Study Of Superplastical Behavior Of In-Situ Particle Reinforced 7715D Matrix Tatunium Composites

Titanium alloys, with high specific strength, good specific modulus and resistance to elevated temperatures, become to one of the most popular materials in aerospace and chemical industrial, medical and domestic applications. Particle reinforced titanium matrix alloy, prepared by in situ technique, have considerable potential for improvement in properties and service temperature, while conventional high temperature titanium material can't meet some application requirements in aerospace industry because of its limited use temperature as 500oC.But its application is also limited for its poor machining performance. In order to solve these problems, superplastic deformation (one way of near-net shape)of materials can be used, which will save raw material and reduce the cost.In this paper, in order to improve the operating temperature of 7715D, an alloy applicated in aerospace industry, TiB, TiC, TiB and TiC particales have been prepared by general casting technology utilizing the self-propagation high-temperature synthesis reactions between titanium and B4C, graphite by in situ technique. XRD results show that those particale reinforced titanium matrix composites were produced successfully with different types and volumes. Materials with basket-weave microstructures were got by heat-treatment and then mechanical properties were tested, and results show that:with 2vol.% particales, the best overall mechanical properties belong to TiC/7715D and the second to TiB/7715; with particles included increasing, the mechanical properties of TiC/7715D improve little while TiB reinforced improve significantly; with 5vol. % particles, the high temperature mechanical properties of (TiC+TiB)/7715D at 650oC are similar to those of 7715D alloys at 600oC.The superplasticity of those composites and the matrix alloy has been studied respectively. With initial equiaxed microstructure, a maximum elongation of 1144% was obtained with different types of true strain-true stress curves in 7715D alloy and a maximum elongation of 802% was observed in 2vol.%TiC/7715D concurrent with a turning point in its elongation-temperture curve and same different true strain-true stress curves,while the second work hardening behavior was observed when the temperature is above 950oC with initial strain rate not above 10-3s-1;with basket-weave initial microstructure, a maximum elongation of 730% and 625% were obtained with similar true strain-true stress curves in 7715D alloy and 2vol. %(TiC+TiB)/7715D,respectively. But the second work hardening behavior was observed while the temperature is above 1025oC with initial strain rate slower than 10-3s-1.Superplastic average activation energies involved were calculated and microstructure involved were observed.Analyse shows that, good superplasticity depends on microstructures during deformation process other than initial shapes.The superplastical deformation mechanism of those composites is mainly Dynamic Recovery / Dynamically Recrystallization concurrent with Grain Boundary Sliding /Grain Coarsening controlled by different diffusion mechanisim,which is effected by temperature and initial strain rate.Precipitates involved effect the ratio of Dynamically Recrystallization by pinning dislocation,inhibiting grain growth,changing the microstructure shapes by making the local phase-transition point changed,thus result in the second work harding presented in changing conditions.