Cost Resistant Titanium Alloy Microstructure And Mechanical Properties

15 Kg ingots of Ti-25V-15Cr-2Al-0.2C-x alloy(x=0,2wt%Mo,0.2wt%Si plus 2wt%Mo) which is a stabilizedβnon-burning titanium alloy with lower cost were produced by vacuum arc remelting(VAR) in this study.These ingots were forged and hot-rolled into bars with 18.5mm diameter.Various mechanical properties were tested for the bars in different heat treatment conditions.The phase constituent and the effect of heat treatments and alloy elements on microstructures and mechanical properties of Ti-25V-15Cr-2Al-0.2C-x alloy have been studied using SEM,TEM,XRD and EDX.The precipitating behavior of different phases,fracture modes,deformation and strengthening mechanisms were discussed in this paper.The results show that besides theβmatrix,Ti-25V-15Cr-2Al-0.2C-x alloy contains other precipitation phases such asα,(Ti,V)C,TiCr₂ and TiCr₂ was first found in the Ti-V-Cr-Al system alloy.Al addition reduces the production cost of the alloy,but it increases the tendency to formαprecipitates and leads to ordering of theβphase into B₂ and the precipitating of orderedα₂.It was found that the occurrence of precipitation on grain boundaries leads to the apparent decrease in the ductility of Ti-25V-15Cr-2Al-0.2C-x alloy.Oxygen is considered to be the main factor which promotes the formation ofαprecipitates.(Ti,V)C can act as a stable sink for dissolved oxygen in theβmatrix,thus suppressingαprecipitation to a certain extent.It is interesting to note that carbides with smaller size and finer distribution can more effectively reduce the level of grain boundariesαprecipitation,thereby significantly improving the ductility of the alloy after exposure at high temperatures.αand(Ti,V)C are two main dispersion-strengthening phases in Ti-25V-15Cr-2Al-0.2C-x alloy.It was found that the number and distribution of(Ti,V)C particles are almost not affected by heat treatment conditions,when compared with those ofαprecipitates. Therefore,the difference in mechanical properties of the alloy in different heat treatment conditions is closely related to the amount ofαphase.The experimental alloy were subjected to different heat treatments.The results showed that the occurrence ofαprecipitation can not be prohibited by air cooling after solution treatment(Process 1^#).The single-stage aging treatment following solution treatment(Process2^#) resulted in a large number ofαprecipitates within grains and on the grain boundaries.Moreover,a unique multiplex heat treatment process(Process 3^#) has been developed in this study,which can significantly suppress the formation ofαprecipitation,thus improving the microstructural stability of the alloy.The samples heat treated by Process2^# have the highest strength and creep-resistance,but the ductility is poor,especially the ductility after exposure.In contrast, the excellent ductility was obtained in the samples heat treated by Process3^#(although the strength of the samples in heat treatment condition3 is not as high as that of the samples in heat treatment condition2),due to the decrease in the amount ofαprecipitation.The properties of the samples heat treated by Process1^# are between above two heat-treated samples.Mo addition has a suppressive effect on the formation ofαprecipitation.Si addition can strengthen the alloy by both solution and dispersion strengthening,especially markedly improving the creep properties.The dislocation slip is the main mode of room plastic deformation of the Ti-25V-15Cr-2Al-0.2C-x alloy.The ordering degree of theβmatrix andαphase increases after longterm exposure,thus leading to planar slip in the deformation structure of the alloy. Dislocation cross-slip and climb are the typical modes of high temperature deformation of the alloy because of atomic thermal activation.Dislocation loop groups are the important dislocation morphologies of creep deformation structure of the alloy,which are effective obstacles to dislocation motion.It was found that the more the number of dislocation loop groups,the higher creep-resistance of the alloy.