Publications
51 results found
Liu C, Pham M-S, 2024, Spatially Programmable Architected Materials Inspired by the Metallurgical Phase Engineering., Adv Mater, Vol: 36
Programmable architected materials with the capabilities of precisely storing predefined mechanical behaviors and adaptive deformation responses upon external stimulations are desirable to help increase the performance and the organic integration of materials with surrounding environments. Here, a new approach inspired by the physical metallurgical principles is proposed to allow the materials designers to not only enhance the global strength but also precisely tune mechanical properties (such as strength, modulus, and plastic deformation) locally in architected materials to create a new class of intelligent mechanical metamaterials. Such programmable materials not only have high strength and plastic deformation stability but also the ability to regulate the local deformation states and spatially control the internal propagation of deformation. This innovative approach also provides new and effective ways to enhance the adaptivity of the materials thanks to responsive strengths that not only make the materials increasingly stronger but also allow threshold-based adaptive responses to external loading.
Piglione A, Bellamy T, Yu J, et al., 2024, Dislocation Arrangements and Cyclic Microplasticity Surrounding Stress Concentration in a Ni-Based Single-Crystal Superalloy, Advanced Engineering Materials, Vol: 26, ISSN: 1438-1656
Local cyclic plasticity near stress concentrations governs the fatigue crack initiation in cyclicly loaded Ni-based single-crystal superalloys, but has not been well studied and understood. The first of its kind transmission electron microscopy (TEM)-based site-specific study of plasticity in the crack initiation region in a notched single-crystal superalloy subjected to fatigue testing at 800 °C, coupling it with microstructure-based crystal plasticity modeling, is presented. Detailed TEM examinations show that local plasticity near the notch significantly differs from bulk plasticity, featuring high dislocation densities and distinctive arrangements of dislocation pairs within γ’ precipitates. It further shows that the increased local stresses alone are responsible for the increase in dislocation density and extensive γ’ shearing, but not solely for the distinctive arrangement of dislocation pairs seen in the notch vicinity, thus highlighting the considerable role played by the local variations in loading rates and stress state surrounding the notch. The results of this work provide new fundamental insights into the deformation micromechanisms leading to fatigue crack initiation in single-crystal superalloys.
Banait S, Liu C, Campos M, et al., 2023, Effect of microstructure on the effectiveness of hybridization on additively manufactured Inconel718 lattices, Materials and Design, Vol: 236, ISSN: 0264-1275
This work aims to investigate the effect of the material microstructure on the effectiveness of hybridization in additively manufactured Inconel718 strut-based lattices. Two microstructures, namely the as-built solid solution and a peak aged condition, are coupled with three hybrid architectures formed by alternating face centred cubic (matrix) and octet-truss (reinforcement) domains containing 1, 8, and 64 unit cells. Digital image correlation is utilized to map the local strain distribution during compression, and finite element analysis is utilized to map stress localization. It is shown that in the as-built condition the mechanical behavior of all the hybrid lattices was very similar to that of the single-oriented components, irrespective of the reinforcement size. In the peak-aged lattices, on the contrary, including octet-truss reinforcements of sufficiently small size resulted in an increase in mechanical stability, damage tolerance, and absorbed energy, as interphase boundaries act as obstacles to shear band propagation. It is also shown that effect of hybridization is a function of the size of the reinforcements. In particular, below a critical size, the lattice behaves as a reinforced “single-phase” structure, rather than a two-phase architecture. This work defines guidelines for the design of robust lattices manufactured by laser powder bed fusion.
