Columnar Grain Boundaries Research Articles

Quantitative study of the thickness-dependent stress in indium tin oxide thin films

A quantitative analysis of stress in indium tin oxide (ITO) films with varying thicknesses was conducted using high-precision stylus profilometry. The experimental results revealed a transition of the stress type from tensile to compressive as the ITO film thickness increased. Further investigation of the surface morphology of the ITO films indicated that the tensile stress originated from the impingement and coalescence of newly deposited equiaxed grains, while the observed compressive stress was attributed to the incorporation of excess material in the boundaries of columnar grains. The ability to monitor stress evolution in sputtered ITO films on glass substrates provides valuable insights for process control in ITO device manufacturing.

Electron beam melting of (FeCoNi)86Al7Ti7 high-entropy alloy

The development of traditional alloy materials has reached the bottleneck due to the constructive interaction between strength and plasticity. Hence, novel alloy systems like (FeCoNi)86Al7Ti7 high-entropy alloys (HEAs) were fabricated using selective electron beam melting (SEBM). The (FeCoNi)86Al7Ti7 HEA dominantly consisted of ordered L12 and disordered fcc matrix phases. The bottom surface (SEBM-bottom)) of the (FeCoNi)86Al7Ti7 HEA blocks presents an equiaxed microstructure; however, coarse columnar grains across layers were observed along the building direction (SEBM-side). Island-like precipitates (ILPs) are distributed evenly within the equiaxed grain. The boundaries of columnar grains and high-density multicomponent cubic nanoparticles (MCCNPs) are dispersed uniformly in the matrix. A strong< 100 > // building direction (BD) fiber texture can be observed. The (FeCoNi)86Al7Ti7 HEA exhibits superior strengths of 2048 MPa and up to 12% ductility at ambient temperature. However, the SEBM (FeCoNi)86Al7Ti7 HEA exhibits anisotropy in its mechanical properties.

Resistance of Austenitic Stainless Steels to Ductility-Dip Cracking: Mechanisms

In Ni-based alloys, precipitates that form along grain boundaries (GBs) during terminal solidification have been shown to pin GBs and resist GB sliding, which can cause ductility-dip cracking (DDC). As a result, it is often suggested that the stainless steel skeletal/lacy  in a  matrix resists DDC because it pins GBs. In the present study, austenitic stainless steels 304, 316, 310, and 321 were quenched with liquid Wood’s metal (75˚C) during welding. Quenching captured the elevated-temperature micro-structure and simultaneously induced cracking, thus revealing the mechanisms of the resistance to DDC. In addition, DDC was much higher in 310 than 304, 316, and 321, which is consistent with results of conventional tests. Both 304 and 316 solidified as columnar  grains, with continuous  formed along GBs soon after solidification to resist DDC along the GBs. 321 solidified as equiaxed grains of  instead of columnar, and the tortuous GBs associated with equiaxed grains resisted DDC. 310, however, solidified as coarse, straight  grains with little  along the GBs, and solidification GBs migrated to become locally straight. The resulting GBs were long, straight, and naked, which is ideal for DDC. In 304, 316, or 321, skeletal/lacy  in a  matrix did not exist in the fusion zone near the mushy zone, where DDC occurs. This proved skeletal/lacy  cannot resist DDC as often suggested. Instead, the present study identified two new mechanisms of resistance to DDC: 1) formation of continuous or nearly continuous  along boundaries of columnar  grains and 2) solidification as equiaxed  grains.

