Anomalous Yield Research Articles

Segregation-assisted yield anomaly in a Co-rich chemically complex intermetallic alloy at high temperatures

Chemically complex intermetallic alloys (CCIMAs) have emerged as promising materials for achieving exceptional softening resistance at elevated temperatures, owing to their unique superlattice structures and elemental synergism. However, the lack of experimental endeavors and understanding regarding their temperature-dependent mechanical behaviors hinders their advancement for high-temperature applications. Here, we conducted a systematic investigation on the mechanical properties and associated deformation mechanisms of an L12-type Co-rich CCIMA using transmission electron microscopy. The Co-rich CCIMA exhibits superior strength across a wide temperature range of 700-900°C, with an anomalous peak yield strength (YS) of ∼1.0 GPa at 700°C, which surpasses most previously reported L12-type intermetallic alloys. This superior softening resistance can be primarily attributed to a combination of cross-slip hardening and segregation-assisted hardening. Specifically, massive Kear-Wilsdorf (K-W) locks formed by multiple cube cross-slips of screw dislocations prevail at 700-900°C, providing strong barriers for dislocation movements and enhancing the yield strength (YS) accordingly. More interestingly, we revealed a segregation-assisted formation of superlattice stacking faults and nanotwins at the peak yield temperature (700°C). The interlocking of these substructures and the associated element dragging effect further impeded the propagation of dislocations, contributing to the observed yield anomaly. This work provides in-depth insight into the high-temperature performance and deformation behaviors of CCIMAs, which paves the way for the future development of novel heat-resistant structural alloys.

Study on the High Temperature Tensile Properties and Strengthening Mechanism of Nitrogen Containing Nickel‐Based Deposited Metals

This article investigates the tensile properties and strengthening mechanism of nitrogen‐containing nickel‐based flux cored welding wire deposited metal from room temperature to 900 °C. The results showed that the high‐temperature strength of the deposited metal increased with the addition of Tungsten and Nitrogen. When the temperature reaches 700 °C, M23C6 carbides begin to decompose. The alloying elements dissolve back into the matrix, and the solid solution strengthening effect minimizes the plasticity of the alloy. More carbonitrides are precipitated within the crystal with temperature. During high‐temperature deformation, carbonitrides fractures at the interface with the substrate due to difficulty in coordinating deformation. Therefore, the crack source formed greatly reduces the strength of the material. In addition, the stacking faults present in the low‐temperature zone have a significant impact on the work hardening of materials. The main deformation mechanism is the unit dislocation a/2 < 110> cutting precipitation phase. It can be observed that the dislocation cutting precipitation mechanism system has transformed into a dislocation bypassing precipitation mechanism system at intermediate. Finally, the anomalous yield strength (YS) and intermediate temperature brittleness phenomenon are reasonably explained. The main deformation mechanism gradually transforms into dislocation slip and climbing with temperature.

Accurate complex-stacking-fault Gibbs energy in Ni3Al at high temperatures

To gain a deeper insight into the anomalous yield behavior of Ni3Al, it is essential to obtain temperature-dependent formation Gibbs energies of the relevant planar defects. Here, the Gibbs energy of the complex stacking fault (CSF) is evaluated using a recently proposed ab initio framework [Acta Materialia, 255 (2023) 118986], accounting for all thermal contributions—including anharmonicity and paramagnetism—up to the melting point. The CSF energy shows a moderate decrease from 300K to about 1200K, followed by a stronger drop. We demonstrate the necessity to carefully consider the individual thermal excitations. We also propose a way to analyze the origin of the significant anharmonic contribution to the CSF energy through atomic pair distributions at the CSF plane. With the newly available high-temperature CSF data, an increasing contribution to the energy barrier for the cross-slip process in Ni3Al with increasing temperature is unveiled, necessitating the refinement of existing analytical models.

