Effect of Substrate Characteristics on Interdiffusion Coefficients of Ni and Al Atoms inβ-NiAl Phase of Aluminide Coatings

Interdiffusion coefficients at 950℃ and 1050℃ are calculated by Wagner analysis method as a function of composition of β-NiAI phase. Theβ-NiAI phase is formed by pack cementation on surface of superalloy. Results of the calculation show that interdiffusion coefficients inβ-NiAI phase strongly depend on the compositions and vary over several orders of magnitude. Compared with the interdiffusion coefficients in the stoichiometricβ-NiAI phase, the interdiffusion coefficients in β-NiAI phase formed on superalloy is obviously small, probably due to the composition, complicated microstructure and precipitates. However, it could be seen clearly that the shapes of the diffusivity curves are very similar to each other. The similarity of the diffusion curves and the difference between interdiffusion coefficients imply that the compositions, microstructures and precipitates of superalloy have a distinctly adverse effect on the interdiffusion of Ni and Al atoms during aluminization, but do not change the essential characteristics of β-NiAI phase.     

On the γ' precipitates of the normal and inverse Portevin-Le Chatelier effect in a wrought Ni-base superalloy

The Portevin-Le Chatelier(PLC) effects in a wrought Ni-base superalloy with different γ' precipitates contents have been investigated. Detailed analysis on the serration type of the tensile curves indicates that the γ' precipitates have a decisive influence on the transformation from normal to inverse PLC behavior, which is rarely proposed in other works. It is considered that the γ' precipitates play the same role in PLC effect as temperature and strain rate for the investigated wrought Ni-base superalloy.     

Oxidation of NiAl-Ag Alloys at 1000℃ in 0.1 MPa O_2

Small amounts of silver have been added to the intermetallic compound NiAl with the purpose of improving its mechanical properties. Four ternary NiAI-Ag alloys containing 0.5, 1, 5 and 10 at. pct Ag, denoted as NiAI-0.5Ag, NiAI-1Ag, NiAI-5Ag and NiAI-10Ag, and the Ag-free NiAl have been oxidized at 1000℃for 24 h in 0.1 MPa O2 to study the effect of the presence of silver on the oxidation behavior of NiAl. All the NiAI-Ag alloys are composed of a matrix of β-NiAI containing a dispersion of isolated particles of a second silver-rich phase. A continuous external layer of AI2O3 formed on all the alloys. In addition, the scales formed on NiAI-5Ag contained a thin and discontinuous layer of pure silver located at the alloy/AI2O3 interface, while those formed on NiAI-10Ag contained isolated particles as well as discontinuous layers of silver at various locations in the scale extending up to the gas/scale interface. The kinetic curves of all the alloys were generally composed of two main parabolic stages with smaller parabolic rate constants for the final stage. The addition of silver does not significantly affect the oxidation behavior of the NiAl intermetallic compound in all cases, as expected because silver is essentially present only as a second phase due to its very small solubility in β-NiAI.     

XPS and PAT Study on NiAl Phase Formed in a Superalloy by Pack Cementation

The NiAl phase prepared by pack cementation （PC） on a nickel base superalloy was investigated by X-ray photoelectron spectroscopy （XPS） and positron annihilation technique （PAT）. The focus was on the effect of the solid solution of the alloying element from substrate on the binding energy of Ni （Al） 2p peaks and vacancy concentration of the NiAI phase formed in a superalloy. The results showed that the binding energy of Ni 2p peak of the NiAI phase grown in a superalloy was shifted by up to 0.55 eV at the temperature from 850 to 1050℃ towards higher energies and the binding energy of Al 2p peak by up to 1.09 eV in comparison with the NiAl phase formed in pure Ni. The positron lifetimes obtained from the NiAl phase formed in a superalloy were found to be markedly lower than the theoretical values, indicating the decrease in vacancy concentration. The variation of binding energies and vacancy concentration are possibly due to the solid solution of the alloying atoms from the substrate into the NiAI lattice.     

Weakening Mechanisms of Platelike σ Phase on Ni-base Superalloys

On the basis of an investigation on σ phase inNi-base cast superalloy K24 and the results aboutσ phase in other Ni-base superalloys,an embrittl-ing mechanism and a softening mechanism,bywhich platelike σ phase weakens the Ni-basesuperalloys,have been proposed.It is consideredthat the platelike morphology and the habit precipi-tation along{111}of σ phase are necessary condi-tions for both mechanisms.The embrittling mecha-nism is dominant at room temperature and highstrain rate,and the softening mechanism is domi-nant at high temperature and low strain rate.Ac-cording to the idea of the softening mechanism andthe analyses of σ phase and alloy compositions,it isconsidered that Nb,Mo and W in the alloys may beresistant to the detrimental effect of σ phase on thestress-rupture properties of the alloys.     

