Graded Microstructure Research Articles

Microstructural evolution and its influence on the wear resistance of a laser directed energy deposited Ni-based single crystal superalloy

The wear behavior of Ni-based single crystal (NBSC) superalloy SRR99 fabricated by laser-directed energy deposition (LDED) is investigated and compared with that of its cast counterpart. While γ’ precipitate size in the latter is > 400 nm, that in the former is an order of magnitude lower. Dry sliding wear tests reveal that the wear rate and coefficient of friction of the LDED alloy are 75 % and 20 % lower than that of its cast counterpart, respectively. Detailed transmission electron microscopy investigation of the wear-tested cast alloy indicates that there is orientation change and formation of nanoscale grains only at the top layer of the worn surface, whereas regions below undergo moderate plastic deformation via dislocation slip. In contrast, the sub-surface of the worn LDED alloy has a graded microstructure, with a composite of NiO/γ-Ni on the top, γ’ free nano-grains in the middle, and a highly deformed nanoscale layer at the bottom. The improved wear behavior of the LDED alloy is attributed to its higher dislocation density, finer γ’ precipitates, and the formation of this graded microstructure. Finally, a detailed description of mechanisms that lead to the formation of this unique graded microstructure is provided.

Unraveling microstructure and mechanical response of an additively manufactured refractory TiVHfNbMo high-entropy alloy

Refractory high-entropy alloys (RHEAs) have attracted considerable interest due to their elevated melting points and exceptional softening resistance. Nevertheless, the ambient-temperature brittleness and inadequate high-temperature oxidation resistance commonly restrict the processability of RHEAs. Direct energy deposition (DED) additive manufacturing technology is ideal for fabricating refractory alloys due to design flexibility and oxygen-free environment. In this work, a novel Ti41V27Hf13Nb13Mo6 RHEA was successfully manufactured by DED, and a comprehensive investigation was conducted to explore the microstructure evolution and mechanical response during tension. The as-deposited RHEAs exhibit a grain-size graded microstructure with a body-centred-cubic (BCC) matrix and precipitates. Increasing laser energy density suppressed the grain boundary precipitation, effectively enhancing the ambient-temperature tensile ductility to ∼11.3%. The optimized specimens achieved an unprecedented yield strength of ∼1.2 GPa among the DEDed RHEAs, which can be attributed to a significant solid solution strengthening from the volume misfit of 5.03%. We revealed that dislocation interactions maintained the working hardening capacity. Moreover, in-situ characterization indicated that slip transfer, grain rotation, and kinking accommodated the plastic deformation. Crack nucleation was caused by slip inhibition at grain boundaries and dislocation pile-ups at intragranular precipitates. The kink band formation relieved stress concentration induced by intragranular precipitates and promoted a ductile fracture. These exceptional outcomes provide opportunities for additive manufacturing RHEAs and greatly advance understanding of their strengthening and deformation mechanisms.

Mechanism of thermal gradients in spark plasma sintering of titanium and the resultant graded microstructure: A novel strategy of incorporating master sintering curve into finite element modeling

Spark plasma sintering (SPS) is an efficient consolidation process for metals and ceramics because of its high heating rates, lower sintering temperatures and shorter holding time than other sintering processes. To understand and utilize the thermal gradients that occur in SPS, the present work developed a strategy to incorporate master sintering curve (MSC) into a thermal-electric (TE) finite element modeling (FEM) to enable continuous nodal-specific simulation of the evolution of thermal and electrical properties of the powder compact with respect to both relative density and temperature during SPS. This combined model is proposed as an alternative to the thermal-electric-mechanical fully coupled model that is more computationally expensive and difficult to parameterize experimentally. Pure Ti was selected as the material for experimental verification because of its technological importance for aerospace and defense applications. While lab-scale Ti samples were sintered for construction and validation of MSC, a scaled-up Ti sample with diameter of 70 mm was made via SPS, and its microstructure were characterized to verify the accuracy of the MSC - TE FEM combined model. Both modeling and experimental results showed that the local relative density of the pure Ti decreased as the location moved radially from center to surface. The location-specific relative densities predicted by the combined model were within ±1.5% deviation from the experimental results, which verified the ability and accuracy of the MSC-TE FEM combined model to guide experimental design and parameter selection for control and utilization of gradients in SPS to achieve graded microstructure. • SPS of pure Ti has an apparent sintering activation energy of of 156.5 kJ/mol • Master sintering curve is incorporated into FEM for improved prediction of gradients • The accuracy of MSC-FEM combined model is experimentally validated • Deviation between the experimental and simulated results are <1.5%

