Comparison Study of Microstructure and Phase Evolution in Mg-Nd- and Mg-Gd-Based Alloys

Abstract

Microstructure and phase evolution in Mg-Nd-, Mg-Gd-, and Mg-Gd-Nd-based alloys with additions of Zn, Zr, and Y were analyzed in the as-cast, solution-treated and aged conditions. Close similarity between the as-cast microstructures and precipitation sequence during aging was revealed. Along with this, distinct features of eutectic compounds and of crystal structure, composition, and orientation relationships of β″, β′, and β ₁ phases were established. Formation of Zn₂Zr₃ rods in Mg-Nd-based alloy and cuboid-shaped particles in Mg-Gd- and Mg-Gd-Nd-based alloys is discussed. The features of the age hardening curves were connected with differences between β″ → β′ transformation and different diffusivities of Gd and Nd in Mg-matrix.

1 Introduction

Magnesium alloys containing rare earth (RE) elements have received extensive attention in recent years because of their excellent mechanical properties, especially high-temperature creep resistance, light weight, and good die-castability.[1–10] The high solid solubility of the RE elements in Mg matrix at high temperature and its substantial decrease with temperature result in remarkable age hardening response during isothermal aging at temperatures ranging from 448 to 473 K (175 to 200 °C). Among the Mg-RE systems, the Mg-Nd-, Mg-Gd-, and Mg-Gd-Nd-based alloys with small additions of Zn, Zr, and Y were believed to have considerable promising potential. While these systems have been extensively investigated, a combined effect of alloying elements on the phase evolution and the precipitation sequence is not fully understood. Microstructural characterization of the Mg-RE alloys has been performed by many researchers.[3–16] It is generally agreed that precipitation process consists of three main stages: [α-Mg supersaturated solid solution (S.S.S.S.) → β″(DO₁₉) → β′(bco) → β(fcc)], which was recently modified by Nie and Muddle[2] to a four-stage precipitation sequence with another intermediate phase β ₁(fcc) forming between β′ and β phases and further confirmed by other researchers.[4,5,11,12] Among the four precipitated phases, the coherent β″ and coherent/semicoherent β′ phases are considered to be the primary strengthening phases.[4,6,10,12] The yield strength or hardness usually reaches a maximum, as the materials form a microstructure with fine β′ precipitates during aging.[3–6] The characteristics of these precipitates (coherency, shape, size, and density) play a significant role in the hardening effect. Detailed investigation of the phase composition and crystallographic orientation of precipitates in Mg-Nd alloys with small additions of Zn and Zr was performed by Atiya et al.[17–20] The investigation was continued on the Mg-Gd- and Mg-Gd-Nd-based alloys[21–23] to understand the combined effects of Nd and Gd on the precipitation sequence, the crystal orientation relationship (OR) of precipitates, exact chemical composition of the metastable phases β″, β′, and β ₁ and explain the corresponding microhardness curves in Mg-RE-based alloys.

2 Experimental

Alloys with the chemical compositions of Mg-3.1Nd-0.45Zr-0.25Zn (wt pct) were prepared by the Dead Sea Magnesium Research Institute (MRI), Mg-5Gd-0.3Zn-0.18Y-0.15Zr and Mg-6Gd-3.7Nd-0.3Zn-0.18Y-0.15Zr (wt pct) were prepared by the Technion Research & Development Foundation (TRDF). The samples were encapsulated in quartz tubes under 300 mmHg argon atmosphere and were solution treated (ST) for 24 hours at 813 K (540 °C) followed by water quenching at 298 K (25 °C). The treatment included heating to 498 K (225 °C) (heating rate of 10 °C/hour), then heating to 813 K (540 °C) at a rate of 15 °C/hour, followed by water quenching at 298 K (25 °C). Then, the samples were aged in an oil bath at 448 K (175 °C) for up to 125 days. Phase analysis was carried out using X-ray diffractometer (XRD), the microstructure was examined using optical microscopy FEI Scanning Electron Microscope (SEM), Quanta 200, and Ultra plus FEG SEM, and EDS Analysis was conducted on TEM samples with a special specimen holder to increase the reliability and ensure that the photons are scattered from a specific analyzed region. Vickers Hardness testing was performed using 50 grams load and holding time of 15 seconds. Precipitate microstructures were examined in FEI Tecnai G2 T20 S-Twin (TEM) and FEI Titan 80-300 FEG-S/TEM (HRTEM).

