查看更多>>摘要：Revealing the oxidation behavior of superalloys is crucial for optimizing material properties and extending service life.This study investigated the oxidation behavior of superalloy GH4738 under stress states at 850 ℃.High-throughput speci-mens were fabricated to withstand different stresses at the same time.Isothermal oxidation samples were analyzed using the mass gain method to obtain oxidation kinetic curves.The results show that the external stress below 200 MPa could improve the oxi-dation resistance of the GH4738.With tensile stress increasing,the oxide layer becomes thinner,denser and more complete,while internal oxidation decreases.The tensile stress alters the structure of the external oxide layer from a two-layer to a three-layer configuration.The Cr2O3 oxide layer inhibits the outward diffusion of Ti,leading to Ti enrichment at the oxide-matrix interface and altering the oxidation mechanism of GH4738.
查看更多>>摘要：Oxidation resistance enhancement of pure Ti often comes at the cost of reduced ductility,which is fre-quently the problem through alloying with sole Al,Si,W,Mo and B.To overcome the short coming of single element alloying,this paper proposes a multi-element low-alloying strategy to take advantage of synergistic effects and resolve the conflict between oxidation resistance and ductility.It demonstrates that the addition of a small quantity of Ta(0.51 wt％)can boost both oxidation resistance and ductility in comparison to pure Ti.Furthermore,the combined addition of a small amount(0.54 wt％)of Ta,Nb and Si not only preserves good ductility of pure Ti,but also reduces mass gains to 14％-67％of pure Ti during 100 h oxidation at 650-850 ℃ in air.This indicates even better oxidation resistance than that obtained through the use of Ta,Nb,or Nb+Ta additions.The Ta+Nb+Si alloying creates an oxide layer that is less porous and more resistant to stratification and spalling.Consequently,a 3-pm N-rich layer can form in the Ti substrate beneath the oxide scale,in which phase transformation generates coherent Ti2N with(0001)Ti as the habit plane,with N atoms prefers to diffuse along[2110]Tithan along[0001]Ti.The completely transformed Ti2N region or partially transformed Ti+Ti2N region can effectively impede oxygen invasion.Therefore,the multi-element low-alloying strategy is promising for enhancing both oxidation resistance and mechanical properties of metallic materials in the future.
查看更多>>摘要：Thermal stable intermetallic particles are important for the heat resistance of magnesium(Mg)alloys.In this work,many lath-like particles formed in α-Mg grains of a Mg-8Gd-3Sm-0.7Al casting alloy when heat-treated at 873 K.Atomic-resolution high-angle annular dark field scanning transmission electron micro-scopy(HAADF-STEM)characterizations indicate that most of them are Mg-containing Al2(Gd,Sm),with the atomic ratio of Mg∶Al∶(Gd,Sm)being～1∶1∶1;a small part of them with relatively wider thickness are long-period stacking ordered(LPSO)phases simultaneously containing both 14H and 18R structures.Both followed common orientation relationships with Mg matrix as those reported in previous work.In addition,many Mg laths were observed in the primary blocky Al2(Gd,Sm)phase at grain boundaries,where the atomic ratio of Al∶(Gd,Sm)in the Al2(Gd,Sm)matrix was 2∶1.Finally,density functional theory(DFT)calculations illustrated the detail structure of the re-constructed Mg/Al2RE interface and simultaneously deduced the underlying reason for the re-dissolution of the newly formed Mg-containing Al2(Gd,Sm)plates in α-Mg matrix.
查看更多>>摘要：Increasing the recrystallization temperature to achieve better high-temperature performance is critical in the development of molybdenum alloys for ultrahigh-temperature applications,such as the newest generation of multitype high-temperature nuclear reactors.In this study,an innovative strategy was proposed to improve the per-formance of molybdenum alloys at high temperature by using the two-dimensional MAX(where M is an early transition metal,A is an A-group element and X is C or N)ceramic material Ti3AlC2.The relationships between flow stress,strain rate and temperature were studied.The microstructure,distribution of misorientation and evolution of dislocations in the Mo-Ti3AlC2 alloy were analyzed.The microscopic mechanism of the Ti3AlC2 phase in the molybdenum alloy at high temperatures was clarified.The experimental results showed that the peak flow stress of Mo-Ti3AlC2 at 1600 ℃ reached 155 MPa,which was 161.8％greater than that of pure Mo.The activation energy of thermal deformation of Mo-Ti3AlC2 was as large as 537 kJ·mol-1,which was 17.6％more than that of pure Mo.The recrystallization temperature reached 1600 ℃ or even higher.The topological reaction of the Ti3AlC2 phase consumed a large amount of energy at high temperatures,resulting in increases in the deformation activation energy.Nanolayer structures of AlTi3 and Ti-O Magnéli-phase oxides(TinO2n-1)were formed in-situ,which relied on kink bands and interlayer slip,resulting in many dislocations during deformation.Therefore,the special two-dimen-sional of the structure Ti3AlC2 ceramic inhibited the recrystallization behavior of the Mo alloy.The results of this study can provide theoretical guidance for the devel-opment of a new generation of molybdenum alloys for use in ultrahigh-temperature environments.
查看更多>>摘要：A promising method for the preparation of pure metallic vanadium via V2CO anode electrolysis was pro-posed and confirmed.The simple and controllable syn-thesis of a molten chloride salt containing VCl2 by an in-situ reaction between VCl3 and metallic vanadium was verified through thermodynamic analysis and experiments.NaCl-KCl-VCl2 molten salt exhibited excellent recycling performance and could be repetitively used to prepare metallic vanadium by V2CO electrolysis.The V2+ions remained in the molten salt after electrolysis.The elec-trolysis conditions,such as the cathode diameter,current density,V2+ion concentration,and temperature,were optimised to maximise the current efficiency.Electrolysis was also simulated to reveal the regulatory mechanism.The highest current efficiency was 85％.The purity of metallic vanadium was up to 99.8％.In this study,an easy and efficient preparation of pure metallic vanadium was achieved.
查看更多>>摘要：Finite-temperature ductility-brittleness and electronic structures of Al3Sc,Al2Sc and AlSc are studied comparatively by first-principles calculations and ab initio molecular dynamics.Results show that Al3Sc and Al2Sc are brittle at both ground state and finite temperatures,while AlSc possesses a significantly superior ductility.At ground state,AlSc is ductile from Pugh's and Poisson's criteria,while it is brittle in Pettifor's model.The ductility of all Al3Sc,Al2Sc and AlSc improves greatly with the elevated temperature.Especially,the Cauchy pressure of AlSc undergoes a transition from negative to positive.At T＞600 K,AlSc is unequivocally classified as ductile from all criteria considered.In all compounds,the Al-Al bond originated from s-p and p-p orbital hybridizations,and the Al-Sc bond dominated by p-d covalent hybridization,are the first and second strongest chemical bonds,respectively.To explain the difference in mechan-ical properties,the mean bond strength(MBS)is intro-duced in this work.The weaker Al-Al bond in AlSc,leading to a smaller MBS,could be the origin of the softer elastic stiffness and superior intrinsic ductility.The longer length of the Al-Al bond in AlSc is responsible for its weaker bond strength.Furthermore,the enhanced metal-licity of the Al-Al bond in AlSc would also contribute to its exceptional ductility.The longer length of the Al-Al bond in AlSc is responsible for its weaker bond strength.Furthermore,the enhanced metallicity of the Al-Al bond in AlSc would also contribute to its exceptional ductility.
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