Flow Behavior and Hot Workability of Nb-15Si-22Ti-5Cr-3Al-2.5Hf Alloy
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Nb-Si-BASED ultrahigh temperature alloys with high melting points of ~ 1750 C have drawn extensive attention as a promising category of structural materials at elevated temperatures because of their remarkable high-temperature strength and creep resistance as well as proper densities (~ 7 g/cm3).[1–4] However, their poor room-temperature fracture toughness deriving from high volume fraction (40 to 65 pct[1]) of silicides and weak oxidation resistance have impeded the practical application of these alloys. In order to get a better balance among room-temperature fracture toughness, high-temperature creep strength and oxidation resistance, alloying elements (such as Ti, Cr, Hf, and Al) have been added to Nb-Si binary system, and multicomponent Nb-Si-based ultrahigh temperature alloys have been developed.[5–9] The constituent phases of Nb-Si-based ultrahigh temperature alloys include niobium solid solution (Nbss), niobium silicides (Nb3Si or Nb5Si3 with different crystal structures, depending on compositions of alloys) as well as Laves Cr2Nb phase when a high Cr content occurs.[5–9] The microstructures of these alloys usually comprise Nbss/Niobium silicide YE TANG and XIPING GUO are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China. Contact e-mail: [email protected] Manuscript submitted March 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
eutectics and primary Nbss (in hypoeutectic alloys) or silicide (in hypereutectic alloys) phase.[5–9] It has been reported that relatively high room-temperature fracture toughness (KQ = 13 to 17 MPa m1/2) and high-temperature compressive strength (200 to 400 MPa at 1250 C) are achieved for the annealed Nb-Ti-Si-Hf-Cr alloy and the arc-melted Nb-Ti-Si-Cr-Al-B-Hf alloy.[6,9] On the other hand, various processing technologies (such as vacuum arc-melting, induction skull melting, directional solidification, and powder metallurgy technology) have been extensively explored to fabricate the Nb-Si-based alloys to improve their mechanical properties both at room and elevated temperature via ameliorating the microstructures.[10–13] In addition, hot working is widely adopted for shaping metals and alloys. Furthermore, this process can also contribute to the development of homogeneous and fine microstructures and the improvement in mechanical properties of metallic materials through the synthetical effects of heat and stress. In order to ascertain the favorable conditions for hot working of an alloy, constitutive models are usually established to predict flow stress at different temperatures, strains and strain rates, and its hot workability is also assessed in advance by establishing a processing map based on the dynamic materials model (DMM).[14,15] In the previous studies, Nb-27Ti-8Cr-9Al-12Si alloy and Nb-10Si-2Fe alloy have been hot extruded,[3,16] and Nb-12Si-24Ti-4Cr-4Al-2Hf alloy has been isothermally forged.[17] The hot-extruded alloys have been reported
to show superior room-temperature fracture toughness
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