Columnar grain-driven plasticity and cracking in nanotwinned FCC metals

The mechanisms of strengthening and plasticity in columnar-grained metals with preferentially oriented nano-sized twins have been examined traditionally by considering dislocation processes, but rarely from the perspective of grain boundary (GB) deformation. Here, the effects of GB strain accommodation on plastic deformation in four different columnar-grained nanocrystalline nanotwinned (nt) face-centered-cubic metals (Cu, Ag, Al, and Ni) were studied by large-scale molecular dynamics simulations. It is observed that in tensile deformation parallel to coherent twin boundaries (CTBs), the dislocation mechanisms in each metal are identical and associated with GB emissions of jog and threading dislocations at small and large CTB spacings, respectively. However, CTB strengthening effects are increasingly more pronounced in columnar-grained nt metals as their shear modulus increases, which is rationalized by the dependence of GB stress concentrations on twin size, metal type and strain rate. Also, while flow stresses in nt-Cu, nt-Ag, and nt-Al metals increase linearly with decreasing CTB spacing, a maximum strength limit is reached in nt-Ni below a critical CTB spacing of 6 nm. The strength limit in nt-Ni results from columnar GB cracking induced by prominent GB sliding. For columnar-grained microstructures, GB sliding is equivalent in nt-Ag and nt-Al and slightly lower in nt-Cu but markedly higher in nt-Ni. These findings underscore the importance of new GB deformation mechanisms on plasticity and fracture in columnar-grained nt metals and enrich our understanding of CTB strengthening in fcc metals synthesized in the literature.

Thermal Properties of Selectively Laser-Melted AlSi10Mg Products with Different Densities

The thermal diffusivity and thermal conductivity of selectively laser-melted AlSi10Mg samples with different target relative densities, 99 and 99.5%, have been studied in the temperature range from 25 to 400 °C. The properties were measured in the build direction and orthogonal to the build direction, for the samples in the as-built and heat-treated conditions. The 99.5% dense samples are characterized by a noticeably higher thermal diffusivity and higher thermal conductivity than the 99% dense samples. For each sample, it is found that the applied heat treatment improves the thermal diffusivity and thermal conductivity of the as-built material, but also results in pronounced anisotropy, with greater thermal diffusivity and conductivity in the build direction, especially for the 99.5% dense sample. The anisotropy is attributed to the presence of Si particles along the grain boundaries of columnar grains. Since the columnar grains are elongated along the build direction, a smaller spacing between particle-decorated boundaries in the plane perpendicular to the build direction presents a higher resistance to the heat conduction.

Process-Dependent Composition, Microstructure, and Printability of Al-Zn-Mg and Al-Zn-Mg-Sc-Zr Alloys Manufactured by Laser Powder Bed Fusion

Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribute to future alloy development. In this study, two different alloys, Al-6Zn-2Mg and Al-6Zn-2Mg-0.7Sc-0.3Zr in wt. pct, representing traditional and AM-specific novel aluminum alloys, respectively, were manufactured by LPBF technique using pre-alloyed gas atomized powders. The Al-Zn-Mg ternary alloys exhibited cracks at various LPBF processing parameters, primarily along the grain boundaries of the large columnar grains that extended across multiple melt pools. The severity of cracking in LPBF Al-Zn-Mg alloys was process-dependent and could be correlated to the change in alloy composition due to evaporation of Zn and Mg with high vapor pressure. The Scheil–Gulliver non-equilibrium solidification calculations showed that the Al-Zn-Mg alloys with lower Zn and Mg concentrations had smaller solidification range (i.e., ΔT) and steepness values (i.e., $$ \left| {{\text{d}}T / {\text{d}}f_{\text{s}}^{1/2} } \right| $$ near $$ f_{\text{s}}^{1/2} = 1 $$), which corresponded to a lower cracking severity. On the other hand, no cracks were observed in Al-Zn-Mg-Sc-Zr alloys, although their solidification range and steepness values were similar to the ternary Al-Zn-Mg alloys. The microstructure of Al-Zn-Mg-Sc-Zr alloys exhibited a much refined heterogeneous microstructure containing small equiaxed and columnar grains within each melt pool, owing to the heterogeneous nucleation upon primary Al3(Sc,Zr) particles. Process-dependent microstructure was observed as a result of variation in thermal gradient and cooling rate associated with LPBF parameters. Findings from this study provide guidance for the future design of AM-specific aluminum alloys and insights into the microstructural control by LPBF.

Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld

A ductility-dip-cracking (DDC)-concentrated zone (DCZ) in a width of about 3 mm was observed adjacent to the AISI 316 L/52 Mw fusion boundary (FB) in 52 Mw. The morphology, microstructure, mechanical and thermal properties and corrosion behavior in simulated primary water of DDC/DCZ were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), 3D X-ray tomography (XRT), 3D atom probe (3DAP), slow strain rate tensile (SSRT) testing and thermal dilatometry. The results indicate that DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains, ranging from micrometers to millimeters in size. Large-size M23C6 carbides dominate on the grain boundaries rather than MC (M=Nb, Ti), and thus the bonding effect of carbides is absent for the straight grain boundaries. The impurity segregation of O is confirmed for the inner surfaces of DDC. The oxide film formed on the inner surface of DDC (about 50 nm) is approximately twice as thick as that on the matrix (about 25 nm) in simulated primary water. The yield strength, tensile strength and elongation to fracture of 52 Mw-DCZ (400 MPa, 450 MPa and 20 %, respectively) are lower than those of 52 Mw-MZ (460 MPa, 550 MPa and 28 %, respectively). The intrinsic high-restraint weld structure, the additional stress/strain caused by the thermal expansion difference between AISI 316 L and 52 Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.

Interfacial structure evolution of Ti-coated diamond particle reinforced Al matrix composite produced by gas pressure infiltration

Abstract The interfacial structure evolution of Ti-coated diamond particle reinforced Al matrix composite produced by gas pressure infiltration has been studied systematically, aimed to provide an insight into the optimization of interfacial structure and properties of the composite. The results show that the Ti coating layer on diamond particle consists of three layers, including fine TiC grains, TiC columnar grains and dispersed Ti particles. During the infiltration of molten Al, the dispersed Ti particles react with Al to form Al 3 Ti particles in Al matrix, while both TiC layers remain unchanged. However, in the subsequent cooling process, Al 4 C 3 particles are formed along the grain boundaries of TiC columnar grains as a result of Al diffusion and the subsequent Al/TiC reaction. By controlling the thickness of the TiC layers on diamond surface, an optimized thermal conductivity of 650 W m −1 K −1 is obtained for the composite. The finding is beneficial to the understanding of diamond coating engineering in the diamond particle reinforced Al matrix composite.

Multilayered scale formation during Zr-based metallic glass oxidation in the supercooled liquid region

During the oxidation of Zr55Cu30Al10Ni5 metallic glass in its supercooled liquid region, multilayered scale structures were found. An amorphous oxide layer forms first, beneath which, a layer of t-ZrO2 columnar dendrites nucleate and grow into the supercooled amorphous substrate. After the substrate fully crystallizes, a layer of flaky t-ZrO2 grains forms beneath the columnar dendrite layer. The continuous outward diffusion of Cu2+ cations from the grain boundaries of columnar grains results in interconnected fissures inside this layer, which provide pathways for gaseous O2 transportation. Thus, another distinguishable layer is generated between the columnar grain layer and the flaky grain layer.

Anisotropic and inhomogeneous thermal conduction in suspended thin-film polycrystalline diamond

While there is a great wealth of data for thermal transport in synthetic diamond, there remains much to be learned about the impacts of grain structure and associated defects and impurities within a few microns of the nucleation region in films grown using chemical vapor deposition. Measurements of the inhomogeneous and anisotropic thermal conductivity in films thinner than 10 μm have previously been complicated by the presence of the substrate thermal boundary resistance. Here, we study thermal conduction in suspended films of polycrystalline diamond, with thicknesses ranging between 0.5 and 5.6 μm, using time-domain thermoreflectance. Measurements on both sides of the films facilitate extraction of the thickness-dependent in-plane (κr) and through-plane (κz) thermal conductivities in the vicinity of the coalescence and high-quality regions. The columnar grain structure makes the conductivity highly anisotropic, with κz being nearly three to five times as large as κr, a contrast higher than that reported previously for thicker films. In the vicinity of the high-quality region, κr and κz range from 77 ± 10 W/m-K and 210 ± 50 W/m-K for the 1 μm thick film to 130 ± 20 W/m-K and 710 ± 120 W/m-K for the 5.6 μm thick film, respectively. The data are interpreted using a model relating the anisotropy to the scattering on the boundaries of columnar grains and the evolution of the grain size considering their nucleation density and spatial rate of growth. This study aids in the reduction in the near-interfacial resistance of diamond films and efforts to fabricate diamond composites with silicon and GaN for power electronics.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.

