Search

Due to its service background, the thermal behavior of Ta films is substantially concerning to researchers, especially the heat-induced β-Ta to α-Ta phase transformation. In this work, the nanocrystalline Ta films with varying β/α phase ratios (0.85–0) were deposited and vacuum annealed at 200–800 °C to study the phase transformation behavior and mechanical properties. Partial phase transformation of β-Ta to α-Ta occurred at low temperature (200 °C) and complete transformation when the annealing temperature was raised to 800 °C. Moreover, an orientation relationship of β-<110>//α-<111> and β-{330}//α-{110} was concluded, and a hypothesis theory about the low-temperature phase transformation was proposed. Annealing hardening took place at 200–600 °C and softening at 800 °C in all the samples. The hardening is mostly due to the ordering of intergranular amorphous phases with the temperature increased, simultaneously bringing the increase of modulus. The annealing softening is governed by both grain coarsening and phase transformation, where the latter is dominated in high β fraction samples. • High-temperature induced grain boundary structure ordering promotes the hardening of Ta films at 200 °C–600 °C. • β-Ta to α-Ta phase transformation was found at a wide temperature range, which partially governs the softening of Ta at 800 °C. • A preferred orientation relationship of β-<110>{330}//α-<111>{110} was concluded • The influence of phase transformation on mechanical properties was independently quantified using a simple comparative method. [ABSTRACT FROM AUTHOR]

Hard β-Ta phase is generally produced during Ta film deposition. While in some cases, soft α-Ta phase is preferred to ensure deformation capability. In this work, templated growth strategy by introducing Cr buffer layer is proposed to tailor the phase composition in Ta films. A broad spectrum of β-Ta/α-Ta ratio from 90% to 0% can be continuously and precisely achieved when the buffer layer thickness is correspondingly controlled from 0 to ∼90 nm. Hardness of the Ta films is well proportional to the β-Ta/α-Ta ratio. The underlying mechanism of phase tailoring is rationalized by demonstrating the thickness-dependent crystallization of buffer layer. [Display omitted]. [ABSTRACT FROM AUTHOR]

Abstract: The strengthening mechanism of σ phase in a Fe20Cr9Ni cast austenite stainless steel used for primary coolant pipes of nuclear power plants has been investigated. The yield and ultimate tensile strengths of aged specimens increased comparing with those of the unaged ones. It was found that the increase of strengths is due to the hard and brittle (σ+γ2) structure which decomposed from α phase in the steel. Fracture surfaces of specimens after in situ tensile test showed that the inhibition of (σ+γ2) structure on the dislocation movements was more significant than ferrite although cracks started predominately at σ/γ2 interfaces. The (σ+γ2) structure behaves like a fiber reinforced composite material. [Copyright &y& Elsevier]

Abstract: The purpose of this study is to investigate the role of stoichiometry in crystalline structure transformations in derivative fluorite compounds known as δ–γ–β phases in Sc2O3–ZrO2 phase diagram. Different composition polycrystalline δ-Sc4Zr3O12, γ-Sc2Zr5O13 and β-Sc2Zr7O17 samples were synthesized by mixing 40% Sc2O3 +60% ZrO2, 16.67% Sc2O3 +83.33% ZrO2, or 12.5% Sc2O3 +87.5% ZrO2, respectively. The pressed and polished pellets of these compounds were then irradiated under liquid nitrogen temperature with 200keV Ne+ ions at a dose rate of ∼1×1012 ions/cm2/s. An order-to-disorder (O–D) transformation was observed for all compositions, as determined using grazing incidence X-ray diffraction (GIXRD) at an incident angle of 0.25°. The O–D transformation threshold dose for δ-Sc4Zr3O12 (∼0.2dpa) was found to be noticeably higher than that for γ-Sc2Zr5O13 and β-Sc2Zr7O17 (∼0.04dpa) based on both GIXRD and cross-sectional transmission electron microscopy (TEM) measurements. However, due to experimental difficulty and uncertainty, our GIXRD and TEM data were unable to conclusively confirm that the O–D transformation threshold dose for γ-Sc2Zr5O13 is in fact slightly higher than that for β-Sc2Zr7O17 as expected from the phase diagram of Sc2O3–ZrO2 system. [Copyright &y& Elsevier]

