Abstract The effects of Al and Hf impurities on the (111) antiphase boundary (APB) energy of metastable FCC Co 3 W are investigated via ab initio calculations. Cluster expansions are used to predict the total energies of supercells containing non-dilute concentrations of impurities using Monte Carlo simulations at relevant temperatures, giving APB energies as a function of impurity concentration and temperature for each ternary system. The cluster expansions are validated by comparing with direct energy calculations of supercells of pure L1 2 Co 3 W and the effects of each impurity are compared. Two sets of compositions are explored for each system — constant ratio (constant ratio between Co and W) and sacrificial W (constant Co). It is found that sacrificial W compositions stabilize the L1 2 structure over a wider range of compositions than constant ratio compositions (given the restriction to the FCC lattice) in both systems and should be preferred. In sacrificial W compositions, Hf increases the APB energy far more than Al, particularly at higher concentrations of the impurity, and both systems exhibit little variation with respect to temperature. It is further shown that at higher concentrations of Hf, and most noticeably for Co 3 (W 0.5 Hf 0.5 ), Hf and W tend to segregate into alternating planes, unlike the corresponding Co 3 (W 0.5 Al 0.5 ), which explains the different impacts of the two impurities on the APB energy. Finally, the ratio of (111) to (100) APB energies is studied for sacrificial W compositions to understand cross slip behavior in both ternary systems.
通过从头计算研究了Al和Hf杂质对亚稳FCC Co 3w(111)反相边界(APB)能量的影响。在相关温度下，利用蒙特卡罗模拟，利用团簇展开来预测含有非稀浓度杂质的超级电池的总能量，给出了每个三元体系中杂质浓度和温度的函数APB能量。通过与纯l2co3w超电池的直接能量计算进行比较，验证了团簇扩展的正确性，并比较了每种杂质的影响。对每个体系探索了两组成分——恒定比(Co和W之间的恒定比)和牺牲W(恒定Co)。在两种体系中，牺牲W组分远比定比组分稳定，应优先选择牺牲W组分。在牺牲W组分中，Hf对APB能量的增加远远大于Al，特别是在杂质浓度较高时，两种体系对温度的变化不大。进一步表明，在较高的Hf浓度下，特别是Co 3 (w0.5 Hf 0.5)， Hf和W倾向于在交替平面上分离，而不像Co 3 (w0.5 Al 0.5)，这解释了两种杂质对APB能量的不同影响。最后，研究了牺牲W组分的(111)与(100)APB能量比，以了解这两种三元体系的交叉滑移行为。
{"title":"First-Principles Study of the Effect of Al and Hf Impurities on Co 3W Antiphase Boundary Energies","authors":"C. Nataraj, R. Sun, C. Woodward, A. van de Walle","doi":"10.2139/ssrn.3807770","DOIUrl":"https://doi.org/10.2139/ssrn.3807770","url":null,"abstract":"Abstract The effects of Al and Hf impurities on the (111) antiphase boundary (APB) energy of metastable FCC Co 3 W are investigated via ab initio calculations. Cluster expansions are used to predict the total energies of supercells containing non-dilute concentrations of impurities using Monte Carlo simulations at relevant temperatures, giving APB energies as a function of impurity concentration and temperature for each ternary system. The cluster expansions are validated by comparing with direct energy calculations of supercells of pure L1 2 Co 3 W and the effects of each impurity are compared. Two sets of compositions are explored for each system — constant ratio (constant ratio between Co and W) and sacrificial W (constant Co). It is found that sacrificial W compositions stabilize the L1 2 structure over a wider range of compositions than constant ratio compositions (given the restriction to the FCC lattice) in both systems and should be preferred. In sacrificial W compositions, Hf increases the APB energy far more than Al, particularly at higher concentrations of the impurity, and both systems exhibit little variation with respect to temperature. It is further shown that at higher concentrations of Hf, and most noticeably for Co 3 (W 0.5 Hf 0.5 ), Hf and W tend to segregate into alternating planes, unlike the corresponding Co 3 (W 0.5 Al 0.5 ), which explains the different impacts of the two impurities on the APB energy. Finally, the ratio of (111) to (100) APB energies is studied for sacrificial W compositions to understand cross slip behavior in both ternary systems.