Coarse-grained Polycrystalline Research Articles

Role of nanostructured surface layers in enhancing pure titanium diffusion bonding above their destabilization temperatures

The benefits of the nanostructured materials are believed to be probably absent if they are used at high temperatures. In this study, enhanced diffusion bonding of pure Ti was investigated by introducing a nanostructured surface layer through the surface mechanical attrition treatment (SMAT) technique. Complete recrystallization and remarkable grain growth have taken place in the original nanostructured surface layer due to the diffusion bonding conditions far beyond the nanostructured Ti destabilization temperatures. However, they still played a significant role in enhancing the Ti/Ti interfacial bonding. The consistency analysis of relative grain boundary energy, relaxation time, and microstrain reveals that although the original nanocrystalline has disappeared, the newly formed grain boundaries (GBs) during recrystallization probably inherit part of non-equilibrium state of the corresponding GBs in the as-SMATed surface layer, where the atomic diffusivity may increase greatly compared to the general relaxed GBs in their coarse-grained polycrystalline counterparts. The presence of these high-energy non-equilibrium GBs retains ultrafast atomic diffusion paths, leading to a diffusion bonding temperature of at least 100 °C lower than the traditional approach. The joint achieved at a low temperature of 750 °C exhibited a shear strength approximately 2 times higher than that prepared using raw substrates.

Interaction and superposition of creep damage and high-cycle fatigue of coarse-grained nickel-base alloy 247

ABSTRACT The cast coarse-grained polycrystalline Alloy 247 is investigated, which is typically used in the rear turbine blades of land-based gas turbines. High rotational speeds up to 10,000 rpm in combination with high temperatures induce creep deformation. Additionally, gas turbine blades are subjected to high-frequency cyclic stresses. This complex loading state requires the investigation of the interaction of creep damage and high-cycle fatigue (HCF). Therefore, HCF tests with a stress ratio of Rσ = −1 were carried out on as-received and pre-crept specimens. In comparison, an HCF test series with high mean stresses (Rσ = 0.5) was conducted to consider the superposition of creep and fatigue. Since nickel-base alloys exhibit a pronounced elastic anisotropy, the grain orientations were considered in the damage analysis. The results show that creep-induced grain boundary damage is a dominant structural parameter for the HCF behaviour of Alloy 247 in both, sequential and superpositioned creep-fatigue loading.

Nonmetallic trace elements induced rhenium co-segregation in nickel Σ5 [001](210) symmetrical tilt grain boundary

The thermally/mechanically-driven structural instability of nano- and coarse-grained polycrystalline metals originates from the unstable grain boundaries (GBs). GB segregation of solute elements could not only stabilize but also embrittle GBs. To improve GB stability and cohesion in parallel via solute segregations, in this work the segregation behavior of six non-metallic impurities X at the Σ5 [001](210) GB of nickel-based alloys, their co-segregations with Re element that has no GB segregation tendency, and segregation-induced changes in GB properties were investigated comprehensively from first-principles calculations. We discovered that these nonmetallic elements can segregate to the Ni GB and thus stabilize it, but they are GB embrittlers except B. Annealing temperature and bulk concentration have no effect on the S segregation. With the X pre-segregations, induced GB segregations of Re were observed, which lowers the excess energy and enhances the cohesive strength of all X-segregated GBs. The attractive interactions between C/N/B and Re atoms promote the Re segregation. The stabilizing and strengthening mechanisms of Re segregation were mainly attributed to the changes in Fermi level location and the increased interplanar bonding, respectively. Our findings may gain more insights into the atomic-scale GB co-segregations and stabilization of polycrystalline Ni-based alloys.

Dislocation evolution in copper in the absence and presence of hydrogen

In the absence and presence of hydrogen, the change of dislocation structure as increasing strain in a coarse-grained polycrystalline Cu was compared. By classifying dislocation structure with crystallographic orientation, it was found that hydrogen has almost no effect on the tensile property and dislocation evolution in coarse-grained Cu. Density functional calculation was used to reveal the underlying dislocation-H interaction mechanism in Cu. The electron structure interaction between Cu and H was the main factor that inhibited the hydrogen accumulation around dislocation, and it was attributed as the principal reason for high resistance to hydrogen embrittlement of coarse-grained Cu.

