Alloy Single Crystals Research Articles

Improving mechanical properties and achieving structural stability of large-size molybdenum single crystal during hot deformation by adding Nb element

The occurrence of recrystallization will cause catastrophic damage to the structure of the metallic single crystal, which can be avoided by alloying reasonably. In this work, a new strategy is proposed to improve mechanical properties and achieve the structural stability of Mo single crystal during hot deformation. Thermal simulation tests show that the mechanical property of Mo single crystal is significantly improved by adding the Nb element. The Nb element is beneficial to inhibit the dynamic recrystallization of Mo single crystals through blocking the development of high-angle grain boundaries and dislocation cells, leading to Mo-Nb alloy single crystals still retaining monocrystal structure.

Effects of aluminum and oxygen additions on quenched-in compositional fluctuations, dynamic atomic shuffling, and their resultant diffusionless isothermal [formula omitted] transformation in ternary Ti–V-based alloys with bcc structure

Diffusionless isothermal ω transformation is a recently discovered ω transformation that occurs isothermally during aging near room temperature in Ti alloys with a bcc (β-phase) structure. Unlike conventional displacive phase transitions, the diffusionless isothermal ω transformation occurs in locally unstable nanoscale β-phase regions formed by quenched-in statistical compositional fluctuations. In the present study, the effects of Al and oxygen additions in ternary β-phase Ti–V-based alloys on the diffusionless isothermal ω transformation and its attempt process, called dynamic atomic shuffling, were studied. Elastic constant and internal friction measurements for Ti–16.6V–2.1Al and Ti–17.5V–2.5O (at.%) alloy single crystals and analysis of quenched-in compositional fluctuations using the thermodynamic theory of fluctuations indicated that the oxygen addition increases the β-phase stability, and it selectively increases the activation energy of dynamic atomic shuffling in locally unstable Ti-rich regions, which is caused by the strong attractive interaction between Ti and oxygen elements. As a result, oxygen addition suppresses the diffusionless isothermal ω transformation that occurs during aging at 298 K. In contrast, low β-phase stability, which promotes ω-phase nucleation, is retained by Al addition. Furthermore, dynamic atomic shuffling with a low activation energy is retained even though the average activation energy is enhanced by the Al addition, originating from the relatively weak atomic interaction between Ti and Al. Therefore, the diffusionless isothermal ω transformation cannot be suppressed by Al addition.

Molecular dynamics study of the effect of rolling process on subsurface strengthening of nickel-based superalloy GH4169 plastic deformation

In this paper, the molecular dynamics (MD) method is used to establish a rolling model at the nanoscale, the same depth of rolling is rolled, and three rolling methods are used, namely, one time, two times and three times respectively. The effects of three rolling methods on the workpiece are studied, and the evolution of dislocation and defect atoms, subsurface damage, surface quality and subsurface micro-strengthening mechanism of the rolled workpiece are mainly analyzed. The results show that the dislocation density increases most significantly by one rolling, and the difficulty of material deformation is reduced by multiple rolling, with the increase of rolling times, the surface roughness increases. We found that the rolling process has a directional effect on the micro-strengthening of nickel-based single crystal alloys, and the strengthening effect is most significant in the normal direction. The subsurface strengthening effect of rolled workpiece is positively correlated with dislocation density and rolling force. Compared with the previous macro rolling test research, we observed the strengthening effect of dislocation density on rolling plastic deformation of nickel-based single crystal alloy from the microscopic level, which can provide a reference for understanding the strengthening mechanism of rolling process on nickel-based single crystal alloy and the optimization of rolling parameters.

In-situ SEM study on fatigue crack behavior of a nickel-based single crystal alloy at 950 °C and 1050 °C

As the blade material of engine, nickel-base single crystal superalloy has been subjected to complex service environment such as high temperature and alternating load for a long time. In this work, the mechanism of fatigue microcrack initiation and propagation in a nickel-based single crystal at 950 °C and 1050 °C was studied by using the in-situ scanning electron microscope high temperature fatigue test system. At 950 °C, the alloy deformation was controlled by slip and the crack shear γ/γ’ phase cracking, showing Stage I mode. At 1050 °C, the alloy crack tearing occurred at the γ phase parallel to the loading direction and γ/γ' interface perpendicular to the loading direction and presents a Stage II mode perpendicular to the loading axis, then changes into Stage I in the accelerating stage. The deformation mechanism of fatigue cracking at two temperatures was compared. Besides, the influence of the microstructure defects on the fatigue crack behavior was discussed. The results show that the propagation of the microcrack below 100 μm shows evident fluctuations, which is closely related to the microstructure and the development stage of the fatigue crack.