Effects of rejuvenation heat treatment on microstructure and creep property of a Ni-based single crystal superalloy

Both surface and internal microstructures of a second-generation Ni-based single crystal(SX) superalloy were studied after creep and rejuvenation heat treatment(RHT).It is indicated that the microstructures,such as the dislocation network,the γ phase and the γ' phase,can be recovered to those after the standard heat treatment(SHT).It is found that RHT affected zone(RAZ) formed at the surface is composed of theγ'-free layer,the transition layer and the recrystallization(RX),which are less than 20 μm in depth totally.Such depth of the RAZ doesn't affect the properties of the superalloy.The morphology of γ' phase at the RAZ is related to the composition of the elements.The average creep life after RHT is close to the average life after SHT.It is concluded that RHT could effectively repair SX parts and increase the total life of the sample after a damage by creep.     

Phase decomposition behavior and its effects on mechanical properties of TiNbTa0.5ZrAl0.5 refractory high entropy alloy

The mechanical properties of refractory high entropy alloys(RHEAs) strongly depend on their phase structures. In this work, the phase stability of a BCC TiNbTa0.5ZrAl0.5 refractory high entropy alloy subjected to thermomechanical processing was evaluated, and the effects of phase decomposition on room/high temperature mechanical properties were quantitatively studied. It was found that, the thermomechanical processing at 800℃and 1200℃ leads to phase decomposition in the TiNbTa0.5ZrAl0.5 alloy. The phase decomposition is caused by the rapid rising of free energy of the primary BCC phase. The effect of the precipitates on room temperature strength is determined by the competition between the increasing in precipitation strengthening and the decreasing in solid solution strengthening. But at high temperatures(800-1200℃), the phase decomposition causes significant reduction in strength, mainly due to the grain boundary sliding and the decreasing in solid solution strengthening.     

Precipitate coarsening and its effects on the hot deformation behavior of the recently developedγ’-strengthened superalloys

Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^■718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^■718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.     

Morphology and Microstructure of Sputtered Superalloy Coating

The morphology and microstructure of magnetron sputtered coating of superalloy K38G on cast al-loy K38G have been investigated by means of SEM, TEM and XRD. The results indicate that thesputtered coating is identical to the substrate in chemical composition but it is composed only of γphase without γ'. The sputtered superalloy coating presents columnar structure and the grain size isin the range of 20-100 nm. There are a great number of planar defects such as stacking faults andmicrotwins in the coating. The morphology and microstructure of the sputtered coating may haveclose relation to the remarkable increase of oxidation resistance of the coating.     

Effect of Heat Treatment on the Microstructure of Multiphase NiAl-based Alloy

The alloy Ni-Al26.6-Cr13.4-Co8.1-Ti4.3-W1.3-Mo0.9 （at. pct） was fabricated from superalloy K44 and Al element using vacuum induction and casting technique. Investigations to this alloy reveal that a new phase Cr3Ni2 possessing low melting point and poor ductility is formed, which is distributed as a network along NiAI matrix grain boundaries. Subsequent different solution and aging treatments are carried out and lead to microstructural changes to various extents. Rapid cooling after solution at 1250℃ for 20 h gives rise to macrocracks in the specimen while slow cooling after the same treatment results in the formation of spheric α-Cr solid solution and needle-like Ni3Al phase, which are embedded in NiAl matrix. It is comfirmed that aging treatments initiate lath-shaped Ni3Al phase and pearl-shaped α-Cr phase to precipitate from the NiAl matrix, which own orientation relationships with these precipitates.     