The evolution of dual-phase core–shell structure and mechanical properties induced by Co addition in as-cast high-entropy intermetallic

In this work, three as-cast L12-type HEIs with different content of Co element were carefully examined combined with TEM, SEM and XRD technologies. The main matrix of three alloys was L12 ordered phase, and Si,Ti-rich grain-boundary precipitate phase generated with the addition of Co. In Co1 alloy, distinct dual-phase core–shell structure was found within grain due to the precipitation of extra FCC disordered secondary phase, which leads to a better strength-plasticity combination (compressive strength of 3441 ± 50 MPa, and fracture strain of 46.0 ± 1.5%) than Co0 and Co0.5 alloy at room temperature. With the increase of temperature, particular anomalous yield effect was discovered in Co1 alloy. It is clear that SFs became main deformation substructure in Co1 alloy at elevated temperature, and the interaction between non-coplanar SFs motivated the generation of L-C Locks, which assists the K-W Locks to pin dislocations movement and enhance the yield strength from room temperature to about 650 ℃. In addition, the interaction between SFs resulted in the formation of nano-spaced SF network, which contributes to the dynamic Hall-Petch effect. Finally, excellent high-temperature yield strength was achieved in as-cast Co1 alloy, which is higher than room-temperature yield strength until 950 ℃.

Microstructural Evolution and Tensile Properties of a Corrosion-Resistant Ni-Based Superalloys Used for Industrial Gas Turbines

As an important mechanical property, tensile behavior has been regarded as an indicator for the creep and thermal mechanical fatigue properties of Ni-based superalloys. The tensile property of Ni-based superalloys is closely related to the amount, size, and distribution of γ'-phase and carbides. To further clarify the tensile deformation mechanism of the CM247LC alloy, this study investigated its solidification characteristics and directionally-solidified and heat-treated microstructure. The dependence of tensile properties on the varied temperature ranging between 650 and 950 ℃ is discussed in detail. It was found that the deformation mechanism at 650 ℃ is dominated by the shearing of dislocations into the γ'-precipitates to form the superlattice stacking faults. At 800 ℃, the K-W lock leads to the anomalous yield effects. At 950 ℃, the deformation mechanism is dominated by the dislocations bypassing the γ'-precipiates. The results provide a comprehensive understanding of the CM247LC alloy and are beneficial for the development of corrosion-resistant Ni-based superalloys.

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.

The anomalous annealing hardening behaviors in commercial pure Tantalum foil

Annealing softening phenomenon is usually observed in pure metals. However, in this study, the anomalous annealing hardening behavior is found in commercial pure Tantalum (Ta) foil. When the cold-rolled Ta foils are annealed at a temperature lower than 700 °C, both the yield strength and ductility gradually increase with the rise in annealing temperature, resulting in an anomalous "low-temperature annealing hardening" phenomenon. The yield strength of Ta foils annealed at 700 °C reaches the maximum value of about 750 MPa. This phenomenon in Ta foil is closely related with the shear bands and the pinning behavior between interstitial atoms and dislocations. When annealing temperature is higher than 700 °C, the yield strength decreases. When annealing temperature exceeds 900 °C, recrystallization occurs in Ta foil. Anomalous yield point phenomenon appears in the recrystallization foils during stretching, which is also related with the pinning of dislocations by interstitial atoms. As annealing temperature further increases, the volume fraction of surface grains gets larger. And {112}<111> slip systems are preferred over {110}<111> slip systems in the surface grains.

Study on lattice discreteness effect on superdislocation core properties of Ni3Al by improved semi-discrete variational Peierls-Nabarro model

Nickel-based superalloys (NBSAs) serve as hot-end component materials of aero engines. NBSAs generally consist of γ and γ′ phases, the γ′ phase (i.e. Ni3Al) presents anomalous yield strength and non-Schmid effect, influencing the mechanical behaviors of NBSAs significantly. These phenomena are closely related to the ⟨110⟩{111} superdislocations. In the present work, based on the quasiharmonic density functional theory (DFT), the improved semi-discrete variational Peierls-Nabarro (SVPN) model is employed to systematically investigate the properties of such superdislocations, in terms of dislocation density distribution, splitting distance (i.e., anti-phase boundary APB, super-lattice intrinsic stacking fault SISF and complex stacking fault width CSF), total energy, Peierls stress and so on. It is found that the predicted superdislocation characters, especially the Peierls stress, are in good agreement with previous experimental results, considering the lattice discreteness effect. This implies that the lattice discreteness plays an important role in the total dislocation energy and Peierls stress, which has however been ignored in previous studies. Although such superdislocation may have several possible core configurations, the dissociation of four 1/6⟨112⟩ Shockley partials separated by two CSFs and an APB may be more energetically favorable at both 300 K and 1200 K. In addition, the superdislocation properties with such the core structure may be easily affected by applied stress. The present work provides an improved SVPN model with consideration of lattice discreteness to describe the superdislocation properties in Ni3Al, which may be helpful for understanding the plastic deformation behavior of the γ′ phase and NBSAs.

