Grain boundary cracking

A chronological summary is given of the various types of grain boundary fracture found in metals. In each case, there is an impurity that adsorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphorous on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger reduction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fracture in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (no void nucleation), in which diffusion of solute to the new surface limits crack growth. Numerous examples of this are also discussed. Dr. Shewmon studied metallurgical engineering at the University of Illinois (B.S. 1952) and Carnegie Institute of Technology (Ph.D. 1955). His first job was at the Westinghouse Research Laboratory, where he studied thermal diffusion in alloys and surface diffusion. In 1958, he moved to the Carnegie Institute of Technology, where he served as a professor until 1967. The text “Diffusion in Solids” was published in 1963. An NSF Fellowship was used to study at Professor C. Wagner’s Max Planck Institute (Goettingen, Germany) in 1963. From 1968 to 1973, he was at Argonne National Laboratory, serving successively as Associate Director of the Metallurgy Division, Associate Director of the EBR-2 Project, and Director of the Materials Science Division. The text “Transformations in Metals” was published in 1969. Materials behavior in fast breeder reactors was the main theme of his work during this period. He was the director of the Division of Materials Research at the National Science Foundation from 1973 to 1975. From 1975 to 1993, he was Professor at Ohio State University in the Department of Metallurgical Engineering (later Materials Science and Engineering), serving as Chairman from 1975 to 1983. Research interests during this period were hard particle erosion and hydrogen-induced cracking of steel (“hydrogen attack”). From 1977 to 1993 he served on the Advisory Committee on Reactor Safety for the United States Nuclear Regulations Committee, serving as Chair for three of those years. Dr. Shewmon was elected to the National Academy of Engineering in 1979 and has been awarded the standing of Fellow in TMS, ASM, ANS, and AAAS. He has received several outstanding paper awards (Noble-AIME, Raymond—TMS, Mathewson—TMS, and Howe—ASM). He received the Distinguished Alumnus Award of the University of Illinois in 1981 and a Humboldt Foundation Senior Scientist Prize in 1984. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. The Institute of Metals Lecture was established in 1921, at which time the Institute of Metals Division was the only professional division within the American Institute of Mining and Metallurgical Engineers. It has been given annually since 1922 by distinguished people from this country and abroad. Beginning in 1973 and thereafter, the person selected to deliver the lecture will be known as the “Institute of Metals Division Lecturer and R.F. Mehl Medalist” for that year.     

Grain boundary cracking

A chronological summary is given of the various types of grain boundary fracture found in metals. In each case, there is an impurity that adsorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphorous on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger reduction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fracture in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (no void nucleation), in which diffusion of solute to the new surface limits crack growth. Numerous examples of this are also discussed. Dr. Shewmon studied metallurgical engineering at the University of Illinois (B.S. 1952) and Carnegie Institute of Technology (Ph.D. 1955). His first job was at the Westinghouse Research Laboratory, where he studied thermal diffusion in alloys and surface diffusion. In 1958, he moved to the Carnegie Institute of Technology, where he served as a professor until 1967. The text “Diffusion in Solids” was published in 1963. An NSF Fellowship was used to study at Professor C. Wagner’s Max Planck Institute (Goettingen, Germany) in 1963. From 1968 to 1973, he was at Argonne National Laboratory, serving successively as Associate Director of the Metallurgy Division, Associate Director of the EBR-2 Project, and Director of the Materials Science Division. The text “Transformations in Metals” was published in 1969. Materials behavior in fast breeder reactors was the main theme of his work during this period. He was the director of the Division of Materials Research at the National Science Foundation from 1973 to 1975. From 1975 to 1993, he was Professor at Ohio State University in the Department of Metallurgical Engineering (later Materials Science and Engineering), serving as Chairman from 1975 to 1983. Research interests during this period were hard particle erosion and hydrogen-induced cracking of steel (“hydrogen attack”). From 1977 to 1993 he served on the Advisory Committee on Reactor Safety for the United States Nuclear Regulations Committee, serving as Chair for three of those years. Dr. Shewmon was elected to the National Academy of Engineering in 1979 and has been awarded the standing of Fellow in TMS, ASM, ANS, and AAAS. He has received several outstanding paper awards (Noble-AIME, Raymond—TMS, Mathewson—TMS, and Howe—ASM). He received the Distinguished Alumnus Award of the University of Illinois in 1981 and a Humboldt Foundation Senior Scientist Prize in 1984. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering.     

