly higher than the hardness maxima of both the Mg–3% Nd and the Mg–8.5% Y alloys. The hardness maximum for the alloy Mg–3% Nd–8.5% Y is reached after a somewhat longer ageing time than that required for the alloy Mg–3% Nd. For the alloy Mg–8.5% Y, the hardness maximum is reached after a significantly longer time (in the longest ageing time we used it was not reached at all). The slight increase of hardness of the alloy Mg–8.5% Y at the initial ageing time corresponds to the preliminary stage of the solid solution decomposition [5] and is not accompanied by any essential decrease of electrical resistivity. In general, the results obtained here agree with the results of other investigations [6]. Structure investigations showed that the hardness maximum during the ageing of the ternary alloy Mg–3% Nd–8.5% Y resulted from the formation of fine plate-like precipitates in the Mg solid solution. Between the precipitates and the Mg solid solution matrix there existed coherency, which was revealed in the specific “ripple” aspect of the image and the close correspondence between matrix reflections and superstructure reflections of the precipitates. A typical micrograph and an electron diffraction pattern from the alloy Mg–3% Nd–8.5% Y after ageing to reach the hardness maximum are shown in Fig. 4. Analysis of
the electron diffraction patterns showed that the precipitates in the ternary alloy had the same crystal structure as the precipitates formed during solid solution decomposition in binary Mg–Y alloys [5]. The significant changes of the volume and the decomposition kinetics of the Mg supersaturated solid solution can only be explained by the dissolution of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates.
4. Summary Decomposition of Mg supersaturated solid solution in ternary Mg–Nd–Y and binary Mg–Nd and Mg–Y alloys was compared. In the ternary alloys the volume of precipitation during ageing becomes larger. The kinetics of precipitation in the ternary Mg–Nd–Y alloys differ from the kinetics of precipitation in the corresponding binary Mg–Nd and Mg–Y alloys. Precipitates with the same crystal structure form in a shorter ageing time in ternary Mg–Nd–Y alloys than in binary Mg–Y alloys. The change of volume and the kinetics of solid solution decomposition in the ternary Mg–Nd–Y alloys as compared with the binary ones suggests solubility of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates.
References [1] W. Unsworth, Met. Mater. 4 (2) (1988) 83–86. [2] M.E. Drits, L.L. Rokhlin, E.M. Padezhnova, I.I. Gurev, N.V. Miklina, T.V. Dobatkina, A.A. Oreshkina, Magnievye Splavys Ittriem (Magnesium Alloys with Yttrium), Nauka, Moscow, 1979 (in Russian). [3] Z.A. Sviderskaya, E.M. Padezhnova, Izv. Akad. Nauk SSSR, Metally, (6) (1971) 200–204 (in Russian). [4] S. Delfino, A. Saccone, R. Ferro, Metall. Trans. 21A (8) (1990) 2109– 2114. [5] M.E. Drits, L.L. Rokhlin, I.E. Tarytina, Izv. Akad. Nauk SSSR, Metally, (3) (1983) 111–116 (in Russian). [6] P. Vostry, I. Stulikova, B. Smola, M. Cieslar, B.L. Mordike, Z. Metallkd. 79 (5) (1988) 340–344.
Peculiarities of the phase relations in Mg-rich alloys of the Mg–Nd–Y system L.L. Rokhlin∗ , T.V. Dobatkina, I.E. Tarytina, V.N. Timofeev, E.E. Balakhchi Baikov Institute of Metallurgy and Material Science, Russian Academy of Science, Leninsky prospect 49, 119991 GSP-1, Moscow, Russia
Abstract The decomposition at 250 ◦ C of the Mg supersaturated solid solution in the ternary Mg–Nd–Y system and in binary Mg–Nd and Mg–Y alloys was studied. The kinetics of precipitation in ternary Mg–Nd–Y alloys differs from the kinetics of precipitation in binary Mg–Nd and Mg–Y alloys. Precipitates with the same crystal structure form in a shorter ageing time in ternary Mg–Nd–Y alloys than in binary Mg–Y alloys. The change of volume and the kinetics of solid solution decomposition in the ternary Mg–Nd–Y alloys as compared with the binary ones suggest that there is a certain solubility of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Rare-earth metals; Solid solution decomposition; Phase relations
1. Introduction Magnesium alloys containing both neodymium and yttrium are interesting as light structural materials with high strength properties at near room and elevated temperatures [1,2]. The high strength properties of these alloys are related to particular characteristics of the Mg–Nd–Y phase diagram. The Mg–Nd–Y phase diagram was studied in the Mg-rich region [3]. It is characterized by the existence of binary compounds of the systems Mg–Nd and Mg–Y in equilibrium with the Mg solid solution (Fig. 1). No other phase was observed in equilibrium with the Mg solid solution. The existence of solubility of Nd in the equilibrium phase Mg24 Y5 and solubility of Y in the equilibrium phase1 Mg41 Nd5 was assumed in [3]. This is in agreement with the chemical similarity of the two rare-earth metals and their ability to form continuous solid solutions with each other, and is supported by the trends of the tie-lines in the Mg corner. In this work, this was confirmed from the investigation of the Mg solid solution decomposition in ternary alloys. Such decomposition is possible, since the solubility of both neodymium and yttrium in solid Mg decreases with decreasing temperature. ∗
Corresponding author. E-mail address: [email protected] (L.L. Rokhlin). 1 Formula of the Mg-rich compound in the Mg–Nd system is assumed according to [4]. 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.08.004
The results of this investigation are interesting, moreover, because the highest strength properties of the alloys are reached when this process occurs.
