铂丝退火过程中的微观形貌演变和位错密度研究

铂丝的微观形貌和位错密度与退火工艺密切相关,研究铂丝退火过程中微观形貌和位错密度的变化,可以优化退火工艺和提高铂丝电阻性能,为研发高性能铂电阻温度计提供支撑。研究表明:初始铂丝表面有黑色斑驳区域和划痕,有自生颗粒和外来颗粒。随着退火时间的延长,铂丝表面的黑色斑驳区域和划痕消失。退火过程中,铂晶粒晶界处的铂原子很不稳定,逐渐与碳原子形成含Pt和C的颗粒,晶界则变宽边深,产生许多孔隙;随着退火时间的延长,铂丝主体中的杂质元素只剩下C。初始铂丝晶粒的内部和晶界处均存在大量的位错,位错密度达到3.69×10¹² cm^-2,经历退火热处理后晶粒内部和晶界处的位错大量消失。铂丝被充分退火后,位错密度低至无法测出。     

Life Prediction of Tungsten Filaments in Incandescent Lamps

The phenomenon of grain boundary sliding with diffusional accommodation has been applied to the problem of incandescent lamp burn-out with good results. Accordingly, the results suggest that the extent to which the grain boundary shape deviates from a planar interface perpendicular to the wire axis is the critical factor in determining the life of the filament. In addition, a method for the optimization of the coil geometry, microstruc-ture, and temperature for increasing the filament life are suggested.     

Metallurgical factors affecting the crack growth resistance of a superalloy

During creep loading of IN-792, grain boundary morphology in conjunction with grain size strongly affected crack propagation. Compositional variations and fabrication techniques showed no significant effect. A primary requirement for materials to be used in gas turbine engine discs is satisfactory resistance to crack growth resistance in the 650 to 760°C range. Both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in this study. Creep fractures took place by grain boundary cracking followed by rapid transgranular fractures. Composition variations had only very slight effects on crack propagation. Materials hot worked from castings had the same properties as those made by powder metallurgy techniques. The primary factors influencing the crack growth behavior were the grain size and grain shape. Increasing grain size markedly improved the toughness. By slow cooling through the gamma prime solvus a serrated grain boundary structure was developed that also improved the cracking resistance. Earlier creep fracture toughness studies have shown that the slow crack growth behavior can be described by a critical strain model in which the crack propagation is controlled by the yield strength, grain size, and a critical strain parameter. The present results are consistent with this model, with serrated grain boundaries introducing a four-fold increase in the critical strain parameter over that of smooth grained material.     

Formation of annealing twins,faceting, and grain boundary pinning in copper bicrystals

In the course of a recent investigation of the kinetics of grain boundary migration in copper bicrystals, formation of annealing twins, faceting and grain boundary pinning have been observed. Stability and frequency of formation of annealing twins are related to boundary misorientation and temperature of anneal. Tendency for grainboundary faceting decreases with increasing temperature indicating that anisotropy of grain boundary energy is more pronounced at lower temperatures. In general, orientations of faceted boundaries corresponded to higher-order twin planes with respect to shrinking grains. At relatively high temperatures (950°C), specimen-thickness-dependent pinning effects also are observed. These observations are described and analyzed in terms of grain boundary structure, energy, and migration behavior.     

Effect of Sulfur and Antimony on the Intergranular Fracture of Iron at Cathodic Potentials

Antimony, segregated to grain boundaries of iron, was found to be five times more effective than sulfur in promoting intergranular fracture of iron when tested in IN H₂SO₄ at cathodic potentials. A decrease in the ductility of iron accompanied the fracture mode change at increasing cathodic potentials. The effectiveness of antimony relative to sulfur was determined from straining electrode tests on iron and iron + 250 appm antimony alloys heat treated at 800 °C and 600 °C to produce different grain boundary chemical compositions. Grain boundary compositions were determined by Auger Electron Spectroscopy (AES). Similar grain boundary sulfur concentrations of 0.2 monolayers were observed by AES for the iron and iron + 250 appm antimony alloy after an anneal of 240 hours at 600 °C, while 0.08 monolayers of antimony was observed for the iron + 250 appm antimony alloy. These results suggest that sulfur and antimony do not compete for grain boundary sites.     