Effect of Grain Boundaries and Grain Orientation on Structure and Properties

The evolution of deformation microstructures in metals follows a universal pattern of grain subdivision. However, the structure in the grain boundary region may be different from that in the grain interior, although a characteristic region cannot be identified for polycrystals with medium to high stacking fault energy. In the grain interior, the dislocation structure is predominantly composed of almost planar boundaries (geometrically necessary boundaries) and cell boundaries (incidental dislocation boundaries) forming a cell block structure. For grains with grain sizes reaching down to about 4 μm deformed in tension and by rolling, a clear correlation has been established between the characteristics of the deformation structure and the orientation of the grain in which it evolves. A similar correlation is observed for single crystals of different orientations. Such correlations form the basis for a general analysis of active slip systems and for modeling of the flow stress and flow stress anisotropy of polycrystalline samples.     

Grain Cluster Microstructure and Grain Boundary Character Distribution in Alloy 690

The effects of thermal-mechanical processing (TMP) on microstructure evolution during recrystallization and grain boundary character distribution (GBCD) in aged Alloy 690 were investigated by the electron backscatter diffraction (EBSD) technique and optical microscopy. The original grain boundaries of the deformed microstructure did not play an important role in the manipulation of the proportion of the Σ3 ⁿ (n = 1, 2, 3…) type boundaries. Instead, the grain cluster formed by multiple twinning starting from a single nucleus during recrystallization was the key microstructural feature affecting the GBCD. All of the grains in this kind of cluster had Σ3 ⁿ mutual misorientations regardless of whether they were adjacent. A large grain cluster containing 91 grains was found in the sample after a small-strain (5 pct) and a high-temperature (1100 °C) recrystallization anneal, and twin relationships up to the ninth generation (Σ3⁹) were found in this cluster. The ratio of cluster size over grain size (including all types of boundaries as defining individual grains) dictated the proportion of Σ3 ⁿ boundaries.     

CSP线高强度细晶热轧板的混晶和变形拉长晶粒的成因

对CSP线生产的高强度细晶热轧板的混晶和拉长晶粒的成因进行了分析,用有限元分析法模拟了热轧带钢的变形区的剪切应变场和温度场,用Gleeble实际模拟轧制工艺和组织变化。结果表明,CSP线高强度细晶热轧板的混晶和拉长晶粒的形成与钢板轧制过程中的钢板表层的变形场及温度场有关,也与先析出铁素体的形成后再进行轧制变形的过程有关;采用奥氏体深过冷轧制,既保证得到细晶粒又避免产生混晶和被变形拉长的晶粒。新的CSP轧制工艺,成功地生产了高强度高成形性细晶粒C-Mn热轧板。     

Linear Relationship Between dV/dt and Grain Volume During Grain Growth

Metallurgical and Materials Transactions A - The volumetric growth rate of individual grains has been found empirically to be directly proportional to their individual volume, dV/dt = β(V0...     

Grain Selection During Casting Ni-Base, Single-Crystal Superalloys with Spiral Grain Selector

The behavior of grain selection in a spiral grain selector during investment casting of a Ni-base, single-crystal (SX) superalloy, DD3, has been investigated by electron backscattered diffraction (EBSD) techniques and optical microscopy. The results indicated that the main function of starter block is to optimize the crystal orientation. During the process of grain selection in spiral passage, the grain near the inner wall of spiral passage was usually selected as the final single crystal. It was found that the dendrites near the inner wall could develop new tertiary dendritic arms that paralleled the primary dendrites from the secondary dendritic arms to overgrow the dendrites far away from the inner wall. The crystal orientation that was examined by X-ray diffraction revealed that (1) the crystal orientation did not change obviously with increasing spiral thickness or angle and (2) the crystal orientation could be optimized by increasing the withdrawal rate and ceramic mold temperature. The influence of pouring temperature on crystal orientation was also discussed.     

Phenomenology of Abnormal Grain Growth in Systems with Nonuniform Grain Boundary Mobility

We have investigated the potential for nonuniform grain boundary mobility to act as a persistence mechanism for abnormal grain growth (AGG) using Monte Carlo Potts model simulations. The model system consists of a single initially large candidate grain embedded in a matrix of equiaxed grains, corresponding to the abnormal growth regime before impingement occurs. We assign a mobility advantage to grain boundaries between the candidate grain and a randomly selected subset of the matrix grains. We observe AGG in systems with physically reasonable fractions of fast boundaries; the probability of abnormal growth increases as the density of fast boundaries increases. This abnormal growth occurs by a series of fast, localized growth events that counteract the tendency of abnormally large grains to grow more slowly than the surrounding matrix grains. Resulting abnormal grains are morphologically similar to experimentally observed abnormal grains.     

