Effect of Heat Treatment on Microstructures and Mechanical Properties of a Novel β-Solidifying TiAl Alloy

Abstract

The effect of heat treatment on the microstructures and mechanical properties of a novel β-solidifying Ti–43Al–2Cr–2Mn–0.2Y alloy was investigated. A fully lamellar (FL) microstructure with a colony size of about 100 μm was obtained by heat treatment at 1320 °C/10 min/furnace cooling (FC). A duplex (DP) microstructure with globular γ grains and γ/α2 lamellae was obtained by heat treatment at 1250 °C/4 h/FC. The residual hard–brittle β0 phase was also eliminated after heat treatment. The mechanical properties of the β-solidifying TiAl alloy depended closely on the heat treatment. The FL alloy had better fracture toughness, and the fracture toughness (KIC) value was 24.15 MPa·m1/2. The DP alloy exhibited better ductility, and the room temperature (RT) elongation of the alloy could reach 1%. The elongation of the alloy with different microstructures sharply increased when the temperature increased from 700 to 750 °C, indicating that the microstructure had no effect on the ductile–brittle transition temperature of the β-solidifying TiAl alloy. The fracture morphologies of different tensile specimens were observed. Interlamellar and translamellar fractures were the main fracture features of the FL alloy. Intergranular, translamellar, and interlamellar fractures were the main fracture features of the DP alloy.

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

“…Apart from Ti and Al, TNM alloys contain alloying elements such as Nb, Mo and minor additions of B [ 8 , 12 , 15 , 16 , 17 , 18 ]. This was developed primarily to be amenable to mechanical working while providing the best combinations of mechanical properties that could be achieved through manufacturing processes with subsequent heat treatments.…”

“…Thus, the curiosity in understanding the microstructural evolution and phase compositions. At elevated temperatures, TNM alloys display satisfactory quantities of disordered β-phase because of the β-stabilizing effects that Mo and Nb offer in promoting hot-workability of TiAl-based alloys [ 7 , 15 , 16 , 17 , 18 ]. TNM alloys containing micro-alloys of Si and C called TNM + are said to demonstrate excellent creep resistance properties and exceeding the present service temperature limits for TiAl alloys [ 5 ].…”

“…This detrimental effect emanates from the disordered A2 structure transforming at ambient temperature to the ordered B2 structure (β 0 -phase) which is brittle and harder than γ and α 2 phases. Also, the β 0 -phase is incompatible with the α 2 (D0 19 structure) and γ (L1 0 structure) phases leading to crack nucleation and propagation at the β 0 /γ interfaces, thereby, causing a reduction in ductility at the service temperatures [ 16 , 20 , 21 , 22 ]. This crack initiation has a significant influence on the fracture of TNM alloys [ 22 ].…”

“…Furthermore, β 0 -phase has a body-centred cubic (BCC) structure which is similar to conventional β-phase Ti alloys with B2 superlattice structure [ 33 ]. As stated earlier, the disordered elevated temperature β-phase changes to a very brittle and hard β 0 -phase at room temperature (RT) [ 16 ]; while ordered β 0 -phase dissolving to disordered β-phase at about 1175–1205 °C [ 29 ]. In typical Ti alloys, there are both high-temperature BCC β-phase and low-temperature HCP α-phase [ 41 ].…”

“…Four possible microstructures are usually formed by TiAl-based intermetallic alloys are shown in Figure 1 . It is reported that the nearly fully lamellar and/or fully lamellar microstructures could provide the much-needed combination of mechanical properties using a viable production method and provide structural integrity during the components’ lifetime [ 5 , 7 , 14 , 16 , 22 , 30 , 46 , 51 , 52 ]. The duplex or equiaxed microstructure (γ+α 2 ) provides adequate ductility and fatigue but it exhibits low creep resistant and fracture toughness; while the microstructure with fully lamellar (with alternating plates of γ and α 2 phases) demonstrates superior fracture toughness and creep resistant but demonstrates poor ductility [ 30 , 31 , 52 ].…”