Abstract
The influence of B and Y on the microstructure, microsegregation, and tensile behaviour of the Ti45Al8Nb0.2W0.25Cr (at%) alloy was investigated. The β-stabilizer elements in the high-Nb TiAl alloy promote the formation of the γ phase in the microsegregation region and lead to the formation of large blocky microsegregation areas. The large blocky microsegregation regions with low specific surface areas reduce the nucleation rate of cavities and cracks at the interfaces of the microsegregation, which are harmful to colony boundary strengthening and decrease tensile resistance. The addition of B and Y affords an obvious refinement in the lamellar colony, renders an increasing opportunity for cavity nucleation at the colony boundary, and thus improves the tensile resistance. The tensile mechanisms of the alloys before and after (B, Y) addition were also compared and analysed.
Keywords
Science Press
TiAl alloys have gained great interest for research on aerospace applications due to their low density and high specific strength in recent year
It is recognized that the as-cast high-Nb TiAl alloy, which possesses a good balance in mechanical properties, needs to be refined by the addition of refining elements such as B and
Based on a high-Nb TiAl alloy, the nominal composition of investigated alloys in this study was Ti45Al8Nb0.2W0.25Cr (TWC), and Ti45Al8Nb0.2W0.25Cr0.2B0.02Y (TWC-BY). The materials used in this investigation were produced by a vacuum induction suspension melting technique. Suspension duration was 100 s for homogenization. The size of ingots was about Φ100 mm×150 mm. The tensile test samples were cut from the as-cast ingots. The chemical compositions of investigated alloys were measured by the spectrofluorometry method, which are summarized in
accurately measured by the used method.
Specimens for room temperature tensile tests, with a gauge section of 1 mm×4 mm×10 mm, were sampled from the core of the alloys. The room-temperature tensile tests were carried out on Instron 5569 universal material testing machine with an initial strain rate of 2.5×1
Fig.1 SEM images of initial microstructures of TWC (a) and TWC-BY (b, c) alloys
Analysis of EDS spectrum data (
TEM images in
Fig.2 TEM images of lamellar microstructures of as-cast TWC (a) and TWC-BY (b) alloys; selected area electron diffraction patterns
of β/B2 phase (c) and γ phase (d)
It can be seen from the liquid phase projection diagram of the Ti-Al-B ternary phase diagra
of β→α→α+γ→(α2+γ). During the solidification of the alloy, two-phase transformation processes of L→β+TiB2 and L+β → α+TiB2 may occur while B is added. A small amount of TiB2 phase nuclei are formed from the liquid during the L→β+TiB2 reaction, while most of the TiB2 phase is formed by the reaction of L+β→α+TiB2, and it grows as a secondary phase. As can be seen from
Y as a surface-active element can reduce the surface tension of the liquid metal. The smaller the surface tension of the interface, the smaller the energy fluctuation required to form crystal nucleus. That is, the addition of Y reduces the nucleation of the alloy to form the critical crystal nucleus and increases the crystal nucleus. In addition, the Y element has a high activity coefficient and a strong affinity with oxygen atoms. In the early stage of solidification, Y is easily concentrated at the front edge of the solid-liquid interface, thereby forming a stable oxide Y2O3, inhibiting the β phase crystal nucleus from growing and increasing the number of β-phase nucleation. Also besides, the enrichment of Y at the solid- liquid interface will cause the constitutional supercooling of the crystallization front, which branches out the precipitated phase, forms grain boundary segregation, hinders the move-ment of the grain boundary, and reduces the growth rate of the crystal. At the same time, the dissolved Y atoms will cause elastic distortion around them and be pinned at the α interface, resulting in a decrease in the layering energy of the α phase interface, and forming a high phase interface energy. Y atoms will hinder the dislocations and step movement at the interface, which reduces the rate of lateral thickening of the γ phase, thereby refining the lamellar spacing.