Modelling of short crack growth in single crystal Ni [formula omitted] microstructure

Mechanistic short crack growth in single crystal Ni γ−γ′ microstructure is investigated using Crystal Plasticity (CPFEM) and eXtended Finite Element Method (XFEM). Maximum slip and stored energy density are hypothesised to be the mechanistic drivers for the crack path and the growth rate that are studied in γ−γ′ microstructures. This mechanistic basis is shown to capture the effect of γ′ anomalous yield strengthening on the crack path and crack growth rate. Crack growth in the γ matrix and shearing of γ′ precipitates is predicted to occur at high applied stress at low or high temperature, while low applied stress is shown to lead to cracks which are confined to the γ channels. Critical stored energies (Gc) are determined for the γ and γ′ phases to predict experimentally observed crack growth rate at low temperature and high stress. The crack growth model offers good prediction of both the crystallographic crack paths and growth rates in γ−γ′ microstructures, thereby supporting its mechanistic basis.

A dislocation density-based model for the temperature dependent anomalous behaviors of nickel-based single-crystal superalloy

A dislocation density-based single crystal plasticity constitutive model was developed to capture the temperature dependent anomalous yield and tension/compression asymmetry behavior of nickel based single crystals. Firstly, the mathematical description of physical mechanism of anomalous yield behavior was considered in the hardening rules, i.e. thermally activated cross-slip mechanism. Secondly, a temperature dependent function was established to describe the transformation of dislocation motion mode from shearing to by-passing. Thirdly, three non-Schmid stress tensors were introduced into the constitutive model to describe the tension/compression asymmetric phenomenon. The model considers the contributions of various strengthening mechanisms, including solid solution, precipitates and base metal. Furthermore, the evolution of microstructural features (such as dislocation density, etc.) and contribution of each mechanism to yield stress with increasing temperature were further analyzed. The model has been implemented via crystal plasticity framework and can predict the temperature dependent anomalous characteristics of yield stress of Ni-based single crystal.

Orientation dependence of microstructure deformation mechanism and tensile mechanical properties of Nickel-based single crystal superalloys: A molecular dynamics simulation

In this paper, molecular dynamics (MD) simulations are performed to investigate the tensile deformation mechanisms and mechanical properties of superalloys with different orientations. The research results show that the superalloys exhibit anisotropy of mechanical properties and are consistent with previous experimental research results. The anisotropy of mechanical properties is related to the differences of stability at the interfacial dislocation network and the 1/6〈112〉{111} slip system startup of superalloys with different orientations. When the yield point is reached, dislocations and stacking faults on both sides of the slip plane will contact each other and merge into a stacking fault band to penetrate into the γ' precipitates. Meanwhile, the dislocation density and the proportion of face-centered cubic (FCC) structure with different orientation models have a sudden change at the yield point. As the temperature increases, the yield strength and elastic modulus decrease in the (110) and (111) orientation models, whereas the (100) orientation model has an anomalous yield behavior. This is mainly related to the destruction of dislocation network, the change of deformation mechanism and the thermally activated cross-slip mechanism when the temperature increases.

Tensile Deformation and Fracture Behavior of Nickel-Based Superalloy DZ951G.

