Mg-RE based alloys

The RE elements can significantly improve the mechanical properties of magnesium alloys.

An Mg-RE based ultra-high alloy has been reported with the properties of 610 MPa in tensile yield stress and 5 % in elongation [102]. Since the Mg-RE based alloys show the greater potential for developing ultra-high strength magnesium alloys via precipitation strengthening, many works have been conducted to study the Mg-RE based alloys [103-111].

The RE elements can be divided into two subgroups according to their atomic number, those from La to Sm (lower atomic numbers and masses) being referred to as the light RE elements and those from Gd to Lu (higher atomic numbers and masses) being referred to as the heavy RE elements [112]. According to Rokhlin [113] and Hadorn et al. [114], the intermediate phase formation amongst the Mg-RE alloy in the same sub-group shows great similarity.

11 2.1.2.5.1 Mg-Gd based alloys

Gadolinium belongs to the heavy RE elements; its maximum solubility in Mg is 23.67 wt. % and decreases to 1.64 wt. % at 200 °C. Many works have been devoted to investigating the precipitation process of binary Mg-Gd alloy [115-118]. The precipitation sequence of binary Mg-Gd alloy is considered as SSSS → 𝛽^′′ Mg3Gd → 𝛽^′ Mg7Gd → 𝛽₁ Mg3Gd → 𝛽 Mg5Gd [5].

Although both the 𝛽^′′ and the 𝛽₁ precipitates have the same Mg3Gd composition. Their lattice structure is different, the 𝛽^′′ phase has an HCP structure while the 𝛽₁ phase has an FCC structure. The 𝛽^′′ phase precipitates at the early stage of ageing, according to Gao et al.[116], it coexists in the matrix with the 𝛽^′ phase after ageing at 250 °C for 0.5 hours. When the ageing time is extended to 2 hours the only existing precipitates is the 𝛽^′ phase. The 𝛽^′ phase has a base-centred orthorhombic structure and it is the key strengthening precipitate phase [119-121].

Recent studies revealed that the 𝛽^′ phase includes two types of precipitates, the 𝛽_𝑆^′ and 𝛽_𝐿^′ phase [122-124]. They have the same base-centered orthorhombic structure but different in the lattice parameters, the lattice parameters of 𝛽_𝐿^′ phase are: a = 0.64 nm, b = 2.22 nm, c = 0.52 nm while a = 0.64 nm, b = 1.11 nm, c = 0.52 nm for 𝛽_𝑆^′ phase [123]. The precipitation of 𝛽₁ phase is somehow under debate, Nie et al. [125] and Gao et al. [116] believed that the 𝛽₁ phase nucleates at the necks of the decomposed 𝛽^′ precipitates and grows at the expense of 𝛽^′, it is supported by the fact that the 𝛽₁ phase is always attached to two 𝛽^′ particles; this is also consistent to other Mg-RE alloys [126]. However, Apps et al. [127] disagreed with that; they assumed that both 𝛽₁ and 𝛽^′ all nucleate on the 𝛽^′′ phase and further ageing caused the two 𝛽^′ particles attached to the 𝛽₁ phase. Additionally, Meng et al. [128] even concluded that the precipitation of 𝛽₁ phase is impossible in binary Mg-Gd alloy due to its high formation energy and low vibrational entropy according to their first-principles calculation. The formation of the equilibrium 𝛽 phase caused the over-aged of Mg-Gd alloy. It is believed to be transformed in situ from the 𝛽₁ phase and the orientation relationship between the 𝛽 phase and matrix is the same as that the orientation relationship between 𝛽₁ phase and the matrix.

Recently, a study on the precipitation of binary Mg-Gd alloy associated with HAADF-STEM (High-angle annular dark-field scanning transmission electron microscopy) was conducted by Zhang et al. [129]. They proposed a very different precipitation sequence as follows: SSSS → ordered solute clusters → G.P. zones → 𝛽^′ → 𝛽_𝑆^′ + tail-like hybrid structures → 𝛽₁ → 𝛽.

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Additionally, an unusual FCC-structured Gd platelets was found to precipitate when Mg-Gd alloy was rapidly heated to 250 °C and held for 2 hours [130].

2.1.2.5.2 Mg-Nd based alloys

Nd is one of the light RE elements; its maximum solubility in Mg is 3.68 wt. % and decreases to 0.01 wt. % at 200 °C. The precipitation sequence in the Mg‒Nd alloy is suggested to be:

SSSS → G.P. zones → 𝛽^′′ Mg3Nd → 𝛽^′ Mg7Nd → 𝛽₁ Mg3Nd → 𝛽 Mg12Nd → 𝛽_𝑒 Mg41Nd5

[5]. Compared to the precipitation in binary Mg-Gd, the difference in binary Mg-Nd is the formation of G.P. zones.

The information on the precipitation of G.P. zones is limited due to the small size of these features and instrumentation restrictions [131], Saito et al. [132] and Lefebvre et al. [133]

found that the G.P. zones were needle-shaped with long axes parallel to the [0001]Mg, but the driving forces for G.P. zones remained a mystery. The 𝛽^′′ phase was determined by Lefebvre et al. [133], it had an FCC structure and the composition of Mg3Nd. It formed on the prismatic planes and was fully coherent with the matrix [134]. Ma et al. [134] thought the formation of the 𝛽^′′ phase was mainly responsible for the precipitation strengthening in Mg-Nd alloy.

Nevertheless, Satio et al. [132] disagreed with that. They reported that 𝛽^′′ phase was not formed in Mg-Nd alloy was when ageing at temperatures ranging from 170 °C to 250 °C, the peak-aged was due to the coexistence of G.P. zones and the 𝛽^′ phase. They also concluded that when the Mg-Nd alloy was over-aged, both the G.P. zones and the 𝛽^′ phase disappeared and coarse stable 𝛽₁ phase was precipitated. Therefore, they assumed that the 𝛽₁ phase was harmful to the precipitation strengthening in Mg-Nd alloy. However, a study by Zhu et al. [135]

concluded that the 𝛽₁ phase was the key strengthening phase in Mg-Nd alloy. They believed that the 𝛽₁ phase had six variants and formed on the {01-10} planes. They also found an unreported phase designated as 𝛽₂, the 𝛽₂ phase always formed in connection points of two 𝛽₁ particles of the same variant or different variants but having opposite shears directions. The 𝛽 phase coexisted with the 𝛽₁ phase and was considered to be the equilibrium phase by Zaden et al. [131]. However, it is confirmed that the equilibrium phase is in fact the 𝛽_𝑒 phase, but it is formed only at high heat treatment temperatures and sufficiently long durations [136].

The precipitation process in magnesium alloys has been extensively studied with the help of TEM [95, 137], HAADF-STEM [138, 139], DSC (Differential scanning calorimetry) [140], synchrotron radiation [141] and dilatometry [142]. Despite the traditional methods, electrical

13 resistivity has been successfully introduced in investigating the precipitation process in steel [143] and Al alloys [144, 145]. However, limited work has been performed to study electrical resistivity in Mg alloys. Therefore, the current study targets understanding the electrical resistivity changes in Mg alloys and exploring the possible use of the electrical resistivity in Mg alloys.