The room temperature tensile curves of the two alloys are shown in Fig.3. It can be seen that the performance is improved with (B, Y) addition. The tensile strength and elongation of the alloy both increase. The tensile strength increases from 582 MPa to 613 MPa, and it is brittle fracture. Compared with the TWC alloy, it is found that the fracture morphology of TWC-BY alloy is still a mixed fracture of interlaminar cleavage fracture and intergranular brittle fracture, neither of which exhibit the existence of dimples, as shown in
Fig.4 Tensile fracture morphologies at room temperature of TWC (a) and TWC-BY (b) alloys
The grain size and interlamellar spacing of the TWC-BY alloy are significantly refined, and the room temperature tensile properties of the alloy are also greatly improved. However, the as-cast TWC-BY alloy still cannot satisfy the requirements for engineering applications. Moreover, β/B2 phase segregation in the as-cast structure of the alloy seriously reduces the room temperature plasticity and fracture toughness of the alloy material. Therefore, in order to optimize the alloy microstructure, improve the mechanical properties of the alloy, and accelerate the industrial application of the β-phase solidified as-cast high-Nb TiAl alloy, it is necessary to study the elimination process of β-phase segregation.
1) The β-stabilizer elements in the TWC alloy promote the formation of the γ phase in the microsegregation region and lead to the formation of large blocky microsegregation areas.
2) The large blocky microsegregation regions with low specific surface areas reduce the nucleation rate of cavities and cracks at the interfaces of the microsegregation, which are harmful to colony boundary strengthening, thus decreasing the tensile resistance.
3) The addition of B and Y in the TWC-BY alloy provides an obvious refinement in the lamellar colony, renders an increasing opportunity for cavity nucleation at the colony boundary, and thus improves the tensile resistance. The room temperature tensile properties of both alloys are brittle fracture.
References
Kim Y W. Materials Science and Engineering A[J], 1995, 192-193: 519 [Baidu Scholar]
Liu C T, Schneibel J H, Maziasz P J et al. Intermetallics[J], 1996, 4(6): 429 [Baidu Scholar]
Gong Shengkai, Shang Yong, Zhang Ji et al. Acta Metallurgica Sinica[J], 2019, 55(9): 1067 [Baidu Scholar]
Zhang Xiwen, Wang Hongwei, Zhu Chunlei et al. Rare Metal Materials and Engineering[J], 2020, 49(1): 138 [Baidu Scholar]
Appel F, Oehring M, Wagner R. Intermetallics[J], 2000, 8(9): 1283 [Baidu Scholar]
Jiang Haiyin, Xu Lianlian, Li Yucuan et al. Journal of Psychiatric Research[J], 2016, 83: 160 [Baidu Scholar]
Helmut Clemens, Svea Mayer. Advanced Engineering Materials[J], 2013, 15(4): 191 [Baidu Scholar]
Imayev R M, Imayev V M, Oehring M et al. Intermetallics[J], 2007, 15(4): 451 [Baidu Scholar]
Li Wei, Liu Jie, Zhou Yan et al. Scripta Materialia[J], 2016, 118: 13 [Baidu Scholar]
Wang Qi, Chen Ruirun, Yang Yaohua et al. Journal of Alloys and Compounds[J], 2018, 747: 640 [Baidu Scholar]
Gussone J, Garces Gerardo, Haubrich J et al. Scripta Materialia[J], 2017, 130: 110 [Baidu Scholar]
Chen Ruirun, Dong Shulin, Guo Jingjie et al. Journal of Alloys and Compounds[J], 2015, 648: 667 [Baidu Scholar]
Kartavykh A V, Asnis E A, Piskun N V et al. Journal of Alloys and Compounds[J], 2015, 643: 182 [Baidu Scholar]
Gong Ziqi, Chen Ziyong, Chai Lihua et al. Acta Metallurgica Sinica[J], 2013, 49(11): 1369 (in Chinese) [Baidu Scholar]
Xiang L L, Zhao L L, Wang Y L et al. Intermetallics[J], 2012, 27: 6 [Baidu Scholar]
Michael Oehring, Andreas Stark, Paul J D H et al. Intermetallics[J], 2013, 32: 12 [Baidu Scholar]
Chen Y Y, Li B H, Kong F T. Journal of Alloys and Compounds[J], 2008, 457(1-2): 265 [Baidu Scholar]
Chen G L, Xu X J, Teng Z K et al. Intermetallics[J], 2007, 15(5-6): 625 [Baidu Scholar]
Valencia J J, Löfvander J P A, McCullough C et al. Materials Science and Engineering A[J], 1991, 144(1-2): 25 [Baidu Scholar]