Isothermal Section of the Al-Mn-Dy System at 500 °C

Abstract

Phase relationships in the Al-Mn-Dy ternary system at 500 °C have been investigated by X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and electron probe microanalysis. From the experimental results it was concluded that the isothermal section consists of 16 single-phase regions, 26 two-phase regions and 12 three-phase regions. Two extensive solid solutions, (Al _x Mn_1−x )₁₂Dy and (Al_1−x Mn _x )₂Dy, were observed. The solid solution (Al _x Mn_1−x )₁₂Dy forms by Al replacing Mn in Mn₁₂Dy, while the continuous solid solution (Al_1−x Mn _x )₂Dy forms by Mn and Al mutually substituting in Al₂Dy and Mn₂Dy, respectively. The maximum solid solubility of Al in Mn₁₂Dy is 79.3 at.%.

Introduction

Phase diagrams and thermodynamics provide basic knowledge for alloy design. For example, thermodynamic descriptions of multi-component Al-based alloys have been applied to alloy development.[1] Rare earth (RE) elements and transition metals play important roles in the improvement of the microstructure as well as the mechanical properties of Al-based alloys. Scandium was reported to greatly improve the properties of Al alloys due to the formation of coherent Al₃Sc.[2,3] The presence of Al₃Dy was observed to significantly reduce the microelectronic resistivity of the Al-Dy alloy thin films.[4]

A major alloying element in 3000 series commercial Al alloys is Mn. Work on the interaction of RE metals with transition metals and aluminum has been reported.[5-11] However, little work on phase equilibria of RE metals, manganese and aluminum was found in the open literature. In order to clarify the interaction of Mn and RE metals in commercial aluminum alloys, it is necessary to investigate phase equilibria in the Al-Mn-RE ternary system. Specifically, this work aims to investigate the phase equilibria in the Al-Mn-Dy system.

Five intermetallic compounds have been reported in the Al-Dy system[12] at 500 °C, i.e. Al₃Dy (rhombohedral Al₃Ho type), Al₂Dy (cubic Cu₂Mg type), AlDy (orthorhombic AlEr type), Al₂Dy₃ (tetragonal Al₂Zr₃ type) and AlDy₂ (orthorhombic Co₂Si type); three intermediate phases have been reported in the Mn-Dy system,[13] i.e., Mn₂Dy (cubic Cu₂Mg type), Mn₂₃Dy₆ (cubic Mn₂₃Th type) and Mn₁₂Dy (tetragonal Mn₁₂Th type); and six intermetallic compounds have been reported in the Al-Mn system, i.e.Al₁₂Mn (cubic Al₁₂W type), Al₆Mn (orthorhombic Al₆Mn type), λ-Al₄Mn, μ-Al₄Mn, Al₁₁Mn₄ (triclinic Al₁₁Mn₄ type), and Al₈Mn₅ (rhombohedral Al₈Cr₅ type).[14,15] The λ-Al₄Mn and μ-Al₄Mn phases are compositionally and structurally different. For example, λ-Al₄Mn is in the Al-rich side with space group P6₃/mmc, while μ-Al₄Mn is in the Al-poor side with space group Pnnn. With temperature decreasing, Al₁₁Mn₄ transforms from the high temperature structure (Space group Pnma) to the low temperature form (space group P $\bar{1}$) at 916 °C. Table 1 lists crystallographic data of compounds stable at 500 °C in Al-Mn-Dy and one can see that there are no ternary compounds in this system.

Table 1 Crystallographic data of the compounds in Al-Mn-Dy system

Experimental Procedure

The starting materials were high purity Al (99.95%), Mn (99.8%) and Dy (99.95%) (in mass%) and the total weight of each sample was 1.5 g. Specimens were melted in an arc furnace under an environment of pure Ar (99.99%). To compensate for the loss of Mn, an additional 20 wt.% Mn was added to each alloy. To guarantee homogeneity, the alloy buttons were melted at least three times. The final measured compositions were observed to slightly deviate from the nominal or synthesis composition of alloy, which shows that the loss of Mn in the alloy is compensated by the above mentioned method. After melting, the buttons were sealed in vacuum quartz tubes and then annealed at 500 °C for 30 days. The tubes were quenched into water after annealing, and then broken to recover the samples. The quenched alloys were cut into two pieces, one for x-ray examination and the other for microscopic analysis. The x-ray diffraction radiation was Cu_Kα.

