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James S Speck - One of the best experts on this subject based on the ideXlab platform.
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effect of carrier gas and substrate misorientation on the structural and optical properties of m plane ingan gan light emitting diodes
Journal of Crystal Growth, 2010Co-Authors: Robert M Farrell, Kathryn M Kelchner, Daniel A. Haeger, Kenji Fujito, Asako Hirai, Arpan Chakraborty, S Keller, X. Chen, James S SpeckAbstract:Abstract The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.
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effect of carrier gas and substrate misorientation on the structural and optical properties of m plane ingan gan light emitting diodes
Journal of Crystal Growth, 2010Co-Authors: Robert M Farrell, Kathryn M Kelchner, Daniel A. Haeger, Kenji Fujito, Asako Hirai, Arpan Chakraborty, S Keller, X. Chen, Steven P Denbaars, James S SpeckAbstract:Abstract The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.
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influence of the substrate misorientation on the properties of n polar gan films grown by metal organic chemical vapor deposition
Journal of Applied Physics, 2007Co-Authors: S Keller, James S Speck, N A Fichtenbaum, David F Brown, A Rosales, Steven P Denbaars, U K MishraAbstract:Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar Ga...
S Keller - One of the best experts on this subject based on the ideXlab platform.
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effect of carrier gas and substrate misorientation on the structural and optical properties of m plane ingan gan light emitting diodes
Journal of Crystal Growth, 2010Co-Authors: Robert M Farrell, Kathryn M Kelchner, Daniel A. Haeger, Kenji Fujito, Asako Hirai, Arpan Chakraborty, S Keller, X. Chen, James S SpeckAbstract:Abstract The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.
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effect of carrier gas and substrate misorientation on the structural and optical properties of m plane ingan gan light emitting diodes
Journal of Crystal Growth, 2010Co-Authors: Robert M Farrell, Kathryn M Kelchner, Daniel A. Haeger, Kenji Fujito, Asako Hirai, Arpan Chakraborty, S Keller, X. Chen, Steven P Denbaars, James S SpeckAbstract:Abstract The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.
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influence of the substrate misorientation on the properties of n polar gan films grown by metal organic chemical vapor deposition
Journal of Applied Physics, 2007Co-Authors: S Keller, James S Speck, N A Fichtenbaum, David F Brown, A Rosales, Steven P Denbaars, U K MishraAbstract:Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar Ga...
Yasuhiko Arakawa - One of the best experts on this subject based on the ideXlab platform.
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Misorientation-angle dependence of GaN layers grown on a-plane sapphire substrates by metalorganic chemical vapor deposition
Applied Physics Letters, 2001Co-Authors: T. Someya, K Hoshino, Yasuhiko ArakawaAbstract:High-quality GaN with smooth surface morphology has been grown on vicinal a-plane sapphire substrates by metalorganic chemical vapor deposition. The misorientation angles of vicinal a-plane sapphire substrates were changed systematically and the results were compared with the growth on exact a- and c-plane sapphire substrates. Surface morphology and crystalline qualities are found to be very sensitive to misorientation angles of a-plane sapphire substrates and the misorientation angle was optimized to be 0.25°.
Steven P Denbaars - One of the best experts on this subject based on the ideXlab platform.
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effect of carrier gas and substrate misorientation on the structural and optical properties of m plane ingan gan light emitting diodes
Journal of Crystal Growth, 2010Co-Authors: Robert M Farrell, Kathryn M Kelchner, Daniel A. Haeger, Kenji Fujito, Asako Hirai, Arpan Chakraborty, S Keller, X. Chen, Steven P Denbaars, James S SpeckAbstract:Abstract The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1 0} m -plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.
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influence of the substrate misorientation on the properties of n polar gan films grown by metal organic chemical vapor deposition
Journal of Applied Physics, 2007Co-Authors: S Keller, James S Speck, N A Fichtenbaum, David F Brown, A Rosales, Steven P Denbaars, U K MishraAbstract:Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar Ga...
Qiang Fengming - One of the best experts on this subject based on the ideXlab platform.
