Random Crystallographic Orientation Research Articles

Insight into the oxidation behavior and excellent internal oxidation and nitridation resistance of Fe71.5-x(Ni, Cr, Ti)28.5Alx complex concentrated alloys by advanced characterization

For precipitate-strengthened alloys, the decomposition of the precipitates at the subsurface during high-temperature oxidation is detrimental to mechanical properties. In this study, we adjusted Al concentrations in Fe71.5-x(Ni, Cr, Ti)28.5Alx (x = 8, 12, and 16 at. %) complex concentrated alloys (CCAs) to precipitate elongated cuboidal L21 with higher density and Al concentration, as well as reduced inter-precipitate spacing. This is more conducive to the rapid diffusion of Al to the matrix and grain boundaries, promoting the formation of a continuous and compact Al2O3 layer with random crystallographic orientation at 800 °C in air. This not only inhibits the further decomposition of the L21 phase but also avoids internal oxidation and nitridation.

Oriented Acicular Rutile Inclusions in Eclogites: Exsolutions From Majoritic Garnet or Shock Needles?

AbstractOriented rutile needles (ORNs) forming a triangular network on the cross sections of garnet crystals have been widely used together with omphacite inclusions as evidence for exsolution from a majoritic garnet and exhumation of the host rocks from great depths (>200 km) in the Earth. A coronitic eclogite at Yangkou in the Chinese Su‐Lu high‐pressure metamorphic belt contains ORNs that are only found in the reddish cores of garnet porphyroblasts. The texture formed by the ORNs is not restricted to garnet but extends into the coexisting other minerals, which together form pseudomorphs after augite. Therefore, the ORNs are not specifically related to the host garnet and cannot be exsolutions therefrom. The outer zones of the garnet porphyroblasts in contact with plagioclase pseudomorphs are pale and rutile‐free but contain minute inclusions of omphacite, quartz, kyanite, phengite, and K‐feldspar, typical of coronitic garnet between augite and plagioclase. Electron backscatter diffraction reveals no optimum matching of the low index crystallographic directions of rutile and garnet as required by an exsolution mechanism. On the other hand, the ORNs resemble the amorphous lamellae in quartz and zircon in meteorite and seismic shocked rocks, and are inferred to have crystallized earlier in seismic shocked augite and were then overgrown by the host minerals. By contrast, the rutile particles in garnet cataclasites in a nearby eclogite breccia display deformed and explosive patterns and random crystallographic orientations. All these observations are best explained by the seismic shock compression and rarefaction scenario proposed earlier.

Unveiling the Microstructural Features during Compression of a High-Modulus Mg-15Gd-8Y-6Al-0.3Mn Alloy Reinforced by a Large Volume of Al2RE Phases

Magnesium alloys with a high volume fraction of secondary phases exhibit inferior formability. Therefore, investigating their thermal deformation characteristics is critical for optimizing thermal processing techniques. In this work, isothermal compression experiments were performed on a Mg-15Gd-8Y-6Al-0.3Mn alloy with an elastic modulus of 51.3 GPa with a substantial volume of aluminum-rare earth (Al2RE) phases. The rheological behavior and microstructural evolution of the material were systematically investigated at varying temperatures (350–500 °C) and strain rates (0.001–1.000 s−1). The calculated thermal processing diagram indicates that the unstable region gradually enlarges with increased strain, and all unstable regions appear within the high-strain-rate, low-temperature domain. The ideal thermal processing range of the alloy is 350–500 °C at strain rates ranging from 0.001 to 0.016 s−1. Particle-stimulated nucleation and discontinuous dynamic recrystallization are both verified to be responsible for the recrystallized microstructure of the alloy. The recrystallized grains exhibit a relatively random crystallographic orientation. As recrystallization proceeds, the texture gradually transitions from a typical [0001] texture in the compression direction to a random texture accompanied by decreased texture intensity. This work sheds new light on the thermo-mechanical processing of high-modulus Mg alloys, which could help design suitable processing techniques for related materials.

