Decrease In Grain Boundary Energy Research Articles

The effect of Hafnium on the microstructure and tensile property in a Ni-based superalloy

The effects of Hafnium on the microstructure and tensile property of a Ni-based superalloy after solution heat treatment and long-term thermal exposure were investigated with field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), X-ray diffractometer (XRD). The results revealed reveal that Hf intensified intensifies the element segregation and promoted promotes the precipitation of MC carbide and honeycomb-like Ni5Hf phase in the as-cast alloy. The solid solution strengthening and second phase (MC) strengthening effects are both enhanced in the alloy doping 0.5 wt% Hf after solution heat treatment. Therefore, the tensile yield strength at 700 °C of the alloy with 0.5 wt% Hf is higher than that in the Hf-free alloy. Otherwise, a large amount of the low melting point Ni5Hf phases appear in the alloy with 0.8 wt% Hf. Ni5Hf phase is prone to become the crack initiation, which decreases diminishes the yield strength in the alloy with 0.8 wt% Hf. The addition of 0.5 wt% Hf can inhibit the formation of needle-like δ phase at grain boundary due to the decrease of grain boundary energy and diffusion rate at grain boundary. Therefore, the 700 °C tensile strength and ductility increase simultaneously in the alloy with 0.5 wt% Hf after thermal exposure at 700 °C.

Evolution of crystalline misorientations in polycrystalline samples of pure ice

There are few studies where the evolution of grain orientations and crystal misorientations of neighboring grains in a polycrystalline ice sample during grain growth are analyzed. In this work, crystal orientations and misorientations were studied in two ice cylinders obtained from bidistilled water using the techniques of plastic replicas and chemical etching pits. The cylinders were grown, one through fast freezing and the other through slow freezing. It was observed that the initial crystal orientations of the grains varied from one cylinder to the other, but this did not result in significant differences in the final crystal misorientations between neighboring grains. Such crystal misorientations could be special because they are related to a coincidence sites lattice (CSL), which could be associated with low-energy grain boundaries. Therefore, the decrease in grain boundary energy during grain growth could be influenced by these special crystal misorientations.

Microstructure criterion for the preferential locations of helium bubble precipitation in a polycrystal

Helium bubble distribution at grain boundaries and inside grains affects materials' susceptibility to helium embrittlement. In this paper, we propose a microstructure criterion to quantitatively analyze the helium bubble preference for precipitation locations. The results show that the grain-boundary energy and the helium-to-vacancy ratio are the key factors in determining bubble formation mainly at the grain boundaries or in the grain interiors. Experimental observation based on the electron backscatter diffraction technique confirmed that helium bubbles are more likely to precipitate in the grain interiors of Inconel X750 alloy than pure nickel due to grain-boundary segregation-induced decrease in grain-boundary energy. The precipitation of intergranular bubbles can be reduced by lowering grain-boundary energy and reducing the helium-to-vacancy ratio at grain boundaries. This work provides a general framework for explaining the intrinsic relation between the formation of inter- and intra-granular helium bubbles and promising strategies for designing highly helium-enduring materials.

A quasi-in-situ EBSD study of the thermal stability and grain growth mechanisms of CoCrNi medium entropy alloy with gradient-nanograined structure

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

EFFECT OF GRAIN SIZE ON ATMOSPHERIC CORROSION RESISTANCE OF ULTRA--LOW CARBON IF STEEL

Three kinds of ultra-low carbon IF steel with different grain sizes,and same chemical composition were prepared by different rolling and heat treat process.The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test,cyclic immersion corrosion test,AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test,the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15μm to 220μm.The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15μm to 46μm.As grain size increase from 15μm to 220μm,the whole compactness of rust are increased,the rust resistance and the atmospheric corrosion resistance are increased.The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed.The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased.Meanwhile,the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased.The atmospheric corrosion resistance is influenced by the two factors simultaneously.