Flexible pressure sensor for high-precision measurement of epidermal arterial pulse

The rapid development of flexible pressure sensors (FPSs) has driven pulse-based personalized health monitoring, which is extremely important for the diagnosis and prevention of cardiovascular diseases. However, developing a FPS with a convenient fabrication process to form a natural graded microstructure to capture high-quality physiological signals is still a challenge. In this study, by utilizing polytetrafluoroethylene and Cu powder with a conductive double-sided adhesive, a FPS was developed for high-precision measurement of epidermal arterial pulse based on the principle of triboelectric nanogenerators. By using a convenient coating operation, the Cu powder layer with a 500-nm-scale natural microstructure was formed on the conductive double-sided adhesive. This graded microstructure is composed of Cu powder that enables the FPS to respond sensitively to weak pressure signals, endowing the FPS with a sensitivity of 1.65 V/kPa, response time of 17 ms, and decent stability of up to 4500 cycles. Given its compelling performance, the FPS holds the capability to precisely detect epidermal pulse wave with rich details. The consistency between the pulse waveform at the same position measured by the FPS and a high-precision laser vibrometer (with a displacement resolution of 0.05 pm) is up to 0.9949. Furthermore, the FPS can be applied to acquire pulse wave on different human body parts and monitor arterial pulse changes in real time under different opening and closing behaviors of the arterial vessels. The graded microstructure formed by the Cu powder coating provides a convenient and highly efficient method for mass production of FPSs, which will provide new inspiration for the practical applications of flexible sensors to human health.

Graded microstructure and properties of TiCp/Ti6Al4V composites manufactured by laser melting deposition

In this paper, 50 vol% TiCp/Ti6Al4V composites without pores, cracks or poor fusion were fabricated by laser melting deposition (LMD) technology using 500 W laser power (lower power, LP) and 1000 W laser power (higher power, HP). The microstructure of different deposited layers was analyzed and the corresponding microhardness was measured. The results show that the composites are composed of undissolved TiC, in-situ TiC (eutectic TiC and primary TiC), α Ti and β Ti. It is interesting that the microstructure from the bottom layer to the top layer presents a graded distribution when all layers have the same composition. The eutectic TiC is mainly formed in lower layers while the primary TiC is mainly in upper layers. With increasing layer number, the size and number of the primary TiC grains increase and the primary TiC grows gradually from granules into dendrites, however, the number of the undissolved TiC particles decreases along the depositing direction. The simulation results show that the peak temperature of deposited layer increases while the cooling rate decreases with increasing layer number. Different thermal history affects the diffusion of carbon and the evolution of TiC, and then affects the microstructure distribution of the composites. This graded microstructure leads to the graded distribution of microhardness. The microhardness of the LP composite rises from 432.5 HV to 488.1 HV, while the HP composite rises from 435.5 HV to 604.3 HV from the bottom layer to the top layer, respectively.

The tension-compression behavior of gradient structured materials: A deformation-mechanism-based strain gradient plasticity model

Gradient structured (GS) metals can simultaneously achieve high strength and high ductility, which is the pursuit of structural materials. Abundant studies have pointed out that significant kinematic hardening is key to their strength-ductility combination. Unfortunately, few constitutive models were established to simulate and analyze the characteristic kinematic hardening behavior of GS metals. Thus, a systematic and deep understanding of the relationship between graded microstructure and the resulting macroscopic response needs further effort. In this work, we develop a strain gradient plasticity model that considers plasticity heterogeneities from the grain to the sample scale. A back stress model, which accounts for the dependency of dislocation pileups on grain size, is established to describe the cyclic deformation properties of GS materials. The established model unifies the concepts of geometrically necessary dislocations accommodating plasticity heterogeneities, grain size-dependent back stress, and reversible dislocation motion during reverse loading into a strain gradient plasticity framework. A finite element implementation of the model quantitatively predicts the uniaxial tensile and tensile-compressive responses of a GS copper bar as well as of homogeneously grained reference samples. The simulation indicates that the enhanced kinematic hardening of GS copper results mainly from fine grains in the GS layer and contributes to the considerable ductility of the GS material. The strain gradient cyclic plasticity model enables future investigation of the cyclic/fatigue behavior of GS materials, which is vital for the engineering application.