3 Microstructure and Phase Composition of Mg-Nd-Based Alloy

The microstructure of the Mg-3.1Nd-0.45Zr-0.25Zn (wt pct) alloy in as-cast condition (Figure 1) consists of α-Mg matrix, eutectic compounds and Zr-rich particles.[17–20]

Fig. 1

(a) Optical micrograph and (b) BSE SEM micrograph of the Mg-3.1Nd-0.45Zr-0.25Zn alloy showing as-cast microstructure

It was revealed by SEM EDS analysis performed on the TEM sample that Zn atoms substitute Mg atoms in the eutectic compound (~2.6 at. pct). After solution treatment for 24 hours at 813 K (540 °C), only α-Mg phase was recognized from XRD data (Figure 2).[18]

Fig. 2

(a) XRD patterns of the Mg-3.1Nd-0.45Zr-0.25Zn as-cast alloy; (b) TEM BFI and corresponding SAED of the eutectic compound zone axis $[001]_{{{\text{Mg}}_{12} {\text{Nd}}}}$

However, detailed SEM and TEM investigations revealed residual eutectic compounds and Zn₂Zr₃ particles that are gathered in clusters. The aging of solution-treated samples was performed at 448 K (175 °C) up to 32 days. The microhardness increases and reaches the maximum value of about 80 HV after 8 days (Figure 3) and then decreases with the aging time.

Fig. 3

Microhardness of the Mg-3.1Nd-0.45Zr-0.25Zn alloy as a function of time during isothermal aging at 448 K (175 °C)

The peak-aged sample was investigated by TEM and HRTEM (Figures 4 and 5).

Fig. 4

(a) BF TEM micrograph of β″ precipitates in the Mg matrix after 8 days of aging and (b) corresponding SAED $[\overline{2} 110](0001)_{\text{Mg}} {\parallel} [\overline{2} 110](0001)_{{\beta^{\prime \prime } }}$

Fig. 5

(a) HRTEM micrograph of β″ precipitates in the Mg matrix and (b) corresponding fast Fourier transform (FFT) $[\overline{1} 010](\overline{1} 2\overline{1} 0)_{\text{Mg}} {\parallel} [\overline{1} 010](\overline{1} 2\overline{1} 0)_{{\beta^{\prime \prime } }}$

TEM micrograph and corresponding SAED from grain interior indicate the presence of metastable β″ (Mg₃Nd)_HCP (DO₁₉ structure) precipitates. The precipitates have a plate-like shape and are fully coherent with the Mg matrix, with the orientation relationship (OR) [$\overline{2} 110(0001)_{\rm Mg}{\parallel}[\overline{2} 110](0001)_{{\beta^{\prime \prime } }}$ and [$[\overline{1} 010](\overline{1} 2\overline{1} 0)_{\rm Mg} {\parallel} [\overline{1} 010](\overline{1} 2\overline{1} 0)_{{\beta^{\prime \prime } }} .$ HRTEM image along $[\overline{1} 010]_{\rm Mg}$ zone axis (Figure 5) reveals β″ plate precipitate that is fully coherent with the Mg matrix.

The Zn₂Zr₃ rods, distributed in the grain interior, serve as additional nucleation sites for precipitates. The tetragonal Zn₂Zr₃ particles (P4₂/mnm: a = 0.768 nm and c = 0.699 nm), which seem to have a near octagon cross section (Figure 6), are elongated along [001] direction, have an orientation relationship with the Mg matrix [$[\overline{2} 110](0001)_{\rm Mg} {\parallel} [001](110)_{{{\rm Zn}_{2} {\rm Zr}_{3} }},$ and appear to be stable in the Mg matrix.

