Columnar Microstructure Research Articles

Effect of laser pulse power on solidification cracking susceptibility in surface processing of a high carbon high chromium tool steel

High carbon, high chromium tool steels are deep hardening steels widely used for cold forming dies. A 400 W Nd:YAG laser was employed to melt the surface of the AISI D2 tool steel with 5 and 6 ms single pulses of laser at 4–10 J energy levels. The microstructural study showed that complete dissolution of large primary carbides occurred during melting, but the cooling rate was too fast to form any primary carbide. Increasing the laser energy (peak power) led to a coarser dendritic microstructure and a reduction in solidification cracking intensity. The analyses indicate that the coarser dendritic structure has promoted the backfilling effect. However, despite having a coarser dendritic structure, the material laser processed with a preheat of 300 °C showed the most severe crack intensity due to the provision of sufficient time in cooling for the formation of a network of brittle carbides in the inter-dendritic regions. The HAZ was found to be sound although reaching a hardness of 750 HV. • A fine columnar dendritic microstructure was formed by laser remelting, which became coarser with increased laser peak power. • Facilitation of the backfilling effect in higher laser peak powers reduced the intensity of solidification cracks.. • The formation of the inter-dendritic carbide phase in the preheated sample increased the solidification cracking intensity.

Influences of the substrate strain on microstructures of L10 FePt-X thin films by using phase field simulation

The microstructure evolution of the FePt-X (segregant) thin films is studied by employing a two-dimensional phase field model based on the elastic effect. Simulated results show that the lattice mismatch between the FePt-X thin film and substrate leads to slight rafting (preferential coarsening) of columnar FePt grains along the direction of the misfit strain, and produces more rounded grains, while an amorphous segregant is doped. The elastic energy primarily affects the areas between the FePt grains, where more than 90% of the elastic energy is concentrated. Moreover, the elastic energy is relaxed gradually with the formation of columnar periodic arrays of FePt grains. To generate columnar FePt grains microstructure in the FePt-X thin films, the lattice mismatch between the thin film and substrate should be controlled within 3% when the thermal expansion difference between the thin film and substrate is ignored.

High temperature oxidation kinetics of Fe-10Al-4Cr-4Y2O3 ODS alloy at 1200–1400 °C

The high temperature oxidation kinetics of the Fe-10Al-4Cr-4Y2O3 ODS alloy (FeAlOY) with excellent creep and oxidation resistance at temperatures exceeding 1100 °C is investigated at 1200 °C, 1300 °C and 1400 °C. A compact layer with columnar grain microstructure consisting of α-Al2O3 and YAG (Y3Al5O12) grains is grown at the surface. The oxidation resistance of FeAlOY is compared with other Fe-Cr-Al alloys. The oxidation mechanism and roles of individual elements in the FeAlOY are discussed in detail. Despite containing 3–4 times less Cr, the FeAlOY has the same or even better oxidation resistance when compared with classical Fe-Cr-Al alloys.

Modeling of machining of EB-PVD ceramic coatings using smoothed particle hydrodynamics method

In this work, an improved Smoothed Particle Hydrodynamics (SPH) based model for simulating the machining process of the thermal barrier coatings is presented. The columnar grain microstructure in the electron-beam physical vapor deposition (EB-PVD) coating is constructed. The Johnson-Holmquist 2 (JH-2) material model coupled with the Johnson-Holmquist plasticity damage model is used for the ceramic coating. The model validation is conducted by the comparison between the SPH model and qualitative agreement with experimental observation, and a quantitative agreement with the one-dimensional stress wave analytical model. In the SPH model, the cutting processing parameters, such as cutting depth, cutting speed, cutting tool's edge radius, rake angle on the cutting force, and temperature change are studied. The results show that the fracture of the columnar grains during the cutting process is done through deflection and fracture of the grains, followed by pushing against neighboring grains. Both the cutting force and temperature of the coating increase with cutting depth due to increased cutting work which is transferred to heat. The cutting forces from the SPH model at the stable stage are in excellent agreement with the fracture mechanics analytical solution. The cutting force and temperature increase are higher for a larger edge radius, due to increased friction between the cutting tool and coating. The SPH model result follows the same trend as the main cutting force calculated by the analytical expression. The SPH cutting model developed in this work can be used as a design tool to optimize the coating machining process.

