PSEUDO-MOLECULAR DYNAMICS STUDY 0F GRAIN BOUNDARY SEGREGATION

The segregation of bismuth atoms on the [101] tilt copper grain boundaries Σ3 (111) 70.53°, Σ33 (545) 58.99°, Σ11(323) 50.48° and Σ9 (212) 38.94° has been studied by pseudo-molecular dynamics using the empirical N-body potentials. The relationship between bismuth segregation and grain boundary structure has been discussed in detail.

Insights into grain boundary segregation and solubility limit of Cr in(TiZrNbTaCr)C

Dense(TiZrNbTaCr)C with Cr segregation along grain boundaries(GBs)has been first proposed and fabri-cated by pressureless sintering at 1800-2000℃,utilizing the self-synthesized carbide powders obtained by carbothermal reduction.Cr segregation along GBs is successfully realized as expected via optimizing the initial Cr content.When Cr content is more than 11.12 at.%,Cr addition is excessive and results in Cr-rich second phase formation at triple junctions.To analyze the Cr solubility dependence on tempera-ture and initial Cr content,the Cr content in(TiZrNbTaCr)C grains is investigated by EDS.The solubility limit of Cr in(TiZrNbTaCr)C is about 3.8 at.%at 1900℃.Finally,Vickers hardness of all the samples is measured to assess the mechanical property of(TiZrNbTaCr)C ceramics.The basic understanding of the Cr solubility limit and GB segregation feature in(TiZrNbTaCr)C have been preliminarily clarified,which may pave a potential way to design and tailor microstructure and GB feature of(TiZrNbTaCr)C for the purpose of enhancing its properties in the future.

Effect of grain boundary segregation on aging resistance and mechanical properties of tetravalent element-doped 3Y-TZP ceramics for dental restoration

In the humid oral environment,3Y-TZP ceramics always suffer from low-temperature degradation(LTD)for a long time,which results in the degradation of mechanical properties and catastrophic failure.The low-temperature degradation(LTD)and mechanical properties of low-cost tetravalent(Ge^(4+),Ti^(4+))element-doped 3Y-TZP were investigated by analysing grain boundary segregation in samples with deferent contents.The results show that GeO_(2) is superior to TiO_(2) in limiting LTD but results in lower flexural strength and fracture toughness when the content is≥1.5 mol%.This dilemma can be improved by adding only 0.1%-0.5 wt%Al_(2)O_(3),and the flexural strength and fracture toughness of 0.25 wt% Al_(2)O_(3) zirconia are then increased to 898 MPa and 4.68 MPa·m^(1/2) compared with 1Ge-3Y,respectively.This work is expected to provide an effective reference for the development and application of budget dental materials.

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony induced embrittlement kinetics in the nick- el-chromiunl steel. Based on these, the activation energy and frequency factor of diffusion of antimony vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.

Effect of Annealing and Hot Rolling on Grain Boundary Segregation of Arsenic in an Mn-Steel Microalloyed by Ti,Cr and Nb

Steel samples with size of 10 mm× 10 mm×5 mm were cut clown from a hot rolled Mn steel microalloyed by Ti, Cr and Nb and produced by compact strip production （CSP） technology. The samples were annealed at 950 ℃ for different time firstly, and then hot rolled or cooled in the air, in water and in furnace, respectively. Auger elee tron spectroscopy （AES） was used to study the effects of annealing and hot roiling on the segregation of arsenic at grain boundary （GB） in the steel. The results indicated that a higher content of arsenic was found at grain boundaries than in the matrix when the steel was annealed at 950 ℃ for 2 h and then cooled to room temperature by water quenching. But the content of arsenic at grain boundaries was similar to that in the matrix when the steel was an- nealed at 950 ℃ for 2 h and then cooled to room temperature by furnace cooling. A longer holding time, such as 12 h and 36 h at 950 ℃, resulted in a similar arsenic content at grain boundaries to that in the matrix of the steels. Hot rolling led to a similar content of arsenic at grain boundaries and within grains in the steels as well.

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms （Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti） near Z3 （ 111 ） [1]-0] tilt symmetric grain boundary （GB） in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atom＇s metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.