Fig. 6

BF TEM micrograph of Zn₂Zr₃ rod particle elongated to [001] direction and corresponding SAED of Mg and Zn₂Zr₃ with the OR $[\overline{2} 110](0001)_{\text{Mg}} {\parallel} [001](110)_{{{\text{Zn}}_{2} {\text{Zr}}_{3} }}$

Nd-rich plate-like precipitates nucleate on the side and basal planes of the Zn₂Zr₃ as identified by EDS mounted on the TEM (Figure 7).[19] It is accompanied by the formation of precipitation-free zones (PFZs) near Zn₂Zr₃ rod particles. These plate-like precipitates were identified by SAED as β ₁(Mg₃Nd)(DO₃ structure) precipitates with FCC crystal structure and lattice parameter of a = 0.739 nm. The following orientation relationships of β ₁ and α-Mg were found: $[0001](2\overline{1} \overline{1} 0)_{\rm Mg} {\parallel} [101](11\overline{1} )_{{\beta_{1} }} \;{\rm and}\;[\overline{1} 010](0001)_{\rm Mg} {\parallel} [112](\overline{1} 10)_{{\beta_{1} }}$ (Figure 8).[17,19]

Fig. 7

DF STEM micrograph of Zn₂Zr₃ particles with Nd-rich plate-like precipitates on their basal and side planes

Fig. 8

(a) TEM micrograph and (b) corresponding SAED of β ₁ precipitate nucleated on Zn₂Zr₃ rod particle

It should be noted that this β ₁ phase was mentioned as β′ in our previous publications,[17–20] but after comparison with phases in Mg-Gd- and Mg-Gd-Nd-based alloys (see below), it was renamed to β ₁.

A small number of large β ₁ precipitates (few hundreds of nm in size) and a high density of fine β ₁ precipitates were observed in the grain boundary regions. After 32 days of aging, it was found that β″ phase transforms into β ₁ in the grain interior and β ₁ precipitates transform into β in the grain boundary regions. In the late stage of aging, the β ₁ precipitates in the matrix transform into a stable incoherent β (Mg,Zn)₁₂Nd phase.

4 Microstructures and Phase Compositions of Mg-Gd- and Mg-Gd-Nd Alloys with Additions of Zn, Y, and Zr

Two Mg-RE alloys, Mg-5Gd and Mg-6Gd-3.7Nd with 0.3 pct of Zn, 0.18 pct of Y and 0.15 pct of Zr (wt pct), were investigated.[21–23] The microstructures of the alloys have been investigated in the as-cast condition and after solution treatment at 813 K (540 °C) for 24 hours followed by isothermal aging at 448 K (175 °C) up to 125 days by means of Vickers hardness, optical microscopy, scanning electron microscopy equipped with energy dispersive X-ray spectrometry, X-ray diffraction, and transmission electron microscopy (TEM).

The microstructure of the as-cast alloys consisted mainly of the α-Mg-matrix, eutectic-like structures, cuboid-like phases, and Zr-rich clusters (Figure 9).

Fig. 9

Microstructures of TEM specimens of the as-cast samples for Mg-6Gd-3.7Nd-based alloy (a) and Mg-5Gd-based alloy (b). SEM images show eutectic-like structure (1) and cuboid-like phase (2)

The cuboid-like particles in micrometer scale are frequently observed in the association with Zr-rich clusters and eutectic-like structures as shown in Figure 9. The compositions of the phases were analyzed by EDS SEM using TEM samples (Tables I and II) with SD = 0.03–1.6.

Table I Compositions of Different Phases in Mg-5Gd-based Alloy Measured by EDS SEM

Table II Compositions of Different Phases in Mg-Gd-Nd-Based Alloy Measured by EDS SEM

The composition of the cuboid-like particles in the as-cast Mg-5Gd-based alloy can be presented as Gd_0.7(Y _x Mg_1−x )_0.3 with x = 0.5, while in the as-cast Mg-6Gd-3.7Nd-based alloy the cuboids are Y-rich phases containing Gd and Nd with the composition Y_0.75Gd_0.25.

The homogenized and quenched alloys contained primary α-Mg solid solution, smaller amount of divorced eutectic compounds, enlarged cuboid-like particles, and Zr-rich clusters (Figures 10 and 11).

