The Experts below are selected from a list of 1020 Experts worldwide ranked by ideXlab platform
P D Lee - One of the best experts on this subject based on the ideXlab platform.
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4d synchrotron x ray tomographic quantification of the transition from cellular to Dendrite growth during directional solidification
Acta Materialia, 2016Co-Authors: Biao Cai, Jianpeng Wang, A Kao, K Pericleous, A B Phillion, R C Atwood, P D LeeAbstract:Solidification morphology directly impacts the mechanical properties of materials; hence many models of the morphological evolution of dendritic structures have been formulated. However, there is a paucity of validation data for directional solidification models, especially the direct observations of metallic alloys, both for cellular and dendritic structures. In this study, we performed 4D synchrotron X-ray tomographic imaging (three spatial directions plus time), to study the transition from cellular to a Columnar dendritic morphology and the subsequent growth of Columnar Dendrite in a temperature gradient stage. The cellular morphology was found to be highly complex, with frequent lateral bridging. Protrusions growing out of the cellular front with the onset of morphological instabilities were captured, together with the subsequent development of these protrusions into established Dendrites. Other mechanisms affecting the solidification microstructure, including Dendrite fragmentation/pinch-off were also captured and the quantitative results were compared to proposed mechanisms. The results demonstrate that 4D imaging can provide new data to both inform and validate solidification models.
Pulin Nie - One of the best experts on this subject based on the ideXlab platform.
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relationships among charpy impact toughness microstructure and fracture behavior in 10crni3mov steel weld joint
Materials Letters, 2020Co-Authors: Shiwei Zhang, Yadong Wang, Minhao Zhu, Zhijie Zhang, Pulin NieAbstract:Abstract Charpy impact toughness of 10CrNi3MoV steel weld joint was investigated, and microstructure and fracture behavior were considered as reasons for differences in Charpy impact toughness. Charpy impact toughness of fusion zone (FZ) was lower than that of heat affected zone (HAZ) due to combinations of microstructure and fracture behavior. Acicular ferrite (AF) and grain boundary ferrite (GBF) with granular bainite (GB) in Columnar Dendrite pattern of FZ and a mixed mode of quasi cleavage fracture and ductile fracture was responsible for the low toughness; while tempered martensite with some bainite in fine dispersed equiaxed Dendrite pattern of HAZ was contributed to absorb crack energy and improve the toughness. Tortuous crack propagation path of zigzag pattern confirmed the crack deviated at large angle grain boundary and depressed crack growth.
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numerical modeling of microstructure evolution during laser additive manufacturing of a nickel based superalloy
Acta Materialia, 2014Co-Authors: Pulin Nie, O A OjoAbstract:A multi-scale model that combines the finite element method and stochastic analysis is developed to simulate the evolution of the microstructure of an Nb-bearing nickel-based superalloy during laser additive manufacturing solidification. Through the use of this model, the nucleation and growth of Dendrites, the segregation of niobium (Nb) and the formation of Laves phase particles during the solidification are investigated to provide the relationship between the solidification conditions and the resultant microstructure, especially in the morphology of Laves phase particles. The study shows that small equiaxed Dendrite arm spacing under a high cooling rate and low temperature gradient to growth rate (G/R) ratio is beneficial for forming discrete Laves phase particles. In contrast, large Columnar Dendrite arm spacing under a low cooling rate and high G/R ratio tends to produce continuously distributed coarse Laves phase particles, which are known to be detrimental to mechanical properties. In addition, the improvement of hot cracking resistance by controlling the morphology of Laves phase particles is discussed by analyzing the cracking pattern and microstructure in the laser deposited material. This work provides valuable understanding of solidification microstructure development in Nb-bearing nickel-based superalloys, like IN 718, during laser additive manufacturing and constitutes a fundamental basis for controlling the microstructure to minimize the formation of deleterious Laves phase particles.
Biao Cai - One of the best experts on this subject based on the ideXlab platform.
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4d synchrotron x ray tomographic quantification of the transition from cellular to Dendrite growth during directional solidification
Acta Materialia, 2016Co-Authors: Biao Cai, Jianpeng Wang, A Kao, K Pericleous, A B Phillion, R C Atwood, P D LeeAbstract:Solidification morphology directly impacts the mechanical properties of materials; hence many models of the morphological evolution of dendritic structures have been formulated. However, there is a paucity of validation data for directional solidification models, especially the direct observations of metallic alloys, both for cellular and dendritic structures. In this study, we performed 4D synchrotron X-ray tomographic imaging (three spatial directions plus time), to study the transition from cellular to a Columnar dendritic morphology and the subsequent growth of Columnar Dendrite in a temperature gradient stage. The cellular morphology was found to be highly complex, with frequent lateral bridging. Protrusions growing out of the cellular front with the onset of morphological instabilities were captured, together with the subsequent development of these protrusions into established Dendrites. Other mechanisms affecting the solidification microstructure, including Dendrite fragmentation/pinch-off were also captured and the quantitative results were compared to proposed mechanisms. The results demonstrate that 4D imaging can provide new data to both inform and validate solidification models.
