Processing, microstructure and mechanical behavior of bulk nanostructured Mg alloy

Due to lightweight and high specific strength, the research and development of magnesium-based alloys has been widely expanded. New magnesium alloys and novel processing technology also have been developed to satisfy the need for applications in the automotive, communications and aerospace industries. In particular, ultrafine grain sized (UFG) and even nanostructured (NS) magnesium alloys fabricated via severe plastic deformation have attracted most of researchers’ attention because of impressive mechanical properties compared to micro sized Mg alloys. With a UFG, NS or a mixed microstructure, exceptional high strength with good ductility could be achieved.
A combination of cryomilling and spark plasma sintering (SPS) was employed in this project to fabricate a NS magnesium alloy. Nanocrystalline (NC) AZ31 powders were produced by cryomilling. A minimum average grain size of approximately 26 nm was obtained when the cryomilling time was extended to 6 hours or longer. Cold welding played a dominant role in the early stage of cryomilling, while fracture took place in the late stage and surpassed cold welding. The former increased while the latter decreased the particle size. The highest hardness of around 160 HV was obtained after 6 hours cryomilling.
This cryomilled NC powder showed excellent thermal stability during annealing at elevated temperatures. There were two separate growth stages with a transition point around 400 °C. More specifically, between 350 and 400 °C, NC Mg grains stabilized around 32 nm, even after 1h heating. At 450 °C, the nano grains grew to 37 nm in the first 5 minutes and grew quickly to approximately 60 nm after 15 minutes. Nevertheless, the average grain size was still less than 100 nm even after 60 minutes annealing at 450 °C.
Bulk nanostructured (NS) Mg AZ31 alloy was produced by spark plasma. The bulk samples consolidated at 400 °C with an average grain size of 45 nm showed exceptional average true compressive yield strength of 408.7 MPa and true ultimate compressive strength of 504.0 MPa. These values were superior to published results for most of conventional Mg alloys. Higher sintering temperature (425-450 °C) improved compressive strain at the sacrifice of strength, while samples consolidated at 350 °C displayed brittle behavior with low strength. However, true compressive strains of these four samples were all less than 0.06 at true ultimate compressive strength.
To enhance the ductility of bulk NS Mg AZ31 alloy in this study, a facile strategy, in situ powder casting during SPS, was introduced. Different amounts of eutectic Mg-Zn alloy powders with low melting temperature approximately 350 °C were mixed with cryomilled powder. During SPS at 400 °C, the low melting temperature eutectic alloy particles melted, and flowed along cryomilled powder particle boundaries and partly dissolved into the Mg matrix. The compressive strain was improved by in situ powder casting during SPS without loss of strength, especially when 20 % (wt. %) of eutectic alloy powder was added. Compared to samples sintered by pure cryomilled powder, its compressive strain was extended from 3.6% to 6.6%. The reason for this was in situ powder casting can simultaneously significantly remove the artifacts such as porosity, enhance the inter-particle bounding between nanostructured Mg particles and introduce very small dense precipitates into bulk NS Mg alloy.