Fig. 10

Microstructure of the ST Mg-5Gd-based alloy (a) optical image and (b) SEM image showing the eutectic structure (1) and cuboid-shaped phase (2). (c) X-ray diffraction

Fig. 11

Microstructure of the ST Mg-6Gd-3.7Nd-based alloy (a) optical image and (b) SEM image showing the eutectic structure (1) and cuboid-shaped phase (2). (c) X-ray diffraction

The compositions determined by EDS SEM analysis are presented in Table II.

The eutectic structures were the products of a “quasibinary eutectic reaction” L → α-Mg + β-Mg₅RE. The eutectic structure in the Mg-5Gd-based alloy was characterized to be α-Mg + Mg₅Gd. The microstructure of the homogenized Mg-6Gd-3.7Nd-based alloy contained skeleton-like eutectic structures (Figure 12) with the Mg₅Gd-prototype phase of the composition Mg₅(Gd _x Nd_1−x , x = 0.2) and FCC crystal structure.

Fig. 12

(a) BF TEM micrograph of the ST Mg-Gd-Nd alloy eutectic phase from the ST sample and (b) corresponding SAED pattern of the sample in zone axis [011], identifying Mg₅RE cubic eutectic phase

It was found by SAED patterns that the cuboid-shaped phase has a FCC crystal structure with a = 0.54 nm and is a GdH₂-type compound as shown in Figure 13.

Fig. 13

BF TEM micrograph of the ST Mg-Gd-Nd alloy with cuboid-shaped phase and corresponding SAED patterns with zone axes [$\overline{1} \overline{1}{\user2 {2}}$] in (a, b) and [100] in (c, d)

Since the EDS analysis cannot be used for the detection of element H, the compositions of the cuboid-shaped particles after homogenization (Tables I and II) are characterized by enlarged amount of Gd and can be presented Gd_0.7(Y _x Mg_1−x )_0.3 with x = 0.5 in the Mg-5Gd-based alloy, and Gd₄(Y _x Nd_1−x ) with x = 0.5 in the Mg-6Gd-3.7Nd-based alloy. The cuboid-shaped particles appeared to be stable in Mg matrix after ST, and SEM/HRSEM + EDS analysis revealed that the Gd/Nd ratio did not change during aging. The Zn₂Zr₃ (as in the Mg-3.1Nd-0.45Zr-0.25Zn alloy) or any other Zn-Zr phases were not recognized by XRD and SEM + EDS analyses.

Crystal structure of precipitates formed during aging was investigated by TEM using SAED patterns. TEM image and corresponding SAED pattern of precipitates in a specimen aged for 16 hours (pre peak-aged specimen) are shown in Figure 14. It was observed that the microstructure contains a high number density of fine plate shape β″ precipitates uniformly distributed within the α-Mg matrix (Figure 14). The β″ precipitates with DO₁₉ structure $\left( {a\sim 2a_{\upalpha{\text{-Mg}}} = 0.64\,{\text{nm}}, \;c\sim c_{{\upalpha{\text{-Mg}} }} = 0.52\;{\text{nm}}} \right)$ correspond to diffuse spots at 1/2 distance of {$01\overline{1} 0$} _α or {$2\overline{1} \overline{1} 0$} _α reflections, with the following orientation relationships (OR) to α-Mg matrix: $[0001]_{{\beta^{''} }} //[0001]_{\alpha } ,\;\left\{ {01\overline{1} 0} \right\}_{{\beta^{''} }} //\left\{ {01\overline{1} 0} \right\}_{\alpha } .$

Fig. 14

(a) TEM image and (b) corresponding SAED pattern of Mg-Gd-Nd alloy aged for 16 h at 448 K (175 °C); zone axis [2110] _α

The HAADF STEM image and SAED pattern typical of microstructure in Mg-Gd-Nd specimen aged for 16 days are presented in Figure 15. As seen from the image, the microstructure in the peak-aged condition contained three kinds of precipitates which can be recognized as β′, β ₁ phases, and very thin precipitates with few tens of nanometer size that are probably β″ precipitates.

