Orientation Of Columnar Grains Research Articles

Synergetic regulation of hierarchical microstructure to improve the 704 ℃ tensile property in laser directed energy deposition IN718Plus superalloy

IN718Plus superalloy with hierarchical microstructure has been successfully manufactured by laser directed energy deposition (LDED). Along building direction, microstructure varies from composing of epitaxial columnar grains oriented along <001>γ direction to equiaxed grains in flower like configuration. Striped Laves phase and fine γ'/γ'' precipitates form in the columnar grains, and would transform into fine Laves phase particles and nanoscale γ' precipitates at equiaxed layer. To improve the compositional homogeneity and microstructural stability, high temperature solution treatment has been explored on the superalloy. The treatment would not affect the morphology and orientation of columnar grains, but would redissolve Laves phase precipitates. Double aging treatment has also been performed to obtain homogeneous distributed γ' precipitates, and the synergistic regulation of microstructure enhances the mechanical property significantly, as causing the room temperature yield strength and tensile strength increased from 0.88 GPa and 1.14 – 1.04 GPa and 1.28 GPa, respectively. Moreover, the excellent mechanical property would be maintained upon 704 ℃, with yield strength of 0.93 GPa, tensile strength of 1.04 GPa and high elongation of 27.3%.

Compensating the anisotropic mechanical properties of electron beam melting-based Gyroid scaffolds using structural design

In recent years, scaffolds based on triply periodic minimal surface (TPMS) have been identified as one of the candidates for bone repair materials because of their excellent mechanical and biological properties. In addition, additive manufacturing (AM) enables the production of porous scaffolds with complex microstructures. However, some AM methods (e.g., electron beam melting, EBM) tend to produce a significant anisotropic discrepancy in the mechanical response of scaffolds, and neither the underlying mechanisms nor the compensation method for this discrepancy is clear so far. The aim of this study was to compensate for this discrepancy using structural design and to reveal the underlying mechanisms of each impact factor. Four types of Gyroid scaffolds ranging from rotational symmetric to obvious anisotropic were produced along the height direction of the scaffold using EBM technique, and the relationship between the degree of structural anisotropy (DA) and compressive modulus was determined. When the value of DA was 1.08, the compressive modulus in the building and non-building directions were equal, which means that the anisotropic mechanical discrepancy generated by EBM process was compensated. On the other hand, the decoupling of factors affecting mechanical properties showed that the effective compressive modulus of the rotational symmetric scaffold in the building direction was 42.6% lower than that of non-building direction in the finite element model with highly reductive geometry, while the discrepancy in experimental method was only 20%. The results suggest that the geometric discrepancy is the main factor of the mechanical discrepancy, and the orientation of columnar grains is inclined to the building direction which helps to alleviate the mechanical anisotropy caused by geometry discrepancy. The findings of this study provide important information for the application of TPMS scaffolds in bone implants.

Critical role of subgrain orientation on the stability of mechanical properties of selective laser melting manufactured alloys

In this work, it was found that the orientation of columnar grains is determined by the growth direction of subgrains within them. The difference in deformation between <001> orientation grains and other orientation grains is an important factor that affects the stability of mechanical properties of SLM manufactured alloys.

Molecular Dynamics Simulations of the Tensile Mechanical Responses of Selective Laser-Melted Aluminum with Different Crystalline Forms

The mechanical deformation of cellular structures in the selective laser melting (SLM) of aluminum was investigated by performing a series of molecular dynamics (MD) simulations of uniaxial tension tests. The effects of crystalline form, temperature, and grain orientation of columnar grains on the mechanical properties of SLM aluminum were examined. The MD results showed that the tensile strength of SLM aluminum with columnar grains at different temperatures was lower than that of single-crystal aluminum, but greater than that of aluminum with equiaxed grains. The tensile strength and Young’s modulus both decreased approximately linearly upon increasing the temperature. The deformation mechanisms of equiaxed and columnar grains included dislocation slip, grain boundary migration, and torsion, while the deformation mechanisms of single crystals included stacking fault formation and amorphization. Finally, the influence of the columnar grain orientation on the mechanical properties was studied, and it was found that the Young’s modulus was almost independent of the grain orientation. The tensile strength was greatly affected by the columnar grain orientation. Reasonable control of the grain orientation can improve the tensile strength of SLM aluminum.

