Thermal Grooving Research Articles

Oxide precipitation strengthening in Cu alloy modified with trace amounts of Nd during dynamic impact

In this study, three kinds of Cu-Nd alloys were prepared. The effect of Nd content on the dynamic impact behavior of the Cu-Nd alloys and its strengthening mechanism were studied. The results show that during the dynamic impact process, nano-scale oxide particles are precipitated in the alloy matrix, and a large number of cracks are formed along the grain boundaries, and the formation of shear bands and thermal grooves are also observed in local areas. With the increase of Nd content, the compressive strength of the alloys increases significantly, the grain size is refined, the number of twins increases, the grooves are much deeper and the number of the precipitated nano-oxide particles also increase significantly. In addition, due to the refinement of the grain size, the crack length is reduced. The increase of the alloy strength is mainly attributed to the nano-scale oxide precipitation strengthening and grain refinement strengthening.

Prediction analysis and an experimental investigation of precision process of CO2-laser micro-turning for Armox 500 T accompanied via N2 gas

The production of micro grooves on extremely hard workpieces like Armox 500 T, commonly used in military sectors like armored protective vehicles, poses challenges in conventional machining processes. Accordingly, an innovative technique called CO2 laser turning process (LTP) assisted by N2 gas was used for manufacturing micro grooves on Armox 500 T to confront the restrictions of traditional machining processes. This study was concerned with investigating the impact of various operating parameters related to the thermal grooving machining process such as: (i) laser power (P), (ii) gas pressure (GP), (iii) feed rate (F), and (iv) motor speed (S). These parameters were examined under various operating conditions like: (i) cutting depth (DC), (ii) upper width cut (UC), (iii) bottom width cut (BC), (iv) roundness error (RE), and (v) metal removal rate (MRR). Thus, response-surface-methodology (RSM) was used to examine the experimental results, enabling the mathematical modeling of the results, and reducing the need for additional experiments. The findings revealed that the optimal combination of condition settings for the CO2-LTP of Armox 500 T (assisted by N2 gas) included a CO2 laser power of 3000 Watts, N2 gas pressure of 17.2370 bar, a feed rate of 400 mm/min, and a motor speed of 10 RPM. Furthermore, the optimized combinations of thermal machining conditions resulted in increasing MRR, DC, BC, UC, DC, and reducing RE, with values of 0.0163 µm, 0.2333 mm, 0.2511 mm, 0.5241 mm, and 19.403 µm, respectively. Additionally, the correctness of the optimization procedure was demonstrated by the difference between the optimal experimental findings and the forecasted magnitudes for MRR, DC, BC, UC, and RE, which were up to 7.9 %, 6.71 %, 6.1 %, 3.6 %, and 2.98 %, respectively.

Reconstruction of the time-dependent source in thermal grooving by surface diffusion

In hot polycrystalline materials, when a vertical flat grain boundary meets a horizontal surface the grain boundary forms a groove in the surface. Mathematically modelling features of such thermal grooving mechanism is therefore very important in characterizing polycrystalline materials composed of tiny grains intersecting an external free surface. With this aim in mind, we formulate and investigate a novel inverse problem of reconstructing the unknown time-dependent source term entering the fourth-order parabolic equation of thermal grooving by surface diffusion from a given integral observation. We formulate and prove in Theorems 2.3–2.7 that this linear inverse problem is well-posed. However, in practice, the ideal regularity of data under which the inverse source problem is stable is never satisfied due to the inherent non-smoothness of the measurement. Consequently, this leads to the inverse problem with raw data becoming ill-posed. In order to obtain accurate and stable solutions, we develop and compare two numerical methods, namely, a time-discrete method and an optimization method. We obtain error estimates and convergence rates for the time-discrete method. For the optimization method, an objective functional, which is proved to be Fréchet differentiable, is introduced and the conjugate gradient method (CGM), regularized by the discrepancy principle, is developed to compute the minimizer yielding the source term. The results of two numerical tests illustrate the performance of the two methods for both exact and noisy measured data.