Physical and thermodynamic simulations of gamma-prime precipitation in Haynes® 282® using arc heat treatment

Haynes 282 is a Ni-based gamma prime (γ′) strengthening alloy with a balanced combination of high temperature properties and fabricability. This paper aims to study the evolution of the microstructure and hardness using a novel physical simulation method called arc heat treatment, followed by thermodynamic modeling of γ′ precipitation. For the arc heat treatment, a steady state temperature gradient was generated using a stationary tungsten inert gas arc on a sample mounted onto a water-cooled chamber. The steady state condition ranged from room temperature to the liquidus and was achieved within the first few seconds. Aged and solutionized samples were arc heat treated for 1.5 min, 30 min, and 4 h. The experiments were complemented with temperature modeling, equilibrium calculations, and γ′ precipitation simulations. A unique graded microstructure formed in the arc heat-treated samples. It consisted of a fusion zone with a dendritic microstructure; a region with the dissolution of all secondary phases (MC carbides, grain boundary carbides, and γ′); a region with MC and grain boundary carbides; a γ′ precipitation zone; and the base metal. The temperature range of the dissolution area extended to lower temperatures with increasing arc heat treatment dwell time. The γ′ precipitation zone showed a distinct etching response coupled with high hardness. The hardness and the temperature range of the γ′ precipitation zone increased with increasing arc heat treatment time. The γ′ radii increased with increasing time and temperature. The γ′ precipitation model, simulated with TC Prisma, showed very good agreement with the experimental results. Finally, the results were used to develop time-temperature precipitation and hardness diagrams.

Microstructure and Mechanical Properties of V-Alloyed Rebars Subjected to Tempcore Process

Two B400B-R and B500B grade rebars were industrially produced through a Tempcore process. The standard chemical composition of B500B grade was additionally alloyed with 0.067 wt.% V to enhance its mechanical properties. A set of optimized processing parameters were applied to manufacture two different diameters D20 (Ø 20 mm) and D32 (Ø 32 mm). The microstructure -mechanical properties relationships were evaluated using optical and scanning electron microscopes, hardness, and tensile testing. In addition, a thermal model was developed to define the thermal cycle evolution during cooling in the quenching &amp; tempering box (QTB) to simulate the kinetics of V(C,N) precipitation. The microstructure observations showed a typical graded microstructure consisting of ferrite-pearlite core and outer tempered martensite ring for both grades of both diameters. The optimized processing parameters for B400B-R of D32 (compared with D20) resulted in softening of the core (from 160 to 135 HV10) and tempered martensite surface (from 220 to 200 HV10) as well as in decreasing the yield strength (from 455 to 413 MPa) and tensile strength (from 580 to 559 MPa). On the contrary, an increase in hardness of the core (from 165 to 175 HV10) and the outer tempered martensite (from 240 to 270 HV10), in addition to an increase in yield strength (from 510 to 537 MPa) at almost the same level of tensile strength of 624–626 MPa are observed for B500B grade D32 compared with D20. The modeling and simulation calculations suggest that the manufacturing D32 rebars of B500B grade involves longer quenching time in the QTB which allow deeper tempered martensite surface along with a relatively higher core temperature that renders faster kinetics and larger volume fraction of V(C,N) precipitates. The current study demonstrates that the full potential of V-alloying can be exploited when a sufficient quenching time at the equalization temperature is achieved, which is valid for D32 rebars.