Fig. 15

(a) HAADF STEM image and (b) corresponding SAED pattern of Mg-Gd-Nd alloy aged for 16 days at 448 K (175 °C); zone axis [1010] _α

The platelets β′ have $\left\{ {\bar{2}110} \right\}_{\alpha }$ habit planes and are extending along [0001] _α direction, while β ₁ precipitates are extending along $\langle \bar{2}110\rangle_{\alpha }$ directions perpendicular to the [0001] _α axis, and $\langle \bar{2}113\rangle_{\alpha }$ directions with the angle of about 30 deg to the [0001] _α direction. As can be seen from the SAED pattern in Figure 15, the precipitate reflections are from β′ and β ₁ precipitates.

The following orientation relationships can be obtained:

For β′ precipitates: [100] _β′ ║ $[10\overline{1} 0]_{{\upalpha{\text{-Mg}}}}$ and (001) _β′ ║ $\left( {0001} \right)_{\alpha } ;$

For β ₁ precipitates: $[112]_{{\beta_{1} }}$ ║ $[10\overline{1} 0]_{{\upalpha {\text{-Mg}}}} ;$ and $(\overline{1} 10)_{{\beta_{1} }}$ ║ $\left( {0001} \right)_{\alpha } .$

Typical high angle annular dark-field (HAADF) STEM image and corresponding SAED pattern of the Mg-Gd-Nd-based alloy specimen aged for 32 days are shown in Figure 16. As can be seen, the microstructure contained β′ and β ₁ precipitates, while β ₁ precipitates appear often to be associated with β′ precipitate.

Fig. 16

(a) HAADF STEM image and (b) corresponding SAED pattern of Mg-Gd-Nd alloy aged for 32 days at 448 K (175 °C); zone axis [2110] _α

β′ precipitates are elongated along [0001] _α direction, while β ₁ precipitates are elongated along two directions which are apparently the projections of $\langle \bar{2}113\rangle_{\alpha }$ directions on the $(2\overline{1} \overline{1} 0)_{\alpha }$ plane.

The OR $[100]_{{\beta^{'} }} {\parallel} [111]_{{\beta_{1} }} ,\; \left( {00\bar{2}} \right)_{{\beta^{'}}}{\parallel} \left( {20\bar{2}} \right)_{{\beta_{1} }}$ between β′ and β ₁ precipitates with very small lattice mismatch can allow for nucleation of β ₁ on the β′ precipitates (Figure 16).

After aging for 64 days, the β′ precipitates almost totally transformed to platelet β ₁ precipitates. The typical microstructure (TEM image) and corresponding SAED pattern of Mg-Gd-Nd alloy specimen aged for 64 days (Figure 17) show that the platelet β ₁ precipitates are elongated along three equivalent directions of $\langle \overline{2} 110\rangle_{\alpha }$ with habit planes $\left\{ {10\overline{1} 0} \right\}_{\alpha }$. The angles between the variants are 60 or 120 deg.

Fig. 17

(a) TEM image and (b) corresponding SAED pattern of Mg-Gd-Nd alloy aged for 64 days at 448 K (175 °C); zone axis [0001] _α

The OR between β ₁ precipitate and the matrix is $\left[ {101} \right]_{{\beta_{1} }}$║$\left[ {0001} \right]_{{\upalpha {\text{-{\rm Mg}}}}}$ and $\left( {11\overline{1} } \right)_{{\beta_{1} }}$║$\left( {11\overline{2} 0} \right)_{\alpha }$, which, in fact, is the same as mentioned above.

In 448 K (175 °C)/125 day-aged specimen of the Mg-Gd-Nd-based alloy, the microstructure contained long needle-like precipitates much coarser than those observed in 448 K (175 °C)/64 days aged specimen (Figure 18).

Fig. 18

TEM image of Mg-Gd-Nd alloy aged for 125 days at 448 K (175 °C), showing needle-like β precipitates

These precipitates seem to grow from the eutectic structure as the continuation of its β phase in the process of coarsening when the smaller β ₁ and β precipitates presented in the α-Mg matrix are dissolved. The β phase has a crystal lattice with FCC (a = 2.22 nm) structure and composition of Mg₅RE.

