Alloy Single Crystals Research Articles

Effects of machining parameters on Ni-Mn-Ga-based alloys for fabrication of multifunctional micro devices using femtosecond pulse width laser

Ni-Mn-Ga-based magnetic shape memory (MSM) alloy single crystals are known for their large magnetic-field-induced strain (MFIS). This quality makes them a promising material for use in micro actuators and devices. However, the manufacturing of single-crystal-based Ni-Mn-Ga micro actuators is challenging due to their high brittleness and other material properties – numerous machining techniques that are successfully used for the deep engraving of conventional engineering materials cannot be directly applied to Ni-Mn-Ga-based alloys. Nevertheless, previous studies have shown that a femtosecond pulse width laser (FPWL) can be successfully utilized for the defect-free micromachining various materials. This work studies the effects of different engraving parameters and introduces a novel scanning-based method for the deep micromachining of Ni-Mn-Ga-based MSM alloys with maximum surface quality. Results show that a 4-layer strategy with a 0.01 mm hatch distance provides excellent machining in terms of surface quality and dimensional accuracy. This study can be utilized within design stages to estimate minimum margin based on required machined depth and avoid defects that occur in the sample preparation stage. Additionally, evolution of structures generated by FPWL machining are characterized. The results highlight how FPWL can be considered a highly capable process for the micromachining and surface structuring of Ni-Mn-Ga-based single crystals for manufacturing multifunctional MSM microdevices.

Atomic-scale study of linear reciprocating friction of TCP/γ phase in nickel-based single crystal alloy

In order to systematically investigate the role of TCP (topologically close-packed) phases in the fretting wear process of nickel-based single crystal alloys (NBSC), this study employed molecular dynamics to conduct comparative analyses of mechanical properties, atomic displacements, wear depth, defects, dislocation density, and the influence of temperature under constant load on the friction process in material wear. The research revealed that during the repetitive friction process, the friction force exhibited a peak at the extreme positions of reciprocating friction on the workpieces, and this peak increased with the number of friction cycles. The dislocation density in the worn area increased, resulting in hardening, and the removal rate of material decreased. At the initial stages of friction, the presence of interfaces notably hindered the transfer of temperature, defects, and atomic displacements in the workpiece, and this inhibitory effect weakened with an increasing number of friction cycles. The TCP phases experienced stratification due to the overall deformation they underwent. Furthermore, as the relaxation temperature increased, the workpiece exhibited enhanced plastic deformation capacity, an increase in dislocation density, and adhesion between abrasive particles and the grinding ball occurred.

Fabrication of single-crystal Fe-Mn-Al-Cu alloys by cyclic heat treatments

In general, single crystal shape memory alloys exhibit superior shape memory effect and superelasticity when compared to their polycrystalline counterparts. However, it is challenging and costly to prepare a single crystal in traditional ways, such as the Czochralski method and directional solidification technique. Recently, cyclic heat treatment (CHT) has emerged as a facile and cost-effective method to fabricate single crystal by inducing abnormal grain growth. In the present work, we aimed to fabricate Fe-Mn-Al-Cu single crystals utilizing the CHT technique. Single crystal Fe-Mn-Al-Cu shape memory alloys were successfully prepared, and the effect of CHT parameters on abnormal grain growth was investigated. It was revealed that increasing the cooling rate resulted in a non-monotonic response in the maximum grain size, which initially increased and was followed by a subsequent decrease. This behavior was attributed to the subgrain refinement, increase in subgrain misorientation angles, and the narrowing of the low density zones (LDZs) of subgrains. Decreasing the lower temperature of CHT caused similar trends in maximum grain size, due to the increase in subgrain misorientation angle and the expansion of the LDZs. Extending the holding time at the lower temperature of CHT had minimal impact on grain size. Increasing the heating rate of CHT resulted in a reduction in the maximum grain size because of the narrowing of the LDZs. Moreover, the addition of 2 wt% Cu led to a reduction of the maximum grain size, attributing to the decreased solvus temperature of the γ phase as a result of Cu addition. Consequently, the γ phase and subgrains were refined, and the LDZs were narrowed.

