Microstructure And Mechanical Properties Of Electromagnetic Coldcrucible Directionally Solidified Ti44Al6Nb1Cr2V-(B,Y) Alloys

Ti Al-based alloy is a kind of new-type material and it is the most promising one to replace Ni-based high temperature alloys partly at 600°C-1000°C, since it yields the advantage of lighter weight and high specific strength, and it is a kind of new type of high temperature structural material. However, it also yields some disadvantages, e.g. poor plasticity at room temperature（RT） and weak machinability, leading to the restriction of the application. High-Nb contained Ti Al-based alloys yields the higher service temperature and high-temperature strength and it can be considered as the milestone in the development history of Ti Al-based alloys. However, higher content of Nb element will induce the increased melting point that makes its preparation much harder, and more B2 phase may further deteriorate the RT plasticity. These problems can be coped with by cold crucible directional solidification（CCDS） techneque. The CCDS billets have the industry size, homogeneous and directionally solidified macrostructure, but without contamination and defects, can be manufactured by this technology. These billets yield better axial mechanical properties, suited for aero-engine blade. This paper mainly involve in Ti44Al6Nb1Cr2 V and Ti44Al6Nb1Cr2V0.1B0.15 Y alloys and CCDS related content, and it mainly discusses and studies B/Y alloying, directionally solidification course, microstructural evolution, RT/high-temperature mechanical properties and thermo-compressive behavior and mechanism in detail and systematically.From the research about B/Y alloying, the conclusions can be drawn as follows: B and Y can refine the microstructure effectively and mainly exist in the form of Al₂ Y, Ti B and Y₂O₃（a little）. When Y content is in the range of 0.3%-0.5%, Al Y and Al3Y5 phases can be detected, too. Al3Y5 yields the hexagonal structure, some Ti B phase has the orthorhombic structure and all the other phases have the cubic structure. Except Y₂O₃ phase, the other B/Y-contained precipitations commonly yield a certain orientation relationship with matrix. Most of the precipitations exist in the grain boundaries. Slight addition of B/Y element can improve the RT mechanical properties. The RT compressive strength and tensile property are generally improved gradually with the increase of Y content in the range of 0-0.5%, and the condition is nearly the same for the B addition. B/Y addition simultaneously can lead to more obvious mechanical properties improvement. For all the alloys studied in this research, Ti44Al6Nb1Cr2V0.15Y0.1B alloy has the best performance, it yields the compressive strength of 2294 MPa, tensile strength of 555 MPa and tensile elongation of 0.72%.CCDS experiments are carried out on the two kinds of alloys, and it finds that lower pulling velocities（0.3-0.5mm/min） and proper input power（45k W） are helpful for directional macro/microstructure. The well-DS billets yield a higher percentage of small-angle lamellas, homogeneous microstructure and small interlamellar space. The slight addition of B and Y shrinks the DS window, leading to the refined columnar grains, slightly increased lamella angle and decreased interlamellar space, and meanwhile, a few B/Y-contained precipitation phases and relatively more blocky γ phases form. Interlamellar space and pulling velocity yield the function relationship of d=a V-b（b>0）, and nanoindentation hardness and interlamellar space yield the function relationship of HN=kd-b（b>0）. The nanoindentaiton hardness of B2 phase and γ phase in the same alloy is nearly the same, but their hardness in No.1 alloy is slightly higher than that in No.2 alloy. The CCDS billets yields better RT tensile property, toughness, and high temperature tensile property. For part of the well-DS billets, they can yield 520 MPa at most of the tensile strength at 900°C. The stress-induced holes mainly contribute to the higher elongation, and the fracture presents the dimple morphology indicating the ductile fracture. Slightly addition of B/Y element can increase the stability of mechanical properties.The thermo-compressive experiment shows that the two CCDS alloys yield much higher stress under the same deformation conditions. Via constitutive derivation, CCDS alloys yield higher deformation resistance, and moreover, the deformation resistance of No.1 alloy is stronger than that of No.2 alloy. It is calculated that the deformation activation energy Q of No.1 alloy in radial is about 729.6KJ/mol and that of No.2 alloy in radial is about 689.8KJ/mol, which indicates that slightly B and Y addition has weakened the high temperature performance in some degree. The stress in axial is obviously higher than that in radial indicating a higher deformation resistance and creep resistance. As heavier axial compression, the obvious shear deformation characteristic will be presented, most residual lamellas are parallel with the shear band and there are still some heavy distorted lamellas. It is deduced that the ordering temperature of β/B2 phase is in the range of 1200°C-1250°C by investigating the twinning behavior. Under a certain thermo-compressive conditions, lots of thinner secondary lamellas can be formed, they also have the（α+γ/γT） structure like the common lamellas, and there are even nanoscale lamellas in some regions.