Characterisation of high temperature oxidation using electron backscatter diffraction

Abstract

Electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) has been used to characterise the oxide scales formed on a low alloy steel. The technique provides a powerful combination of local phase information and orientational relationships both within and between phase layers. It has revealed that hematite grain growth occurs almost exclusively along the 0001 direction for the entire range of samples examined. Wüstite and magnetite grains were also found to grow preferentially along orientations close to the 001 direction.

EBSD is also well suited to characterising more complex scales such as those formed during hot working (e.g. millscale), and those formed on Fe–Ni alloys. In the latter complementary chemical information from energy dispersive spectroscopy (EDS), which was acquired simultaneously with the EBSD, enables the identity of crystallographically similar phases to be distinguished. EDS also shows that no nickel exists in the external scale and that it instead accumulates at the interface with the scale and at adjacent grain boundaries.     

Dynamic recrystallization of Ni-base alloys—Experimental results and comparisons with simulations

The dynamic recrystallization of the Ni-base alloy Böhler L306 VMR (Alloy 80A) in a transient state was investigated both by light microscopy and electron backscatter diffraction (EBSD), and the experimental results were compared with those from simulations. Subgrain structures of the size of the recrystallized grains were observed close to the grain boundaries of the original grains. With increasing strain a texture developed in the deformed fraction. Strong twinning was found in the recrystallized fraction, with area fractions of the twinned grains of around 80% for higher strains. Thus the measured grain sizes strongly depend on the handling of the twins. A pronounced increase in the average grain size of the recrystallized fraction with increasing strain (time) was only observed after twin removal. There was generally good agreement between the measured and the simulated results.     

Effect of Si Content on Dynamic Recrystallization of Al-Si-Mg Alloys During Hot Extrusion

By using electron backscatter diffraction（EBSD） and transmission electron microscopy（TEM）, the effect of Si content on microstructure characteristics of three as-extruded Ale SieM g alloys was investigated. Results showed that the density of coarse Si particles played a critical role in dynamic recrystallization. Dynamic recrystallization rarely occurred in S1 alloy with less Si content; however, it happened in the Si-rich zones in S2 alloy with a medium Si content. And a mature recrystallization was observed in S3 alloy with high Si content. Although deformation was carried out at high temperature, particle-stimulated dynamic recrystallization occurred in Si-rich zones.     

铝合金的织构及测试分析研究进展

织构广泛存在于铝合金制品中,并影响着材料的一系列性能,包括强度、延展性、成形性和腐蚀性等。介绍了目前织构的一般测试分析方法,主要阐述了铝合金制造加工过程中产生的各种类型织构,如铸造织构、变形织构和再结晶织构等以及它们的演变规律,分析了铝合金加工工艺对材料最终的织构成分及织构强弱的影响,并揭示了厚板铝合金轧制和铝合金的搅拌摩擦焊接过程中产生的不均匀分布织构,以及其对材料性能的影响。最后介绍了铝合金织构的应用情况,并对铝合金织构的研究前景进行了展望,提出了在线织构检测技术,将是未来铝合金织构测试分析的主要发展方向之一。     

Ti150合金离心叶轮锻件工艺优化

目的 解决Ti150合金离心叶轮锻件低倍组织不均匀和锻件力学性能离散性较大的问题。方法 通过对锻件进行金相组织分析,再利用EBSD、DEFORM等工具分析锻件成形时应变分布及组织特点,探索低倍组织不均匀和锻件力学性能离散性较大的原因及解决方法。通过对棒材进行六方拔长、倒棱、平头和滚圆等处理,实现从圆棒到六方形再到圆棒的变形过程（改锻）,使棒材各部位的变形均匀,达到消除微织构的目的。结果 对棒材进行反复镦拔（改锻）,可有效消除棒材中的微织构并改善低倍组织的不均匀,是控制锻件力学性能离散性的重要措施。结论 中间坯组织的均匀性是影响低倍组织不均匀和锻件力学性能离散性的主要因素,通过对棒材进行改锻,Ti150合金离心叶轮锻件的组织及力学性能优异,满足相应的技术要求。     

铝合金熔焊区结构稳定性的测试技术

在航空航天领域中,轻合金激光熔焊区的结构稳定性具有重要意义.因此,本文从晶界面结构视角具体审视这一问题.本研究以一种5系铝合金的激光熔焊区为研究对象,分别利用扫描电镜、电子背散射衍射(EBSD)和统计性的晶界面结构分析方法,首先实现了热影响区、等轴区和熔合区在空间上的准确划分;其次对各子区分别计算了穿过晶界面的取向差角...     