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Évolution de la microstructure et de la texture de la phase α à haute température dans l'alliage TiAl au cours du traitement thermomécanique
HAL CCSD, 2021Co-Authors: Qiang FengmingAbstract:Increasing demands on modern turbines require (α2+γ) lamellar-structured TiAl alloys with fine colony size and properly aligned lamellae. According to the α → α2+γ phase transformation, the lamellar structure depends directly on the high-temperature α phase. Thus, the lamellar structure optimization could be realized by the modification of high-temperature α phase through thermomechanical processing. In this work, a thorough investigation was conducted on the high-temperature α phase in two TiAl alloys in terms of the deformation behavior, dynamic recrystallization (DRX), grain growth and texture evolution.Under uniaxial compression, the DRX of the α phase is a continuous fragmentation process (CDRX) in three characteristic stages: ⅰ) serrations of grain boundary and formation of symmetrical-tilt boundaries with Disorientation axis near boundary bulging regions; ⅱ) formation of subgrains by evolving symmetrical-tilt boundaries into asymmetrical-tilt boundaries with Disorientation axis by absorbing basal dislocations, or tilt-twist boundaries with Disorientation axis by rotational movements of the bulged parts; ⅲ) detachment of subgrains from the parent grain with gradually increased misorientation, and mixture with the ones fragmented from other grains by grain boundary sliding. These three processes happened repeatedly from grain boundary regions toward grain interiors until the whole initial microstructure was replaced by the DRXed one. The plastic deformation of the α phase is closely related to the crystallographic orientations of the initial α grains that play an important role in the deformation mechanism and the CDRX progress (softening). For the soft α grains, CDRX was completed quickly at a relatively small macroscopic strain by intragranular dislocation slip. The hard α grains demonstrated two ways: ⅰ) grains with //LD, in which dislocation accumulation was only assisted by the local strain accommodation with the neighboring α grains from boundary regions; ⅱ) grains with //LD, in which dislocation accumulation was achieved by kinking through basal slip and dislocation slip in the boundary regions from incompatible local strain. These grains required large strain to accumulate sufficient dislocations for CDRX. The strain-resolved contribution of the deformation (hardening) and CDRX (softening) result in the specific flow stress states. The texture evolution is mainly induced by dislocation slip. The orientations of the DRXed grains were largely inherited from those of the parents. With the deformation, the tilted basal fiber typed orientations developed in both the retained coarse α and the formed DRXed α grains. The hot compression produces refined α grains but not expected texture to align (α2+γ) lamellae. The microstructure and texture of the α phase during hot extrusion in the (α+β) phase region exhibit different features. The extruded microstructure was very heterogenous, comprising a large population of unRXed α grains and fine primary RXed α grains with //ED and a small population of grown α grains with //ED. The two texture components are beneficial for lamella alignment. Besides, the thermally-induced α → β phase transformation interweaving with the abnormal α grain growth happened during the transition from extrusion to water-quenching, producing two types of β particles: i) intergranular β particles with the Burger OR with their neighboring α grain; ii) intragranular β particles without the OR with the hosts from the intergranular β ones after being swallowed by the abnormally grown α grains.La demande de performances accrues pour les turbines d’avion nécessitent d’utiliser des alliages TiAl à structure lamellaire (α2+γ) en contrôlant la taille des colonies et l’orientation des lamelles. Comme la structure lamellaire dépend directement de la phase haute température (HT), l'optimisation de la structure lamellaire peut être réalisée en modifiant la phase HT par un traitement thermomécanique adéquat. Dans ce travail, une étude approfondie a été menée sur la phase HT α dans deux alliages TiAl en termes de comportement lors de la déformation, de la recristallisation dynamique (DRX), de la croissance des grains et d'évolution de la texture.Sous compression uniaxiale, la DRX de la phase α se déroule selon un processus de fragmentation continue en trois étapes caractéristiques: ⅰ) déformation des joints de grain initiaux en dentures et formation de sous-joints de flexion symétrique avec axe de désorientation ; ⅱ) Évolution des joints de flexion symétriques en joints de flexion asymétrique avec axe de désorientation ou joints de flexion-torsion avec axe de désorientation ; ⅲ) détachement des sous-grains du grain parent et mélange avec les fragments des autres grains par glissement