2. Materials and methods Two ternary alloys, Mg–3% Nd–4.5% Y, Mg–3% Nd–8.5% Y, and three binary alloys, Mg–3% Nd, Mg–4.5% Y, Mg–8.5% Y were chosen for investigation. The alloys were prepared by melting in an electrical resistance furnace in steel crucibles under flux in order to prevent burning of the melts. The flux consisted of chlorides and fluorides of alkali and alkali–earth metals in appropriate proportions (38–46% MgCl2 , 32–40% KCl, 3–5% CaF2 , 5–8% BaCl2 , 1.5% MgO, <8% (NaCl + CaCl2 )). Metals of high purity were used as starting materials, Mg 99.96%, Nd 99.85% and Y 99.87%. During melting, neodymium and yttrium were added as master alloys, Mg–33.4% Nd and Mg–35.1% Y, prepared beforehand. In equilibrium conditions, the alloys prepared were within the limits of the Mg solid solution area at high temperatures and outside this area at low temperatures. The alloys were cast into steel moulds. The ingots obtained were about 15 mm in diameter and 90 mm long. They were cut into pieces for the investigation. The specimens were solution-treated by heating at 510 ◦ C for 6 h, followed by quenching in cold water. They were then isothermally aged at 250 ◦ C for up to 128 h.
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L.L. Rokhlin et al. / Journal of Alloys and Compounds 367 (2004) 17–19
Fig. 1. Isothermal section of the Mg–Nd–Y phase diagram at 500 ◦ C, from [3].
The process of the Mg solid solution decomposition during ageing was checked by hardness and electrical resistivity measurements. Hardness was measured using the Brinel method with a 250 kg load and an indenting steel ball of 5 mm in diameter. The hardness values are presented in kg/mm2 . Electrical resistivity was measured by a compensation method using equipment that made it possible to obtain a voltage sensitivity of ±0.05 mV. Taking into account the errors in the determination of the specimen dimensions and instrumental errors, the overall error of the electrical resistivity determination was estimated to ±0.7%. The electrical resistivity measurement enabled us to check depletion of the supersaturated solid solution. As the electrical resistivity decreases during ageing, the solid solution is depleted. The hardness changes during the decomposition of the supersaturated solid solution, usually following a curve with a maximum. The height and place of the hardness maximum during ageing enables us to estimate the kinetics of the solid solution decomposition and formation of precipitates. The structure of the alloys was studied using a transmission electron microscopy with JEM-1000 equipment from Jeol (Japan). The thin foils for investigation were prepared by thinning small plates cut from the specimens. Thinning was performed in a 20% solution of nitric acid in alcohol. The structure of the alloys and the electron diffraction patterns were observed using an accelerating voltage of 500 kV.
Fig. 2. Change of hardness during ageing at 250 ◦ C.
relative changes of these properties with respect to the low hardness and electrical resistivity of Mg–3% Nd as cast, the observed effects appear to be quite significant. This is not the case for the alloys Mg–4.5% Y and Mg–8.5% Y. In these
3. Results and discussion The results of the hardness and electrical resistivity measurements at 250 ◦ C are presented in Figs. 2 and 3. They show typical resistivity and hardness curves for the decomposition of the supersaturated solid solution, but with some peculiarities for every alloy. The alloy Mg–3% Nd shows the lowest electrical resistivity and hardness after quenching. In contrast, the alloy with 3% Nd + 8.5% Y showed the highest hardness and electrical resistivity values after quenching. Other alloys showed intermediate values of hardness and electrical resistivity in correlation with the contents of the alloying elements. All binary alloys showed small absolute changes of the properties during ageing. However, in the case of the alloy Mg–3% Nd these small effects should be compared with the initially low values. Considering the
Fig. 3. Change of electrical resistivity during ageing at 250 ◦ C.
