Imperial College London

Dr Minh-Son (Son) Pham

Faculty of EngineeringDepartment of Materials

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 9529son.pham Website

 
 
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Location

 

B301FBessemer BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

42 results found

Jin M, Hosseini E, Holdsworth SR, Pham M-Set al., 2022, Thermally activated dependence of fatigue behaviour of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion, Additive Manufacturing, Vol: 51, Pages: 1-13, ISSN: 2214-8604

The CrMnFeCoNi high-entropy alloy demonstrates a promising potential for applications over a range of temperature. The alloy also shows excellent printability to be fabricated by additive manufacturing for complex structures. Nevertheless, there are limited studies on the thermo-mechanical behaviour of the alloy, in particular when fabricated by laser powder-bed fusion. This study provides an in-depth understanding of the relationship between as-built cellular microstructures and fatigue behaviour at a range of temperatures (22–600 °C) in particular concerning the stability of dislocation cells and thermo-mechanical dependence of the fatigue behaviour of the alloy. At all tested temperatures, the alloy exhibits a very short duration cyclic hardening with a low hardening rate followed by a cyclic softening. The high density of dislocations already existing in as-built condition were able to accommodate most of the prescribed strain. Hence, only a small number of mobile dislocations needs to be generated, causing a short cyclic hardening phase. Upon further loading, the back stress associated with the long-range stress field was dominant factor governing the cyclic softening behaviour. The similitude relationship provided insights into the stability of as-built cells, in particular it explains why the size of as-built cells did not change during cyclic loading at 22 °C. The significant reduction in dislocation density due to the increased annihilation rate and untanglement of dislocation substructures thanks mainly to thermal assistance at elevated temperatures led to a decrease in cyclic strength and related properties (yield stress, friction and back stress, hysteresis loop shape parameter and energy per cycle). The LPBF HEA shows an insignificant strain rate dependence of the primary cyclic hardening and softening in the range of 10−3s−1 and 10−2s−1. However, the dynamic strain ageing results in a secondary cyclic hardening at

Journal article

Rielli VV, Piglione A, Pham M-S, Primig Set al., 2022, On the detailed morphological and chemical evolution of phases during laser powder bed fusion and common post-processing heat treatments of IN718, Additive Manufacturing, Vol: 50, Pages: 1-17, ISSN: 2214-8604

IN718 is the most common Ni-based superalloy for manufacturing aircraft engine parts via thermo-mechanical treatments. The evolution of nanoscale strengthening phases is well researched, enabling optimization of strength, fatigue, and creep properties. Recently, IN718 has shown great viability for laser powder bed fusion (LPBF) additive manufacturing of aerospace parts. However, the detailed microstructure-property relationships during thermal profiles typical to LPBF are not yet well understood. Previous works reported interdendritic precipitation of Laves phase. These detrimental particles can be dissolved by heat treatments, however, the detailed nanoscale phase evolution remains unknown. Using atom probe microscopy, we report on the detailed morphological and chemical evolution of phases in IN718 after LPBF with chessboard versus meander scanning strategies, and direct ageing versus homogenization and ageing treatments. Due to differences in scanning vector length, up to 3.6 times larger dendritic structures, double volume fractions of Laves particles, and Al clusters are found in the chessboard strategy. Coarser matrix grains and a higher dislocation density are detected in the meander strategy. The precise chemical composition and morphology evolution of the matrix, Laves, MC, γ′, and γ′′ phases are obtained and correlated to hardness. Retained Laves phase after direct ageing causes precipitation of 4% volume fraction of γ′′, with additional coarsened precipitates formed along dislocations. Direct ageing leads to an increase in hardness corresponding to roughly 190 HV. Due to Laves phase dissolution, a volume fraction of 16% of compositionally stable, larger γ′′ precipitates is found after homogenization and ageing, also causing partial matrix recrystallization.