Investigation of constitutive models and microstructure evolution during hot deformation and solution treatment of Al–Zn–Mg–Cu alloy

In this paper, isothermal thermal compression tests were carried out on Al–Zn–Mg–Cu alloys at varying deformation temperatures (360°C∼440°C) and strain rates (0.001s−1∼10s−1) using a Gleeble-3500C thermal simulation tester. Based on the true strain stress data obtained from the tests, the flow behavior of Al–Zn–Mg–Cu alloys during thermal deformation was investigated. Through a comprehensive consideration of deformation temperature, strain rate, and strain compensation, a high-precision constitutive model was established. Additionally, the specimens subjected to hot compression were further treated with solution annealing for different durations (0 h–2 h) to investigate the static recrystallization (SRX) behavior and the associated microstructural evolution of the alloy during the solution treatment process. Electron Backscatter Diffraction (EBSD) was employed to characterize the microstructure evolution of Al–Zn–Mg–Cu alloy under various deformation and solution treatment conditions. The EBSD data were further processed and analyzed using the MTEX toolbox. The results revealed that dynamic recovery (DRV) is the predominant softening mechanism during the deformation process, accompanied by a small amount of dynamic recrystallization (DRX). The dynamically recrystallized grains exhibit a random crystallographic orientation. Further investigation showed that low temperature and high strain rate conditions are unfavorable for the occurrence of dynamic recovery and dynamic recrystallization during the thermomechanical processing. However, dislocations accumulated during deformation provided sufficient stored energy for the growth of static recrystallized grains during subsequent solution treatment, with static recrystallized grains predominantly forming at grain boundaries.

Strengthening and precipitation hardening mechanisms of surface-mechanically treated 17-4PH stainless steel

Achieving ultra-high strength without sacrificing too much ductility is the focus of attention in nanostructured materials. Here, the strengthening mechanism and property enhancement of surface-mechanically treated 17-4PH stainless steel (SS17-4PH) were investigated. Our findings show that a grain refinement and elongated lath-like martensitic grain (~ 50 nm thick) could be produced after surface treatment. The grain size remains in the nanoscale, and random crystallographic orientations with the presence of nanocrystallites characterize the nanocrystalline grains formed on the treated sample. This contributes to the property enhancement with a yield strength of about 901 MPa and a reduced elongation to failure of about 17%. The atom probe tomography (APT) characterization unveiled the emergence of high-density precipitate (Cu-rich) at the material surface, with a number density of about 2.6255 × 1024 m−3 and an average radius of 2.22 nm. Besides, the dislocation activities caused by SMAT result in the gradual breakdown of precipitates into smaller sizes and final dissolution in the matrix, increasing the number of nucleation sites and leading to more grain refinement processes. The grain boundary, dislocation densities, and the Cu-rich precipitate greatly influence the strengthening mechanism of surface-treated SS17-4PH.

Phase Field Fracture Modelling of Crack Initiation and Propagation in Dual-Phase Microstructures

Dual-phase (DP) steel's spread in the industry has been fueled by its highly desirable qualities, which include a unique balance of strength and ductility, making it a valuable material for applications ranging from automotive manufacturing to construction. Despite the continued progress in the research of DP steels, the complex failure mechanisms at the microscale, resulting from the coexistence of brittle martensite phases within the ductile ferrite phase, remain an area of ongoing exploration. This study aims to investigate the micro structural evolution, as well as crack initiation and propagation, in DP steels at the microscale. In this context, to accurately capture the microstructural evolution of DP steels, a rate-dependent crystal plasticity formulation is utilized for the ferrite phase alongside a phenomenological isotropic J2 plasticity model for the martensite phase. A novel ductile phase-field fracture framework has been implemented to predict crack initiation and propagation, integrating both crystal plasticity and J2 plasticity constitutive models with the phase field fracture model. Generic 3D polycrystalline Representative Volume Elements (RVEs) are created and simulated to analyze the trends influencing the failure behavior of DP steels. The numerical study includes examples with two different martensite volume fractions (15% and 37%), each characterized by varying random crystallographic orientation sets and morphologies. The obtained results suggest that despite the similarity in stress concentration regions within two different random orientation sets in a simulation with fixed volume fraction and morphology, the resulting crack paths exhibit significant differences. Additionally, it is inferred that damage appears to accumulate at the junction points of the martensite islands. Moreover, an increase in the martensite volume fraction is found to diminish the influence of crystallographic orientation on the resultant crack path. Therefore, given the aforementioned findings, it is essential to analyze the microstructural evolution, as well as crack initiation and propagation in DP steels, utilizing appropriate material and fracture modeling frameworks at the microscale.

