Fraction Of Grain Boundaries Research Articles

Grain boundary engineering in Ni-carbon nanotube composite coatings and its effect on the corrosion behaviour of the coatings

Coatings have been traditionally used for protection against corrosion. One possible way to enhance the corrosion resistance performance of coatings is by grain boundary engineering (GBE). This study illustrates the role of optimum amount of carbon nanotubes (CNTs) in modifying the grain boundary constitution in nickel-CNT composite coatings thereby enhancing its corrosion resistance performance. Compact and crack free Ni-CNT composite coatings were electrodeposited on mild steel substrates using electrolyte bath with different concentrations of dispersed CNTs (4, 6, 8, 10, 20, 30, 40, 50, 70 and 100 mg/L). Corrosion performance of coatings exposed to 3.5 wt.% NaCl medium was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy techniques. Among the 10 different Ni-CNT composite coatings, relatively low corrosion rates were observed for two distinct CNTs concentrations. Microstructural characterization conducted using the electron back scatter diffraction method (EBSD) suggested that the grains in all coatings grew with 〈110〉 orientation. Ni-CNT3 coating (from 8 mg/l of CNT in electrolyte) which showed low corrosion rate (corrosion current density of 0.98 µA/cm2) primarily contained very high fraction of low angle grain boundaries. Ni-CNT8 coating (from 50 mg/l of CNT in electrolyte) which exhibited the lowest corrosion rate (corrosion current density of 0.66 µA/cm2) contained high fraction of Σ3 coherent twins. Ni-CNT10 coating (from 100 mg/l of CNT in electrolyte) which exhibited the highest corrosion rate (corrosion current density of 14.08 µA/cm2) contained random high energy high angle grain boundaries (HAGBs) along with high energy symmetrical and asymmetrical tilt boundaries.

Microstructural characterization of copper coatings in development for application to used nuclear fuel containers

In the design of used nuclear fuel containers for deep geological repositories, copper is considered to be a suitable and long-lived barrier for corrosion resistance. The microstructures of state-of-the-art copper materials used in this application produced through extrusion, a grain boundary engineered electrodeposition technique and cold spraying were studied via electron backscattered diffraction. Desirable microstructural characteristics for localized corrosion resistance of pure copper were compiled from the literature considering grain size, grain boundary character distribution, and crystallographic texture. The subject copper materials were found to have favourable microstructures for localized corrosion resistance, in particular, a high fraction of special grain boundaries, especially Σ3 twins, rendering them suitable for the given application.

Surface modification of aluminum alloy by incorporation of AlCoCrFeNi high entropy alloy particles via underwater friction stir processing

AlCoCrFeNi high entropy alloy (HEA) particles were incorporated into AA5083 matrix by underwater friction stir processing (UFSP) to modify the surface properties of aluminum matrix. The effects of incorporation of HEA particles on microstructural evolution, micromechanical properties and wear performance were investigated. Microstructural observations showed that HEA particles were homogeneously distributed in the matrix with refined grains, the interface between the HEA particles and matrix showed a good metallurgical bonding with the thickness of approximately 200 nm. The incorporation of HEA particles can effectively accelerate the dynamic recrystallization (DRX) due to particle stimulated nucleation (PSN) mechanism and result in higher fraction of high-angle grain boundary (HAGBs). The fabricated composite showed 68.8% higher microhardness compared with as-received Al. Nanoindentation test results showed that the hardness, elastic modulus, H/E and H3/E2 ratios of the matrix were also improved. The wear test results showed that the fabricated composite exhibited the lowest friction coefficient and wear loss in all samples, and the wear rate was decreased by 48.6% compared with that of the as received Al.

Investigation of porosity, microstructure and mechanical properties of additively manufactured graphene reinforced AlSi10Mg composite

Currently, additive manufacturing is a rapidly growing technique that should be explored for the development of various composites and alloys. Graphene is also simultaneously gaining considerable attention as a reinforcement material for metals due to its superior properties. In this study, a graphene/AlSi10Mg composite was developed using the powder bed fusion (PBF) technique. The effect of graphene reinforcement and laser power variation was studied on the basis of the porosity, microstructure and mechanical properties of the composite. First, graphene (0.1 and 0.2 wt.%) was mixed in AlSi10Mg powder by conducting low-energy ball milling. The resultant mixture was used for PBF at laser power values of 200, 300 and 400 W. The prepared samples were characterised by synchrotron-based X-ray computed tomography to observe their pore distribution and morphology. The observation results reveal that the energy (laser power) required for appropriate melting of the powder was increased after graphene incorporation. Electron backscattered diffraction analysis revealed grain refinement and increase in fraction of high angle grain boundaries due to progressive addition of graphene. The strain developed after graphene incorporation was also validated using X-ray diffraction analysis. The uniform distribution of graphene and its structural inherency was confirmed by Raman analysis. Moreover, transmission electron microscopy revealed a suitable graphene-matrix interface. The tensile properties were significantly influenced by the porosity induced in the samples irrespective of graphene reinforcement. However, a yield strength increase of 22 % was observed in the composite compared with the strength of unreinforced sample of equivalent density. Hardness increased progressively with the graphene content and was unaffected by variation in the laser power.

