Richard G. Castro

Dislocations, twins, grain boundaries and precipitates in MoSi2

Abstract Hot-pressed and plasma-sprayed specimens of MoSi2 have been examined by transmission electron microscopy both in the undeformed condition and when they were deformed by bending or hot hardness indentation. Hot-pressed specimens are found to contain 〈100〉 dislocations after deformation at lower temperatures (about 600°C or lower) and 1 2 〈111〉 dislocations after deformation at higher temperatures. High energy 1 2 〈331〉 dislocations previously reported in the literature are not observed but this may be due to their intrinsic instability. Twins are commonly observed, especially in undeformed plasma-sprayed MoSi2. Analysis of diffraction patterns shows that the twin plane is {112} and the twinning direction is 〈111〉; the twin boundary is {111} corresponding to a rotationh twin. The twins are suggested to be variants resulting from the hexagonal-to-tetragonal transformation at high temperatures. Silica particles are frequently seen in hot-pressed material on the grain boundaries and at triple points. However, no grain boundary phase is observed, even by high resolution electron microscopy, suggesting that silica does not wet the grain boundaries. Silica is less common in plasma-sprayed MoSi2 but there is a high density of Mo5Si3 particles due to silicon loss during processing.

Ductile phase toughening of molybdenum disilicide by low pressure plasma spraying

The low fracture toughness of MoSi{sub 2} at ambient temperature has prompted investigations into new processing methods in order to impart some degree of fracture toughness into this inherently brittle material. In the following investigation, low pressure plasma spraying was employed as a fabricating technique to produce spray-formed deposits of MoSi{sub 2} and ductile reinforced MoSi{sub 2} composites containing approximately 10 and 20 volume percent of a discontinuous tantalum lamelli reinforcement. Fracture toughness (K{sub 1C}) measurements of MoSi{sub 2} and the MoSi{sub 2}/Ta composites were done using a chevron notched 4-point bend fracture toughness test in both the as-sprayed condition and after hot isostatic pressing at 1200{degrees}C/206 MPa for 1 hour. Results from the ductile reinforced MoSi{sub 2}/Ta composites have shown fracture toughness increases on the order of 200% over the as-sprayed MoSi{sub 2}. In addition, a marked anisotropy in fracture toughness was observed in the spray-formed deposits due to the layered splat structure produced by the low pressure plasma spray process.

The effect of substrate temperature on the thermal diffusivity and bonding characteristics of plasma sprayed beryllium

Abstract Plasma spraying is under investigation as a method for in-situ repair of damaged beryllium and tungsten plasma facing surfaces for the International Thermonuclear Experimental Reactor (ITER), the next generation magnetic fusion energy device, and is also being considered as a potential fabrication method for beryllium and tungsten plasma-facing components for the first wall of ITER. Investigators at Los Alamos National Laboratorys Beryllium Atomization and Thermal Spray Facility have concentrated on investigating the structure-property relationship between the as-deposited microstructures of plasma sprayed beryllium coatings and the resulting thermal properties of the coatings. In this study, the effect of substrate temperature on the resulting thermal diffusivity of the beryllium coatings and the thermal diffusivity at the coating/beryllium substrate interface was investigated. Results have shown that increases in the beryllium substrate temperature can improve the thermal diffusivity of the beryllium coatings. Results also indicate that the thermal resistance at the interface between the beryllium coating and the beryllium substrate were minimal and showed little dependence on the substrate temperature. The effective bond strength and failure characteristics of plasma spray beryllium on beryllium surfaces were predominately dominated by mechanical interlocking at low substrate temperatures and increased metallurgical bonding at higher substrate temperatures.

The structure, properties and performance of plasma-sprayed beryllium for fusion applications

Plasma-spray technology is under investigation as a method for producing high thermal conductivity beryllium coatings for use in magnetic fusion applications. Recent investigations have focused on optimizing the plasmaspray process for depositing beryllium coatings on damaged beryllium surfaces. Of particular interest has been optimizing the processing parameters to maximize the through-thickness thermal conductivity of the beryllium coatings. Experimental results will be reported on the use of secondary H2 gas additions to improve the melting of the beryllium powder and negative transferred-arc cleaning to improve the bonding between the beryllium coatings and the underlying surface. Information will also be presented on thermal cycle tests which were done on beryllium coated ISX-B beryllium limiter tiles using 10s cycle times with 60s cooldowns using a heat flux slightly in excess of 5 MW/m2.

{112} 〈111〉 twins in tetragonal MoSi2

Abstract Plasma-sprayed tetragonal MoSi2 has been examined by transmission electron microscopy. Growth twins are commonly observed in the microstructure. Analysis of diffraction patterns using a stereographic projection indicates that the twinning plane is {112} and the twinning direction is 〈111〉. Atomic modelling of the twin interface shows that a key element is the pseudo-hexagonal nature of the MoSi2{110} planes which allows a ∑2 coincidence site relationship between matrix and twin. The habit plane for the twins is commonly observed to be {110}, implying that the plane corresponds to a twist boundary. The rotation angle is almost exactly 60°. It is likely that the twins result from the hexagonal-to-tetragonal transformation of MoSi2 at high temperatures.

