Effects of the extrusion parameters on microstructure,texture and room temperature mechanical properties of extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy

Microstructure,texture,and mechanical properties of the extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy were investigated at different extrusion temperatures(260 and 320℃),extrusion ratios(10:1,15:1,and 30:1),and extrusion speeds(3 and 6 mm/s).The experimental results exhibited that the grain sizes after extrusion were much finer than that of the homogenized alloy,and the second phase showed streamline distribution along the extrusion direction(ED).With extrusion temperature increased from 260 to 320℃,the microstructure,texture,and mechanical properties of alloys changed slightly.The dynamic recrystallization(DRX)degree and grain sizes enhanced as the extrusion ratio increased from 10:1 to 30:1,and the strength gradually decreased but elongation(EL)increased.With the extrusion speed increased from 3 to 6 mm/s,the grain sizes and DRX degree increased significantly,and the samples presented the typical<2111>-<1123>rare-earth(RE)textures.The alloy extruded at 260℃ with extrusion ratio of 10:1 and extrusion speed of 3 mm/s showed the tensile yield strength(TYS)of 213 MPa and EL of 30.6%.After quantitatively analyzing the contribution of strengthening mechanisms,it was found that the grain boundary strengthening and dislocation strengthening played major roles among strengthening contributions.These results provide some guidelines for enlarging the industrial application of extruded Mg-RE alloy.

Understanding the creep behaviors and mechanisms of Mg-Gd-Zn alloys via machine learning

Mg-Gd-Zn based alloys have better creep resistance than other Mg alloys and attract more attention at elevated temperatures.However,the multiple alloying elements and various heat treatment conditions,combined with complex microstructural evolution during creep tests,bring great challenges in understanding and predicting creep behaviors.In this study,we proposed to predict the creep properties and reveal the creep mechanisms of Mg-Gd-Zn based alloys by machine learning.On the one hand,the minimum creep rates were effectively predicted by using a support vector regression model.The complex and nonmonotonic effects of test temperature,test stress,alloying elements,and heat treatment conditions on the creep properties were revealed.On the other hand,the creep stress exponents and creep activation energies were calculated by machine learning to analyze the variation of creep mechanisms,based on which the constitutive equations of Mg-Gd-Zn based alloys were obtained.This study introduces an efficient method to comprehend creep behaviors through machine learning,offering valuable insights for the future design and selection of Mg alloys.

Microstructure Evolution during Solidification of AZ91D Magnesium Alloy in Semisolid

The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.

Effect of the precipitation state on high temperature tensile and creep behaviors of Mg-15Gd alloy

Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.

QoS-Aware Power Allocation Scheme for Relay Satellite Networks

This paper investigated a QoS-aware power allocation for relay satellite networks.For the given QoS requirements,we analyzed the signal model of relay transmission and formulated the power minimization problem which is non-convex and difficult to solve.To find the optimal solution to the considered problem,we first analyzed the optimization problem and equivalently turn it into a convex optimization problem.Then,we provided a Lagrangian dual-based method to obtain the closed-form of the power allocation and provided an iterative algorithm to the optimal solution.Moreover,we also extended the results to the cooperative transmission mode.Finally,simulation results were provided to verify the superiority of the proposed algorithm.

Additive manufacturing of Ti-6Al-4V lattice structures with high structural integrity under large compressive deformation

Additively manufactured Ti-6 Al-4 V lattice structures have found important niche applications. However, they often show insufficient compressive ductility or insufficient structural integrity. In this study,a batch of 45 octahedral Ti-6 Al-4 V lattice structures was manufactured in three different strut diameters(0.5, 1.0, 1.5 mm) by selective electron beam melting(SEBM). The influence of post-SEBM annealing on the compressive deformation characteristics of the lattice structure was investigated. The as-built Ti-6 Al-4 V lattices fragmented when the compressive strain reached 13%–23% depending on strut diameter.Annealing at 950?C(β transus temperature: 995?C) only slightly improved the compressive ductility of the lattice structures. However, annealing at 1050?C(β-annealing) fundamentally changed the compressive deformation mode of the lattice structures. The resultant compressive stress-strain curve was featured by a long smooth plateau and no facture occurred even after significant densification of the lattice structure had taken place(>50% of compressive strain).