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"30 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83543048","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract The ability to quickly analyze large imaging datasets is vital to the widespread adoption of modern materials characterization tools, and thus the development of new materials. Image segmentation can be the most subjective and time-consuming step in the data analysis workflow. A promising approach to segmentation of large materials datasets is the use of convolutional neural networks (CNNs). However, a major challenge is to obtain the images and segmentations needed for CNN training, since this requires segmentations performed by humans. We show that it is possible to segment experimental materials science data using a SegNet-based CNN that was trained only using simple phase field simulations. A test image from an in-situ solidification experiment of an Al-Zn alloy was used to parameterize the phase field simulations. The most important microstructural features required for the best CNN to “understand” the contents of the image are ranked as: (1) having training images with diffuse particle-background interfaces, (2) modifying the images by adding noise, (3) removing particles at the image edges, and (4) adding sub-images to the particles to account for the feint bands present on some dendrites. The CNN trained on phase field images segmented the experimental test image with 99.3% accuracy, comparable to CNNs trained on experimental data. This approach of using computationally generated images to train CNNs capable of segmenting experiments will accelerate the rate of materials design and discovery.
{"title":"Segmentation of Experimental Datasets Via Convolutional Neural Networks Trained on Phase Field Simulations","authors":"Jiwon Yeom, T. Stan, Seungbum Hong, P. Voorhees","doi":"10.2139/ssrn.3680400","DOIUrl":"https://doi.org/10.2139/ssrn.3680400","url":null,"abstract":"Abstract The ability to quickly analyze large imaging datasets is vital to the widespread adoption of modern materials characterization tools, and thus the development of new materials. Image segmentation can be the most subjective and time-consuming step in the data analysis workflow. A promising approach to segmentation of large materials datasets is the use of convolutional neural networks (CNNs). However, a major challenge is to obtain the images and segmentations needed for CNN training, since this requires segmentations performed by humans. We show that it is possible to segment experimental materials science data using a SegNet-based CNN that was trained only using simple phase field simulations. A test image from an in-situ solidification experiment of an Al-Zn alloy was used to parameterize the phase field simulations. The most important microstructural features required for the best CNN to “understand” the contents of the image are ranked as: (1) having training images with diffuse particle-background interfaces, (2) modifying the images by adding noise, (3) removing particles at the image edges, and (4) adding sub-images to the particles to account for the feint bands present on some dendrites. The CNN trained on phase field images segmented the experimental test image with 99.3% accuracy, comparable to CNNs trained on experimental data. This approach of using computationally generated images to train CNNs capable of segmenting experiments will accelerate the rate of materials design and discovery.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"102 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90803075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. S. Samantaray, P. Anees, Vinayan Bhaghavathi Parambath, R. S.
Abstract Experimental studies along with density functional theory (DFT) calculations have been performed to probe and understand the hydrogen storage properties of graphene supported MgNi alloy nanoparticles nanocomposites. The main highlight of the present work is that nanostructuring and alloying of Mg with Ni and its dispersion on graphene (MgNi/G) & nitrogen/boron doped graphene (MgNi/NG, MgNi/BG) resulted in a substantial enhancement in the hydrogen storage capacity. The hydrogen storage capacity measured at room temperature and at 3 MPa pressure for MgNi/G, MgNi/BG and MgNi/NG are ∼2.5 wt. %, 3.5 wt. % and 5.4 wt. % respectively. The large improvement in hydrogen storage capacity compared to the reported values of G (0.5 wt. %), BG (0.7 wt. %) and NG (0.9 wt. %) under the same experimental conditions is noteworthy. DFT calculations shed light on the adsorption mechanism underlying this enhanced hydrogen uptake capacity and corroborate the experimental results.