Atomic-scale study of the repeated friction processes of γ/γ' phase nickel-based single crystal alloys

The molecular dynamics method was used to simulate the micro-motion friction process of nickel-based single-crystal alloy. With the repeated friction processes increases, the peak friction force gradually increases, the friction coefficient changes more drastically, the transmission of atomic displacement gradually changes from having discontinuity to having continuity, the increment of abrasion depth gradually decreases, the dislocation entanglement is more likely to appear near the two-phase interface, and the friction hardening effect is weakened when the position of the grinding ball is closer to the two-phase interface. The transfer of temperature is relatively linear, which is different from the effect of the amorphous interface layer. Under constant load, the faster the friction speed, the smaller the wear of the workpiece.

Creep Behavior Characterization of Nickel-Based Single-Crystal Superalloy DD6 Thin-Walled Specimens Based on a 3D-DIC Method.

The thickness debit effect of creep behavior has been a focal point of nickel-based single-crystal superalloy research, and there is a need for an advanced creep deformation measurement method. This study developed a novel high-temperature creep test system based on a single-camera stereo digital image correlation (DIC) method with four plane mirrors to conduct creep tests on thin-walled specimens of a nickel-based single-crystal alloy, DD6, with thicknesses of 0.6 mm and 1.2 mm under experimental conditions of 980 °C/250 MPa. The reliability of the single-camera stereo DIC method in measuring long-term deformation at a high temperature was experimentally verified. The experimental results show that the creep life of the thinner specimen was significantly shorter. It was found the lack of coordination in the creep deformation process of the edge and middle section of the thin-walled specimens may be an important factor in the thickness debit effect according to the full-field strain contour. By comparing the local strain curve at the rupture point with the average creep strain curve, it was found that the creep rate at the rupture point was less affected by the specimen thickness during the secondary creep stage, while the average creep rate in the working section significantly increased as the wall thickness decreased. The thicker specimen usually had a higher average rupture strain and higher damage tolerance, which prolonged the rupture time.

Combining MD-LAMMPS and MC-McChasy2 codes for dislocation simulations of Ni single crystal structure

The unique capability of the new version of the McChasy code (called McChasy2) is to provide the possibility to simulate experimental energy spectra delivered by Rutherford Backscattering Spectrometry in channeling direction (RBS/C) using large atomic structures (ca. 108 atoms).Ni-based alloys are nowadays one of the most studied and promising materials that can be used in the power generation sector and in general for high-temperature applications because of their radiation resistance and proof against harsh environmental conditions.In this work, we present recent results of investigations regarding simulations of extended structural defects (edge dislocations and loops) developed in the directions typically observed in the fcc systems that are formed inside nickel-based single-crystal alloys. The extended defect models are created using ATOMSK and the Molecular Dynamics (MD)-LAMMPS thermalization process. The models are then used to create virtual samples and fit experimental RBS/C spectra.

Growth of single-crystal black phosphorus and its alloy films through sustained feedstock release

Black phosphorus (BP), a fascinating semiconductor with high mobility and a tunable direct bandgap, has emerged as a candidate beyond traditional silicon-based devices for next-generation electronics and optoelectronics. The ability to grow large-scale, high-quality BP films is a prerequisite for scalable integrated applications but has thus far remained a challenge due to unmanageable nucleation events. Here we develop a sustained feedstock release strategy to achieve subcentimetre-size single-crystal BP films by facilitating the lateral growth mode under a low nucleation rate. The as-grown single-crystal BP films exhibit high crystal quality, which brings excellent field-effect electrical properties and observation of pronounced Shubnikov-de Haas oscillations, with high mobilities up to ~6,500 cm2 V-1 s-1 at low temperatures. We further extend this approach to the growth of single-crystal BP alloy films, which broaden the infrared emission regime of BP from 3.7 μm to 6.9 μm at room temperature. This work will greatly facilitate the development of high-performance electronics and optoelectronics based on BP family materials.

