Study On The Microstructure And Mechanical Properties Of High Strength Heat Resistant Mg-Gd-Nd-Zr Alloys

Magnesium-rare earth alloys are considered to be a kind of novel high strength and heat resistant magnesium alloys with promising applications in industry. In this paper, Magnesium alloys added with a mixture of heavy and light rare earth elements were designed with nominal composition of Mg-xGd-2Nd-Zr(x=6, 8, 11, wt.%). The alloys were subjected to heat and thermo-mechanical treatment in order to strengthen them by various structures. The microstructures and their evolution, strengthening mechanism, processing for the performance improvement, microstructure dependence of performance were studied by using optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, thermal analysis and mechanical test of hardness, tension and creep. The primary attempt was to gain knowledge on the relationship between structure and performance of the alloys and provide instructive information for the development of new high performance magnesium-rare earth alloys.The as-cast microstructure of the Mg-Gd-Nd-Zr alloys consists of primaryα-Mg solid solution, skeleton-like eutectic structures, small cuboid-shaped phases and Zr-rich clusters. Eutectic structures can be identified asα-Mg+β-Mg₅RE, and the eutectic phase was characterised to have Mg5Gd-type FCC crystal structure with a～2.22nm. Microstructural evolution of the alloys during homogenisation involves three coinstantaneous processes: dissolving of eutectic phases, growth of cuboid-shaped phases and coarsening ofα-Mg grains. Cuboid-shaped phase in the alloys has a FCC crystal structure with a～0.55nm and is a GdH2-type compound with partial replacement of Gd atoms by Nd and Mg atoms. The alloys could be substantially homogenised by the optimised homogenisation treatment leaving little eutectic phases and relatively fine grains.The decomposition of supersaturated solid solution of GN112K alloy during isothermal aging at 250℃consists of four stages of precipitation transformation as follows: supersaturatedα-Mg solid solution→β″(D0₁₉, a～0.64nm, c～0.52nm, {2110}αplate)→β′(CBCO, a～0.64nm, b～2.2nm, c～0.52nm, ellipsoidal)→β1(FCC, a～0.74nm, {0110}αplate, isomorphous with Mg₃Nd)→β(FCC, a～2.22nm, {0110}αplate, isomorphous with Mg₅Gd)。The evolution of grain boundary structure during precipitation aging is characterized by the formation of GBPs and PFZs, and they grow with prolonged aging time. Among the four types of precipitate phases, theβ′phase acts as the key contributor to the high strength of the GN112K alloy. The existence of GBPs and PFZs deteriorates the mechanical properties and promotes the occurrence of intergranular fracture of the alloy.The cast alloys were primarily strengthened by the precipitates in the microstructure. The optimum heat treatment condition for the cast alloys are as follows: GN62K—500℃/6h + 200℃/24h, GN82K—515℃/4h + 225℃/12h, GN112K—525℃/4h + 250℃/2h, corresponding to tensile properties as follows: GN62K—TYS～182MPa, UTS～342MPa, E～7.9%; GN82K—TYS～200MPa, UTS～342MPa, E～5.0%; GN112K—TYS～224MPa, UTS～353MPa, E～3.7%. The strength lowers quickly as temperature goes above 200℃, which gets more drastic above 250℃, whilst the elongation goes up.Thermo-mechanical treatment was used to produce a variety of microstructures in the alloys, and proper treatment can further improve the mechanical properties. Cold stretching deformation between solution treatment and aging at 200℃creates high density of dislocations and twins. Pre-deformation accelerates the overaging process of the alloy due to heterogeneous precipitation ofβ1 phase at dislocations. The improvement of strength with pre-deformation is attributed to the increased dislocation density, the presence of twins and the heterogeneous precipitation ofβ1 phase in the pre-deformed specimens. Small pre-deformation can improve the yield strength of the alloys with slight damage of the ductility. The grains can be refined to below 10μm by hot extrusion. The typical tensile properties of hot extruded and aged alloys are as follows: GN62K(350℃extrusion+200℃/24h aging)—TYS～273MPa, UTS～381MPa, E～17.6%; GN82K(350℃extrusion +200℃/24h aging)—TYS～304MPa, UTS～397MPa, E～10.6%; GN112K(350℃extrusion +200℃/24h aging)—TYS～328MPa, UTS～422MPa, E～4.3%. Hot rolling can be used to refine the grains of homogenized GN62K plate and strengthening the alloy by work hardening at the same time. The hot rolled plates show isotropic tensile properties. The tensile properties of the rolled plate are as follows: TYS=330～356MPa, UTS=385～394MPa, E=5～10%.The creep behavior of the Mg-Gd-Nd-Zr alloys was studied in a temperature range of 250～300℃and a stress range of 50～100MPa. The activation energies for creep of the alloy were found to be 110～280KJ/mol at temperature range of 250～300℃. The stress exponents for creep were measured to be～4, indicating a dislocation creep mechanism. A sigmoidal creep stage appears between primary and steady-state creep stage in the creep curves for specimens in unaged, underaged, peak aged and slightly overaged conditions. On contrary, there is no evidence of such a creep stage for overaged specimens. The sigmoidal creep stage is induced by the dynamic precipitation during creep. The thermally stable equilibriumβ-phase is regarded as the primary contributor to the creep resistance of the alloy. The substantially overaged alloys show better creep resistance. Creep failure of the alloy is mainly initiated by means of the nucleation and growth of triple point wedge cracking. Cavity nucleation and growth at triple point and cracking along grain boundary facets are also the fracture mechanism. Precipitate free zones at the grain boundaries perpendicular to the applied stress can facilitate the intergranular fracture of the specimens during creep.