Density changes accompanying the annealing of wrought thoriated tungsten wire

Density measurements were made on 0.762 mm W-l wt pct ThO₂ wires which had been annealed at temperatures between 1000°C and 2000°C for times up to 120 min. Selected specimens were examined using optical and scanning electron microscopy. The results of this study indicate that the low, as-worked wire density is a result of large cracks associated with the ThO₂ particles. Time-temperature annealing regimes, significantly below those necessary for recrystallization of the wire, result in partial healing of the cracks. This crack healing process can be followed qualitatively by observing longitudinal fracture surfaces in the scanning electron microscope and quantitatively by changes in the density of the wire. The apparent activation energy for the crack healing process was found to be 57.8 kcal/mole. The results suggest that crack closure is a result of both shear and grain boundary diffusion transport processes. It is concluded that the presence of cracks can influence the morphology of the recrystallized grains.     

Recrystallization of molybdenum wire doped with potassium-silicate

The doping effect of the bubble formation oxide on the recrystallization of Mo wire was investigated. Five different wires of 1 mm in diameter were prepared through sintering, swaging, and drawing processes. Each wire was doped with various amounts of potassium (K) plus silicon (Si), i.e., 0, 0.028, 0.14, 0.28, and 0.49 by weight percent, and annealed for 30 minutes at given temperatures. To understand the overall recrystallization phenomena, changes in hardness and in optical microstructures were examined. Transmission electron micrographs were taken for the specimens in the as-drawn state and at the beginning of the decrease in hardness. And also, the relative excess resistivity was measured as a function of heating temperature to confirm the occurrence of the abnormal grain growth. During the grain growth, bubble dispersion was evaluated through fractography by scanning electron microscopy (SEM). Primary re-crystallization started at 750 °C regardless of the amount of dopants. For the specimens doped with 0.14 and 0.28 (K + Si), large elongated and interwoven grain structures indicating ab-normal grain growth developed over 1400 °C and 1600 °C, respectively. For the specimens doped with 0.028 and 0.49 (K + Si), however, small equiaxial grain structure developed similar to pure molybdenum wire. Such a difference was understood through the relationship between grain structures and bubble dispersion parameters (the average bubble diameter, the bubble row density, the columnar bubble spacing, and the bubble row distance). It was concluded that two of the most important parameters to develop a grain structure of high aspect ratio were bubble row density and bubble row distance. At a high bubble row density, irregularity in bubble row distance induced the higher aspect ratio (length/width (L/W)) of grain.     

The influence of Sc on thermal stability of a nanocrystalline Al-Mg alloy processed by cryogenic ball milling

The microstructural evolution during annealing of a cryogenically ball-milled Al-7.5Mg-0.3Sc (in wt pct) was examined using differential scanning calorimetry and transmission electron microscopy (TEM). The as-milled alloy was a supersaturated fcc solid solution with an average grain size of ∼25 nm and heterogeneous grain morphologies and size distributions. Calorimetric measurements at a constant heating rate of 32 K/min indicated two exothermic events in association with recovery from 100 °C to 240 °C and recrystallization from 300 °C to 450 °C. Prior to recrystallization, the precipitation of Al₃Sc may occur at low annealing temperatures producing a nonuniform dispersion of approximately spherical particles with diameters of 4 to 5 nm. Recrystallization gave rise to heterogeneous microstructures with bimodal grain size distributions, which may result from the heterogeneity of microstructure in the as-milled state. The heterogeneous microstructures of the recrystallized Al-Mg-Sc alloy were similar to those observed in the recrystallized Sc-free Al-Mg alloy.     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced «110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the «110» drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced ?110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the ?110? drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

Creep of fine wires of W-ThO2 alloys

The increasing interest in the application of fine wires as high strength structural components,e.g., in high temperature composite materials, makes an understanding of the differences between the creep behavior of large specimens and that of fine wires desirable. In this investigation, the creep properties of fine wires of recrystallized W-1 wt pct ThO₂ were studied over the temperature range between 1800° to 2750°C. In tungsten wires in which the dispersion of fine thoria particles stabilized a fine grained structure, the stress dependence of the creep rate varies with test temperature and stress. For test temperatures below 2500°C, a stress dependence ofn ~ 5 was found, indicative of creep deformation due to dislocation climb processes, while for temperatures above 2500°C and low stresses, values ofn < 2 were obtained, indicative of deformation by grain boundary sliding and diffusional creep processes. In wires which recrystallized to a large-grained structure, having a large aspect ratio, a high stress dependence of 15 was found when tested at 1800°C.     