Crossover from mechanical stabilization of martensite to spanning avalanche-type reverse transformation of deformed Cu-Al-Ni shape memory crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.

The Effect of Strain Rate on the Tensile Deformation Behavior of Single Crystal, Ni-Based Superalloys

Single crystal Nickel-based superalloys exhibit an anomalous yield point, the yield stress increasing with temperature to a maximum at around 750 ºC. Here, we demonstrate in the alloy CMSX-4 at 750 ºC that, although there is virtually no effect of strain rate on the initial yield point, at slow strain rates a second mechanism can initiate leading to a considerable softening effect. By examining the microstructures of a series of interrupted tests, this is attributed to the initiation of stacking fault shear after the operation of a secondary slip system. Using high-resolution TEM, the dislocation structures are shown to be identical in both structure and in the segregation of Co, Cr, and W, to those observed during creep deformation of single crystal alloys, although the conformation of the dislocations and faults differs from that observed during creep. This drop in flow stress at low strain rates is not observed in the alloys TMS138A and SRR99, in the former case, the improved creep resistance of this fourth-generation alloy would require a much slower strain rate to match the creep rate achievable at this temperature.

Morphological stability of solid-liquid interfaces under additive manufacturing conditions

Understanding rapid solidification behavior at velocities relevant to additive manufacturing (AM) is critical to controlling microstructure selection. Although in-situ visualization of solidification dynamics is now possible, systematic studies under AM conditions with microstructural outcomes compared to solidification theory remain lacking. Here we measure solid-liquid interface velocities of Ni-Mo-Al alloy single crystals under AM conditions with synchrotron X-ray imaging, characterize the microstructures, and show discrepancies with classical theories regarding the onset velocity for absolute stability of a planar solid-liquid interface. Experimental observations reveal cellular/dendritic microstructures can persist at velocities larger than the expected absolute stability limit, where banded structure formation should theoretically appear. We show that theory and experimental observations can be reconciled by properly accounting for the effect of solute trapping and kinetic undercooling on the velocity-dependent solidus and liquidus temperatures of the alloy. Further theoretical developments and accurate assessments of key thermophysical parameters – like liquid diffusivities, solid-liquid interface excess free energies, and kinetic coefficients – remain needed to quantitatively investigate such discrepancies and pave the way for the prediction and control of microstructure selection under rapid solidification conditions.

A microscopy study of nickel-based superalloys performance in type I hot corrosion conditions

ABSTRACT Alloy material selection for sustainable, efficient, and cost-effective use in components is a key requirement for both power generation and aerospace sectors. Superalloys are manufactured using a combination of different elements, selected carefully to balance mechanical performance and environmental resistance to be used in a variety of different service conditions. Therefore, a fundamental understanding of each element is critical to alloy design. In this paper, the interaction of alloy chemistry, particularly chromium as a corrosion-resistant element along with titanium and molybdenum, and their effect on alloys performance for the relevant gas turbine industries were discussed. Based on the findings, the single-crystal alloy is found to be a better corrosion resistant alloy exhibited higher corrosion resistance in comparison to polycrystal alloys and proved that microstructure has a significant impact on alloy performance. This study also established that molybdenum level in chromia former alloys can significantly enhance the corrosion damage.

Effect of cyclic heat treatment on abnormal grain growth in Fe-Mn-Al-based shape memory alloys with different Ni contents

