SiC纤维增强Ti17合金复合材料轴向残余应力的拉曼光谱和X射线衍射法对比研究

准确测量和分析SiC纤维增强Ti合金复合材料(SiC_f/Ti)中残余应力状态对优化复合材料的成型工艺和理解其失效模式具有重要意义,但其残余应力的实验测量和分析仍是一个挑战.石墨C涂层作为SiC纤维与Ti17基体合金之间必需的扩散障涂层,承载了由纤维与基体之间热不匹配引入的残余应力.本文采用显微拉曼光谱法对比测量纤维表面C涂层在复合材料中和去掉基体无应力态下G峰的峰位,通过石墨C涂层应力态下峰位移动计算出SiCf/C/Ti17复合材料中SiC纤维受到～705.0 MPa的残余压应力.采用X射线衍射方法测量了不同方向上该复合材料中基体钛合金的晶面间距以获取其空间应变,根据三轴应力模型分析了复合材料中基体钛合金沿轴向方向的残余应力为～701.3 MPa的张应力,并通过线性弹性理论转化为SiC纤维的残余压应力为～759.4 MPa.两种测试方法都确定了SiC纤维在成型过程中受到残余压应力,且获得的应力值较为接近,都可以用于对SiC_f/Ti复合材料的残余应力测量.

Comparison between axial residual stresses measured by Raman spectroscopy and X-ray diffraction in SiC fiber reinforced titanium matrix composite

Accurate measurement and analysis of residual stress state in the SiC_f/Ti composites are crucial to optimizing their fabrication process and to understanding their failure mode, but they are still a challenge. In this work, SiC_f/C/Ti17 composites with~48% fiber volume fraction, consisting of W-core SiC fibers (~100 μm in diameter), turbostratic C coating (~2.5 μm in thickness) and Ti17 matrix, are prepared by consolidating precursor wires fabricated by matrix-coated fiber method through hot isostatic pressing at 920℃/120 MPa/2 h; these samples are used for measuring their stresses. It is noted that turbostratic C coating, a necessary diffusion barrier layer between SiC fiber and Ti17 alloy matrix, bears the residual stress caused by the mismatch of thermal expansion coefficients between fiber and matrix during consolidation. It is found that the graphene planes are almost parallel to the axial direction of SiC fibers in the turbostratic C coating revealed by high magnification transmission electron microscope, and thus G peak position of C coating would be sensitive to stress state. Accordingly, micro-Raman spectroscopy is first used to measure the G peak positions of C coating under stress and stress-free state in the SiC_f/C/Ti17 composite, respectively. Based on the position shift of G band caused by residual stress, the axial residual compressive stress of SiC fiber in SiC_f/C/Ti17 composite is calculated to be~705.0 MPa. For comparison, X-ray diffraction method is also adopted to measure the interplanar spacing values of the Ti17 alloy matrix in different directions to obtain the spatial strains. During measurement, α -Ti (213) high-angle diffraction peak is chosen to reduce test error, and then the different interplanar spacing values of α -Ti (213) are obtained by varying the values of ψ in three different directions at φ=0°, 45° and 90°. As three-axis-stress model is employed, the residual tensile stress of Ti17 alloy matrix in the axial direction of SiC_f/C/Ti17 composite is~701.3 MPa, which is transformed through linear elastic theory into the residual compressive stress of SiC fiber of~759.4 MPa. The similar results confirm that it is reliable to characterize the residual stress in the SiC_f/C/Ti17 composite with high-texture turbostratic carbon by both the Raman spectroscopy and the X-ray diffraction method.