Dendritic Grains Research Articles

Hot deformation behavior of Sc/Nb modified AA2195 Al–Li–Cu alloys

The hot workability of third generation AA2195 (Al–Li–Cu–Ag–Mg–Zr) alloy with different Sc (0.15–0.25 wt%) and Nb (0.15–0.25 wt%) additions in the homogenized condition was investigated using processing maps approach. The additions of Sc and Nb were found to be beneficial in refining the as-cast grain size of AA2195 and changing the morphology from less dendritic grain structure to a globular or equiaxed form. Hot compression tests carried out in the temperature, T, range of 300–450 °C and strain rate, ε˙, of 10−3 – 10 s−1 were utilized to generate processing maps at a true strain, ε = 0.5. The flow curves of the alloys exhibit a typical dynamic recovery (DRV) characteristics and Arrhenius type constitutive equations successfully predicted flow stress of the alloys. The results obtained from the processing maps and complementary microstructural analysis of the deformed specimens revealed that 0.15 wt% additions of Sc and Nb (separately or in combination) are optimum to improve its workability. However, increasing Sc and Nb contents to 0.25 wt% relatively expanded flow instability regions. With the aid of the processing maps, deformed specimens microstructural analysis, activation energy and Zener-Hollomon parameter, Domain D-3 occurring at higher temperatures and strain rates were identified as the “safe” regimes for hot working of AA2195 with optimum Sc and Nb contents (0.15 wt% added separately or in combination). Limited pancake type grain morphology with higher fractions of dynamically recrystallized (DRX) grains was observed in Domain D-3. The microstructural manifestation in the flow instability regions was flow localization (at higher strain rates), cavities formation at the grain boundaries & precipitates, particles breakage and wedge cracking at the triple junctions at lower strain rates.

Multi-scale simulation of microstructure evolution during direct laser deposition of Inconel718

As a free forming additive manufacturing technology for high performance metal parts, laser deposition manufacturing has a broad application prospect in aerospace and other fields. The solidification behavior and microstructure of molten pool determine the mechanical properties of the deposited parts. A multi-scale Cellular Automaton (CA)- Finite Element (FE) model is developed to simulate the microstructure evolution with different process parameters during the Direct Laser Deposition (DLD) process. Temperature data and solidification parameters under different process parameters are obtained by finite element method, which establish connections between macroscopic temperature field and microstructure simulation. In addition, the effects of different process parameters on dendrite morphology, element segregation and grain structure are investigated. The primary dendrite arm spacing (PDAS), Nb element precipitation and grain size all increase with the increase of laser power, which are caused by the decrease of cooling rate. Moreover, the trend of the simulated results of microstructure under various process parameters are consistent with the experimental results.

Microstructure and mechanical properties of twinning induced plasticity steel prepared by optical floating zone method

The three twinning induced plasticity (TWIP) steels were directionally solidified (DS) using the optical floating zone (OFZ) method at the withdrawal rates of 7 (DS7), 15 (DS15), and 30 μm s-1 (DS30) for the first time. The results showed that the relatively low withdrawal rate of 7 μm s-1 was unfavorable to the growth of DS, and the hybrid morphology was observed, which included equiaxed grains, columnar grains, amorphous coarse grains, and a few stray grains. With the increase of withdrawal rate, the volume fraction of columnar grain and dendrite grain increases, and the spacing decreased correspondingly, increasing microhardness and tensile strength. The samples exhibited outstanding plasticity, especially the DS15 had the highest elongation of 96%. This is attributed to the strong TWIP effect with a higher work hardening rate and the strongest ability of uniform plastic deformation at medium to large strains. Compared with the other two samples, the twin structure formed in the DS15 is more homogeneous and prevalent. The interaction between high-density dislocations and increased grain boundaries from the refined columnar grains and dendrites coupled with activated deformation twins, which contributes to the highest work hardening rate in the whole plastic deformation process of DS30.