Spatial Variation of Thermokinetics and Associated Microstructural Evolution in Laser Surface Engineered IN718: Precursor to Additive Manufacturing

The spatial variation of thermokinetic parameters has a significant influence on solidification and microstructural aspects such as grain orientation, types and dimensions of the microstructural features, and crystallographic defects. In laser-based additive manufacturing, these factors are mainly dependent on the process parameters and have a wide implication on the microstructural aspects and, in turn, on the mechanical properties. In view of this, the current study focuses on the spatial variation on thermokinetic parameters such as cooling rate, thermal gradient (G), and solidification velocity (R) within the melt pool formed during laser processing of IN718. The continuous-wave Nd-YAG laser was employed at a laser fluence of 14.85, 19.10, and $$23.34\text { J/mm}^2$$ with varying power (700, 900, 1100 W) at a constant scanning speed of 100 mm/s. The finite element method-based multiphysics heat transfer model, coupled with the dynamic fluid flow, was developed to predict these parameters. The model was correlated with microstructural aspects such as melt pool dimensions, orientation of columnar grains, and secondary dendritic arm spacing. The cumulative diffusion length of Nb obtained via thermo-diffusion calculation during multiple heating/cooling cycles was enough to dissolve the fine intragranular plate-shaped $$\delta $$ precipitates in the heat-affected zone.The spatial variation of the G/R ratio recognized the transition of columnar to equiaxed solidification grains which was associated with the G/R ratio lower than $$10 \text {K s/mm}^2$$ in the top region ( $$\sim 25 \mu \text {m}$$ ) of the melt pool. In addition, the coupled solid mechanics model predicted the evolution of thermal stresses during solidification of the melt pool under a high thermal gradient, which marked the generation of high dislocation density in the solidified melt pool.

Loading Direction-Dependent Mechanical Properties of Columnar Polycrystal: A Molecular Dynamics Study

In this work, mechanical properties and plastic deformation behaviors of columnar polycrystal are examined by compression tests using molecular dynamics simulation. The results show that anisotropic mechanical properties occur to the fcc FeCrNi samples with different inclined angles between the orientation of columnar grains and loading direction. The yield stress and flow stress of the α = 0° sample are the highest, while the values of α = 45° sample are the lowest among the tested samples. The reasons for these differences derive from the geometric constraint of grain boundaries (GBs) and the available slip systems of different grains. The GBs serve as obstacles where extended dislocations are incorporated rather than dislocation pile-ups at GBs. For the α = 0° sample, the GBs bending toward the radius direction coupled with less partial dislocation motion and twinning compensates the compression strain. The strain-assisted GBs migration along loading direction extensively takes place in α = 90° sample, whereas the numerous partial dislocation activity and GBs gliding occur in samples with the angle close to 45°. This results from the higher Schmid factors of slip systems of columnar grains within these samples and long mean free sliding path for GBs gliding. Besides, the influence of radius size is probed, exhibiting a constant Young’s modulus, but slightly different plastic deformation characteristics.

Experimental and Numerical Modeling for Flattening and Rapid Solidification with Crystallization Behavior of Supersonic Ceramic Droplets

Successive impingement of droplets after refining in supersonic plasma jet generally yields a submicron-sized lamellar coating with excellent comprehensive properties. Nevertheless, physical insight into the flattening and rapid solidification with crystallization behavior of supersonic impingement of refined droplets is difficult to understand. In this research, the content of refinement droplets reached 90% and displayed the multi-scale distribution of equiaxed grains. The boundary migration of equiaxed grains and anisotropic coalescence was found in the dynamic temperature gradient. Furthermore, an optimized model was established in order to accurately reproduce the multi-physical coupling process of supersonic impingement of single or two refined droplets, which was based on the numerical calculation of nonlinear equations (including the Mass and momentum, energy balance, Cahn–Hilliard, phase-field and orientational field equations). The size distribution and growth orientation of columnar grains within single or two flattened droplets were in good agreement with the experimental results. Epitaxial growth of columnar grains was found in the two-flattened droplet interface during the extremely rapid cooling stage. This optimized model could be an effective method in predicting the flattening and solidification with crystallization behavior of droplets during plasma spraying.