Failure Analysis of Weld Cracking in a Thick-Walled 2.25Cr-1Mo Steel Pressure Vessel

A crack in thick-walled 2.25Cr-1Mo steel pressure vessel girth weld was found during manufacturing. To investigate the cause of failure, optical microscopy, scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, and microhardness tester were used in this study. According to test results, the fracture is classified as reheat cracking with multiple origins. The cracking occurred during surfacing or final post-weld heat treatment process. Coarse-grains in the weld and bulk-carbides precipitated along grain boundaries induced by multiple heating are main causes of the fracture from material aspect, while high level of the hoop stress component and excess localized deformation in stress relief procedure are mechanical aspect causes of the cracking. The fracture surfaces present major intergranular feature with a small fraction of transgranular morphologies. Large numbers of M3C and M23C6 carbides particles were found on the fracture surface, these carbides mainly precipitated on prior austenite-grain boundaries, columnar-grain boundaries, and sub-grain boundaries. Additionally, several proposals were also offered to reduce weld cracking of 2.25Cr-1Mo steel pressure vessels.

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ∼5 nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si.

Failure of Hastelloy C-276 pump impeller in hydrochloric acid

This paper presents the results of a failure investigation carried out on a cast Hastelloy C-276 pump impeller that suffered general corrosion in 32% hydrochloric acid at ambient temperature in a chemical plant. Microscopic examination revealed that the attack was crystallographic in nature. The attack was more enhanced at the interdendritic arm spacing as well as the boundaries of columnar grains, resulting in deep grooves. Taking into consideration that the local temperature could reach up to 50 °C in the summer months, Hastelloy C-276 was judged to be unsuitable material for applications involving 32% HCl. A search into a suitable alternative alloy did not provide a satisfactory answer. Therefore, rubber-lined pump impeller was considered the better alternative.

ＭｇＢ２超伝導体の性質とその可能性 ＭｇＢ２薄膜における柱状晶結晶配向とＪｃの異方性

We prepared MgB2 thin films using an electron-beam evaporation technique. TEM observation of the detailed microstructure for the films with and without substrate-spin during deposition revealed that the MgB2 columnar grains align in the direction of B flux. Jc of the film is higher in perpendicular fields than in parallel fields, whereas Bc2 and Birr have the opposite anisotropy. A detailed study on the relationship between the Jc anisotropy and the texture geometry of MgB2 columnar grains revealed that Jc becomes maximal where the magnetic field direction aligns in the direction of the MgB2 columnar-grain boundaries. The results indicate conclusively that the columnar-grain boundaries in MgB2 act as effective pinning centers.

Grain-boundary recombination in Cu(In,Ga)Se2 solar cells

Two-dimensional simulations are performed to investigate the impact of grain boundaries (GBs) on Cu(In,Ga)Se2 (CIGS) solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1μm, the effective grain-boundary recombination velocity must be less than 104cm∕s to allow for record-efficiency devices. The majority-hole repulsion (additional donors at the GB) and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion (additional acceptors at the GB) will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block holes from the GB region and allow efficiencies comparable to GB-free material.