Samples for x-ray diffraction were directly ground into powder, while samples for microstructure analysis were prepared by grinding and polishing. The microstructure of unetched samples was observed on the Zeiss EVO-18 scanning electronic microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Some samples were analyzed using the JEOL JXA-8230 electron probe microanalyzer (EPMA), as well. Reported compositions are the average of three measurements.

Results and Discussions

Table 2 lists the experimental results obtained from XRD and SEM/EDS analysis of 22 alloy buttons, as well as EPMA measurements on selected alloys.

Table 2 Phase identification by using XRD, SEM/EDS and EPMA for the selected Al-Mn-Dy alloys

A phase with nearly constant Dy (about 6-10 at.%) and various Mn/Al ratios was observed in alloys 7-22, as shown in Table 2. Some peaks of the x-ray pattern in these alloys are a match to Mn₁₂Dy, with slightly different lattice parameters. It was concluded that a solid solution, corresponding to (Al _x Mn_1−x )₁₂Dy, formed by Al substituting for Mn in Mn₁₂Dy in this system. The maximum solid solubility of Al in Mn₁₂Dy is 79.3 at.%, which is the average of alloys 20 and 21.

Note that both Al₂Dy and Mn₂Dy have the same cubic Cu₂Mg-type crystal structure. Similarly, phases with constant Dy content (about 30-33 at.%) and different Al/Mn ratio were observed in alloys 1-9,15-16 and 22, as listed in Table 2. The x-ray diffraction patterns of these phases were indexed by Al₂Dy or Mn₂Dy with slightly different lattice parameters. Thus it was concluded that, Al₂Dy and Mn₂Dy form a continuous solid solution, (Al_1−x Mn _x )₂Dy, in all these alloys.

Figure 1 is the backscattered electron (BSE) image and x-ray diffraction pattern of alloy 14. The light gray phase is (Al _x Mn_1−x )₁₂Dy, and the gray phase is Al₈Mn₅, while the dark needle phase is Al₁₁Mn₄. These three phases were identified by x-ray diffraction. Identification of phases in alloy 14 showed that there is a tie-triangle of [(Al _x Mn_1−x )₁₂Dy + Al₈Mn₅ + Al₁₁Mn₄] in this system.

Fig. 1

(a) BSE image of the microstructure and (b) XRD results of alloy 14 annealed at 500 °C for 30 days

The phases (Al _x Mn_1−x )₁₂Dy, Al₁₁Mn₄, and Al₄Mn were observed to co-exist in alloy 17, as shown in Fig. 2. The EDS analysis showed that the white gray phase is (Al _x Mn_1−x )₁₂Dy, and the gray phase is Al₁₁Mn₄, while the dark gray phase is Al₄Mn. The dark gray phase was identified as Al₄Mn from its lower Al content.

Fig. 2

(a) BSE image of the microstructure and (b) XRD results of alloy 17 annealed at 500 °C for 30 days

The co-existence of (Al _x Mn_1−x )₁₂Dy, Al₆Mn, and (Al) was found in alloy 19, as shown in Fig. 3. Compositions of the identified phases are listed in Table 2. X-ray diffraction and EPMA confirmed the results of SEM/EDS analysis.

Fig. 3

(a) BSE image of the microstructure and (b) XRD results of alloy 19 annealed at 500 °C for 30 days

Direct evidences to support the tie-triangles of $\left[ {\left( {\text{Al}} \right) + {\text{Al}}_{ 3} {\text{Dy}} + \left( {{\text{Al}}_{x} {\text{Mn}}_{ 1- x} } \right)_{ 1 2} {\text{Dy}}} \right],\,\left[ {{\text{Al}}_{ 3} {\text{Dy}} + \left( {{\text{Al}}_{ 1- x} {\text{Mn}}_{x} } \right)_{ 2} {\text{Dy}} + \left( {{\text{Al}}_{x} {\text{Mn}}_{ 1- x} } \right)_{ 1 2} {\text{Dy}}} \right],{\text{ and }}\left[ {{\text{Mn}}_{ 2 3} {\text{Dy}}_{ 6} + \left( {{\text{Al}}_{ 1- x} {\text{Mn}}_{x} } \right)_{ 2} {\text{Dy}} + \left( {{\text{Al}}_{x} {\text{Mn}}_{ 1- x} } \right)_{ 1 2} {\text{Dy}}} \right]$ are shown in Fig. 4, 5 and 6.

