Microstructure formation and interface characteristics of directionally solidified TiAl-Si alloys in alumina crucibles with a new Y2O3 skull-aided technology
Abstract:The microstructure evolution and interface characteristics of a directionally solidified Ti-43Al-3Si (at.%) alloy in an alumina (Al2O3) crucible with new Y2O3 skull-aided technology were investigated. The Y2O3-skull that is in contact with the TiAl-melt is relatively stable, which results in a more controlled reaction between the skull and the melt than in the case of an Al2O3 crucible is used. A thin reaction layer was formed between the mould and the melt through mutual diffusion. The layer thickness increas… Show more
“…Yield stress increases with grain refinement and the decrease in interlamellar spacing. Conversely, ductility decreases with the increase in current density, which could be caused by contamination from the crucible 32–34 .Figure 8True stress-true strain relationships for Ti-48Al-2Cr-2Nb solidified at different current densities.…”
In this article, microstructural evolution during the solidification of Ti-48Al-2Cr-2Nb with current density, as well as the formation mechanisms, are discussed, along with the impacts on microhardness and hot compression properties. The applied electric current promotes the solidification from the α primary phase to a largely β solidification in Ti-48Al-2Cr-2Nb. With an increase in supercooling, the solidification process have a tendency to change from an α-led primary phase to (α + β)-led primary phase. The primary dendrites, grain size, and lamellar spacing show a tendency to decrease first before increasing with increasing current density. Microhardness and high-temperature yield strength increase with a decrease in primary dendrite spacing, grain size, and lamellar spacing. Correlations between primary dendrite spacing, lamellar spacing, microhardness, yield strength, and current density are described by a fitting formula. An increase of α
2
phase, due to the application of electric current, results in improved microhardness. The yield strength of Ti-48Al-2Cr-2Nb alloy increases linearly with microhardness. Yield stress increases with a decrease in microstructure parameters, in accordance with the Hall–Petch equation. The predominant modification mechanism with electric current application for TiAl solidification is the variation of supercooling and temperature gradients ahead of the mush zone due to Joule heating.
“…Yield stress increases with grain refinement and the decrease in interlamellar spacing. Conversely, ductility decreases with the increase in current density, which could be caused by contamination from the crucible 32–34 .Figure 8True stress-true strain relationships for Ti-48Al-2Cr-2Nb solidified at different current densities.…”
In this article, microstructural evolution during the solidification of Ti-48Al-2Cr-2Nb with current density, as well as the formation mechanisms, are discussed, along with the impacts on microhardness and hot compression properties. The applied electric current promotes the solidification from the α primary phase to a largely β solidification in Ti-48Al-2Cr-2Nb. With an increase in supercooling, the solidification process have a tendency to change from an α-led primary phase to (α + β)-led primary phase. The primary dendrites, grain size, and lamellar spacing show a tendency to decrease first before increasing with increasing current density. Microhardness and high-temperature yield strength increase with a decrease in primary dendrite spacing, grain size, and lamellar spacing. Correlations between primary dendrite spacing, lamellar spacing, microhardness, yield strength, and current density are described by a fitting formula. An increase of α
2
phase, due to the application of electric current, results in improved microhardness. The yield strength of Ti-48Al-2Cr-2Nb alloy increases linearly with microhardness. Yield stress increases with a decrease in microstructure parameters, in accordance with the Hall–Petch equation. The predominant modification mechanism with electric current application for TiAl solidification is the variation of supercooling and temperature gradients ahead of the mush zone due to Joule heating.
“…During the solidification process, silicon will be squeezed from α phase and segregated in the liquid melt between the dendrites. When the content of silicon is higher than 4% in the segregation area, Ti 5 Si 3 phase will form at the interface of the colonies [25][26][27] . Thus, when the content of silicon is < 0.3at.%, the solidification process of the Ti48Al6NbxSi alloys is as follows, L → L+β → L+α+β r → α+β r →α+γ+β r → (α 2 +γ)+γ+B2.…”
In order to improve mechanical properties of TiAlNb alloys, different contents of silicon were added into Ti48Al6Nb alloy. The Ti48Al6NbxSi (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5, at.%) alloys were prepared by vacuum arc melting. The phase constitution, microstructure evolution and mechanical properties of the alloys were studied. Results show that the Ti48Al6NbxSi alloys consist of γ-TiAl phase, α 2 -Ti 3 Al phase and B2 phase, and Ti 5 Si 3 silicide phase is formed when the addition of silicon is higher than 0.3at.%. The addition of silicon leads to the decrease in γ phase and increase in α 2 phase. The formation of silicide decreases the amount of Nb dissolved in the TiAl matrix, and therefore decreases B2 phase. Compressive tests show that the ultimate strength of the alloys increases from 2,063 MPa to 2,281 MPa with an increase in silicon from 0 to 0.5at.%, while the fracture strain decreases from 34.7% to 30.8%. The increase of compressive strength and decrease of fracture strain can be attributed to the decrease of B2 phase and the formation of Ti 5 Si 3 phase by the addition of silicon. The strengthening mechanism is changed from solid solution strengthening when the addition of silicon is less than 0.3at.% to combination of solid solution strengthening and secondary phase strengthening when the addition of silicon is higher than 0.3at.%.
“…Barbosa et al [49] prepared zirconia crucible with Y 2 O 3 coating (about 200μm) by dipping method which could reduce the number of Y 2 O 3 particles in TiAl alloys after melting. Fan et al [30,62,63] have reported that no obvious oxide pollution was found in directionally solidified TiAl alloy prepared by using micro/nano double scale Y 2 O 3 /Al 2 O 3 composite crucible. The reason is that the directional solidification process promotes the re-sintering of the surface of the double scale Y 2 O 3 layer and forms a relatively dense isolation layer.…”
Section: Oxide Casting Mould Materialsmentioning
confidence: 99%
“…Al 2 O 3 has high thermal stability, good formability and low cost [25][26][27][28][29][30][31][32][33]. At the same time, the thermal expansion coefficient of Al 2 O 3 is similar to that of TiAl alloys, which can reduce the cracking probability during casting process [34].…”
Intermetallic TiAl alloys are new generation high-temperature material. However, extensive application of TiAl alloys is hindered by some disadvantages, especially the high processing cost. Currently, precision casting is an effective method to manufacture TiAl components with complex shape. However, the interfacial reaction between the TiAl alloy melt and mould affects the quality of the castings and hinders extensive applications of casting TiAl components. In this paper, the research status of mould materials for the casting of TiAl alloys is reviewed. Performances of present used mould materials are compared in details. Reaction mechanisms between mould materials and the melts of TiAl alloys are also summarised. Finally, the future development tendency and prospect are pointed out.
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