DZ951G is a novel developed nickel-based directional solidified superalloy with an incipient high melting point and low density. Compared with DZ417G superalloy, DZ951G superalloy has a higher ultimate tensile strength. At intermediate temperatures, the plasticity and strength were both markedly improved, and an obviously anomalous yield behavior could be observed where the yield strength reached its maximum at 760 °C. Below 600 °C, two competitive modes of dislocations shearing γ′ particles existed, in which one was the formation of stacking faults and another was a/2<101> dislocations shearing. At intermediate temperatures, a transitional phase between shearing γ′ particles and bypassing appeared, and the fracture translated from brittle fracture into ductile fracture. Exceeding 900 °C, bypassing of dislocations was operated under thermal activation. Moreover, short continuous stacking faults still existed at 760 °C. Finally, the various dislocation configurations were rationally illuminated and explained with the intrinsic connection of mechanical properties.

Drainage feasibility of a Carboniferous thin-layer limestone aquifer based on a dewatering test: Luxi coal mine, China

Mine water damage is one of the main disasters threatening mine safety during production. The key to mine water hazard prevention and control is determining the main aquifer characteristics that threaten the mine and the hydraulic connections between aquifers and water diversion channels and proposing appropriate preventive measures. This paper focuses on the 16104 working face of the lower coal group in the Luxi coal mine of Shandong Province. The main water-filled aquifer that threatens the no. 16 coal mine is the Carboniferous no. 14 limestone thin-karst aquifer. Before adopting hydrophobic pressure-lowering measures, it is necessary to determine the water yield characteristics of the no. 14 limestone aquifer, the strength of recharging, and the hydraulic connection with the Ordovician limestone aquifer and to analyze and evaluate the feasibility of dredging. First, various geophysical exploration methods were utilized to comprehensively identify the anomalous water yield areas of the aquifer, which were verified by drilling methods. By combining these results with drilling and geophysical results, appropriate drainage and observation holes were selected. Second, a special water discharge stage and sequence were designed to carry out water discharge testing. By analyzing the change of water discharge and water pressure in dewatering test and the change of conventional ion concentration in water samples, the hydrogeological parameters were obtained, the hydrogeological situation was ascertained and the hydraulic connection with the Ordovician limestone aquifer was determined. The drainage time required for the aquifer to attain a safe water pressure and the amount of hydrophobicity were calculated. Through combination with the obtained hydrogeological parameters, the feasibility of depressurization was analyzed. The results show that the water yield property of the no. 14 limestone aquifer is not high, but the connectivity is good. There is a hydraulic connection between the no. 14 and the Ordovician limestone aquifers, but the connection is weak. The no. 14 limestone aquifer of the entire working surface was drained at a rate of 240 m3/h, indicating that the aquifer has good depressurization properties.

Modelling high temperature deformation of lamellar TiAl crystal using strain-rate enhanced crystal plasticity

Multi-phase lamellar γ-TiAl alloys are highly anisotropic and rate sensitive. At high temperature, the strain rate sensitivity (SRS) increases, which eventually influences the yield behaviour of the alloy. With increasing temperature, the onset of yield changes anomalously showing an increasing or sometimes a decreasing trend. This anomalous behaviour depends on alloy chemistry, lamellar microstructure and anisotropy due to lamellar orientations. To capture the strain rate sensitive deformation behaviour over a wide temperature range and to understand distinct nature of yield anisotropy, a new formulation of the strain rate sensitivity (SRS) expressed by a temperature dependent exponent has been proposed within the framework of the classical power-law based visco-plastic Crystal Plasticity Finite Element Method (CPFEM). In the presented formulation, the evolution of SRS parameter was deduced from laws of thermally activated dislocation motion to obtain temperature-enhanced strain rate sensitivity. Moreover, a dislocation pile-up assisted mechanical threshold stress was incorporated to obtain an “anomalous yield” response. The model was validated for a lamellar PST-TiAl, which is a single crystal consisting of γ-TiAl and α2-Ti3Al lamellar phases. Detailed predictive numerical analysis was performed for an in-depth understanding of temperature enhanced strain-rate sensitivity during local deformation behaviour of TiAl alloy based on slip activities.

