25 results found
Williams R, Ronneberg T, Piglione A, et al., In-situ thermography for laser powder bed fusion: effects of layer temperature on porosity, microstructure and mechanical properties, Additive Manufacturing, ISSN: 2214-8604
In laser powder bed fusion(LPBF)the surface layer temperature is continually changing throughout the build process. Variations in part geometry, scanned cross-section and number of parts all inffluence the thermal field within a build. Process parameters do not take these variations into account and this can result in increased porosity and differences in local microstructure and mechanical properties, undermining confidence in the structural integrity of a part. In this paper a wide-field in-situ infra-redimaging system is developed and calibrated to enable measurement of both solid and powder surface temperatures across the full powder bed. The influence of inter-layer cooling time is in-vestigated using a build scenario with cylindrical comp onents of differing heights. In-situ surface temperature data are acquired through out the build process and are compared to results from porosity, microstructure and mechanical property investigations. Changes in surface temperature of u to 200°C are attributed to variation in inter-layer cooling time and this is found to correlate with density and grain structure changes in the part. This work shows that these changes are significant and must be accounted for to improve the consistency and structural integrity of LPBF components.
Chen L, James Edwards TE, Di Gioacchino F, et al., 2019, Crystal plasticity analysis of deformation anisotropy of lamellar TiAl alloy: 3D microstructure-based modelling and in-situ micro-compression, International Journal of Plasticity, ISSN: 0749-6419
Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of lamellar microstructure with the interfaces being aligned and to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the micropillar while the anisotropy of the micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the micropillar less localised than that in the micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.
Pham M-S, Liu C, Todd I, et al., 2019, Publisher Correction: Damage-tolerant architected materials inspired by crystal microstructure., Nature, Vol: 567
In Fig. 4a of this Article, owing to an error in the production process, the scale bar inadvertently read 1 mm instead of 1 m. This error has been corrected online.
Minh-Son P, Liu C, Todd I, et al., 2019, Damage-tolerant architected materials inspired by crystal microstructure, Nature, Vol: 565, Pages: 305-311+, ISSN: 0028-0836
Architected materials that consist of periodic arrangements of nodes and struts are lightweight and can exhibit combinations of properties (such as negative Poisson ratios) that do not occur in conventional solids. Architected materials reported previously are usually constructed from identical ‘unit cells’ arranged so that they all have the same orientation. As a result, when loaded beyond the yield point, localized bands of high stress emerge, causing catastrophic collapse of the mechanical strength of the material. This ‘post-yielding collapse’ is analogous to the rapid decreases in stress associated with dislocation slip in metallic single crystals. Here we use the hardening mechanisms found in crystalline materials to develop architected materials that are robust and damage-tolerant, by mimicking the microscale structure of crystalline materials—such as grain boundaries, precipitates and phases. The crystal-inspired mesoscale structures in our architected materials are as important for their mechanical properties as are crystallographic microstructures in metallic alloys. Our approach combines the hardening principles of metallurgy and architected materials, enabling the design of materials with desired properties.
Piglione A, Dovgyy B, Liu C, et al., 2018, Printability and microstructure of the CoCrFeMnNi high-entropy alloy fabricated by laser powder bed fusion, Materials Letters, Vol: 224, Pages: 22-25, ISSN: 0167-577X
The CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bi-directional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations.
Dovgyy B, Pham MS, 2018, Epitaxial growth in 316L steel and CoCrFeMnNi high entropy alloy made by powder-bed laser melting, ISSN: 0094-243X
© 2018 Author(s). This study presents our fundamental study to understand the crystal formation in rapid cooling of cubic crystalline alloys (namely, 316L steel and NiCoCrFeMn) and how crystal microstructure evolves during the repeated deposition of material in powder-bed laser melting. The rapid cooling results in extremely fine rod-like cells. Cells in a fresh meltpool epitaxially grow from existing grains in the substrate (or existing cells in previously solidified meltpools). It is found that the orientation of existing crystals and the thermal gradient are two governing variables for the evolution of cells in meltpools.
