Simulation of pre-precipitation in Ni_(75)Al_(14)Mo_(11) alloy by microscopic phase-field model

The early precipitation process of Ni（75）Al（14）Mo（11） alloy was simulated by microscopic phase-field model at different temperatures.The microstructure of the alloy,the precipitation time of Llo structure and occupation probability of the three kinds of atoms were investigated.It is indicated that the non-stoichiometric Ll0（Ⅰ/Ⅱ） phases are found in the precipitation process.With the temperature increasing,the appearance time of Ll0 is brought forward.The Ll0（Ⅱ） structure always precipitates earlier than the Ll0（Ⅰ） structure.Compared with lower temperature,higher temperature brings the formation time of Ll0 phase forward and makes Ll0 phase have a higher order degree.But lower temperature shortens the process time of the Ll0 phase to the Ll2 phase.Al and Mo atoms tend to occupy γ site,Ni atom tends to occupy a and β sites.At the same temperature,Al atom has stronger occupation ability than Mo atom in the same site.Ni,Al and Mo collectively form the composited Ll2 structure.

Site occupation evolution of alloying elements in L1_2 phase during phase transformation in Ni_(75)Al_(7.5)V_(17.5)

Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.

Site occupation evolution of alloying elements in Ni_3 V phase during phase transformation in Ni_(75)Al_(4.2)V_(20.8)

Based on the microscopic phase-field model, the correlation between site occupation evolution of alloying elements in Ni3V-DO22 phase and growth of Ni3Al-L12 phase was studied during the phase transformation of Ni75Al4.2V20.8. The results demonstrate that the growth of L12 phase can be divided into two stages： at the early stage, the composition of alloying elements in DO22 phase almost remains unchanged; at the late stage, the compositions of Ni and Al decrease while V increases in DO22 phase. Part of alloying elements for L12 phase growth are supplied from the site occupation evolution of alloying elements on three kinds of sublattices in DO22 phase. Ni is mainly supplied from V sublattice, and part of Al is supplied from NiⅠ and V sites at the centre of DO22 phase. The excessive V from the decreasing DO22 phase migrates into the centre of DO22 phase and mainly occupies V and NiII sites. It is the site occupation evolution of antisite atoms and ternary additions in DO22 phase that controls the growth rate of L12 phase at the late stage.

Corrosion performance and tribological behavior of diamond-like carbon based coating applied on Ni−Al−bronze alloy

The effect of diamond-like carbon(DLC)coating(fabricated by cathodic arc deposition)on mechanical properties,tribological behavior and corrosion performance of the Ni−Al−bronze(NAB)alloy was investigated.Nano-hardness and pin-on-plate test showed that DLC coating had a greater hardness compared with NAB alloy.Besides,the decrease in friction coefficient from 0.2 for NAB substrate to 0.13 for the DLC-coated sample was observed.Potentiodynamic polarization and EIS results showed that the corrosion current density decreased from 2.5μA/cm2 for bare NAB alloy to 0.14μA/cm2 for DLC-coated sample in 3.5 wt.%NaCl solution.Moreover,the charge transfer resistance at the substrate−electrolyte interface increased from 3.3 kΩ·cm2 for NAB alloy to 120.8 kΩ·cm2 for DLC-coated alloy,which indicated an increase in corrosion resistance due to the DLC coating.

Microstructure features and mechanical/electrochemical behavior of directionally solidified Al−6wt.%Cu−5wt.%Ni alloy

The effects of the addition of 5.0 wt.%Ni to an Al−6wt.%Cu alloy on the solidification cooling rate(T),growth rate(V_(L)),length scale of the representative phase of the microstructure,morphology/distribution of intermetallic compounds(IMCs)and on the resulting properties were investigated.Corrosion and tensile properties were determined on samples solidified under a wide range of T along the length of a directionally solidified Al−6wt.%Cu−5.0wt.%Ni alloy casting.Experimental growth laws were derived relating the evolution of primary(λ_(1))and secondary(λ_(2))dendritic spacings with T and V_(L).The elongation to fracture(δ)and the ultimate tensile strength(σ_(U))were correlated with the inverse of the square root of λ_(1) along the length of the casting by Hall−Petch type experimental equations.The reinforcing effect provided by the addition of Ni in the alloy composition is shown to surpass that provided by the refinement of the dendritic microstructure.The highest corrosion resistance is associated with a microstructure formed by thin IMCs evenly distributed in the interdendritic regions,typical of samples that are solidified under higher T.