aux joints de grains. Ce processus de fragmentation se produit d’abord dans les régions proches des joints de grain initiaux puis de proche en proche vers l'intérieur des grains. La déformation de la phase α est étroitement liée aux orientations des grains α initiaux. Pour les grains mous, la DRX les transforme complètement pour une déformation macroscopique relativement petite. Les grains durs sont de deux types: ⅰ) grains avec //LD, pour lesquels l'accumulation de dislocations n’est assistée que par l'accommodation locale des déformations avec les grains α voisins; ⅱ) grains avec //LD, pour lesquels l'accumulation de dislocations peut aussi être obtenue par pliage par glissement basal. Ces grains nécessitent une grande contrainte pour accumuler suffisamment de dislocations pour amorcer la DRX. Le comportement mécanique résulte des contributions respectives de la déformation et de la DRX au durcissement et à l’adoucissement. L'évolution de la texture est principalement induite par le glissement des dislocations. Les orientations des grains recristallisés sont largement héritées de celles de leurs parents. Avec la déformation, des orientations de type fibre avec plan basal incliné se sont développées à la fois dans les gros grains retenus (grains durs) et dans les grains formés par DRX (grains mous). Cette texture ne permet pas d’aligner suffisamment les lamelles (α2+γ). Toutefois la compression à chaud produit des grains fins.La microstructure et la texture de la phase après extrusion à chaud dans le domaine de phases (α+β) présentent des caractéristiques différentes. La microstructure extrudée est très hétérogène, comprenant d’une part une population majoritaire constituée à la fois de grains non recristallisés et de petits grains recristallisés avec une texture de fibre //ED et d’autre part une population minoritaire de grains ayant subi une croissance anormale avec une texture de fibre //ED. Les deux composantes de texture sont bénéfiques pour l'alignement des lamelles. La transformation de phase α → β induite thermiquement interfère avec la croissance anormale des grains pendant le transfert de l'extrusion à la trempe à l'eau, produisant deux types de particules β: i) particules β intergranulaires avec relation d’orientation de type Burger avec leur grain voisin; ii) particules β intragranulaires sans relation d’orientation avec le grain hôte qui correspondent à des particules β intergranulaires qui ont été ensuite envahies par des grains α en croissance anormale
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Évolution de la microstructure et de la texture de la phase α à haute température dans l'alliage TiAl au cours du traitement thermomécanique
2021Co-Authors: Qiang FengmingAbstract:La demande de performances accrues pour les turbines d’avion nécéssitent d’utiliser des alliages TiAl à structure lamellaire (α2+γ) en contrôlant la taille des colonies et l’orientation des lamelles. Comme la structure lamellaire dépend directement de la phase haute température (HT), l'optimisation de la structure lamellaire peut être réalisée en modifiant la phase HT par un traitement thermomécanique adéquat. Dans ce travail, une étude approfondie a été menée sur la phase HT α dans deux alliages TiAl en termes de comportement lors de la déformation, de la recristallisation dynamique (DRX), de la croissance des grains et d'évolution de la texture.Sous compression uniaxiale, la DRX de la phase α se déroule selon un processus de fragmentation continue en trois étapes caractéristiques: ⅰ) déformation des joints de grain initiaux en dentures et formation de sous-joints de flexion symétrique avec axe de désorientation ; ⅱ) Evolution des joints de flexion symétriques en joints de flexion asymétrique avec axe de désorientation ou joints de flexion-torsion avec axe de Disorientation ; ⅲ) détachement des sous-grains du grain parent et mélange avec les fragments des autres grains par glissement aux joints de grains. Ce processus de fragmentation se produit d’abord dans les régions proches des joints de grain initiaux puis de proche en proche vers l'intérieur des grains. La déformation de la phase α est étroitement liée aux orientations des grains α initiaux. Pour les grains mous, la DRX les transforme complètement pour une déformation macroscopique relativement petite. Les grains durs sont de deux types: ⅰ) grains avec //LD, pour lesquels l'accumulation de dislocations n’est assistée que par l'accommodation locale des déformations avec les grains α voisins; ⅱ) grains avec //LD, pour lesquels l'accumulation de dislocations peut aussi être obtenue par pliage par glissement basal. Ces grains nécessitent une grande contrainte pour accumuler suffisamment de dislocations pour amorcer la DRX. Le comportement mécanique résulte des contributions respectives de la déformation et de la DRX au durcissement et à l’adoucissement. L'évolution de la texture est principalement induite par le glissement des dislocations. Les orientations des grains recristallisés