Abstract: In this study, 200keV Ne+ ion irradiations were performed under cryogenic conditions (∼77K) on δ-phase oxide compounds with formula, A4Zr3O12 (A=Sc, Lu and Dy). An order-to-disorder (O–D) phase transformation was observed in δ-Lu4Zr3O12 and δ-Sc4Zr3O12 after Ne+ ion irradiation, but no phase change was observed in Dy4Zr3O12. The latter compound exhibited a disordered fluorite structure before and after irradiation. The threshold doses to produce O–D phase transformations using Ne+ ions were found to be significantly lower than for similar irradiations (reported previously) using Kr++ ions. This suggests that light ions are more efficient than heavy ions in producing the retained defects that are ultimately responsible for the observed O–D transformation. The ion irradiation-induced O–D transformations observed here are discussed in terms of analogous transformations found in temperature–composition (T–C) phase diagrams for A4Zr3O12 compounds. [Copyright &y& Elsevier]

Abstract: The purpose of this study is to investigate the role of stoichiometry on crystal structure transformations in derivative fluorite compounds known as delta (δ) phase. In this study, polycrystalline δ-phase ceramic pellets were prepared with stoichiometries given by Sc4− x Zr3+ x O12+ x/ 2 (x= 0, 0.77 and 1.20). The pressed and polished pellets were then irradiated under cryogenic conditions with 200keV Ne+ ions to fluences ranging from 1–5×1014 Ne/cm2. An order–to–disorder (O–D) transformation was observed for all compositions, as determined using grazing incidence X-ray diffraction (GIXRD). However, the transformation threshold dose was found to systematically decrease with increasing ZrO2 content: ∼0.2, ∼0.16, and ∼0.08dpa for Sc4− x Zr3+ x O12+ x/ 2 with x= 0, 0.77, and 1.20, respectively. These irradiation-induced phase transformation results are discussed in terms of the crystal structure of the δ-phase. [ABSTRACT FROM AUTHOR]

Abstract: An investigation was conducted to synthesize simple β-CoAl or β-(Co,Ni)Al coatings and platinum (Pt)-modified aluminide coatings on cobalt-base superalloys via an industrial vapor phase aluminizing process. Surface morphology and cross-section microstructure of the developed coatings were studied and compared by using optical microscope, scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that Al increase in Al–Cr nuggets increases the coating thickness; substrate elements influence the formation of coatings; the increase of the electrodeposited Pt thickness increases the amount of ξ-PtAl2 phase after aluminizing process; and the transformation of ξ-PtAl2 phase into β-(Co,Ni,Pt)Al phase can be obtained via a heat treatment process. [Copyright &y& Elsevier]

The Al3+ doping effects on the ferromagnetic metallic state of La 0.55 Ca 0.45 MnO 3 have been studied by preparing the La 0.55 Ca 0.45 Mn 1- x Al x O 3- δ (x = 0, 0.05, 0.1, 0.15) ceramics. Various measurements such as ac susceptibility, relaxation, magnetization and resistivity were performed to study the magnetic and magnetotransport properties. The magnetic disorder produced by the Al3+ dopants and the oxygen vacancies suppress the long range ferromagnetic ordering and induce a short range charge ordered antiferromagnetic clusters, leading to a phase separated behavior in the Al3+-doped samples. A Griffith-like behavior is observed due to the occurrence of short range ferromagnetic clusters in the paramagnetic phase. The 15% Al3+ doping sample exhibits a spin-glass state. The 10% Al3+ doping system is blocked in a metastable state upon cooling at zero field, and a sharp metamagnetic transition is induced by applying a high magnetic field, in which the antiferromagnetic phase is transformed into ferromagnetic phase and the ferromagnetic clusters grow in size. The ferromagnetic domains grows rapidly when applying a high magnetic field. Once the conducting ferromagnetic domains are interconnected throughout the sample, a percolative insulating-to-metallic state transition would occur, accompanied by a sharp drop in resistivity. • Al3+ doping induces charge ordered AFM clusters in La 0.55 Ca 0.45 Mn 1- x Al x O 3- δ. • The magnetic disorder caused by Al3+ doping suppresses the long rang FM ordering. • The AFM clusters transform into FM phase via field driven metamagnetic transition. • Percolative insulator-metallic transition occurs when applying a magnetic field up to H C. [ABSTRACT FROM AUTHOR]