Deformation nanomechanics and dislocation quantification at the atomic scale in nanocrystalline magnesium

Abstract Classical molecular dynamics (MD) simulation method is employed to study the uniaxial tensile deformation of nanocrystalline magnesium (Mg) of varying grain size levels. The mean grain size of the sample is varied from 6.4 nm to 45 nm, with each sample containing about 43 million atoms in the modeling system. The deformation nanomechanics reveals two distinct deformation mechanisms. For larger grain-sized samples, dislocation dominated deformation is observed while, in smaller grain-sized samples, grain boundary-based mechanisms such as grain boundary sliding, grain boundary rotation are observed. The transition of normal and inverse Hall–Petch relation occurs at around 10 nm. Dislocation density quantification shows that the dislocation density in the sample drastically reduces with decreasing grain size. Elastic modulus of nanocrystalline Mg with mean grain size above 20 nm remains comparable to that of the coarse-grained polycrystalline bulk, followed by a rapid reduction below that grain size. The present work reveals the nanomechanics of nanocrystalline Mg, facilitating the design and development of Mg-based nanostructured alloys with superior mechanical properties.

Design optimisation of permanently installed monitoring system for polycrystalline materials

This article presents a design procedure for structural health monitoring systems based on bulk wave ultrasonic sensors for structures fabricated from polycrystalline materials. When designing a monitoring system, maximum coverage per transducer is a general requirement in order for the system to be economic. For coarse-grained polycrystalline materials, monitoring is often made challenging by low signal-to-noise ratios caused by grain scattering. Therefore, when designing a monitoring system for these materials, in addition to the economic requirement, it needs to be ensured that an adequate signal-to-noise ratio can be obtained throughout the monitoring volume. This typically introduces a trade-off between volume coverage per transducer and sensitivity that must be investigated. In this article, this trade-off is studied and a methodology using signal-to-noise maps is presented to design the system, that is, choose the optimal transducer parameters and placement. First, a combined analytical and numerical approach is used to generate a signal-to-noise map. Then, the influence of various factors on signal-to-noise ratio is investigated. Finally, two representative examples, with different criteria, are given to illustrate the methodology. In one example, the full surface area of the testpiece is covered with transducers and the optimum gives the deepest coverage. The other one aims to achieve the minimum fractional surface area that has to be covered with transducers to monitor a narrow depth range far from the surface, which has a potential application in weld monitoring. Results show that the optimum is likely to be at much lower frequency than typically used in inspection, as tracking signals with time gives sensitivity gains. Experiments were carried out to illustrate that higher volume coverage can be obtained at lower frequencies.

XPS study on the passivity of coarse-grained polycrystalline and electrodeposited nanocrystalline nickel-iron (NiFe) alloys

Electrodeposited nanocrystalline NiFe alloys are compared to their polycrystalline, coarse-grained counterparts to understand the effects of grain size reduction on the passivation kinetics and the chemical structure of the passive film. The passive film formed in acidic sulfate environment was characterized using parallel angle-resolved XPS (PARXPS) to obtain non-destructive compositional depth profiles. The composition (Ni:Fe) of the passive film was insensitive to grain size; however, grain size reduction resulted in increased anodic activity producing thicker films that contained increased concentrations of higher oxide species.

Surface formation mechanism in ultraprecision diamond turning of coarse-grained polycrystalline ZnSe

Zinc selenide is an excellent infrared optical material. In this study, ultraprecision diamond turning experiments were carried out on coarse-grained polycrystalline ZnSe (p-ZnSe) under various conditions and the corresponding surface formation mechanisms were investigated by examining the surface topography, chip morphology, material microstructural change, and cutting forces. Two kinds of surface defects were observed (i.e. plowing-induced micron-scale cleavage craters and tearing-induced submicron-scale tearing pits). It was determined that plowing induced micron-scale cleavage craters could be suppressed by reducing undeformed chip thickness. Furthermore, it was ascertained that tearing-induced submicron-scale tearing pits could be restrained by using a cutting tool with a zero rake angle. The minimal surface roughness was dominated by grain boundary steps formed when the cutting tool crossed twin boundaries at a large angle. A model was proposed for correlating surface defects and boundary steps with crystal orientations and cutting directions. By using a large-nose diamond tool for cutting, a smooth surface of 1.5 nm Sa was obtained. In addition, a zinc blende to cinnabar phase transformation was observed in the cutting chips, and metallization of cutting chips occurred at an extremely small undeformed chip thickness (~20 nm). Moreover, it was discovered that phase transformation inside the workpiece depends on the radius of the tool nose. The findings of this study provide an important reference for ultraprecision machining of brittle polycrystalline materials.