The micro-structure and mechanical performance of Nickel-based single crystal alloy

Because of the increasing temperature in front of turbine in aero-engine field, Ni-base single crystal alloys are of high demand, which achieve high strength at high temperature. Therefore, a variety of methods have been developed to detect or describe nickel-base single crystal alloys. This article will take stock of the alloy valve and the mainstream description of valve methods. This paper will describe the basic micro-mechanism of rafting. Then the channel width method, which is the most widely used method to describe the degree of rafting, is introduced. After that, the updated version of the channel width method, the method to describe the rate of rafting and the effect on its strength, and the related methods of image processing are described. It is hoped that this paper can help readers to have a preliminary understanding of the principle and mathematical description method of nickel-base alloy rafting. There are many branches of mathematical description, but the main ideas are very similar. It is hoped that the researchers concerned will be able to develop some mathematical models that require fewer measurement parameters but not reducing the overall accuracy of the measuremen.

Selective Laser Melting of Non-Weldable Nickel Superalloy: Microstructure, Cracks and Texture

Additive manufacturing, particularly selective laser melting, presents exciting possibilities for fabricating components from high-temperature nickel-based superalloys. Controlling microstructural features and minimizing defects in fabricated specimens are critical challenges. This study explores the influence of process parameters on microstructure and defect formation in directionally solidified nickel-based superalloy specimens. We conducted a comprehensive analysis of selective laser melting process variables, including interdendritic spacing, crystallization times, and volumetric energy density. Electron backscatter diffraction analysis was employed to assess the feasibility of obtaining a directional structure in single-crystal nickel-based heat-resistant alloy specimens using selective laser melting. The study shows a significant correlation between reduced interdendritic spacing and increased defect formation. Longer crystallization times and higher volumetric energy density lead to decreased defect volumes and sizes. Electron backscatter diffraction analysis confirms the maintenance of preferential growth direction across subsequent layers. Our research underscores the importance of optimizing selective laser melting parameters, balancing refractory elements in alloy composition, and adopting strategies for enhancing crystallization times to minimize structural defects. This comprehensive approach ensures both heat resistance and minimal defects, facilitating the production of high-quality components. These findings contribute to advancing selective laser melting applications in critical industries like aerospace and power generation, where heat-resistant materials are paramount.

Elastic constants of equiatomic Fe–Ni Invar alloy single crystal

In a disordered fcc structure, the equiatomic Fe–Ni alloy is of great interest as a soft magnetic Invar alloy with a near zero or low thermal expansion coefficient. In the L10 ordered phase, it is of interest as a potential high-performance permanent magnet alloy. In this work, the first detailed measurements of the elastic constants in a [001]-oriented Fe-50 at. % Ni alloy single crystal under a no magnetic field condition were made using the resonant ultrasound spectroscopy technique. The elastic constants measured allow one to understand the nature of atomic bonding in a material and the mechanical response to applied stress. The single crystal sample having a rectangular parallelepiped shape was prepared from a single crystal grown using the vertical Bridgman technique. The elastic constants C11, C12, and C44 were obtained from the resonant frequencies observed over a frequency range of 100–900 kHz. Using these C11, C12, and C44 values, Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio were calculated to be 99.1 GPa, 73.4 GPa, 176.7 GPa, and 0.20, respectively. The large anisotropy factor of 3.29 highlights the strong directional dependence of the material’s mechanical behavior.

Tensile Properties of DD15 Single Crystal Superalloy at Different Temperature

DD15 single crystal nickel-based alloy was prepared by a directional solidification induction heating furnace and its tensile properties were investigated at the 23°C, 400°C, 800 °C, 1100°C and 1140°C. The tensile fracture mechanism of the alloy at different temperature was investigated. Results indicate that tensile strength of the alloy reduces first, increases afterward, and decrease sharply at last as the test temperature rises, while the tensile plasticity exhibit the reverse change trend. The extension strength of the alloy is superior to that of EPM-102 alloy. At 23°C, 400°C and 800°C, the tensile fracture mode of the alloy is quasi-cleavage mechanism, and at 1100°C and 1140°C that is dimple model. As tensile fracture at 23°C, 400°C and 800°C, the γ′ precipitate nearly keeps cube shape, while that becomes rectangular after tensile fracture at 1100°C and 1140°C. The APB and stacking fault can be seen in the γ′ phase at 23°C, 400°C and 800°C. There is dislocation networks at the γ/γ′ interface and the deformation characteristic is dislocation by-passing mechanism at 1100°C and 1140°C.

