Development Of Columnar Grains Research Articles

Development of an additively manufactured metastable beta titanium alloy with a fully equiaxed grain structure and ultrahigh yield strength

Metastable β titanium alloys usually suffer from relatively low yield strengths, which restricts their applications as a structural material. Additive manufacturing (AM), due to its extremely high cooling rates, can generate a refined microstructure that is beneficial to yield strength. However, the intrinsic steep thermal gradients within melt pools often lead to development of columnar grains that can result in mechanical anisotropy. To address these issues, we propose to use potent β-stabilizing elements with large growth restriction factors as the main solute elements in Ti, specifically Fe and Co. d-electron theory is also used to design the detailed compositions of the new titanium alloys for AM. A novel metastable titanium alloy, Ti-xFe-xCo-1Mo (1.5< x <3.5 at%), is thus developed by laser powder bed fusion (L-PBF). With process optimization, the L-PBF-processed alloy was found to contain fully equiaxed β grains embedded with α laths and ω precipitates and the grain boundaries decorated by Ti 2 Co precipitates. The matrix consists of a certain number of micro-sized β flecks and a high density of Fe and Mo atomic clusters. Upon solution treatment (ST), the microstructure turned into equiaxed β grains embedded with ultrafine ω precipitates and Mo atomic clusters. While the L-PBF-processed alloy shows poor tensile properties probably due to the presence of isothermal ω precipitates, the L-PBF-ST-processed alloy demonstrates an unprecedented yield strength of 1.2 GPa and a decent elongation of 10~12%. The alloy deformed by dislocation slipping and failed in a ductile fracture mode.

Solid solution coating of (TiVCrZrHf)N with unusual structural evolution

In this study, we investigated the microstructure of (TiVCrZrHf)N multi-element coating deposited on Si(100) substrate by magnetron sputtering system. Interestingly, the film showed an unusual structure evolution. A continuous and amorphous initial (TiVCrZrHf)N layer formed on the substrate. As film thickness increased, randomly oriented small (TiVCrZrHf)N grains formed, followed by the development of columnar grains with FCC crystal phase. In contrast to previous studies, the nitride coatings grew on substrates with a 270-nm amorphous interlayer. Under appropriate parameters, the occurrence of the unusual microstructure could be attributed to the sluggish diffusion of the multi-principal components. The maximum hardness of 33.5GPa was obtained when coating was deposited under substrate bias of −100V and at substrate temperatures of 723K.

Three‐dimensional EBSD study on the relationship between triple junctions and columnar grains in electrodeposited Co–Ni films

Electrodeposited nanocrystalline materials are expected to have a homogeneous grain size and a narrow grain size distribution. In Co-Ni electrodeposited films, however, under certain conditions an undesired columnar grain structure is formed. Fully automated three-dimensional (3D) orientation microscopy, consisting of a combination of precise material removal by focussed ion beam and subsequent electron backscatter diffraction (EBSD) analysis, was applied to fully characterize the grain boundaries of these columnar grains in order to gain further understanding on their formation mechanisms. Two-dimensional orientation microscopy on these films indicated that the development of columnar grains could be related to the formation of low-energy triple junctions. 3D EBSD allowed us to verify this suggestion and to determine the boundary planes of these triples. The triplets are formed by grain boundaries of different quality, a coherent twin on the {1011} plane, an incoherent twin and a large-angle grain boundary. These three boundaries are related to each other by a rotation about the <1120> direction. A second particularity of the columnar grains is the occurrence of characteristic orientation gradients created by regular defects in the grain. Transmission electron microscopy was applied to investigate the character of the defects. For this purpose, a sample was prepared with the focussed ion beam from the last slice of the 3D EBSD investigation. From the TEM and 3D EBSD observations, a growth mechanism of the columnar grains is proposed.

Columnar grain development in C-Mn-Ni low-alloy weld metals and the influence of nickel

This paper discusses the development of columnar grains in as-deposited C-Mn-Ni(-Mo) low-carbon low-alloy weld metals and the influence of the alloying elements, particularly nickel. It was found that the austenite columnar grain size prior to the γ-α transformation of the weld metals was mainly controlled by the alloying contents rather than HAZ grains adjacent to the fusion boundaries, although the latter determined the size of the initial columnar grain size. The addition of nickel initially depressed the prior austenite grain size and subsequently dramatically coarsened it. This was related to the nickel equivalent (Nieq) of the weld metals and the peritectic reaction during the solidification process. Small columnar grains were associated with a Nieq between 3.4 and 6.2% which resulted in a peritectic reaction when the weld melt solidified, whilst a Nieq higher than 6.2% produced very large columnar grains because the weld pool would directly solidify into austenite and have a subsequent continuous growth.