Magnetization relaxation, ac susceptibility, dc magnetization and magnetoelectrical transport measurements have been carried out on Al3+-doped manganites La 0.5 Ca 0.5 Mn 1- x Al x O 3- δ (x = 0.05, 0.07, 0.09, 0.10, 0.15). Experimental results suggest that nonmagnetic Al3+ substituting Mn suppresses the ferromagnetic phase and induces a charge ordered antiferromagnetic phase. The coexistence and competition of these two kinds of interactions result in a spin-glass like state in x = 0.1 and 0.15 system. Obvious hysteresis was observed in both magnetization and resistivity versus temperature and magnetic field curves, suggesting an inhomogeneous metastable phase transition. Upon applying a magnetic field of H C A-F, the Al3+-doped samples undergo an irreversible metamagnetic transition from the charge ordering antiferromagnetic insulating to ferromagnetic metallic phase in the low temperature region. Phase diagram in the H C - T plane has been determined according to the magnetization and magnetoelectric transport measurements. The magnetic disorders and antiferromagnetic matrix produced by Al3+ dopants and spontaneous oxygen vacancies play an important role in the pinning of the FM phase. [ABSTRACT FROM AUTHOR]

In this study, the crystallography of the nugget zone of bainitic steel was examined by applying friction stir welding (FSW) and submerging in various cooling mediums. In air, the variants mainly occurred in a single Bain group because of the plastic accommodation of transformation strain. However, in water and a mixture of water and CO 2 , the variants generated in two and four close-packed (CP) groups, respectively, and this was attributed to the self-accommodation of transformation strain. In this case, the variant selection was inhibited by both the severe deformation during FSW and low temperature of cooling medium. Furthermore, the reduced cooling-medium temperature resulted in refined bainitic and martensitic blocks with an increased fraction of high-misorientation variant pairs owing to the increased transformation driving force. • Previous studies on the FSW bainitic steel were focused on the phase transformation, and the crystallography was ignored. • The crystallographic characteristics and its effect on microstructure refinement for FSW bainitic steel was studied. • In air, the plastic deformation in austenite easily accommodates the transformation strain, causing the variant selection. • In water and water+CO 2 , refined blocks were obtained with an increased fraction of high-misorientation variant pairs. [ABSTRACT FROM AUTHOR]

We show that the replacement of Mn with Al strongly affects the magnetization and electrical transport behaviors in La 0.5 Ca 0.5 Mn 1- x Al x O 3- δ (x = 0, 0.05, 0.07 and 0.09). Nonmagnetic Al3+ ions substitution at Mn sites dilutes Mn3+-O2--Mn4+ network, thus suppresses the ferromagnetic metallic state in La 0.5 Ca 0.5 MnO 3- δ and causes a phase separation phenomenon, in which ferromagnetic phase coexists with antiferromagnetic charge ordered phase at low temperatures. With applying sufficiently high magnetic field, step-like metamagnetic transitions were observed in x = 0.05–0.09 systems below helium temperature, in which the antiferromagnetic charge ordered phase collapsed into ferromagnetic phase. Corresponding to the sharp step-like metamagnetic transitions, the resistivity decreases dramatically with increasing magnetic field, exhibiting a percolative insulator-metal transition. The variation of temperature and magnetic field changes the relative fractions of ferromagnetic and charge ordering phases, and percolative insulator-metal transition occurs due to the development of percolation paths between the growing FM domains. [ABSTRACT FROM AUTHOR]

In this work, hcp-to-bcc Ti phase transformation was uncovered in Ti/Ta nanostructured multilayers. The Ti nanolayers were fully transformed to bcc phase when the layer thickness ( h ) smaller than ~7.5 nm, accompanied with the formation of fully coherent interfaces. While in the greater layer thickness, Ti layers were partially phase-transformed, resulting in the simultaneous existence of bcc/bcc Ti/Ta coherent interfaces and bcc/hcp Ti/Ti phase boundaries. A thermodynamic model was proposed to account for the layer thickness h -dependent Ti phase transformation. The phase transformation caused an unusual size-dependent hardness, i.e., the smaller the layer thickness and the lower the hardness, which was quantitatively addressed by an ad hoc strengthening model. [ABSTRACT FROM AUTHOR]