Gadolinia Doped Ceria with Engineered Grain Boundaries as Potential Electrolyte Material for Thin-Film Solid Oxide Fuel Cells

Thin-film solid oxide fuel cells are receiving increasing attention within the scientific community thanks to their high energy density and high efficiency in different temperature ranges. To be utilized as power supply systems for portable electronic devices, thin-film solid oxide fuel cells must demonstrate high power supply and low-temperature operation capabilities. Both capabilities are highly dependent on the function of the electrolyte layer, commonly comprised of ceramic mixed-ionic-electronic conductors [1]. The state-of-the-art thin-film solid oxide fuel cells utilize yttrium stabilized zirconia as an electrolyte material, despite its relatively low ionic conductivity within the typical working-temperature range of solid oxide fuel cells (up to 1000 °C). Moreover, the ionic conductivity of yttrium stabilized zirconia dramatically reduces with decreased temperature, limiting its performance as an electrolyte material for low-temperature (below 600 °C) solid oxide fuel cells [2].Gadolinia doped ceria is an alternative promising electrolyte material for low temperature solid oxide fuel cells. Its relatively high ionic conductivity below 600 °C makes it attractive as an electrolyte material although its partial electronic conductivity is a significant drawback [3]. This drawback is more pronounced in ultra-thin layers (sub-micron scale) solid oxide fuel cells devices. As a result, gadolinia doped ceria have been utilized only to a limited extent for thin-film solid oxide fuel cells.The issue of partial electronic conductivity might be resolved thanks to a non-conventional deposition technique of polycrystalline gadolinia doped ceria thin films. The proposed sputtering technique includes metallic alloy target and results in a granular morphology, as opposed to the columnar morphology obtained using the conventional sputtering method. As electrical conductivity of polycrystalline thin films is higher along the grain boundaries [4], gadolinia doped ceria layers with random crystallographic orientation are expected to be less electronically conductive. Here, we investigated the integration and the performance of granular gadolinia doped ceria as an electrolyte material for thin-film solid oxide fuel cells. The electronic-conduction properties along the grain boundaries of gadolinia doped ceria were studied, and a thin-film solid oxide fuel cell prototype was fabricated. Such devices with engineered grain boundaries hold the potential for low-temperature operation, high power output, and silicone-compatibility for novel integration opportunities and bulk-fabrication.

Analysis of the influence of a pseudo-random number generator type on the kinetics of the cellular automata recrystallization model

Models based on discrete simulation methods, i.e., cellular automata (CA) or Monte Carlo (MC), often contain probabilistic elements in the code to capture the stochastic character of material behaviour. They can be related to various physical aspects reflected in the model, e.g., heterogeneous energy distribution or random crystallographic orientation assignment to subsequent grains. Therefore, the paper's main aim is to analyze the influence of such random elements in the CA recrystallization model used to predict the material microstructure morphology after thermo-mechanical treatment. The first part of the work presents the available tools for pseudo-random number generation and the main elements of the investigated CA recrystallization model. Different variants of input data preparation prior to the simulation, including deformation energy distribution, are shown. The role of the investigated pseudo-random number generators (rand, lcg, mt, ranlux) and their distribution (uniform, normal, lognormal) is highlighted in this part. Finally, the input data are used to perform simulations of recrystallization progress, and the effect of pseudo-random number generation parameters on transformation kinetics and growth of recrystallized grains is evaluated. With this work, we have demonstrated that the choice of the generator type, particularly the method of its distribution, influences the simulation results in the form of final material morphology.