On the thermal stability and performance evaluation of Si doped transition metal nitride/oxide nanolayered multilayer-based spectrally selective absorber for high-temperature photothermal applications

Developing an absorber coating with high absorptance (α) in the solar spectrum region and low thermal emittance (ε) in the infrared region for concentrated solar power (CSP) applications, operating at temperatures >400 °C, is still a great challenge. Herein, we describe a multilayer solar selective coating on stainless steel 304 (SS 304) substrates with α of 0.954 and ε of 0.07. The multilayer solar selective coating consists of: (1) tungsten (W) infrared reflector layer, (2) titanium aluminum nitride (TiAlN) absorber layer, (3) titanium aluminum silicon nitride (TiAlSiN) absorber layer, (4) titanium aluminum silicon oxy-nitride (TiAlSiON) semi-absorber layer and (5) titanium aluminum silicon oxide (TiAlSiO) anti-reflection layer. The compositions of the individual layers have been selected in such a way that they easily form protective layers of Al2O3, TiO2 and SiO2 on the coating surface when exposed to high temperature in air. Further, addition of Si in different layers not only improves the thermal stability but also helps in densifying the microstructure of the layers. Moreover, the presence of multilayer structure hinders the formation of pinholes and pores along with columnar microstructure, a typical characteristic of the sputter deposited transition metal nitrides and oxides. This unique coating design, thus, leads to high spectral selectivity (α/ε) of 13.6 on SS 304 substrate along with thermal stability up to 600 °C for 1000 h in vacuum under cyclic heating conditions. These properties of the developed solar absorber coating demonstrate its suitability for evacuated receiver tubes in CSP plants.

High power laser powder bed fusion of AlSi10Mg alloy: Effect of laser beam mode

High power laser powder bed fusion (HP-LPBF) is a burgeoning additive manufacturing technology for the high-efficiency and high-accuracy build of metallic parts. However, studies about the effect of laser beam mode on build quality is still insufficient. In this paper, both a 2 kW multi-mode fiber laser and a 2 kW Gaussian mode fiber laser are used for the HP-LPBF of AlSi10Mg alloy. Firstly, the relative density (RD) of the HP-LPBF samples has been studied for parameter optimization. The RD of the samples built with the multi-mode laser beam (hereafter called as Multi-mode sample) first increases and then remains stable with the increase of laser energy density Ev. When Ev is no less than 50 J/mm3, the RD of the Multi-mode sample is higher than 99.5%. In contrast, the RD of the samples built with the Gaussian mode laser beam (hereafter called as Gaussian sample) first increases and then decreases with the increase of Ev, and the highest RD is just 95.87 ± 0.2% at the Ev of 50 J/mm3. Based on this, the Multi-mode sample and Gaussian sample built with the same parameters (Ev=50 J/mm3) are chosen as the typical contrast samples for further studies of surface quality, microstructure and tensile property. The results show that the typical Multi-mode sample presents a flatter surface and a lower surface roughness compared with the Gaussian sample. The phase composition of both the two kinds of samples consists of α-Al matrix and eutectic Si. However, the solidification microstructure varies with laser beam mode. The Multi-mode sample presents a columnar solidification microstructure with a strong< 100 > texture along the build direction. In contrast, the solidification microstructure of the Gaussian sample is a mixture of equiaxed grains and short columnar grains. The crystallographic orientations of the two kinds of grains are multifarious, leading to a much weaker texture. The tensile strengths and elongation of the Multi-mode sample are significantly better than those of the Gaussian sample. The effect mechanisms of the laser beam mode on relative density, surface quality, microstructure and tensile property have also been revealed.