Polycrystalline δ-phase Sc 4 Hf 3 O 12 was irradiated with light and heavy ions to study the radiation stability of this compound. In order to explore the ion species spectrum effect, the irradiations were performed with 400 keV Ne 2+ ions to fluences ranging from 1 × 10 14 to 1 × 10 15 ions/cm 2 , 600 keV Kr 3+ ions to fluences ranging from 5 × 10 14 to 5 × 10 15 ions/cm 2 , and 6 MeV Xe 26+ ions to fluences ranging from 2 × 10 13 to 1 × 10 15 ions/cm 2 . Irradiated samples were characterized by various techniques including grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). A complete phase transformation from ordered rhombohedral to disordered fluorite was observed by a fluence of 1 × 10 15 ions/cm 2 with 400 keV Ne 2+ ions, equivalent to a peak ballistic damage dose of ∼0.33 displacements per atom (dpa). Meanwhile, the same transformation was also observed by 600 keV Kr 3+ ions at the same fluence of 1 × 10 15 ions/cm 2 , which however corresponds to a peak ballistic damage dose of ∼2.2 dpa. Only a partial O-D transformation was observed for 6 MeV Xe 26+ ions in the fluence range used. Experimental results indicated that the O-D transformation is observed under both electronic and nuclear stopping dominant irradiation regimes. It was also observed that light ions are more efficient than heavy ions in producing the retained defects that are presumably responsible for the O-D phase transformation. The O-D transformation mechanism is discussed in the context of anion oxygen Frenkel defects and cation antisite defects. We concluded that the irradiation induced O-D transformation is easier to occur in δ-phase compounds with partial order of cations than in that with fully disordered cation structures. [ABSTRACT FROM AUTHOR]

Abstract: Radiation damage mechanisms in single crystal SrTiO3 irradiated with 250keV Ne ions to a fluence of 1.11×1020 ions/m2 at both room temperature and 773K were systematically investigated. The irradiation-induced microstructural evolution was characterized using transmission electron microscopy. Ion irradiation at room temperature results in amorphization of crystalline SrTiO3 near the peak damage region at this fluence. On the other hand, ion irradiation at high temperature leads to less irradiation-induced damage in SrTiO3 due to the higher recovery rate of defects. Nevertheless, the formation of dislocation loops has been observed in the SrTiO3 crystals irradiated at high temperature. These dislocation loops were determined to be unfaulted loops with Burgers vector along 〈011〉. [Copyright &y& Elsevier]

In this study, polycrystalline δ-phase Lu4Hf3O12 was irradiated with 6MeV Xe26+ ions to fluences ranging from 2×1013 to 1×1015 ions/cm2. Ion irradiation-induced microstructural evolution was examined by using grazing incidence X-ray diffraction (GIXRD). A complete phase transformation from ordered rhombohedral to disordered fluorite (O–D) was observed by a fluence of 1×1015 ions/cm2, equivalent to a peak ballistic damage dose of ∼3.49 displacements per atom (dpa). To research the different irradiation effect between light ion and heavy ion on δ-Lu4Hf3O12, 400keV Ne2+ ions were implanted to ion fluences ranging from 1×1014 to 1×1015 ions/cm2. A complete O–D crystal structure transformation was observed by a fluence of 5×1014 ions/cm2 (∼0.22dpa). This threshold dose was found to be observably lower than the threshold dose to produce order-to-disorder transformation using Xe26+ ions on δ-Lu4Hf3O12. This suggests that heavy ions are less efficient than light ions in producing the retained defects that are responsible for the O–D transformation. The theoretical calculations show that the O–D transformation of δ-phase was attributed to the anion oxygen Frenkel pair defect. The ion irradiation-induced transformation of δ-phase Lu4Hf3O12 into disordered fluorite structure observed here is also discussed in relation to the temperature–composition (T–C) phase diagrams for the compound. [ABSTRACT FROM AUTHOR]