6061棒材晶粒度大、粗晶环严重工艺研究

阐述了6061合金生产时粗晶环产生的机理,结合正交实验及多年的生产实践经验,提出了解决6061合金棒材晶粒粗大的措施.     

Grain boundaries in recrystallization

Abstract

In this brief survey, the theoretical models for nucleation of new grains in annealed cold-worked metals are reviewed in the light of recent researches in the author's laboratory. The importance of strain-induced boundary migration is gradually emerging. The role of vacancies in enhancing the mobility of grain boundaries is reviewed, and this leads on to a survey of the influence of pores in particular, and precipitates more generally, on the motion of grain boundaries. Areas of ignorance which call for further experiments are pointed out.

The influence of dissolved impurities on grain-boundary motion is excluded from this survey.

Résumé

Dans cette brève synthèse, l'auteur passe en revue, à la lumière de récents travaux effectués dans son laboratoire, les modèles théoriques de germination des grains dans les métaux déformés à froid et recuits. L'importance d'une migration des joints induite par la déformation émerge graduellement. Le rôle des lacunes, qui favorisent la mo bilite des joints de grains, est passé en revue et ceci conduit à une étude de l'influence des pores en particulier, et de précipités plus généralement, sur la migration des joints de grains. L'auteur souligne les domaines, où notre ignorance nécessite d'autres expériences. Est exclue de cette synthèse l'influence des impuretés dissoutes sur la migration des joints de grains.     

回火对超细贝氏体晶粒取向和尺寸的影响

用50kg真空感应炉冶炼的钢锭,经1150℃开轧,两阶段控制轧制,830℃终轧成12mm钢板,驰豫27s,720℃进水快冷(冷却速率30～35℃/s)后,超低碳贝氏体钢(成分%:0.023C、1.63Mn、0.52Cu、0.25Mo、0.055(Nb+Ti)、0.0011B)具有板条贝氏体+少量粒状贝氏体和不规则铁素体的复合组织,贝氏体束团尺寸为3～5μm,块状和不规则铁素体尺寸为1～2.5μm。650℃和700℃回火对贝氏体晶粒取向无显著影响。     

粒状陶瓷纤维浇注料

介绍了一种新型陶瓷纤维浇注料,将陶瓷纤维棉制成高强度的球柱状纤维颗粒,然后再用这种纤维颗粒置换耐火浇注料中的骨料而制成的粒状陶瓷纤维浇注料。它集重质耐火浇注料和轻质隔热浇注料的优点于一体,既具备强度高、耐高温的机械性能,又具备低体积密度、低导热率的隔热性能。     

Grain refinement of magnesium alloys

The literature on grain refinement of magnesium alloys is reviewed with regard to two broad groups of alloys: alloys that contain aluminum and alloys that do not contain aluminum. The alloys that are free of aluminum are generally very well refined by Zr master alloys. On the other hand, the understanding of grain refinement in aluminum bearing alloys is poor and in many cases confusing probably due to the interaction between impurity elements and aluminum in affecting the potency of nucleant particles. A grain refinement model that was developed for aluminum alloys is presented, which takes into account both alloy chemistry and nucleant particle potency. This model is applied to experimental data for a range of magnesium alloys. It is shown that by using this analytical approach, new information on the refinement of magnesium alloys is obtained as well as providing a method of characterizing the effectiveness of new refiners. The new information revealed by the model has identified new directions for further research. Future research needs to focus on gaining a better understanding of the detailed mechanisms by which refinement occurs and gathering data to improve our ability to predict grain refinement for particular combinations of alloy and impurity chemistry and nucleant particles. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.     

铝钛碳晶粒细化剂的研究

在铝工业中,晶粒细化剂的使用量是比较大的,而不同的细化剂其细化效果明显不同.目前国外研究和应用较为广泛的是Al-Ti-C细化剂.本文采用原位-复合的方法制备Al-Ti-C细化剂,对制得的细化剂进行了物相分析和组织形貌观察,分析了该合金的合成反应机理,并对其综合细化能力进行了试验检测,进一步探索了这种细化剂的细化机理和本质.     

Grain growth inhibition by porosity

A model of pore or other second phase inhibited grain growth has been developed based on shape independent stereologically measurable parameters such as pore surface area and the degree of pore/grain boundary contact. The model prediction of a linear relationship between pore surface area and the inverse of the grain size was observed experimentally with sintered copper, tungsten, Al₂O₃ and UO₂. The model also quantifies the conditions for pore controlled grain growth and pore separation in terms of the pore and grain boundary mobilities and grain boundary curvature and provides a means for quantifying pore mobility from grain growth studies on porous and dense materials.