Journal article

Iantaffi C, Leung CLA, Chen Y, Guan S, Atwood RC, Lertthanasarn J, Pham M-S, Meisnar M, Rohr T, Lee PDet al., 2021, Oxidation induced mechanisms during directed energy deposition additive manufactured titanium alloy builds, Additive Manufacturing Letters, Vol: 1, Pages: 100022-100022, ISSN: 2772-3690

Journal article

Piglione A, Attard B, Vieira Rielli V, Santos Maldonado C-T, Attallah MM, Primig S, Pham M-Set al., 2021, On the constitutive relationship between solidification cells and the fatigue behaviour of IN718 fabricated by laser powder bed fusion, Additive Manufacturing, Vol: 47, Pages: 1-19, ISSN: 2214-8604

IN718 combines excellent mechanical properties with a good weldability and is therefore an ideal alloy for laser powder bed fusion (LPBF). Knowledge of the relationship between its as-built microstructure, particularly solidification cells, and its fatigue properties is needed to better utilise additively manufactured microstructures and guide their further optimisation. This study presents a comprehensive investigation of the as-built microstructure and the associated monotonic and fatigue properties of LPBF IN718 aimed at highlighting the influential effect of solidification cells on monotonic and cyclic plasticity. In monotonic tension, cells induced pronounced strain hardening and good ductility by acting as strong yet not impenetrable obstacles to dislocation slip. In fatigue loading, cyclic hardening followed by cyclic softening was linked to the stability of the as-built solidification cells, the high initial dislocation densities and the subsequent rearrangements of such dislocations during cyclic loading using the similitude relation and the evolution of friction and back stresses. By thoroughly investigating the evolution of the cyclic response of samples printed using two different scanning patterns, the relationship between process (scanning line length and thus local substrate temperature), microstructure (dislocation cell size and their spatial arrangement) and mechanical properties (cyclic hardening and softening responses) was comprehensively discussed.

Journal article

Lertthanasarn J, Liu C, Pham MS, 2021, Influence of the base material on the mechanical behaviors of polycrystal-like meta-crystals, Journal of Micromechanics and Molecular Physics, Vol: 6, Pages: 1-1, ISSN: 2424-9130

Architected lattice metamaterials offer extraordinary specific strength and stiffness that can be tailored through the architecture. Meta-crystals mimic crystalline strengthening features in crystalline alloys to obtain high strength and improved post-yield stability of lattice materials. This study investigates synergistic effects of the base material's intrinsic crystalline microstructure and architected polycrystal-like architecture on the mechanical behavior of architected metamaterials. Four different polygrain-like meta-crystals were fabricated from 316L, Inconel 718 (IN718) and Ti6Al4V via laser powder bed fusion (L-PBF). While the elastic modulus of the meta-crystals did not vary significantly with the base material or the number of meta-grains, the strength of the meta-crystals showed strong increasing correlation with reducing the size of meta-grains. The differences between meta-crystals made by the three alloys were the most substantial in the post-yield behavior, where the 316L meta-crystals were the most stable while Ti6Al4V meta-crystals were the most erratic. The differences in the post-yield behavior were attributed to the base material's ductility and intrinsic work-hardening. For all base materials, increasing the number of meta-grains improved the post-yield stability of meta-crystals. The tolerance to the processing defects also differed with the base material. Detrimental defects such as the high surface roughness on the downskin of the struts or the large, irregularly shaped pores near the surface of the struts led to early strut fracture in Ti6Al4V meta-crystals. In contrast, ductile IN718 was able to tolerate such defects, enabling the most significant synergistic strengthening across lengthscales to achieve architected materials of low relative density, but with a very high strength and an excellent energy absorption.