Abstract: In this report, we present radiation damage effects in a thin film, tri-layer structure, HfO2/MgO/HfO2. Irradiations were performed with 10MeV Au ions in a recently developed medium energy ion irradiation facility at Los Alamos National Laboratory, which is described in this paper. Energy deposition by 10MeV Au ions corresponds to a mixed regime, wherein electronic and nuclear stopping contribute to radiation damage. In this study, we investigated modifications of both surface and bulk properties in order to assess the structural stability of our oxide tri-layers under the severe irradiation conditions employed here. The most dramatic structural changes were observed to occur on the surfaces of the tri-layer samples. Surface features consisted of large craters and spires. The dimensions of these craters and spires exceed those of the individual ion tracks by almost three orders of magnitude. As for the bulk tri-layer structure, our conclusions are that this structure is stable in terms of: (i) resistance to amorphization; (ii) resistance to compositional mixing and (iii) resistance to pronounced nucleation and growth of extended defects. The main effect observed in the tri-layer structure was the transformation of the first HfO2 layer from a monoclinic to either a tetragonal or cubic form of HfO2. [Copyright &y& Elsevier]

Abstract: At zero magnetic field, a series of five phase transitions occur in . The Néel temperature, , is followed by four additional phase changes at , , , and . The different phases are distinguished by the commensurability of the b-component of its spin density wave vector. We investigate the stability of these various phases under magnetic fields through dielectric constant and magnetic susceptibility anomalies. The field-temperature phase diagram of is completely resolved. The complexity of the phase diagram results from the competition of different magnetic states with almost equal ground state energies due to competing exchange interactions and frustration. [Copyright &y& Elsevier]

• The charge ordering (CO) phase in Nd 0.6- x Bi x Ca 0.4 MnO 3 is enhanced with Bi3+ doping. • The enhancement of CO phase is explained by the s-p hybridization model. • Pulsed high magnetic fields up to 50 T were used to study the CO phase melting. • H C - T phase diagrams of Nd 0.6- x Bi x Ca 0.4 MnO 3 (0 ≤ x ≤ 0.4) were obtained. We report the doping effects and pulsed high magnetic field-induced charge ordering to ferromagnetic phase transitions in manganites Nd 0.6- x Bi x Ca 0.4 MnO 3 (0 ≤ x ≤ 0.4). The experimental results reveal that the charge ordering is significantly enhanced upon Bi3+ doping, and the charge ordering temperature increases with increasing Bi3+ content. With applying pulsed high magnetic field, the charge ordering phase collapses into ferromagnetic phase through a metamagnetic transition. The phase diagram of Nd 0.6- x Bi x Ca 0.4 MnO 3 has been determined by the high magnetic field studies, in which the charge ordering and ferromagnetic phase boundary is clearly figured out. The critical fields for melting the charge ordering phase of Nd 0.6- x Bi x Ca 0.4 MnO 3 increase significantly with increasing Bi3+ content, which is revealed to reach up to 33.5 T for the sample with x = 0.3. Based on the hybridization between Bi-6s orbitals and O-2p orbitals, we assume that the 6s2 lone-pair character of Bi3+ plays a crucial role in favoring the charge ordering in Nd 0.6- x Bi x Ca 0.4 MnO 3 (0 ≤ x ≤ 0.4). [ABSTRACT FROM AUTHOR]

The tuning effects of Al3+ doping on the magnetic and magnetotransport properties of La 0·6 Ca 0·4 MnO 3- δ have been studied by preparing series La 0·6 Ca 0·4 Mn 1- x Al x O 3- δ (x = 0, 0.1, 0.125 and 0.15). Al3+ doping suppresses the ferromagnetic metal phase of La 0·6 Ca 0·4 MnO 3- δ by weakening the double-exchange interaction of Mn3+-O2--Mn4+, leading to a phase separation state. Al3+ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x = 0.125 and 0.15 system below 4.2 K. Phase diagrams of La 0·6 Ca 0·4 Mn 1- x Al x O 3- δ (0.1 ≤ x ≤ 0.15) in the B - T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3+ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition. • Phase separation scenario is induced by Al3+ doping in La 0·6 Ca 0·4 Mn 1- x Al x O 3- δ. • Martensitic-like transition was observed in x = 0.125 and 0.15 systems below 4.2 K. • The pinning effects of disorder play a crucial role in martensitic-like transition. • Phase diagram of La 0·6 Ca 0·4 Mn 1- x Al x O 3- δ (0.1 ≤ x ≤ 0.15) in B - T plane is obtained. [ABSTRACT FROM AUTHOR]