Grain boundary segregation induced strong UHTCs at elevated temperatures:A universal mechanism from conventional UHTCs to high entropy UHTCs

Ultra-high temperature ceramics(UHTCs)exhibit a unique combination of excellent properties,including ultra-high melting point,excellent chemical stability,and good oxidation resistance,which make them promising candidates for aerospace and nuclear applications.However,the degradation of hightemperature strength is one of the main limitations for their ultra-high temperature applications.Thus,searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed.To achieve this goal,grain boundary segregation of a series of carbides,including conventional,medium entropy,and high entropy transition metal carbides,i.e.,Zr_(0.95)W_(0.05)C,TiZrHfC_(3),ZrHfNbTaC_(4),TiZrHfNbTaC_(5),were studied by atomistic simulations with a fitted Deep Potential(DP),and the effects of segregation on grain boundary strength were emphasized.For all the studied carbides,grain boundary segregations are realized,which are dominated by the atomic size effect.In addition,tensile simulations indicate that grain boundaries(GBs)will usually be strengthened due to segregation.Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs,since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.

Grain boundary segregation and relaxation in nano-grained polycrystalline alloys

The present study carries out systematic thermodynamics analysis of Grain Boundary(GB)segregation and relaxation in NanoGrained(NG)polycrystalline alloys.GB segregation and relaxation is an internal process towards thermodynamic equilibrium,which occurs naturally in NG alloys without any applied loads,causes deformation and generates internal stresses.The analysis comprehensively investigates the multiple coupling effects among chemical concentrations and mechanical stresses in GBs and grains.A hybrid approach of eigenstress and eigenstrain is developed herein to solve the multiple coupling problem.The analysis results indicate that the GB stress and grain stress induced by GB segregation and relaxation can be extremely high in NG alloys,reaching the GPa level,which play an important role in the thermal stability of NG alloys,especially via the coupling terms between stress and concentration.The present theoretic analysis proposes a novel criterion of thermal stability for NG alloys,which is determined by the difference in molar free energy between a NG alloy and its reference single crystal with the same nominal chemical composition.If the difference at a temperature is negative or zero,the NG alloy is thermal stable at that temperature,otherwise unstable.

Effect of nickel segregation on CuΣ9 grain boundary undergone shear deformations

Impurity segregation at grain boundary（GB） can significantly affect the mechanical behaviors of polycrystalline metal. The effect of nickel impurity segregated at Cu GB on the deformation mechanism relating to loading direction is comprehensively studied by atomic simulation. The atomic structures and shear responses of Cu Σ9（114） 110 and Σ9（221） 110 symmetrical tilt grain boundary with different quantities of nickel segregation are analyzed. The results show that multiple accommodative evolutions involving GB gliding, GB shear-coupling migration, and dislocation gliding can be at play, where for the 2ˉ21ˉ shear of Σ9（114） 110 the segregated GBs tend to maintain their initial configurations and a segregated GB with a higher impurity concentration is more inclined to be a dislocation emission source while maintaining the high mechanical strength undergone plastic deformation for the 11ˉ4ˉ shear of Σ9（221） 110. It is found that the nickel segregated GB exerts a cohesion enhancement effect on Cu under deformation： strong nickel segregation increases the work of separation of GB, which is proved by the first-principles calculations.

Segregation behavior and embrittling effect of lanthanide La, Ce, Pr,and Nd at Σ3(111) tilt symmetric grain boundary in α-Fe

The migration of lanthanide fission products to cladding materials is recognized as one of the key causes of fuel–cladding chemical interaction(FCCI) in metallic fuels during operation. We have performed first-principles density functional theory calculations to investigate the segregation behavior of lanthanide fission products(La, Ce, Pr, and Nd) and their effects on the intergranular embrittlement at Σ3(111) tilt symmetric grain boundary(GB) in α-Fe. It is found that La and Ce atoms tend to reside at the first layer near the GB with segregation energies of-2.55 eV and-1.60 eV, respectively,while Pr and Nd atoms prefer to the core mirror plane of the GB with respective segregation energies of-1.41 eV and-1.50 eV. Our calculations also show that La, Ce, Pr, and Nd atoms all act as strong embrittlers with positive strengthening energies of 2.05 eV, 1.52 eV, 1.50 eV, and 1.64 eV, respectively, when located at their most stable sites. The embrittlement capability of four lanthanide elements can be determined by the atomic size and their magnetism characters. The present calculations are helpful for understanding the behavior of fission products La, Ce, Pr, and Nd in α-Fe.