Continuum damage mechanics of cyclic viscoplasticity using asymptotic numerical method

Conscious efforts on reduction of greenhouse gas emissions have led to an energy transition to renewable energy, however uncertainties of renewable energy production have resulted in higher thermal cycling demands from conventional power plants. Thermal load cycling at high temperature regions of steam turbine components leads to enhanced creep-fatigue damage accumulation. It is well established that such damage mechanism is numerically best predicted by unified constitutive modeling including damage as a variable as per the formalism of continuum damage mechanics at an expense of considerable computational efforts using finite element analysis. In this paper, the non-iterative Asymptotic Numerical Method (ANM), currently limited to partial cycle analysis with linear hardening plasticity model, is proposed for the first time to address cyclic viscoplasticity problems including damage capable of handling multiple cycles and most generalized loading conditions. Regularization techniques additionally necessary to implement loading-unloading-reloading criteria and advanced constitutive models etc. are presented. The constitutive model chosen for the formulation includes the non-linear multiple back stress variable modified Chaboche model to include damage combined with modified Chaboche-Rousselier isotropic hardening model to include damage, power law for viscoplasticity, Lemaitre’s damage potential and Kachanov-Rabotnov’s creep damage law. The method is verified with defined error measures and then applied to two high pressure steam turbine rotors, one with and another without thermal stress relief groove (TSRG) at the inlet under service type loading conditions to study the beneficial effect of the TSRG on creep-fatigue damage evolution. The accumulated errors of the proposed ANM and computational time are compared to a conventional Newton-Raphson solution.

Effect of concurrent thermal grooving and grain growth on morphological and topological evolution of a polycrystalline thin film: Insights from a 3D phase-field study

Experiments and computer simulations of bicrystals have been extensively used to study the role of thermal grooving on grain boundary motion. Although these studies provide a fundamental understanding of the factors that affect boundary motion, they do not capture interactions between the grain boundary network and (solid-vapor) surface on the overall grain growth kinetics. To understand the complex interactions between the thermal groove and the grain boundary, we developed a three-dimensional phase-field model to simulate concurrent capillary-driven grain growth and surface diffusion-controlled thermal grooving. We systematically studied the role of film thickness and surface diffusivity on the interactions between thermal grooving and grain growth in polycrystalline films. To make outcomes statistically significant, we implement an efficient active parameter tracking algorithm that can simulate a large number of grain orientations. Our results show an increase in the degree of stagnation with the reduction in film thickness due to the modification of boundary curvature normal to the surface of the film. Large-scale simulations of polycrystalline films show the development of nonuniform groove surface profiles with varying degrees of asymmetry that correspond to steady state, decelerating, and stationary boundaries. The stagnation of grain boundaries shows dependence on groove depth and topological distribution of grains. The critical grain size for grain boundary stagnation and grain size distribution varies with film thickness. We also demonstrate, using various tailored configurations, how concomitant thermal grooving can lead to the complete arrest of grain growth.

The evolution of grain boundary energy in textured and untextured Ca‐doped alumina during grain growth

AbstractThe role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility.

Dynamic α globularization in laser powder bed fusion additively manufactured Ti-6Al-4V

As a dual-phase α+β titanium alloy broadly used in the aerospace and biomedicine sectors, Ti-6Al-4V offers a variety of attractive mechanical properties imparted by its tailorable microstructures typically developed via thermomechanical processing. Such a capability in microstructural control, however, is still beyond the reach of laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ti-6Al-4V. Here we demonstrate for the first time the development of near-equiaxed α microstructure in situ via dynamic globularization in the as-built state of LPBF Ti-6Al-4V. Akin to globularization during thermomechanical processing, the globularization process during LPBF is also governed by a series of mechanisms such as sub-grain boundary formation, boundary splitting, thermal grooving, lamellar termination migration and epitaxial growth. This is largely driven by continuous introduction of excess dislocations at the mismatched α/β interphase boundaries while the α↔β phase transformation takes place repeatedly during LPBF. The findings in this study provide new knowledge towards the development of a holistic microstructural control strategy in additively manufactured Ti-6Al-4V.