Effect of gas bubbles on recrystallization of tungsten

Tungsten sheet deposited from WF₆ vapor was rolled at temperatures up to 1000°C and reductions in thickness up to 95 pct. Recovery and recrystallization were studied using hardness measurements and transmission electron microscopy. The temperatures for the onset of recrystallization during 1 hr anneals were 1600°C or more. Recrystallization was sluggish, requiring temperatures of 2100°C or more for completion. Transmission electron microscopy revealed that many small (< 500Å diam) gas bubbles were present on dislocations and recovered subgrain boundaries and caused scalloping of these boundaries and the advancing recrystallized grain boundaries. Coarsening of the bubbles to a relatively few larger bubbles permitted complete recrystallization. No solid inclusions were found. Annealing treatments that coarsened the gas bubbles prior to rolling caused accelerated recovery and complete recrystallization was observed at approximately 1200°C. It is concluded that small gas bubbles were the cause of the original high recrystallization temperatures and also influenced the size and shape of the recrystallized grains.     

OBSERVATIONS ON THE EFFECT OF VACUUM HEAT-TREATMENT (UP TO 4000°F) ON DISPERSIONS OF VARIOUS OXIDES,NITRIDES, AND BORIDES IN MOLYBDENUM

Abstract

The compatibility between molybdenum and several refractory compounds present as dispersed phases in the 3000-4000°F (1650–2210°C) temperature range has been evaluated. Compounds included in the study were BN, ZrN, HfN, LaB₆, ZrB₂, TaB₂, Y₂O₃, MgO, ZrO₂, and ThO₂. Hot-pressed alloys, consisting of nominally 2.5 vol.%, of the refractory phase in a molybdenum matrix, were vacuum-annealed to determine elevated-temperature stability. The evaluation was based on changes in grain size, dispersed-phase morphology, hardness, and gross macroscopic deterioration. The materials HfN, ZrO₂, and ThO₂ showed promise for dispersion-strengthening and were recommended for further development.     

Crystallization of an Fe-Ni base amorphous alloy

The crystallization of an amorphous Fe-Ni base alloy was studied in a dynamic heating mode from room temperature to 700°C and during isothermal annealing at 400°C. Differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, and hardness measurement were used to characterize the crystallization process under two heating conditions. In the dynamic heating condition, structural relaxation or atomic regrouping was thought to occur belowT _c. AboveT _c, crystallization occurred spontaneously and four crystalline phases were formed. The number of phases and the relative amount of these phases varied with the heating temperature. At a higher temperature, recrystallization occurred which resulted in grain growth. The final matrix phase was observed to coexist with other phases after crystallization. In the isothermal heating condition, it was found that the transformation of the alloy from amorphous state to crystalline state was through the nucleation and growth process. The first crystallization steps were via the formation of metastable phases. The final matrix phase than nucleated from the existing metastable phases. Hardness measurements in both heating conditions indicated that the alloy attained its peak hardness immediately after complete crystallization.     

Effects of heating rate on densification and grain growth during field-assisted sintering of α-Al2O3 and MoSi2 powders

Two types of powders, electrically conductive MoSi₂ and insulating α-Al₂O₃, were sintered by a field-assisted sintering technique (FAST) using heating rates from 50 °C to 700 °C/min. The Al₂O₃ powders were sintered to 99 pct density at 1100 °C for 2 minutes under 45 Mpa pressure. For Al₂O₃, no exaggerated grain growth was observed and the final grain size inversely scaled with the heating rate. Such a grain growth behavior fits the literature models based on multiple transport mechanisms for constant-heating-rate sintering. The density reached by MoSi₂ under similar sintering conditions was 91 pct. The grain size was independent of the heating-rate value. Specific electrical field and pressure effects are shown to contribute to enhanced densification and minimal coarsening in each material.     

Production of chromium base alloys by ball milling in hydrogen iodide

The effects of processing variables on the tensile properties and ductile-to-brittle transition temperature (DBTT) of Cr+4 vol pct ThO₂ alloys and of pure Cr produced by ball milling in hydrogen iodide were investigated. Hot rolled Cr+ThO₂ was stronger than either hot pressed Cr+ThO₂ or pure Cr at temperatures up to 1540°C. Hot pressed Cr+ThO₂ had a DBTT of 500°C as compared with ?8°, to 24°C for the hot rolled Cr+ThO₂ and with 140°C for pure Cr. It is postulated that the dispersoid in the hot rolled alloys lowers the DBTT by inhibiting recovery and recrystallization of the strained structure.     