{"title":"Graphene Supported MgNi Alloy Nanocomposite as a Room Temperature Hydrogen Storage Material – Experiments and Theoretical Insights","authors":"S. S. Samantaray, P. Anees, Vinayan Bhaghavathi Parambath, R. S.","doi":"10.2139/ssrn.3762207","DOIUrl":"https://doi.org/10.2139/ssrn.3762207","url":null,"abstract":"Abstract Experimental studies along with density functional theory (DFT) calculations have been performed to probe and understand the hydrogen storage properties of graphene supported MgNi alloy nanoparticles nanocomposites. The main highlight of the present work is that nanostructuring and alloying of Mg with Ni and its dispersion on graphene (MgNi/G) & nitrogen/boron doped graphene (MgNi/NG, MgNi/BG) resulted in a substantial enhancement in the hydrogen storage capacity. The hydrogen storage capacity measured at room temperature and at 3 MPa pressure for MgNi/G, MgNi/BG and MgNi/NG are ∼2.5 wt. %, 3.5 wt. % and 5.4 wt. % respectively. The large improvement in hydrogen storage capacity compared to the reported values of G (0.5 wt. %), BG (0.7 wt. %) and NG (0.9 wt. %) under the same experimental conditions is noteworthy. DFT calculations shed light on the adsorption mechanism underlying this enhanced hydrogen uptake capacity and corroborate the experimental results.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"75 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86173780","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Defects play a key role in deciding the mechanisms and kinetics of phase transformations. In this paper, we show how dislocations influence phase separation in alloys with miscibility gap. Specifically, depending on the ratio of pipe mobility to bulk mobility, it is seen that even in a system with nominal compositions outside the spinodal limit, spinodal phase separation is possible. Surprisingly, phase separation through both nucleation and growth, and spinodal decomposition, is seen concurrently (for the case of intersecting dislocations). Finally, the prominent role played by dislocations in influencing the morphology of precipitates is explored. We show that these results agree qualitatively with recent experimental results in iron based systems obtained using Atom Probe Tomography (APT).
{"title":"Dislocation Assisted Phase Separation: A Phase Field Study","authors":"R. Arjun Varma, P. Pant, M. Gururajan","doi":"10.2139/ssrn.3910594","DOIUrl":"https://doi.org/10.2139/ssrn.3910594","url":null,"abstract":"Defects play a key role in deciding the mechanisms and kinetics of phase transformations. In this paper, we show how dislocations influence phase separation in alloys with miscibility gap. Specifically, depending on the ratio of pipe mobility to bulk mobility, it is seen that even in a system with nominal compositions outside the spinodal limit, spinodal phase separation is possible. Surprisingly, phase separation through both nucleation and growth, and spinodal decomposition, is seen concurrently (for the case of intersecting dislocations). Finally, the prominent role played by dislocations in influencing the morphology of precipitates is explored. We show that these results agree qualitatively with recent experimental results in iron based systems obtained using Atom Probe Tomography (APT).","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"135 ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-07-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91551593","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Ahmad, P. Strak, P. Kempisty, K. Sakowski, J. Piechota, Y. Kangawa, I. Grzegory, M. Leszczyński, Z. Zytkiewicz, G. Muzioł, E. Monroy, A. Kamińska, S. Krukowski
In this work, we study the emergence of polarization doping in AlxGa1−xN layers with graded composition from a theoretical viewpoint. It is shown that bulk electric charge density emerges in the graded concentration region. The magnitude of the effect, i.e., the relation between the polarization bulk charge density and the concentration gradient is obtained. The appearance of mobile charge in the wurtzite structure grown along the polar direction was investigated using the combination of ab initio and drift-diffusion models. It was shown that the ab initio results can be recovered precisely by proper parameterization of drift-diffusion representation of the complex nitride system. It was shown that the mobile charge appears due to the increase of the distance between opposite polarization-induced charges. It was demonstrated that, for sufficiently large space distance between polarization charges, the opposite mobile charges are induced. We demonstrate that the charge conservation law applies for fixed and mobile charge separately, leading to nonlocal compensation phenomena involving (i) the bulk fixed and polarization sheet charge at the heterointerfaces and (ii) the mobile band and the defect charge. Therefore, two charge conservation laws are obeyed that induces nonlocality in the system. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities.