The influence of the microtexture and orientation of columnar grains on the fatigue crack growth of directed energy deposited Ti-6.5Al-2Zr-Mo-V alloys

This work aims to investigate the influence of the orientation and microtexture of columnar grains on the fatigue crack growth of a Ti-6.5Al-2Zr-Mo-V titanium alloy fabricated by directed energy deposition. In this paper, the fatigue crack growth rate test in three sampling directions in a directed energy deposited Ti-6.5Al-2Zr-Mo-V titanium alloy using compact specimens was carried out. The crack length was measured visually, and the fatigue crack growth rate of the stable crack growth stage was obtained. During the test, the influence of the microstructure on the crack growth was directly observed. In addition, the complete crack front shape was indicated on the fracture surface by the marker load technique, and the crack growth behavior was obtained. An optical microscopy, a scanning electron microscopy and a laser confocal microscopy were used to observe and clarify the influence of the columnar grain boundary on the crack growth behavior and the interaction between the crack front and microstructure. The results show that the fatigue crack growth rate in the three sampling directions is different in the low ΔK region; the columnar grain boundary has no significant effect on the fatigue crack growth behavior, but the columnar grain itself has an effect on the fatigue crack growth behavior, which is indicated by the irregularity of the crack front shape in different columnar grains. Microhardness testing and electron backscattered diffraction were used to explain the above phenomena based on static and orientation characteristics. It was found that the microtexture and orientation of the columnar grains are responsible for differences in the crack growth rates, and the orientation of the columnar grains also determines the extent of the difference.

Microstructure evolution and mechanical properties of laser metal deposition of Invar 36 alloy

Laser metal deposition (LMD) technology was used to fabricate single-track-multi-layers samples using Invar 36 alloy powder by two different deposition patterns: one-way laser scanning pattern and back and forth laser scanning pattern. The microstructure characteristics, grain morphology, formation mechanism and mechanical properties of the samples fabricated in two different deposition patterns were systematically investigated. It was shown that there was a thin layer of equiaxed grains in the top zone, an alternative arrayed grain morphology of columnar and cellular grains in the middle and periodic circular growth of small columnar grains at the bottom, which was generated by different temperature gradient in different location. The crystal orientation of columnar grains was distinct due to different deposition patterns. X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) showed that the deposited specimen was composed of austenite-single phase. Samples built by back and forth laser scanning pattern showed higher microhardness and tensile strength but lower elongation compared with samples built by one-way pattern, the horizontally built sample showed higher Ultimate tensile strength and yield strength but lower elongation compared with vertically built sample in the same laser scanning pattern. Laser metal deposited Invar 36 showed more excellent mechanical properties compared to the forged and casted counterparts.

Mechanism of Competitive Grain Growth in 8YSZ Splats Deposited by Plasma Spraying

The competitive growth mechanism of columnar grains generated from rapid solidification in plasma-sprayed yttria-stabilized zirconia (YSZ) splats collected on stainless steel at different temperatures was studied. The microstructure of YSZ splats was examined quantitatively by scanning electron microscope and x-ray diffraction. The crystalline orientation of columnar grains was investigated by electron backscattered diffraction. It was found that the competition between the columnar grains was governed by their relative growing rate. The columnar grains with growing face (001) had the slowest growing rate, while those with growing face {011} had the fastest growing rate. The degree of competition between the columnar grains was related to the nucleation rate for the splat at different substrate temperatures (T s). When the splat was at T s = 300 K or T s = 373 K, the relatively low nucleation rate resulted in less competition between the columnar grains; thus, no preferential orientation was observed. In contrast, for the splat at T s = 473 K, the higher nucleation rate resulted in more competition between the columnar grains, increasing the proportion of grains with the faster growing face {011}, and therefore, the columnar grains show 〈011〉 preferential orientation.

Numerical simulation of microstructure evolution during directional solidification process in directional solidified (DS) turbine blades

Directional solidified (DS) turbine blades are widely used in advanced gas turbine engine. The size and orientation of columnar grains have great influence on the high temperature property and performance of the turbine blade. Numerical simulation of the directional solidification process is an effective way to investigate the grain’s growth and morphology, and hence to optimize the process. In this paper, a mathematical model was presented to study the directional solidified microstructures at different withdrawal rates. Ray-tracing method was applied to calculate the temperature variation of the blade. By using a Modified Cellular Automation (MCA) method and a simple linear interpolation method, the mushy zone and the microstructure evolution were studied in detail. Experimental validations were carried out at different withdrawal rates. The calculated cooling curves and microstructure agreed well with those experimental. It is indicated that the withdrawal rate affects the temperature distribution and growth rate of the grain directly, which determines the final size and morphology of the columnar grain. A moderate withdrawal rate can lead to high quality DS turbine blades for industrial application.

