Formation mechanisms of Ni-Al intermetallics during heat treatment of Ni coating on 6061 Al substrate

The formation mechanisms and growth kinetics of Al3 Ni and Al3Ni2 in Ni-Al diffusion couple prepared by electrodeposition of Ni on Al substrate were investigated. The nickel coating with 20 μm thickness was applied on 6061 aluminum alloy by direct current electroplating. The samples were then heat-treated for different durations at 450, 500 and 550 °C under argon atmosphere. The intermetallic phases were identified by means of scanning electron microscopy（SEM）, energy dispersive spectrometry（EDS） and X-ray diffraction（XRD）. The results showed that the formation of intermetallic phases consisted of two important steps. The first step was the lateral growth of intermetallic phase from separate sites, resulting in the formation of a continuous layer. The second step was the growth of the continuous intermetallic layer in the direction perpendicular to the interface. However, excessive increase in thickness of intermetallic phases led to the detachment of reaction products, i.e., Al3 Ni and Al3Ni2, from the substrate. It was also observed that aluminum was the dominant diffusing element during Al3 Ni growth, while nickel diffusion was dominant during Al3Ni2 growth. The growth kinetics of both Al3 Ni and Al3Ni2 phases obeyed a parabolic law.

Preparation and properties of the Ni-Al/Fe-Al intermetallics composite coating produced by plasma cladding

A novel approach to produce an intermetallic composite coating was put forward.The microstructure,microhardness,and dry-sliding wear behavior of the composite coating were investigated using X-ray diffraction （XRD）,scanning electron microscopy （SEM）,energy dispersive spectrum （EDS） analysis,microhardness test,and ball-on-disc wear experiment.XRD results indicate that some new phases FeAl,Fe0.23Ni0.77Al,and Ni3Al exit in the composite coating with the Al2O3 addition.SEM results show that the coating is bonded with carbon steel metallurgically and exhibits typical rapid directional solidification structures.The Cr7C3 carbide and intermetallic compounds co-reinforced composite coating has a high average hardness and exhibits an excellent wear resistance under dry-sliding wear test compared with the Cr7C3 carbide-reinforced composite coating.The formation mechanism of the intermetallic compounds was also investigated.

In-situ formation of Ni-Al intermetallics-coated graphite/Al composite in a cold-sprayed coating and its high temperature tribological behaviors

In this study, cold spraying（CS） was used to deposit a mixture of nickel-coated graphite and 40 vol.% Al powder（Ni-Gr/Al） on a steel substrate aiming to effectively preserve a certain volume fraction of graphite in the deposited Ni-Gr/Al composite coating. The microstructure of the as-sprayed coating and the effect of post-spray heat-treatment（PSHT） temperatures on the in-situ formation of Ni-Al intermetallic phases in coating were studied. The tribological behaviors of the as-sprayed coating and the PSHTed coating under 450？C were tested at 25？C, while the as-sprayed coating was tested at 450？C for comparison.As a result, the Ni-Gr particles showed a homogenous distribution in the coating. The multilayer Ni-Al intermetallics-coated graphite/Al composite coating was achieved in situ after the PSHT of 450？C, where the graphite did decompose at 550？C leaving big pores in the coating. The coefficients of friction（COF）of the CSed coating and the PSHTed coating were measured at 450？C as well as 25？C, which showed a similar tendency, much higher than that of the CSed coating tested at 25？C. The lubrication phase（graphite） improved the formation of a graphite film during sliding friction and decreased the COF, while the hard Ni-Al intermetallic phases contributed to the increase of COF.

Effect of Fe and Mo additions on microstructure and mechanical properties of TiAl intermetallics

The ductility of TiAl intermetallics can be improved through stabilizing the ductile β phase.New β-stabilized Ti-45Al-xFe-yMo（x,y=1,2,3,4） alloys were designed through adding the β stabilizing elements Fe and Mo.The microstructural evolution and deformation behavior of the Ti-45Al-xFe-yMo alloys were investigated.The results show that the amount of β（B2） phase is increased with the increase of alloying elements.Mo shows a higher capability for stabilizing the β phase than Fe.In the optimized Ti-45Al-3Fe-2Mo alloy,the grains are significantly refined to about 12 μm,and this alloy shows a very good hot ductility at the elevated temperature.