4.1 Microhardness

The microhardness evolution of the Mg-5Gd and Mg-6Gd-3.7Nd-based alloys during isothermal aging at 448 K (175 °C) up to 125 days with error bars of ±3HV is shown in Figure 19. This shows that the Mg-5Gd-based alloy displays a two-stage behavior typical for Mg-Gd alloys.[24]

Fig. 19

Age hardening curves of the Mg-6Gd-3.7Nd and Mg-5Gd-based alloys aged at 448 K (175 °C)

In the first stage, the microhardness increased and, after 2 days of aging, reached a first maximum, then it slightly decreased, and increased again up to a second maximum value after ~16 to 64 days of aging. Further aging led to a decline in the microhardness. During aging of Mg-6Gd-3.7Nd-based alloy, the microhardness reached a first maximum (of about 90 HV) after 8 hours, then it decreased, but, after 16 hours, increased again until reaching a second maximum (~100 HV) after 2 days of aging; the third peak was reached after ~16 days of aging. Further aging resulted in the decrease of the microhardness.

5 Discussion

The microstructure of the as-cast Mg-RE-based alloys was similar for the Mg-Nd- and Mg-Gd-based alloys: it consists of α-Mg matrix, eutectic compounds, and Zr-rich particles. The presence of Y in Mg-Gd-based alloys led to formation of cuboid-shaped particles containing Y, Gd, Nd, and Mg. However, the eutectic compounds are different: for the case of Mg-3.1Nd-based alloy, the compound is Mg₁₂Nd where Zn atoms may substitute Mg atoms (up to ~2.6 at. pct); for the case of Mg-5Gd and Mg-6Gd-3.1Nd-based alloys, it is Mg₅Gd-prototype phase of the composition Mg₅(Gd _x Nd_1−x ) with x = 1 for the first alloy and x = 0.5 for the second one. After solution treatment followed by water quenching, supersaturated solution of RE metals in the Mg-matrix was attained. Besides, the Zn₂Zr₃ particles formed in the grain interior of the ST Mg-3.1Nd-0.45Zr-0.25Zn alloy, while no Zn-Zr phases were found in the ST Mg-5Gd-0.3Zn-0.18Y-0.15Zr and Mg-6Gd-3.7Nd-0.3Zn-0.18Y-0.15Zr alloys. The absence of Zn₂Zr₃ phase in the two last alloys may be connected with higher Zn solubility in the Mg-Gd matrix compared with Mg-Nd or pure α-Mg matrix (or/and with smaller amount of Zr there). The Zn₂Zr₃ rods influence on the precipitation of metastable phases during aging, with these rods providing additional places for nucleation of these phases.

Formation of the cuboid-shaped particles in the Mg-RE-based alloys has been investigated in a number of studies.[6,25] They were identified to have a fcc crystal structure (a = 0.56 nm), and the composition depending on the alloy constitution and thermal treatment conditions. Zheng et al.[6] assumed that these are the RE-hydrides of the GdH₂ type. Since the hydrogen canot be detected by EDS analysis, a hydride may be assumed for a cuboid-shaped phase. (GdH₂ compound has a fcc structure with a = 0.5303 nm; different compositions of the cuboid phases can be obtained if part of Gd atoms is replaced by Nd, Y or Mg).

Formation of cuboid-shaped particles in the investigated Mg-Gd- and Mg-Gd-Nd-based alloys was connected with the presence of Y and Gd—all the cuboid-shaped particles in the as-cast condition contained enlarged amount of Y and Gd. One can assume that the Y and/or Gd hydrides formed during melting or casting. YH₂ phase has a fcc crystal structure with a = 0.525 nm. Part of yttrium atoms can be replaced by Gd, Nd, and Mg. In the as-cast Mg-5Gd-based alloy, the Gd atoms in the RE-hydride (e.g., GdH₂ phase) were partially replaced by Y and Mg to form the compound Gd_0.7(Y _x Mg_1−x )_0.3, with x = 0.5, this phase was found to be stable during ST applied. In the as-cast Mg-6Gd-3.7Nd-based alloy, the RE elements in the REH₂-type compound were mainly Y and Gd with relative composition of Y_0.75Gd_0.25.