Changes in the microstructure and density of Li2O during irradiation in BEATRIX-II, phase I

Abstract BEATRIX-II, phase I was an in situ tritium recovery experiment carried out on Li2O to lithium burnups in excess of 4%. Solid Li2O specimens were irradiated under a temperature gradient in the range from 700 K at the outer diameter to 1270 K at the center. A ring specimen of Li2O operated at a nearly uniform temperature with the capability of changing the temperature in the range 780–920 K. Li2O single crystals with two different 6Li enrichments (0.07 and 1.8 at.%) were irradiated at 650 K in non-vented canisters. The microstructures of these specimens before and after irradiation were investigated using optical and scanning electron microscopy. Substantial microstructural change was found to have occurred in the solid specimen during irradiation and this change was characterized by the development of large columnar grains, lenticular pores and a center void. The development of columnar grains and the formation of the center void was attributed to the migration of lenticular pores up the thermal gradient by an evaporation-condensation process. Morphological changes on fracture surfaces were observed for both polycrystalline (solid and ring) specimens and single-crystal specimens. Measurements of density were used to characterize further the effects of irradiation on the open and closed porosity distributions in all the specimens. Although extensive microstructural and density changes occurred during irradiation, these changes had no significant impact on the integrity of Li2O during irradiation, and therefore these results support the use of Li2O as a fusion solid breeder.

High Resolution Electron Microscopy of thin Co-Cr films

Co-Cr thin films have been studied extensively as leading candidates for perpendicular recording media. The development of microstructure in this system has been reported by a number of investigators. These studies have revealed a general growth scenario in which a fine grained, randomly oriented "transition layer" forms first, followed by the development of columnar grains having their hcp c axes textured normal to the film plane. However, it has been found that such columns are not present in very thin (∽10 nm) layers. Also, it has been inferred from electron microdiffraction studies that both c and a axis texturing occurs in these same films. However, direct evidence for this texturing is still needed. In this work, high resolution electron microscopy (HREM) has been employed to examine very thin (5-10 nm) Co-Cr layers.

Texture and columnar structure of evaporated cobalt films: I

The geometric and crystallographic structures of crystallites in cobalt films about 6000 Å in thickness have been investigated. Films were evaporated at an angle of incidence of 70° and an evaporation rate of 1700 Å min -1. The temperature T s of the glass substrate was varied from 100 to 450°C. The crystallographic structure was analysed by plotting pole figures obtained using the Schulz X-ray method, and the geometric structure was observed by replica electron microscopy of the surfaces and of cross sections of the films. The results obtained can be summarized as follows. (1) There was a critical substrate temperature T sc of about 200–250°C, below which the texture was in the final orientation and above which it was in the initial orientation. (2) The development of columnar grains was clearly seen in films evaporated below T sc but this did not occur directly on the substrate in the case of evaporation near T sc. Above T sc, the observation of the columns became difficult and no grains were recognizable as T s was increased.

On the Microstructure of UO2Pellets Irradiated at High Temperature

The microstructure of sintered UO2 pellets irradiated at high temperature in GETR were studied. Two groups of UO2 pellets prepared from the Mme lot were irradiated under similar thermal conditions for different periods. The effect of hurnup on the microstructure of the two groups were studied. An extensive development of columnar grains with lenticular voids were observed in the pellets from the lower burnup group, while those from the higher burnup group had large columnar grains characterized by many radial cracks. Detailed discussion is given. Autoradiographic techniques were also applied on the specimens to examine the microstopic structure. No appreciable migration of the fission products was detected.

On the Microstructure of UO2 Pellets Irradiated at High Temperature

The microstructure of sintered UO2 pellets irradiated at high temperature in GETR were studied. Two groups of UO2 pellets prepared from the Mme lot were irradiated under similar thermal conditions for different periods. The effect of hurnup on the microstructure of the two groups were studied. An extensive development of columnar grains with lenticular voids were observed in the pellets from the lower burnup group, while those from the higher burnup group had large columnar grains characterized by many radial cracks. Detailed discussion is given. Autoradiographic techniques were also applied on the specimens to examine the microstopic structure. No appreciable migration of the fission products was detected.