Microstructure Characterization and Thermal Stability of TNM Alloy Fabricated by Powder Hot Isostatic Pressing

A TNM alloy ingot was fabricated with powder hot isostatic pressing (P-HIP) and short-time exposure treatment conducted at 750–1050 °C for 2–5 h. The tensile mechanical properties were investigated at room temperature and 800 °C. The results revealed that a fully lamellar microstructure of P-HIPed TNM alloy with only 0.3 vol.% β0 phase could be obtained by hot isostatic pressing at 1260 °C, under the pressure of 170 MPa, held for 4 h. When the exposure temperature was below 850 °C, the α2 lamellae were transformed into nano-scaled (α2 + γ) lamellae (i.e., the α2→α2 + γ transformation). With increases in the exposure temperature, the β0 phase began to precipitate within the α2 lamellae (α2→β0 transformation) at 950 °C. The α2→γ and the α2→β0 transformation both happened at 950–1050 °C, and the higher exposure temperature accelerated the diffusion of Mo and facilitated the α2→β0 transformation. The yield strength and elongation at RT and 800 °C were both improved after short-time high-temperature exposure treatment. The uniform distribution and nano-scaled interfacial β0 phase provided precipitation strengthening and were not harmful to the elongation.