Five-parameter grain boundary distribution of commercially grain boundary engineered nickel and copper

The five-parameter grain boundary distributions of grain boundary engineered nickel and copper specimens have been analyzed in detail. The relative areas of {1 1 1} planes in the entire population did not increase as a result of grain boundary engineering (GBE) and, in the Σ3-excluded population, decreased after GBE. This decrease occurred because the majority of the newly generated Σ3 grain boundaries were not coherent twins with {1 1 1} grain boundary plane orientations. GBE increased the proportion of Σ3 boundary length that was vicinal-to-{1 1 1} and the proportion of asymmetrical 〈1 1 0〉 tilt boundaries. There was a clear propensity for selection of particular planes or plane combinations which were associated with low energy. These plane types were analyzed in some detail, and it was shown that many of these boundaries were asymmetrical tilts comprising (or vicinal to) at least one low-index plane.     

Comparing calculated and measured grain boundary energies in nickel

Recent experimental and computational studies have produced two large grain boundary energy data sets for Ni. Using these results, we perform the first large-scale comparison between measured and computed grain boundary energies. While the overall correlation between experimental and computed energies is minimal, there is excellent agreement for the data in which we have the most confidence, particularly the experimentally prevalent Σ3 and Σ9 boundary types. Other CSL boundaries are infrequently observed in the experimental system and show little correlation with computed boundary energies. Because they do not depend on observation frequency, computed grain boundary energies are more reliable than the experimental energies for low population boundary types. Conversely, experiments can characterize high population boundaries that are not included in the computational study. Together the experimental and computational data provide a comprehensive catalog of grain boundary energies in Ni that can be used with confidence by microstructural scientists.     

Relative grain boundary area and energy distributions in nickel

The three-dimensional interfacial network of grain boundaries in polycrystalline nickel has been characterized using a combination of electron backscatter diffraction mapping and focused ion beam serial sectioning. These data have been used to determine the relative areas of different grain boundary types, categorized on the basis of lattice misorientation and grain boundary plane orientation. Using the geometries of the interfaces at triple lines, relative grain boundary energies have also been determined as a function of lattice misorientation and grain boundary plane orientation. Grain boundaries comprising (1 1 1) planes have, on average, lower energies than other boundaries. Asymmetric tilt grain boundaries with the Σ9 misorientation also have relatively low energies. The grain boundary energies and areas are inversely correlated.     

Effect of grain boundary fraction on castability of a directionally solidified nickel alloy

The effect of grain boundary (GB) fraction on hot tearing during directional solidification was explored. The increase of GB fraction was found to reduce the hot tearing tendency. The eutectic melt is finely dispersed and increasingly discontinuous at the GBs when the GB fraction is increased. The change of eutectic melt at the GBs is related to the reduced concentration of GB elements or impurities. The better castability is attributed to the uniform distribution of strain due to the presence of more GBs and the stronger GB cohesion because of the larger bridging areas.     

Reversed anisotropy of grain boundary properties and its effect on grain boundary engineering

Data on anisotropy of grain boundary properties are frequently published. In some cases, when they show the reverse course of structural dependence than is expected, they can seem confusing. Examples of this “reversed anisotropy” found for grain boundary segregation, diffusion and migration are presented. We demonstrate that the reversed anisotropy of grain boundary properties is caused by the compensation effect. This can have serious consequences for grain boundary engineering. It is also suggested that grain boundaries can be classified specifically and generally solely on the basis of well-defined thermodynamic parameters.     

基于CSP工艺一段式脱碳退火对取向硅钢初次再结晶晶界特征的影响

利用背散射衍射技术(EBSD),在一段式840 ℃不同时间脱碳退火条件下,研究了基于CSP工艺取向硅钢初次再结晶过程中的组织和结构变化。结果表明,在初次再结晶退火时间为4 min时织构类型较多,分别为{332}<`533>、{554}<225>、{111}<110> 、{001}<100>、 {111}<112>、{001}<110>、{110}<001>、{110}<110> 、 {112}<110>、{110}<112>、{112}<1`10>、{012}<001>和{111}<231>等。当初次再结晶退火时间延长为5 min时, {111}<112>取向晶粒数量明显增多,而{332}<`533>和{012}<001>取向晶粒比例下降。同时Σ3、Σ5和Σ9晶界比例升高,小角度晶界比例较少,而大角度晶界比例较多,这将有助于在二次再结晶退火时发生高斯织构。继续延长退火时间到6 min以后,Σ3、Σ5和Σ9晶界比例下降,小角度晶界比例提高,此时再结晶晶粒长大。     

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4 wt. ppm S) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550 °C/0, 550 °C/3 × 10^?5 s^?1, 550 °C/3 × 10^?4 s^?1 and 450 °C/3 × 10^?5 s^?1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ～10³ to a few 10⁵, depending on the deformation conditions. Very high levels of segregation (～0.8 monolayer of sulphur) are obtained after very low deformation (～5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.     

晶界特征分布对哈氏合金力学性能的影响

采用扫描电镜、背散射电子衍射、透射电镜等手段对Hastelloy X(HX)合金的晶界特征分布、拉伸断口形貌及位错分布等进行研究。结果表明,HX合金室温拉伸断口由局部裂纹、微孔和大量的韧窝构成,微孔的形成与材料内部第二相颗粒分布有关。组织为晶内共格孪晶型的材料开裂是裂纹源扩展引起的,而组织为非共格孪晶型的样品是微孔聚集导致的开裂。分析表明,不同晶界特征分布的样品室温力学性能的差异,主要是晶粒尺寸和一次碳化物分布作用的结果。     

Increase in grain size during recrystallization of heat resistant nickel alloys

Conclusions We have shown that the critical increase in grain size during recrystallization occurs only when recrystallization proceeds during plastic deformation or during the cooling of the alloy after plastic deformation or under certain conditions (heating rate) during repeated heating. If recrystallization does not occur during plastic deformation (cold deformation) and the rate of repeated heating is such that polygonization processes have time to occur then there is no critical increase in the grain size.Central Scientific Institute of Ferrous Metallurgy Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 22–24, January, 1966     

Structure and properties of high-temperature nickel alloys with hafnium

The present work is devoted to the effect of modification of high-temperature alloys ZhS6U-VI and ZhS3DK-VI with hafnium on their properties and structure. Alloys ZhS6U-VI and ZhS3DK-VI were molten in a vacuum induction furnace and modified with hafnium in an amount of 0–1.5% (as calculated) using a nickel-hafnium alloying composition. It is shown that hafnium alloying diminishes the sizes and amount of the brittle component in a fracture and the distance between the second-order dendrite axes and changes the morphology of the carbides. With growth of the hafnium content the mechanical properties of both alloys and the high-temperature strength of alloy ZhS3DK-VI at room temperature increase. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 29–32, January, 1999.