Atomic insights into the effects of Al element on the nanoindentation behavior of single-crystal FeNiCoCr-based multicomponent alloys

The effects of added Al element on the nanoindentation behavior of single-crystal FeNiCoCr-based multicomponent alloys (MCAs) loaded along different crystal orientations, were investigated via using molecular dynamic (MD) simulation. It is found that the added Al with a composition from ∼2.44 to ∼11.1 at. percentage, decreases the stacking fault energy (SFE) and enhances the lattice distortion of the face-centered cubic (FCC) alloys, accordingly affecting the dislocation-medium plastic behavior. Especially, via using the Hertz elastic theory to fit the load-displacement curve, the onset of incipient plasticity triggered by first dislocation nucleation exhibits a significant dependence on the Al concentration. As increasing the Al contents, the critical force for incipient plasticity decreases, mainly owing to the Al-enhanced lattice distortion reducing the atomic displacement necessary for homogeneous dislocation nucleation. Moreover, due to the different Schmidt factors and various slip systems activated for plasticity, dislocations are most difficult to be generated when the (111) lattice plane is indented, but easiest when the (100) plane is indented, leading to the orientation-dependence nano-mechanical response of these MCA. This suggests that whether the nanoindentation is easy in one orientation is trivially connected to the crystalline structure, and mostly has nothing to do with the chemical composition altered by the added Al. Besides, due to the lowered SFE value by the Al added to the alloy, the generation and propagation of extended dislocation structures (composed of Shockley partials and in-between stacking faults) that transfer most plastic strains on deformation, are significantly promoted, consequently increasing the plastic zone size beneath the indenter and lowering the nano-harness of the Al-doped specimens. The simulation results verify the strong composition dependence of mechanical properties for single-crystal MCAs and contribute to the composition design of MCAs for real applications.

Unveiling mechanisms of self-healing in CoCrFeMnNi/HfNbTaTiZr dual-phase high-entropy alloys: A molecular dynamics simulation study

This study investigates the mechanical properties and self-healing behavior of FCC CoCrFeMnNi/BCC HfNbTaTiZr dual-phase high-entropy alloy (DP-HEA) multilayers using molecular dynamics simulations. The relaxed DP-HEA multilayers have low hexagonal close-packed (HCP) percentages and decreasing "Other" fractions with increasing thickness. DP-HEA multilayers exhibit ductile behavior under uniaxial tensile loading, with stress increasing linearly until the yield strain and then gradually decreasing. Dislocation motion, twinning in the FCC phase, and BCC to HCP local structural transformation contribute to this behavior. Single crystal FCC CoCrFeMnNi and BCC HfNbTaTiZr alloys show different stress-strain responses, while DP-HEAs exhibit no significant stress drop beyond the ultimate strain. Self-healing is observed in DP-HEAs with 5 and 10 nm thickness, with a transformation from HCP to BCC structures. The DP-HEA with 15 nm thickness shows less pronounced self-healing. The interlayer distance between BCC/FCC interfaces influences the self-healing ability of the BCC phase. These findings enhance our understanding of the mechanical behavior and self-healing mechanisms in DP-HEA multilayers, facilitating the design of advanced materials with improved properties.

Thickness debit effect and multi-factor coupling mechanism of Ni3Al-based single crystal alloy

With the continuous development of aircraft engine generations, single-crystal alloy blades with more complex shapes and reduced wall thickness are urgently requested. However, the deteriorated performance associated with the wall thickness reduction has not been effectively resolved until now. In this research, the creep behavior of a [001]-oriented Ni3Al-based single crystal alloy with a thin-wall thickness was explored under the environment of 980 °C/220 MPa. A multi-factor coupling mechanism was proposed to explain the premature failure of the thin-walled specimen, providing a comprehensive understanding of the reasons behind the occurrence of the thickness debit effect. The study elucidated the influences of surface oxidation, the stress state of TCP phases, pore growth, and crack propagation on the creep performance. The findings serve as valuable theoretical references for the design and related research of future blade materials.

Coalescence of Al0.3CoCrFeNi polycrystalline high-entropy alloy in hot-pressed sintering: a molecular dynamics and phase-field study

Existing hot sintering models based on molecular dynamics focus on single-crystal alloys. This work proposes a new multiparticle model based on molecular dynamics to investigate coalescence kinetics during the hot-pressed sintering of a polycrystalline Al0.3CoCrFeNi high-entropy alloy. The accuracy and effectiveness of the multiparticle model are verified by a phase-field model. Using this model, it is found that when the particle contact zones undergo pressure-induced evolution into exponential power creep zones, the occurrences of phenomena, such as necking, pore formation/filling, dislocation accumulation/decomposition, and particle rotation/rearrangement are accelerated. Based on tensile test results, Young’s modulus of the as-sintered Al0.3CoCrFeNi high-entropy alloy is calculated to be 214.11 ± 1.03 GPa, which deviates only 0.82% from the experimental value, thus further validating the feasibility and accuracy of the multiparticle model.