Lifetimes and failure mechanisms of w/re hairpin filaments

A life test of tungsten hairpin filaments used in a high-reliability electron beam instrument has been carried out to improve knowledge of filament life and failure mechanisms as a function of temperature. These filaments are made of nonsag tungsten/3 pct rhenium (W/3 pct Re) wire. A steep reduction in filament life is observed at 2760 K, which is not predicted by models which assume thermal evaporation as the principal failure mechanism. Failure analysis of the filaments shows that the sudden loss of life at 2760 K is the result of localized hot-spot formation caused by the accumulation of voids at grain boundaries. Examination of the crystal growth and growth rates indicates that the recrystallization temperature, T_Rec, occurs near 2760 K for the nonsag W/3 pct Re wire used in these filaments. This suggests that void accumulation acts as the principal, life-limiting failure mechanism; spontaneous recrystallization at 2760 K increases the rate of void growth and causes a severe reduction of filament life.     

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.     

Optimization of Nb3SnCu superconductor wire prepared by the in situ process

The in situ process for preparing Nb₃SnCu superconductor wire produces the Nb₃Sn filaments in a bronze matrix from as-cast NbCu ingots. The ingots are drawn to produce a composite of aligned Nb filaments in a Cu matrix and following Sn plating a diffusion anneal converts Nb filaments to Nb₃Sn filaments. This work shows that an optimum size Nb filament is required to maximize critical current capacity. If the filaments are too big resistance of the Nb₃SnCuNb₃Sn junctions limit J_c. while if the filaments are too small coarsening occurs during the diffusion anneal and the junctions so formed limit J_c. It is shown that J_c values for wires of optimum filament size compare quite favorably to bronze processed Nb₃SnCu wire at fields as high as 15T.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Fatigue Initiation Study of TMT Eutectoid Steel

Eutectoid steel and 1060 steel samples were given a variety of thermomechanical treatments (TMT), described in Table I, which varied the pearlite interlamellar spacing, the cementite and inclusion orientation, the degree of cold-work in the ferrite matrix, and (for 1060 steel only) the proeutectoid grain boundary ferrite network. These samples were then evaluated as to resistance to fatigue crack initiation. The TMT designated, G, involves a subcritical anneal and results in a partially recrystallized condition which shows not only excellent resistance to fatigue crack initiation in the near threshold region but is second only to a fine pearlite microstructure, B, in fatigue crack propagation threshold value. It is believed that the excellent properties of the TMT, G, are related to the formation of subgrains in the interlamellar ferrite. On a scale normalized to tensile strength,(ΔK/√ρ)/σ_u, fine oriented pearlite in a soft ferrite matrix (rapid up-quench TMT such as E, F) shows the best resistance to fatigue crack initiation. A proeutectoid ferrite grain boundary network is poor at resisting fatigue crack initiation but good at resisting fatigue crack propagation. It should be emphasized that the combined high resistance of the subcritical TMT (G) toboth fatigue crack initiation and propagation, coupled with its much easier implementation relative to similar microstructures produced by difficult rapid up-quench TMT's (E, F) make it a very promising approach to improving overall fatigue resistance.     

Evolution of microstructure and properties in alpha-brass after iterative processing

Iterative strain-recrystallization cycles have been applied to alpha-brass in order to enhance ductility while maintaining tensile strength. Five iterations of 25 pct uniaxial strain followed by a 300 second anneal at 665 °C were used to achieve the desired properties. This article concentrates on assessment of the effect of the processing on microstructure evolution and crystallographic details of the grain boundary population, after each strain-recrystallization cycle. The overall aim of the work is to provide further knowledge on the mechanisms of grain boundary engineering. The results demonstrate that there is a distinctive pattern in both the Σ3 population density (in coincidence site lattice notation) and the proximity to the Σ reference structure, as a function of treatment cycle iteration. During the first two treatment cycles, the proportion of Σ3s drops, which the present work shows is an essential step to homogenize the microstructure in preparation for the subsequent treatment iterations required for the property enhancements to develop. It is proposed that this is a general feature of all grain boundary engineering by iterative processing where cold reduction is involved. Furthermore, Σ3 ⁿ (n>1) boundaries do not build up in the microstructure concomitant with the Σ3 fraction because they are removed by the “Σ3 regeneration model.”     