The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals

Additively manufactured (AM) metals exhibit highly complex microstructures, particularly in terms of grain morphology which typically features heterogeneous grain size distribution, irregular and anisotropic grain shapes, and the so-called columnar grains. The conventional morphological descriptors based on grain shape idealization are generally inadequate for representing complex and anisotropic grain morphology of AM microstructures. The primary aspect of microstructural grain morphology is the state of grain boundary spacing or grain size whose effect on the mechanical response is known to be crucial. In this paper, we formally introduce the notion of axial grain size from which we derive mean axial grain size, effective grain size, and grain size anisotropy as robust morphological descriptors capable of effectively representing highly complex grain morphologies. We instantiated a discrete sample of polycrystalline aggregate as a representative volume element (RVE) featuring random crystallographic orientation and misorientation distributions. However, the instantiated RVE incorporates the typical morphological features of AM microstructures including distinctive grain size heterogeneity and anisotropic grain size owing to its pronounced columnar grain morphology. We ensured that any anisotropy observed in the macroscopic mechanical response of the instantiated sample primarily originates from its underlying anisotropic grain size. The RVE was then employed for mesoscale full-field crystal plasticity simulations corresponding to uniaxial tensile deformation along various axes via a spectral solver and a physics-based crystal plasticity constitutive model which was developed, calibrated, and validated in earlier studies. Through the numerical analyses, we isolated the contribution of anisotropic grain size to the anisotropy in the mechanical response of polycrystalline aggregates, particularly those with the characteristic complex grain morphology of AM metals. This contribution can be described by an inverse square relation.

Fabrication of TiB-reinforced titanium matrix composite via spark plasma sintering and vertical bidirectional reactive hot rolling

TiB/Ti6Al4V composite was prepared using spark plasma sintering followed by bidirectional reaction hot rolling. The TiB whiskers were synthesized in situ during the rolling process, thereby preserving a relatively high structural integrity. TiB whiskers were distributed within the composite in three distinct locations: intragranular, intergranular, and transgranular distribution. After eight passes of vertical bidirectional reaction hot rolling, the average grain size of the matrix was refined to 1.36 μm, and the grains exhibited random crystallographic orientations. Tensile properties in different directions demonstrated high consistency, with a strength of 1220 MPa ± 30 MPa and an elongation of 8.0% ± 0.5%. The primary strengthening mechanisms in the composite included grain refinement, dislocation strengthening, and load transfer strengthening from the TiB.

Novel composition of Nd-Fe-B gas atomized powder to produce compression bonded magnets

New compositions of Nd-Fe-B powders were produced via gas atomization. Helium and argon were used as atomizing gases. For the same process parameters, helium resulted in a powder that is 6 times finer than the powder produced with argon. In both cases, the atomization conditions were set to achieve fine powders and, therefore, fine microstructures, since this is critical to obtain a suitable coercivity. The powders were sieved to separate the fraction below 20 μm (∼90 % of powder atomized with He). The microstructure is formed by a mix of elongated and equiaxial grains with random crystallographic orientation and an average grain size of ∼ 1 μm, which was measured by Electron Backscattered Diffraction. This kind of measurements and images were taken for the first time for fine helium atomized Nd-Fe-B powders. The embedding of the powder in copper was found to be useful, as the conventional mounting in metallographic conductive resin produced charging of the sample. Annealing up to 1000 °C did not cause any significant grain growth that could deteriorate the magnetic properties. Annealing at 700 °C for 15 min improved the magnetic properties due to the crystallization of residual amorphous phase. Warm compaction was used to produce magnets with a high volume fraction of magnetic powder (>70 %) and low real porosity (∼5 %). These magnets exhibit a high remanence (∼0.5 T) and maximum energy product (∼35 kJ/m3), combined with a suitable intrinsic coercivity (>500 kA/m). The new compositions and processing route could cover a market niche. The thoroughly analyzed powders could be also suitable for some additive manufacturing technologies that require spherical powders with very specific particle size distributions.