Fig. 4

BSE image of the microstructure of alloy 21, the dark phase is Al, the phase grey with point dispersed is Al₃Dy, the light gray phase is (Al _x Mn_1−x )₁₂Dy

Fig. 5

BSE image of the microstructure of alloy 16, the gray phase is Al₃Dy, the light gray phase is (Al_1−x Mn _x )₂Dy, and the dark gray matrix phase is (Al _x Mn_1−x )₁₂Dy

Fig. 6

BSE image of the microstructure of alloy 9, the gray phase is Mn₂₃Dy₆, the light gray phase is (Al_1−x Mn _x )₂Dy, and the matrix phase is (Al _x Mn_1−x )₁₂Dy

According to the Al-Dy phase diagram,[12] Al₂Dy₃ is formed by the peritectic reaction AlDy + Liquid → Al₂Dy₃, but Al₂Dy₃ was not observed in the current work. It is most likely that the phase does not form during rapid cooling following melting. Although AlDy and AlDy₂ will not co-exist when the alloy is in equilibrium, the co-existence of AlDy, AlDy₂ and (Al_1−x Mn _x )₂Dy is observed in alloy 1, as shown in Table 2. That means that alloy 1 is off-equilibrium.

Phases with ~64 at.% Dy in alloys 1-6 were proven by XRD to be AlDy₂ rather than Al₂Dy₃. The crystal structure of Al₂Dy₃ was determined before in Ref 26 and 27. The Al₂Gd₃ type compounds Al₂Ho₃ and Al₂Er₃ form in the Al-Ho and Al-Er systems,[26,40] respectively. These are examples of the similarity between heavy RE metal compounds. Also, Al₂Dy₃ was observed in the determination of the Al-Mo-Dy ternary phase diagram at 600 °C.[5] Finally, Al₂Dy₃ appears stable below 600 °C according to the Al-Dy phase diagram in Ref 12. Thus, it is concluded that Al₂Dy₃ is stable at 500 °C.

The similar compound, Al₂Ho_3, forms by the peritectic reaction AlHo + liquid → Al₂Ho₃ at 994 °C.[41] Also, Al₂Ho₃ was confirmed to exist at 500 °C in the Al-V-Ho system.[42] The Al-V-Ho alloys were heated at 900 °C for 20 days to guarantee homogeneity before the 500 °C final heat treatment. The temperature of 900 °C was high enough for diffusion to enable the formation of Al₂Ho₃. In this work, alloys were kept at 500 °C and had no high temperature anneal. Apparently 500 °C is too low for sufficient diffusion to occur. Therefore it is assumed that Al₂Dy₃ is a stable phase at 500 °C and should be included in Fig. 7, although it was absent in the current work.

Fig. 7

The phase diagram of the Al-Mn-Dy system at 500 °C

In the Al-Dy system,[12] the solubility of Al in (Dy) is negligible. A large amount of Al was detected in (Dy) in alloys 3, which shows that this alloy is off-equilibrium. Contents of Al in (Dy) in alloys 4-6 are smaller than that in alloy 3, which suggests that these alloys are closer to equilibrium.

Based on data available in Table 2, the phase diagram of the Al-Mn-Dy system was drawn, as shown in Fig. 7. It should be noted that the tie-triangles with solid lines are directly supported by the equilibrated alloys, and those with dash lines were deduced from the surrounding phase relationships.

The maximum solubility of Mn in Al₃Dy is 4.8 at.%, which was observed in alloy 16. Thus, the composition at the Al₃Dy corner of the tie-triangle $\left[ {{\text{Al}}_{ 3} {\text{Dy}} + \left( {{\text{Al}}_{ 1- x} {\text{Mn}}_{x} } \right)_{ 2} {\text{Dy}} + \left( {{\text{Al}}_{x} {\text{Mn}}_{ 1- x} } \right)_{ 1 2} {\text{Dy}}} \right]$ is 72.1 at.% Al, 4.8 at.% Mn and 23.1 at.% Dy.