Journal article

Liu C, Lertthanasarn J, Pham MS, 2021, The origin of the boundary strengthening in polycrystal-inspired architected materials, Nature Communications, Vol: 12, ISSN: 2041-1723

Crystal-inspired approach is found to be highly successful in designing extraordinarily damage-tolerant architected materials. i.e. meta-crystals, necessitating in-depth fundamental studies to reveal the underlying mechanisms responsible for the strengthening in meta-crystals. Such understanding will enable greater confidence to control not only strength, but also spatial local deformation. In this study, the mechanisms underlying shear band activities were investigated and discussed to provide a solid basis for predicting and controlling the local deformation behaviour in meta-crystals. The boundary strengthening in meta-crystals was found to relate to the interaction between shear bands and polygrain-like boundaries. More importantly, the boundary type and coherency are found to be influential as they govern the transmission of shear bands across meta-grains boundaries. The obtained insights in this study provide crucial knowledge in developing high strength architected materials with great capacity in controlling and programming the mechanical strength and damage path.

Journal article

Lertthanasarn J, Liu C, Pham MS, 2021, Synergistic effects of crystalline microstructure, architected mesostructure, and processing defects on the mechanical behaviour of Ti6Al4V meta-crystals, Materials Science and Engineering: A, Vol: 818, Pages: 1-11, ISSN: 0921-5093

The mimicry of crystalline microstructure at the meso-scale creates a new class of architected materials, termed meta-crystals, and offers effective ways of significantly improving the toughness and eliminating the post-yield collapse of architected materials. The application of meta-crystal approach to crystalline alloys provides exciting opportunities for high strength structural components. This study investigated the mechanical behaviour of polycrystal-like meta-crystals fabricated from a widely used alloy, Ti6Al4V, by laser powder bed fusion (LPBF). The use of Ti6Al4V in fabricating meta-crystals created materials containing hierarchical lattice structures across length-scales: the atomic lattice, the intrinsic crystalline microstructure, and the architected polycrystal-like mesostructure, with each hierarchical feature playing an influential role in the mechanical behaviour of meta-crystals. This present study examined the hierarchical lattice structures at different lengthscales and their contribution to the behaviour of meta-crystals. In particular, the presence of acicular α’ martensitic microstructure was responsible for low ductility in the as-printed meta-crystals. Although heat-treatment was able to transform the martensitic microstructure to a typical α+β microstructure thus increasing the ductility, it was found that notch-like defects from lack of fusion at the free surface of struts were significantly detrimental. The study subsequently altered the meso-structural parameters to reduce the influence of the process defects and explored the effects of the heat treatment on the altered meta-crystals. Such alternations of structural design and crystalline microstructure appeared to be successful in minimising the processing effect, enabling the crystal-mimicry approach to effectively improve the toughness of Ti6Al4V meta-crystals.

Journal article

Dovgyy B, Pham MS, Simonelli M, 2021, Alloy design against the solidification cracking in fusion additive manufacturing: An application to FeCrAl alloy, Materials Research Letters, Vol: 9, Pages: 350-357, ISSN: 2166-3831

This study developed a design methodology against liquid-state cracking by combining the Scheil–Gulliver solidification simulations and Machine Learning analysis to design alloys for Fusion Additive Manufacturing. Applying this design approach resulted in a Fe–20Cr–7Al–4Mo–3Ni. The alloy was successfully printed with relative densities of over 99%. Microstructure of printed material was extensively characterised through scanning and transmission electron microscopy, energy dispersive spectroscopy and x-ray diffraction, confirming a single-phase material with low texture and negligible chemical segregation. Neither solidification nor liquation cracks were detected, supporting the validity of the methodology, however, the alloy suffered from solid-state cracking, hindering the ductility.

Journal article

Wang H, Chen L, Dovgyy B, Xu W, Sha A, Li X, Tang H, Liu Y, Wu H, Pham M-Set al., 2021, Micro-cracking, microstructure and mechanical properties of Hastelloy-X alloy printed by laser powder bed fusion: As-built, annealed and hot-isostatic pressed, Additive Manufacturing, Vol: 39, Pages: 1-14, ISSN: 2214-8604