A thermokinetic description of nanoscale grain growth: Analysis of the activation energy effect

The inhibition of grain growth by solute segregation in nanoscale materials has often been described using kinetic models (e.g. Acta Mater 1999;47:2143) or thermokinetic models (e.g. Acta Mater 2009;57:1466), in which constant activation energy and a negligible effect of solute segregation on activation energy were assumed. In this paper, an intact thermokinetic model for nanoscale grain growth was developed by incorporating mixed effects of activation energy and grain boundary (GB) energy. By application of the model to nanoscale grain growth in Ni–P, Pb–Zr, Fe–Zr and Ru–Al alloys, the validity of the present model was confirmed, in combination with verification of the initial condition of GB segregation. On this basis, the increase of activation energy and the decrease of GB energy are interrelated and thus the kinetics and the thermodynamics of normal grain growth are linked. Based on a comparison of three characteristic velocities VTK, VGE and VAE of GB derived from the present thermokinetic model, grain boundary energy model and activation energy model, a mechanism of controlled nanoscale grain growth was proposed, which indicated a transition from a kinetic-controlled to a thermodynamic-controlled process.

Effect of grain boundary structures on grain boundary sliding in magnesium

The effect of grain boundary structures on the deformation behavior at the grain boundaries in magnesium was examined by the nanoindentation creep test. The results of the nanoindentation creep test showed that the dominant deformation mechanism around the grain boundary was grain boundary sliding; however, the occurrence of grain boundary sliding was closely related to the grain boundary energy. The grain boundary with high energy showed high strain rate sensitivity, which was the same tendency as that of the other metallic materials. Furthermore, the addition of aluminum atoms into magnesium tended to prevent the grain boundary sliding due to the decrease in grain boundary energy.

Solute Effects on β-Sn Grain Boundary Energy and Shear Stress

The excess enthalpy and shear strain behavior of the β-Sn (101) grain boundary containing various amounts of Ag and Cu solute atoms was investigated using molecular simulation. An increase in either type of solute at the interface lowers the excess enthalpy of the grain boundary. The enthalpies of segregation for both solute types are computed as the slope of excess enthalpy of the grain boundary per area versus interfacial solute concentration per area. For Ag, ΔHSEG=0.64(9)  eV/atom , and for Cu, ΔHSEG=0.45(8)  eV/atom , indicating a preference for grain boundary segregation. These results agree with other simulation work that attributes a larger decrease in grain boundary energy to a solute with a larger atomic radius. A (301) special boundary of β-Sn was also investigated and found ΔHSEG=0.12(9)  eV/atom for Ag and ΔHSEG=0.06(9)  eV/atom for Cu. Statistically, this shows the possibility for excess enthalpy increase when Cu is present at the (301) boundary. Under constant shear strain, addition of solu...

Influence of an applied dc electric field on the plastic deformation kinetics of oxide ceramics

A modest dc electric field markedly reduced the tensile flow stress at high temperatures in three polycrystalline oxides, i.e. MgO, Al2O3 and yttria-stabilized tetragonal ZrO2 (Y-TZP). The reduction in flow stress ΔσE in Y-TZP consisted of three components: (i) ΔσT due to Joule heating, (ii) a rapid, reversible component obtained in on-off and electric field step tests and (iii) the cumulative effect of the field on microstructure. Only ΔσT and occurred in MgO and Al2O3. It is concluded that results from a reduction in the electrochemical potential for the formation of vacancies corresponding to the diffusion of the rate-controlling ion in the space-charge at the grain boundary. The calculated magnitude of the space-charge zone width and its temperature and solute composition dependence are in accord with theory and experiment; is attributed mainly to the retardation of grain growth by the field. The retardation could be due to one or more of the following effects of the field on the space-charge zone: (i) an increase in the segregated solute ions, (ii) a decrease in grain boundary energy and (iii) a decrease in solute ion mobility.

On the stagnation of grain growth in nanocrystalline materials

The stagnation of grain growth with solute segregation in nanocrystalline materials is often analyzed by using two kinetic models [J.E. Burke, Trans. Metall. Soc. AIME 180 (1949) 73; A. Michels, C.E. Krill, H. Ehrhardt, R. Birringer, D.T. Wu, Acta Mater. 47 (1999) 2143], in which it is supposed that solute drag stops grain growth. However, we show that the drag force in these kinetic models is not equivalent to the solute drag force, but to the reduced driving force due to the decrease in grain boundary energy.