Single-crystal nickel-rich layered-oxide battery cathode materials: synthesis, electrochemistry, and intra-granular fracture

Electro-mechanical degradation is commonly observed in various battery electrode materials, which are often prepared as polycrystalline particles consisting of nanoscale primary grains. The anisotropic volume change during lithium extraction/insertion makes these materials intrinsically vulnerable to grain-boundary (inter-granular) fracture that leads to rapid impedance growth and capacity decay. Here, guided by fracture mechanics analysis, we synthesize microsized single-crystal Ni-rich layered-oxide (NMC) cathode materials via an industrially-applicable molten-salt approach. Using single-crystal LiNi0.6Mn0.2Co0.2O2 as a model material, we show that the cycle performance of the Ni-rich NMC can be significantly improved by eliminating the internal grain boundaries and inter-granular fracture. The single-crystal LiNi0.6Mn0.2Co0.2O2 cathodes show high specific capacity (183 ​mAh g−1 ​at 0.1 ​C rate, 4.3–2.8 ​V) and excellent capacity retention (94% after 300 cycles at 1C/1C cycling). Further, it is confirmed for the first time that the single-crystal LiNi0.6Mn0.2Co0.2O2 particles are stable against intra-granular fracture as well under normal operating conditions but do crack if severely overcharged. Electrochemical-shock resistant single-crystal NMC reveals an alternative path towards developing better battery cathode materials, beyond the traditional one built upon polycrystalline NMC.

Effects of Magnetic Field on the Residual Stress and Structural Defects of Ti-6Al-4V

In this work, the influences of a magnetic field of 2.4 T on the macro residual stress and the status of structural defects, including grain boundaries, dislocations and the Fe-rich clusters of Ti-6Al-4V were investigated by X-ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and magnetic measurement. The XRD test results show that the applied magnetic field can cause the relaxation and homogenization of macro residual stress. The maps of Kernel Average Misorientation (KAM) values obtained by EBSD tests present a significant dislocation multiplication caused by a magnetic field, and the rise of dislocation density was estimated to be about 32% by XRD tests. The EBSD test results also show an increase in the fraction of Coincidence Site Lattice (CSL) grain boundaries and a decrease in the fraction of low-angle grain boundaries. The results of magnetic measurement show that Ti-6Al-4V has mixed magnetism consisting of paramagnetism and weak ferromagnetism, and that the ferromagnetic saturation magnetization decreased after exposing the alloy to the magnetic field, which suggests the dissolution of the Fe-rich clusters in the alloy. These magnetically-induced changes are related to magnetoplastic effects, a kind of phenomena on which there have been some research, and the possible mechanism of them is discussed in this paper.

Effect of martensite–austenite constituents on impact toughness of pre-tempered MnNiMo bainitic steel

The martensite-austenite constituents are known to deteriorate the impact toughness of alloyed steel. In order to reduce this adverse effect, the pre-tempering process at low temperatures was introduced before the tempering of intercritical heat treatment process of a MnNiMo bainitic steel. The effects of pre-tempering temperature on the decomposition of martensite-austenite constituents and impact toughness properties of the pre-tempered specimens were investigated. The results of instrumented Charpy V-notch impact testing show that the impact toughness of the materials can be significantly improved by the pre-tempering process, especially for the condition of pre-tempering at 400 °C. After pre-tempering at 400 °C, more martensite-austenite constituents are decomposed into ferrite and small carbides, resulting in the reduction of microcracks nucleation sites. The residual martensite-austenite constituents become mainly the island-like, which can effectively hinder the propagation of secondary cracks. In addition, the pre-tempering at 400 °C increases the fraction of high-angle grain boundaries and refines the size of blocks, which can effectively prevent or deflect the propagation of microcracks. All of those microstructure changes are beneficial to the crack propagation energy. The results suggest that the application of the pre-tempering process is a feasible method to enhance the impact toughness of the studied alloyed steel.