Dongdong Gu - One of the best experts on this subject based on the ideXlab platform.
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relation of microstructure microhardness and underlying thermodynamics in molten pools of laser melting deposition processed tic inconel 625 composites
Journal of Alloys and Compounds, 2017Co-Authors: Dongdong GuAbstract:Abstract Laser melting deposition (LMD) was applied to deposit nano-TiC particles reinforced Inconel 625 composite parts. The mechanisms of microstructure evolution and microhardness distinction in the different zones of the individual molten pool which was produced in LMD-processed composites were investigated. The layer-wise microstructural features of the manufactured parts were generally observed with clear outline curves of the molten pool as a result of the layer-by-layer deposition manner of the LMD shaping process. It could be observed that the microstructures in the upper part of the molten pool were mainly cellular structures, whereas which in the bottom and edge region were predominantly Columnar Dendrites. The increasing ratio of the temperature gradient to the solidification velocity (G/R), which resulted in a gradual change from Columnar Dendrite growth to cellular grain growth in the solidification regime, accounted for this phenomenon. The different sizes of cellular grains and Dendrite spacing were ascribed to the varied cooling rates of diverse regions in the molten pool as well as the heat affecting nearby the overlapping zone. The factors contributing to microhardness variety could be summed up in three aspects, which were sizes of grains, TiC reinforcing particles and solid solution strengthening.
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selective laser melting additive manufacturing of tic inconel 718 bulk form nanocomposites densification microstructure and performance
Journal of Materials Research, 2014Co-Authors: Dongdong GuAbstract:Selective laser melting (SLM) process was used to prepare the nanocrystalline titanium carbide (TiC)-reinforced Inconel 718 matrix bulk-form nanocomposites in the present study. An in-depth relationship between SLM process, microstructures, properties, and metallurgical mechanisms was established. The insufficient laser energy density (η) input limited the densification response of shaped parts due to the formation of either larger-sized pore chains or interlayer micropores. The densification of SLM-processed part increased to a near-full level as the applied η was properly settled. The TiC reinforcements generally experienced successive changes from severely agglomerated in a polygon shape to the uniformly distributed with smoothened and refined structures on increasing the applied η, while the Columnar Dendrite matrix exhibited strong epitaxial growth characteristic concurrently. The optimally prepared fully dense part achieved a high microhardness with a mean value of 419 HV 0.2 , a considerably low friction coefficient of 0.29, and attendant reduced wear rate of 2.69 × 10 −4 mm 3 /N m in dry sliding wear tests. The improved densification response, SLM-inherent nonequilibrium metallurgical mechanisms with resultant uniformly dispersed reinforcement microstructures, and elevated microhardness were believed to be responsible for the enhancement of wear performance.
Fusheng Pan - One of the best experts on this subject based on the ideXlab platform.
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analysis about forming mechanism of equiaxed crystal zone for 1cr18ni9ti stainless steel twin roll thin strip
Journal of Materials Processing Technology, 2009Co-Authors: Mingbo Yang, Fusheng PanAbstract:Abstract Based on a simple macro/micro mathematical model developed in this paper and its predicted results, the forming mechanism of equiaxed crystal zone for 1Cr18Ni9Ti stainless steel twin-roll thin strip is investigated and analyzed. In the developed model, the latent heat is treated with the enthalpy method, the grid and nodes are divided by the assumed streamlines. Moreover, the heterogeneous nucleation and Columnar-to-equiaxed transition (CET) models are also introduced, together with the revising of Dendrite growth dynamic model of Kurz–Giovanola–Trivedi (KGT). Finally, with the help of solid fraction, the coupling of macro/micro models is realized by using different grid-sizes and time-steps, macro and micro, together with the Columnar Dendrite front tracing. The predicted results of mathematical models indicate that, regardless of the solidification types (semi-solid, rolling or ideal type) of 1Cr18Ni9Ti stainless steel thin strip, the thin strips include the equiaxed crystal zone. The forming of equiaxed crystal zone is possibly related to the following mechanisms: (1) the sedimentation and accumulation of free crystals floating in molten pool onto the Dendrites solidification fronts and the accumulated crystals growth and (2) the suppression of the growth of dendritic solidification front and preferential growth of fine free crystals in unsolidified layer near the strip center by the abrupt decrease in the heat transfer coefficient on the strip surface after leaving the minimum gap between the rolls.