Abstract
The effects of the package materials on the hot workability and stress-strain characteristics of high-Nb TiAl alloy with a nominal composition of Ti-46Al-8Nb (in at.%) were systematically studied via “sandwich structure” hot compression. TiAl sheet fabrication was conducted by hot pack rolling, and the microstructural characteristics and deformation mechanisms were investigated. Based on the analysis of compressed samples and stress-strain curves, the stainless steel/TiAl structure showed better deformation compatibility with homogeneous deformation and decreasing resistance. However, severe interfacial reactions were inevitable. Meanwhile, for the titanium alloy/TiAl structure, few interfacial reactions happened, but wavy deformation and high resistance complicated the compression process. Finally, a package structure with an outer stainless steel isolation layer and inner titanium alloy was determined for the pack rolling process. A TiAl sheet with no crack defects was obtained with 80% reduction. The pack-rolled TiAl sheet took on alternate microstructure of the grain-boundary Al-enriched ribbons and elongated lamellar colonies ribbons. The grain-boundary recrystallized α2 phase, lumpy γ phase, and massive α2/γ lamellae could be observed, which led to the scatter microstructure. The microstructural characteristics mainly resulted from the solute segregations of as-cast Ti-46Al-8Nb alloys, which triggered the local flow softening and deformation incompatibility during hot pack rolling.
Highlights
TiAl-based alloys have attracted much attention as potential candidates for high temperature structural applications due to their excellent low density, high-temperature properties and creep resistance [1,2]
The 10 × 10 × 20 mm piece of TiAl alloy and the 4 × 10 × 20 mm piece of the packaging material were designed for the hot compression experiment, during which the stainless steel (2520, Fe-25Cr-20Ni) and titanium alloy (TC4, Ti-6Al-4V) were selected as the packaging materials to investigate their effects on the workability of the TiAl alloy
It was seen that the packaging materials with different strengths directly determined the heat-deformed conditions of TiAl alloy, which always plays the function of preventing the heat dissipation, oxidation, and crack initiation during the pack rolling process
Summary
TiAl-based alloys have attracted much attention as potential candidates for high temperature structural applications due to their excellent low density, high-temperature properties and creep resistance [1,2]. TiAl sheet fabrication has been limited by its poor ductility, low workability, and high-temperature resistance [5,6]. TiAl sheet fabrication, such as the isothermal rolling [7,8], direct casting [9], and hot pack rolling [10], among which hot pack rolling is considered the most low-cost and widespread method. The packaging materials can be used for preventing heat loss, surface oxidation, and crack initiation, which makes it possible to produce TiAl sheets on a conventional mill, and the only successful production of large-scale sheets (1800 × 500 × 1 mm3 ) was achieved by GKSS and Plansee (company) with the advanced plate rolling process (ASRP).
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