Effects Of Alloying Elements On Microstructures And Properties Of Cast Ti-1100 Alloys

600℃ high temperature titanium alloys are extensively used in aerospace due to the combination of superior properties, such as lightweight, heat-resistance and high strength. To date, researches on this alloy mainly focus on wrought process. Unfortunately, little attention has been paid to the casting method. In this paper, the effects of solidification condition and alloying elements on microstructures and properties of cast high temperature titanium alloys have been investigated. The interfacial reactions between the CaO crucible and Ti-1100 alloy were investigated firstly. The results demonstrate that the interactions between crucible and molten alloys are attributed to slight chemical dissolution and weak physical erosion. The formation of a-case was caused by the diffusion of interstitial oxygen atoms at the interface. The standard Gibbs energy of reaction for Ti-1100 alloy and CaO crucible was also calculated. The equilibrium constant and the interaction parameter between calcium and oxygen were obtained as logK=-3.14 and eOCa=-3.54.The solidification microstructures of different casting modulus Ti-1100 alloys are all belong to widmanstatten structure. The prior β grain size and a-laths spacing become small with the decrease of the casting modulus. The prior β grain size and a-laths spacing for 2.5mm alloys are approximately 249.4μm and 5.3μm, which decreased 43.7% and 22.1%, respectively, compared to 5.6mm alloy. The microhardness increases with the decrease of the casting modulus. The β transus temperature of the alloys decrease with the increase of the casting modulus and increase with the increase of the heating rates. After heat treatments, the microstructures of the alloys are still widmanstatten structure. The prior β grain size and a-laths spacing decrease with the decrease of the casting modulus. The microhardness values are improved after heat treatments, and the microhardness values increase with the decrease of the casting modulus in the same heat treatment condition.The effects of Zr addition on the microstructure and mechanical properties in as-cast Ti-1100 alloys were systematically investigated. The microstructures of the alloys are refined with the addition of the Zr content. The TiSi and S2 type silicides are found to precipitate heterogeneously in the β phase for the 6wt.%Zr composition alloy. The TiSi phase has an orthorhombic structure, and the orientation relationships between the TiSi phase and the retained β phase layers are identified as (102)TiSi//(110)β, [2 41]TiSi//[001]β and (100)TiSi//(101)β,[011]TiSi//[111]β.The S2 type silicide has a hcp structure, and the long axis direction of S2 type silicides has geometric connection with the direction of plates of a phase due to the orientation relationships between the S2 phase and the a phase:(0110)S2//(2110)α, [2113]S2//[0001]α and (1010)S2//(1210)α, [1213]S2//[1011]α. The microhardness increases with the increase of the Zr content. The β transus temperature decrease with the increase of the Zr content. After heat treatments, the microstructures of all the alloys are still belong to widmanstatten structure. The prior β grain size and a-laths spacing decrease with the increase of the Zr content and the microhardness values are also improved compared to the as-cast condition. The microhardness values increase with the increase of the Zr content in the same heat treatment condition.The solidification microstructures of Ti-1100 alloys can be refined significantly by the addition of minor B. The TiB phase is readily to grow along the [010] direction with the morphology of needle-like shape to minimize the lattice strain at the interfaces. The interfaces between the TiB and Ti matrix are clean without precipitation phase. The stacking faults are observed in the (100) plane of the TiB phase, which are ascribed to the locations of boron atoms in the crystal structure of TiB and the lattice mismatch energy between TiB and Ti matrix. The microhardness and the β transus temperature of the Ti-1100 alloys increase with the increase of the B content. After heat treatments, the a-laths spacing of the alloys decrease with the increase of the B content and the microhardness values are also improved compared to the as-cast condition. The microhardness values increase with the increase of the B content in the same heat treatment condition.The effects of Nb addition on the microstructures and mechanical properties of the Ti-1100 alloy castings were systematically examined. The results demonstrate that the microstructures of the alloys are refined due to the addition of Nb element, and Nb can induce the precipitation of S2 type silicide. The microhardness of the Ti-1100 alloy increases first and then decreases with the increase of the Nb content. The fracture toughness of the Ti alloy increases with the increase of Nb content. The fracture toughness of the Ti-1100-1.0Nb alloy can reach 44.5MPa-m1/2, which is 15.6MPa-m1/2 higher than that of Ti-1100 alloy. This increase can be attributed to the improved propensity for crack-tip tortuousity caused by grain refinement and the silicide precipitation. The fracture morphologies of these alloys reveal cleavage fracture mode. The microstructures of all the alloys are still belong to widmanstatten structure after heat treatments. The prior β grain size and a-laths spacing of the same composition alloy change little after heat treatments. The prior β grain size and a-laths spacing decrease with the increase of the Nb content in the same heat treatment condition. The EBSD analysis results show that no obvious texture exists in the heat treated alloys. Low and high angle grain boundaries formed in the alloys, and the proportion of the two kinds of grain boundaries are slightly different. β/α transformation occurs in the vicinity of 60° high angle grain boundaries of all the heat treated alloys to form β/α transformed structure. The microhardness values are also improved compared to the as-cast condition. The microhardness values of the 1.0wt.% Nb composition alloy are lower than the 0.5wt.% Nb composition alloy in the same heat treatment condition.