Secondary recrystallization of pure molybdenum wires

The secondary recrystallization of drawn pure Mo wires has been studied; a particular attention has been paid on the effect of temperature gradients in the annealing process. The secondary recrystallized grains being very elongated along the wire axis have been obtained by annealing in a moderate temperature gradient furnace at about 2000°C. Orientations of the wire axis of the secondary recrystallized grains are mainly 〈023〉 or 〈135〉, while 〈011〉 oriented grains are less frequently observed. In contrast, the primary recrystallized texture consists of the 〈011〉 main component and 〈023〉-〈012〉 sub-component; many 〈011〉 oriented grains have large grain sizes. The observed discrepancy in orientation between the primary and secondary recrystallized grains is explained in terms of the difference in grain boundary mobilities which depend on the character of grain boundaries.     

Effect of Scandium on the Interaction of Concurrent Precipitation and Recrystallization in Commercial AA3003 Aluminum Alloy

In the current study, the effect of Sc addition on the interaction of concurrent precipitation and recrystallization in commercial AA3003 aluminum alloy was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. In case of AA3003 alloy, which was cold rolled to a true strain of 2.20 and heated at a heating rate of 150 K/s, the onset of precipitation and ending of recrystallization are signified by the critical temperature, T _C ~740 K (467 °C). There is a change in the shape of the recrystallized grains from pancake-like to equiaxed shape, as the annealing temperature increases greater than T _C. In case of AA3003 alloy microalloyed with 0.4 wt pct of Sc, the high no. density precipitation of coherent Al₃Sc precipitates always occurs before recrystallization because of the small nucleation barrier and high rate of decomposition. This leads to extremely coarse pancake-like recrystallization grains with high fraction of low-angle grain boundaries in the entire annealing temperature range, even at a high brazing temperature of 883 K (610 °C).     

The recrystallization of commercially pure and doped tungsten wire drawn to high strain

The recrystallization processes in both undoped and doped tungsten wire after drawing to a true strain of 7.7 were examined by light microscopy and transmission electron microscopy. High angle grain boundary migration commenced at approximately the same temperature in both materials, but proceeded much more rapidly in the undoped wire, where the absence of a potassium bubble dispersion allowed a coarser, more equiaxed grain structure to form. No change from the (110) deformation texture was observed in either case. Recrystallization in the undoped wire was dominated at lower temperatures (1100 to 1200°C) by the growth of large grains into a much finer structure. As the annealing temperature was increased, this process was replaced by a general grain coarsening which eventually produced a relatively equiaxed recrystallized grain structure. It appeared probable that it was the second phase dispersion inhibition alone that prevented similar structural changes in the doped wire. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

The dependence of normal and abnormal grain growth in silver on annealing temperature and atmosphere

When polycrystalline pure Ag specimens are compressed to 40 pct and annealed, there is no noticeable texture in the recrystallized structure, and normal or abnormal grain growth occurs during annealing. When annealed further in low vacuum (10⁻³ to 10⁻⁴ Torr) after the completion of recrystallization, normal grain growth occurs at 920 °C and 800 °C, but abnormal grain growth (AGG) occurs at 700 °C, 600 °C, and 500 °C. When annealed in O₂ atmosphere, normal grain growth occurs at 920 °C and AGG at 800 °C, 700 °C, 600 °C, and 500 °C. At temperatures close to the melting point (960.5 °C), the grain boundaries are expected to be rough at atomic scales and hence have nearly isotropic boundary energy. The normal growth of the grains with such atomically rough boundary structures is consistent with some theoretical analysis and simulation. At low temperatures, the grain boundaries can be faceted with probably singular structures. Because these grain boundaries apparently migrate by the movement of boundary steps, AGG occurs. The observations with optical microscopy indeed indicate that some grain boundaries are faceted at low temperatures and all of them are smoothly curved indicating an atomically rough structure at high temperatures close to the melting point. Although the results are not conclusive, they support the hypothesis that AGG occurs because the faceted singular grain boundaries migrate by the step mechanism.     

Enhanced Densification of Carbonyl Iron Powder Compacts by the Retardation of Exaggerated Grain Growth through the Use of High Heating Rates

An investigation of the effect of heating rates on the densification behavior of carbonyl iron powder compacts, particularly on the exaggerated grain growth during the α-γ phase transformation, was carried out in this study. Compacts heated at 1200 °C/min and then sintered for 90 minutes at 1200 °C attained 7.14 g/cm³, while those heated at 10 °C/min reached only 6.61 g/cm³. Dilatometer curves using heating rates of 2 °C/min, 5 °C/min, 10 °C/min, 30 °C/min, and 90 °C/min demonstrate that 90 °C/min yields the highest sintered density. The microstructure analysis shows that high heating rates inhibit exaggerated grain growth during the phase transformation by keeping the interparticle neck size small and pinning the grain boundaries. This explanation is supported by the calculation that shows that the energy barrier preventing the grain boundary from breaking away from the neck is reduced hyperbolically as the neck size and the amount of shrinkage increase. The high heating rate, however, shows little beneficial effect for materials that have no allotropic phase transformation or have less drastic grain growth during heating, such as nickel and copper. Thus, bypassing the low temperatures to suppress the surface diffusion mechanism, which does not contribute to densification, is ruled out as the main reason for the enhanced densification of carbonyl iron powders.     