Plastic and energy absorption behavior of compressed circular rings with locally nanocrystallized segments

By utilizing the technology of surface nanocrystallization, the distribution of mechanical properties on metal structures can be changed. In this paper, a theoretical study is presented to investigate the large plastic deformation and energy absorption behavior of circular rings with locally nanocrystallized segments crushed between two rigid, flat plates. The theoretical results are compared with the finite element analysis and a good agreement is obtained when the structural plastic deformation is not too large. The results show that the deformation modes of locally nanocrystallized circular rings are quite different from those of circular rings made of conventional coarse-grained poly-crystalline materials. The dependence of load-deflection relationship and typical energy absorption metrics of circular rings on several key material parameters and distribution strategy of locally nanocrystallized segments is discussed. Through properly designing the distribution of locally nanocrystallized segments, the crushing behaviors of circular rings can be optimized and the comprehensive energy absorption performance can be significantly improved. The present theoretical study can serve as guidance for developing superior energy absorbers utilizing modern advanced material processing techniques.

Grain morphology reconstruction of crystalline materials from Laue three-dimensional neutron diffraction tomography

The macroscopic properties of advanced engineering and functional materials are highly dependent on their overall grain orientation distribution, size, and morphology. Here we present Laue 3D neutron diffraction tomography providing reconstructions of the grains constituting a coarse-grained polycrystalline material. Reconstructions of the grain morphology of a highly pure Fe cylinder and a Cu cube sample are presented. A total number of 23 and 9 grains from the Fe and Cu samples, respectively, were indexed and reconstructed. Validation of the grain morphological reconstruction is performed by post-mortem EBSD of the Cu specimen.

Sites of Initiation of Explosive-Emission Processes on the Surface of Single-Crystal and Coarse-Grained Polycrystalline Copper

Results of a study of sites of vacuum breakdown initiation on the electrolytically polished surface of single-crystalline and coarse-grained polycrystalline copper are presented. The coincidence of cathode erosion traces with sites of dislocation outcrops has been established.

High-temperature internal friction and dynamic moduli in copper

New measurements of dynamic shear and Young's moduli and their associated internal frictions were made with torsional and flexural forced-oscillation methods, respectively, on three polycrystalline specimens of pure copper. The tests spanned the 1–1000 s range of oscillation periods, at temperatures ranging from those of annealing close to the melting point (1050 °C) down to room temperature. A broad internal friction peak, found at temperatures around 700 °C (at 1 Hz) for samples annealed at 1050 °C, is superimposed on a monotonic relaxation background. Non-linear viscoelastic behaviour is observed above strains around 5×10−6. The period- and temperature-dependence of both shear modulus and internal friction is adequately captured by an extended “background plus peak” Burgers model for viscoelastic rheology. Activation energies are found to be around 200 kJ mol−1 for both the high-temperature peak and monotonic damping background, consistent with common diffusional control of both the dissipation background and superimposed peak, plausibly involving stress-induced migration of dislocations. Complementary torsional microcreep tests at selected temperatures reveal that most of the inelastic strain is anelastic (viscous) for loading durations less (greater) than 1000 s. Such linear viscous deformation, observed at low stress in coarse-grained polycrystalline copper, involves much higher strain rates than expected from published rheology, plausibly attributable to the Harper-Dorn mechanism.

Testing of time-of-flight neutron diffraction imaging using a high-resolution scintillator detector with wavelength-shifting fibers

In this study, time-of-flight (TOF) neutron diffraction images of a β-Sn single crystal and a coarse-grained polycrystalline steel plate were obtained using a high-resolution wavelength-shifting fiber based scintillator detector. An inhomogeneous intensity distribution of the 200 reflection from the β-Sn single crystal indicated a large defect density near the surface of the crystal. In addition, the diffraction spots from the individual grains of the steel plate were successfully observed separately in both the normal and width directions. This imaging technique has the potential to be used for evaluating the distribution of material characteristics in different regions inside bulk materials.

Grain boundary character distribution in electroplated nanotwinned copper

The grain boundary character distribution (GBCD) of nanotwinned copper, fabricated by electroplating inside small-scale through-wafer vias, was characterized using a stereological interpretation of electron backscatter diffraction maps. The GBCD of electroplated nanotwinned copper, specified by five macroscopic parameters (three for the lattice misorientation and two for the grain boundary plane inclination), is similar to the GBCD of coarse-grained polycrystalline copper used here as a reference material. The GBCD was compared to calculated grain boundary energies determined from atomistic simulations. We find that the grain boundary population in the electroplated nanotwinned and coarse-grained reference copper is both on average inversely correlated to the grain boundary energies. The slopes of the relationships between grain boundary population and energy for the most highly populated misorientations (Σ3, Σ9, and Σ11) are different. The relationships are strongly influenced by the geometric constraints at the triple junctions and multiple twinning, which enhanced the observed frequencies of Σ9 boundaries. The results suggest that the grain boundary network and the GBCD in the polycrystalline specimens are strongly influenced by the microstructure, grain boundary energy, and multiple twining.