Cyclic heat treatment that can continuously promote abnormal grain growth is widely used for the preparation of single‐crystal Fe‐Mn‐Al‐based shape memory alloys. However, it takes a long time to prepare large‐size Fe‐Mn‐Al‐based alloy single crystals via the reported cyclic heat treatments. Meanwhile, the long‐time cyclic heat treatment at high temperatures leads to the development of defects including oxidation and a decrease in Mn, which would deteriorate superelasticity in the Fe‐Mn‐Al‐based shape memory alloys. To shorten the fabrication time of single crystals, the effect of the cyclic heat treatment process on the abnormal grain growth in the Fe‐Mn‐Al‐based alloys with different Ni contents was systematically investigated. It is found that the abnormal grain growth of Fe‐Mn‐Al‐based alloys was not significantly affected by the Ni contents (within 2.1 at.%–6.2 at.%). In addition, the abnormal grain growth could be promoted by 1–2 °C min–1 cooling rate, high solution temperature, and multiple cycles, while it was insensitive to other processes including heating rate, dual‐phase time as well as long‐time solution treatment. These findings can guide optimizing the fabrication process of single crystals by cyclic heat treatment. Finally, the Fe41.9Mn37.8Al14.1Ni6.2 single crystal prepared by the optimized cyclic heat treatment showed a recoverable strain of about 4%.

Investigation of external capacitance effects on electrochemical discharge drilling

ABSTRACT Electrochemical discharge drilling (ECDD) involves a combination of electrical discharge drilling (EDD) and electrochemical drilling (ECD) to remove the recast layer while drilling at high speed. Here, a T-RC power supply similar to the EDD method is adopted. However, the solution resistance characteristics change as a result of utilizing a weak conductivity solution instead of deionized water with insulating properties. Different external capacitances will impact ECDD. In this study, through experiment and simulation, the influence of capacitance on the ECDD performance of Ni-based single-crystal alloy is investigated compared to EDD. In the open-circuit stage, the ECDD current shows an exponential relationship, and the voltage drops slightly. The highest line material removal rate reaches 326 μm/s, which is higher than EDD, and the tool wear ratio drops by 40%. The aperture and taper continue to ascend. Finally, no recast layer can be realized over 680 nF.

Experimental investigations into effect of milling parameters on milling force and burr micromorphology for Zr-based bulk metallic glass by using groove milling

Zr-based bulk metallic glass is an amorphous alloy. In order to study the burr problem in the groove milling process of Zr-based bulk metallic glass, the milling force in the process is studied experimentally. Then the effect of milling parameters on the burr of Zr-based bulk metallic glass was studied, and the relationship between milling force and burr was analyzed. In order to further study the milling burr of amorphous alloy, under the same test parameters, the single crystal alloy material (nickel base single crystal superalloy DD5) and the traditional polycrystalline metal material (304 stainless steel) were selected for comparative test and analysis. The results show that Zr-based bulk metallic glass has good groove milling machinability. The test results provide some experimental basis for groove milling of Zr-based bulk metallic glass.

Optimization of Femtosecond Laser Drilling Process for DD6 Single Crystal Alloy

In this paper, we explore the optimal combination of femtosecond laser drilling parameters for micro-hole processing on DD6 single-crystal high-temperature alloy and analyze the significance of parameter variations on the microstructure characteristics of the holes. The L25(56) orthogonal test was performed by controlling six parameters during femtosecond laser ring processing: average power; overlap rate; defocus rate; feed amount; gas pressure; and end position. The significance of the influence of the factors was analyzed by ANOVA, and the parameters were optimized by genetic algorithm. Scanning electron microscopy was performed on the micropores and the salient features of the pores were analyzed. Finally, we calculated the extreme differences and conducted single-factor effect analysis. We conclude that the defocus rate has the most significant level on the hole drilling by femtosecond laser ring processing for DD6 single crystal high-temperature alloy; and the effect of the end position is smaller than others. The optimized parameters are power 6.73 W; overlap 99%, defocus 0 mm; pressure 0.2 MPa; feed 0.02 mm, and end −0.4 mm.

A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property

Severe intermediate-temperature embrittlement as well as rapid softening and oxidation at high temperatures are significant problems for high-entropy alloys (HEAs) in polycrystalline structures, which greatly limit their applications in modern industries. In this study, a novel Co-rich single-crystal high-entropy alloy (HEA), Co41Ni35Al11·5Ta2·5Cr4Ti6 (at.%), was innovatively developed. After isothermally aged at 1250 °C, this alloy exhibits an ultrahigh volume fraction of cuboidal L12-type nanoprecipitates (∼80%) which is higher than those of previous Co-based superalloys. More importantly, the present HEA shows excellent mechanical performance at both ambient and elevated temperatures, and no intermediate temperature embrittlement and rapid softening were observed. Noticeably, the yield strength can still maintain at ∼800 and ∼500 MPa when tested at 900 and 1000 °C, which are superior to other conventional single-crystal alloys. Moreover, extraordinary oxidation resistance can be achieved at the high temperature of 1000 °C due to the formation of a compact sandwich-structured oxide layer.

