HREM STUDY OF GRAIN BOUNDARY PHASES IN Fe-Ni-Co-Nb-Ti SUPERALLOY

The microstructures of the grain boundary phases in Fe-Ni-Co-Nb-Ti superalloy have been studied by high resolution electron microscopy (HREM).Besides the Laves phase,three other grain boundary phases have been found and analyzed. The ε phase particles are needle-like with some stacking faults along the(0001) direction. The dominant phase at the grain boundaries is the orthorhombic phase that is also rod-like. High reselution image also confirms the existence of the triclinic phase at the grain boundaries that is irregular and scarce.

Coercivity enhancement of sintered Nd–Fe–B magnets by grain boundary diffusion with Pr_(80-x)Al_(x)Cu_(20)alloys

A grain boundary diffusion(GBD)process with Pr_(80-x)Al_(x)Cu_(20)(x=0,10,15,20)low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets.The best coercivity enhancement of a diffused magnet was for the Pr_(65)Al_(15)Cu_(20)GBD magnet,from 16.38 kOe to 22.38 kOe.Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary(GB)phase,optimizing the microstructure to enhance the coercivity.The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains.Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process.Meanwhile,the introduction of Al can effectively promote the infiltration of Pr into the magnet,which increases the diffusion rate of rare-earth elements within a certain range.This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.

Grain boundary construction and properties enhancement for hot deformed(Ce,La,Y)-Fe-B magnet by a two-step diffusion process

The rare earth-iron-boron magnets based on high abundance rare earths(REs)show potential for costeffective permanent magnets but their hard magnetic properties have to be greatly improved.The grain boundary diffusion process(GBDP)is known as an effective way to improve the coercivity of Nd-Fe-B magnets,however,the conventional diffusion method faces a challenge for Ce-based magnets since there is no enough continuous GB layer as the diffusion channel.Here,a two-step(Nd-Cu doping followed by Nd-Cu diffusion)GBDP was introduced for hot deformed(Ce,La,Y)-Fe-B magnet,and the excellent magnetic properties ofμ0Hc=0.63 T,μ0Mr=0.68 T,and(BH)max=72.4 kJ/m^(3)were achieved.The Nd-Cu doping helps the formation of RE-rich GB layer,and then it acts as the diffusion channel for increasing the ef-ficiency of the subsequent Nd-Cu diffusion and results in the increased volume fraction of continuously distributed GB phase,whose paramagnetism was verified by 57Fe Mössbauer spectrometry.Those paramagnetic GB phases help to form the discontinuous domain walls,as observed by Lorentz transmission electron microscopy,and break the magnetic exchange coupling of RE2Fe14B grains.It thus contributes to the coercivity enhancement of the hot deformed magnet with two-step diffusion,which is further proved by micromagnetic simulation.This study proposes a potential technique to prepare anisotropic hot deformed(Ce,La,Y)-Fe-B magnet with high cost-performance.

Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength.However,the mechanism has not been fully elucidated so far.In this work,the role of phase/grain bound-ary in the static globularization of TC17 alloy was systematically studied by setting differentαphase con-tent before annealing through low-and high-temperature deformation.Isothermal compression causes the parallel distribution and fragmentation of 3Dαplates and few globularαparticles are formed at a strain rate of 1 s^(-1).Post-deformation annealing promotes the static globularization ofαphase while it is affected by initialαphase content.After 730°C deformation,the development ofα/αinterface by absorbing dislocations promotes the formation of globularαgrains based on the nucleation of sepa-ratedαparticles and pre-recoveryαsubgrain during subsequent annealing.Theα/α/βandα/β/βtriple junctions formed due to highαcontent with about 36%volume fraction are favorable for the further nucleation and growth of globularαgrains by reducing interface energy,forming a 3D irregularαplate.Then nucleation and growth of theβphase dominate the microstructure evolution during subsequent an-nealing,resulting in the local dissolution of the plate and formation ofαrods.After 850°C deformation,theαphase tends to nucleate at theβ/β/βtriple junctions and grow into a lamellar shape along the high energyβ/βgrain boundary due to lowαcontent with about 7%volume fraction.Theαnucleation that maintains the Burgers orientation relationship(BOR)with the surroundingβphase grows along the habit plane and thickens slowly,resulting in the formation of a precipitatedαplate with a flat surface and the suppression of static globularization.The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.