Extraordinary radiation tolerance of a Ni nanocrystal-decorated carbon nanotube network encapsulated in amorphous carbon

Nanostructured materials with abundant defect sinks show good radiation tolerance due to their efficient absorption of irradiation-induced interstitials and vacancies. However, the poor thermal stability and limited size of such nanomaterials severely limit their practical applications. Herein, we report a novel flexible free-standing network-structured hybrid consisting of amorphous carbon encapsulated nickel nanocrystals anchored on a single-wall carbon nanotube scaffold with excellent radiation tolerance up to 5 dpa at 673 K and exceptional thermal stability up to 1073 K. The nano-scale Ni-SWCNT network with abundant Ni-SWCNT interfaces and grain boundaries provides effective sinks and fast transportation channels for defects, which effectively absorb irradiation-induced defects and improved the irradiation tolerance. Furthermore, the formation of a low-energy Ni-C interface and surface thermal grooves significantly reduces the system free energy and increased thermal stability. The amorphous carbon layer produces an external compressive radial stress that inhibits Ni grain boundaries from migrating, which greatly improves the thermal stability of the hybrid by pinning GBs at grooves between grains and facilitates the annihilation of irradiation-induced defects at the sinks. This work provides a new strategy to improve the thermal stability and radiation tolerance of nano-materials used in an irradiation environment.

материаловЭволюция границ зерен алюминия в процессе изотермического отжига

Using an example of textured aluminum foils, the possibilities of using the reflected electron diffraction method for in situ studies of the behavior of grain boundaries during isothermal annealing have been experimentally demonstrated. It is shown that there is no homogeneity of the microstructure in space and no continuity of its change in time during grain growth. The experimentally discovered rotation of the crystal lattice near the grain boundary in the region of a thermal etching groove can have a retarding effect on the movement of the boundary.

Development of microstructure and high hardness of Ti6Al4V alloy fabricated using laser beam powder bed fusion: A novel sub-transus heat treatment approach

The present work aims to study the effect of novel sub-transus heat treatment on the microstructure evolution and hardness of Ti6Al4V alloy fabricated using laser based powder bed fusion. The results revealed that the as-built microstructure is α′ martensite which decomposes to α + β during sub-transus heat treatment at 970 °C for different soaking times. The formation of a bi-modal structure with approximately 25% globular primary α and transformed β is observed. The development of globular primary alpha is attributed to thermal grooving and boundary splitting. The resultant coarsening of these α/β grains was associated with a time-temperature combination and orientation relation (OR) to 45° [100]. Moreover, precipitation of fine Ti3Al intermetallic (α2) in plate form is observed predominantly inside the primary α grains. Owing to the faster cooling rate post soaking time resulted in Ti3Al precipitates which is the conversion of α to α2 precipitates. The average micro-hardness of the specimen heat-treated for 12 h at 970 °C is found to be 45% higher in comparison to the as-built specimen. Whereas, the micro-hardness differs between the primary α and transformed β region due to the alloy element partitioning effect. The average micro-hardness of 592 HV is observed in primary α grains in the specimen heat-treated for 12 h at 970 °C. The prior results are associated with the formation of intermetallic nano-sized Ti3Al (α2).