Effects of rare-earth micro-alloying on microstructures, carbides, and internal friction of 51CrV4 steels

The effects of rare-earth (RE) micro-alloying on the microstructures, mechanical properties, and internal friction (IF) of 51CrV4 steels after quenching and tempering were investigated. The addition of RE elements was beneficial for refining the size of the martensite and increasing the fraction of twins in the 51CrV4 steels. The RE-51CrV4 steel possessed a high dislocation density and a large fraction of low-angle grain boundaries after quenching. Cr7C3 carbides of RE-51CrV4 steel tended to precipitate along the lath martensite interfaces and were easily spheroidized during tempering. With the addition of RE elements, the Snoek–Köster–Kê (SKK) peak was increased after quenching but decreased after different tempering times. The results of the IF and high-resolution transmission electron microscopy analyses indicated that RE atoms formed micro-segregations at the lath martensite interfaces and resulted in a pinning effect on the carbon atoms. Therefore, the hardenability of the RE-51CrV4 steel was reduced, whereas its elongation improved significantly due to the presence of high-density twins and spheroidized carbides.

In Situ Transmission Electron Microscopy for Ultrahigh Temperature Mechanical Testing of ZrO2.

This work demonstrates a novel approach to ultrahigh-temperature mechanical testing using a combination of in situ nanomechanical testing and localized laser heating. The methodology is applied to characterizing and testing initially nanograined 10 mol % Sc2O3-stabilized ZrO2 up to its melting temperature. The results suggest that the low-temperature strength of nanograined, d < 50 nm, oxides is not influenced by creep. Tensile fracture of ZrO2 bicrystals produce a weak-temperature dependence suggesting that grain boundary energy dominates brittle fracture of grain boundaries even at high homologous temperatures; for example, T = 2050 °C or T ≈ 77% Tmelt. The maximum temperature for mechanical testing in this work is primarily limited by the instability of the sample, due to evaporation or melting, enabling a host of new opportunities for testing materials in the ultrahigh-temperature regime.

The precipitation control of grain boundary M23C6 phases and the ductility improvement in aged 22Cr-25Ni-WCuNbN austenitic stainless steel by Co addition

Addition of 5 wt% Co to 22Cr-25Ni-WCuNbN austenitic stainless steel resulted in a precipitation inhibition of M23C6 phases along the grain boundary and a large fraction of high angle grain boundaries after aging treatment. Co-added steels showed better ductility than the alloys without Co addition, due to the control of grain boundary M23C6 phases.

Unravelling Microstructure Evolution and Grain Boundary Misorientation in Coarse-Grained Heat-Affected Zone of EH420 Shipbuilding Steel Subject to Varied Welding Heat Inputs

Microstructure evolution and grain boundary misorientation in coarse-grained heat-affected zone of EH420 shipbuilding steel have been investigated at different welding heat inputs. As heat input increases, main constituents transform from lath bainites to granular bainites, and to acicular ferrites as a result of reduced cooling rate. Corresponding fraction of high angle grain boundaries decreases from 28.24 to 20.04 pct as heat input is raised from 50 to 150 kJ/cm but reverses to 29.04 pct at 200 kJ/cm.

Texture evolution during Isothermal compression process of Ti-22Al-25Nb alloy in B2 phase region

In the present work, the hot deformation behavior, dynamic recovery, dynamic recrystallization and texture evolution of Ti-22Al-25Nb alloy on the conditions of 1100°C with four different thickness reductions (35%, 50%, 65% and 80%) are investigated by isothermal compression testing on Gleeble-3500 thermo-mechanical simulator. The strain rate is 0.1mm/s-1. Subsequently, metallographic observation and EBSD analysis are carried out. The results show that during the hot deformation, the dynamic recovery (DRV) and dynamic recrystallization (DRX) strongly affect the microstructure and texture evolution. It is observed that with the strain increasing, the intensity of ηbcc-fiber increases firstly (crystallographic fiber axis &lt;100&gt; parallel to the compression direction). When the thickness reduction reaches to 80%, the intensity of &lt;001&gt; pole becomes stronger expectedly. Whereas the ηbcc-fiber transform into cube components ({100} &lt;001&gt;) unexpectedly. In addition, as the strain increases through 35%-80%, the fraction of large misorientation grain boundaries and fraction of DRX grains gradually increase due to continuous recrystallization. The evolution mechanism of grain orientations and texture during the DRX process will be discussed.