Effect of Zirconium on Tensile Property,Microstructure and Fracture Behaviour of Cast Ni_(3)Al Based Alloy

The effect of Zr on tensile property, microstructure and fracture behaviour of cast Ni 3Al based alloy strengthened with carbides has been studied. It was found that Zr distributes at interdendrites and grain boundaries in the form of ZrC particle and Ni 5Zr eutectic phase which can refine microstructure. Ni 5Zr phase can alleviate the crack initiation at grain boundaries and dendrite boundaries, which helps deformation to be harmonical between matrix and precipitates at elevated temperature.

Microscopic phase-field study on aging behavior of Ni_(75)Al_(17)Zn_8 alloy

The precipitating kinetics of Ni75Al17Zn8 alloy was studied at both 873K and 973K by microscopic phase-field model.The calculation results show that the order-disorder transformation experiences the matrix→lowly-ordered L10 phase→L12 phase at 973 K.And the nucleation of L12 particles belongs to the spinodal decomposition mechanism.As temperature increases,orderings of Al and Zn atoms are resisted,but coarsening of L12 particles is promoted.The value of coarsening kinetic exponents approaches to 1/2.In addition,the discussions about Ni-Al anti-site defect and Zn substitutions for Ni site and Al site exhibit that the higher the temperature,the more distinctive the Ni-Al anti-site defect,but the less the Zn substitution.

Al-doping influence on crystal growth of Ni–Al alloy: Experimental testing of a theoretical model

Recently, a condensing potential model was developed to evaluate the crystallization ability of bulk materials [Ye X X, Ming C, Hu Y C and Ning X J 2009 J. Chem. Phys. 130 164711 and Peng K, Ming C, Ye X X, Zhang W X, Zhuang J and Ning X J 2011 Chem. Phys. Lett. 501 330], showing that the best temperature for single crystal growth is about0.6Tm, where Tm is the melting temperature, and for Ni–Al alloy, more than 6 wt% of Al-doping will badly reduce the crystallization ability. In order to verify these predictions, we fabricated Ni–Al films with different concentrations of Al on Si substrates at room temperature by pulsed laser deposition, and post-annealed the films at 833, 933, 1033（- 0.6Tm）,1133, and 1233 K in vacuum furnace, respectively. The x-ray diffraction spectra show that annealing at 0.6Tm is indeed best for larger crystal grain formation, and the film crystallization ability remarkably declines with more than 6-wt% Al doping.

Modification of short-range repulsive interactions in ReaxFF reactive force field for Fe–Ni–Al alloy

The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics(MD)simulations.In this work,the short-range repulsive interaction of a reactive force field(ReaxFF),describing Fe-Ni-Al alloy system,is well modified by adding a tabulated function form based on Ziegler-Biersack-Littmark(ZBL)potential.The modified interaction covers three ranges,including short range,smooth range,and primordial range.The short range is totally predominated by ZBL potential.The primordial range means the interactions in this range is the as-is ReaxFF with no changes.The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function.Both energies and forces are guaranteed to be continuous,and qualified to the consistent requirement in LAMMPS.This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects'booming and recombination effectively.

Diffusion of Hydrogen along the Grain Boundaries in Ni_3Al Alloys

The diffusivity of hydrogen in two Ni3Al alloys (No.1 and No.2) has been measured in the temperature range of 100 degreesC to 420 degreesC using an ultrahigh vacuum gaseous permeation technique. The diffusivity data fall into two segments, in which the hydrogen diffusivity adheres to the Arrhenius form, respectively. From the hydrogen diffusivity, it is conjectured that the hydrogen diffusivity reflects the hydrogen transportation along the grain boundaries at lower temperature and the hydrogen transportation in the lattice at higher temperature. The intergranular fracture of Lit-type intermetallics induced by hydrogen at relative low temperature results from hydrogen transportation along the grain boundaries and not in the lattice.

