Microstructural Evolution and Refinement Mechanism of a Beta–Gamma TiAl-Based Alloy during Multidirectional Isothermal Forging

Kai Zhu,Aihan Feng,Jun Shen,Shoujiang Qu,Jingli Sun

doi:10.3390/ma12152496

Abstract

Multidirectional isothermal forging (MDIF) was used on a Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B (at. %) alloy to obtain a crack-free pancake. The microstructural evolution, such as dynamic recovery and recrystallization behavior, were investigated using electron backscattered diffraction and transmission electron microscopy methods. The MDIF broke down the initial near-lamellar microstructure and produced a refined and homogeneous duplex microstructure. γ grains were effectively refined from 3.6 μm to 1.6 μm after the second step of isothermal forging. The ultimate tensile strength at ambient temperature and the elongation at 800 °C increased significantly after isothermal forging. β/B2→α2 transition occurred during intermediate annealing, and α2 + γ→β/B2 transition occurred during the second step of isothermal forging. The refinement mechanism of the first-step isothermal forging process involved the conversion of the lamellar structure and discontinuous dynamic recrystallization (DDRX) of γ grains in the original mixture-phase region. The lamellar conversion included continuous dynamic recrystallization and DDRX of the γ laths and bugling of the γ phase. DDRX behavior of γ grains dominated the refinement mechanism of the second step of isothermal forging.

Highlights

Introduction γTiAl-based alloys are considered highly promising materials for aeroengine and automotive applications because of their attractive properties, including low density, high-temperature strength, good oxidation, and creep resistance at elevated temperatures [1,2,3,4]
Different from traditional γ-TiAl-based alloys, which contain two basic phases (i.e., γ-TiAl and α2 -Ti3 Al) and often small amounts of the β0 phase, the beta–gamma TiAl-based alloy is a multiphase alloy that consists of three major phases (i.e., γ-TiAl, α2 -Ti3 Al and β0 -Ti) and an additional phase (i.e., ω0 -Ti4 Al3 Nb), which usually co-exists with the β0 phase [6,7]
multidirectional isothermal forging (MDIF) was applied to a beta–gamma TiAl-based alloy to achieve a fine and homogeneous microstructure on a large scale and with good mechanical properties

Summary

Introduction

TiAl-based alloys are considered highly promising materials for aeroengine and automotive applications because of their attractive properties, including low density, high-temperature strength, good oxidation, and creep resistance at elevated temperatures [1,2,3,4]. An advanced γ-TiAl-based alloy called beta–gamma TiAl-based alloy has gradually aroused researchers’ interest because it has good hot workability, homogeneous microstructures, weak textures, and minimal segregation [5]. The β phase has a body centered cubic (bcc) structure and can work as a lubricant during hot processing, thereby facilitating the plastic flow and improving the formability of γ-TiAl-based alloys at elevated temperatures [8]. The beta–gamma TiAl-based alloy is suitable for hot processing without cracks, and can produce a fine-grained (FG) microstructure with improved mechanical properties [9,10,11]. The typical hot Materials 2019, 12, 2496; doi:10.3390/ma12152496 www.mdpi.com/journal/materials

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Conclusion