{"title":"Polarization Doping - Ab Initio Verification of the Concept: Charge Conservation and Locality","authors":"A. Ahmad, P. Strak, P. Kempisty, K. Sakowski, J. Piechota, Y. Kangawa, I. Grzegory, M. Leszczyński, Z. Zytkiewicz, G. Muzioł, E. Monroy, A. Kamińska, S. Krukowski","doi":"10.2139/ssrn.3920962","DOIUrl":"https://doi.org/10.2139/ssrn.3920962","url":null,"abstract":"In this work, we study the emergence of polarization doping in AlxGa1−xN layers with graded composition from a theoretical viewpoint. It is shown that bulk electric charge density emerges in the graded concentration region. The magnitude of the effect, i.e., the relation between the polarization bulk charge density and the concentration gradient is obtained. The appearance of mobile charge in the wurtzite structure grown along the polar direction was investigated using the combination of ab initio and drift-diffusion models. It was shown that the ab initio results can be recovered precisely by proper parameterization of drift-diffusion representation of the complex nitride system. It was shown that the mobile charge appears due to the increase of the distance between opposite polarization-induced charges. It was demonstrated that, for sufficiently large space distance between polarization charges, the opposite mobile charges are induced. We demonstrate that the charge conservation law applies for fixed and mobile charge separately, leading to nonlocal compensation phenomena involving (i) the bulk fixed and polarization sheet charge at the heterointerfaces and (ii) the mobile band and the defect charge. Therefore, two charge conservation laws are obeyed that induces nonlocality in the system. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"38 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82089974","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Aradi, J. Lewis-Fell, G. Greaves, Stephen E. Donnelly, J. Hinks
Abstract Using in-situ transmission electron microscopy (TEM) with ion irradiation, we investigated the microstructural changes in silicon carbide nanowhiskers (SiC NWs) which were used as a model system for nanoporous SiC. Irradiations were carried out using 6 keV He ions at temperatures between 500 and 1000°C and doses up to 20 dpa. These results are compared with the irradiation effects in SiC thin foils under the same conditions to establish differences in their response to radiation damage. The irradiation temperature played a significant role in the evolution of different microstructures; at 500°C, small defect clusters were observed in the NWs together with a segregation of carbon at the surface of the NWs mapped using energy-filtered TEM (EFTEM). At 800°C, small He bubbles (2–4 nm in diameter) were observed in the NW matrix while He platelets and bubble discs formed in the foils. At 1000°C, several changes were observed in the NWs including bubbles at twin boundaries, voids and oxygen-rich precipitates. The large surface area to volume ratio enhances defect recombination supressing the defect density in the SiC NWs compared to the foils indicating high radiation tolerance; however, elemental segregation and precipitation may limit its application in advanced nuclear reactors.
