Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACT

Intriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ～ 25?nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ～ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.     

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.     

Mechanically driven grain boundary relaxation: a mechanism for cyclic hardening in nanocrystalline Ni

Molecular dynamics simulations are used to show that cyclic mechanical loading can relax the non-equilibrium grain boundary (GB) structures of nanocrystalline metals by dissipating energy and reducing the average atomic energy of the system, leading to higher strengths. The GB processes that dominate deformation in these materials allow low-energy boundary configurations to be found through kinematically irreversible structural changes during cycling, which increases the subsequent resistance to plastic deformation.     

Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.     

Effect of initial texture on deformation-induced grain growth in pulsed electrodeposited microcrystalline copper

Owing to the presence of large fraction of grain boundaries, deformation-induced grain growth is commonly observed in fine-grained electrodeposited metals. Here we demonstrate that microcrystalline copper (d～1–10?µm) with different textures produced by electrodeposition exhibit significant deformation-induced grain growth in tension by coalescence along with twin boundary migration and detwinning. Oriented growth with the formation of a cube texture was noted in the deformed samples. There was an increased fraction of twin boundaries with large angular deviation from Brandon's criterion during deformation.     

Stress-enhanced grain growth in a nanostructured aluminium alloy during spark plasma sintering

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration.     

Enhanced grain growth in yttria-doped zirconia thin film structures synthesized under photon irradiation

Enhanced grain growth in ultra-thin yttria-doped zirconia (YDZ) films synthesized under ultraviolet (UV) irradiation is reported. The mean grain size in UV-synthesized 7.5 mol% YDZ films nearly 56 nm thick increased to 85 nm upon annealing in an oxygen-rich ambient at 900°C for 1 h, while in thermally grown YDZ they grew only to ～15 nm under identical annealing conditions. In situ electron microscopy kinetic studies reveal an enhanced kinetic constant and self-limiting grain growth behaviour in the UV-synthesized oxide films. The difference between UV and thermally grown films was not significant in undoped films when compared to the case of 7.5 mol% YDZ.     

New understanding of the role of coincidence site lattice boundaries in abnormal grain growth of aluminium alloy

The sequential microstructure evolution of abnormal grain growth (AGG) in the aluminium alloy (AA5052) was investigated to analyse the migration behaviour of coincidence site lattice (CSL) boundaries, which are known to play an important role in inducing AGG. The sequential evolution showed that CSL boundaries tend to disappear more slowly than general boundaries at the growth front of abnormally growing grains. Especially, the migration rate of Σ9 boundaries was noticeably low, which is contrary to the previous suggestions.     

Pathways for psychological adjustment in breast cancer: A longitudinal study on coping strategies and posttraumatic growth

This longitudinal study examined the role of coping strategies and posttraumatic growth (PTG) on the psychological adjustment to breast cancer trajectory. The participants were 50 women assessed at the time of surgery (T1), during adjuvant treatment (T2) and six months after the end of treatment (T3). Women completed questionnaires assessing coping strategies, PTG and psychological adjustment (psychological quality of life, anxiety and depression). Results showed that the greatest impact of breast cancer on women's adjustment occurred at T1, when patients were significantly more anxious than in the other phases of the disease. The type of surgery and adjuvant treatment did not account for the course of PTG and adjustment. Coping through seeking social support and using cognitive strategies at T1 were linked to psychological quality of life and depression at T3 via PTG dimension of personal resources and skills at T2. Findings emphasise the value of promoting adaptive coping strategies and PTG in order to improve psychological adjustment in breast cancer patients.     

Posttraumatic growth in survivors of a natural disaster: the role of social axioms of religiosity,reward for application,and social cynicism

The positive psychology movement has impelled trauma research to focus not only on debilitating aspects of trauma experiences, but also on its positive outcomes such as posttraumatic growth (PTG). We investigated how the social themes of religiosity, reward for application, and social cynicism, as part of the individual’s distal culture, contribute to PTG among natural disaster survivors. We tested a model wherein social axioms of religiosity, reward for application, and social cynicism predict PTG, and their relationships are mediated by adaptive cognitive processing of trauma. The model was tested in a sample of Filipino survivors of typhoon Haiyan using structural equation modeling. The results support the proposed model except for the influence of social cynicism. The findings suggest that generalized beliefs about the social world play an important role in the development of PTG, and must be considered to understand how growth occurs in the aftermath of a traumatic event.