Li J, Pham MS, Tian GF, et al., 2018, Creep deformation mechanisms and CPFE modelling of a nickel-base superalloy, Materials Science and Engineering: A, Vol: 718, Pages: 147-156, ISSN: 0921-5093
Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. The microstructures have been evaluated in terms of γ’ size distribution, morphology and volume fraction for the different heat treatments using SEM and post digital image software. The results illustrate that γ’ size decreases with cooling rate and interrupted cooling leads to a higher tertiary γ’ but lower secondary γ’ volume fraction. The heat treated samples were creep tested under 690 MPa at 700 °C. It was found that the creep response it inversely proportional to secondary γ’ size. Tertiary γ’ volume fraction was also found having a great impact on the material's creep properties. The TEM micrographs show that matrix dislocations could be the precursor of microtwin formation and creep deformation modes are closely related to tertiary γ’ precipitates. Tertiary γ’ size determines whether a/2<110> dislocations shear or dissociate before entering tertiary γ’ and the tertiary γ’ volume fraction determines how matrix dislocation dissociates. A CPFE model has been developed based on the experimental results. The simulation results for both the single element and the poly-crystalline model indicated that the modelled secondary stage creep rate is in very good agreement with the experimental results. The modelling enables the consequences of this to be investigated for representative different spatial microstructural disc conditions.
Xu WY, Peng ZC, Li MZ, et al., 2018, Microstructure analysis and creep behaviour modelling of powder metallurgy superalloy, Pages: 134-140, ISSN: 0255-5476
© 2018 Trans Tech Publications, Switzerland. Microstructure analysis of Ni-based superalloy FGH96 under different ageing treatments were carried out in order to understand the microstructure-creep strength relationships of the alloy. It was found that the volume fraction of tertiary γ′ and the mean γ-channel width was significantly varied with different ageing treatments, leading to the changes in creep behavior. The dislocation/γ′ shearing mechanisms were also changed with ageing treatment. The volume fractions of both secondary and tertiary γ′ and the mean γ-channel width were quantitatively analyzed by electron microscopy. The quantified microstructures were used for a crystal plasticity-based constitutive model. It was observed that the crystal plasticity model can accurately simulate experimentally observed creep behavior of aged samples showing significant secondary creep stage.
Minh-Son P, Hooper P, 2017, Roles of Microstructures on Deformation Response of 316 Stainless Steel Made by 3D printing, 20th International ESAFORM Conference on Material Forming, Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Pham MS, Dovgyy B, Hooper PA, 2017, Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 704, Pages: 102-111, ISSN: 0921-5093
Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.
Pham MS, Creuziger A, Iadicola M, et al., 2016, Roles of texture and latent hardening on plastic anisotropy of face-centered-cubic materials during multi-axial loading, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 99, Pages: 50-69, ISSN: 0022-5096
Jeong Y, Minh-Son P, Iadicola M, et al., 2016, Forming limit prediction using a self-consistent crystal plasticity framework: a case study for body-centered cubic materials, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, Vol: 24, ISSN: 0965-0393
Minh-Son P, Rollett AD, Creuziger A, et al., 2016, Crystal Plasticity Analysis of Constitutive Behavior of 5754 Aluminum Sheet, 19th International ESAFORM Conference on Material Forming (ESAFORM), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Pham MS, Iadicola M, Creuziger A, et al., 2015, Thermally-activated constitutive model including dislocation interactions, aging and recovery for strain path dependence of solid solution strengthened alloys: Application to AA5754-O, International Journal of Plasticity, Vol: 75, Pages: 226-243, ISSN: 0749-6419
© 2014 Elsevier Ltd. All rights reserved. A thermally-activated constitutive model is developed based on dislocation interactions, crystallographic orientations and microstructural evolution to describe the elasto-plastic stress-strain behavior during multi-axial loading. The aim is to contribute to the quantification of complex strain path response in solid solution strengthened alloys. In detail, dislocation/dislocation interactions are incorporated in the model to quantify latent and kinematic hardening phenomena during loading path changes. Dislocation density-based constitutive relations are included to account for dislocation features such as dislocation forests, walls and channels. Moreover, dislocation/solute atom interactions are also considered in order to account for both dynamic and static strain aging as well as static recovery. The model is validated against multiple multi-axial data sets for AA5754-O with changes of loading path and various degrees of pre-strain and time intervals between tests.