{"title":"In Situ TEM Investigations of the Microstructural Changes and Radiation Tolerance in SiC Nanowhiskers Irradiated with He Ions at High Temperatures","authors":"E. Aradi, J. Lewis-Fell, G. Greaves, Stephen E. Donnelly, J. Hinks","doi":"10.2139/ssrn.3674678","DOIUrl":"https://doi.org/10.2139/ssrn.3674678","url":null,"abstract":"Abstract Using in-situ transmission electron microscopy (TEM) with ion irradiation, we investigated the microstructural changes in silicon carbide nanowhiskers (SiC NWs) which were used as a model system for nanoporous SiC. Irradiations were carried out using 6 keV He ions at temperatures between 500 and 1000°C and doses up to 20 dpa. These results are compared with the irradiation effects in SiC thin foils under the same conditions to establish differences in their response to radiation damage. The irradiation temperature played a significant role in the evolution of different microstructures; at 500°C, small defect clusters were observed in the NWs together with a segregation of carbon at the surface of the NWs mapped using energy-filtered TEM (EFTEM). At 800°C, small He bubbles (2–4 nm in diameter) were observed in the NW matrix while He platelets and bubble discs formed in the foils. At 1000°C, several changes were observed in the NWs including bubbles at twin boundaries, voids and oxygen-rich precipitates. The large surface area to volume ratio enhances defect recombination supressing the defect density in the SiC NWs compared to the foils indicating high radiation tolerance; however, elemental segregation and precipitation may limit its application in advanced nuclear reactors.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"31 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87517266","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Subing Yang, Y. Nakagawa, Minako Kondo, T. Shibayama
Abstract Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He+ irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He+-irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He+-ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([ 11 2 ¯ 0 ] and [ 10 1 ¯ 0 ]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He+-irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He+-irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([ 10 1 ¯ 0 ] and [ 11 2 ¯ 0 ])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.
{"title":"Anisotropic Defect Distribution in He+-Irradiated 4h-Sic: Effect of Stress on Defect Distribution","authors":"Subing Yang, Y. Nakagawa, Minako Kondo, T. Shibayama","doi":"10.2139/ssrn.3751554","DOIUrl":"https://doi.org/10.2139/ssrn.3751554","url":null,"abstract":"Abstract Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He+ irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He+-irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He+-ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([ 11 2 ¯ 0 ] and [ 10 1 ¯ 0 ]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He+-irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He+-irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([ 10 1 ¯ 0 ] and [ 11 2 ¯ 0 ])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"25 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81715501","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Wenwu Xu, A. Maksymenko, Shahrier Hasan, J. Meléndez, E. Olevsky
Abstract The atomistic structural modification and ionic diffusivity in 8YSZ at elevated temperature with the presence of an external electric field (E-field) were investigated by molecular dynamics (MD) simulations. As an example, a sufficiently large system of atomic configuration for a bi-crystal ∑ 5 ( 310 ) / [ 001 ] grain boundary (GB) model was studied. MD results show that the E-field promotes the formation of Frenkel pair defects. Notably, some Zr ions diffuse out of GB regions at E-field > ∼ 500-1000V/cm, resulting in an “avalanche” of cation vacancies at GBs and reduction of GB space charge. Consequently, the diffusivities of cations and anions are enhanced in the 8YSZ system with the presence of E-field. The atomistic level understanding of E-field induced structural modifications and ionic diffusivities provide an in-depth insight to unravel the flash sintering mechanisms of ionic ceramics, especially the coupled thermal-field effect during the flash sintering process.