Coercivity enhancement of Ce-containing hot-deformed magnets by grain boundary diffusion of DyF_(3)

The samples with full density were prepared by hot pressing the the melt-spun powders mixed with DyF_(3)powders of different mass fractions followed by hot-deformation process.The magnetic properties and temperature dependence of coercivity were obtained by BH tracer and VSM,respectively.The microstructure were analyzed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The coercivity of Ce-containing hot-deformed magnets is increased from 1.41 to 1.95 T by grain boundary diffusion of 3 wt%DyF_(3),and is further enhanced to 2.05 T after annealing treatment.The thermal stability of coercivity and remanence is improved.The annealing condition in this work crucially plays a role in thickening the grain boundary phase.Microstructure analysis reveals that the continuous and thick grain boundary phase formed after DyF_(3)diffusion can weaken the magnetic coupling between grains,and suppress the platelet shaped grain size and the aspect ratio.The Dycontaining shell structure formed by the partial diffusion of Dy into the main phase can increase the magnetic anisotropy field,which is the main reason for the coercivity improvement.After optimizing the structure by DyF_(3)diffusion,the"dendritic-like"reverse domain is transformed into the"dot scatteredlike"reverse domain.

^(57)Fe Mössbauer spectrometry: A powerful technique to analyze the magnetic and phase characteristics in RE–Fe–B permanent magnets

This review summarizes the recent advances on the application of ^(57)Fe Mössbauer spectrometry to study the magnetic and phase characteristics of Nd–Fe–B-based permanent magnets. First of all, the hyperfine structures of the Ce_(2)Fe_(14)B,(Ce,Nd)_(2)Fe_(14)B and MM_(2)Fe_(14)B phases are well-defined by using the model based on the Wigner-Seitz analysis of the crystal structure. The results show that the isomer shift δ and the quadrupole splitting öEQ of those 2:14:1 phases show minor changes with the Nd content, while the hyperfine field Bhfincreases monotonically with increasing Nd content and its value is influenced by the element segregation and phase separation in the 2:14:1 phase. Then, the hyperfine structures of the low fraction secondary phases are determined by the ^(57)Fe Mössbauer spectrometry due to its high sensitivity. On this basis,the content, magnetic behavior, and magnetization of the REFe_(2) phase, the amorphous grain boundary(GB) phase, and the amorphous worm-like phase, as well as their effects on the magnetic properties, are systematically studied.

Anisotropic alumina ceramics with isotropic optical properties

Excellent mechanical and isotropic optical properties are achieved simultaneously from fully dense polycrystalline advanced alumina ceramics with a hexagonal crystal structure,which are optically anisotropic.A small amount(240 ppm)of ZrO_(2) additive is used to synthesize the transparent alumina ceramics with an average grain size of 0.7µm,and locally detected optical distortion(birefringence)is extremely minimized,less than 20 nm/pass.Total transmittance(86%)of the alumina ceramics(thickness=0.3 mm)in ultraviolet(UV)to infrared(IR)regions is similar to that of commercial c-axis sapphire single crystals produced by Czochralski(CZ)method,while the extinction ratio in the visible wavelength area is over 25 dB.Like glass and the c-axis sapphire single crystals,the alumina ceramics can clearly display texts and images on liquid crystal display(LCD)screens.A grain boundary phase of nano-sized Al_(2)O_(3)-ZrO_(2) composition(amorphous)is formed at the grain boundary of the advanced alumina ceramics,which enhances four-point bending strength and fracture toughness(KIC)simultaneously to 921 MPa and 6.8 MPa·m^(0.5),respectively,and hence the mechanical properties are superior to those of the sapphire single crystals.

Deformation and cracking behaviors of proton-irradiated 308L stainless steel weld metal strained in simulated PWR primary water

The proton-irradiated 308L stainless steel weld metal was strained by using constant extension rate tensile testing in simulated PWR primary water, and its deformation microstructures and irradiation assisted stress corrosion cracking(IASCC) behavior were investigated. The results suggest that the irradiation significantly increases the SCC susceptibility of 308L weld metal and causes various deformation microstructures including lathy faulted planes, dislocation channels and deformation twins in austenite and atomic plane rotation in δ-ferrite. The propagation of intergranular IASCC cracks is closely related to the location of the crack tip. For the crack tip in the specimen matrix interior, localized deformation is likely the key factor responsible for the crack growth. For the crack tip close to the specimen surface, however, localized corrosion along the grain boundary rather than the localized deformation appears to dominate the crack propagation. Unlike the intergranular cracks, the IASCC cracks along the δ-ferrite/austenite phase boundary can initiate either by crack initiation at the phase boundary or by crack propagation from the grain boundary. In both cases, the cracked phase boundaries contain a large number of carbides and are severely corroded, but no deformation microstructures are observed, which implies that the localized corrosion may play an important role in the IASCC along the phase boundary. In addition, δ-ferrite can retard the IASCC crack propagation along the grain boundary, which is probably related to the reduction of localized deformation by δ-ferrite.