Strengthening mechanisms of Al wires processed by equal channel angular torsion drawing

ABSTRACTThis paper reports the microstructure evolution, mechanical properties and strengthening mechanisms of a commercial purity Al alloy (Al 1350) after severe plastic deformation by a novel continuous method called ‘Equal Channel Angular Torsion Drawing (ECATD)’. Electron backscatter diffraction (EBSD) results revealed an inhomogeneous grain refinement including a large fraction of low-angle grain boundaries. After four passes, the microhardness increases from the initial value of 35 HV up to 44 and 62 HV at the centre and near to wire surface, respectively. A combination of high strength and ductility can be achieved based on the results of a Taguchi design of experiment (DoE) for mechanical properties. The strengthening occurs due to increment of the dislocation density, and development of mainly new low-angle grain boundaries.

Microstructure and microtexture evolution of dynamic recrystallization during hot deformation of a nickel-based superalloy

Effect of strain rate route on the microstructure and microtexture evolution during the dynamic recrystallization (DRX) of a nickel-based superalloy subjected to the isothermal compression tests was investigated using electron back-scatter diffraction. The evolutions of microtexture components and fiber textures and the role of Σ3 boundaries in microstructure on the pole density in the partially and fully DRX processes were explored. The α-fiber in Euler space is regarded as the compression microtexture. The locally organized substructures in the matrix grains are formed on certain {111} slip planes with the high Schmid factor. Although the dislocation-free DRX grains show the random orientated distribution, the weak recrystallization 〈001〉 fiber parallel to the compression axis (ND) is developed at the high strain in the fully DRX processes. Whereas the compression 〈101〉 fiber parallel to ND in the partially DRX processes is sharpened. Moreover, the instantaneous decrease of strain rate leads to the highest fraction of Σ3 grain boundaries, indicating the lowest pole density. Cube-Twin component adjacent to Cube component contributes partly to the formation of Σ3 grain boundaries. The findings provide insights into the microtexture characteristics and enhance the understanding of the orientation dependence of the mechanical behavior of nickel-based superalloys.

Recrystallization mechanisms of Zircaloy-4 alloy annealed by electric current

In this paper we present recrystallization mechanisms in Zircaloy-4 alloy subjected to electric current annealing while maintaining the surface temperature below 135 °C. Electron backscatter diffraction (EBSD) was used to characterize and study the evolution of the microstructure and microtexture. We observed a continuous dynamic recrystallization (cDRX), and full recrystallization was achieved at a current density of 5x106 A/mm2. This was attributed to the synergistic effects of athermal (electron-wind force) and thermal (Joule heating) analogous to the effects of deformation strain and temperature during warm/hot working process. While the boundary density and fraction of low-angle grain boundaries decreased with current density, a gradual increase in the fraction of high-angle boundaries was noted. The dominant recrystallization mechanism was volumetric rotation-coalescence-induced grain growth. The <0001>//z microtexture of electric current annealed Zr-4 alloy remained virtually the same before and after recrystallization and was attributed to the grain rotational mechanism. These results show the potential of using this process for tailoring the microstructure and grain boundary engineering while retaining the desired texture.

Effect of Travel Speed on Microstructure and Mechanical Properties of FSW Joints for Al-Zn-Mg Alloy.

The microstructures and mechanical properties of friction stir welded (FSW) Al–Zn–Mg alloy plate under different travel speeds were investigated. Both the average grain sizes (AGSs) of the shoulder affected zone (SAZ), nugget zone (NZ), and the widths of thermo-mechanically affected zone (TMAZ) decreased with the increase of travel speed. Moreover, the AGSs of NZ are always about 60% of that of SAZ at different travel speeds. The fractions of high-angle grain boundaries (HAGBs) in the FSW joints reduce with the distance away from the stir zone (SZ). Furthermore, the initial η’ strengthening precipitates in NZ and TMAZ dissolve and GP zones form during subsequent natural aging, so that the hardness is similar in the two zones. The precipitate evolution in the heat-affected zone (HAZ) at hardness minima are affected by travel speeds, which induce the hardness minima and ultimate tensile strength (UTS) of FSW joints and increase with the increase of travel speed, and a fracture tends to occur at hardness minima location of HAZ during tensile testing.

Achieving good tensile properties in ultrafine grained nickel by spark plasma sintering

In this work, the tensile properties of ultrafine grain Ni samples processed by spark plasma sintering are investigated. To this aim, a high purity Ni powder was nanostructured by ball milling using different processing conditions, and consolidation of the powder was performed by spark plasma sintering. Milling parameters were varied to produce samples with different microstructures. Each sample was characterised in terms of grain size and grain boundary character distribution to study the influence of milling parameters on the microstructure. Results show that suitable ball milling and subsequent sintering can be employed to obtain specimens with grain sizes in the ultrafine grain range with a high fraction of Σ3 grain boundaries. All samples exhibit a low internal stress state, which is evaluated in terms of grain orientation spread computed from electron backscatter diffraction and dislocation observation by transmission electron microscopy. Uniaxial tensile testing shows an increase in yield strength with grain refinement with a pronounced deviation from the Hall-Petch relation, for samples with grain sizes below 1.2 μm. Heterogeneity in deformation at yielding seems responsible for the deviation due to differences in the strain hardening behaviour between ultrafine grains and conventional coarse grains. All samples display high ductility with values of elongation to fracture above 35% as well as good toughness. The combination of ball milling with spark plasma sintering is therefore a promising tool to produce ultrafine grain Ni with good mechanical properties.