Tuning the morphology of 2D transition metal chalcogenides via oxidizing conditions

Two-dimensional transition metal chalcogenides (TMCs) are emerging as an intriguing platform to realize nascent properties in condensed matter physics, materials science and device engineering. Controllable growing of TMCs becomes increasingly important, especially for the layer number, doping, and morphology. Here, we successfully tune the morphology of MoS2, MoSe2, WS2 and WSe2, from homogenous films to individual single crystalline grains only via changing the oxidizing growth conditions. The oxidization degrees are determined by the oxygen that adsorbed on substrates and the oxygen concentrations in reaction gas together. We find the homogenous films are easily formed under the reductive conditions, triangular grains prefer the weak oxidizing conditions, and medium oxidizing conditions bring in dendritic grains with higher oxygen doping and inhomogenous photoluminescence intensities from edge to interior regions shown in the dendritic grains. These growth rules under different oxidizing conditions are easily generalized to other TMCs, which also show potential for growing specific TMCs with designed oxygen doping levels.

Failure cause assessment of steam reformer radiant tube after long‐term service

AbstractThis study investigates the damage, microstructure, and mechanical evolution of service‐exposed HP40NbTi radiant tubes of a steam reformer furnace. Tube failure was reported after 6‐year of working at 525°C–830°C in the form of visible longitudinal and transverse cracks. The microstructure was evaluated using optical and scanning electron microscopy equipped with energy‐dispersive X‐ray spectrum analysis. The carbide phase transformation was determined by the method based on the matrix dissolution technique and X‐ray diffraction. The results of this study showed that the primary cause of the longitudinal crack formation is the local overheating and the oxidation of the dendritic grain boundaries. The M7C3 carbide transforms completely into the metastable M23C6 phase during high‐temperature service. Metallographic examinations revealed the shrinkage cavities and creep voids that are nucleated on both Cr‐ and Nb‐carbides. The continuous oxide layers of Cr2O3 and SiO2 formed near the inner and outer tube surfaces and the matrix's chemical composition near the outer tube surface was found to be depleted of Cr and C. Mechanical properties are irregular across the samples. A little margin of safety is noted, while the complete ductility dip is detected at room temperature. The root cause analysis of the failure under investigation showed that the tube failed due to the creep.

Multi-phase field model simulation based on MPI+OpenMP parallel: Evolution of seaweed and dendritic structure in directional solidification

A multi-phase model was established to imitate the growth of algal and dendritic grains during directional solidification. We studied the effects of temperature on the growth of bi-crystals and quantitatively analyzed the influence of anisotropic strength, thermal gradient, and pulling velocity on the evolution of bi-crystals. The results show that both weaker anisotropy strength and smaller pulling velocity can maintain the formation of seaweed tissue. The increase in the pulling velocity can degenerate the seaweed grains into dendrites and improve the growth rate of the dendrites, which make grain B produce more spindles, thereby accelerating the elimination of grain A. The thermal gradient is inversely proportional to the average initial spacing of dendrites. When the thermal gradient is too small, dendritic dendrites produce developed secondary dendrite arms, which, in turn, develop into tertiary dendrite arms to occupy the grain boundary, accelerating the elimination of seaweed grains. In addition, the multi-phase field model is solved by using central processing unit serial computation, single MPI (message passing interface) parallel programming method calculation, and MPI+OpenMP hybrid parallel programming structure, and the relevant factors affecting the efficiency of program operation are analyzed and tested. By comparing the computational efficiency of the three methods, it can be seen that the MPI+OpenMP hybrid parallel programming technology can make full use of computing resources in the case of large computing scale, further optimize the MPI parallel model, and obtain a higher acceleration ratio.

Closed-loop control of meltpool temperature in directed energy deposition

The objective of this work is to mitigate flaw formation in powder and laser-based directed energy deposition (DED) additive manufacturing process through close-loop control of the meltpool temperature. In this work, the meltpool temperature was controlled by modulating the laser power based on feedback signals from a coaxial two-wavelength imaging pyrometer. The utility of closed-loop control in DED is demonstrated in the context of practically inspired trapezoid-shaped stainlesssteel parts (SS 316L). We demonstrate that parts built under closed-loop control have reduced variation in porosity and uniform microstructure compared to parts built under open-loop conditions. For example, post-process characterization showed that closed-loop processed parts had a volume percent porosity ranging from 0.036% to 0.043%. In comparison, open-loop processed parts had a larger variation in volume percent porosity ranging from 0.032% to 0.068%. Further, parts built with closed-loop processing depicted consistent dendritic microstructure. By contrast, parts built with open-loop processing showed microstructure heterogeneity with the presence of both dendritic and planar grains, which in turn translated to large variation in microhardness.