Synthesis of High Sn Content Ge1-x-ySixSny (0.1 < y < 0.22) Semiconductors on Si for MWIR Direct Band Gap Applications.

A systematic effort has been described to grow ternary Ge1-x-ySixSny semiconductors on silicon with high Sn concentrations spanning the 9.5-21.2% range. The ultimate goal is not only to produce direct band gap materials well into the infrared region of the spectrum but also to approach a critical concentration (yc) for which further additions of Si would decrease─rather than increase─the band gap. This counterintuitive behavior is expected as a result of the giant bowing parameter in the compositional dependence of the band gap associated with the presence of Si-Sn pairs. The growth approach in this study was based on a chemical vacuum deposition method that uses Si4H10, Ge3H8, and SnD4 or SnH4 as the sources of Si, Ge, and Sn, respectively. A fixed Si concentration near x = 0.05-0.07 was chosen to focus the exploration of the compositional space. A first family of samples was grown of Ge-buffered Si substrates. For Sn concentrations y < 0.12, it was found that the samples relaxed their mismatch strain in situ during growth, resulting in high Sn content films that had relatively low levels of strain and exhibited photoluminescence signals that demonstrated direct band gap behavior for the first time. The device potential of these materials was also demonstrated by fabricating a prototype photodiode with low dark currents. The optical studies suggest that the above-mentioned critical concentration is close to yc = 0.2. As the growth temperature was lowered in an effort to reach such values, Sn concentrations as high as y = 0.15 were obtained, but the films grew fully strained with compressive levels as high as 1.7%. To increase the Sn concentration beyond y = 0.15, a new strategy was adopted, in which the Ge buffer layer was eliminated, and the ternary alloy was grown directly on Si. The much higher lattice mismatch between the Ge1-x-ySixSny layer and the Si substrate caused strain relaxation right at the film/substrate interface, and the subsequent films grew with much lower levels of strain. This made it possible to lower the growth temperatures even further and achieve a comprehensive series of strained relaxed samples with tunable Sn concentrations as high as y = 0.21 (and beyond). The latter represent the highest Sn contents in crystalline Ge1-x-ySixSny attained to date and reach the desired yc = 0.2 range. The synthesized films exhibited significant thickness, allowing a thorough determination of composition, crystallinity, morphology, and bonding properties, indicating the formation of single-phase single-crystal alloys with random cubic structures. Further work will focus on optimizing the latter samples to explore the optical and electronic properties.

Crystallographic and dimensional characteristics of structure elements in welds of high-temperature nickel alloy single crystals

Crystallographic and dimensional characteristics of structure elements in welds of high-temperature nickel alloy single crystals

Кристалографічні та розмірні характеристики елементів структури зварних швів монокристалів жароміцних нікелевих сплавів

Кристалографічні та розмірні характеристики елементів структури зварних швів монокристалів жароміцних нікелевих сплавів

Dislocation evolution of a novel Co-Al-W based single crystal alloy with a γ-CoSS/γ’-Co3(Al, W) coherent microstructure around flow stress anomaly temperature from yielding to fracture

In this study, a novel single-crystal (SC) Co-9Al-9W-2Ta-0.02B-0.02Ce alloy with a growth direction of [001] was prepared by selecting crystal method (SCM). The tensile strength, ductility and dislocation evolution around temperatures of the flow stress anomaly from deformation to fracture were investigated. Aged at 900 °C for 100 h, the microstructure of the aged SC alloy is composed of Co solid solution γ-CoSS matrix and nano-scale cuboidal γ’-Co3(Al, W) intermetallic precipitates, while the two phases are coherent. The aged SC alloy exhibits tensile flow stress anomaly at 800 °C, with a peck 0.2% yield strength of 685 MPa. The SC alloy always shows strain-hardening state after yielding and excellent elongation larger than 18.9% from RT to 900 °C. At the stress anomaly temperature of 800 °C, the dislocation evolution is dominated by the 〈001〉 dislocation in the {001} plane of the γ’ phase pining back the dislocation in the {111} plane of the γ' phase in a strain range of ∼5%. With the further increase of the strain until fracture, the dislocation configuration is controlled by formation of the superlattice intrinsic stacking faults (SISF) through the frontal a/3 〈121〉 dislocations extending in the γ' phase. At 900 °C, the mobility of dislocation increases by the strong thermal activation and the Orowan mechanism appears due to the dislocations bowling the γ' phase. Both subsequently, reduce the strength and the strain-hardening effect of the SC alloy.