Distribution of Be in a Ti-Based Bulk Metallic Glass Composite Containing B-Ti

In order to obtain a glassy matrix during quenching, Be is often selected as a constituent of the compositions of Ti/Zr-based bulk metallic glass composites(BMGCs). The in situ formed β phase in Be-bearing BMGCs was reported to be Be-free. However, a thorough investigation of the distribution of Be in BMGCs is still missing to date. In this work, the distribution of Be in a Ti_(47.5)Zr_(33)Cu_(5.8)Co_3Be_(12.5)(at.%) BMGC was studied by the secondary ion mass spectrometry(SIMS) and the electron energy loss spectroscopy(EELS).It is found that Be almost totally dissolves in the glassy matrix, but a very weak intensity of Be in β phase is still detectable by SIMS, and the content of Be in β-Ti is estimated to be about 0.3 at.%. Based on the recently established two-phase quasi-equilibrium of BMGCs, the distinct solubility of Be in the glassy matrix and in β-Ti has been explained.     

Oxidation of NiAl-Ag Alloys at 1000℃ in 0.1 MPa O2

Small amounts of silver have been added to the intermetallic compound NiAl with the purpose of improving its mechanical properties. Four ternary NiAl-Ag alloys containing 0.5, 1, 5 and 10 at. pct Ag, denoted as NiAl-0.5Ag, NiAl-1Ag, NiAl-5Ag and NiAl-10Ag, and the Ag-free NiAl have been oxidized at 1000℃ for 24 h in 0.1 MPa O2 to study the effect of the presence of silver on the oxidation behavior of NiAl. All the NiAl-Ag alloys are composed of a matrix of β-NiAl containing a dispersion of isolated particles of a second silver-rich phase. A continuous external layer of Al2O3 formed on all the alloys. In addition, the scales formed on NiAl-5Ag contained a thin and discontinuous layer of pure silver located at the alloy/Al2O3 interface, while those formed on NiAl-10Ag contained isolated particles as well as discontinuous layers of silver at various locations in the scale extending up to the gas/scale interface. The kinetic curves of all the alloys were generally composed of two main parabolic stages with smaller parabolic rate constants for the final stage. The addition of silver does not significantly affect the oxidation behavior of the NiAl intermetallic compound in all cases, as expected because silver is essentially present only as a second phase due to its very small solubility in β-NiAl.     

γ″Precipitate in Inconel 718

The γ"precipitate in Inconel 718 Ni-base superalloy has been investigated using TEM. Based on a calculation of diffraction pattern. the orientation relations between precipitates and matrix are given in detail. The influence of solution temperature on growth of γ" precipitates was investigated. and a γ" precipitate free area near δ phase was found. The coarsening behaviour of γ" precipitates during short time aging is discussed     

Diffraction and Morphology Study of Surface Structures Accomponying with the Formation of α''''Plate(Bainite)in a Cu-Zn-AI Alloy

In the present paper the morphologies of sur-face structures and the anomalous contrast of β_1phase accompanying with the formation of bainiteplate in the Cu-Zn-Al shape memory alloy havebeen investigated in detail by the transmission elec-tron microscopy and diffraction.The experimentalresults have proved that the extra diffraction andanomalous contrast results from the surfacemartensite formed on the β_1 phase.However,thecrystal structure and morphology of the surfacemartensite are varied at different stage of bainiteformation.The observation of interfacial disloca-tion structure of bainite/β_1 matrix reveals thatthere is a large strain in the bainite transformationof β CuZnAl alloy.Based on the experiment resultsthe formation of surface martensite and bainitictransformation were discussed.     

Effect of a High Magnetic Field on the Shape of the γ' Precipitates in Cast Nickel-based Superalloy K52

The shape change of the γ' precipitates of cast Ni-based superalloy K52 after aging treatment under a high magnetic field was investigated.The results show that duplex γ' precipitates are present in the γ matrix after aging treatment with or without the magnetic field.One is the coarse particles with average size of 500 nm; the other is fine spherical γ' precipitates with average of 100 nm in diameter.The application of a 10T magnetic field only results in the shape of the coarse γ' particles changing from spherical to cuboidal when the alloys subjected to the same heat treatments.This shape change was mainly discussed based on the strain energy increase caused by the difference in magnetostriction between the γ matrix and γ' precipitates.The fine γ'particles still keep spherical.Further TEM observations shows that a number of γ particles in nano-scale precipitate in the coarse γ' particles in the specimens treated without the magnetic field.In addition, it was found that the magnetic field caused the decrease of the hardness in the alloy, and the hardness was associated with the field direction.     