Tortuosity factor for porous FeAl intermetallics fabricated by reactive synthesis

The tortuosity factor is the most critical parameter for the pore characteristic of porous materials. The tortuosity factor for porous FeAl intermetallics was studied based on the Darcy law and Hagen-Poiseuille equation. Porous stainless steel with the same pore structure parameter as porous FeAl was fabricated by powder metallurgy method for comparison. The results show that the tortuosity factor of porous FeAl intermetallics is smaller than that of porous stainless steel when their pore structure parameters are the same. The average tortuosity factor is 2.26 for the porous FeAl material and 2.92 for the porous stainless steel, calculated by Hagen-Poiseuille equation. The reason of the different tortuosity factors for porous FeAl and porous stainless steel was also explored through studying the pore formation mechanisms of the two types of porous materials.

Effect of pore structures on corrosion resistance of porous Ni_3Al intermetallics

Porous Ni3Al intermetallics were fabricated by elemental powder reactive synthesis method, using carbamide powders as space holders. Corrosion behavior of porous Ni3Al intermetallics was investigated in a 6 mol/L KOH solution using electrochemical methods and immersion test. Effect of porous structures on the corrosion behavior of the porous Ni3Al intermetallics was studied. The results indicate that the porous Ni3Al intermetallics with higher porosities suffer more serious corrosion than the ones with lower porosities because the complicated interconnected porous structures and the large true surface areas exist in the samples with a higher porosity. But the corrosion rates of the porous Ni3Al intermetallics are not proportional to the true surface areas. The reason is that the pore size, pore size distribution and pore shape of the porous Ni3Al intermetallics change with the increasing porosity. All the porous Ni3Al intermetallics with different porosities exhibit excellent corrosion resistance in a strong alkali solution.

Intermetallics and phase relations of Mg-Zn-Ce alloys at 400 ℃

The crystal structures,compositions and phase relations of the intermetallics of Mg-Zn-Ce system in the Mg-rich corner at 400 ℃ were identified through equilibrium alloy method.For Mg-Zn-Ce system,there is a linear ternary compound（T phase）,whose chemical formula is（Mg1-xZnx）11Ce.The range of Zn content in T phase is from 9.6% to 43.6%（molar fraction）.The crystal structure of T phase is C-centered orthorhombic lattice with lattice parameters of a=0.96-1.029 nm,b=1.115-1.204 nm,c=0.940-1.015 nm.And the lattice parameters of T phase are decreasing a little with increasing Zn content.According to the results of composition and crystal structure,the maximal solubility of Zn in Mg12Ce is about 7.8%（molar fraction）,and the chemical formula of the solid solution can be identified as（Mg1-xZnx）12Ce.The isothermal section of Mg-Zn-Ce system in Mg-rich corner at 400 ℃ was constructed.

Dynamic behavior and fracture mode of TiAl intermetallics with different microstructures at elevated temperatures

Experimental studies were conducted on the tensile behaviors and fracture modes of TiAl（Ti-46.5Al-2Nb-2Cr） alloys with near gamma（NG） equiaxed and near lamellar（NL） microstructures over a temperature range from room temperature to 840 ℃ and a strain rate range of 0.001-1 350 s-1.The results indicate that the alloys are both temperature and strain rate dependent and they have a similar dependence.The dynamic strength is higher than the quasi-static strength but almost insensitive to high strain rate range of 320-1 350 s-1.The brittle-to-ductile transition temperature（BDTT） increases with increasing strain rates.NG TiAl yields obviously,while NL TiAl does not.Below BDTT,as the temperature increases,the fracture modes of the two alloys change from planar cleavage fracture to a mixture of transgranular and intergranular fractures,and finally to totally intergranular fracture.

Formation and sedimentation of Fe-rich intermetallics in Al-Si-Cu-Fe alloy

Formation and sedimentation of Fe-rich intermetallics were studied in a commercial Al-Si-Cu-Fe alloy with extra additions of Mn. It is found that the introduction of extra Mn is an effective approach to lower the Fe level in the equilibrium liquid phase after sedimentation of solid Fe-rich phase at a temperature between its liquidus and solidus. The higher Mn/Fe mass ratio results in the lower Fe content in the retained alloy, during which Mn is also consumed and settled at the bottom of the melt as solid Fe-rich intermetallics. Therefore, the final Fe content in the alloy can be controlled by the Mn content and the holding temperature of the melt. The results confirmed a good agreement of the theoretical calculation and the experimental test with a specially designed 50 mm cylindrical casting. The sedimentation of Fe-rich intermetallics in the Al-Si-Cu-Fe alloy is completed at 600 °C after 10 min. The reduction of Fe content in the retained alloy is 31.4% when m（Mn）/m（Fe）=0.5 and 53.3% when m（Mn）/m（Fe）=1.0 in comparison with that in the original alloy. The settled Fe-rich intermetallics were identified as α-Al15（Fe,Mn）3Si2, which provided the lower balanced Fe concentration in the melt in comparison with other Fe-rich intermetallics.