Role of extrusion rate on the microstructure and tensile properties evolution of ultrahigh-strength low-alloy Mg-1.0Al-1.0Ca-0.4Mn(wt.%)alloy

Mg-1.0Al-1.0Ca-0.4Mn(AXM1104, wt.%) low alloy was extruded at 200 ℃ with an extrusion ratio of 25 and different ram speeds from 1.0 to 7.0 mm/s. The influence of extrusion rate on microstructure and mechanical properties of the AXM1104 alloy was systematically studied. With the increasing of extrusion rate, the mean dynamically recrystallized(DRXed) grain size of the low alloy and average particles diameter of precipitate second phases were increased, while the degree of grain boundary segregation and the intensity of the basal fiber texture were decreased. With the rising of extrusion rate from 1.0 to 7.0 mm/s, the tensile yield strength(TYS) of the as-extruded AXM1104 alloy was decreased from 445 MPa to 249 MPa, while the elongation to failure(EL) was increased from 5.0% to 17.6%. The TYS, ultimate tensile strength(UTS) and EL of the AXM1104 alloy extruded at the ram speed of 1.5 mm/s was 412 MPa, 419 MPa and 12.0%, respectively,exhibiting comprehensive tensile mechanical properties with ultra-high strength and excellent plasticity. The ultra-high TYS of 412 MPa was mainly due to the strengthening from ultra-fine DRXed grains with segregation of solute atoms at grain boundaries. The strain hardening rate is increase slightly with increasing extrusion speed, which may be ascribed to the increasing mean DRXed grain size with rising extrusion speed. The higher strain hardening rate contributes to the higher EL of these AXM1104 samples extruded at higher ram speed.

Underlying slip/twinning activities of Mg-xGd alloys investigated by modified lattice rotation analysis

The inconsistencies regarding the fundamental correlation between Gd content and slip(twinning)activities of Mg alloys appeal further investigations.However,the traditional slip dislocations analysis by TEM is time-consuming,and that by SEM/EBSD cannot recognize the partial slip modes.These urge a more efficient and comprehensive approach to easily distinguish all potential slip modes occurred concurrently in alloy matrix.Here we report a modified lattice rotation analysis that can distinguish all slip systems and provide statistical results for slip activities in Mg alloy matrix.Using this method,the high ductility of Mg-Gd alloy ascribed to the enhanced non-basal slips,cross-slip,and postponed twinning activities by Gd addition is quantitatively clarified.

Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

A typical G-phase strengthened ferritic model alloy(1Ti:Fe-20Cr-3Ni-1Ti-3Si,wt.%)has been carefully studied using both advanced experimental(EBSD,TEM and APT)and theoretical(DFT)techniques.During the classic“solid solution and aging”process,the superfine(Fe,Ni)_(2)TiSi-L2_(1)particles densely precipitate within the ferritic grain and subsequently transform into the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase.In the meanwhile,the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase are also observed.These nanoscale microstructural evolutions result in a remarkable increase in hardness(100-300 HV)and severe embrittlement.When the“cold rolling and aging”process is used,the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening ef-fect.Superhigh yield strength of 1700 MPa with 4%elongation at break is achieved.This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary.These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.

Segregation of Phosphorus and Precipitation of MNP-Type Phosphide at the Grain Boundary of IN706 Superalloy

The effect of phosphorus content and heat treatment on the segregation of phosphorus and precipita- tion of phosphide at the grain boundary of IN706 alloy has been investigated, The phosphide had a stoichiometry of MNP （M = Nb, Ti and N = Ni, Fe, Cr） and an orthorhombic crystal structure. The solubil- ity of phosphorus in the grain matrix of IN706 alloy was determined to be between 0.008% and 0.013%. When soaked at 980 ℃ for 5 min, the phosphide was noticeably precipitated at grain boundaries with the grain size unchanged. When soaked at 980 ℃ for 3 h, the grains grew significantly, but the phos- phide stayed at original grain boundaries which outlined the grain shape before soaking. Soaking at 1060 ℃ for 2 h fully dissolved the pre-existing phosphide but phosphorus atoms were not distributed uni- formly in the grain matrix. When the holding time at 1060 ℃ was extended to 10 h, the phosphorus atoms were distributed uniformly in the grain matrix by sufficient diffusion, and the phosphide could only be formed at grain boundaries during the 980 ℃ soaking. The precipitation of phosphide mainly relied on the phosphorus segregation which was built up by diffusion.

Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttriastabilized zirconia

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries（GBs） and the solidvapor interfaces（free surfaces）, each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies（differently）as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.

Mg在Ni基高温合金中的晶界偏析行为和强化机制的计算机模拟

This paper used EAM and static relaxation method to simulate the grain boundary segregation behavior of Mg in Ni-based superalloys. The results offer a better understanding in the strengthening mechanism of Mg addition in superalloys. The segregation of Mg increases the grain boundary cohesive bond and the vacancy formation energy, and decreases the mobility of grain boundary dislocation. It results in the retardation of creep voids initiation and growth.

