Effect Of β Heat Treatment On The Growth Of Grain And The Formation Of Lamellar Structure For Several Kinds Of Titanium Alloys

It is very necessary for aviation aircraft materials to realize a transformation from traditional strength design to damage tolerance design criteria to meet the requirements of high stability and extending service life. At present time, the laminarization is one of the effective routes to obtain titanium alloys with high damage tolerance. In this paper,the growth of β grains and the microstructure evolution of lamellars are investigated based on kinds of typical titanium alloys, such as TA15、TB6 and TC11. The results are expected to provide some technical and theorial support for the formation of lamellars and preparation of titanium alloys with high damage tolerance. The main findings are summarized as follows:The results showed that the microstructure of TB6 mainly consisted of β and that of TA15 and TC11 mainly consisted of martensite when these three titanium alloys were quenched after solution treated. Initially, the β grains in three alloys grew very fast with increase of remperature. However, the growth rate slowed down when the annealing temperature was over 1090℃ for TA15 alloy, 890℃ for TB6 alloy and1090℃ for TC11 alloy. It was also found the growth of these titanium alloys follow n<l model functions. The growth kinetics model of TA15 alloys was D-D0=4.357t0.602, that of TB6 alloy was D-D0=26.34t0.277 and that of TC11 alloy was D-D0=18.168t0.4105.The effects of β annealing treatment, β double annealing treatment and the β triple annealing treatment on the microstructures and the formation mechanism of lamellars for these three titanium alloys were observed. The results showed that lamellars can be obtained by annealing treatments and the nucleations mainly began between the boundaries and a few started inner crystals. These nucleations grew along the habit plane until touched. During β annealing treatment, the lamellar sapces increased linearly and the growth rate of cluster size was increasing and then slowing with the slowing of cooling rate. During β double annealing treatment, the thickness of lamellar increased with the increasing of annealing temperature and annealing time. Some bi-lamellar microstructure which was consisting of precipitated small secondary α needles were observed among α-lamellars. After the β triple annealing treatment, the growth rate of cluster size was increasing with the increasing of annealing temperature and annealing time and the lamellar space kept stable. For the β triple annealing treatment, less time was needed to obtain the similar structure with that of the β annealing treatment sample.The effects of lamellars structure on the strength and plasticity were also analysed.It was found that the tensile strength and plasticity enhanced as the lamellar space reducing. The strength and plasticity presented a linear relationship with grain diameter,cluster size and lamellar thinkness of β structure.The fracture mechanisms of the samples were cleavage and dimple fracture mixed fracture mechanism. But the fracture platform reduced, the shear lip zone became larger and the dimples became deeper as the decreasing of lamellar thickness.