Microstructural control during laser powder fusion to create graded microstructure Ni-superalloy components

This work explores the feasibility of additively manufacturing tailored microstructures through varying process parameters, to eventually control the mechanical properties and performance. The investigation focuses on controlling the heat input and thermal history during laser-powder bed fusion of IN718 through process parameter manipulation; notably the heat input parameters (power, scan speed, and hatch spacing) and island scanning parameters (island size, shift, and island overlap). The changes in preferred orientation, morphology and grain size were characterised in both the transverse and build cross-sections using scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD), while the texture development was comparatively characterised using X-ray diffraction (XRD). The solidification cell size was quantified to estimate the influence of the process parameters on the cooling rates. This was also rationalised using a thermal model resolving the scan characteristics to provide the transient temperature distribution to a numerical grain growth model. Based on the obtained microstructures, graded microstructures were generated using the island strategy and identical laser parameters throughout but changing subtle features such as the island size and shift. A suitable post-process heat treatment was applied to retain the tailored microstructures, while obtaining the required hardness.

On the comparison of graded microstructures developed through High Reduction (per pass) Cold Rolling (HRCR) and Ultrasonic Nanocrystal Surface Modification (UNSM) in nickel-base Alloy 602CA

The development of graded microstructure in Ni-base Alloy 602CA, processed through the new technique of high reduction (per pass) cold rolling (HRCR) and the severe surface plastic deformation technique of ultrasonic nanocrystal surface modification (UNSM) was investigated. Clear through-thickness gradients in residual stress, dislocation structure and boundary density with misorientation >2° were observed after the HRCR and UNSM. However, the gradient in HRCR did depend on the roll-pass schedule: relative reduction per pass, total reduction and direction of the roll pass. Applying maximum reduction per pass and minimum number of passes exhibited an improved graded microstructure for 40% total reduction in both unidirectional (UR) and cross rolling (CR) cases. The UR40 and CR40 in HRCR process led to graded microstructures that compared well with those produced by UNSM.

A novel asymptotic-analysis-based homogenisation approach towards fast design of infill graded microstructures

Graded microstructures have demonstrated their values in various engineering fields, and their production becomes increasingly feasible with the development of modern fabrication techniques, such as additive manufacturing. With the use of asymptotic analysis, we propose in this article a homogenisation framework to underpin the fast design of devices filled with quasi-periodic microstructures. With the introduction of a mapping function which transforms an infill graded microstructure to a spatially-periodic configuration, the originally complicated cross-scale problem can be asymptotically decoupled into a macroscale problem within a homogenised media and a microscale problem within a representative unit cell. For a given graded microstructure, the stress field and overall compliance computed by the proposed method are shown, both theoretically and numerically, to be consistent with the underlying fine-scale results. Upon linearisation, the computational cost associated with the proposed formulation is found to be as low as that in existing asymptotic-analysis-based homogenisation approaches, where only spatially periodic microstructures are considered. The present framework also exhibits interesting features in several other aspects. Firstly, smooth connectivity within graded microstructures is automatically guaranteed. Secondly, the configuration obtained here is naturally characterised by a finite length scale associated with the resolution of fabrication. The proposed approach effectively reproduces the optimal microstructure for the case of uniaxial loading where explicit solutions are available, and other numerical results are further provided.

Development of gradient microstructure in mild steel and grain size dependence of its electrochemical response

This work discusses a novel technique to develop graded microstructure consisting of fine grains on the surface and coarse grains in the interior. It consisted of preferential surface deformation by placing hardened steel rods and subsequent forging to flatten it out. Using the above technique, four microstructures consisting of different grain sizes were fabricated. Dependence of their corrosion behavior on grain size and boundary fraction was examined using 0.6 M NaCl and 15.6 M HNO3 solutions. Corrosion resistance was found to be lowest for the fine grained steel specimen as compared to that of the specimens with relatively coarser grains.

TiB 2 -based Ceramic/1Cr18Ni9Ti Stainless Steel Composite Produced by Combustion Synthesis in Ultra-high Gravity Field

Abstract Based on fusion bonding and atomic interdiffusion between the ceramic and stainless steel, the TiB2-based ceramic/1Cr18Ni9Ti stainless steel composite with chemical composition gradient was produced by combustion system in ultra-high gravity field (CSUGF). The presence of chemical reaction in explosive combustion and the subsequent thermal-vacuum circumstance induced by liquid products in ultra-high gravity make stainless steel partially fused, resulting in fusion bonding. XRD, FESEM and EDS results show that the interface has a good bonding, and the intermediate exhibits three dimensional network ceramic-metal graded microstructure considered as a result of intensive interdiffusion of atomics. Vickers hardness profile reveals the quasi-parabola relationship of the hardness to the testing distance from the ceramic matrix to the steel substrate. Meanwhile, interfacial shear fracture of the composite presents the mixed mode consisting of intercrystalline fracture along TiB2 platelets and ductile fracture in Fe-Ni-Cr alloy, presenting interfacial shear strength (325±25) MPa.