Microstructural Evolution, Mechanical Properties and Tribological Behavior of B4C-Reinforced Ti In Situ Composites Produced by Laser Powder Bed Fusion.

Based on the advantage of rapid net-shape fabrication, laser powder bed fusion (LPBF) is utilized to process B4C-reinforced Ti composites. The effect of volumetric energy density (VED) on the relative density, microstructural evolution, tensile properties and wear behaviors of B4C-reinforced Ti composites were systematically investigated. The LPBF-ed samples with high relative density (>99%) can be achieved, while the pores and un-melted powders can be observed in the sample owing to the low energy input (33 J/mm3). The additive particulates B4C were transformed into needle-like TiB whiskers with nano-scale while C dissolved in the Ti matrix. Fine-scale grains (<10 μm) with random crystallographic orientation can be achieved and the residual stress shows a downtrend as the VED increases. Through the analysis of the tensile and wear tests, the sample at 61 J/mm3 VED showed a good combination of strength and wear performance, with an ultimate tensile strength of 951 MPa and a wear rate of 3.91 × 10-4 mm3·N-1m-1. The microstructural evolution in VED changes and the corresponding underlying strengthening mechanisms of LPBF-ed Ti + B4C composites are conducted in detail.

Influence of Process Parameter and Alloy Composition on Misoriented Eutectics in Single-Crystal Nickel-Based Superalloys.

The nucleation and the growth of misoriented micro-structure components in single crystals depend on various process parameters and alloy compositions. Therefore, in this study, the influence of different cooling rates on carbon-free, as well as carbon-containing, nickel-based superalloys was investigated. Castings were carried out using the Bridgman and Bridgman-Stockbarger techniques under industrial and laboratory conditions, respectively, to analyze the impact of temperature gradients and withdrawing rates on six alloy compositions. Here, it was confirmed that eutectics could assume a random crystallographic orientation due to homogeneous nucleation in the residual melt. In carbon-containing alloys, eutectics also nucleated at low surface-to-volume ratio carbides due to the accumulation of eutectic-forming elements around the carbide. This mechanism occurred in alloys with high carbon contents and at low cooling rates. Furthermore, micro-stray grains were formed by the closure of residual melt in Chinese-script-shaped carbides. If the carbide structure was open in the growth direction, they could expand into the interdendritic region. Eutectics additionally nucleated on these micro-stray grains and consequently had a different crystallographic orientation compared with the single crystal. In conclusion, this study revealed the process parameters that induced the formation of misoriented micro-structures, which prevented the formation of these solidification defects by optimizing the cooling rate and the alloy composition.

Electrochemical-mechanical coupling failure mechanism of composite cathode in all-solid-state batteries

Composite cathode composed of active particles and solid electrolytes (SEs) can considerably enlarge the particle-SE contact areas and achieve high areal loadings in all-solid-state batteries (ASSBs). However, the challenging interfacial instability and particle damage problems remain unsolved. Herein, we establish a 3D electrochemical-mechanical coupled model to investigate the underlying failure mechanism by considering the governing electrochemical and physics processes. Micro-scale heterogeneous primary particles with random crystallographic orientation and size inside the LiNi1/3Co1/3Mn1/3O2 (NCM111) secondary particle of the model result in the anisotropic Li diffusion and volume variation within the secondary particle, leading to significant nonuniformity of the Li concentration, and GPa-level stress distributions at primary particle boundaries, and finally causing the particle internal cracks. The particle volume shrinkage under the constraint of stiff Li7La3Zr2O12 (LLZO) SE triggers the interface debonding (gap>50 nm) with increased interfacial impedance to degrade cell capacity. Higher C-rates result in larger residual stress (∼100 MPa)/strain/debonding gap at discharging end, more likely to deteriorate the cell performance. Increasing the interfacial strength between the particle and SE can suppress the interface debonding but induces high stress (up to 10 GPa). Results reveal the underlying mechanism of the electrochemical-mechanical coupling failure mechanism for composite cathode and provide promising guidance on the further improvement of a more robust composite cathode for ASSBs.

