The influence of microstructure on fracture of drawn tungsten wire

The microstructures and longitudinal fracture resistances of 0.635 mm diam lamp-doped and undoped tungsten wire were examined in the as-drawn condition and after anealing at temperatures between 600 and 1500°C. A variety of experimental techniques were employed, including Auger Electron Spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and a newly developed mechanical testing technique. The longitudinal fracture mode was intergranular for all wires and a second phase was observed on the grain boundaries of all doped wires. High concentrations of the dopant element potassium were present on the fracture surfaces of doped wires and experimental evidence was obtained which suggests they may be due to postfracture surface diffusion. Doped wires demonstrated increasing amounts of structure coarsening up to 1500°C whereas large equiaxed grains were formed in undoped wires annealed at 1300 and 1500°C. The longitudinal fracture resistance of undoped wire was unaltered by annealing at 1050°C and below, but decreased dramatically after annealing at 1300 and 1500°C. In contrast the fracture resistance of doped wire decreased after annealing at 1050 and 1300°C, but increased after annealing at 1500°C. Fracture resistance is discussed in terms of microstructure and fracture surface chemistry. A. W. FUNKENBUSCH, formerly Research Metallurgist with General Electric Refractory Metals Product Department     

Dopant particle characterization and bubble evolution in aluminum-potassium-silicon-doped molybdenum wire

The present article describes the creation of dopant inclusions in aluminum-potassium-silicon (AKS)-doped molybdenum powder and the generation of potassium bubbles in doped molybdenum wire. Molybdenum wire is used extensively in the incandescent lamp industry for coiling mandrels, filament support wires, and foil seals. The AKS-doped molybdenum wire is an important product, because it possesses greater high-temperature strength and a higher recrystallization temperature than undoped molybdenum; both of these properties are important for structural applications in lamps. The AKS-doped molybdenum wire is produced in a similar manner to AKS-doped tungsten wire, but lower processing temperatures are typically used for the production of molybdenum wire. Previous studies on AKS-doped tungsten wire have shown that the dispersion which provides the interlocking grain structure in recrystallized tungsten wire is bubbles of elemental potassium; these enhance incandescent lamp filament life. However, there is little previous work on the potassium-containing dispersion in AKS-doped molybdenum wire. In AKS-doped molybdenum, the dispersion can be either potassium bubbles, or solid oxide particles, depending on the processing method. This article will describe a series of analyses of doped molybdenum wire and its precursors, namely, doped powder and sintered ingots. The roles of high- and low-temperature sintering are also described.     

Porosity and anomalous recrystallization behavior in doped tungsten wire

An anomalous recrystallization behavior has been observed in 0.010 in. diam K?Si?Al doped tungsten wire. Recrystallization to relatively equiaxed grains occurred only in the core at temperatures of 1400° to 1600°C, but not at lower temperatures or higher temperatures up to 1900°C. The anomalous recrystallization reaction was suppressed when the wire was preannealed at 1700° to 1900°C, but not by preanneals at temperatures between 500° and 1100°C. Critically shadowed electron fractographs indicated that linear arrays of microporosity (≈100Å diam) were responsible for the stabilization of the fibrous structures, Recrystallization occurred when these strings of pores were absent.     

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.     

Characterization of recrystallization and grain growth in Ti-7.4 At. Pct Al (cph) and Ti-15.2 At. Pct Mo (bcc) alloys

Quantitative microscopy, texture and grain growth kinetic studies were made on swaged and recrystallized Ti-7.4 at. pct Al and Ti-15.2 at. pct Mo alloys. The quantitative microscopy studies indicated that the grain size distribution in both alloys is a constant for a given grain size, independent of annealing time and temperature and follows a log normal distribution. Moreover, there exists a range of grain sizes in space; the relative quantities of each size in the range varies with average grain size. Also, the grain shape factor decreases with increase in annealing time (grain size) at a constant temperature and with decrease in temperature for a constant grain size. The values of the shape factor for a given grain size and temperature were approximately the same for the two alloys. The quantitative microscopy features were essentially the same as those observed by Okazaki and Conrad for unalloyed titanium. The texture of the as-swaged Ti-7.4 at. pct Al wire specimens and the changes in this texture during grain growth were in accord with those previously reported for deformed and recrystallized titanium. The Ti-15.2 at. pct Mo alloy retained the deformation texture even after recrystallization. At ^1/3 time law was found to hold for the grain growth over most of the grain sizevs time curve. The values of the activation energy for grain boundary migration were 25.2 Kcal/mole for the Ti-7.4 at. pct Al alloy and 29 Kcal/mole for the Ti-15.2 at. pct Mo alloy. These are similar to those for diffusion of Al and Mo in titanium, indicating that the diffusion of these substitutional elements controls the rate of boundary migration in these alloys. 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.     

