Dilute Mg alloys processed by the rapidly solidified ribbon consolidation (RSRC) technique are candidate materials for structural applications due to their enhanced mechanical performance. The thermal stability of the structure in these alloys strongly influences their mechanical performance at elevated temperatures. In this study, an RSRC-processed Mg–1% Ca–0.5% Zn–0.1% Y–0.03% Mn (at%) alloy was heated at a constant rate up to 833 K, and concurrently in situ X-ray diffraction (XRD) measurements were performed using synchrotron radiation in order to monitor the changes in the structure. In addition, ex situ electron microscopy investigations were carried out before and after annealing to complete the XRD study. On the basis of XRD results, the stages of the microstructure evolution during heating were identified. In addition, the thermal expansion coefficients of the matrix and the Mg₂Ca secondary phase were determined. Between 299 and 400 K, the lattice constants of both the matrix and the Mg₂Ca phase increased due to thermal expansion. In the temperature range of 400-673 K, the increase of the lattice constants with increasing the temperature continued, but their rate was different for the two phases which can induce thermal stresses. Between 673 and 753 K, the lattice constants of the secondary phase did not change most probably due to the compensating effects of the thermal expansion and the decrease of the Ca content. In the temperature range of 753–793 K, the Mg₂Ca phase started to dissolve. Between 793 and 833 K the dissolution continued, and additionally the matrix was partially melted.

Mg-Y-Zn-Al alloys processed by rapidly solidified ribbon consolidation (RSRC) technique exhibit an exceptional mechanical performance indicating promising application potential. This material has a bimodal microstructure consisting of fine recrystallized and coarse non-recrystallized grains with solute-rich stacking faults forming cluster arranged layers (CALs) and nanoplates (CANaPs), or complete long period stacking ordered (LPSO) phase. In order to reveal the deformation mechanisms, in-situ synchrotron X-ray diffraction line profile analysis was employed for a detailed study of the dislocation arrangement created during tension in Mg - 0.9% Zn - 2.05% Y - 0.15% Al (at%) alloy. For uncovering the effect of the initial microstructure on the mechanical performance, additional samples were obtained by annealing of the as-consolidated specimen at 300 and 400 ℃ for 2 h. The heat treatment at 300 ℃ had no significant effect on the initial microstructure, its evolution during tension and, thus, the overall deformation behavior under tensile loading. On the other hand, annealing at 400 ℃ resulted in a significant increase of the recrystallized grains fraction and a decrease of the dislocation density, leading to only minor degradation of the mechanical strength. The maximum dislocation density at the failure of the samples corresponding to the plastic strain of 10–25% was estimated to be about 16–20 × 10¹⁴ m⁻². The diffraction profile analysis indicated that most dislocations formed during tension were of non-basal 〈a〉 and pyramidal 〈c + a〉 types, what was also in agreement with the Schmid factor values revealed independently from orientation maps. It was also shown that the dislocation-induced Taylor hardening was much lower below the plastic strain of 3% than above this value, which was explained by a model of the interaction between prismatic dislocations and CANaPs/LPSO plates.