Elasticity-based microstructural optimization: An integrated multiscale framework

An integrated multiscale computational framework is developed to identify material microstructures which minimize target field variables at the microstructural level aimed at enhancing structural performance. The computational framework is comprised of the Particle Swarm Optimization algorithm, a structural-level analytical solution to a technological problem, and the elasticity-based locally-exact homogenization theory at the microstructural level. The excellent stability and quick convergence of the homogenization theory with the concomitant rapid execution times enables repetitive solutions of the unit cell problem with variable geometric and material parameters within a multiscale framework. The unit cell solution provides accurate homogenized moduli for input into the structural-level analysis, as well as accurate local stress fields for the calculation of objective functions by the optimization algorithm. The integrated multiscale optimization capability is demonstrated in the context of the classical Kirsch problem with an underlying graded microstructure aimed at stress concentration reduction. The results reveal the limitations of stress field optimization in graded materials solely at the homogenized level, pointing to the need for repetitive micro-level stress field recovery from an accurate and efficient homogenization theory within multiscale optimization analysis.

Continuum damage modeling and simulation of hierarchical dental enamel

Dental enamel exhibits high fracture toughness and stiffness due to a complex hierarchical and graded microstructure, optimally organized from nano- to macro-scale. In this study, a 3D representative volume element (RVE) model is adopted to study the deformation and damage behavior of the fibrous microstructure. A continuum damage mechanics model coupled to hyperelasticity is developed for modeling the initiation and evolution of damage in the mineral fibers as well as protein matrix. Moreover, debonding of the interface between mineral fiber and protein is captured by employing a cohesive zone model. The dependence of the failure mechanism on the aspect ratio of the mineral fibers is investigated. In addition, the effect of the interface strength on the damage behavior is studied with respect to geometric features of enamel. Further, the effect of an initial flaw on the overall mechanical properties is analyzed to understand the superior damage tolerance of dental enamel. The simulation results are validated by comparison to experimental data from micro-cantilever beam testing at two hierarchical levels. The transition of the failure mechanism at different hierarchical levels is also well reproduced in the simulations.

Reaction Fusion and 3D-Network Span-Scale Graded Microstructure Evolution in Laminated Composite of TiB2-Based Ceramic and 42CrMo Steel

Based on liquid fusion and atomic interdiffusion of TiB2-based ceramic and 42CrMo steel, the ceramic-metal laminated composites with interfacial 3D-network span-scale graded microstructures were achieved by combustion synthesis in high gravity field. The presence of respective atomic concentration gradient of Ti, B, C, Fe and the others between the ceramic and the steel reasoned for continuously-graded interfacial microstructure in which the TiB2 and TiC phases transform sub-micrometer, micro-nanometer grains from the micrometer ones. The Fe-based liquid flow from the ceramic to the steel substrate resulted in the 3D-network distribution of Fe-based alloy phases from the ceramic to the steel substrate. Hence, interfacial shear fracture presented the mixed mode consisting of intercrystalline fracture along fine TiB2 platelets and ductile fracture in Fe-based alloy phases, presenting interfacial shear strength 415 ± 25 MPa of the laminated composite.

Sintering Behaviour, Graded Microstructure and Corrosion Performance of Sintered Fe-Mn Biomaterials

Sintering Behaviour, Graded Microstructure and Corrosion Performance of Sintered Fe-Mn Biomaterials

Characterization Of The Graded Microstructure In Powder Sintered Porous Titanium

Abstract The proposed sintering process produce porosity and functional graded microstructure in the sinterd titanium powders. Titanium powders with different micro sizes were sintered at the proposed temperature region at 1200 and 1300°C for 2h. The apatite-forming on the graded microstructure is observed by immersion test in Hanks balanced salt soluion at 37°C. Sintering condition of titanium powders is estimated by thermogravitmetry-differential thermal analysis (TG-DTA). The synthersied surface structures and apatite-forming ability were characterized by a field emission scanning electron microscopy (FE-SEM) observation and energy dispersive X-ray spectroscopy (EDS) analysis. As results, these graded microstructure of sintered porous titanium powders reveals apatite-forming ability as osseointegration by calcification in Hanks balanced salt soluion(HBSS) at 37°C.

