Quantitative phase analysis of Ti-3Al-5Mo-4.5 V dual phase titanium alloy by XRD whole pattern fitting method

Abstract

The reliable and convenient quantitative phase analysis (QPA) of titanium alloys is a meaningful work. In this study, the relative phase fraction of the bulk aviation serviced Ti-3Al-5Mo-4.5 V alloy was investigated by various methods, including Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Electron Back-Scattered Diffraction (EBSD) and X-ray diffraction (XRD). OM, SEM and TEM can scarcely do phase quantification. From the convincing results of EBSD test, the specimen was primarily constituted by fine, uniformly distributed and equiaxed α grains embedded in β matrix and α/β phase respectively accounted for 74.14% and 25.86%. The obstacles of Peaks overlapping and PO (Preferred Orientation) to XRD methods were greatly dealt with employing XRD whole pattern fitting method according to the 2 θ diffraction pattern by careful scanning. Through initial fitting without PO correction, the calculated α/β phase fractions were 62.25% and 37.75%, respectively. The main PO could be inferred along the directions of (002) α , (101) α and (110) β . When modifying PO of these peaks stepwise utilizing March-Dollase function, the difference of the calculated profile to the observed one was gradually reduced to a low level and the calculated α/β phase components were respectively 73.04% and 26.96%. With the optimization of adding (200) β for PO correction or utilizing spherical harmonics function, the fitting quality was further improved and the quantitative results tended to stability. It was found that PO of α phase was suitable to be modified by eighth order spherical harmonics function because it has multiple diffraction peaks with complicated PO, while β phase adopting March-Dollase function. The final α/β phase contents determined were 73.65% and 26.35% respectively, which was greatly consistent with EBSD results. • The discussion of PO correction in the practical Rietveld refinement process. • A combination of March-Dollase and spherical harmonics function for PO correction. • The relative phase composition of Ti-3Al-5Mo-4.5 V alloy was determined.