It was found from SAED patterns after ST (Figures 13(b) and (d)) that the cuboid-shaped phase has a FCC crystal structure with a = 0.54 nm corresponding to a GdH₂-type compound. The heat treatment caused diffusion of Gd and Nd atoms in α-Mg matrix toward the cuboid-like phases and partial replacement of Y by Gd and Nd atoms resulting in relative composition of Gd₄(Y _x Nd_1−x ) with x = 0.5. After ST, the cuboid-shaped particles appeared to be stable during aging at 448 K (175 °C) that corresponds to stability of RE hydrides at this temperature. SEM/HRSEM + EDS analysis revealed that the Gd/Nd ratio did not change during aging.

Decomposition of supersaturated solid solution during aging resulted in the precipitation of coherent plate-shaped β″(DO₁₉) phase with the composition Mg₃RE. The β″ phase then may transform to the β′ phase with the composition Mg₇RE and the base-centered orthorhombic (BCO) crystal structure (a ~ 2a _α-Mg = 0.64 nm, b ~ 2.2 nm, and c ~ c _α-Mg = 0.52 nm)[26] as in the Mg-Gd and Mg-Gd_Nd-based alloys. The β′ precipitates have a few hundreds of nanometers platelet-like shape in habit planes $\left\{ {\overline{2} 110} \right\}_{{\upalpha {\text{-Mg}}}}$ and grow along $\langle 0001\rangle_{{\upalpha {\text{-Mg}}}} .$ It was found that β′ precipitates are coherent with the Mg matrix, with the following orientation relationship: $\left[ {10\overline{1} 0} \right]\left( {0001} \right)_{{\upalpha {\text{-Mg}}}}$║$[100](001)_{{\beta^{\prime } }} .$ At the next stage of aging, β′ transforms to a semicoherent β ₁(DO₃) phase with the composition Mg₃RE and FCC (a = 0.74 nm) structure. In the investigated Mg-Nd-based alloy, β″ transformed directly to the β ₁ phase.

The β ₁ precipitates have a hundreds of nanometers platelet-like shape in habit planes $\left\{ {0\overline{1} 10} \right\}_{{\upalpha {\text{-Mg}}}}$, grew mainly along three directions of $\langle 11\bar{2}0\rangle_{{\upalpha {\text{-Mg}}}}$ and are semicoherent with the α-Mg matrix, with the following orientation relationship: $[110]_{{{{\upbeta}}_{1} }} {\parallel} [0001]_{\alpha }$ and $(1\overline{1} 1)_{{{{\upbeta}}_{1} }} {\parallel} \left( {11\bar{2}0} \right)_{\alpha } .$

OR of the β ₁(Mg₃RE)_FCC with the α-Mg matrix provides small lattice misfits in [0001] _α directions (0.31 pct), but rather large misfits in $[\bar{2}110]_{\alpha }$ directions (11.4 pct).[17] This explains why the β ₁ precipitates are semicoherent. Another situation is realized for the bco β′ in the α-Mg matrix: in [0001] _α directions lattice misfits are small ($c_{{\beta^{\prime } }} \sim c_{{\upalpha {\text{-Mg}} }}$), in $[\bar{2}110]_{\alpha }$ and $[\overline{1} 010]_{\alpha }$ directions they are also small ($a_{{_{{\beta^{\prime } }} }} \sim 2a_{{\upalpha{\text{-Mg}} }}$ and $b_{{\beta^{\prime } }} \sim 8d_{{[\bar{1}010]\upalpha {\text{-Mg}} }}$). Thus, bco β’ can be coherent with α-Mg matrix.

At the overaging stage, metastable phases transform to a stable incoherent β phase (Mg₁₂Nd in the Mg-Nd-based alloys and Mg₅Gd in the Mg-Gd- and Mg-Gd-Nd-based alloys).