This study analyses literature data to identify optimised print parameters and assesses the consolidation, microstructure, and mechanical properties of Hastelloy-X printed by laser powder bed fusion. Effects of post annealing and hot-isostatic pressing (HIP) on the microstructure and mechanical properties are also revealed. The susceptibility to the solidification cracking was evaluated on the basis the solidification gradient and freezing range obtained via the calculation of thermodynamic phase diagrams (CALPHAD). In addition, the microstructure such as precipitation and chemical segregation were predicted using CALPHAD. The distribution of solidification cracks throughout the builds was quantified for the as-built, annealed and HIP conditions. The assessment reveals the variation of crack density towards the bottom, top and free surface of solid builds. This distribution of cracks is found to be associate with the thermal gradient and effective thermal conductivity, which were estimated by analytical thermal calculations. While the annealing and HIP both can alter the as-printed microstructure thanks to recovery and recrystallisation, the internal micro-cracks and internal pores were only successfully removed by the HIP. In addition to the removal, recrystallisation and precipitation in the HIP (stronger than in annealing), resulting in optimal mechanical properties including a substantial increase in elongation from 13% to 20%, significant improvement of ultimate tensile stress from 965 MPa to 1045 MPa with moderately high yield stress thanks to precipitation.

Journal article

Calvet T, Wang Y, Pham MS, Davies CMet al., 2021, Prediction of j-integrals at defects in W-9CR steel sandwich-type cooling pipes, ISSN: 0277-027X

Sandwich-type cooling pipes of the first wall of future fusion nuclear reactors (i.e. DEMO) will likely consist of tungsten brazed to a Reduced Activation Ferritic Martensitic (RAFM) steel. Under a high heat flux (HHF) (1-5 MW/m2) the mismatch in thermal expansion between tungsten and steel results in significant thermal stresses in the brazing region. These stresses can cause crack initiation and growth and thus compromise the structural integrity of such pipes. Finite element analyses have been performed on the brazed joints of a reference cooling assembly under HHF. Thermal stresses and resulting plastic strains were estimated for both the braze interlayer and parent materials. As images of brazed joints revealed, brazing processes are very likely to induce defects near the edges of the joints. A crack is therefore introduced in the brazed region where simulated stresses and strains are found to be the highest. Jintegrals were calculated for cracks growing from an edge to the center of the considered piping assembly. The results are discussed in relation to the current sandwich-type piping design of the DEMO reactor.

Conference paper

Williams RJ, Al-Lami J, Hooper PA, Pham M-S, Davies CMet al., 2021, Creep deformation and failure properties of 316 L stainless steel manufactured by laser powder bed fusion under multiaxial loading conditions, Additive Manufacturing, Vol: 37, Pages: 1-11, ISSN: 2214-8604

316 L stainless steel has long been used in high temperature applications. As a well-established laser powder bed fusion (LPBF) alloy, there are opportunities to utilise additive manufacturing in such applications. However, the creep behaviour of LPBF 316 L under multiaxial stress conditions must first be quantified before such opportunities are realised. Uniaxial and double notched bar creep tests have been performed and characterised using power-law relations to evaluate the creep strain and rupture properties of LPBF 316 L. The creep response was found to be anisotropic with specimen build orientation, with samples loaded perpendicular to the build direction (Horizontal) exhibiting 8 times faster minimum creep rates than samples built parallel to the build direction (Vertical) and significantly shorter rupture lives. This was mainly attributed to the columnar grain structure, which was aligned with the build direction of the LPBF samples. The multiaxial creep rupture controlling stress was determined and found to be a combination of the equivalent and max. principal stress. X-Ray CT measurements in selected samples illustrated that the samples were approximately 99.6% dense post-build and the quantity of damage post testing was determined. Optical and EBSD microstructural characterisation revealed intergranular creep damage present in the specimens, however rupture was ultimately trans-granular in nature and influenced by the presence and orientation of pre-existing processing defects relative to the sample build and loading direction.

