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

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 2θ 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.