Thickness Debit Effect in Creep Performance of a Ni3Al-Based Single-Crystal Superalloy with [001] Orientation

With the complexity of the structure of aero-engine turbine blades, the blade wall thickness continues to decrease. It is found that when the blade wall thickness decreases to a certain extent, its mechanical properties will decline significantly. It is extremely important to study this phenomenon of a significant decline in mechanical properties caused by wall thickness. In this paper, the creep behavior of a second-generation Ni3Al-based single crystal superalloy with different wall thicknesses and [001] orientation at 980 °C/220 MPa has been studied and compared with the creep life of Φ4 round bar. The experimental results show that the second orientation and the surface affected zone are not the main reasons for the reduction of the life of thin-walled samples under this experimental condition. By analyzing the fracture morphology and deformed microstructure of thin-walled samples with different thicknesses, it is found that the thickness debit effect of the single crystal alloy occurs since the effective stress area of the alloy changes due to internal defects and surface affected zone during the creep process. For thicker samples, the creep life of the alloy can be extended by making the samples undergo certain plastic deformation through better plastic deformation coordination ability, while for thinner samples, the plastic coordination ability is poor, and the ability to extend the creep life through plastic deformation is also weaker when the effective stress area of the alloy changes, which leads to the thinner samples being more prone to fracture.

A Novel Yield Criterion for Nickel-Based Superalloys

Abstract With the significant evolution of modern gas turbine engines, selection of high-temperature resistant alloys in the hot section is known to be the fundamental solution to enhance the capabilities of these engines. In general, the high-temperature components are mainly comprised of polycrystalline, directionally solidified, and single crystal superalloys. Single crystal (SX) superalloys were developed in the 1980s to achieve high fatigue resistance and substantial creep rupture strength by eliminating grain boundaries. Directional solidification methods enabled the solidification arrangement of the materials to be comprised of columnar grains which are aligned parallel to the 〈001〉 direction. These casting types have been frequently used with nickel-based superalloys (NBSAs) to develop modern gas turbine blades. In this work, the yield behavior of generic SX and directionally solidified (DS) NBSAs is studied. By observing various SX and DS alloys, it was concluded with need for a novel criterion that can present anisotropic and tensile/compressive asymmetric yield surfaces. This novel criterion is comprised of the criterion proposed by Hill for anisotropic materials and the method developed by Drucker and Prager for alloys that have different tensile and compressive yield strengths. Additional terms to Hill's criterion are introduced to capture the coupling effect of normal stress and shear stress when the applied loads are not in the direction of principal axes of the material coordinate system for single crystal alloys. The parameters for the criterion are obtained from simple uniaxial tension and compression experiments. Results are compared with various well-established yield criterions. Additionally, the novel criterion is utilized to capture the effective stress and strain of multi-axial loading of turbine blades under nonisothermal conditions

Effect of martensite ageing on the orientation dependence of the B2-phase yield stress and the shape memory effect in Ti-40Ni-10Cu alloy single crystals

For the [001]- and [123]-oriented single crystals of the Ti-40Ni-10Cu alloy, it was shown for the first time that martensite ageing at 250 MPa leads to a strong orientation dependence of the B2-phase yield stress under tension, which is weakly manifested in the initial crystals. The maximum increase in the B2-phase yield stress by 350 MPa compared with the initial state was found in the [001]-oriented crystals. At external tensile stresses of 100 MPa, the shape memory effect in [001]-oriented crystals was 4.0± 0.2% in the initial state and increased to 5.0± 0.2% after martensite ageing, while in [123] orientation, on the contrary, it decreased from 8.7± 0.2% to 5.4± 0.2%. Keywords: Single crystals, Martensite ageing, B2-phase, Shape memory effect, Tension.