Research On The Effects Of Crystallographic Orientation Of ? Phase On Variant Selection Of Secondary ? Phase In High Temperature Titanium Alloy

In titanium alloys,there is an association between the microstructure of transformed a phase and the crystallographic orientation of ? grains during solid phase transformation.In this thesis,the ?+? TC25G titanium alloy was chosen to investigate the influence of two kinds of ? welding textures on the microstructure of transformed a phase,characterized by OM,SEM and EBSD with a "orientation-trace" method.Further,the forged TC25G without texture was used to study variant selection on ?/?grain boundaries which could affect the final microstructure during phase transformation.A special orientation relationship of neighboring ? grains was proposed which could predict variant selection.Thus,we could adjust the microstructure of ?phase to improve mechanical properties by controlling the crystallographic orientation of ? grains.The influence of ? textures on the microstructure of a phase in welding seams of two kinds of welding joints was investigated.In the electron beam welded TC25G joint,the traces of two kinds of ? laths were nearly perpendicular to each other which appearing in cross configuration in columnar ? grains located in fusion zone.It showed that there was a ? cube texture in this area,and strong variant selection occurred.Meanwhile,in the inertia friction welded TC25G/Ti65 dissimilar joint,there were triangular configuration consisted of three kinds of ? laths in equiaxed grains located in dynamic recrystallization zone.It showed there was a strong<111>texture and variant selection also occurred in this area.We calculated the frequency distribution of angle between the trace direction of a laths and X-axis from ? phase with<100>or<111>fiber textures through "orientation-trace" method.The results showed that the angle of<100>fiber frequency curve concentrated on range of 40°?60° and 120°?140°,while in case of<111>fiber the angle mainly distributed in 60°?75°,-60°?75° and-10°?10°.It meant that the microstructure of transformed a phase from ? phase with textures would naturally appear in special morphology.A statistical research of variant selection in grain boundaries of forged TC25G titanium alloy without texture was studied.It found that there was a crystallographic orientation relationship between the selected grain boundary a phase and neighboring? grains.Based on Burgers orientation relationship,we established a crystallographic orientation relationship of neighboring ? grains to describe the relative amount of free energy change during phase transformation.It was named misorientation of phase transformation which was defined by the minimum angle around a specific crystallographic orientation axis of grain rotates which could produce subphase with the same orientation as the adjacent grain,or the minimum misorientation of two adjacent grains which could produce subphase with the same orientation.The angle of misorientation of phase transformation showed,to some extent,the amount of ?/? phase boundaries' energy.If the angle is relatively small,the atoms between ?/? phase boundaries will be high matched,leading to low phase boundaries' energy and high driving force of phase transformation.Things are opposite in case of a high angle of misorientation of phase transformation.Thus,we could use this variable to control,design,improve the desired ?/? grain boundaries for arranging the variant selection.