| As a Ti Al-based alloy for casting, the advantages such as high specific strength and modulus, good high temperature oxidation resistance and fracture resistance, etc, make Ti-48Al-2Cr-2Nb alloy the most attractive candidate material for aerospace and automobile application. However, the coarse columnar microstructure of the as cast Ti-48Al-2Cr-2Nb alloy limits its performance of mechanical properties, and results in the anisotropy of mechanical properties. Now researchers focous on the methods to obtain refined and uniform equiaxed microstructure of as cast Ti Al alloys. In this work, Ti B2 and Ni element were chose to added in the Ti-48Al-2Cr-2Nb alloy during melting to optimize the solidification microstructure and mechanical properties. Meanwhile, the effect of Ti B2 and Ni addition on microstructure evolution, microstrucure refinement mechanism, machanical properties and investment casting have been investigated systematically.Ti-48Al-2Cr-2Nb buttons with different Ti B2 additions were produced in a laboratory arc furnace with a non-consumable tungsten electrode. With the increase of Ti B2 addition, the microstructure transforms from coarse columnar microstructure to uniform equiaxed microstructure. With 0.3%(atomic ratio) and more Ti B2 addition, the colony size is refined to 200μm. With equivalent amount of boron, the pure B and Ti B2 powders present the same grain refinement effect, while th e alloys with B addition contain a certain amount of cavities. The borides in this work are identified as Ti B2 with C32 structure, and the morphology of these borides can be observed as flake, block and needle. Furthermore, all of them are proved to be the production of dissolution-reprecipitation of the initial Ti B2 additives. The Ti B2 flakes consist of Ti B2 plates and B2 plates with nanoscale, which are the production of Lâ†'β+Ti B2 eutectic reaction, and the orientation relationship between Ti B2 phase and B2 phase is clarified as [1(?)1(?)]Ti B2//[011]B2,(1(?)2)Ti B2//(0(?)2)B2. The assisted nucleation by the β/B2 containing Ti B2 flakes and the constitutional undercooling induced by boron atoms in the solid-liquid front appear to be the refinement mechanism in the Ti B2 containing Ti Al alloys.Ti-48Al-2Cr-2Nb ingots with different Ti B2 additions were prepared by ISM. With 0.4% and more Ti B2 addition, the microstructure transforms from coarse columnar microstructure to uniform equiaxed microstructure, and the colony size is refined from 700μm to 100μm, and the lamellar size is refined from 400 nm to 185 nm. The tensile properties and fracture toughness of Ti-48Al-2Cr-2Nb ingots are anisotropic. The hard-mode specimens present the best room temperature UTS and KIC as 600 MPa and 20.89 MPa·m1/2, respectively, while the corresponding properties of the soft-mode specimens are the worst. T4822-0.4Ti B2 alloy presents the favorable comprehensive mechanical properties and the toughening mechanisms in the Ti B2-containing alloy as ligament toughening, crack bowing induced by Ti B2 flakes and bridge force of Ti B2 needles are proposed. In the stair-step castings, grain size and boride size are further refined under the highest cooling rate, while the medium cooling rate(1.2×103 K/s) results in the best mechanical properties. Under the medium cooling rate, the room temperature UTS and εf are 550 MPa and 0.36%. Moderate Ti B2 addition can improve the fluidity of the Ti Al melt.The effects of Ni alloying on microstructure and properties in Ti-48Al-2Cr-2Nb alloy are investigated by ISM process. With 3.0% and more Ni additions, the microstructure transforms from coarse columnar microstructure to uniform equiaxed microstructure, and the colony size is refined from 700μm to 50μm. Ni addition leads to the formation of Ï„3 phase with hexagonal structure and the increase of bulk γ phase volume fraction. In T4822-4.0Ni alloy, the volume fraction of Ï„3 phase and bulk γ phase reach 7.9% and 39.5%, respectively. The nano-hardness of Ï„3 phase is tested as 8.6GPa, much higher than that of lamellar colony and bulk γ phase. The room temperature UTS and εf of T4822-0.5Ni alloy are 630 MPa and 0.15%, while with the increase of Ni addition the room temperature tensile properties of the Ni-containing alloys decrease sharply. The hard and brittle Ï„3 phase may be the main reason. The Ni alloying improves the high temperature εf and decrease the brittle-ductile transition temperature. With the increase of Ni addition the room temperature KIC of the Nicontaining alloys decrease sharply. T4822-0.5Ni alloy presents the favorable room temperature KIC, as 20.5 MPa·m1/2, while the KIC of T4822-4.0Ni is only 11.74MPa·m1/2. The bulk γ phase transformed to lamellar microstructure after the heat treatment of 1380℃/30min/FC, and the Ï„3 phase was remelted during the heat treatment following the reaction: Ï„3+γâ†'α+L. Ni elements were distributed in the matrix alloy or reprecipitated in the lamellar interface and grain boundaries as needlelike and random shape, respectively. The tensile properties are improved after heat treatment. Moderate Ni addition can improve the fluidity of the Ti Al melt.Corresponding to the influence of Ti B2 and Ni addition on the microstructure and properties in Ti-48Al-2Cr-2Nb alloy, the optimum contents of Ti B2 and Ni additions are 0.4% and 0.5%, respectively. The Ti-48Al-2Cr-2Nb-0.5Ni-0.4Ti B2 ingot was prepared by ISM. The microstructure consists of lamellar colony, Ï„3 phase and bulk γ phase with the mean colony size as 100μm and lamellar size as 210 nm. The mechanical properties of this alloy is improved comparing with matrix alloy, which can be mainly attributed to the uniform and refined microstructure, minor Ï„3 phase and bulk γ phase. The Ti-48Al-2Cr-2Nb-0.5Ni-0.4Ti B2 bent tube component was prepared by investment casting by industrial grade ISM furnace without observable defects. The casting presents favorable tensile properties. The room temperature UTS and εf are 590 Mpa and 0.55%, and the 700℃ UTS and εf are 565 MPa and 2.23%, respectively. |
PDF Full Text Request