Environmental Embrittlement of Fe-based, Co-based, Ni-based and Ti-based Intermetallics in Gaseous Hydrogen

The environmental embrittlement of intermetallics Co3Ti, Ni3Al, Fe3Al and TiAl has been investigated by measuring the tensile properties in oxygen and hydrogen at 2×l0-4/s strain rate. The results show that the hydrogen embrittlement factor in gaseous hydrogen (IH2 ) defined as[(δO2 -δH2 ) / δH2, ] ×l00% of above mentioned four intermetallics is decreased in the sequence of Co3Ti> Ni3Al> Fe3Al> TiAl. This phenomena can be explained by the different catalytic reaction on the surface of matrix metals (such as Ni, Co, Fe, Ti) with decomposition of H2 into atommic hydrogen, leading to hydrogen embrittlement.

SYNTHESIS OF INTERMETALLICS BY MECHANICAL ALLOYING

Mechanical alloying (MA), a solid-state powder processing method, is a 'far from equilibrium' synthesis technique which allows development of novel crystal structures and microstructures, leading to enhanced physical and mechanical properties. The ability to synthesize a variety of alloy phases including supersaturated solid solutions, nanocrystalline structures, amorphous phases and intermetallic compounds themselves is discussed. No extension of solubility using MA has been observed in the intermetallics studied. Nanostructured grains were observed in all compositions. Long time milling generally resulted in amorphous phase formation in large part because of the increase in grain boundary energy/mole with reduced grain size; good agreement with the Miedema model for amorphization was obtained in the Al-Fe system. Generally an anneal was required to form the intermetallic after MA; however,intermetallics with a large negative enthalpy of formation were detected in the MA condition. A study of the hot isostatic pressing of γ-TiAl powders produced by MA demonstrated that full density can be achieved at least 400℃ below the normal temperature required for conventional powder, that is 725℃ or below. Nanometered sized grains (≤100nm)were observed after HIP'ing up to 850℃.

EFFECT OF ALLOYING ELEMENTS ON SENSITIVITY TO ENVIRONMENTAL EMBRITTLEMENT OF L1_2 INTERMETALLICS

The effect of Zr doping in Ni 3Al and B doping in Co 3Ti intermetallics on the sensitivity to moisture induced environmental embrittlement and on the hydrogen diffusivity was investigated. The results show that both B in Co 3Ti and Zr in Ni 3Al do not reduce the hydrogen diffusivity along the grain boundaries, therefore can not suppress the moisture induced environmental embrittlement. The above mentioned behavior of Zr in Ni 3Al and B in Co 3Ti is attributed to the fact that Zr and B are not segregated on the grain boundaries.

INTERMETALLICS AS SUBSTITUTES FOR SUPERALLOYS

The application advances of TiAl, Ti3Al and Ni3Al base aUoys were denumstrated by Central Iron and Steel Research Institute, China. The recent research progresses on improving the reliability of cast TiAI were mainly presented and discussed. The characteristics of the self-oriented lamellar microstructure in cast TiAI were investigated in both as cast and as HlPed states. Based on the mechanical anisotropy of the cast lamellar microstructure, the component specific microstrueture design was proposed for a better performance and reliability of cast TiAl.

STUDY OF THE EFFECT OF BORON CONTENT ON THE ORDERED EXTENT OF THE Ni_3Al INTERMETALLICS COMPOUND BY AP-FIM

By using AP-FIM the varity of the ordered degree of Ni_3Al with L1_2 structure with B content was studied. The possibility of boron improving ductility was also discussed from bonding between Ni and Al atoms, and antisite defects in Ni_3Al. The extent of ordering is reduced with an increase in boron content and the autisite defects are most obvious for 0.52at.K B-doped sample that has the best ductility. Some results were verified by X-ray diffraction non. The addition of boron not only influences electron environment at grain boundary but also in the interior of Ni_3Al gm ins, the latter is favorable to improve the ductility of Ni_3Al grains.

Influence of mechanical alloying time on the properties of Fe_3Al intermetallics prepared by spark plasma sintering

The Fe3Al-based intermetallics were prepared by mechanical alloying and spark plasma sintering （SPS）, and the influence of milling time on the properties of materials was investigated. The phase identification was investigated by X-ray, and the surface morphology and fractography were observed by scanning electron microscope （SEM）. The mechanical properties such as bending strength, strain, and microhardness were tested. The results show that Fe reacts with Al completely to form Fe3Al during short SPS processing time. The relative densities of the sintered samples were nearly 100%. The mechanical properties of the sintered samples can be improved along with the milling time. The representative values are the bend strength of 1327 MPa and the microhardness of 434.