Multiaxial fatigue of Inconel 718 produced by Selective Laser Melting at room and high temperature

This work investigates the fatigue and cyclic plasticity of Inconel 718 produced by Selective Laser Melting (SLM) at room and elevated temperature. Strain-controlled axial, torsional and proportional tests with Rɛ=0 were carried out at 20 °C and 450 °C using thin-walled specimens, resulting in fatigue lives ranging from 103 to 105 cycles. SLM Inconel 718 exhibits a columnar microstructure along the build direction, and the dominant defect related to the manufacturing process was gas pores. Fatigue crack initiation occurred a carbides and/or metallic inclusions for all tests and, therefore, intrinsic defects related to the manufacturing process did not induce premature initiation of fatigue cracks. The orientation of macroscopic fatigue cracks was consistent with the plane of maximum shear stress for all loading conditions. The fatigue crack propagation mechanism of SLM Inconel 718, on the other hand, depends on the loading path and temperature. Both the equivalent strain amplitude and the Fatemi–Socie criterion could correlate fatigue life of all loading conditions at 20 °C and 450 °C within factor-of-two boundaries. As a general trend, the fatigue endurance of SLM Inconel 718 is inferior to its forged counterpart, especially at 450 °C. The cyclic plasticity behaviour of SLM and forged Inconel 718 is similar at both 20 °C and 450 °C and can be simulated by the Chaboche model.

Effect of target power ratio on microstructure and deposition rate of TiN film deposited by dual pulse power magnetron sputtering

This work focused on removed mode of titanium target material and deposition process of the TiN film in a novel dual pulse power magnetron sputtering (DPPMS), which could improve deposition rate and properties of the TiN film. The study was carried out using DPPMS with different target power ratios between the first and the second ionized periods (Pw/Ps). It was found that when Pw/Ps was changed from 0.9/1.6 to 0.3/2.2, the removed mode of target material changed from sputtering to sputtering and evaporation, and the deposition rate of TiN films increased from 26 to 61 nm/min. All TiN films showed face center cubic (fcc) crystal structure and typical columnar microstructure. The compactness and size of columnar structure were improved with Pw/Ps changed from 0.9/1.6 to 0.3/2.2. Additionally, the hardness of TiN film deposited at Pw/Ps of 0.3/2.2 reached up to 25.4 GPa.

Influence of the Template Layer on the Structure and Ferroelectric Properties of PbZr0.52Ti0.48O3 Films.

The microstructure of the PbZr0.52Ti0.48O3 (PZT) films is known to influence the ferroelectric properties, but so far mainly the effect of the deposition conditions of the PZT has been investigated. To our knowledge, the influence of the underlying electrode layer and the mechanisms leading to changes in the PZT microstructure have not been explored. Using LaNiO3 (LNO) as the bottom electrode material, we investigated the evolution of the PZT microstructure and ferroelectric properties for changing LNO pulsed-laser deposition conditions. The explored deposition conditions were the O2 pressure, total pressure, and thickness of the electrode layer. Increasing both the O2 pressure and the thickness of the electrode layer changes the growth of PZT from a smooth, dense film to a rough, columnar film. We explain the origin of the change in PZT microstructure as the increased roughness of the electrode layer in relaxing the misfit strain. The strain relaxation mechanism is evidenced by the increase in the crystal phase with bulk LNO unit cell dimensions in comparison to the crystal phase with substrate-clamped unit cell dimensions. We explain the change from a dense to a columnar microstructure as a result of the change in the growth mode from Frank–van der Merwe to Stranski–Krastanov. The ferroelectric properties of the columnar films are improved compared to those of the smooth, dense films. The ability to tune the ferroelectric properties with the microstructure is primarily relevant for ferroelectric applications such as actuators and systems for energy harvesting and storage.

Effect of Metal-Precursor Stacking Order on Volume-Defect Formation in CZTSSe Thin Film: Formation Mechanism of Blisters and Nanopores.

