Journal of Magnesium and Alloys最新文献

In this study, the tensile creep behavior of a hot-rolled Mg-4Y-3.5Nd alloy subjected to different prior thermo-mechanical treatments was investigated at 220 ℃. Five groups of samples were prepared using different combinations of the solid solution (S), aging treatment at 220 ℃ for 30 h (A), and hot compression at 490 ℃ to a true strain of 0.25 (C). The abbreviations for the samples are S, SA, SC, SAC, and SCA. Upon examining the yield strength and creep resistance, it was found that creep resistance could not be directly predicted by the yield strength. The stability of the deformation bands (DBs) induced by prior thermo-mechanical treatment plays an important role in determining the creep resistance. The dislocation of the DBs and demonstrated the best creep resistance in the SAC sample, which were prepared using a solid solution, aging treatment, and subsequent hot compression. However, despite the highest yield strength, frequent dislocation motions destroyed the stability of the DBs and deteriorated the creep resistance of the SCA sample, which were prepared using a solid solution, hot compression, and subsequent aging treatment. Among the thermo-mechanical treatments used in this study, the application of aging treatment was important to obtain the resultant creep resistance. When the aging treatment was performed prior to hot compression, the creep resistance could be further enhanced based only on hot compression. Accordingly, the sequence from the strongest to the weakest creep resistance was SAC > SC > S > SCA > SA.

Currently, the hierarchical structure is one of the most effective means to enhance the strength and plasticity of metal materials, since the strain localization can be effectively delayed by the coordination of the unique microstructure. In this study, a hierarchical structure of Mg-15Gd-1Zn-0.4Zr (GZ151K) alloys containing grain, twin, and precipitation structural units was prepared by ultrasonic surface rolling process (USRP) and recrystallization annealing (RU). The results showed that the stress gradient generated by USRP formed a twin gradient structure, which will activate the twin-assisted precipitation (TAP) effect and twin-induced recrystallization (TIR) effect during RU. Then, the twin gradient structure transformed into a twin-precipitation gradient structure, and finally into a hierarchical structure with grain-twin-precipitation as the increasement of recrystallization degree. Besides, the dual gradient structure with twin and precipitation structural units had the highest strength and microhardness owing to the precipitation strengthening. However, the hierarchical structure with grain, twin, and precipitation structural units exhibited the most excellent combination of strength and plasticity under grain refinement and precipitation strengthening.

The Mg-7Gd-4Y-2Zn-0.5Zr alloy chips were successfully recycled through isothermal sintering and equal channel angular pressing (ECAP). The mechanical properties and microstructure evolution of samples during the recycling process were studied in detail. The eutectic phases in the as-cast alloy transform into long period-stacking ordered (LPSO) phases after homogenization, which can improve the plasticity of the material. After isothermal sintering, the density of the sample is lower than that of the homogenized sample, and oxide films are formed adjacent to the bonding interface of the metal chips. Hence, the plasticity of the sintered sample is poor. Dense samples are fabricated after ECAP. Although the grains are not refined compared to the sintered sample, the microstructure becomes more uniform due to recrystallization. Fiber interdendritic LPSO phase and kinked 14H-LPSO phase are formed in the alloy due to the shear deformation during the ECAP process, which improves the strength and plasticity of the sample significantly. Furthermore, the basal texture is weakened due to the Bc route of the ECAP process, which can increase the Schmid factor of the basal slip system and improve the elongation of the sample. After 2 ECAP passes, the fully densified recycled billet shows superior mechanical properties with an ultimate tensile strength of 307.1 MPa and elongation of 11.1%.

In this work, anisotropic fatigue crack growth rate (FCGR) behaviour in a hot-rolled Mg-3wt%Al-0.5wt%Ce alloy was investigated using compact tension (CT) specimens with notch (a_n) parallel to the rolling direction (RD) and transverse direction (TD). The FCGR tests were conducted at a constant load ratio (R = 0.1) and maximum stress intensity factor (K_Max = 15.6 MPa√m) to investigate the crack closure effect. For both constant R and K_Max conditions: (i) the load-displacement curves for every loading cycle were linear for a_n ∥ to RD and TD, indicating no crack closure; (ii) the FCGR was found to be lower for a_n ∥ RD than a_n ∥ TD over the entire stress intensity factor range (ΔK). The hot-rolled sample contained long-aligned Al₁₁Ce₃ intermetallic phase within grain boundaries that are elongated along RD. During the FCGR test, $\left\{10\overline{1}2\right\}〈10\overline{1}1〉$ extension twins (ET) with lamellae ∼⊥ and c-axis $\sim \parallel$ to these elongated intermetallics along RD developed irrespective of the notch orientation. During the loading cycle, these intermetallics along RD generate back-stresses, reducing the in-plane tensile stress $\sim \parallel$ and $\sim \perp$ to crack-tip to $\sim 0$ for a_n ∥ to RD and TD, respectively. Hence, lenticular ET $\sim \perp$ and $\sim \parallel$, with c-axis $\sim \parallel$ and $\sim \perp$ to crack path activates, leading to trans and inter lamellar crack for a_n ∥ to RD and TD, respectively, and anisotropic FCGR. Translamellar crack in a_n ∥ RD reduces the FCGR due to plastic energy dissipation as perceived by comparatively more geometrically necessary boundaries (GNBs). On the other hand, faster FCGR was obtained for a_n ∥ TD due to interlamellar cracking. Thus, the crack growth through the matrix-ET interfaces was favoured due to strain incompatibility. The Fractography for a_n ∥ RD shows smaller elongated grooves along crack propagation, which indicates crack arrest. However, larger elongated grooves for a_n ∥ TD indicated easy crack propagation due to favourable interlamellar crack.

According to a high-temperature compression test of rare earth magnesium alloy (WE43), a strain-compensated constitutive model of the Arrhenius equation based on Zener-Hollomon parameters was established, and the rheological behaviors were predicted. The model exhibited relatively serious prediction distortion in the low-temperature and high-strain rate parameter interval, and its accuracy was still unsatisfactory even after modification by a correction operator considering the coupling of temperature and strain rate. The microstructure characterization and statistical analysis showed that a large number of twinning occurred in the parameter intervals with prediction deviation. The occurrence of twinning complicated the local internal stress distribution by drastically changing the crystal orientation and led to significant fluctuations in the macroscopic strain-stress and hardening curves relative to the rheological processes dominated by the dislocation and softening mechanisms, making the logarithm of the strain rate and stress deviate from the linear relationship. This twinning phenomenon was greatly influenced by the temperature and strain rate. Herein, the influence mechanism on twinning behavior was analyzed from the perspective of the interaction of dislocation and twinning.