A Novel Composition Design Method for Beta-Gamma TiAl Alloys with Excellent Hot Workability
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TRODUCTION
c-TiAl alloys are important light-weight materials for high-temperature (HT) structural applications in the aerospace and automobile industries owing to their excellent properties, such as low density, high specific strength, and good oxidation resistance.[1,2] Despite these advantages, low room-temperature (RT) ductility has always limited the engineering application of TiAl alloys.[3,4] Thermomechanical processing is an effective method to refine the microstructure and improve the mechanical properties of the alloys. But conventional TiAl alloys generally exhibit poor hot workability. Thus, novel beta-gamma TiAl alloys, which exhibit excellent hot workability owing to the introduction of b phase, have attracted special attention. Disordered b phase in beta-gamma TiAl alloys is a ductile phase at HT, which can provide a sufficient number of independent slip systems and act as a
FANTAO KONG, YUYONG CHEN, and XIAOPENG WANG are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. Contact e-mail: [email protected] NING CUI is with the School of Mechanical Engineering, Qingdao University of Technology, Qingdao 266520, China. Manuscript submitted April 2, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
lubricant layer, and thereby improve the hot workability of TiAl alloys.[5–7] b phase can be introduced in TiAl alloys by adding b stabilizers, such as Cr, V, Mn, Nb, Mo, and W.[6] It is worth noting that b phase exists as an ordered b0 phase with a B2 structure (CsCl) at RT, which is detrimental to RT ductility.[8] There is as yet no effective way to eliminate b0 phase completely. It is desirable to increase disordered b phase at HT while simultaneously controlling ordered b0 phase at RT by alloying. However, very little research has been reported on how to quantitatively control b and b0 phase by alloying. The synergistic effects of various b stabilizers on b and b0 phases in complex alloy systems are also not at all understood. Moreover, the compositional design of beta-gamma TiAl alloys relies mainly on qualitative analysis, which has low efficiency and high cost. To improve efficiency and avoid resorting to expensive trial and error methods, a quantitative alloy design method for beta-gamma TiAl alloys is urgently needed. Much research has been devoted to the quantitative alloy design strategy in recent decades. Equivalent methods have been proven to be an effective quantitative method to guide composition design, such as C equivalent for carbon steel and Al equivalent and Mo equivalent for Ti alloys. Regarding Ti alloys, H.W. Rosenberg expressed the effect of a stabilizers as equivalent aluminum content in designing Ti alloys for elevated temperature applications.[9] A Mo equivalent
rule describing the effects of b stabilizers was constructed for multi-component Ti alloys by Kolachev.[10] The Mo equivalent and Al equivalent permit the evaluation of the expected constitution of a Ti alloy with a given chemistry. Moreover, Sun et al. evaluated the solute s
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