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

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.

Microstructure Evolution and Mechanical Properties of 0.4C-Si-Mn-Cr Steel during High Temperature Deformation

“…Forging is a method for refining the grain size of steel via severe plastic deformation to enhance the strength and toughness. Internal cracks and defects of raw materials can be reduced during forging, and the microstructure and grain distribution are more uniform and compact after forging [9,10].…”

Characteristic effects of alloying elements on β solidifying titanium aluminides: A review

“…Several authors [ 4 , 9 , 24 , 26 , 61 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 100 , 101 , 102 ] have used a dual-stage heat treatment process in trying to reduce the β-phase. These TiAl-based alloys were first heated to a temperature above the γ solvus temperature (Tγ solv ), held for a brief period to avoid grain growth with subsequent fast cooling.…”

“…For the past three decades, titanium aluminides intermetallic alloys have been of major interests especially in the aerospace and automotive industries. This is essentially due to their attractive mechanical properties like high strength-to-weight, low density, excellent high-temperatures stiffness and strength retention [ 15 , 28 , 31 , 43 , 46 , 47 , 48 , 50 , 56 , 62 , 67 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 ]. This makes TiAl-based alloys a type of material with very high prospects of replacing nickel-based superalloys typically used for making aircraft-engine turbine parts [ 43 ].…”

“…The intermetallic Ti–Al ordered system is still a topic of extensive study because of its inherent applications for structural materials operating at elevated temperatures [ 6 , 23 , 47 , 53 , 54 , 60 , 69 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 ]. The successful integration of TiAl-based alloys parts into aircraft engines is predicted to tremendously decrease greenhouse gas emission and fuel consumption [ 5 ].…”

“…Thus, titanium aluminide has found application in making seal supports, cases and frames of aircraft engines where clearances are crucial [ 54 ]. Also, it is applied in turbocharger wheels and turbine blades of automobile and aero engines respectively [ 5 , 7 , 21 , 43 , 50 , 58 , 73 , 79 , 80 , 81 , 87 , 88 , 90 , 91 ]. Considerable decrease in fuel consumption and greenhouse gases have been achieved due to the lightweight of rotating components.…”