A general thermodynamic model for the long-period stacking ordered phases in magnesium alloys

A thermodynamic model Mg x(Xs,Mg)6(Xl,Mg)8(Xs and Xl are elements smaller and larger than Mg)for long-period stacking ordered phases(LPSOs)was proposed based on two key factors:the Xs 6 Xl 8-type L12 clusters and the variation of chemical compositions.In general,all available LPSOs can be described with this model.As a representative system,Mg-Y-Zn with three LPSOs was investigated using the CALPHAD(calculation of phase diagram)approach aided with first-principles calculations.Two new three-phase equilibria were predicted and were validated by key experiments.The model-based descriptions will be the basis for the research and development of magnesium alloys.

Review: Application of the CALPHAD Approach and First-Principles Calculations to Electrode Materials in Li Ion Batteries

Design and development of novel electrode materials is one of several hot topics in studying Li ion battery. The CALPHAD(CALculation of PHAse Diagrams) approach enables calculation of stable and metastable phase equilibria,as well as thermodynamic properties for various materials,w hich has been applied to accelerate modern materials design in recent years. The traditional trial-and-error method is being replaced by the integration of CALPHAD with first-principles calculations,as well as empirical methods and key experiments. The CALPHAD approach and first-principles calculations have been proved to be a powerful tool in studying electrode materials, not only for calculation of phase equilibria and thermodynamic properties,but also for prediction of cell voltages in Li ion batteries,which allows for the design of future electrode materials with improved stability and efficiency. Examples of the cathode systems(Li-O,Li-Co-O and Li-Ni-O) and anode systems(Li-Sb and Li-Sn),which are studied by applying the CALPHAD approach and first-principles calculations,are presented.

Design of novel NiSiAlY alloys in marine salt-spray environment:PartⅡ.Al-Ni-Si-Y thermodynamic dataset

The NiSiAlY material is a promising candidate to replace NiCrAlY,which can withstand the harsh saltspray conditions in marine environment.To efficiently design novel NiSiAlY alloys,this work establishes a thermodynamic dataset of the Al-Ni-Si-Y quaternary system using the CALPHAD(CALculation of PHAse Diagrams)approach.We employ this database to calculate and predict the phase constitutions and solidification behaviors of different NiSiAlY alloys concerning the content variations of Al and Si.We have further proposed the selection of the NiSiAlY alloys for serving in marine salt-spray environment with three constraints:(i)outstanding mechanical property;(ii)good high-temperature anti-oxidation;(iii)excellent corrosion resistance.This results in a compositional range of the NiSiAlY alloys with 1 wt%<w(Si)<5 wt%,11 wt%<w(Al)<20 wt%and w(Y)=1 wt%,which corresponds the L1_(2)+bcc B2+Ni_(5)Y ternary phase region at temperatures ranging from~500 to~1000℃.Our predictions are validated by key experiments,suggesting that the model-based description of the Al-Ni-Si-Y system can serve as a guidance to design the novel NiSiAlY alloys in resisting harsh salt-spray environment.

High-temperature wear mechanisms of TiNbWN films:Role of nanocrystalline oxides formation

Réfractory high/medium entropy nitrides(HENs/MENs)exhibit comprehensive application prospects as protective films on mechanical parts,particularly those subjected to sliding contacts at elevated temperatures.In this study,a new MEN system TiNbWN,forming a single fc solution,is designed and its wear performance at temperatures ranging from 25 to 750℃is explored.The wear mechanisms can be rationalized by examining the subsurface microstructural evolutions using the transmission electron microscopy as well as calculating the phase diagrams and interfacial adhesion behavior employing calculation of phase diagram(CALPHAD)and density functional theory(DFT).To be specific,increased wear losses occur in a temperature range of 25-600℃,being predominantly caused by the thermally-induced hardness degradation;whereas at the ultimate temperature(750℃),the wear loss is refrained due to the formation of nanocrystalline oxides(WnO_(3n-2r)TiO_(2),and TiOx),as synergistically revealed by microscopy and CALPHAD,which not only enhance the mechanical properties of the pristine nitride film,but also act as solid lubricants,reducing the interfacial adhesion.Thus,our work delineates the role of the in situ formed nanocrystalline oxides in the wear mechanism transition of TiNbWN thin films,which could shed light on the high-temperature wear behavior of refractory HEN/MENfilms.