Microstructure and grain boundary morphology evolution of a novel Co-9Al-9W-2Ta-0.02B alloy doped with yttrium

The microstructures and grain boundary morphologies of a novel Co-9 Al-9 W-2 Ta-0.02 B alloy doped with yttrium(Y)(0.01,0.05,0.10,and 0.20; at%) were investigated as functions of aging temperatures(900 and1000 ℃) and time(50 and 150 h). The aged alloys all exhibit a γ/γ’-Co(Al,W) coherent microstructure in grain interiors, whereas an intermetallic κ-Co(W) phase precipitates at grain boundaries. Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies. For 0.01 Y alloy, bright κ-Co(W) stripes precipitate along grain boundaries, where a needlelike κ-Co(W) phase grows from grain boundaries or κ-Co(W) stripes toward grain interior. As the nominal Y content increases, the stripe and needlelike κ-Co(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20 Y alloy, leaving fine κ-Co(W) particles scattered at grain boundaries. It is noted that more Y segregation may increase the number of low-angle grain boundaries(LABS, with misorientations of <15°), whereas it eliminates O impurities from grain boundaries. Finally,the effect of Y segregation on tensile behavior of Co-AlW-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies, grain boundary character distribution(GBCD), and impurity segregation.

Effect of Constituent Core-sizes on Microstructure and Dielectric Properties of BaTiO_3@(0.6BaTiO_3-0.4BiAlO_3) Core-Shell Material

The fundamental characteristics of varied initial core-sizes of Ba Ti O3（BT） and its influential role on the morphology and dielectric properties of Ba Ti O3@0.6 Ba Ti O3-0.4 Bi Al O3（BT@0.6 BT-0.4 BA） ceramic samples were studied. Alkoxide sol-precipitation method was adopted as revised chemical route to synthesize the constituent ＂core＂ BT powders in a dispersed phase, whereas the distinctive initial nano-sized particles were affected by the pre-calcination temperatures（600-900 ℃）.The microstructure of the uncoated BT ceramics revealed an exaggerated grain growth with an optimized dielectric constant（ε（max） 〉9 000） whilst the coated ceramics behaved otherwise（grain growth inhibited） when sintered at an elevated temperature. Regardless of the previously studied solubility limit（about 0.1%） of BT-BA samples, BT@0.6 BT-0.4 BA maintained a maximum dielectric constant（ε（max）） ranging from 1 592 to 1 708 and tan δ less than 2% under a unit mole ratio at room temperature. In view of all these analyses, the initial nanometer sizes of the as-prepared BT-core powders combined with the increase effect of cation substitutions of Bi^（3＋） and Al^（3＋） in the shell content, induced the diffuse transition phase of BT@0.6 BT-0.4 BA composition.

Metallurgical Source of Cryogenic Intergranular Fracture of Fe-38Mn Austenitic Alloy

SEM and Field emitting TEM-EDAX were used to investigate the fracture surface of series impact specimens and the grain boundary chemistries of VIM(vacuum-inductionmelted) Fe-38 Mn austenitic alloy before and after ESR(electroslag remelting,).The quantity and the size of inclusions were also examined.The results show that the VIM Fe-38 Mn austenitinic alloy water-quenched from 1 100 C undergoes an obvious ductile-to-brittle transition,and the impact work at ambient temperature is 242 J,the corresponding fracture surface exhibits a dimple character.However,the impact work at 77 K of VIM alloy is only 25 J and the fracture mode is IGF(intergranular fracture).After ESR,the impact work at ambient temperature is 320 J and the fracture surface exhibits a character of "volcano lava"(meaning excellent toughness);The impact work at 77 K is up to 300 J and the fracture mode is micro void coalescence mixed with quasi-cleavage.The segregation of Mn is not found in all specimens,but the segregation of S is observed,and the S segregation is decreased after ESR.The examined results of inclusions show that ESR reduces the quantity and improves the morphology of inclusions.From the above results it can be seen that the cryogenic IGF of VIM Fe-38 Mn austenitic alloy is related to the S segregation at grain boundary.After ESR the decrease in the quantity and size of inclusion results in the increase of the impact work at ambient temperature,while the restriction of IGF is related to the decrease in the total level,and hence in the grain boundary segregation of S.

IMPROVEMENT ON SUSCEPTIBILITY OF ULTAR-HIGH STRENGTH STEEL TO STRESS CORROSION CRACKING BY HIGH TEMPERATURE QUENCHING

The effect of quenching temperature on the stress corrosion cracking of 30Cr3SiNiMoV ultra-high strength steel in 3.5% NaCl aqueous solution has been studied.The threshold K_(ISCC) may continuously increase with the quenching temperature raised from 870 to 1200℃ . All of the fractures are intergranular.The analyses of the segregation along prior austenitic grain boundaries,grain size and other microstructural factors reveal that the inerease of K_(ISCC) is mainly due to the coarsening of prior austenitic grains.