sont largement héritées de celles de leurs parents. Avec la déformation, des orientations de type fibre avec plan basal incliné se sont développées à la fois dans les gros grains retenus (grains durs) et dans les grains formés par DRX (grains mous). Cette texture ne permet pas d’aligner suffisamment les lamelles (α2+γ). Toutefois la compression à chaud produit des grains fins.La microstructure et la texture de la phase après extrusion à chaud dans le domaine de phases (α+β) présentent des caractéristiques différentes. La microstructure extrudée est très hétérogène, comprenant d’une part une population majoritaire constituée à la fois de grains non recristallisés et de petits grains recristallisés avec une texture de fibre //ED et d’autre part une population minoritaire de grains ayant subi une croissance anormale avec une texture de fibre //ED. Les deux composantes de texture sont bénéfiques pour l'alignement des lamelles. La transformation de phase α → β induite thermiquement interfere avec la croissance anormale des grains pendant le transfert de l'extrusion à la trempe à l'eau, produisant deux types de particules β: i) particules β intergranulaires avec relation d’orientation de type Burger avec leur grain voisin; ii) particules β intragranulaires sans relation d’orientation avec le grain hôte qui correspondent à des particules β intergranulaires qui ont été ensuite envahies par des grains α en croissance anormale.Increasing demands on modern turbines require (α2+γ) lamellar-structured TiAl alloys with fine colony size and properly aligned lamellae. According to the α → α2+γ phase transformation, the lamellar structure depends directly on the high-temperature α phase. Thus, the lamellar structure optimization could be realized by the modification of high-temperature α phase through thermomechanical processing. In this work, a thorough investigation was conducted on the high-temperature α phase in two TiAl alloys in terms of the deformation behavior, dynamic recrystallization (DRX), grain growth and texture evolution.Under uniaxial compression, the DRX of the α phase is a continuous fragmentation process (CDRX) in three characteristic stages: ⅰ) serrations of grain boundary and formation of symmetrical-tilt boundaries with Disorientation axis near boundary bulging regions; ⅱ) formation of subgrains by evolving symmetrical-tilt boundaries into asymmetrical-tilt boundaries with Disorientation axis by absorbing basal dislocations, or tilt-twist boundaries with Disorientation axis by rotational movements of the bulged parts; ⅲ) detachment of subgrains from the parent grain with gradually increased misorientation, and mixture with the ones fragmented from other grains by grain boundary sliding. These three processes happened repeatedly from grain boundary regions toward grain interiors until the whole initial microstructure was replaced by the DRXed one. The plastic deformation of the α phase is closely related to the crystallographic orientations of the initial α grains that play an important role in the deformation mechanism and the CDRX progress (softening). For the soft α grains, CDRX was completed quickly at a relatively small macroscopic strain by intragranular dislocation slip. The hard α grains demonstrated two ways: ⅰ) grains with //LD, in which dislocation accumulation was only assisted by the local strain accommodation with the neighboring α grains from boundary regions; ⅱ) grains with //LD, in which dislocation accumulation was achieved by kinking through basal slip and dislocation slip in the boundary regions from incompatible local strain. These grains required large strain to accumulate sufficient dislocations for CDRX. The strain-resolved contribution of the deformation (hardening) and CDRX (softening) result in the specific flow stress states. The texture evolution is mainly induced by dislocation slip. The orientations of the DRXed grains were largely inherited from those of the parents. With the deformation, the tilted basal fiber typed orientations developed in both the retained coarse α and the formed DRXed α grains. The hot compression produces refined α grains but not expected texture to align (α2+γ) lamellae. The microstructure and texture of the α phase during hot extrusion in the (α+β) phase region exhibit different features. The extruded microstructure was very heterogenous, comprising a large population of unRXed α grains and fine primary RXed α grains with //ED and a small population of grown α grains with //ED. The two texture components are beneficial for lamella alignment. Besides, the thermally-induced α → β phase transformation interweaving with the abnormal α grain growth happened during the transition from extrusion to water-quenching, producing two types of β particles: i) intergranular β particles with the Burger OR with their neighboring α grain; ii) intragranular β particles without the OR with the hosts from the intergranular β ones after being swallowed by the abnormally grown α grains