Journal article

Jin M, Piglione A, Dovgyy B, Hosseini E, Hooper P, Holdsworth S, Pham MSet al., 2020, Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion, Additive Manufacturing, Vol: 36, Pages: 1-15, ISSN: 2214-8604

The CrMnFeCoNi high-entropy alloy is highly printable and holds great potential for structural applications. However, no significant discussions on cyclic plasticity and fatigue damage in previous studies. This study provides significant insights into the link between print processes, solidification microstructure, cyclic plasticity and fatigue damage evolution in the alloy fabricated by laser powder bed fusion. Thermodynamics-based predictions (validated by scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the core of the solidification cells, whilst Mn and Ni to the cell boundaries in all considered print parameters. Both dislocation slip and deformation twinning were found to be responsible for plastic deformation under monotonic loading. However, the former was found to be the single dominant mechanism for cyclic plasticity. The surface finish helped to substantially delay the crack initiation and cause lack-of-fusion porosity to be the main source of crack initiation. Most significantly, the scan strategies significantly affect grain arrangements and grain dimensions, leading to noticeable effects on fatigue crack propagation; in particular, the highest resistance crack propagation was seen in the meander scan strategy with 0° rotation thanks to the most columnar grains and the smallest spacing of grain boundaries along the crack propagation path.

Journal article

Bahshwan M, Myant CW, Reddyhoff T, Pham MSet al., 2020, The role of microstructure on wear mechanisms and anisotropy of additively manufactured 316L stainless steel in dry sliding, Materials and Design, Vol: 196, ISSN: 0264-1275

Wear control, which relies on understanding the mechanisms of wear, is crucial in preserving the life of mechanical components and reducing costs. Additive manufacturing (AM) techniques can produce parts with tailored microstructure, however, little has been done to understand how this impacts the mechanisms of wear. Here we study the impact of initial grain arrangement and crystal orientation on the wear mechanisms of austenitic stainless steel (SS) in dry sliding contact. Specifically, the anisotropic sliding wear behavior of as-built, AM-ed 316L SS is compared against annealed, wire-drawn counterparts. We describe, in-detail, how the sliding wear mechanisms of delamination, abrasion, oxidation, and plastic deformation are attributed to the initial surface microstructure under different loading conditions using a number of techniques. This new understanding sheds light on how different AM-induced microstructures affect wear, thereby allowing for better utilization of this technology to develop components with enhanced wear properties.

Journal article

Dovgyy B, Piglione A, Hooper P, Pham M-Set al., 2020, Comprehensive assessment of the printability of CoNiCrFeMn in laser powder bed fusion, Materials and Design, Vol: 194, ISSN: 0264-1275

This study assesses the printability including the consolidation, solidification microstructure, and mechanical properties of the CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion. A range of print parameters was used for a comprehensive assessment of printability, providing a basis to establish the relationship between process, microstructure, and mechanical properties. The study demonstrates a high relative density of the alloy fabricated with energy density in the range 62.7-109.8 J/mm3. It is shown that the scan strategy plays an important role in consolidation. For the same energy density, the rotation of 67° between two consecutive layers tends to yield higher consolidation than other considered strategies. Moreover, the scan strategy is found to be most influential in microstructure development. The scan strategy rotation angle controls the extent to which epitaxial growth can occur, and hence the crystallographic texture and the grain morphology. Amongst four considered strategies, the 0°- and 90°-rotation meander led to the strongest preferred texture while the 67°-rotation resulted in weaker texture. The 67°-rotation strategies led to broadened grains with lower aspect ratios. The understanding of texture and grain size provides explanations to the observed mechanical properties (such as flow stress and plastic anisotropy) of the alloy.

Journal article

Pham M-S, Dovgyy B, Hooper P, Christopher G, Alessandro Pet al., 2020, The role of side-branching in microstructure development in laser powder-bed fusion, Nature Communications, Vol: 11, ISSN: 2041-1723

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (i.e. 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67 rotation between layers.