Jeong Y, Pham MS, Iadicola M, et al., 2015, Forming limit diagram predictions using a self-consistent crystal plasticity model: A parametric study, Key Engineering Materials, Vol: 651-653, Pages: 193-198, ISSN: 1013-9826
© (2015) Trans Tech Publications, Switzerland. A numerical model to predict forming limit diagrams (FLD) for polycrystalline metal sheets is presented. In it, the Marciniak-Kuczyński (MK) approach  is incorporated into the framework of the viscoplastic self-consistent (VPSC) crystal plasticity model . The current model, dubbed the VPSC-FLD, can run simulations along individual loading paths in parallel, which can make use of a CPU-cluster to enhance the computational speed. The main objective of the current work is to provide a detailed sensitivity report based on the VPSC-FLD. First of all, the influence of the initial inhomogeneity, f, as defined in the MK approach, is illustrated. Secondly, FLDs resulting from various sizes of the statistical population for the crystallographic texture are examined. Lastly, the computation time spent for various sizes of the statistical population is given.
Pham M-S, Rollett AD, Creuziger A, et al., 2014, Constitutive Modeling based on Evolutionary Multi-junctions of Dislocations, MATERIAL FORMING ESAFORM 2014, Vol: 611-612, Pages: 1771-1776, ISSN: 1013-9826
Pham M-S, Holdsworth SR, 2014, Evolution of Relationships Between Dislocation Microstructures and Internal Stresses of AISI 316L During Cyclic Loading at 293 K and 573 K (20 degrees C and 300 degrees C), METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 45A, Pages: 738-751, ISSN: 1073-5623
Facheris G, Pham M-S, Janssens KGF, et al., 2013, Microscopic analysis of the influence of ratcheting on the evolution of dislocation structures observed in AISI 316L stainless steel during low cycle fatigue, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 587, Pages: 1-11, ISSN: 0921-5093
Pham MS, Holdsworth SR, Janssens KGF, et al., 2013, Cyclic deformation response of AISI 316L at room temperature: Mechanical behaviour, microstructural evolution, physically-based evolutionary constitutive modelling, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 47, Pages: 143-164, ISSN: 0749-6419
Pham MS, Holdsworth SR, 2013, Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 degrees C, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 51, Pages: 36-48, ISSN: 0142-1123
Pham MS, Holdsworth SR, 2012, Dynamic strain ageing of AISI 316L during cyclic loading at 300 degrees C: Mechanism, evolution, and its effects, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 556, Pages: 122-133, ISSN: 0921-5093
Pham MS, Solenthaler C, Janssens KGF, et al., 2011, Dislocation structure evolution and its effects on cyclic deformation response of AISI 316L stainless steel, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 528, Pages: 3261-3269, ISSN: 0921-5093
Pham MS, Holdsworth SR, 2011, Change of stress-strain hysteresis loop and its links with microstructural evolution in AISI 316L during cyclic loading, 11th International Conference on the Mechanical Behavior of Materials (ICM), Publisher: ELSEVIER SCIENCE BV, Pages: 1069-1074, ISSN: 1877-7058
Holdsworth SR, Mayer T, Pham MS, et al., 2010, The effect of sub-grain formation and development on cyclic response in engineering steels
A series of elevated temperature strain-controlled low cycle fatigue (LCFtests has been performed on two steels, one which cyclic softens (2CrMoNiWV one which both hardens and softens during its lifetime (17Cr12Ni2Mo. Multi-specimen tests have been conducted at specific conditions of temperature, strain rate and strain amplitude. In addition to the determination of crack initiation endurance, tests have been taken to, and discontinued at, various life fractions to provide samples for TEM, EBSD and SEM-BSE microstructural examination. The development of sub-grain microstructures ultimately controls the way in which both steels respond to strain-controlled cyclic plastic loading, but in very different ways. In the case of the 2CrMoNiWV steel at 565°C, continuous cyclic softening from the first load reversal is the consequence of progressive dynamic recovery and sub-grain growth. The more complex cyclic hardening/softening response of 17Cr12Ni2Mo steel at room temperature is a result of the development first of vein and channel/wall structures which are subsequently broken down into sub-grains of progressively reducing size by cross-slip.
Pham MS, Park K-W, Yoo B-G, et al., 2009, Plasticity Improvement of Amorphous Alloy via Skim Cold Rolling, METALS AND MATERIALS INTERNATIONAL, Vol: 15, Pages: 209-214, ISSN: 1598-9623
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