{"title":"Effect of External Electric Field on Diffusivity and Flash Sintering of 8ysz: A Molecular Dynamics Study","authors":"Wenwu Xu, A. Maksymenko, Shahrier Hasan, J. Meléndez, E. Olevsky","doi":"10.2139/ssrn.3711245","DOIUrl":"https://doi.org/10.2139/ssrn.3711245","url":null,"abstract":"Abstract The atomistic structural modification and ionic diffusivity in 8YSZ at elevated temperature with the presence of an external electric field (E-field) were investigated by molecular dynamics (MD) simulations. As an example, a sufficiently large system of atomic configuration for a bi-crystal ∑ 5 ( 310 ) / [ 001 ] grain boundary (GB) model was studied. MD results show that the E-field promotes the formation of Frenkel pair defects. Notably, some Zr ions diffuse out of GB regions at E-field > ∼ 500-1000V/cm, resulting in an “avalanche” of cation vacancies at GBs and reduction of GB space charge. Consequently, the diffusivities of cations and anions are enhanced in the 8YSZ system with the presence of E-field. The atomistic level understanding of E-field induced structural modifications and ionic diffusivities provide an in-depth insight to unravel the flash sintering mechanisms of ionic ceramics, especially the coupled thermal-field effect during the flash sintering process.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"51 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86463126","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The activation of non-basal pyramidal slip has been perceived as key to enhance the ductility of magnesium and its alloys. However, there has never been convincing evidence to show the physical existence of dislocations and their involvement in deformation has been a core issue in magnesium research. In the present work, the impossibility of slip is analyzed based on fundamental concepts of dislocation and atomic interactions. The atomic configurations and crystallographic features in association with dislocations are unambiguously revealed for the first time, demonstrating that any possible dislocation core structures would involve too many atoms on multiple lattice planes and are physically impossible. Experiments of magnesium single crystal compression along its c-axis were conducted at temperatures from 20°C to 500°C and the results showed no evidence of the involvement of dislocations in any form as a mechanism of deformation during either plastic flow or fracture. von Mises criterion for compatible deformation, which drives the pursuit of pyramidal slip, is critically discussed.
{"title":"Crystallographic and Experimental Disproof of Pyramidal < c+a > Slip in Magnesium","authors":"Yan Huang, Xinliang Yang, Jun Jiang","doi":"10.2139/ssrn.3793928","DOIUrl":"https://doi.org/10.2139/ssrn.3793928","url":null,"abstract":"The activation of non-basal pyramidal slip has been perceived as key to enhance the ductility of magnesium and its alloys. However, there has never been convincing evidence to show the physical existence of dislocations and their involvement in deformation has been a core issue in magnesium research. In the present work, the impossibility of slip is analyzed based on fundamental concepts of dislocation and atomic interactions. The atomic configurations and crystallographic features in association with dislocations are unambiguously revealed for the first time, demonstrating that any possible dislocation core structures would involve too many atoms on multiple lattice planes and are physically impossible. Experiments of magnesium single crystal compression along its c-axis were conducted at temperatures from 20°C to 500°C and the results showed no evidence of the involvement of dislocations in any form as a mechanism of deformation during either plastic flow or fracture. von Mises criterion for compatible deformation, which drives the pursuit of pyramidal slip, is critically discussed.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85624400","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A combination of high strength and reasonable ductility has been achieved in a copper-1.7 at.%titanium alloy deformed by high-pressure torsion. Grain refinement and a spinodal microstructure provided a hardness of 254 ± 2 Hv, yield strength of 800 MPa and elongation of 10%. The spinodal structure persisted during isothermal ageing, further increasing the yield strength to 890MPa while retaining an elongation of 7%. This work demonstrates the potential for spinodal microstructures to overcome the difficulties in retaining ductility in ultra-fine grained or nanocrystalline alloys, especially upon post- deformation heating where strain softening normally results in brittle behavior.
{"title":"Spinodal Decomposition Stabilizes Plastic Flow in a Nanocrystalline Cu-Ti Alloy","authors":"J. Rosalie, O. Renk, R. Pippan","doi":"10.2139/ssrn.3793931","DOIUrl":"https://doi.org/10.2139/ssrn.3793931","url":null,"abstract":"A combination of high strength and reasonable ductility has been achieved in a copper-1.7 at.%titanium alloy deformed by high-pressure torsion. Grain refinement and a spinodal microstructure provided a hardness of 254 ± 2 H<i>v</i>, yield strength of 800 MPa and elongation of 10%. The spinodal structure persisted during isothermal ageing, further increasing the yield strength to 890MPa while retaining an elongation of 7%. This work demonstrates the potential for spinodal microstructures to overcome the difficulties in retaining ductility in ultra-fine grained or nanocrystalline alloys, especially upon post- deformation heating where strain softening normally results in brittle behavior.","PeriodicalId":7755,"journal":{"name":"AMI: Acta Materialia","volume":"74 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-02-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77383940","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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