Growth of ATiO3 (A = Ba,Sr) dielectric thin film by a hydrothermal–electrochemical method

The hydrothermal–electrochemical growth ofATiO3 (A = Ba, Sr) dielectric thin films on Ti substrates inA(OH)2 solutions is reviewed. The film thickness increased monotonically with increase in the quantityof electricity passed through the Ti substrate, and could be controlled coulometrically by thisfactor. The current efficiency for the film growth was in the range from about 0.6% to 3.0%.(Ba,Sr)TiO3 solid-solution films could be grown with control of the Ba/Sr composition.ATiO3 thin films could also be grown uniformly on porous Ti bodiesusing cyclic electrolysis. A tracer experiment revealed that theATiO3 film grew at the film/substrate interface by transport of both A-site and oxygen atomsfrom the solution to the interface. The films grown were composed of a thin surface layerhaving an isotropic polycrystalline structure and an inner layer having a columnarstructure. Nanopores were found at the boundaries of columnar grains constituting theinner layer. The observed pores are considered to act as short-circuit paths for masstransport from the film surface to the film/substrate interface. The resistivity of theBaTiO3 thin film wasas high as 1012 Ω cm in the voltage range up to 2 V. The breakdown field was higher than30 MV m−1. The dielectric constant and dielectric loss were about 340–350 and 7%–10%, respectively, at 1 kHzand 0.1 Vrms. The polarization–voltage hysteresis loops were those of a paraelectric phase.

Origin of soft-mode stiffening and reduced dielectric response inSrTiO3thin films

The problem of the reduced dielectric response in thin films of high-permittivity materials is analyzed by studying the soft-mode response in several ${\mathrm{SrTiO}}_{3}$ thin films by means of Fourier transform far infrared, monochromatic submillimeter, and micro-Raman spectroscopies. A 300-nm-thick metalorganic chemical vapor deposition film, quasiepitaxially grown on a (0001) sapphire substrate with a perfect 〈111〉 orientation, displays a ferroelectric transition near 125 K induced by a tensile residual stress, appearing apparently simultaneously with the antiferrodistortive transition. On the other hand, polycrystalline chemical solution deposition films grown on (0001) sapphire, and also tensile stressed, show a harder soft mode response without the appearance of macroscopic ferroelectricity. This effect, which increases with the film thickness, is explained by a strong depolarizing field induced by the percolated porosity and cracks (in the 10-nm scale) along the boundaries of columnar grains (normal to the probe field direction). Brick-wall model calculations showed that 0.2 vol. % of such a porosity type reduces the permittivity from 30000 to less than 1000. The activation of the forbidden IR modes in the Raman spectra in the whole 80--300-K temperature range studied is explained by the effect of polar grain boundaries, in analogy with the bulk ceramics.

Microscopic study of failure mechanisms in infiltrated carbon fiber felts

Failure mechanisms in infiltrated carbon fiber felts have been studied by optical light microscopy, transmission electron microscopy and scanning electron microscopy combined with mechanical testing experiments. A model is presented which describes crack generation and propagation at layer–layer and fiber–matrix interfaces as well as within matrix carbon layers with different textures. Intensive cracking occurs within high- and less frequently in medium- and low-textured pyrolytic carbon layers. In particular, fracture does not occur directly at the fiber–matrix interface but within the low-textured matrix layer deposited on the fiber. Crack deflection in interface regions between layers with different textures, crack deflection along boundaries of columnar grains in high -textured layers and at interfaces between polyhedral nanoparticles, and finally crack bridging within high -textured lamellae are cooperative failure mechanisms contributing to the toughness enhancement.

Microstructure of Strontium Titanate Thin Film Grown by the Hydrothermal‐Electrochemical Method

Microstructures of SrTiO3 thin films grown on Ti substrates by the hydrothermal‐electrochemical method have been studied by transmission electron microscopy. The grown films consisted of a thin surface layer having an isotropic polycrystalline structure and a thick inner layer having a columnar structure. Pores of the order of several tens of nanometers were found at boundaries of columnar grains constituting the inner layer. A 20‐ or 30‐nm‐thick polycrystalline layer of Ti oxides contaminated with a small amount of Sr was also found at the SrTiO3 film/Ti substrate interface. During the film growth at this interface, the observed pores are considered to act as short‐circuit paths for mass transport from the film surface to the interface.