L.L. Rokhlin et al. / Journal of Alloys and Compounds 367 (2004) 17–19
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Fig. 4. Microstructure and diffraction pattern of a Mg–3% Nd–8.5% Y alloy (after ageing at 250 ◦ C, 8 h).
alloys, the relative changes of the properties are quite weak. This is particularly apparent if the hardness of the alloys Mg–3% Nd and Mg–4.5% Y is compared. The behavior of the ternary alloys during ageing is quite unusual. The effects differ from those observed in the binary ones. A greater quantity of alloying elements is precipitated from the solid solution during ageing in the ternary alloys than in the binary ones. Another remarkable difference between the ternary and binary alloys is the different kinetics of the solid solution decomposition. In the ternary alloys the solid solution decomposition runs more rapidly than in the corresponding binary Mg–Y alloys, but slower than in the corresponding binary Mg–Nd alloy. In this respect, the hardness maximum of the alloy Mg–3% Nd–8.5% Y is the most impressive. The maximum value is significantly higher than the hardness maxima of both the Mg–3% Nd and the Mg–8.5% Y alloys. The hardness maximum for the alloy Mg–3% Nd–8.5% Y is reached after a somewhat longer ageing time than that required for the alloy Mg–3% Nd. For the alloy Mg–8.5% Y, the hardness maximum is reached after a significantly longer time (in the longest ageing time we used it was not reached at all). The slight increase of hardness of the alloy Mg–8.5% Y at the initial ageing time corresponds to the preliminary stage of the solid solution decomposition [5] and is not accompanied by any essential decrease of electrical resistivity. In general, the results obtained here agree with the results of other investigations [6]. Structure investigations showed that the hardness maximum during the ageing of the ternary alloy Mg–3% Nd–8.5% Y resulted from the formation of fine plate-like precipitates in the Mg solid solution. Between the precipitates and the Mg solid solution matrix there existed coherency, which was revealed in the specific “ripple” aspect of the image and the close correspondence between matrix reflections and superstructure reflections of the precipitates. A typical micrograph and an electron diffraction pattern from the alloy Mg–3% Nd–8.5% Y after ageing to reach the hardness maximum are shown in Fig. 4. Analysis of
the electron diffraction patterns showed that the precipitates in the ternary alloy had the same crystal structure as the precipitates formed during solid solution decomposition in binary Mg–Y alloys [5]. The significant changes of the volume and the decomposition kinetics of the Mg supersaturated solid solution can only be explained by the dissolution of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates.
4. Summary Decomposition of Mg supersaturated solid solution in ternary Mg–Nd–Y and binary Mg–Nd and Mg–Y alloys was compared. In the ternary alloys the volume of precipitation during ageing becomes larger. The kinetics of precipitation in the ternary Mg–Nd–Y alloys differ from the kinetics of precipitation in the corresponding binary Mg–Nd and Mg–Y alloys. Precipitates with the same crystal structure form in a shorter ageing time in ternary Mg–Nd–Y alloys than in binary Mg–Y alloys. The change of volume and the kinetics of solid solution decomposition in the ternary Mg–Nd–Y alloys as compared with the binary ones suggests solubility of Y in the Nd-rich precipitates and of Nd in the Y-rich precipitates.
References [1] W. Unsworth, Met. Mater. 4 (2) (1988) 83–86. [2] M.E. Drits, L.L. Rokhlin, E.M. Padezhnova, I.I. Gurev, N.V. Miklina, T.V. Dobatkina, A.A. Oreshkina, Magnievye Splavys Ittriem (Magnesium Alloys with Yttrium), Nauka, Moscow, 1979 (in Russian). [3] Z.A. Sviderskaya, E.M. Padezhnova, Izv. Akad. Nauk SSSR, Metally, (6) (1971) 200–204 (in Russian). [4] S. Delfino, A. Saccone, R. Ferro, Metall. Trans. 21A (8) (1990) 2109– 2114. [5] M.E. Drits, L.L. Rokhlin, I.E. Tarytina, Izv. Akad. Nauk SSSR, Metally, (3) (1983) 111–116 (in Russian). [6] P. Vostry, I. Stulikova, B. Smola, M. Cieslar, B.L. Mordike, Z. Metallkd. 79 (5) (1988) 340–344.