Journal article

Piglione A, Yu J, Zhao J, Xiao C, Dunne F, Pham MSet al., 2020, Micro-mechanisms of Cyclic Plasticity at Stress Concentrations in a Ni-Based Single-Crystal Superalloy, Pages: 333-340, ISSN: 2367-1181

Ni-based single-crystal superalloys are high-temperature materials used for turbine blades in jet engines. Fatigue damage can pose a major threat to the integrity of such components in operation. Traditionally, TEM-based studies on the fatigue behaviour of superalloys has been studied by investigating cyclic plasticity in the bulk of the material. When the cyclic loads are nominally elastic, however, such investigation may not contribute to the understanding of the alloy’s fatigue behaviour, since plastic micro-strains are confined to regions near stress raisers such as microstructural defects and are therefore randomly distributed. In turn, the plastic micro-strains near the ‘critical’ stress raiser, i.e. the one that acts as nucleation site for the dominant crack, govern fatigue life by inducing early crack initiation, and are therefore the key to capture the material’s fatigue behaviour. Hence, this work is concerned with the experimental characterisation of cyclic plasticity at the initiation site in a Ni-based single-crystal superalloy at 800 °C tested with nominally elastic cyclic loads. Such investigation was carried out by focused ion beam (FIB) lift-outs and subsequent transmission electron microscopy (TEM) studies. It is shown that deformation is significantly more pronounced near the ‘critical’ stress concentration; in addition, deformation is rather homogeneous across large regions surrounding the stress raiser, with remarkably different deformation modes compared to those observed in the bulk of the specimens and from those expected in superalloys tested in similar conditions. The investigation of local cyclic plasticity at stress concentrations promises therefore to provide new insight into fatigue crack initiation in Ni-based superalloys.

Conference paper

Williams R, Ronneberg T, Piglione A, Jones C, Pham M-S, Davies C, Hooper Pet al., 2019, In-situ thermography for laser powder bed fusion: effects of layer temperature on porosity, microstructure and mechanical properties, Additive Manufacturing, Vol: 30, 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.

Journal article

Chen L, James Edwards TE, Di Gioacchino F, Clegg WJ, Dunne FPE, Pham M-Set 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, Vol: 119, Pages: 344-360, 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.

Journal article

Pham M-S, Liu C, Todd I, Lertthanasarn Jet 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.

Journal article

Pham MS, Liu C, Todd I, Lertthanasarn Jet 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.

Journal article

Piglione A, Dovgyy B, Liu C, Gourlay C, Hooper P, Pham Met 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.

Journal article

Li J, Pham MS, Tian GF, Peng ZC, Shollock Bet 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.

Journal article

Xu WY, Peng ZC, Li MZ, Pham MSet al., 2018, Microstructure analysis and creep behaviour modelling of powder metallurgy superalloy, Pages: 134-140, ISSN: 0255-5476

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.

Conference paper

Dovgyy B, Minh-Son P, 2018, Epitaxial Growth in 316L Steel and CoCrFeMnNi High Entropy Alloy Made by Powder-bed Laser Melting, 21st International ESAFORM Conference on Material Forming (ESAFORM), Publisher: AMER INST PHYSICS, ISSN: 0094-243X

Conference paper

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

Conference paper

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.

Journal article

Pham MS, Creuziger A, Iadicola M, Rollett ADet al., 2017, 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

This study investigates the joint impact of preferred texture and latent hardening on the plastic anisotropy of face centered cubic (FCC) materials. The main result is that both aspects have significant impact on the anisotropy, but the two can either counteract each other or synergistically reinforce each other to maximize anisotropy. Preferred texture results in significant anisotropy in plastic yielding. However, the latent hardening significantly alters the texture-induced anisotropy. In addition, one latent hardening type can cancel out the anisotropy of another type. Consequently, if all dislocation-based latent hardening types are included at the same level as the self-hardening, the result might not reveal the complexity of plastic anisotropy. The present study of the synergistic influence of detailed latent hardening and texture presented helps provide new insights into the complex anisotropic behavior of FCC materials during multi-axial forming.

Journal article

Jeong Y, Minh-Son P, Iadicola M, Creuziger A, Foecke Tet 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

Journal article

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