Mechanism of ultrahigh ductility obtained by globularization of αGB for additive manufacturing Ti–6Al–4V

In the present work, two post-processes, the 800 °C and 950 °C for 4 h annealing below β transus, were designed to improve the performance of selective laser melted (SLM) Ti–6Al–4V (Ti64) alloy. The results show that 800 °C annealing raises the ductility to 18%, while 950 °C annealing raises the ductility high up to 26%. Although the residual stress relief and martensite decomposition fully accomplished in both annealed samples, there is still a noticeable difference in ductility. The difference is due to that 950 °C annealing induce noticeable globularization which further alters the fracture mode. Further study indicates that with globularization of αGB phase (α phase along prior β grain boundary), the transgranular fracture replaces the detrimental intergranular fracture. Particularly, the high angle grain boundary (HAGB) between equiaxed αGB phase effectively inhibits the initiation and propagation of crack, when comparing to the low angle grain boundary (LAGB) in α lamella. The reason for globularization in 950 °C annealing could be concluded as the boundary splitting model. The β phase grow along the LAGB in α lamella and form thermal grooving. With the groove deepening, the lamella was separated into equiaxed α phase. In this process, high temperature annealing promotes sufficient bulk diffusion, which is a necessary condition for globularization of lamellar α phase in SLM Ti64 alloy.

Grain Boundary Grooving by Surface Diffusion in Nickel Bicrystals

Surface self-diffusion was investigated via thermal grooving experiments on nickel bicrystals in combination with numerical simulations of the groove profiles. The objective of this work was the clarification of the dominant mechanism of grain boundary grooving in nickel at T=750∘C and the determination of the respective diffusion coefficients with high accuracy. The literature values of surface self-diffusion coefficients in nickel exhibit high scatter most likely associated with adsorption of impurities, which makes a comparison between different sets of data impractical and drawing conclusions about surface diffusion anisotropy impossible. In this work, thermal grooving experiments were performed on nickel bicrystals with different grain boundary and surface orientations at T=750∘C in forming gas. The groove profiles were measured by means of atomic force microscopy. Surface diffusion was clearly identified as the dominant mechanism of grain boundary grooving. Asymmetrical grain boundary grooving by surface diffusion was modelled numerically. From the comparison of simulated and measured groove profiles, the surface diffusion coefficients were determined with high accuracy and for the first time the anisotropy of surface diffusion was quantitatively measured. The surface self-diffusion in nickel is highly anisotropic, varying between DS=0.3×10−21 m3/s and DS=12.0×10−21 m3/s, whereby the surface diffusion near {100} surfaces is an order of magnitude slower than near {110} and {111} surfaces. The present work demonstrates that thermal grooving is an excellent tool to study the surface diffusion and to determine the respective diffusion coefficients with high accuracy from the comparison of simulated and measured groove profiles.

Polymorphic transformation and elemental diffusional controlled phase spheroidization of eutectic VNbTiSi medium entropy alloy during heat treatment

Microstructure evolution of a eutectic VNbTiSi medium entropy alloy (MEA) was systematically investigated during annealing heat treatment. Polymorphic transformation from β-(Nb, X)5Si3, a high-temperature metastable phase, to α- or γ-(Nb, X)5Si3 is revealed and is demonstrated to be closely related to element diffusion. There was mutual diffusion between V and Si, forming a diffusion transition, and the Ti diffusion promoted the formation of γ-(Nb, X)5Si3 mainly distributed at the grain boundary because of the Ti supersaturation in the BCC phase during solidification. Strong elemental diffusion promotes the formation of a thermal groove at the three junctions of sub-grain boundaries, contributing to the fast spheroidization of (Nb, X)5Si3, while the termination migration dominates the subsequent coarsening stage. Polymorphic transformation of silicides brings in sub-grain boundaries or low angle grain boundaries (LAGBs), contributing to silicide refinement. As a result, the compressive strength at elevated temperatures is greatly improved due to the increasing volume fraction of (Nb, X)5Si3 after heat treatment.