Effects of Static Recrystallization and Precipitation on Mechanical Properties of 00Cr12 Ferritic Stainless Steel

The 00Cr12 ferritic stainless steel samples were isothermally held at different temperatures in the range of 700 °C to 1000 °C to investigate the effect of static recrystallization and precipitation on mechanical properties, such as microhardness, tensile strength, and yield strength. The results show that the formation of the fine recrystallized grain, as well as precipitation, coarsening, and dissolution of the second-phase particles, influences the mechanical properties remarkably. The fine recrystallized grain can provide a positive grain boundary-strengthening effect in the sample under a relatively high holding temperature. Coarsening and dissolution of M₂₃C₆ result in partial depletion of precipitate hardening. In contrast, the size and number density of MX particles are almost constant, regardless of the holding temperature; therefore, it can provide a better precipitation-hardening effect.     

Creep deformation of ultrafine grained Ni75B17Si8

Ultrafine grained, polycrystalline materials can be prepared by annealing of melt spun amorphous ribbons under carefully controlled conditions. A grain size of 6.5–12.10⁻⁵ mm was attained in ribbons of Ni₇₅B₁₇Si₈, annealed at 600°C for 15 min, with a heating rate of 0.01–1 K/s. The samples were then creep tested at low stresses (< 400 kg/cm²) and temperatures between 500 and 615°C. Coble creep, with an activation energy of 4 eV, was the dominant mechanism of deformation at low stress, and below 575°C. At higher stress the rate controlling mechanism became a dislocation climb mechanism. The transition was temperature and grain size dependent. Above 575°C, the dominant mechanism was found to be a nondiffusion controlled process, with an activation energy of 0.6 eV, close to dislocation glide activation energy in Ni.     

Microstructure and Mechanical Properties of Ultrafine-Grained Interstitial-Free Steel Processed by ECAP

Equal-channel angular pressing (ECAP) of interstitial-free (IF) steel at equivalent strain, ε_vm = 12 has been employed to develop ultrafine-grained (UFG) microstructure with high fraction of low angle grain boundaries, that enhances strength significantly with reduced tensile ductility. ECAPed IF steel has been deformed further by cold rolling/cryorolling at ?50 °C to >90 % reduction in area. It is observed that the UFG structure gets refined with an improvement in high angle grain boundary fraction and heavily stressed non-equilibrium grain boundaries in cryorolled state resulting in significant strengthening. However, the decrease in grain size to an ultrafine level with the increased lattice strain lowers the work hardening ability of the material that limits its ductility. Hence, the rolled samples are flash annealed at 675 °C in order to recover the ductility of the material by achieving partially recrystallized structures. Consequently, the increased subgrain size as well as the grain size, the reduced residual lattice strain, lower hardness and strength with marginal recovery of ductility is maintained in order to attain the yield strength 2–3 times compared to that of as-received coarse-grained IF steel.     

Structure,Mechanical Properties,and Oxidation Resistance of MoSi<Subscript>2</Subscript>, MoSiB,and MoSiB/SiBC Coatings

Single-layer MoSi₂, MoSiB, and multilayer MoSiB/SiBC coatings are fabricated by magnetron sputtering. Coating structures are investigated using X-ray diffraction, a scanning electron microscopy, and glow-discharge optical emission spectroscopy. Mechanical properties of coatings are determined by nanoindentation. The thermal stability of coatings is studied in a temperature range of 600–1200°C and oxidation resistance is studied upon heating to 1500°C. It is established that single-layer MoSiB coatings possess a hardness of 27 GPa, elasticity modulus of 390 GPa, and elastic recovery of 48%. They can also resist oxidation up to 1500°C inclusively, which is caused by the formation of the SiO₂-based protective film on their surface. The MoSi₂ coatings can have hardness comparable to the hardness of MoSiB coatings, but they are somewhat worse than them in regards to oxidation resistance. Multilayer MoSiB/SiBC coatings have hardness 23–27 GPa and oxidation resistance restricted by 1500°C, but they herewith have higher elastoplastic properties when compared with MoSiB.