Advances in Laser Additive Manufacturing of Cobalt-Chromium Alloy Multi-Layer Mesoscopic Analytical Modelling with Experimental Correlations: From Micro-Dendrite Grains to Bulk Objects.

This study presents two analytical models for the laser powder bed fusion (LPBF) process. To begin, the single layer’s dimensions were measured using principal operating conditions, including laser power, laser scanning speed, powder layer thickness, and hatch distance. The single-layer printing dimensions were transformed into multi-layer printing using the hatch distance. The thermal history of the printed layers was used as an input to the Johnson–Mehl–Avrami-Kolmogorov model to estimate the average dendrite grain size. LPBF experiments were conducted for a Cobalt–chromium (Co–Cr) alloy to validate the developed model. The average dendrite grain size was estimated using a scanning electron microscope (SEM) combined with “Image J” software. The Vickers hardness test was performed to correlate the average dendrite grain size and operating conditions. A 10–15% mean absolute deviation was presented between experiments and simulation results. In all samples, a Co-based γ-FCC structure was identified. An inverse correlation was established between the laser power and smaller average dendrite grain, while a direct relationship has been determined between laser scanning speed and average dendrite grain size. A similar trend was identified between hatch distance and average dendrite grain size. A direct link has been determined between the average dendrite grain size and hardness value. Furthermore, a direct relationship has connected the laser volume energy density and hardness value. This study will help experimentalists to design operating conditions based on the required grain size and corresponding mechanical characteristics.

NbB refining capability: Effects of slow and rapid solidification on dendritic spacings and grain sizes of a 6201 alloy

NbB inoculants are refiners that might be of interest to the Al industry. Grain size (GS) and secondary dendrite arm spacing (SDAS) are two important microstructural variables to consider when assessing a refiner's performance. Both of these attributes may have an impact on the alloy’s properties. The interplay of grain refiner and cooling rate on grain size, SDAS, and grain shape in the 6201 alloy is investigated in this article. It is demonstrated that GS associated with rapid solidification of the 6201 + NbB alloy was roughly 7 times smaller than that associated with slow solidification. Furthermore, regardless of the processing method (either slow or fast solidification), α-Al cells are generated for GS/SDAS ratio below and equal to 6.

Anisotropic cellular structure and texture microstructure of 316L stainless steel fabricated by selective laser melting via rotation scanning strategy

In this work, the effects of solidification and remelting under chessboard 67° rotation scanning strategy on the microstructure and properties of selective laser melted (SLMed) 316L stainless steel (316L SS) were studied, including dendrite microstructure, element distribution, grain orientation, dislocation structure, microhardness and corrosion resistance. We put forward a complete foundation relationship among texture, cellular structure and mechanical property influenced by anisotropy. Main results are summarized as follows: The 〈101〉 texture formation is the result of dendrite epitaxial growth. The cellular dislocation structure highly overlapped with dendrite significantly changed in remelting zone. Hence the high ratio of 〈101〉 texture signifies the anisotropy distribution of epitaxially growth dendrite which released the internal stress. The dislocation structure has a contribution to strength similar work hardening strengthening. Meanwhile, the 316L SS obtains Orowan strengthening via the nano oxide pinning effect in dislocation wall. Thus, the dislocation density and oxide distance obvious changed in different directions of cellular dislocation structure, resulting in the cellular dendrite regions performed higher microhardness than the columnar dendrite regions. The drastic change of dendrite increased the crack sensitivity of SLMed 316L SS, and the junction of different epitaxially growth direction dendrites formed high energy grain boundary.

Microstructure and yield phenomenon of an extruded Mg-Y-Cu alloy with LPSO phase

A yield phenomenon was firstly reported in an extruded Mg-6.8Y-2.5Cu alloy and the corresponding microstructure was also investigated in this work. The cast alloy is mainly composed of α-Mg, 18R long period stacking order (LPSO) phase, eutectic phase (Mg20Cu4Y1), and Mg2Cu phase. The 18R LPSO phase at the dendritic grain boundary transforms into the 14H LPSO phase in the grain interior during homogenization. After extrusion, the grain size of the homogenized alloy is remarkably refined to –3.69 μm and the second phase is significantly broken and distributed in the extrusion direction. Tensile testing curves of the extrude alloy at room temperature indicate that the yield strength and ultimate tensile strength increase while the elongation of the alloy decreases with increasing strain rate. Interestingly, a yield plateau forms and gradually decreases with increasing strain rate. The yield phenomenon is related to the dislocation multiplication and the interaction between the movable dislocations and solute atoms.