Antiphase Boundary Strengthening in a Single Crystal Nickel—base Superalloy

An investigation has been made into strengthening mechanism in a single crystal nickel-base superalloy DD8 by transmission electron microscopy. The results show that the stress rupture strength of the alloy increases with decreasing misfit, and the antiphase boundaries (APBs) formed in the ordered γ' phase, rather than the misfits, play a dominate role in strengthening of the single crystal Ni-base superalloy DD8. There are three kinds of mechanisms for forming the APBs which were observed in the present materials. One is mis-arrangement of the local ordered atoms in the γ' precipitates due to the local strain; the second arises from the 1/2<110> dislocations cutting into the γ', and the third is the formation of the APBs induced by the 1/2<110> matrix dislocation network. The contribution of the antiphase boundary energy to the strength of the alloy can be expressed by: where τ is the resistance to deformation provided by the APB energy; S is the long-range order degree in γ'; Tc is the transition     

Oxidation Behavior of Pd-Modified Aluminide Coating at High Temperature

(Ni,Pd)Al coating, prepared by low pressure pack cementation on the Ni-base superalloy M38 where Pd-20 wt pct Ni alloy was predeposited, consists of a single β-(Ni,Pd)Al phase. The initial isothermal oxidation behavior of (Ni,Pd)Al coating was investigated by TGA, XRD, SEM/EDS at 800～1100℃. Results show that oxidation kinetics accord preferably with parabolic law at 800, 900 and 1100℃, but not at 1000℃.θ-Al2O3 was observed at 800～1100℃. It is found that Pd plays an important role in accelerating the diffusion of Ti from the substrate to the coating surface in the aluminide coating.     

Structural Changes of α Phase in Furnace Cooled Eutectoid Zn-Al Based Alloy

Furnace cooling is a slow cooling process. It is of importance to study structural evolution and its effects on the properties of alloys during the furnace cooling. Decomposition of aluminium rich α phase in a furnace cooled eutectoid Zn-Al based alloy was studied by transmission electron microscopy. Two kinds of precipitates in the α phase were detected in the FCZA22 alloy during ageing at 170℃. One was the hcp transitional α＂ m phase which aooears as directional rods and the round precipitates. The other was the fcc α＇m phase. 〈101〉. The orientation relationship between the a phase and transitional phase α＇m was determined as （022）α＇m （fcc）//（022^-）α（fcc）, [1^-11]α＇m, （fcc）//[2^-33]α（fcc）. The non-equilibrium phase decomposition of the α phase is discussed in correlation with the equilibrium phase relationships.     

Crystallization Behaviour of Laser Synthesized Nanometric Amorphous Si_3N_4 Powders

The crystallization behaviour of the laser synthesized nanometric amorphous Si3N4 powders with the particle size of 15 nm in diameter has been studied between 1200° and 1700℃ by XRD,TEM and FTIR techniques. A small amount of β-Si3N4 formed at 1250℃ and increased slowly until the α- β transformation happened at 1700℃, whereas α-Si3N4 appeared at 1300℃ andincreased rapidly between 1500-1600℃. The formation of β phase at the lower temperature was caused by the nitridation of free Si due to the preexisted β-nuclei in the Si3N4 particles, whereasthe α phase was formed by solid crystallization from the amorphous matrix. There were α and β SiC formed at 1700℃ due to the presence of Sio and Co gases in the system. FTIR analysis shows that two new IR absorption at 1356 and 1420 cm-1, and an overall strong absorption in wide wavenumber range resulted from the powders annealed at 1600 and 1700℃ respectively     

Creep Behavior and Microstructure Evolution of Sand–Cast Mg–4Y–2.3Nd–1Gd–0.6Zr Alloy Crept at 523–573 K

High temperature tensile–creep behavior of Mg–4Y–2.3Nd–1Gd–0.6Zr(wt%,WE43(T6)) alloy at 523–573 K was investigated.The creep stress exponent is equal to 4.6,suggesting the underlying dislocation creep mechanism.The activation energy is(199 ± 23) k J/mol,which is higher than that for selfdiffusion in Mg and is believed to be associated with precipitates coarsening or cross slip.The creep mechanism is further suggested to be dislocation climb at 523 K,while a cross slip at 573 K is possible.The metastable β′ and β1 phases in the WE43(T6) alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb,which contributed to its excellent creep resistance.However,at 573 K it readily transforms into equilibrium βe phase and coarsens within two hours,thereby causing a decrease of creep resistance.In addition,precipitate free zones approximately normal to applied stress direction(directional PFZs) developed during the creep deformation,especially at 573 K.Those zones became preferential sites to nucleate,extend and connect microcracks and cavities,which lead to the intergranular creep fracture.Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.