Design of novel NiSiAlY alloys in marine salt-spray environment:PartⅠ.Al-Si-Y and Ni-Si-Y subsystems

MCrAlY(M=Ni,Co,NiCo)alloys have been widely used as a bond layer for thermal barrier coatings(TBCs).However;oxides form discontinuously on MCrAlY surface in the marine environment since Cr_(2)O_(3)can react with NaCl and H_(2)O at temperatures above 500℃.Replacing Cr with other alloying elements(such as Si)might prevent severe salt-spray corrosion effectively.To save time and efforts in materials design of the Ni-Si-Al-Y system;we obtained the thermodynamic descriptions of the ternary compounds by coupling the CALPHAD(CALculation of PHAse Diagrams)approach and first-principles calculations.Seven and eighteen ternary compounds were evaluated and calculated in the Al-Si-Y and Ni-Si-Y systems;respectively.Phase diagrams of the two systems were depicted after a series of experimental Al/Ni-Si-RE(RE denotes rare earth elements,including La,Ce,Pr,Nd,Sm,Gd,Dy,Ho,and Er)systems being analyzed.It is found that the RE elements with the same reference states tend to form ternary compounds with the same crystal structures.The model-based descriptions are the basis for the research and development of novel NiSiAlY alloys in resisting high-temperature corrosion.

Design of the rare-earth-containing materials based on the micro-alloying phase equilibria, phase diagrams and phase transformations

Rare-earth(RE)elements,known as“industrial vitamins”,have permeated modern lives,especially in high-tech applications.Although the RE elements possess close chemical similarities and have been treated as“one element”in the periodic table,their characteristics differ from each other.The RE microalloying effect is the crux to ameliorate the physicomechanical and thermochemical properties of materials,thereby the study of RE-related phase diagrams becomes indispensable to the design and optimization of RE-containing materials.However,in reality,the knowledge base in this area is considerably scarce compared with that of other commonly-used elements.In this work,the phase equilibria,phase diagrams,phase transformations,and some recent examples of RE-containing materials design are summarized,with which one can predict the RE solubilities,the RE precipitates,as well as the corresponding service behaviors.The attainment of enhanced materials’properties suggests that the thermodynamic rules extracted from the phase diagrams could serve as fundamental criteria for the successful development of novel RE-containing materials.

Efficient prediction of corrosion behavior in ternary Ni-based alloy systems:Theoretical calculations and experimental verification

Pourbaix diagrams are calculated to describe electrochemical processes for alloys in aqueous solution.With the multi-component differentiation of alloy systems,the construction of Pourbaix diagrams is fac-ing challenges,especially for non-single-phase alloy systems.In this study,the simultaneous construction of phase diagrams and Pourbaix diagrams were implemented for predicting the evolution of the phases in the immune and passive regions.The CALPHAD(CALculation of PHAse Diagram)approach was used to quickly access the Gibbs free energies of various phases and the chemical potential of the elements in the phases from the thermodynamic database of the Ni-Si-Al-Y system.The corrosion behavior of two typical Ni-Al-Si and Ni-Al-Y systems was investigated.Si and Y were added to Ni-based alloys to produce the solid solutions L12-Ni_(3)(Al,Si)and L12-Ni_(3)Al+Ni_(5)Y,respectively.Calculations showed that NiO and Al_(2)O_(3)make up the passive area of the Ni_(3)Al 1 alloy.The introduction of SiO_(2)and Y(OH)3 in the passive region separately helped to minimize the alloys’susceptibility to corrosion.However,Si reduced the thermody-namical possibility of NiO for mation in the passive film,and the addition of Y caused extreme galvanic corrosion.Experiments on Ni-based alloys validated the results through electrochemical corrosion.It was also discovered that the presence of Ni_(5)Y produced galvanic corrosion and that Si reduced the oxide in the passive film,causing pitting corrosion.The corrosion prediction of the quaternary alloys indicates that the solid solution of Si in Ni_(5)Y reduces the galvanic corrosion effect and the dissolution of passive film.The current work demonstrates that phase diagrams and Pourbaix diagrams may be efficiently and accurately predicted using a well-constructed thermodynamic database,which has major implications for future studies on the corrosion behavior of multi-component alloys.