Precipitation strengthening in nanostructured AZ31B magnesium thin films characterized by nano-indentation, STEM/EDS, HRTEM, and in situ TEM tensile testing

Thin magnesium (Mg) AZ31B (Mg-3Al-1Zn-<0.4Ca-<0.3Mn-<0.2Si in wt%) foils were sputter deposited, aged, and mechanically tested at quasi-static strain rates. The as-deposited microstructure is a hexagonal close-packed solid solution with no second phase particles and a strong basal texture. Subsequent aging at 200 °C for up to 170 h led to nanocluster and precipitate formation that was characterized by TEM and significant hardening that was characterized by nanoindentation. Precipitates containing Al, Mn, and Mg were clustered around grain boundaries and dislocations in samples aged at 200 °C for 65 h, but were distributed uniformly in a sample aged at 400 °C for 30min. The peak hardness was greater than 2 GPa and is explained by a submicron grain size and precipitation strengthening. In situ straining TEM experiments were performed on samples aged at 200 °C for 65 h, as well as on samples aged at 400 °C for 30min, to examine the effects of nanoclusters and precipitates in obstructing dislocation motion. Dislocation glide along basal planes was identified as the main deformation mechanism in samples aged at 200 °C and 400 °C, and pinning of dislocations by precipitates was observed in the sample age at 400 °C. Dislocations are assumed to cut through the nanoclusters in the 200 °C sample. The fracture mechanism was consistently trans-granular cracking, regardless of the thermal treatment history, and twins were not observed to nucleate or propagate within these fine-grained, highly textured samples.

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