The presence of two age hardening peaks in the Mg-Gd-based alloys, compared with one peak in the Mg-Nd-based alloy, testifies the intermediate transformation β″ → β′ which was absent in the Mg-Nd-based alloy. Formation of β′ phase with the composition Mg₇RE involves additional Mg atoms from the adjacent Mg matrix. It increases precipitated volume and precipitates’ average size and, by these means, provides additional increase of microhardness. Transformation β′ → β ₁ results in decrease of precipitates’ volume and corresponding decrease of microhardness after the second or third maximum in the Mg-Gd- and Mg-Gd-Nd-based alloys, respectively. Heterogeneous nucleation of β ₁ phase on the Zn₂Zr₃ rods may explain early formation of this phase in the Mg-Nd-based alloy, while in the Mg-Gd- and Mg-Gd-Nd-based alloys, it nucleates heterogeneously on the β′ precipitates.

Nucleation and growth of Mg-RE phases are controlled by diffusion of RE elements in Mg matrix. Experimental values for RE diffusion in Mg are very limited. Ab initio calculations of diffusion of several RE impurities in Mg were performed recently by Huber et al.[27] For Nd and Gd, they obtained the diffusion activation energies of 1.05 and 1.17 eV, respectively. Assuming the equal pre-exponential factors, the ratio of diffusion coefficients of Nd and Gd at 448 K (175 °C) can be estimated as D _Nd/D _Gd=exp(0.12 eV/kT) ~ 20. One can assume that the first peak in the Mg-Gd-Nd-based alloy corresponds to formation of β″ that contain mainly Nd (Mg₃Nd phase), while the second peak (first peak in the Mg-Gd-based alloy)—after 2 days of aging, resulted from the diffusion of Gd to form β″ precipitates containing Nd and Gd. In the coarsening regime (the overaging stage), the precipitate number density, N _V, decreases with time, $N_{\text{V}}^{ - 1}$ ∼ α _LSW t ∼ D _RE t, where α _LSW is the coefficient of the LSW-coarsening theory. The microhardness roughly is inversely proportional to a distance between precipitates,[28] and thus, is proportional to $N_{\text{V}}^{1/3} .$ Therefore, one can expect that the overaging stage in the Mg-Gd alloys would be substantially longer (to ~20^1/3 = 2.7 times) than in the Mg-Nd alloys. The microhardness curves for Mg-Nd- and Mg-Gd-based alloys (Figures 3 and 14) confirm this evaluation.

6 Conclusions

Comparison study of phase evolution in Mg-RE alloys revealed close similarity of the alloys microstructure and precipitation sequence during aging among Mg-Nd-, Mg-Gd-, and Mg-Gd-Nd-based alloys. Along with this, on closer investigation of crystal structure, composition, and OR of metastable phases β″, β′, and β ₁, certain distinct features emerged. These features are connected with different structures of eutectic compounds and of final stable phases β (Mg₁₂Nd and Mg₅Gd, respectively), and with the formation of additional metastable β′ phase in the Mg-Gd- and Mg-Gd-Nd-based alloys. Transformation β″ → β ₁ provides one age hardening peak in the Mg-Nd-based alloy.

Transformations β″ → β′ → β ₁ in the Mg-Gd- and Mg-Gd-Nd-based alloys are manifested in two age hardening peaks. The first peak is connected with formation of β″(Mg₃RE) phase which, in turn, is divided in the Mg-Gd-Nd-based alloy into two subpeaks associated with the formation of β″(Mg₃Nd) and β″(Mg₃(GdNd)) phases. The second microhardness peak is connected with transformation β′ → β ₁. Slower diffusion of Gd in Mg matrix (in comparison with Nd) provided a longer overaging stage in Mg-Gd-based alloys. Formation of Zn₂Zr₃ rods, distributed in the grain interior of Mg-Nd-based alloy and served as additional nucleation sites for precipitates, resulted in early heterogeneous nucleation of β ₁ phase and formation of T-like and H-like particles. The absence of Zn-Zr phases in the investigated Mg-Gd- and Mg-Gd-Nd-based alloys can be connected with higher solubility of Zn in the Mg-Gd matrix comparing to Mg-Nd matrix. The cuboid-shaped particles found in the Mg-Gd-and Mg-Gd-Nd-based alloys were the Y- and Gd-containing phases with the composition changing during ST, but nearly constant during aging. It was assumed that these particles are the fcc YH₂-type hydrides where Y atoms can be replaced by Gd, Nd, and Mg. The cuboid-shaped particles are stable during aging at 448 K (175 °C).