Solid‐State Catalytic Hydrogen Sponge Effects in BaInO2.5 Epitaxial Films

AbstractGas sponges capable of absorbing, storing, and releasing ions in a reversible manner are in high demand for advanced electronics, energy devices, and sensors. Here, it is shown that brownmillerite BaInO2.5 epitaxial films exhibit the capability to act as solid‐state catalytic hydrogen sponges at a remarkably low temperature (≈100 °C). Compared to sintered pellets with random crystallographic orientations and many defects, BaInO2.5 epitaxial films give three orders of magnitude higher conductivity of hydrogen ions, which can be linked to the presence of well‐ordered 1D channels and lack of grain boundaries and other defects. Using scanning transmission electron microscopy, it is shown that hydrogen ions can be stored near InO4 tetrahedral layers of brownmillerite without destroying the parent framework. The high performance and reproducibility of the BaInO2.5 epitaxial films coupled with ultralow power consumption make them ideal candidates for neuromorphic devices and beyond.

Texture and High Yield Strength of Rapidly Solidified AZ31 Magnesium Alloy Extruded at 250 °C.

In this study, commercial AZ31B magnesium alloy was used to compare the differences between the microstructure, texture, and mechanical properties of conventional solidification (as homogenized AZ31) and rapid solidification (as RS AZ31). The results demonstrate that a rapidly solidified microstructure leads to better performance after hot extrusion with a medium extrusion rate (6 m/min) and extrusion temperature (250 °C). The average grain size of as-homogenized AZ31 extruded rod is 100 μm after annealing and 4.6 μm after extrusion, respectively, but that of the as-RS AZ31 extruded rod is only about 5 μm and 1.1 μm, correspondingly. The as-RS AZ31 extruded rod attains a high average yield strength of 289.6 MPa, which is superior to the as-homogenized AZ31 extruded rod, and is improved by 81.3% in comparison. The as-RS AZ31 extruded rod shows a more random crystallographic orientation and has an unconventional weak texture component in <112¯1>/<202¯1> direction, which has not been reported yet, while the as-homogenized AZ31 extruded rod has an expected texture with prismatic <101¯0>/<1¯21¯0>//ED.

Processability and characterization of A20X aluminum alloy fabricated by laser powder bed fusion

The processing window for laser powder bed fusion (LPBF) of A20X aluminum alloy was determined to increase the build rate and reduce the fabrication cost for this material. The influence of hatch spacing, laser scan speed, and powder layer thickness on the microstructure and mechanical properties were also systematically investigated. Over a wide processing window (40–115 J/mm3 energy density), near fully dense and crack-free components were achieved. In all cases, the as-built microstructure consisted of an aluminum matrix with TiB2 particles and Al2Cu precipitates, where volume fraction and size of both TiB2 and Al2Cu phases were found to be independent of the LPBF process parameters. A fine equiaxed grain structure with a random crystallographic orientation was observed in the vertical section (XZ-plane) of the as-built samples. This led to an isotropic elastic modulus, ultimate tensile strength, and total elongation. The yield strength, however, was found to be anisotropic, exhibiting a 16 % difference for different orientations. The average grain size decreased 54 % due to increased scan speed and reduced powder layer thickness. This resulted in a remarkable improvement in the alloy micro-hardness value from 108 HV to 124 HV, where the experimental data obeyed the Hall-Petch relationship.