In this study, we investigated the effect of the stacking order of metal precursors on the formation of volume defects, such as blisters and nanopores, in CZTSSe thin-film solar cells. We fabricated CZTSSe thin films using three types of metal-precursor combinations, namely, Zn/Cu/Sn/Mo, Cu/Zn/Sn/Mo, and Sn/Cu/Zn/Mo, and studied the blister formation. The blister-formation mechanism was based on the delamination model, taking into consideration the compressive stress and adhesion properties. A compressive stress could be induced during the preferential formation of a ZnSSe shell. Under this stress, the adhesion between the ZnSSe film and the Mo substrate could be maintained by the surface tension of a metallic liquid phase with good wettability, or by the functioning of ZnSSe pillars as anchors, depending on the type of metal precursor used. Additionally, the nanopore formation near the back-contact side was found to be induced by the columnar microstructure of the metal precursor with the Cu/Zn/Mo stacking order and its dezincification. Based on the two volume-defect-formation mechanisms proposed herein, further development of volume-defect-formation suppression technology is expected to be made.

Effects of Recrystallization on Tensile Anisotropic Properties for IN738LC Fabricated by Laser Powder Bed Fusion

This study demonstrates the effects of recrystallization on tensile properties and the anisotropy of IN738LC, a typical γ’ precipitation-strengthened alloy, at both room and high temperatures via the laser powder bed fusion process. The nonrecrystallized columnar microstructure, subjected to standard IN738LC heat treatment up to 1120 °C, and the almost fully recrystallized microstructure, heat-treated at 1204 °C, were compared. The tensile properties strongly depend on whether recrystallization was completed as well as the tensile direction. This can be explained by microstructure characterization, featuring the Taylor factor in the tensile direction, average grain size estimated by ellipse approximation, and the relationship between the grain shape and tensile direction. The shape of the recrystallized grains and the distribution of coarse MC carbides inside the recrystallized grains were determined by the microstructure in an as-built state. In high-temperature tensile tests conducted in the horizontal direction, the separation of the columnar grains caused a brittle fracture. In contrast, dimples were observed at the fracture surface after recrystallization, indicating scope for further improvement in ductility.

Multi-axial fatigue life assessment of additively manufactured nickel-based superalloys

In the present work, the microstructures and mechanical properties of additively manufactured IN718 alloy were studied and the dendritic columnar microstructures elongated along the building direction are related to macroscopic mechanical properties. Detailed experiments under both proportional and non-proportional multi-axial loading conditions indicate that the influence of the building orientation from manufacturing on mechanical property and fatigue performance is negligible. Fractography analysis reveals that fatigue crack initiates from carbide and oxygen phases. The shear energy-based models reasonably predict the fatigue life and shear failure is the dominant failure mode for the laser melting material.

Chemical Solution Deposition of Barium Titanate Thin Films with Ethylene Glycol as Solvent for Barium Acetate.

Chemical solution deposition (CSD) of BaTiO3 (BT) or BT-based thin films relies on using a carboxylic acid and alcohol as the solvents for alkaline-earth carboxylate and transition-metal alkoxide, respectively; however, the esterification reaction of the solvents may lead to in-situ water formation and precipitation. To avoid such an uncontrolled reaction, we developed a route in which ethylene glycol (EG) is used as the solvent for Ba-acetate. The EG-based BT coating solutions are stable for at least a few months. The thermal decomposition of the BT xerogel obtained by drying the EG-based solutions depends on the choice of the solvent for the Ti-alkoxide as well: in the case of EG and 2-methoxyethanol solvents carbon residues are removed at only about 1100 °C, while in the case of ethanol it is concluded at about 700 °C. About 100 nm thick BT films derived from the EG-ethanol solution deposited on platinized silicon reveal dense, crack-free columnar microstructure. They exhibit local ferro- and piezoelectric properties. The macroscopic polarization-electric field loops were obtained up to a quite high electric field of about 2.4 MV/cm. The EG-ethanol based CSD route is a viable alternative to the established acetic acid–alcohol route for BT and BT-based films.

