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SXES VERSUS WDS IN STEEL SCIENCE: A THAI-GERMAN COOPERATION

机译:钢铁科学中的SXES VS WDS:泰德合作

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High-strength steels play an important role in automotive industry. Especially, the TRIP (transformation induced plasticity) effect delivers remarkable properties, i.e., strain hardening, which is visible in the high energy absorption at high strain rates during a car crash. The TRIP effect is caused by the transformation of retained austenite into martensite during plastic deformation and strongly controlled by the amount and stability of the retained austenite. Carbon partitioning is the main factor influencing both the amount and stability of the retained austenite. The different diffusion and solubility behaviour of carbon in ferrite and austenite phases results in characteristic distributions after the heat treatments and rolling processes of TRIP steels. Viewed from a different angle, studying carbon concentration and its distribution leads to remarkable information about production processes and mechanical properties. Up to now there are only two devices on the market that have the possibility to measure carbon profiles on the sub-micrometre scale: the Schottky emitter electron probe microanalyser (FE-EPMA) and the soft X-ray emission spectrometer (SXES). FE-EPMA is equipped with conventional wavelength-dispersive X-ray spectrometers (WDS) and SXES is installed on a microprobe or a scanning electron microscope (SEM) equipped with a Schottky emitter. In [1] the excellent analytical properties of measuring carbon concentrations at higher resolution were shown. In this work we are focussing more on the comparison between FE-EPMA and SXES and answering the questions: Is it possible to transfer the method of measuring carbon developed by FE-EPMA to SXES? Is there any complementary information since both techniques have their own performances? As test specimens, hot-rolled TRIP steels were processed under different thermomechanical treatments to produce different distributions of carbon. In a first step, mappings of C-Ka were acquired at 15 kV and 100 nA with a JEOL JXA 8530F located in Aachen, Germany. The resulting carbon distribution is shown in Fig. 1. Carbon containing phases are revealed with an enrichment of carbon visible at the boundaries of larger grains. To exclude artefacts due to higher contamination at edges the results were validated by means of phase field simulations described in more detail in [2]. To obtain more precise information about the carbon distribution line scans were performed under the same excitation conditions described above but now at high resolution with a step size of~62 nm and a dwell time of 5 s per point. Data for the first 2 - 3 μm of the line scan were not taken into account due to a non-stabilised situation of the carbon contamination rate [1]. As a result, a characteristic carbon profile is visible in the martensite/austenite structure (middle of the line scan, see Fig. 2 left). With the results of FE-EPMA in mind, experiments were repeated using the same specimen at the National Metal and Materials Technology Center (MTEC) in Thailand but with analytical conditions optimised for the SXES (installed at JEOL JSM-7800F Prime SEM, varied line spacing (VLS) diffraction grating, CCD camera, energy range: 70-210 eV): 5 kV, 190 nA, 20 s and a step size of 125 nm. A carbon profile and associated microstructure are presented in Fig. 2 right. Although the dose (beam current times acquisition time) was a factor of 8 higher, the precision gets worse in comparison to FE-EPMA. The reason might be the use of the 2nd order of C-Ka, the high background intensity, and efficiency of the SXES in general.
机译:高强度钢在汽车工业中起着重要作用。尤其是,TRIP(转变诱导的可塑性)效果提供了显着的性能,即应变硬化,这在汽车碰撞期间在高应变率下的高能量吸收中可见。 TRIP效应是由塑性变形过程中残余奥氏体转变为马氏体引起的,并受残余奥氏体的量和稳定性的强烈控制。碳分配是影响残余奥氏体数量和稳定性的主要因素。碳在TRIP钢的热处理和轧制过程之后,在铁素体和奥氏体相中碳的不同扩散和溶解行为会导致特征分布。从不同的角度来看,研究碳浓度及其分布会导致有关生产过程和机械性能的大量信息。到目前为止,市场上只有两种设备可以测量亚微米级的碳分布:肖特基发射器电子探针显微分析仪(FE-EPMA)和软X射线发射光谱仪(SXES)。 FE-EPMA配备了常规的波长色散X射线光谱仪(WDS),SXES安装在配备了肖特基发射器的微探针或扫描电子显微镜(SEM)上。在[1]中显示了以较高的分辨率测量碳浓度的出色分析性能。在这项工作中,我们将更多的精力放在FE-EPMA和SXES之间的比较上,并回答以下问题:是否有可能将FE-EPMA开发的碳测量方法转移到SXES?由于两种技术都有自己的表现,是否有补充信息?作为试样,对热轧的TRIP钢进行了不同的热机械处理,以产生不同的碳分布。第一步,使用位于德国亚琛的JEOL JXA 8530F在15 kV和100 nA下获得C-Ka的图谱。产生的碳分布如图1所示。含碳相在较大晶粒的边界处可见可见的碳富集。为了排除由于较高的边缘污染造成的假象,通过[2]中更详细描述的相场模拟对结果进行了验证。为了获得有关碳分布线的更精确的信息,在上述相同的激发条件下进行了扫描,但是现在以〜62 nm的步长和每点5 s的停留时间以高分辨率进行了扫描。由于碳污染率不稳定[1],未考虑在线扫描的前2-3μm的数据。结果,在马氏体/奥氏体组织中可见特征碳分布(线扫描的中间,见图2左)。考虑到FE-EPMA的结果,在泰国国家金属和材料技术中心(MTEC)使用相同的样品重复进行了实验,但分析条件针对SXES进行了优化(安装在JEOL JSM-7800F Prime SEM上,不同型号间距(VLS)衍射光栅,CCD相机,能量范围:70-210 eV):5 kV,190 nA,20 s,步长为125 nm。碳分布和相关的微观结构如图2右所示。尽管剂量(光束电流乘以采集时间)高了8倍,但与FE-EPMA相比,精度却变差了。原因可能是使用C-Ka的二阶,背景强度高以及SXES的效率较高。

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