Evaluation of neural network attribution methods for microstructural representation of aero-engine material

Efficiency and accuracy of evaluation for the key to microstructure parameters in aero-engine materials is crucial for understanding the properties and performance. However, it is still a tough research topic to identify the important microstructural variables using conventional methods. In the present work, attribution (variable importance evaluation) methods based on neural networks have been systematically sorted out with a comprehensive understanding of the strengths and limitations. Microstructures of nickel base single-crystal alloys is taken as an example for discuss the variable importance evaluation methods, namely forward stepwise, backward stepwise, and partial derivative. Suggestions are provided for future application in material science and solid mechanics.

Quantitative representation of directional microstructures of single-crystal superalloys in cyclic crystal plasticity based on neural networks

Nickel-based single-crystal alloys undergo microstructural degradation induced by thermal exposure. The directional rafting of microstructures significantly affects the mechanical properties and makes the material anisotropic. For structural design, establishing a quantitative description of microstructural effects in a constitutive model becomes essential and is still a tough research topic in multi-scale materials modeling. In the present work, the fabric tensor was correlated with the anisotropic cyclic crystal plasticity of nickel-based single-crystal alloys with the help of neural networks. The microstructural representative volume elements with various single-crystal morphologies were generated by the phase-field method and the deformation behaviors were studied under different crystal orientations and loading configurations. The neural network analysis confirmed that the fabric tensor can present anisotropic single-crystallographic microstructural features and describe mechanical behavior under both monotonic and cyclic multi-axial loading conditions. The history-dependent anisotropic cyclic hardening or softening behavior of the material can be captured by the introduced microstructural state variable. A principal component analysis (PCA) aided gradient-based attribution method was proposed to evaluate the importance of input variables. The characterization of different material components and their contribution to the stress–strain relationships are investigated and validated. The fabric tensor was verified to be an effective microstructural indicator for the continuum plasticity of single-crystal alloys.

Analytical model for estimating calcium solution strengthening in single-crystal Mg-Ca alloys at finite temperatures

Mg is the lightest structural metal and a suitable replacement for heavy steel and Al alloys. However, Mg exhibits poor plastic formability at room temperature, and the strength of conventional Mg alloys is less than that of most Al alloys. Its low formability at room temperature is attributed to the lack of slip systems because of the extremely large gap in the critical resolved shear stress (CRSS) between the lowest basal system and the other slip and twin systems. This low CRSS of the basal slip usually limits the strength of Mg alloys. Therefore, strengthening the basal slip is important to improve the strength and deformability of Mg alloys and enhance their ductility. In this study, we construct an analytical model for predicting the CRSS in single-crystal Mg–Ca alloys at finite temperatures by quantitatively investigating the CRSS for the 〈a〉 dislocation gliding on the basal plane at temperatures from 1 to 500 K through molecular-dynamics computations. The prediction results agree well with our simulation results and the micropillar compression test results of Mg–0.3at.%Ca and Mg–0.6at.%Ca alloys at room temperature.

Quantifying the Crack-Tip Residual Stress of Nickel-Based Single-Crystal Alloys at the Micron Scale by Focused Ion Beam and Digital Image Correlation

Quantifying the Crack-Tip Residual Stress of Nickel-Based Single-Crystal Alloys at the Micron Scale by Focused Ion Beam and Digital Image Correlation

The widening of the solution heat treatment window by the addition of Ru in Ni-based single crystal superalloy

This work investigates the effect of Ru on the solution heat treatment window of the Ni-based single crystal superalloy. The addition of the Ru influences the solidification microstructure and widens the window. The solidification process is established to occur in the sequence of γ (dendrite) → γ + γ' grid eutectic (inter-dendrite) → γ + γ' lamella eutectic (inter-dendrite) → γ' (inter-dendrite). This is followed by the γ → γ + γ' decomposition of the γ dendrite into the common channel-cuboid structure upon cooling. The solvus of γ' and the solidus of the alloy define the window. In alloys containing inter-dendrite eutectic, the window is in principle zero. In this case, only the dynamic window created by homogenization diffusion may allow heat treatment. Adding Ru decreases the elements' partition ratios between the dendrite core and dendrite edge regions. It also promotes the enrichment of Re at the γ/γ' phase interfaces, which helps to refine the γ + γ' channel-cuboid structure and reduce the differences in both the size and volume fraction of the γ' phase between dendrite core and dendrite edge. Ru is found to inhibit the formation of (Ta, X)C carbide. These help widen the solution treatment temperature window of the Ni-based single crystal alloy.