Effect and mechanism of inter-layer ultrasonic impact strengthening on the anisotropy of low carbon steel components fabricated by wire and arc additive manufacturing

The problem of anisotropy in the microstructure and mechanical property of the fabrications limits the application and development of wire and arc additive manufacturing (WAAM). By introducing inter-layer ultrasonic impact (UI) strengthening in the low carbon steel WAAM process, the anisotropy can be effectively improved. After being treated by UI, the typical columnar microstructure with obvious direction is transformed into a uniform and fine equiaxed microstructure. Digital image correlation results show that the stress concentration in the tensile process is significantly improved. The anisotropy of tensile strength and yield strength is reduced from 4.2% to 1.6% and 10.1%–2.3%, respectively. Scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) are applied to investigate this phenomenon. The results indicate that UI can break the constraint for dislocations to move, promoting dislocation merging and annihilating. And then, numerous substructures are formed, partially recrystallized by the subsequently deposited layers' thermal effect. The transformation can block the columnar microstructure growth and divide the columnar microstructure into the cellular or equiaxed microstructure. In addition, the results show that inter-layer ultrasonic impact strengthening can result in local misorientation reduction, grain refinement (average grain size reduction from 2.18 μm to 1.65 μm), and texture orientation strength weakening.

Tribological behavior investigation of 316L stainless steel samples processed by selective laser melting

Selective Laser Melting (SLM) is a widely used process-based on melting powder material through a high laser beam. Process parameters significantly influence the microstructure, surface texture, and mechanical properties of materials in SLM. In this experimental investigation, we have used SLM core parameters, including scan speed, layer thickness, hatch spacing, and laser power, to observe their influence on the tribological behavior of 316L Stainless Steel (SS) samples. Samples were fabricated using SLM, and the wear test was performed according to the ASTM G-99 test standard on a dry sliding wear testing machine, keeping the process parameters (load, speed, wear track diameter, and time) constant for all samples. The Scanning Electron Microscopic (SEM) analysis showed columnar and cellular microstructure of the samples before the wear. In contrast, shallow plowing grooves, delamination marks, and cracks were observed in worn-out samples caused due to formation of abrasive and adhesive wear. Taguchi's statistical design of experiment was used to optimize the selected parameters for response variables like density and specific wear rate. From the Taguchi analysis the high density and minimum specific wear rate values were achieved at a scan speed of 900 mm/s, a laser power of 300 W, and a hatch spacing of 90 µm.The validation of obtained parameters has been done through Analysis of variance (ANOVA) to develop a linear correlation between the chosen parameters.

Effect of stress-relief heat treatments on the microstructure and mechanical response of additively manufactured IN625 thin-walled elements

Metallic structures fabricated with laser powder-bed fusion (LPBF) often have elemental segregations that are far from equilibrium and high levels of residual stresses due to the localized melting and rapid solidification that occur during printing. These unique characteristics result in mechanical property debits that make it difficult to employ such printed parts in critical applications. However, post-processing heat treatments can minimize the negative effects of residual stresses and reduce segregation. In this study, we measure the effect of two stress-relief heat treatments, 870°C for 1 h (industry-recommended) and 800°C for 1 h (potential alternative), on the microstructure and room temperature, monotonic quasi-static mechanical response of LPBF-processed thin-walled Inconel 625 T-elements. Electron backscatter diffraction analysis revealed an overall columnar microstructure with a strong <001> texture that is closely aligned with the build direction. The well-developed dislocation cell walls in both stress-relieved cases were found to be decorated with two types of blocky precipitates, namely, (Nb, N)-rich MX and (Nb, Mo, Si, N)-rich -M6X with scant evidence of the deleterious δ phase. The slightly larger size and higher volume fraction of precipitates in the 870°C case was found to have a small effect on the mechanical response of the T-elements under the current loading conditions and the difference in mechanical response of the two heat treatments was only significant at the very later stages of deformation. These findings make 800°C for 1 h a viable stress-relief alternative for our IN625 samples under the loading conditions studied here.

The Effect of Bond Coat Roughness on the CMAS Hot Corrosion Resistance of EB-PVD Thermal Barrier Coatings

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat’s microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion.