Fabrication and high heat flux testing of plasma sprayed beryllium ITER first wall mock-ups

Abstract Plasma-sprayed beryllium ITER first wall mock-ups have survived 3000 thermal fatigue cycles at 1 MW/m 2 without damage during testing at the Plasma Materials Test Facility at Sandia National Laboratory in New Mexico. This heat flux level is twice the expected design heat flux for ITER first wall modules. Plasma sprayed beryllium mock-ups were vacuum plasma sprayed at the Los Alamos National Laboratorys Beryllium Atomization and Thermal Spray Facility. Results will be reported on the fabrication, high heat flux testing and post-mortem analysis of two beryllium plasma sprayed mock-ups (1) beryllium plasma sprayed directly on a CuNiBe heat sink and (2) beryllium plasma sprayed on a compliant layer of aluminum which was explosion-bonded to a CuCrZr copper heat sink. The high heat flux tests utilized the 30 kW Electron Beam Test System at a Sandia National Laboratory.

Reactive plasma spraying of MoSi2 using an Ar-10%CH4 powder carrier gas

Abstract Plasma spraying of MoSi 2 using an Ar-10%CH 4 powder carrier gas mixture was investigated as a way of incorporating carbon and carbide particles into spray deposits of MoSi 2 which could subsequently getter SiO 2 after elevated-temperature exposure. After hot isostatic pressing (HIP) at 1800°C and 206 MPa for 1 h a decrease in oxygen from approximately 2580 to 750 ppm resulted in the spray deposits of MoSi 2 produced with the ArCH 4 powder gas carrier. A factor of 3 increase in yield strength was observed in these deposits when compared to conventional hot-pressed MoSi 2 matrix. The temperatures between 1100 and 1400°C. This increase was attributed to a decrease in SiO 2 in the MoSi 2 matrix. The presence of SiC particles was identified in as-sprayed deposits of MoSi 2 and after HIP. Silicon carbide particles in the as-sprayed condition resulted from inflight reactions between MoSi 2 and carbon present in the methane gas. Spray deposits of MoSi 2 , without the use of the ArCH 4 powder carrier gas, were also produced in order to establish a baseline in elevated temperature behavior.

Optimizing the thermal conductivity of vacuum plasma-sprayed beryllium for fusion applications

Plasma-spraying of beryllium is under investigation as a potential method for coating the first wall blanket surface of the International Thermonuclear Experimental Reactor (ITER) and as a technique for regenerating damaged beryllium surfaces as a result of normal and off-normal operating events. Methods to optimize the thermal conductivity of vacuum plasma-sprayed (VPS) beryllium are currently under investigation at the Los Alamos National Laboratorys Beryllium Atomization and Thermal Spray Facility (BATSF). Studies are being conducted to evaluate the effect of the starting beryllium feedstock powder, processing parameters and resulting microstructures on the thermal conductivity of VPS beryllium. The characteristic layered microstructure, typical of plasma-sprayed deposits, contains regions of incomplete bonding between individual splat layers which can decrease the thermal conductivity through the thickness of the deposit. Impurities present in the starting beryllium feedstock powder can also influence the thermal conductivity of VPS beryllium by causing impurity striations throughout the thickness of the deposit.

Durability of molybdenum disilicide in molten alkali borosilicate glass

Abstract The kinetics and mechanisms of corrosion of MoSi 2 materials were investigated in molten alkali borosilicate glass at 1000–1550°C for 0.5–1510 hours. The corrosion behavior of MoSi 2 is compared to an alumina-zirconia-chrome-silica refractory (AZCS). The corrosion rates were evaluated by measuring dimensional changes at, above, and below the molten glass line. MoSi 2 -based materials perfomed at least as well as AZCS above and below the glass line, but were somewhat inferior at the glass line. The kinetics and mechanisms of corrosion of MoSi 2 varied with position in the melt. Above the glass line a semi-protective SiO 2 layer forms. Below the glass line, a protective Mo-rich layer forms. At the glass line, simultaneous dissolution of SiO 2 and vaporization of MoO 3 leads to comparatively rapid loss of material. Based on microstructural studies, a phenomenological model is proposed that illustrates the basic mechanisms of corrosion in these three regions.

Steam chemical reactivity of plasma-sprayed beryllium

Plasma-spraying with the potential for in-situ repair makes beryllium a primary candidate for plasma facing and structural components in experimental magnetic fusion machines. Deposits with good thermal conductivity and resistance to thermal cycling have been produced with low pressure plasma-spraying (LPPS). A concern during a potential accident with steam ingress is the amount of hydrogen produced by the reactions of steam with hot components. In this study the authors measure the reaction rates of various deposits produced by LPPS with steam from 350 C to above 1,000 C. They correlate these reaction rates with measurements of density, open porosity and BET surface areas. They find the reactivity to be largely dependent upon effective surface area. Promising results were obtained below 600 C from a 94% theoretical dense (TD) deposit with a BET specific surface area of 0.085 m{sup 2}/g. Although reaction rates were higher than those for dense consolidated beryllium they were substantially lower, i.e., about two orders of magnitude, than those obtained from previously tested lower density plasma-sprayed deposits.