A study on anomalous yield drop behaviour in modified beta titanium alloys

ABSTRACTThe yield drop phenomenon observed in the Ti–15V-3Al–3Sn-3Cr (Ti–15–3) beta-titanium alloy and its anomalous behaviour in the boron and carbon added Ti–15–3 alloys have been studied. While the base and the carbon containing alloys exhibit yield drop, the boron containing alloy with smaller grain size than base alloy does not appear to show this phenomenon. Tensile tests were interrupted at different stress levels followed by analyses of slip lines and sub-structural characteristics using scanning and transmission electron microscopes to understand this anomalous yield point phenomenon. Infrared thermal imaging technique was used to map the strain localisation and the spatiotemporal evolution of deformation along the gauge length of the specimens during the tensile tests. Deformation in these alloys initiates only in a few grains. Pile-up of dislocations in these grains subsequently triggers the formation of dislocations in other grains and their rapid multiplications. The spreading of deformation by the generation of dislocations from pile up dislocations in one grain to neighbouring un-deformed grains and their rapid multiplication to new regions influence the yield drop phenomenon and its characteristics. It is shown in this study that microscopic instability in the grain level is a necessary, but not the sufficient condition for the manifestation of macroscopic instability during tensile deformation in polycrystalline materials. The presence of boride particles at grain boundaries restricts the slip transfer across the grains as well as the spreading of deformation to new regions, which causes the suppression of yield drop in the boron containing alloy.

Design for anomalous yield in γ′-strengthening superalloys

Anomalous yield is a critical characteristic of temperature-resisted superalloys. However, it is undesirably anisotropic, being absent sometimes in the [111] crystallographic orientation. Here the orientation dependence of the yield behavior and the corresponding mechanism have been investigated. We show how the orientation affects the dislocations' motion in the γ-γ′ structure, where overcoming the critical resolved shear stress associated with anti-phase boundary plays a significant role. The influence of the orientation on anomalous yield is rationalized, and the criterion for the onset or absence of yield anomaly is proposed. An alloy design is then provided to effectively enhance yield strength anomaly.

A simple big data methodology and analysis of the specific yield of all PV power plants in a power system over a long time period

In this work, we have reviewed recent literature concerning the performance of PV plants in different power systems and detected that, usually, the raw data used is not complete in terms of the number of PV plants and also main parameters of poor quality need to be removed. Then, for the first time, a study of the specific yield of all PV plants in a power system with single-PV-plant resolution is presented. Thus, we have analyzed the official 237,588 monthly energy values obtained from 2005 to 2017 for the 1523 PV plants in the Canary Islands. This dataset is obtained from PV plants ranging from 0.53 kWp up to 9 MWp, and it has been supplied by the distribution system operator and main utility (ENDESA). Then, this dataset is compared to 153,120 irradiance and temperature data from a PVGIS database. Results show that the Spanish regulation has a direct effect not only on the development of the PV capacity in the Canary Islands, but also on the specific yields. Moreover, only combining meteorological data (irradiance, temperature and wind speed) from satellites, starting year of operation, and nameplate capacity we have developed a very simple theoretical model to predict the specific yield of a PV plant at any location in the Canary Islands, avoiding the requirement of any data from the owners of the PV plants. The simulation values obtained have been validated with the real specific yields for PV plants assumed to be well managed (multi-MW power plants placed in best locations) showing errors below a 3%. This theoretical model has also been used for detecting suboptimal PV plant designs and anomalous specific yield of PV plants above the clear sky limit. Recommendations to avoid anomalous specific yields in future are included.

Size effects on tensile properties and deformation mechanism of commercial pure tantalum foils

The tensile properties and deformation microstructure of commercial high pure tantalum foils with thickness ranging from 30 μm to 200 μm have been investigated. The origin foils show a large texture intensity of γ fiber. The yield strength and ultimate tensile strength gradually increase with the decrease of thickness due to the change of the scale factor of the surface grains and the increase of volume fraction of passivation film. The constitutive model based on the surface layer theory is well established. Meanwhile, the anomalous yield point phenomenon can be seen. The deformation microstructure is closely related with the orientations. Two sets of microbands are developed in most of the grains. The density of microbands is the largest in the grains with tensile direction // 〈110〉, which belong to α fiber. Microbands are usually concentrated near the grain boundaries, which frequently incline at 25–45° with respect to the tensile direction. The trace of microbands is usually consistent with the slip planes of {110} or {112} with largest or second largest Schmid factor. Therefore, the appearance of wavy slip traces on the fracture is attributed to the dislocation cross-slip on these different kinds of slip planes.