Development and passive exhumation of high-pressure shear zones (Blueschist Unit, Syros): Insights from quartz and columnar calcite microstructures

Quartz and columnar calcite microstructures in two major high-pressure (HP) shear zones (Syros Island, Greece), which carry meta-igneous rocks over calcite marbles, were studied to investigate the deformation conditions associated with their development and exhumation. Quartz-rich bands within the hanging-wall of the shear zones record the deformation during the operation of the shear zones under HP conditions at slightly decreasing temperatures (from 500 °C to 450 °C). At HP conditions, the shear zones are associated with (E)SE-directed shearing under plane strain conditions, as well as with a temporal increase in strain rate of one order of magnitude (from ∼10−12 to ∼10−11). Well-preserved columnar calcite within marbles of the footwall indicates that ductile shearing along the shear zones should have abruptly ceased during the early stages of exhumation, passing to a period of relative quiescence within the stability field of aragonite at temperatures ∼430–450 °C. Progressive exhumation and cooling below 400 °C are associated with weak and distributed (E)NE-directed shearing with strain rate slower than ∼10−14. Our results from north(east) Syros indicate a change in transport direction from (W)NW-(E)SE at the deep subduction levels to (W)SW-(E)NE during exhumation. This change is linked to a change in deformation, from localized to weak and distributed revealing that the Blueschist Unit in north(east) Syros was exhumed as a nearly rigid body.

Relationship between the mechanical properties and structure of a suspension plasma-sprayed thermal barrier coating with columnar microstructure

To investigate the mechanical properties of a suspension plasma-sprayed (SPS) thermal barrier coating (TBC) with a unique cauliflower-like columnar structure, shear and cantilever bending tests were conducted on its single submillimeter-sized columns. In the shear test, the fracture of the single columns occurred at the porous layer near the top-coating/bond-coating interface. Compared with conventional atmospheric plasma-sprayed (APS) TBCs having a lamellar microstructure, the cantilever bending test revealed a significantly low Young's modulus along the out-of-plane direction and strong anisotropy in the elastic modulus of the SPS TBC. The shear strength and out-of-plane Young's modulus of the single columns increased due to the sintering resulting from thermal aging; however, the increasing ratio of the shear strength was higher than that of the Young's modulus. Both experimental and finite element analysis results indicated that the mechanical properties of the SPS TBC, as well as their variation with thermal aging, are dominated by those of the porous layer within the single column undergoing sintering. Moreover, the experimental results suggested that SPS TBCs having a cauliflower-like columnar microstructure have superior durability to thermal cycles compared with APS TBCs.

Improvement of microstructure and tribological properties of titanium nitride films by optimization of substrate bias current

Severe wear is a key factor affecting the work character and service life of the cutting tools used for dry cutting. The deposition of wear-resistant films on the cutting tools via magnetron sputtering is one of the most effective strategies, and the further improvement of the properties of the deposited films has attracted much attention. Plasma-enhanced magnetron sputtering possesses a much higher ionization rate than conventional magnetron sputtering and has been used to deposit dense hard wear-resistant film, but the influence of substrate bias current (Is) on the structure and properties of the films should be further studied. In this paper, plasma-enhanced magnetron sputtering technique was utilized to obtain an individually adjustable Is and TiN film was taken as an example to study the effects of Is on the properties of the films. It is found that the microstructure of the films is transformed from a loose columnar microstructure into a dense and featureless microstructure with increasing Is from 0.1 to 3.0 A, and the preferred growth orientation along TiN(111) is found at a high level of Is. As Is increases from 0.1 to 1.5 A, the grain size of the films is abruptly decreased, and the mechanical properties are significantly improved; but when Is further increases, the mechanical properties stay nearly unchanged. When Is is increased, the wear rate of TiN-coated samples is distinctly decreased at first, and then gradually increased. When Is is 3.0 A, the highest hardness of 38.7 GPa and lowest wear rate of 9.4 × 10–16 m3/(N•m) are obtained for the TiN-coated sample.