掺杂稀土元素的高温钼合金的研究

介绍了国内外掺杂稀土元素的高温钼合金的研究情况。对Y2O3、La2O3、NdO3、Sm2O3、Gd2O3五种稀土元素掺入钼丝和未掺入钼丝了进行研究。结果表明,掺杂钼丝比未掺杂钼丝具有更高的再结晶温度和高温下更好的抗变形性能。目前这种掺有稀土元素的钼合金已被研制出,并得到广泛的应用,市场前景看好。     

Recrystallization of W-1 wt pct ThO2 wire

The recrystallization behavior of W-l wt pct ThO₂ wires of two sizes, 0.457 and 0.178 mm (18 and 7 mil), was studied using optical microscopy, transmission electron microscopy, hardness, and resistivity ratio techniques. For the 0.178 mm wire the effects of heating rate were also analyzed. Recrystallization of the 18 mil wire is characterized by rather gradual changes in hardness, resistivity ratio, and microstructure leading to a small recrystallized grain size. The 0.178 mm wire, on the other hand, exhibits more abrupt changes in resistivity ratio and hardness which coincide with the development of a recrystallized structure having a large grain-size range. Variation of heating rates between about 150,000°C/min and 500°C/min did not significantly affect the grain structure or hardness achieved for annealing temperatures of 2000°, 2500°, and 2700°C. However, very slow heating rates, less than 10°C/min, were shown to prevent the formation of large grains in the 0.178 mm wire. These results are explained on the basis of differences in the effectiveness of ThO₂ particles in hindering grain boundary motion.     

On the short time creep rupture

Short time tensile creep rupture tests were made on 7 mil lamp filament wire and on 7 mil undoped tungsten wire in the temperature range 2500 to 3100 K. Steady-state creep rate was described by an expression having the form: ε =ADσ/E)^m whereA is a constant,D is the self-diffusion coefficient, σ is the creep stress,E is the elastic modulus, andm is 5 for undoped tungsten and 25 for lamp grade wire. Activation energy for time to failure of lamp wire was found to vary from 150 to 185 kcal in the range 2500 to 3100 K. Failure mode in this range was slip at high stresses and intergranular brittle fracture at low stresses. The very high creep-strength observed for lamp wire was attributed to a combination of resistance to diffusional creep at low stresses and resistance to dislocation creep at high stresses. The dispersoid structure of lamp wire was suggested as the origin of strength in both cases. Formerly manager, Research and Development, Refractory Metals Business Section     

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 Behavior and Damage of Ni-Base Superalloys PM 1000 and PM 3030

Two oxide dispersion strengthening (ODS) nickel-base superalloys, a solely dispersion-strengthened alloy (PM 1000) and an additionally γ′-strengthened alloy (PM 3030) are investigated regarding creep resistance at temperatures between 600 °C and 1000 °C. The creep strength advantage of PM 3030 over PM 1000 decreases as the temperature increases due to the thermal instability of the γ′ phase. The particle strengthening contribution in both alloys increases linearly with load. However, solid solution softening leads to an apparent drop in particle strengthening in PM 1000. Deformation concentration in slip bands is more accentuated in PM 3030-R34 due to additional γ′ strengthening combined with strongly textured coarse and elongated grain structure. Finer, equiaxed grains reduce creep strength at higher temperatures due to grain boundary deformation processes and premature pore formation, but have only minor impact at low and intermediate temperatures.     

稀土氧化物粒子对钼合金粉末冶金过程及力学性能的影响

试验研究了掺杂La2O3、Y2O3、CeO2稀土氧化物颗粒对钼合金的粉末物性、烧结进程、制品的烧结致密度及压力加工丝材的室温力学性能的影响规律。试验结果表明,掺杂稀土氧化物粒子细化了钼粉的粒度,降低了松装密度和粒度分布范围,同时导致粉末团聚现象增多;稀土氧化物粒子延迟了钼合金的烧结进程,降低了烧结制品的致密度,同时细化了烧结体晶粒尺寸。稀土氧化物粒子以弥散强化和细晶强化的形式,提高了钼合金丝的室温强度。CeO2显著提高了钼合金丝的室温韧性,La2O3、Y2O3则降低了钼合金丝的室温韧性。     

Effect of Thorium Additions on Metallurgical and Mechanical Properties of Ir-0.3 pct W Alloys

Metallurgical and mechanical properties of Ir-0.3 pct W alloys have been studied as a function of thorium concentration in the range 0 to 1000 ppm by weight. The solubility limit of thorium in Ir-0.3 pct W is below 30 ppm. Above this limit, the excess thorium reacts with iridium to form second-phase particles. Thorium additions raise the recrystallization temperature and effectively retard grain growth at high temperatures. Tensile tests at 650 °C show that the alloy without thorium additions (undoped alloy) fractured by grain-boundary (GB) separation, while the alloys doped with less than 500 ppm thorium failed mainly by transgranular fracture at 650 °C. Intergranular fracture in the doped alloys is supressed by GB segregation of thorium, which improves the mechanical properties of the boundary. The impact properties of the alloys were correlated with test temperature, grain size, and heat treatment. The impact ductility increases with test temperature and decreases with grain size. For a given grain size, particularly in the fine-grain size range, the thorium-doped alloys are much more ductile and resistant to GB fracture. All of these results can be correlated on the basis of stress concentration on GBs by using a dislocation pileup model.     

Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature

Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of ∼26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and ∼24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.     

Influence of Filler Wire Diameter on Mechanical and Corrosion Properties of AA5083-H111 Al–Mg Alloy Sheets Welded Using an AC Square Wave GTAW Process

In this work, the influence of filler wire diameter on AA5083-H111 weldments was studied. For that, square butt joints were made using an AC square wave gas tungsten arc welding process with the addition of filler wires of diameter 1.2 and 2.4 mm separately. The experimental results revealed that changing the filler wire diameter influenced the bead geometry and a complete penetration was achieved in both welds. The weldment processed with smaller diameter filler wire consisted of a wider heat affected zone with recrystallized grains and a fusion zone with coarser grain structure, thus reducing the mechanical properties and corrosion resistance. However, the use of larger diameter filler wire assisted in faster torch speed, resulting in lower heat input and thus finer equiaxed grains were produced in fusion zone. Also, finer grains along with the dispersion of finer Al₆(Fe,Mn) particles supported in obtaining the superior tensile and corrosion properties.     

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.     

The influence of γ′ on the recrystallization of an oxide dispersion strengthened superalloy-ma 6000e

The requirement of large, recrystallized, highly elongated grains is of primary importance to the development of suitable high temperature properties in oxide dispersion strengthened superalloys. In the present study the recrystallization behavior of MA 6000E, a recently developed Y₂O₃ strengthened superalloy produced by mechanical alloying, was examined using transmission and replication microscopy. Gradient and isothermal annealing treatments were applied to extruded and hot rolled product. It was found that conversion from a very fine (0.2 μm) grain structure to a coarse (≅10 mm) grain structure is controlled by the dissolution of the gamma prime phase, while grain shape was controlled primarily by the thermal gradient. The fine uniform oxide dispersion appeared to have only a secondary influence in determining the grain shape as columnar grains could be grown transverse to the working direction by appropriate application of the thermal gradient.     

TA12A钛合金热处理过程中等轴和片层α相演变行为研究

对近α 型TA12A钛合金进行热处理实验,利用扫描电子显微镜(SEM)和电子背散射衍射(EBSD)技术对热处理后的微观组织进行观察,研究了两相区固溶温度和冷却速率对微观组织的影响.研究表明:TA12A钛合金在980和1000℃保温后冷却时,β相向α相转变,一方面可以使得等轴α相长大,另一方面也可析出片层α相.等轴α相长...     

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.     

Measurements and modeling of the microstructural morphology during equiaxed solidification of Al-Cu alloys

The morphology predicted by an equiaxed growth model recently presented elsewhere has been compared with quantitative experimental morphology measurements on a range of Al-Cu alloys. In the experiments, the samples have been solidified with a uniform temperature and quenched from the mushy state at the instant when the eutectic temperature was reached. The copper content and the amount of grain-refiner additions have been varied, resulting in both “clover-leaf” and dendritic equiaxed morphologies. Morphology characterization on both the intragranular and extragranular length scales has been performed on the quenched samples. Average heat-extraction rates, grain densities, and alloy compositions from the experiments have been used as input to the equiaxed grain-growth model, and the resulting morphology predictions have been compared with the morphology measurements. For the morphologies observed in the present study, the equiaxed growth model predicts higher values of the internal solid fraction than observed experimentally. This has been indicated to be due to the failure of commonly made modeling assumptions during the later stages of the solidification.     

Homogeneity and Equiaxed Grain Structure of Billets

The homogeneity of as‐cast structures is essential to the application of advanced steel materials. The relationship between as‐cast equiaxed grain structure and homogeneity was examined by means of OPA (Original Position Analysis) technology. The results indicate that macrosegregation of the solute elements was obviously suppressed in silicon steel billets with equiaxed grain structure. Furthermore, two methods to achieve an equiaxed grain structure were described. One is to use titanium‐based inoculation technology. TiN particles and TiN/ Ti₂O₃ complex particles can precipitate at the early stage of solidification and then play an important role of heterogeneous nucleation when the solidification condition is carefully controlled. The other method is to maintain a small temperature gradient in the mould to enhance the constitutional undercooling As a result, the as‐cast equiaxed grain structure was well developed in the experimental billets.