R&D STATUS ON INTERMETALLICS IN CHINA

This paper briefiy introduces the R&D of intermetallics in China. R&D on intermetallics in a national scale in China began near ten years ago. The investigation in past years focused on the fundamental research and materials development. A significant progress has been made. Various components that made of Ti3Al Ni3Al, TiAl and Fe3Al have been successfully manufactured Some of them have been evaluted. It is expected that some intermetallic alloys will be produced in an industrial scale in the near future.

Phase stability,elastic properties and electronic structures of Mg-Y intermetallics from first-principles calculations

The phase stability,elastic properties and electronic structures of three typical Mg-Y intermetallics including Mg_(24)Y_(5),Mg_(2)Y and MgY are systematically investigated using first-principles calculations based on density functional theory.The optimized structural parameters including lattice constants and atomic coordinates are in good agreement with experimental values.The calculated cohesive energies and formation enthalpies show that either phase stability or alloying ability of the three intermetallics is gradually enhanced with increasing Y content.The single-crystal elastic constants C_(ij) of Mg-Y intermetallics are also calculated,and the bulk modulus B,shear modulus G,Young's modulus E,Poisson ratio v and anisotropy factor A of polycrystalline materials are derived.It is suggested that the resistances to volume and shear deformation as well as the stiffness of the three intermetallics are raised with increasing Y content.Besides,these intermetallics all exhibit ductile characteristics,and they are isotropic in compression but anisotropic to a certain degree in shear and stiffness.Comparatively,Mg_(24)Y_(5) presents a relatively higher ductility,while MgY has a relatively stronger anisotropy in shear and stiffness.Further analysis of electronic structures indicates that the phase stability of Mg-Y intermetallics is closely related with their bonding electrons numbers below Fermi level.Namely,the more bonding electrons number below Fermi level corresponds to the higher structural stability of Mg-Y intermetallics.

A COMPONENT-SPECIFIC ALLOY DESIGN OF TiAl INTERMETALLICS

The metallographic observation and analyses of TiAl alloy cast ingots revealed that the preferably arranged γ/α_2 lamellar microstructure can be obtained in columnar dendritic cast ingot through controlling the Ti/Al atomic ratio. The experiments conf irmed that the preferably arranged γ/α_2 lamellar microstructure has excellent tensile strength and fracture toughness and tolerant tensile plasticity when the stress is applied parallel to the γ/α_2 interface.Based on these results and the working condition of the turbine blades,a component-specific alloy design has been suggested.

Isothermal oxidation behavior of TiAl intermetallics with different oxygen contents

Isothermal oxidation behaviors of Ti-45Al-2Fe-2Mo-1Cr intermetallics with different oxygen contents were studied under the condition of 950 °C, 100 h in air, and the oxidation kinetic parameters were also evaluated. The results show that the oxidation resistance of the TiAl intermetallics is negatively related to the oxygen content, and both the mass gain and thickness of oxide scale increase with the oxygen content. The sub-surface microstructure of the oxide scales varies with the oxygen content. Z phase is observed in the sub-surface area of the low-oxygen-content alloy, while the τ2(Al2FeTi) phase is found in the medium-oxygen-content and the high-oxygen-content alloys. The deterioration of oxidation resistance is due to the enhanced internal oxidation with the increase of oxygen content. It is possible to improve the oxidation resistance by controlling the oxygen content.

Fabrication of porous Fe Al-based intermetallics via thermal explosion

Porous FeAl-based intermetallics were fabricated by thermal explosion（TE） from Fe and Al powders. The effects of sintering temperature on phase constitution, pore structure and oxidation resistance of porous Fe-Al intermetallics were systematically investigated. Porous Fe-Al materials with high open porosity（65%） are synthesized via a low-energy consumption method of TE at a temperature of 636 ℃ and FeAl intermetallic is evolved as dominant phase in sintered materials at 1000 ℃. The porous materials are composed of interconnected skeleton, large pores among skeleton and small pores in the interior of skeleton. The interstitial pores in green powder compacts are the important source of large pores of porous Fe-Al intermetallics, and the in-situ pores from the melting and flowing of aluminum powders are also significant to the formation of large pores. Small pores are from the precipitation of Fe-Al intermetallics particles. In addition, the porous specimens exhibit high resistance to oxidation at 650 ℃ in air.