Investigation on Cracking in the Surfacing Welding Layer of Ni_3Al Based Alloy

Investigation has been made into the causes of cracking in the Surfacing welding layer of Ni3Al based alloy by analysing both the liqu id-to-solid transformation in the molten pool and the distribution of thermal stress within the surfacing welding layer. The results show that cracking in the surfacing welding layer is directly related to the producing of eutectic phase β' (NiAl) in the interdendritic region and high thermal stress within the surfacing welding layer. When the process of electric arc surfacing welding is changed from along straight line to along' Z' pattern, cracking in the surfacing welding layer of Ni3Al based alloy is prevented due to being reduced of both the cooling rate of liquid in the molten pool and the moving speed of the heat source. Reducing the melting volume of the substrate material by lowering the output power of electric arc welding would make the content of iron atoms in the molten pool decrease. and this also can reduce the trend of the eutectic reaction in the interdendfitic region and is helpful to Suppress cracking in the surfacing welding layer.

Cavitation Erosion Behavior of as-Welded Cu12Mn8Al3Fe2Ni Alloy

Cavitation erosion behavior of as-welded Cu12Mn8Al3Fe2Ni alloy in 3.5% NaCl aqueous solution was studied bymagnetostrictive vibratory device for cavitation erosion. The results show that the cavitation erosion resistance ofthe as-welded Cu12Mn8Al3Fe2Ni alloy is much more superior to that of the as-cast one. The cumulative mass lossand the mass loss rate of the as-welded Cu12Mn8Al3Fe2Ni alloy are almost 1/4 that of the as-cast one. SEM analysisof eroded specimens reveals that the as-cast Cu12Mn8Al3Fe2Ni alloy is attacked more severely than the as-weldedone. Microcracks causing cavitation damage initiate at the phase boundaries.

Dissolution Mechanism of a Zr Rich Structure in a Ni_3Al Base Alloy

In the present research, the dissolution mechanism of a Zr rich structure during annealing of a Ni3Al base alloy containing Cr, Mo, Zr and B, was investigated. The annealing treatments were performed up to 50 h at 900, 1000 and 1100℃. The alloy used in this investigation was produced by vacuum-arc remelting technique. The results show that at the beginning of the process, a mixed interface reaction and local equilibrium （long range diffusion） mechanism controls the dissolution process. After a short time, this mechanism changes and the dissolution mechanism of the Zr rich structure changes to only long range diffusion of Zr element. According to this mechanism, the activation energy of this process is estimated to be about 143.3 kJ.mol-1. Also the phases that contribute to this structure and the transformations that occur at the final steps of solidification of this alloy were introduced. According to the results, at the final step of solidification, a peritectic type reaction occurs in the form of L＋ y→Ni7Zr2 and →-Ni7Zr2 segregates from the melt. Following this transformation, →-Ni7Zr2 eutectic separates from the remaining Zr rich liquid. The solidification process will be terminated by a ternary eutectic reaction in the form of L→y＋Ni5Zr＋Ni7Zr2.

In situ Study on the Growth Behavior of Primary Al_3Ni Phase in Solidifying Al–Ni Alloy by Synchrotron Radiography

The growth behavior of Al_3Ni intermetallic compounds（IMCs）during the directional solidification of Al-10 wt%Ni alloy was investigated by synchrotron radiography.Two main growth patterns of primary Al_3Ni phase,I-like and V-like,appeared during solidification,and I-like Al_3Ni phase grew faster than V-like phase,which can be explained by the minimum energy criterion.The growth of I-like phase can be divided into two stages and was mainly affected by undercooling and Ni concentration.We also found that V-like Al_3Ni phase can evolve into M-like phase during solidification.

A user-friendly yield criterion for metals exhibiting tension-compression asymmetry

The aim of this paper is to model the yielding asymmetry of pressure-insensitive metals,including but not limited to Ni3 Al alloys.The main focuses are put on the flexibility and manipulative convenience.The parameters of theory are kept to a minimum and can be determined by as few tests as possible.These requirements are fulfilled by constructing a yield function using the second and third-invariants of a linearly transformed stress tensor.The proposed yield criterion has a simple mathematical form and has only seven parameters when used in three-dimensional stresses.Compared with existing theories,the new yield criterion has much fewer parameters,which makes it very convenient for practical applications.The coefficients of the criterion are identified by an error minimization procedure.Applications to a Ni3 Al based intermetallic alloy as well as a Cu-Al-Be shape memory alloy and comparison to other criteria show that the proposed criterion has nearly the same predictive ability and flexibility with other criteria.The proposed yield criterion can estimate the coefficients by using less data,which is a big advantage compared with other similar theories,especially when there is a limited number of experimental data.