The effects of Mn addition on the room temperature tensile strength and deformation mechanisms of as-cast Mg-8Al-1Nd-1.5Gd-xMn alloys (x = 0, 0.3, 0.5, 1.0 wt.%) are investigated in this paper. The results indicate that the addition of Mn contributes to the precipitation of Al-Mn-RE intermetallics and the refinement of α-Mg matrices, thereby improving the tensile strength of the 1.0 Mn alloy at 190 MPa. The fracture mechanism of Mn-containing alloys transforms from a cleavage fracture to a ductile fracture as the Mn content increases from 0.3 to 1.0 wt.%. The presence of intermetallic particles in the dimples confirms the hindrance effect of Al₁₀Mn₂ (Nd,Gd) on dislocation slips. The novel technology of in-grain misorientation axes (IGMAs) is used to identify activated slip modes and deformation twins. It can be concluded that the activated pyramidal <a> slip during tensile deformation significantly promotes the ductility of the 1.0 Mn alloy with an elongation rate of 9.8%. It is worth noting that reducing the coarse $\left\{10\overline{1}2\right\}$ tensile twins and enhancing the proportion of $\left\{10\overline{1}1\right\}$ compressive twins and $\left\{10\overline{1}1\right\}$-$\left\{10\overline{1}2\right\}$ double twins contributes to maintaining the continuous plastic deformation of Mg alloy.

Ni-CNTs/AZ91 magnesium matrix composites were fabricated by ultrasound treatment combined with a semi-solid stirred method for the first time. The agglomerated spherical Ni-CNTs transferred from spherical shape to clear tubular shape after pre-dispersion treatment. For the Ni-CNTs/AZ91 magnesium matrix composite prepared by semi-solid stirring followed by ultrasonic treatment, Ni-CNTs were evenly distributed in the magnesium matrix or wrapped on the β (Mg₁₇Al₁₂) phase. Mg₂Ni were formed at the interface of the magnesium matrix and CNTs by in-situ reaction, which significantly improved the interface bonding strength of CNTs and the Mg matrix. The tensile strength and elongation of 1.0wt.% Ni-CNTs/AZ91 magnesium matrix composites were improved by 36% and 86%, respectively, compared with those of AZ91 matrix alloy. After Ni-CNTs were added to AZ91 matrix alloy, more dimples were observed at the fracture surface. The fracture behavior of Ni-CNTs/AZ91 composite was transformed from a cleavage fracture of AZ91 matrix alloy to a quasi-cleavage fracture. Meanwhile, the CNTs dispersed near the fracture showed a “pull-out” state, which would effectively bear and transfer loads. The strengthening mechanism of CNTs was also discussed.

Nd³⁺, Y³⁺: SrF₂ transparent ceramics were successfully synthesized by two methods: hot-forming and hot-pressing techniques. The mechanical properties and optical properties of the hot-formed Nd³⁺, Y³⁺: SrF₂ transparent ceramics were much better than that of single crystal. On the other hand, the transmittance of the hot-formed transparent ceramics with different deformation rate reached up to 90% at 1054 nm, which is superior to the hot-pressed ceramics. Furthermore, the fracture toughness of hot-formed Nd³⁺, Y³⁺: SrF₂ transparent ceramics with the deformation rate of 51% reached up to 0.70 MPa m^1/2, which is nearly 1.5 times higher than that of as-grown single crystal. The full width at half maximum (FWHM) of the hot-formed ceramic is larger than that of the single crystal at 1053 nm under continuous-wave (CW) laser operation. The thermal conductivity of Nd³⁺, Y³⁺: SrF₂ single crystal and hot-formed ceramics were also discussed.

TiB₂/Mg-4Al-1.5Si magnesium matrix composites were prepared by semi-solid stirring assisted ultrasonic treatment, the primary Mg₂Si phases in the composites exhibit polygon with sharp corners, and the eutectic Mg₂Si phases appear thin Chinese-script or short-strip shape. To reduce stress concentration around the sharp corners (tips) of the Mg₂Si phases, the morphology of the Mg₂Si phases was further modified by solution treatment at 420 °C for 24 h, and the effects of the morphology modification of the Mg₂Si phases on the mechanical properties of the composites were investigated. The results showed that after the solution treatment, the sharp corners of the primary Mg₂Si phases were blunted, and the partial branches of the eutectic Mg₂Si phases were dissolved into particles. The Vickers-hardness, ultimate tensile strength, yield strength, and elongation of the composites were increased by 11.50%, 33.28%, 28.57%, and 27.17% compared with those of unmodified composites, respectively. The solution treatment exhibits a more significant strengthening effect for the composites in hardness, ultimate tensile strength, and yield strength compared with the matrix alloys.

Compressive creep behavior of a Mg-Al alloy containing a small amount of Nd and Gd (Mg-6Al-1Nd-1.5Gd) was investigated at temperatures from 150 °C to 200 °C under a constant applied stress of 90 MPa, and its microstructure before and after creep testing was compared. Results showed that steady-state creep rate of the alloy was only 1.946 × 10⁻⁸ /s at 150 °C, and was increased by four times and almost one order of magnitude at 175 °C and 200 °C, respectively. The microstructure of the alloy mainly consists of α-Mg, β-Mg₁₇Al₁₂ phases, and Al₂RE phases, which were distributed both in dendrites of α-Mg and at grain boundaries originally. After creep for 120 h, more Al₂RE phases were aggregated at grain boundaries. The continuous β-Mg₁₇Al₁₂ phase turned into dispersed dot-like or blocky particles. As the test temperature increased, the number of dislocation lines gradually increased due to the increase of creep strain. Meanwhile, dislocation tangle and dislocation pile-ups occurred near grain boundaries. However, obvious slip traces and slip lines appeared inside α-Mg dendrites at 175 °C and 200 °C, respectively, indicating that 〈c + a〉 non-basal slip system was activated, creep resistance decreased dramatically.

The effects of Mn/RE (Nd, Gd) multi-modification on the microstructure and high-temperature compressive creep properties of Mg–8.0Al alloys were investigated. The dominant intermetallic phases in the as-cast microstructure are β-Mg₁₇4Al₁₂, Al₂(Gd,Nd), Al₁₁(Gd,Nd)₃, Al₈(Gd,Nd)Mn₄, and Al₁₀Mn₂(Gd,Nd). The detailed structures of various intermetallics were revealed by TEM; the results indicate that Mn addition promotes grain refinement and facilitates the precipitation of lath-shaped and spherical β-Mg₁₇Al₁₂ in as-cast Mg–Al–RE alloys, resulting in increases in the tensile strength and elongation of the 1.0Mn alloy by 26.5% and 92.1%, respectively. Additionally, thermally stable micron-scale Al₈(Gd,Nd)Mn₄ and Al₁₂(Gd,Nd)₂Mn₅, along with dynamically precipitated spherical nano-sized AlGd and AlNd particles in the α-Mg matrix, were innovatively observed in compression-crept specimens tested at 200 °C and 60 MPa; these phases play a key role in improving high-temperature creep resistance. A significant finding is that excessive Mn addition deteriorates creep performance, which is attributed to excessive grain refinement and the consequent increase in the contribution of grain boundary sliding during creep. However, the negative effect of grain boundary sliding—caused by grain refinement—on creep performance can be balanced by the strengthening effect of Al–Mn–Gd phases and the dynamic precipitation of nanoscale Al–RE particles. This paper provides new insights for designing Mg–Al–Nd–Gd–Mn alloys with both excellent high-temperature creep resistance and significantly enhanced mechanical properties.