Deep convolutional autoencoder augmented CFD thermal analysis of bearings with inter pad groove mixing

The treatment of thermal mixing in inter pad grooves of a fluid film bearing is essential due to its influence on the heat transfer with the rotating shaft and stationary bearing. Lower fidelity models that either neglect or over approximate thermal groove mixing may lead to premature bearing or machinery failure, most commonly from babbitt thermally induced fatigue. Conventional models rely on bulk flow and thermal analyses yielding a single temperature at the groove outlet into the pad inlet. The high uncertainty of this approach carries over into downstream predictions for bearing life, stiffness and damping, and machinery vibration predictions. Contrary to a uniform temperature, CFD-Conjugate heat transfer studies reveal large gradient temperature distributions varying in both the radial and axial directions at the groove outlet, especially with jet lubrication implemented with multiple nozzles. These distributions vary continuously with time as the spinning shaft and bearing pads vibrate. A direct CFD simulation thus becomes computationally prohibitive.The present work introduces a novel approach which yields highly detailed lubricant temperature distributions at the pad inlets in a computationally economical manner. This is implemented with a surrogate groove model via a deep convolutional autoencoder neural network based on CFD (Computational Fluid Dynamics) data. The trained Convolutional Neural Network (CNN) shows excellent prediction capability for 2D temperature distribution at a circumferential groove outlet. The trained CNN is combined with a rotor-bearing model, and the combined model is verified by full CFD results and experimental data. In addition, this approach is expanded to include various oil injection types, illustrating their detailed heat transfer to the rotating shaft and bearing.

Heterointerface and grain boundary energies, and their influence on microstructure in multiphase ceramics

Heterointerface energy measurements for multiphase systems are rarely reported in the scientific literature in contrast to grain boundary energies for single phase systems. Hence, thermal groove dimensions on the surfaces of single-phase (YSZ, Al2O3, MgAl2O4), two-phase (YSZ/MgAl2O4, YSZ/Al2O3), and three-phase (YSZ/MgAl2O4/Al2O3) ceramics were measured using atomic force microscopy to calculate relative energies for six different types of interfaces. Average grain boundary and interfacial energies were estimated from relative values calculated using previously published surface energies. Our results show that relative areas of interfaces in a multiphase composite are governed by relative interface energies. We show that Al2O3-Al2O3 homointerfaces occur less frequently in the multiphase systems studied, attributed to their higher grain boundary energies compared to YSZ-YSZ and MgAl2O4-MgAl2O4 boundaries. Using the complex three-phase ceramic system as a model, we also show that heterointerface energies are intermediate between the grain boundary energies of each of the two associated phases. Grain boundary and heterointerface energies for any specific type of boundary were similar for single phases versus multiphases, but the influence of grain boundary chemistry variations is reflected in the energy distribution. Our findings contribute novel insights on grain boundary and heterointerface energies that can inform materials design due to their strong influences on microstructural evolution.

Universally characterizing atomistic strain via simulation, statistics, and machine learning: Low-angle grain boundaries

When applied to catalysis and related materials phenomena, grain boundary (GB) engineering optimizes over many currently disparately defined properties. Such properties include GB mobility, solute diffusivity, and catalytic footprints correlating current density with dislocation-induced strain. A recent universalizing framework has systematically classified low-Σ GBs in relation to analogous high-angle references, distinguishing them using footprints formed from the directional straining needed to reversibly yield bicrystals from their separated grains. Correlating the elastic work profiles derived from this thermodynamic process with matching changes in GB dislocations, strain footprints can comprehensively link formerly disparate catalytic properties and materials phenomena. This research investigates such structure-energy correlations to evaluate differences between low-angle (LAGBs) and high-angle (HAGBs) GBs, systematically delineating LAGB-HAGB transitions, explaining their origins, and connecting transitions to materials phenomena. A hierarchical statistical model, nesting GB degrees of freedom within one another, systematically detects such transitions via simplified strain footprints without failure of a single unique GB structure and material combination. A more comprehensive analysis of footprint directional components and discontinuities links transitions to catalytically relevant materials phenomena, describing thermal grooving, shear coupling, complexions, and defect migration under a single universal atomistic framework. With machine learning and spatially generalized strain footprints, this framework reconciles such phenomena via more comprehensive geometry-energy correlations.