Evaluating grain size, dendritic scale, and tensile properties of a NbB-inoculated 6201 alloy using solidification rate

Previous studies investigated the ability of Nb–B inoculants to refine grain in Al alloys; however, their dendritic network was not considered. In the present study, the synergistic effect of 0.5Nb-0.1B wt.% on the dendritic microstructure scale and grain size (GS) of a 6201-base alloy with an extensive range of solidification cooling rates is investigated. It is demonstrated that 0.5Nb-0.1B wt.% refined GS for either columnar or equiaxed morphology independent of the cooling rate, with a refining capability of 13-fold reduction in GS. The secondary dendritic spacing (λ2) also decreased, specifically for low cooling rates, as compared to the non-modified alloy. It is proposed that the GS to λ2 ratio may characterize α-Al cell micromorphology in the 6201 + NbB alloy, which occurs when this ratio is lower than 6. High-cooling-rate cellular arrays were observed for cooling rates higher than 5.5 °C/s. Although a low GS and λ2 result in an increased strength of the 6201 + NbB alloy, a limit for optimizing this property via grain refining appears to be reached at approximately 190 MPa for the GS ranging from 142 to 239 μm and the λ2 ranging from 22.6 to 24.5 μm.

PBF-LB/M multiphysics process simulation from powder to mechanical properties

The laser powder bed fusion process is modelled holistically. The Discrete Element Method is used for powder spreading simulations yielding realistic powder beds. Smoothed Particle Hydrodynamics simulations are then used to study the thermo-viscous flow in the melt pool considering laser energy absorption, radiation, heat transfer, melting and re-solidification, surface tension, Marangoni currents and vaporization pressure. Material properties are obtained from thermodynamic CALPHAD simulations. The temperature field of the melt pool is coupled to a Cellular Automaton, which calculates the growth of dendritic grains and, thus, provides a prediction for the microstructure formed during solidification. This microstructure serves then as input for Crystal Plasticity Finite Element Analyses to qualitatively describe texture dependent mechanical properties.

Assessing Mottled Indications when Digitally Radiographing Austenitic Stainless Steel Welds

When radiographing an austenitic stainless steel weld with an appreciable weld deposit size, selecting a low radiographic kilovoltage (keV) can contribute to producing a radiographic indication that is not an imperfection. The contributors to this mottled condition are both radiographical and metallurgical. Electrons from low keV can diffract or absorb when penetrating through the dendritic grain structure of a weld. The increase in keV, or using gamma ray–equivalent isotopes, produces a marked change in electron output and penetration in material.

Особенности формирования микроструктуры и механических свойств алюминиевой бронзы при различном тепловложении во время электронно-лучевой аддитивной печати

Microstructure and mechanical characteristics of aluminum bronze fabricated using electron beam additive manufacturing (EBAM) under adjusting the heat input as a function of the sample’s height have been investigated. It was shown that such an adjustment of the heat input had effect on the dendrite grain size and morphology as well as mechanical characteristics. Columnar high aspect ratio grains were formed during deposition at maximum heat input values. The columnar grain width in section parallel to the deposition direction was increasing in EBAM conducted at lower heat input. Tensile static testing showed that the tensile strength values were improving from 435 to 483 MPa in the process. Some moderate degree of anisotropy has been revealed between samples oriented by their tensile axes parallel and perpendicular to the deposition direction. Samples with tensile axes parallel to the deposition direction showed higher tensile strength and less plasticity as compared to those of samples with perpendicular orientations of their tensile axes. In addition, the heat input adjustment had no effect on the mean width of columnar grains measured in a section perpendicular to the deposition direction.

Assessing Mottled Indications When Digitally Radiographing Austenitic Stainless Steel Welds

When radiographing an austenitic stainless steel weld with an appreciable weld deposit size, selecting a low radiographic kilovoltage (keV) can contribute to producing a radiographic indication that is not an imperfection. The contributors to this mottled condition are both radiographical and metallurgical. Electrons from low keV can diffract or absorb when penetrating through the dendritic grain structure of a weld. The increase in keV, or using gamma ray–equivalent isotopes, produces a marked change in electron output and penetration in material.