Akisin CJ, Dovgyy B, Bennett CJ, et al., 2023, Microstructural Study of Cold-Sprayed CoCrFeNiMn High Entropy Alloy, JOURNAL OF THERMAL SPRAY TECHNOLOGY, ISSN: 1059-9630
Al-Lami J, Hoang P, Davies C, et al., 2023, Plastic inhomogeneity and crack initiation in hybrid wrought - additively manufactured Inconel 718, Materials Characterization, Vol: 199, Pages: 1-12, ISSN: 1044-5803
Directed energy deposition (DED) holds great promise for repair applications involving site-specific deposition that creates hybrid components. However, it has been reported that failure in hybrid components occurs in the additively manufactured (AM) side despite its higher strength and excellent consolidation. To unravel the underlying mechanisms responsible for the observed behaviour, we carried out both ex-situ and in-situ mechanical testing complemented with detailed microstructural characterisation of the wrought, AM and bond interface regions in hybrid Inconel 718. The role of the local microstructure in the spatial strain development and crack initiation in the hybrid wrought-AM Inconel 718 is revealed. Most importantly, it is shown that severe inhomogeneous plastic deformation quickly developed in the AM side of the hybrid sample, and this is primarily attributed to few, but very large columnar grains that were preferentially oriented for dislocation slip under external loading. Furthermore, the AM region of the hybrid Inconel 718 had a lower work-hardening rate that caused a higher thinning rate during deformation, promoting strain localisation in the AM side. The degradation in the ductility of AM Inconel 718 is shown to have occurred not because of defects as widely reported for AM components, but rather as a consequence of the interaction between the brittle Laves phase with the localised slip bands which were rapidly intensified by the plastic inhomogeneity. This interaction resulted in early crack initiation in the AM side, primarily leading to the final fracture of the hybrid wrought-AM Inconel 718. The study also demonstrates that the bond interface is the strongest region of the hybrid component under uniaxial loading, and reveals the origins of the strength of the bond interface. The insights revealed advance the understanding of the mechanical performance and the direct failure mechanism in hybrid wrought-AM Inconel 718 components, and opens new p
Li X, Esmaeilizadeh R, Jahed H, et al., 2023, Effects of temperature and strain rate on tensile properties and dynamic strain aging behaviour of LPBF Hastelloy X, ADDITIVE MANUFACTURING LETTERS, Vol: 4, ISSN: 2772-3690
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Banait S, Liu C, Campos M, et al., 2022, Coupled effect of microstructure and topology on the mechanical behavior of Inconel718 additively manufactured lattices, Materials and Design, Vol: 224, Pages: 1-17, ISSN: 0264-1275
LPBF-manufactured Inconel718 lattices with six cubic and hexagonal structures and with two material microstructures (as-built, without γ” precipitation, and heat-treated, with γ” precipitates, but with similar cell/grain size, shape and texture) were compressed at room temperature and at 600 °C. The behavior of the base material under identical microstructure and test conditions was also investigated using dedicated LPBF-manufactured specimens with single strut gauges. Irrespective of topology, precipitation led to a transition in the lattice mechanical behavior from bending-dominated to stretch-dominated, which was associated to a change in the strut deformation mode from plastic hinging to elastic buckling, as well as to a decrease in the base material strain to fracture. For all topologies investigated, precipitation led to lattice strengthening, consistent with particle-strengthening of the base material. Additionally, high temperature straining resulted both in a decrease in the lattice yield strength, consistent with softening of the base material, and in a reduction in the width of the stress oscillations in the stretch-dominated lattices, which is associated to the decrease in the base material ductility. This work proves that material microstructure influences strongly the behavior of additively manufactured architectured structures and that it must be considered as a design criterion for performance optimization.
Jin M, Hosseini E, Holdsworth SR, et 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
Rielli VV, Piglione A, Pham M-S, et 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.
Iantaffi C, Leung CLA, Chen Y, et al., 2021, Oxidation induced mechanisms during directed energy deposition additive manufactured titanium alloy builds, ADDITIVE MANUFACTURING LETTERS, Vol: 1, ISSN: 2772-3690
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Piglione A, Attard B, Vieira Rielli V, et 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.
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.
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.
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.
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.
Wang H, Chen L, Dovgyy B, et 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.
Williams RJ, Al-Lami J, Hooper PA, et 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.
Calvet T, Wang Y, Pham M-S, et al., 2021, PREDICTION OF J-INTEGRALS AT DEFECTS IN W-9CR STEEL SANDWICH-TYPE COOLING PIPES, ASME Pressure Vessels and Piping Conference (PVP), Publisher: AMER SOC MECHANICAL ENGINEERS
Jin M, Piglione A, Dovgyy B, et 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.
Bahshwan M, Myant CW, Reddyhoff T, et 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.
Dovgyy B, Piglione A, Hooper P, et 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.
Pham M-S, Dovgyy B, Hooper P, et 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.
Piglione A, Yu J, Zhao J, et 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.
Williams R, Ronneberg T, Piglione A, et 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.
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, 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.
Pham M-S, Liu C, Todd I, et al., 2019, Damage-tolerant architected materials inspired by crystal microstructure (vol 565, pg 305, 2019), NATURE, Vol: 567, Pages: E14-E14, ISSN: 0028-0836
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Pham MS, 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.
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
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.
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