High-harmonic generation in polycrystalline CdTe nano-films via macroscopic investigations.

Bright high harmonics generation (HHG) in CMOS-compatible nano-films can provide new opportunities for integrated coherent ultra-violet sources and attosecond photonic devices. Up to now, most HHG studies have been limited to single crystals. Polycrystalline materials, which consist of many grains separated by grain boundaries and normally have random crystallographic orientations, have rarely been explored for HHG. Understanding and predicting the HHG properties in polycrystalline nano-films are important owing to its merits of low cost and diversified properties, but challenging due to their complicated electronic structures. Here, we for the first time experimentally discover the correspondence between HHG in polycrystalline matters and macroscopic material parameters, to the best of our knowledge. Pumped by a mid-infrared femtosecond laser centered at 7.1 µm wavelength, bright and long-term stable harmonics extending to 25th orders (284 nm) are demonstrated in polycrystalline cadmium telluride (CdTe) nano-films. It is found that the HHG strengths in the transmission and the reflection behave differently as a function of the material thickness in the range from 6 nm to 4 µm, which is highly correlated to the measured macroscopic conductivity. The experimental findings agree well with the recent theoretical prediction [Phys. Rev. B103(15), 155426 (2021)10.1103/PhysRevB.103.155426]. This work provides a simple gauge to study and predict HHG in complicated polycrystalline and amorphous nano-systems, and paves the way for novel strong-field nanophotonics based on polycrystalline nano-films.

Investigation on intragranular and intergranular void growth and their competition in polycrystalline materials

In polycrystalline materials, void growth is strongly dependent on its surrounding microstructural environment such as grain morphologies, crystallographic orientations and void-grain boundary (GB) relative position. In this study, two kinds of void-GB relative positions are considered to investigate the orientation effect on void growth in FCC polycrystals: intragranular type, i.e., void located in interior of a grain; and intergranular type, i.e., void located at boundary of two adjacent grains. The analysis of orientation effect in intragranular (resp. intergranular) cases is conducted through a comparative study with single-crystal (resp. bi-crystal) counterparts. Moreover, the competition between the intragranular and intergranular void growth behaviors is statistically investigated for the first time. To this end, the representative volume element (RVE) of polycrystalline aggregates containing either intragranular or intergranular void is created by 3D Voronoi tessellation. Multiple realizations with different grain morphologies and crystallographic orientation permutations are implemented using crystal plasticity finite element (CPFE) simulations. We show that the mechanism for orientation effect on void growth in single crystals (resp. bi-crystals) is not preserved in intragranular (resp. intergranular) cases. According to the statistical analysis, both the intragranular and intergranular void growth follows a Gaussian distribution, whose profile is affected by both macroscopic stress triaxiality and Lode parameter. It is revealed that the statistical void growth distribution induced by random grain morphologies and crystallographic orientations is closely associated with the void-GB relative position (intragranular or intergranular). The present study may provide much richer insights into the mechanism of damage evolution in polycrystalline metals.

Characterisation of heat-treated gradient alloy steel fabricated by laser melting deposition

A gradient alloy steel prepared by laser melting deposition (LMD) was subjected to two heat treatments (quenching-intercritical quenching denoted as Q-IQ and quenching-intercritical quenching-tempering denoted as Q-IQ-T). Results showed that martensite microstructure was changed to a duplex microstructure of martensite and ferrite after heat treatment, with a small number of Cr23C6 phases appearing. The samples before and after heat treatment exhibited random crystallographic orientation, while the grain was refined and the fraction of Cr23C6 increased after heat treatment. The samples after heat treatment possessed more slip systems and exhibited a decrease in hardness and wear resistance in comparison with LMDed sample. The corrosion resistance decreased in the order of Q-IQ > Q-IQ-T > LMDed samples in 3.5 wt-% NaCl solution.