Columnar structure of reactively sputtered aluminium nitride films

Columnar structure of thin aluminium nitride (AlN) films is examined by x-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The films were deposited on SiO2/Si(100) substrate using radiofrequency reactive sputtering method. Strong [0001] preferred orientation is observed by XRD and confirmed by selected area diffraction pattern of TEM. Columnar grains of ∼50–100 nm inclined at an angle of ∼10° to the substrate normal are observed by SEM. As revealed by TEM, each columnar grain is composed of nano-grains of the order of 10 nm and no faceting is observed in the nano-grains and columns. The [0001] preferred orientation results as columnar grains are oriented at various azimuthal angles with their c-axes perpendicular to the substrate surface. A slight tilt of a few tenths of a degree between adjacent nano-grains within a column is also observed. The random azimuthal orientation of columnar grains and small tilt between nano-grains in the films are accommodated by the amorphous phase present in the grain boundaries.

Effect of initial orientation on the cold-rolling behavior of solidified columnar crystals in an Fe-36%Ni austenitic alloy

The cold rolling behavior of a solidified columnar-crystal specimen with the 001〈 uv0〉 initial texture in an Fe-36%Ni austenitic (f.c.c.) alloy has been investigated. The rolled microstructure and the crystal rotation strongly depended on the initial orientation of columnar grains. 70% rolled microstructure of the initially (001)[100] oriented grains was composed of three kinds of deformed microstructures: the smoothly-etched matrix with the (001)[100] orientation, the wide deformation bands roughly parallel to the transverse direction (TD) with the (101)[101¯] orientation and the narrow deformation bands parallel to the rolling direction (RD). The shear bands inclined by 30 deg from the rolling plane formed within the initially (001)[110] oriented grains, where the matrix regions rotated to the 113〈332〉 orientation. Each shear band was composed of many elongated cells whose width was less than 0.2 μm. The formation of each deformed microstructure was discussed in connection with the active slip systems. † Present address: Department of Materials Science and Engineering, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka, 565 Japan. ‡ Present address: Sumitomo Metal Industries Ltd, Hazaki-cho, Kashima-gun, Ibaragi, 314 Japan. § Present address: Nissan Motor Co. Ltd, Oppama, Yokosuka, Kanagawa, 273 Japan.

Synthesis and properties of superconducting Bi2Sr2CaCu2Oy films prepared by chemical diffusion

Films of Bi2Sr2CaCu2Oy have been prepared by diffusing Bi2CaCu2O5 into Sr2CaCu2O5. Examination by x-ray diffraction and scanning electron microscopy showed a highly preferred orientation of columnar grains with the [100] axis normal to the surface. The maximum oriented texture was for a film thickness of 76 μm for thicknesses between 51 and 150 μm. The preferred orientation existed up to the surface of the sample, and there was no evidence of an unreacted region at the surface when the diffusion was done for at least 3 days. The films had the usual superconducting transition of the 2212 phase.

Grain structure and solidification cracking in oscillated arc welds of 5052 aluminum alloy

The effect of arc oscillation on grain structure and solidification cracking in GTA welds of 5052 aluminum alloy was investigated using a four-pole magnetic arc oscillator and a modified fish-bone crack test. Two different mechanisms of crack reduction were identified: one in the low frequency range of arc oscillation and the other in the high frequency range. The former was the alteration of the orientation of columnar grains, while the latter was grain refining. Neither mechanism was operative in the intermediate frequency range and solidification cracking was severe, especially when the amplitude of arc oscillation was small. Alteration of grain orientation was obtained in welds made with transverse and circular arc oscillations, but not longitudinal arc oscillation. Grain refining, on the other hand, was achieved in welds made with all three types of arc oscillation patterns. The differences between the response of alloy 5052 to arc oscillation and that of alloy 2014 observed previously were discussed.

Crystallographic Investigations of Columnar Grains in Iron Film Evaporated at Oblique Incidence

It has been found from X-ray analysis and electron microscopic observation that two types of crystallites with different crystallographic orientations, [100]-(100) and [111]-(1\bar10), constitute the columnar grain structure in iron film evaporated at an incidence angle of 45° . In these orientations, (010) and (1\bar10) planes are parallel to the incidence plane, and [100] lies along the direction of the vapor beam in the former and [111] tilts from the film normal toward the vapor beam by 10° in the latter orientations. It has also been verified that, irrespective of orientation, the axis of the column tilts from the film normal toward the vapor beam direction by about 35° and the top of the column is surrounded by {110} planes. The magnetic anisotropy concerned with crystallographic orientation of columnar grains is briefly discussed.