Bulk amorphous materials

This is the final report for a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The objective of this work was to develop the competency for the synthesis of novel bulk amorphous alloys. The authors researched their synthesis methods and alloy properties, including thermal stability, mechanical, and transport properties. The project also addressed the development of vanadium-spinel alloys for structural applications in hostile environments, the measurement of elastic constants and thermal expansion in single-crystal TiAl from 300 to 750 K, the measurement of elastic constants in gallium nitride, and a study of the shock-induced martensitic transformations in NiTi alloys

Prediction of irradiation spectrum effects in pyrochlores

The formation energy of cation antisites in pyrochlores (A2B2O7) has been correlated with the susceptibility to amorphize under irradiation, and thus, density functional theory calculations of antisite energetics can provide insights into the radiation tolerance of pyrochlores. Here, we show that the formation energy of antisite pairs in titanate pyrochlores, as opposed to other families of pyrochlores (B = Zr, Hf, or Sn), exhibits a strong dependence on the separation distance between the antisites. Classical molecular dynamics simulations of collision cascades in Er2Ti2O7 show that the average separation of antisite pairs is a function of the primary knock-on atom energy that creates the collision cascades. Together, these results suggest that the radiation tolerance of titanate pyrochlores may be sensitive to the irradiation conditions and might be controllable via the appropriate selection of ion beam parameters

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl2O4, partially inverted MgGa2O4 and fully inverse MgIn2O4 were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400 eV and 2 keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10 ps. Simulations in the normal MgAl2O4 spinel for the 2 keV energy regime resulted in similar defect structures as obtained at the post-threshold 400 eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitialvacancy cation chains and oxygen crowdions

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. These results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives

Radiation damage effects in ferroelectric LiTaO

Z-cut lithium tantalate (LiTaO{sub 3}) ferroelectric single crystals were irradiated with 200 keV Ar{sup ++} ions. LiTaO{sub 3} possesses a structure that is a derivative of the corundum (Al{sub 2}O{sub 3}) crystal structure. A systematic study of the radiation damage accumulation rate as a function of ion dose was performed using ion-beam channeling experiments. An ion fluence of 2.5 x 10{sup 18} Ar{sup 2+} ions/m{sup 2} was sufficient to amorphize the irradiated volume of a LiTaO{sub 3} crystal at an irradiation temperature of approximately 120K. This represents a rather exceptional susceptibility to ion-induced amorphization, which may be related to a highly disparate rate of knock-on of constituent lattice ions, due to the large mass difference between the Li and Ta cations. The authors also observed that the c{sup {minus}} end of the ferroelectric polarization exhibits slightly higher ion dechanneling along with an apparent greater susceptibility to radiation damage, as compared to the c{sup +} end of the polarization

Relationship between High-Strain-Rate Superplasticity and Interface Microstructure in Aluminum Alloy Composites