Fracture mechanism of Mg–3Al–1Zn sheet at the biaxial state with respect to forming temperatures

Magnesium (Mg) sheet has been of great interest in automobile industries to make a light-weight design although it has low formability at the room temperature compared with steel sheets. It is required to elevate forming temperature to enhance the formability of Mg sheet, which enables increase of active slip systems in Hexagonal close packed (HCP) crystal structure. This paper demonstrates the effect of forming temperature on the formability of Mg–3Al–1Zn sheet, which is evaluated by the Limit Dome Height (LDH) test at temperature of 423 K, 523 K, and room temperature. The variation of dome heights depending on the forming temperature has been investigated to stand for its formability, and punch stroke and loads have been compared with each other. It has been tried to correlate the fracture mechanism with formability of AZ31 sheet with respect to the forming temperature by investigating the fracture surfaces with optical microscopy (OM) and orientation imaging microscopy (OIM) analyses.     

基于EBSD技术的P91钢蠕变过程中小角度晶界演化行为表征

在620℃、145 MPa条件下对给定的P91钢进行高温蠕变持久与间断试验,采用电子背散射衍射(EBSD)技术研究其在蠕变过程中小角度晶界的演化行为。通过引入EBSD图像中的取向差分布来表征小角度晶界处(0.5~5°)的边界位错密度,分析了边界位错密度在蠕变过程中与小角度边界的数量、塑性应变以及内部微观组织演化之间的关系。此外,通过改变EBSD像素点与像素点之间的计算步长,探讨了步长选择对边界位错密度计算结果的影响。结果表明,小角度晶界处的位错密度在蠕变过程中先迅速上升,在最小蠕变率处达到极值后缓慢下降,直到最后基本保持不变;同时,EBSD的计算步长越小,得到的位错密度值越准确。     

Optical properties of self assembled oriented island evolution of ultra-thin gold layers

Gold layers with a thickness of only 8 to 21 nm were sputtered on soda-lime-silica glasses. Subsequent annealing at 300 and 400 °C for 1 and 24 h resulted in the formation of separated round gold particles with diameters from 8 to 200 nm. Crystal orientations were described using X-ray diffraction and electron backscatter diffraction. The gold particles are oriented with their (111) planes perpendicular to the surface. Most gold nano particles are single crystalline, some particles are twinned. Thermal annealing of sputtered gold layers resulted in purple samples with a coloration comparable to that of gold ruby glasses. The color can be controlled by the thickness of the sputtered gold layer and the annealing conditions. The simple method of gold film preparation and the annealing temperature dependent properties of the layers make them appropriate for practical applications.     

基于EBSD方法测定单晶Cu纳米压痕的各向异性实验

为研究单晶Cu材料的各向异性力学特性,针对单晶连铸技术制备的单晶Cu,采用电子背散射衍射(EBSD)法对其3个不同晶粒的晶面进行定向,利用原位纳米压痕仪在不同晶面进行不同压入载荷的纳米压痕实验.通过EBSD分析,发现用单晶连铸技术制备的单晶Cu在拉拔方向上具有较强的择优取向,单个晶粒较大,且晶粒内部没有(亚)晶界存在.纳米压痕实验结果表明单晶Cu样件在各种压痕载荷下的约化模量为50 GPa～120GPa,材料的晶体取向对纳米压痕载荷-位移曲线和约化模量有很大影响,面(032)比面(119)和面(041)有更大的约化模量.不同载荷下,硬度值在0.8 GPa左右变动,晶体取向对硬度的影响较小.实验所得单晶Cu各晶面约化模量与采用金属弹性力学理论计算所得数值吻合较好.     

Effect of carbon content on solidification behaviors and morphological characteristics of the constituent phases in Cr-Fe-C alloys

A combination of transmission electron microscopy, electron backscatter diffraction and wavelength dispersive spectrum has been used to identify crystal structure, grain boundary characteristic and chemical composition of the constituent phases in Cr-Fe-C alloys with three different carbon concentrations. Depending on the three different carbon concentrations, the solidification structures are found to consist of primary α-phase and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-18.4Fe-2.3 C alloy; primary (Cr,Fe)₂₃ C₆ and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-24.5Fe-3.8 C alloy and primary (Cr,Fe)₇ C₃ and [α + (Cr,Fe)₇ C₃] eutectic in Cr-21.1Fe-5.9 C alloy, respectively. The grain boundary analysis is useful to understand growth mechanism of the primary phase. The morphologies of primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides are faceted structures with polygonal shapes, different from primary α-phase with dendritic shape. The primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides with strong texture exist a single crystal structure and contain a slight low angle boundary, resulting in the polygonal growth mechanism. Nevertheless, the primary α-phase with relative random orientation exhibits a polycrystalline structure and comprises a massive high-angle boundary, caused by the dendritic growth mechanism.