The maximum solubility of Mn in AlDy₂ is 5.7 at.%, which was observed in alloy 6. Thus the composition of the AlDy₂ corner in the tie-triangle $[{\text{AlDy}}_{ 2} + \left( {{\text{Al}}_{ 1- x} {\text{Mn}}_{x} } \right)_{ 2} {\text{Dy}} + \left( {\text{Dy}} \right)]$ is 26.4 at.% Al, 5.7 at.% Mn and 67.9 at.% Dy.

The maximum solubility of Al in Mn₂₃Dy₆ is 10.7 at.%, which was observed in alloy 9. Thus the composition of the Mn₂₃Dy₆ corner of the tie-triangle $\left[ {{\text{Mn}}_{ 2 3} {\text{Dy}}_{ 6} + \left( {{\text{Al}}_{ 1- x} {\text{Mn}}_{x} } \right)_{ 2} {\text{Dy}} + \left( {{\text{Al}}_{x} {\text{Mn}}_{ 1- x} } \right)_{ 1 2} {\text{Dy}}} \right]$ is 10.7 at.% Al, 70.9 at.% Mn and 18.4 at.% Dy.

For the Al-rich alloy, the annealing temperature of 500 °C seems high enough for sufficient Al diffusion to achieve equilibrium, since the Al melting point is only 660 °C. The phase boundary in alloy 19 (Fig. 3) is very clear suggesting equilibrium. Results from alloy 19 are consistent with the previous experimental investigation of the Al-Mn phase diagram,[14, 15, 43, 44] which show that only Al₆Mn exists in the Al-rich corner at 500 °C, and disagrees with the thermodynamic description of the Al-Mn system.[45–47]

In addition, Gordillo et al.[6] studied the effect of heat treatment on an Al-5Mn-2Ce (at.%) alloy by using x-ray diffraction and electron microscopy. The master alloy was prepared by powder processing and extruding. The extrudate contained a mixture of fcc Al, Al₂₀Mn₂Ce, Al₆Mn and a small amount of Al₁₂Mn and Al₁₁Ce₃. After heat treatment at 450 °C for 48 h, the Al₂₀Mn₂Ce and Al₆Mn phases decomposed completely and the volume fraction of Al₁₂Mn increased to 72-73 vol.%. Heat treatment at 500 °C for 48 h resulted in the decrease of Al₁₂Mn volume fraction to 9 vol.%, while amounts of each phase in the Al-5Mn-2Ce alloy becomes 60 vol.% fcc Al, 22 vol.% Al₃(Mn,Ce), 8 vol.% Al₆Mn and 1 vol.% Al₁₁Ce₃. After prolonged annealing for 30 days in our work, the Al₁₂Mn phase was not observed. These results suggest that Al₁₂Mn is metastable at 500 °C, and that the temperature of the peritectoid reaction Al + Al₆Mn → Al₁₂Mn is below 500 °C. That is lower than the reaction temperature of 507 °C predicted in the assessment work of Jansson.[45] Thus, Al₁₂Mn is not included in the Al-rich corner in Fig. 7. In addition, it is hard to distinguished λ-Al₄Mn from μ-Al₄Mn. Therefore these two phases are treated as single Al₄Mn in this work.

Summary

• 1. The ternary phase equilibria relationships in the Al-Mn-Dy system at 500 °C have been established by XRD, SEM/EDS combining with EPMA techniques and deduced from the surrounding phase relationships. There are 12 tie-triangles in this system, of which five were measured and seven were deduced.

• 2. Two extensive solid solutions were found in this system. One was a continuous solid solution between Al₂Dy and Mn₂Dy to form (Al_1−x Mn _x )₂Dy. In the other solid solution, Al substitutes for Mn in Mn₁₂Dy to form (Al _x Mn_1−x )₁₂Dy. The maximum solubility of Al in Mn₁₂Dy is 79.3 at.%.

• 3. Al₂Dy₃ was not observed in the current work, which is assumed to be caused by difficulty forming Al₂Dy₃ both by the peritectic reaction AlDy + Liquid → Al₂Dy₃ on cooling and by heat treatment at 500 °C.