Modeling the joint influence of multiple synoptic-scale, climate mode indices on Australian wheat yield using a vine copula-based approach

Abstract Twelve large-scale climate drivers are employed to investigate their spatio-temporal influence on the variability of seasonal wheat yield in five major wheat-producing states across Australia using data for the period 1983–2013. Generally, the fluctuations in the Indian Ocean appear to have a dominant effect on the Australian wheat crop in all states except Western Australia, while the impact of oceanic conditions in the Pacific region is much stronger in Queensland. The results show a statistically significant negative correlation between the Indian Ocean Dipole (IOD) and the anomalous wheat yield in the early growing stage of the crop in the eastern and southeastern wheat belt regions. This correlation suggests that the wheat yield can be skillfully forecast 3–6 months ahead, supporting early decision-making in regard to precision agriculture. In this study, we use vine copula models to capture climate-yield dependence structures, including the occurrence of extreme events (i.e., the tail dependences). The co-occurrence of extreme events is likely to enhance the impacts of climate mode and this can be quantified probabilistically through conditional copula-based models. Generally, the developed D-vine quantile regression model provide greater accuracy for the forecasting of wheat yield at given different confidence levels compared to the traditional linear quantile regression (LQR) method. A five-fold cross-validation approach is also used to estimate the out-of-sample accuracy of both copula-statistical forecasting models. These findings provide a comprehensive analysis of the spatio-temporal impacts of different climate mode indices on Australian wheat crops. Improved quantification of the impacts of large-scale climate drivers on the wheat yield can allow a development of suitable planning processes and crop production strategies designed to optimize the yield and agricultural profit.

Yield degradation in inertial-confinement-fusion implosions due to shock-driven kinetic fuel-species stratification and viscous heating

Anomalous thermonuclear yield degradation (i.e., that not describable by single-fluid radiation hydrodynamics) in Inertial Confinement Fusion (ICF) implosions is ubiquitously observed in both Omega and National Ignition experiments. Multiple experimental and theoretical studies have been carried out to investigate the origin of such a degradation. Relative concentration changes of fuel-ion species, as well as kinetically enhanced viscous heating, have been among possible explanations proposed for certain classes of ICF experiments. In this study, we investigate the role of such kinetic plasma effects in detail. To this end, we use the iFP code to perform multi-species ion Vlasov-Fokker-Planck simulations of ICF capsule implosions with the fuel comprising various hydrodynamically equivalent mixtures of deuterium (D) and helium-3 (3He), as in the original Rygg experiments [J. R. Rygg et al., Phys. Plasmas 13, 052702 (2006)]. We employ the same computational setup as in O. Larroche [Phys. Plasmas 19, 122706 (2012)], which was the first to simulate the experiments kinetically. However, unlike the Larroche study, and in partial agreement with experimental data, we find a systematic yield degradation in multi-species simulations versus averaged-ion simulations when the D-fuel fraction is decreased. This yield degradation originates in the fuel-ion species stratification induced by plasma shocks, which imprints the imploding system and results in the relocation of the D ions from the core of the capsule to its periphery, thereby reducing the yield relative to a non-separable averaged-ion case. By comparing yields from the averaged-ion kinetic simulations and from the hydrodynamic scaling, we also observe yield variations associated with ion kinetic effects other than fuel-ion stratification, such as ion viscous heating, which is typically neglected in hydrodynamic implosions' simulations. Since our kinetic simulations are driven by hydrodynamic boundary conditions at the fuel-ablator interface, they cannot capture the effects of ion viscosity on the capsule compression, or effects associated with the interface, which are expected to be important. Studies of such effects are left for future work.