Mechanistic Prediction of the Effect of Microstructural Coarsening on Creep Response of SnAgCu Solder Joints

Mechanistic microstructural models have been developed to capture the effect of isothermal aging on time dependent viscoplastic response of Sn3.0Ag0.5Cu (SAC305) solders. SnAgCu (SAC) solders undergo continuous microstructural coarsening during both storage and service because of their high homologous temperature. The microstructures of these low melting point alloys continuously evolve during service. This results in evolution of creep properties of the joint over time, thereby influencing the long term reliability of microelectronic packages. It is well documented that isothermal aging degrades the creep resistance of SAC solder. SAC305 alloy is aged for (24–1000) h at (25–100)°C (~0.6–0.8 × Tmelt). Cross-sectioning and image processing techniques were used to periodically quantify the effect of isothermal aging on phase coarsening and evolution. The parameters monitored during isothermal aging include size, area fraction, and inter-particle spacing of nanoscale Ag3Sn intermetallic compounds (IMCs) and the volume fraction of micronscale Cu6Sn5 IMCs, as well as the area fraction of pure tin dendrites. Effects of microstructural evolution on secondary creep constitutive response of SAC305 solder joints were then modeled using a mechanistic multiscale creep model. The mechanistic phenomena modeled include: (1) dispersion strengthening by coarsened nanoscale Ag3Sn IMCs in the eutectic phase; and (2) load sharing between pro-eutectic Sn dendrites and the surrounding coarsened eutectic Sn-Ag phase and microscale Cu6Sn5 IMCs. The coarse-grained polycrystalline Sn microstructure in SAC305 solder was not captured in the above model because isothermal aging does not cause any significant change in the initial grain size and orientation of SAC305 solder joints. The above mechanistic model can successfully capture the drop in creep resistance due to the influence of isothermal aging on SAC305 single crystals. Contribution of grain boundary sliding to the creep strain of coarse grained joints has not been modeled in this study.

X-ray diffraction measurements of the internal stresses in coarse-grained polycrystals

The possibilities of a one-crystal X-ray diffraction analysis of coarse-grained polycrystalline materials are demonstrated for cast 20L steel specimens in order to determine the elastic lattice strains and the residual stresses calculated from them.

原子論に基づく鉄合金のマクロ降伏強度予測のための理論モデルの構築

It is well known that the mechanical strength of iron is significantly changed by alloying. However, atomistic origin and underlying mechanism are still unclear. Since the strength change with respect to solute concentration is very sensitive and highly non-linear, the way of empirical prediction may contribute little to designing the mechanical strength by alloying. In this study, we theoretically construct a model which predicts temperature, strain rate, and solute concentration dependencies on critical resolved shear stress (CRSS) and yield stress of BCC iron alloys with dilute substitutional solutes based on atomistic analysis of the dislocation-solute atom interaction. In the coarse-grained BCC polycrystalline metals, the mechanical strength and deformation are dominated by screw dislocation motion consisting of kink nucleation and migration processes. Thus, our model is based on atomistically computed activation free energies for kink nucleation and migration of screw dislocation. We eventually apply our model to Fe-Si dilute alloy system as a representative example of BCC dilute alloys, and the theoretically predicted CRSS by our model is compared with an experimental one.

Plastic deformation behavior during unloading in compressive cyclic test of nanocrystalline copper

Deformed coarse-grained polycrystalline metals always unload elastically where permanent dislocation network well-developed in the loading regime hinders the movement of dislocations and allows only the elastic relaxation of stress. Such elastic unloading behavior is, however, unexpected in nanocrystalline metals because the dislocation network cannot effectively form inside nanometer-scale grains. In this work, we report the experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test at room temperature of nanocrystalline Cu. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading test and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation–twin boundaries.

Thermomechanical Fatigue of Mar-M247: Extension of a Unified Constitutive and Life Model to Higher Temperatures

The predictive capability of the Sehitoglu–Boismier unified constitutive and life model for Mar-M247 Ni-base superalloy is extended from a maximum temperature of 871 °C to 1038 °C. The unified constitutive model suitable for thermomechanical loading is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500 °C to 1038 °C at different strain rates with and without dwells. The flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. Creep and stress relaxation are critical to capture in this elevated temperature regime. The coarse-grained polycrystalline microstructure exhibits a high variability in the predicted cyclic response due to the variation in the elastic response associated with the orientation of the crystallographic grains. The life model accounts for fatigue, creep, and environmental damage under both isothermal and thermomechanical fatigue (TMF).

Atomistic Simulations of Dislocation Pileup: Grain Boundaries Interaction

Using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation of these kinetic processes with the available slip systems across the GB and atomic structures of the GB.