China Nonferrous Metallurgy

Rare earth doping and deep cryogenic treatment are both advanced methods for improving material performance. In this study, a novel AZ91D-0.1Ti-1.5Sm magnesium alloy was successfully fabricated by introducing Ti and Sm alloying elements into the AZ91D magnesium alloy matrix via an alloying approach. The alloy was subjected to cryogenic treatment in liquid nitrogen at -196℃ for durations of 6 h, 12 h, 18 h, and 24 h, respectively. A comprehensive combination of characterization methods including optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), mechanical property testing, and electrochemical corrosion measurements was employed to systematically investigate the influence and underlying mechanisms of cryogenic treatment time on the microstructure, mechanical properties, and corrosion resistance of the alloy. The results demonstrate that cryogenic treatment refines the α-Mg matrix grains and significantly increases the volume fraction of the β-Mg₁₇Al₁₂ phase and Al₂Sm rare-earth phase with a more dispersed distribution. However, as the treatment time extends to 24 h, the β-Mg₁₇Al₁₂ phase gradually transforms from dispersed particles into a continuous network-like morphology, accompanied by coarsening and agglomeration of the Al₂Sm phase. Regarding mechanical properties, optimal comprehensive performance is achieved at 12 h of cryogenic treatment, with microhardness increasing from 69.54 HV (untreated sample) to 72.38 HV, tensile strength from 189 MPa to 196 MPa, and elongation from 4.90% to 5.13%. Nevertheless, after 24 h of treatment, the mechanical properties deteriorate significantly, with microhardness dropping to 67.12 HV, tensile strength to 176 MPa, and elongation to 4.36%. In terms of corrosion performance, the 12 h cryogenically treated sample exhibits the best corrosion resistance, with corrosion current density decreasing from 15.8 μA·cm^-2 (as-cast) to 10.2 μA·cm^-2, and corrosion potential shifting positively from -0.79 V to -0.69 V. After 24 h, the corrosion resistance deteriorates, with corrosion current density increasing to 43.6 μA·cm^-2. Controlled cryogenic treatment duration (with 12 h identified as the optimal processing window) can effectively enhance both the mechanical properties and corrosion resistance of this magnesium alloy.