Computer simulation of symmetrical tilt grain boundaries in noble metals with MAEAM

This paper reports that an atomic scale study of [10] symmetrical tilt grain boundary (STGB) has been made with modified analytical embedded atom method (MAEAM) for 44 planes in three noble metals Au, Ag and Cu. For each metal, the energies of two crystals ideally joined together are unrealistically high due to very short distance between atoms near the grain boundary (GB) plane. A relative slide between grains in the GB plane results in a significant decrease in GB energy and a minimum value is obtained at specific translation distance. The minimum energy of Cu is much higher than that of Ag and Au, while the minimum energy of Ag is slightly higher than that of Au. For all the three metals, the three lowest energies correspond to identical (111), (113) and (331) boundary successively for two translations considered; from minimization of GB energy, these boundaries should be preferable in [10] STGB for noble metals. This is consistent with the experimental results. In addition, the minimum energy increases with increasing reciprocal planar coincidence density Σ, but decreases with increasing relative interplanar distance d/a.

Energy calculation for symmetrical tilt grain boundaries in iron

Abstract An atomic study of [0 0 1] symmetrical tilt grain boundary (STGB) in iron has been made with modified analytical embedded atom method (MAEAM). The energies of two rigid-body crystals joined together directly are unrealistically high due to very short distance between atoms near grain boundary (GB) plane in either crystal. For each of 27 (h k 0) GB planes, a relative slide between grains could result in a decrease in GB energy and a minimum value could be obtained at specific translation distance Lmin/L(h k 0). Three lowest minimum-energies are corresponding to (3 1 0), (5 3 0) and (5 1 0) boundary successively, from minimization of GB energy, these boundaries should be preferable in (h k 0) boundaries. In addition, the minimum energy increases with increasing ∑, but decreases with increasing interplanar spacing.

A comparison of thermal stability in nanocrystalline Ni- and Co-based materials

In this paper the microstructural development upon annealing of nanocrystalline Ni- and Co-based electrodeposits is described. New investigations on Ni, Ni–Fe, and Co–P are compared with previous results on Ni, Co, and Ni–P in terms of microstructural changes and stabilizing mechanisms. The conclusions are: pure nanocrystalline Ni and Co are stabilized by impurities in the grain boundaries. In the case of Co, also an allotropic phase transformation influences the occurrence of abnormal grain growth. Alloying and/or adding solutes is found to increase thermal stability. While in Ni-20 at.% Fe the ordering transformation is expected to be the reason for stabilization, in strongly segregating systems (Ni–P and Co–P) the stabilizing effect is the decrease in grain boundary energy due to solute segregation. After precipitation, Zener pinning still hinders grain boundary migration, but not sufficient to stabilize the nanocrystalline structure.

A comparison of thermal stability in nanocrystalline Ni- and Co-based materials

Abstract In this paper the microstructural development upon annealing of nanocrystalline Ni- and Co-based electrodeposits is described. New investigations on Ni, Ni – Fe, and Co–P are compared with previous results on Ni, Co, and Ni –P in terms of microstructural changes and stabilizing mechanisms. The conclusions are: pure nanocrystalline Ni and Co are stabilized by impurities in the grain boundaries. In the case of Co, also an allotropic phase transformation influences the occurrence of abnormal grain growth. Alloying and/or adding solutes is found to increase thermal stability. While in Ni-20 at.% Fe the ordering transformation is expected to be the reason for stabilization, in strongly segregating systems (Ni –P and Co–P) the stabilizing effect is the decrease in grain boundary energy due to solute segregation. After precipitation, Zener pinning still hinders grain boundary migration, but not sufficient to stabilize the nanocrystalline structure.

Sintering and Grain Growth Rates of Two Spheres with Different Radii

Rate equations for sintering and grain growth have been proposed on the two-sphere model that simulates shrinkage processes during heating of a ceramic powder compact. In the model it was assumed that total excess energy associated with surfaces and grain boundary drives mass transport in sintering and grain growth. The rate equations formulated here show that sintering and grain growth are activated by the same kinds of driving force and processes. They show quantitatively that the sintering rate increases with a decrease in grain boundary energy and with a decrease in the grain size difference. It is found also that the grain growth of a larger grain is much slower than a smaller grain and that the growth rate of a smaller grain is accelerated remarkably when its grain size becomes smaller compared with a lager grain.