Examination of high-temperature mechanisms and behavior under compression and processing maps of pure copper

The hot compression behavior of pure copper (commercial grade) was studied in a temperature range between 843 and 993K and in a strain rate range between 10−3 and 10s−1. The activation energy for plastic flow was determined to be 215kJ/mol, which is similar to the activation energy for lattice diffusion in copper (=197kJ/mol). The stress exponent values associated with the plastic flow were 7–9, which are larger than the theoretical values associated with lattice-diffusion controlled dislocation climb creep (4.5–5). A comparison of the current data with the data obtained from the creep tests performed on the pure copper at low strain rates by other investigators in a similar temperature range revealed that the most of the data studied in this study belong to the regime where power-law breakdown occurs. Processing map constructed at a strain where steady-state plastic flow was observed showed that the power dissipation efficiency was low (≤21%) in the entire experimental ranges of strain rate and temperature. Discontinuous dynamic recrystallization (DDRX) associated with a single peak or multiple peaks in the stress-strain curves occurred during the first stage of plastic deformation. Dynamic recovery occurred after the DDRX activity ended, leading to the formation of a subgrains with low-and intermediate angle grain boundaries within the recrystallized grains with high-angle grain boundaries. Due to the formation of extensive substructure within DDRX grains, the fractions of high-angle grain boundaries measured after the compressive deformation were low (≤30%).

Effects of cooling condition on microstructural evolution and mechanical properties of friction stir processed 2A14 aluminum alloy

Detailed analysis on the microstructural evolution and mechanical properties of 2A14 aluminum alloy subjected to friction stir processing (FSP) under different cooling conditions has been conducted. SEM, EBSD and TEM were used to reveal the microstructure characteristic, while the microhardness and tensile tests were also performed. The results showed that microstructures formed by fine, equiaxed and highly misoriented grains with the average grain size in the range of 1.19∼3.1 μm were obtained in the stirred zone (SZ). Compared with FSP under air cooling, rapid cooling can significantly reduce grain size, second phase particles size and fraction of high angle grain boundaries in the SZ. Besides, recrystallized grains with random orientation are formed throughout SZ during FSP. The microtexture analysis also demonstrated the dominance of shear texture ({hkl}〈110〉 and {111}〈uvw〉 fibers), with C and A1* components are produced through the geometrically dynamic recrystallization mechanism. And the texture intensity in SZ of the samples is slightly increased with accelerated cooling method, indicating that another mechanism of grain refinement through discontinuous dynamic recrystallization might be involved. Additionally, the strength and microhardness of friction stir processed samples were improved with increasing the cooling rate due to grain refinement, the increase of dislocation density and fine and uniformly distributed second phase particles.

Deformation and fracture of WC grains and grain boundaries in a WC-Co hardmetal during microcantilever bending tests

The deformation and fracture behaviour of constituents of a WC-Co hardmetal were investigated by microcantilever bending technique. The compositions of FIB fabricated microcantilevers were: I) single grains of WC, II) WC grains of different orientations and III) the mixture of WC grains and Co phase. The crystallographic orientation of WC grains and the fracture surface of beams were studied by EBSD and SEM analyses, respectively. It was revealed that the elastic deformation depends mainly on the composition of the beams and the orientation of the WC grains. The Young's modulus of WC grains showed an orientation dependence with decreasing values from the basal (E~800 GPa) towards the prismatic orientations (E~500 GPa), which is in agreement with the theoretical predictions. The deformation behaviour of WC grains exhibited plasticity before their fracture with an average fracture strength of σ = 12.3 ± 3.8 GPa. It was found that the effect of dislocations and nanometre-sized defects (e.g. pores) plays an important role in the bending test of WC grains. Most of the WC/WC boundaries showed brittle failure with an average fracture strength of σ = 4.1 ± 2.5 GPa. It was concluded that the majority of the boundaries in the WC-Co composite are high energy WC/WC boundaries and their fracture strength is generally much lower than that of the WC grains.