Comparing Five and Lower-Dimensional Grain Boundary Character and Energy Distributions in Copper: Experiment and Molecular Statics Simulation

The misorientation of 515 grain boundaries has been determined using electron backscatter diffraction data from an 18 μm thick copper foil with columnar grain structure and a preferential {110} surface orientation. The energy of the grain boundaries was determined from the dihedral angles in the vicinity of grain boundary thermal grooves. The experimental grain boundary energy vs. misorientation angle shows deep minima for the low-angle grain boundaries and small minima corresponding to the Σ3 and Σ9 grain boundaries. Only a small fraction of the coincidence site lattice grain boundaries demonstrate an increased occurrence frequency (compared to a random orientation distribution) and low energy. In parallel, the grain boundary energy for a subset of 400 symmetrical tilt grain boundaries was calculated using molecular statics simulations. There is a good agreement between the experiment and molecular statics modeling.

A novel cyclic thermal treatment for enhanced globularisation kinetics in Ti-6Al-4V alloy: Experimental, constitutive and FE based analyses

Secondary hot-working on dual phase titanium alloys are essential for microstructural modification to tailor mechanical properties, which is typically challenging due to a narrow available processing window, especially during industrial scale manufacturing. Poor workability, strain induced porosity and adiabatic temperature rise in α + β phase region (i.e., sub-transus) are some of the main challenges faced. Cyclic thermal treatment (CTT) is an emerging technology showing potentials for microstructure modification (i.e., globularisation) in Ti‐6Al‐4V with significantly reduced mechanical work in the α + β region. This study summarises the results of CTT investigations conducted on a wrought Ti‐6Al‐4V alloy subjected to various thermo-mechanical conditions to develop different initial microstructures. Samples with uniform strain distributions were extracted from pre-forged samples and subsequently subjected to CTT using both conventional electric furnace (i.e., for slow heating and cooling rates), and induction heating (i.e., for faster heating and cooling rates). CTT of the samples forged at sub-transus temperature in conventional furnace led to maximum (i.e. ~100%) globularisation and significant coarsening of α grains, resulting in an equiaxed bimodal microstructure. On the other hand, CTT with induction heating method has resulted in a maximum of 80% globularisation fraction in samples forged to 60% reduction, and ~35% globularisation fraction in those forged to 20% reduction. The globularisation mechanisms during CTT of the sub-transus forged samples was dominated by the boundary splitting and thermal grooving. A Johnson-Mehl-Avarmi-Kolmogorov (JMAK) based model has been developed to predict the evolution of globularisation and grain growth during CTT. The developed JMAK model was then successfully incorporated into DEFORM® software as post-processing user subroutines. The predicted microstructure evolution by Finite Element (FE) simulations shown a good convergence towards the experimentally measured data following CTT.

A computational analysis of universal behavior of thermal groove in a moving grain boundary

We report the existence of universal behavior of a mobile thermal groove in polycrystalline thin films using a novel three-dimensional phase-field model. The universal behavior is independent of film thickness, surface diffusivity and grain boundary mobility when grain boundary motion is in steady-state. Moreover, the simulated universal curve shows excellent agreement with Mullins’ theory of mobile grooves for any groove shape. Onset of universality of surface profiles is concomitant with the attainment of steady-state motion of the grain boundary and a constant value of groove depth. On the other hand, in the case of thinner films, the interaction between the groove and the grain boundary leads to partial stagnation of the boundary. Even for grooves attached to decelerating boundaries we observe self-similarity of surface profiles, although this self-similar behavior is different from the ones associated with steady-state boundaries.

Microstructure evolution and mechanical behavior of a mullite fiber heat‐treated from 1100 to 1300°C

AbstractIn this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ‐Al2O3 and δ‐Al2O3 with amorphous SiO2 at 1150°C. The stable α‐Al2O3 formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.