A study of the mechanisms of fatigue life improvement in an ion implanted nickel-chromium alloy

Wires of the alloy Ni20Cr with and without carbon ion implantation have been tested in tension-tension fatigue. A 17% increase in endurance limit was found with implantation. The fatigued surface was examined by SEM, and the wire then nickel plated so that transverse sections could be made for TEM study. It was found that bulk slip was unaffected by implantation but slip in surface grains was unable to penetrate the implanted layer to significant degree. Slip band cracking which was found in unimplanted specimens was replaced by grain boundary cracking in the implanted specimens.     

Microstructure and Tensile Ductility in a β Heat Treated Titanium Alloy

The nature of tensile failure in aβ-treated titanium alloy, Ti-6Al-l.6Zr-3.3Mo-0.3Si, is demonstrated to be strongly dependent on cooling rate followingβ treatment. A characterization of microstructure, the fracture surface of tensile tested samples, and the associated slip behavior shows the fracture mode to be determined by the microstructure as well as composition of the a laths. A martensitic matrix with fine grain boundary α particles leads to intergranular failure. At slower cooling rates, α laths form as side plates from grain boundary allotriomorphs, and an fcc interface phase is present at thea lath/retainedβ interfaces. This microstructure results in transgranular fracture caused by ductile failure at the α lath/β interfaces within the grains rather than at grain boundary ?. If the composition is equilibrated across the a laths by a long-term anneal followingβ treatment, the slip mode becomes extremely planar leading to slip band related cracking across the grains.     

High temperature ductility loss in carbon-manganese and niobium-treated steels

The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation. Formerly Graduate Student, Lehigh University     

Mechanisms of strain accumulation and damage development during creep of prestrained 316 stainless steels

The effects of prestraining at room temperature and at the creep temperature of 848 K, as well as the responses to stress reductions during creep, have been studied for 316 stainless steels varying in composition and initial microstructure. The results are analyzed by contrasting the strengthening effects achieved by introducing high dislocation densities prior to creep exposure with the deleterious effects, which can occur when prestraining causes premature void nucleation at grain boundaries. In addition, by recognizing the differing contributions made by the grain interiors and the grain boundary zones to the overall rates of creep strain accumulation, a consistent explanation is provided for the diverse creep behavior patterns reported for different metals and alloys after various prestraining treatments.     

A tight-binding study of grain boundaries in silicon

A simple empirical tight-binding model for silicon is propounded and used to compute the atomic and electronic structures of three symmetrical tilt grain boundaries and the intrinsic stacking fault. The ability of the model to describe silicon in a variety of crystal structures is tested and it is shown to be satisfactory for simulating defects in the diamond structure. The effect of charge transfer on the energy and stability of the grain boundaries is assessed. Interatomic forces and energies are computed in real space using a rotationally invariant formulation of the recursion method. Five proposed reconstructions of the (112̄) symmetrical tilt boundary are studied in detail and good agreement is achieved with results from electron microscopy and diffraction. The (13̄0) and (111) symmetrical tilt boundaries have also been modelled successfully. Comparison is made between the computed electronic structures of the boundaries, reported in this work and by other authors, and experimental measurements of the densities of states at grain boundaries. The existence of band tails and midgap continua in the experimental measurements and the absence of both of these features in the models are two notable points of disagreement. Some fundamental questions about localisation of electronic states at grain boundaries are raised.     

精密异型钛材热处理工艺研究

采用清洗—碱洗—酸洗工艺对眼镜用精密异型钛材进行真空退火前的预处理,给出了工艺控制条件并进行了讨论。对氢脆的危害进行了探讨,得出了适宜的真空退火条件。试验表明,按确定的真空退火前预处理工艺和真空退火工艺条件处理后的精密异型钛材,能够获得非常光亮的表面,满足后续加工工艺和产品质量要求。     

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.     

The effect of deformation on abnormal grain growth in tungsten ingots

This paper reports a study of abnormal grain growth in tungsten ingots that had been deformed in compression at elevated temperatures. The results show that very large grains form in these samples directly from the polygonized structure when the deformed samples are annealed. A critical amount of deformation is required to begin this process. Beyond that point, the resulting grain size decreases with increasing amounts of deformation and decreasing test temperature. Abnormal grain growth occurs first in the regions of the sample that have undergone the most strain. Two factors appear to provide the driving force for the formation of these large grains. One is the elimination of grain boundary area. This effect would be present in any system undergoing regular or abnormal grain growth. The other is the elimination of grains that are more highly strained by grains that are less strained. In this way, this process is similar to the large grain growth resulting from strain annealing.