Microstructure evolution,mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6 Zn-2 Gd-0.5 Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primaryα-Mg matrix, α-Mg +W（Mg;Zn;Gd;） eutectic along grain boundaries, and icosahedral quasicrystalline I（Mg;Zn;Gd） phase within α-Mg matrix. During homogenization process, α-Mg +W（Mg;Zn;Gd;） eutectic and I phase gradually dissolved into a-Mg matrix, while some rod-like rare earth hydrides（GdH2）formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505℃ for 16 h according to the microstructure evolution. Furthermore,the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.

Developing an efficient and visible prime editing system to restore tobacco 8-hydroxy-copalyl diphosphate gene for labdane diterpene Z-abienol biosynthesis

Prime editing(PE)is a versatile CRISPR-Cas based precise genome-editing platform widely used to introduce a range of possible base conversions in various organisms.However,no PE systems have been shown to induce heritable mutations in tobacco,nor in any other dicot.In this study,we generated an efficient PE system in tobacco that not only introduced heritable mutations,but also enabled anthocyanin-based reporter selection of transgene-free T_(1) plants.This system was used to confer Zabienol biosynthesis in the allotetraploid tobacco cultivar HHDJY by restoring a G>T conversion in the NtCPS2 gene.High levels of Z-abienol were detected in the leaves of homozygous T_(1) plants at two weeks after topping.This study describes an advance in PE systems and expands genome-editing toolbox in tobacco,even in dicots,for use in basic research and molecular breeding.And restoring biosynthesis of Z-abienol in tobacco might provide an efficient way to obtain Z-abienol in plants.

Microstructure, Microsegregation, and Mechanical Properties of Directional Solidified Mg–3.0Nd–1.5Gd Alloy

The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary a（Mg） phase and interdendritic a（Mg） · Mg12（Nd, Gd） eutectic and Mg5 Gd phase. The primary dendrite arm spacing k1 and secondary dendrite arm spacing k2 were found to be depended on the cooling rate R in the form k1= 8.0415 9 10-6R-0.279 and k2= 6.8883 9 10-6R-0.205, respectively, under the constant temperature gradient of40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within a（Mg） matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved,while the corresponding elongation decreased with the increase of growth rate.

Switch of substrate specificity of hyperthermophilic acylaminoacyl peptidase by combination of protein and solvent engineering

The inherent evolvability of promiscuous enzymes endows them with great potential to be artificially evolved for novel functions.Previously,we succeeded in transforming a promiscuous acylaminoacyl peptidase(apAAP)from the hyperthermophilic archaeon Aeropyrum pernix K1 into a specific carboxylesterase by making a single mutation.In order to fulfill the urgent requirement of thermostable lipolytic enzymes,in this paper we describe how the substrate preference of apAAP can be further changed from p-nitrophenyl caprylate(pNP-C8)to p-nitrophenyl laurate(pNP-C12)by protein and solvent engineering.After one round of directed evolution and subsequent saturation mutagenesis at selected residues in the active site,three variants with enhanced activity towards pNP-C12 were identified.Additionally,a combined mutant W474V/F488G/R526V/T560W was generated,which had the highest catalytic efficiency(kcat/Km)for pNP-C12,about 71-fold higher than the wild type.Its activity was further increased by solvent engineering,resulting in an activity enhancement of 280-fold compared with the wild type in the presence of 30%DMSO.The structural basis for the improved activity was studied by substrate docking and molecular dynamics simulation.It was revealed that W474V and F488G mutations caused a significant change in the geometry of the active center,which may facilitate binding and subsequent hydrolysis of bulky substrates.In conclusion,the combination of protein and solvent engineering may be an effective approach to improve the activities of promiscuous enzymes and could be used to create naturally rare hyperthermophilic enzymes.