Ratcheting-based microstructure-sensitive modeling of the cyclic hardening response of case-hardened bearing steels subject to Rolling Contact Fatigue

Rolling Contact Fatigue (RCF) is a commonly observed phenomenon in bearings that imposes a highly localized subsurface volume to a complex triaxial and non-proportional fatigue loading. Case-hardened M50-NiL steel, a widely used aerospace bearing material, when subjected to RCF loading shows a definite increase in hardness within the localized RCF-affected zone as a function of cycles. To simulate such cyclic hardening material response during RCF using finite element (FE) modeling, knowledge of localized cyclic hardening properties of bearing steels is required. However, these properties are not available for through-hardened and case hardened steels nor are they readily measurable from the conventional bulk specimen testing methods used in high cycle fatigue. To overcome this challenge, an alternative method to evaluate the material-specific localized cyclic hardening properties as a function of case-layer depth is presented. This method uses the experimentally measured increase in micro-hardness within the RCF-affected zone, in conjunction with elastic–plastic FE simulation of RCF cyclic hardening, to extract localized cyclic constitutive properties, and investigate the underlying physics associated with the local hardness increase. Case hardened bearing steels are complex composites having a heterogeneous graded microstructure with carbide precipitates ranging from nanometer to micrometer length scales that are largely responsible for the material hardening. Micro-plastic strain accumulation via ratcheting at the scale of the carbide microstructure is proposed to be the underlying mechanism for cyclic hardening. This manuscript presents for the first time a procedure to estimate microstructure-sensitive isotropic and kinematic cyclic hardening parameters, from a judicious combination of numerical and experimental methods. Methods presented are applicable to both through-hardened and case hardened bearing steels.

Functionally graded structural members obtained via the low temperature strain induced phase transformation

The notion of functionally graded materials (FGM) covers all domains of discrete and smooth gradation of material microstructure designed in order to obtain macroscopic features suitable for a given application. A special class of multi-phase materials with graded microstructure can be obtained at cryogenic temperatures as a result of smooth transition from the parent phase to the secondary phase. The required continuously graded material features are obtained at low temperatures via the mechanism of controlled strain induced phase transformation from the purely austenitic to the martensitic lattice (γ → α′). Several families of ductile materials are known to behave in a metastable way when strained at very low temperatures. Among them the austenitic stainless steels are extensively used to construct components of the superconducting magnets, cryogenic transfer lines and other structural members loaded in cryogenic conditions. The constitutive model used to describe mathematically the plastic strain induced phase transformation at low temperatures involves strain hardening where two fundamental effects play an important role: interaction of dislocations with the martensite inclusions and increase in material tangent stiffness due to the mixture of harder martensite with softer austenite. The interaction of dislocations with the martensite inclusions is reflected by the hardening modulus that depends on the volume fraction of martensite. Here, a linear approximation, based on the micro-mechanics analysis, is used. On the other hand, evaluation of the material tangent stiffness of two-phase continuum is based on the classical homogenization scheme and takes into account the local tangent moduli of the components, as postulated by Hill [Hill, R., 1965. A self consistent mechanics of composite materials. J. Mech. Phys. Solids 13, 213–222]. In the present paper, the Mori–Tanaka homogenisation scheme is applied. Both effects contribute to strong nonlinear hardening that occurs as soon as the phase transformation process begins. The material model is suitable for a wide range of temperatures, however the best results are obtained at very low temperatures, where the linearized kinetic law of phase transformation is valid [Garion, C., Skoczeń B., 2002. Modeling of plastic strain induced martensitic transformation for cryogenic applications. J. Appl. Mech. 69 (6), 755–762]. As the application field the structural members in the form of rods (cylinders) of circular cross-section, used as parts of the carrying structures, are analyzed. The required graded microstructure of the material is obtained by imposing torsion at cryogenic temperatures. Both the intensity of the phase transformation and the depth of the transformed zone is obtained by suitable kinematic control (angle of twist). The closed form solutions for the stress state and torque as a function of the angle of twist are shown.