Diffracting-grain identification from electron backscatter diffraction maps during residual stress measurements: a comparison between the sin2ψ and cosα methods

X-ray diffraction (XRD) is a widely used technique to evaluate residual stresses in crystalline materials. Several XRD measurement methods are available. (i) The sin(2)psi method, a multiple-exposure technique, uses linear detectors to capture intercepts of the Debye-Scherrer rings, losing the major portion of the diffracting signal. (ii) The cos alpha method, thanks to the development of compact 2D detectors allowing the entire Debye-Scherrer ring to be captured in a single exposure, is an alternative method for residual stress measurement. The present article compares the two calculation methods in a new manner, by looking at the possible measurement errors related to each method. To this end, sets of grains in diffraction condition were first identified from electron backscatter diffraction (EBSD) mapping of Inconel 718 samples for each XRD calculation method and its associated detector, as each method provides different sets owing to the detector geometry or to the method specificities (such as tilt-angle number or Debye-Scherrer ring division). The X-ray elastic constant (XEC) 1/2S(2), calculated from EBSD maps for the {311} lattice planes, was determined and compared for the different sets of diffracting grains. It was observed that the 2D detector captures 1.5 times more grains in a single exposure (one tilt angle) than the linear detectors for nine tilt angles. Different XEC mean values were found for the sets of grains from the two XRD techniques/detectors. Grain-size effects were simulated, as well as detector oscillations to overcome them. A bimodal grain-size distribution effect and `artificial' textures introduced by XRD measurement techniques are also discussed.

X射线衍射（XRD）是一种广泛用于评估晶体材料中残余应力的技术。有几种XRD测量方法可供使用。（i）sin(2)ψ方法是一种多次曝光技术，它使用线性探测器来捕捉德拜 - 谢乐环的截距，会丢失大部分衍射信号。（ii）cosα方法是残余应力测量的一种替代方法，这得益于紧凑型二维探测器的发展，它能够在一次曝光中捕捉整个德拜 - 谢乐环。本文以一种新的方式对这两种计算方法进行了比较，即考察与每种方法相关的可能测量误差。为此，首先从因科镍合金718样品的电子背散射衍射（EBSD）图谱中，针对每种XRD计算方法及其相关探测器确定处于衍射状态的晶粒组，因为由于探测器几何形状或方法特性（例如倾斜角度数量或德拜 - 谢乐环划分），每种方法提供不同的晶粒组。根据EBSD图谱针对{311}晶面计算出的X射线弹性常数（XEC）1/2S(2)被确定，并针对不同的衍射晶粒组进行了比较。据观察，对于九个倾斜角度，二维探测器在一次曝光（一个倾斜角度）中捕捉到的晶粒比线性探测器多1.5倍。对于来自两种XRD技术/探测器的晶粒组，发现了不同的XEC平均值。模拟了晶粒尺寸效应以及探测器振荡以克服这些效应。还讨论了由XRD测量技术引入的双峰晶粒尺寸分布效应和“人为”织构。