The Influence of Nbb Inoculation on Dendritic Spacing and Grain Size of an Aluminum 2017 Alloy at Different Cooling Rates

The Influence of Nbb Inoculation on Dendritic Spacing and Grain Size of an Aluminum 2017 Alloy at Different Cooling Rates

Effect of Zr on the as-cast microstructure and mechanical properties of lightweight Ti2VNbMoZrx refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) have attracted extensive attention due to their outstanding high-temperature performance. However, high density and poor ductility are the two bottleneck problems in industrial application. This study aims to exploit novel RHEAs with high specific strength and good ductility by adding light Group IVB elements. The effect of Zr on the as-cast microstructure, density, and mechanical properties of Ti2VNbMoZrx (x = 1, 1.5, 2, and 3) RHEAs was investigated in detail. All Ti2VNbMoZrx RHEAs exhibited a disordered body-centered-cubic solid-solution phase. The as-cast microstructures changed from dendritic morphology (x = 1 and 1.5) to equiaxed dendritic morphology grain (x = 2 and 3) with the increasing Zr content. The Ti2VNbMoZr2 RHEA displayed optimal mechanical properties with a compressive yield strength of 1421 MPa, a plastic strain of 31.6%, and a specific yield strength of 211 kPa · m3 · kg−1, superior to the most-reported RHEAs. The microbands along {110} and {112} slip planes, which was a typical poly slip configuration, were observed in the Ti2VNbMoZr2 alloy after compression test using ex-situ transmission electron microscopy. Furthermore, the Ti2VNbMoZr2 RHEA exhibited good high-temperature mechanical properties, showing a yield strength of 977 MPa at 600 °C and 386 MPa at 800 °C without fracture at a plastic strain of 50%. The solution strengthening model revealed that the high yield strength of Ti2VNbMoZrx RHEAs was mainly derived from the atomic radius misfit and shear modulus misfit of the Zr atoms.

In-situ precipitation of metastable phases and microstructure control for additively manufactured Al alloy

The Al-Cu6.3-Mg1.3 alloy with high Cu content was designed and additively manufactured by using double wires and arcs. Key findings demonstrated the micro hardness, YS, and UTS achieve 115.6 HV, 206 MPa, and 307 MPa, respectively. Those values are notably increased compared with that of the WAAM Al alloys with traditional compositions. Dendritic grains dominate the microstructure of the designed alloy with Cu and Mg contained eutectic structures. The in-situ precipitated and homogeneously distributed nano-scale needle-shape metastable S′ phases, which are triggered by rapid solidification and intrinsic heat treatment during WAAM, contribute to the mechanical properties improvement of directly deposited alloy. This study provides a promising guidance for the development of additively manufactured components with excellent performance.

Effects of size and cooling rate on solidification behavior of freestanding Sn-3.0Ag-0.5Cu solder balls

The diversity of electronic product types needs variable solder joint sizes and processing conditions, which can lead to the complexity and diversity of solidified microstructure in Sn-based solder joints. Due to the significant anisotropy of β-Sn in physical and mechanical properties, the solidified microstructure of the Sn-based solders after soldering should be carefully controlled. In this paper, Sn-3.0Ag-0.5Cu freestanding solder balls were used to understand the effects of size and cooling rate on the solidification behavior and microstructure. Larger undercooling, ∆T , was detected for smaller solder balls or under larger cooling rate. With the solder ball size increased from small level of 100 and 200 μm to medium level of 400 μm and then to large level of 700 μm and 1200 μm, the dominant solidified microstructure transformed from interlaced grain & beach ball to randomness and then to single grain. Correspondingly, three nucleation & growth models, i.e., single grain ( ∆T ≤ 20 K), cyclic twin (20 K ≤ ∆T ≤ 25 K) and interlaced grain & beach ball (25 K ≤ ∆T ), were proposed to explain the solidification behavior of β-Sn. A mapping of the most likely formed β-Sn microstructure regarding solder ball size and cooling rate was established to not only predict the major microstructure of solder balls but also optimize the process parameters for solder ball fabrication. • Effects of size and cooling rate on undercooling, secondary dendrite arms and grain features of β-Sn were studied systematically. • A mapping of the most likely formed β-Sn microstructure regarding different process parameters was established. • Three nucleation & growth models were proposed to explain the solidification behavior and microstructure of freestanding SAC solder balls.