The Al alloy composites reinforced with Si3N4 or SiC have been reported to exhibit superplasticity at high strain rate of faster than 1x 10-2s-1. It has been shown in many aluminum alloy composites that the optimum superplastic temperature coincides with an incipient melting temperature. The coincidence suggests a contribution of the liquid phase to the superplasticity mechanism. This paper shows a direct evidence of partial melting along matrix grain boundaries and matrix-reinforcement interfaces. Based on the obtained results, the role of the liquid phase in the high-strain-rate superplasticity is discussed. The sample was Al-Mg (5052) alloy reinforced with 20vol% Si3N4 particles, fabricated by a powder metallurgy process. The sample showed an excellent superplasticity under the conditions given in Table 1. Partial melting was confirmed to occur at 821 K by differentail scanning calorimetry. The microstructural changes during heating were observed in situ by TEM using a heating stage. The structure of interfaces and grain boundaries was observed by HREM. Chemical analysis was performed with EDS attached to VG-STEM. A bright-field image of the composite is shown in Fig. 1. Notice that the edge of the Si3N4 particles are fragmented. Fig. 2 (a) shows a selected-area diffraction pattern taken at 821 K. A halo ring appears at this temperature, indicating partial melting. Fig. 2 (b) shows a dark- field image with an inverted contrast, taken from a part of the halo ring. The location of partial melting can be identified by a dark contrast along the matrix grain boundaries and the matrix- reinforcement interfaces. Above this temperature, grain-boundary corners become a rounded shape caused by the formation of the liquid phase at triple grain junctions. Figure 3 shows a concentration profile across a matrix-reinforcement interface. The left side is the aluminum matrix and the right is a Si3N4 particle. In the middle in between two dotted lines, there is a region where the Al and Si concentrations take intermediate flat values, indicating the formation of an interface phase. Mg segregation at the location of the left dotted line is also observed. This solute segregation is considered to be a cause for partial melting. The crystal structure of the interface phase was investigated by HREM and shown in Fig. 4. The figure shows the Al matrix and the interface phase. The interface is severely serrated and may become a stress concentration site during deformation. FFT of various regions reveals that the interface phase is composed of MgO, Al2MgO4, and Mg2Si. In the present work, partial melting was observed along interfaces and grain boundaries. Solute segregation is suggested to be the cause for partial melting. The liquid phase is considered to act as a stress accommodation site that can prevent cavitation failure at the serrated interfaces and triple grain junctions

Microwave sintering of nanophase ceramics without concomitant grain growth

A method of sintering nanocrystalline material is disclosed wherein the nanocrystalline material is microwaved to heat the material to a temperature less than about 70% of the melting point of the nanocrystalline material expressed in degrees K. This method produces sintered nanocrystalline material having a density greater than about 95% of theoretical and an average grain size not more than about 3 times the average grain size of the nanocrystalline material before sintering. Rutile TiO[sub 2] as well as various other ceramics have been prepared. Grain growth of as little as 1.67 times has resulted with densities of about 90% of theoretical

In situ MeV ion beam analysis of ceramic surfaces modified by 100-400 keV ion irradiation

This paper describes use of the in situ ion beam analysis facility developed at Los Alamos National Laboratory for the study of irradiation effects in ceramic materials. In this facility, an analytical beamline of 3 MV tandem accelerator and an irradiation bean-dine of 200 kV ion implanter are connected at 60{degrees} to a common target chamber. This facility provides a fast, efficient, and quantitative measurement tool to monitor changes of composition and crystallinity of materials irradiated by 100-400 keV ions, through sequential measurement of backscattering events of MeV ions combined with ion channeling techniques. We will describe the details of the in situ ion beam analysis and ion irradiation and discuss some of the important issues and their solutions associated with the in situ experiment. These issues include (1) the selection of axial ion channeling direction for the measurement of radiation damage; (2) surface charging and charge collection for data acquisition; (3) surface sputtering during ion irradiation; (4) the effects of MeV analytical beam on the materials; and (5) the sample heating effect on ion beam analysis

In situ beam analysis of radiation damage kinetics in MgTiO{sub 3} single crystals at 170-470 K

Radiation damage kinetics in synthetic MgTiO{sub 3} (geikielite) single crystals have been studied using the in situ ion beam facility at Los Alamos National Laboratory. The geikielite samples were irradiated at temperatures of 170, 300, and 470 K with 400 keV xenon ions and the radiation damage was sequentially measured with Rutherford backscattering using a 2 MeV He ion beam along a channeling direction. Threshold doses of I and 5x l0{sup 15} Xe/cm{sup 2} were determined for the crystalline-to-amorphous transformation induced by Xe ion irradiation at 170 and 300 K, respectively. However, geikielite retained its crystallinity up to a dose of 2.5xl0{sup 16}Xe/cm{sup 2} at the irradiation temperature of 470 K. This study has shown that MgTiO{sub 3}, which has a corundum derivative structure, is another radiation resistant material that has the potential for use in radiation environments