Pearlite-Like Microstructure and B2 Phase in TiAl Alloy with Tungsten Addition AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY25010002

Chinese Library Classification Number:TG146.2

Citation Information:

The addition of tungsten （W） improves the high-temperature mechanical properties of TiAl alloys by effectively suppressing atomic diffusion. However，its low diffusivity and high melting point lead to significant microstructural inhomogeneity and complications during melting and solidification. Consequently，the systematic understanding of how W influences the microstructure of TiAl alloys remains limited. To address these challenges，powder metallurgy offers a promising route for obtaining uniform and fine microstructures suitable for detailed investigation. In this work，the effect of W diffusion on microstructure evolution and micro-/nano-mechanical properties was investigated. To promote W redistribution，solid-solution treatments （ST） were performed at three different temperatures within （α+γ） phase region. After ST，all three alloys exhibited similar microstructures，consisting mainly of a pearlite-like matrix and B2 phase. Within B2 phase，γ laths （denoted γ_p） precipitated in W-deficient zones following the Kurdjumov-Sachs （K-S） orientation relationship，forming B2（γ） region. γ_p phase nucleated directly inside B2 grains，with its growth likely governed by discontinuous coarsening. The volume fraction of γ_p within the B2 phase increased with rising ST temperature. Higher temperatures enhance atomic mobility，increasing the diffusion coefficient of W and promoting γ phase precipitation in W-depleted areas. Moreover，reduced interfacial energy at elevated temperatures provides a stronger driving force for direct nucleation，explaining the observed increase in γ_p content. During ST，the α→γ phase transition expels W into adjacent regions，resulting in a supersaturated α phase and inducing lattice distortion. To relieve the distortion energy，the hexagonal structure transforms into a body-centered cubic （B2） phase，leading to the formation of a pearlite-like microstructure. Prior homogenization annealing at 1250 ℃ for 16 h effectively reduced micro-segregation，as confirmed by a residual segregation index δ close to zero. This homogenization promoted uniform W distribution，suppressed phase transformations during subsequent ST，and ultimately yielded a segregation-free lamellar structure. These observations indicate that W segregation significantly hinders the formation of equilibrium lamellar microstructures. Micro-and nano-mechanical tests revealed that microhardness first increased and then decreased with rising ST temperature. The highest hardness of HV 440.0  was achieved after ST at 1320 ℃，followed by a decline to HV 384.3 at 1340 ℃ and HV 365.8 at 1360 ℃. After prior homogenization，hardness remained stable around HV 350.5  regardless of ST temperature. Similarly，the microhardness of B2（γ） region peaked at HV 509.2 after ST at 1320 ℃，dropping to HV 427.0 and HV 430.0  at higher temperatures. In contrast，the pearlite-like matrix showed little dependence on ST temperature，averaging HV 354.0.Nanoindentation tests were conducted to clarify the mechanisms behind these trends. The nanohardness of B2 phase decreased slightly from （7.80±0.39） GPa （1320 ℃） to （7.49±0.37）  GPa （1340 ℃） and （7.62±0.15）  GPa （1360 ℃）. Conversely，γ_p phase exhibited a notable increase from （5.94±0.53）  GPa to （6.09±0.10）  GPa and further to （6.73±0.48）  GPa under the same conditions. Load-displacement curves showed that B2-phase curves nearly overlapped across temperatures，while γ_p-phase curves gradually approached those of B2 phase with increasing temperature. EDS point analysis indicated that higher ST temperatures raised W content in both B2 and γ_p phases，enhancing solid-solution strengthening and thereby increasing nanohardness. Concurrently，the volume fraction of the softer γ_p phase grew due to the elevated nucleation drive at higher temperatures. Thus，microhardness variation is governed by two competing mechanisms tied to W diffusion：solid-solution strengthening and phase-transformation-induced softening. Furthermore，the decreasing nanohardness difference between B2 and γ_p phases reflects improved micromechanical compatibility，which helps dissipate interfacial stress concentrations and promotes more uniform deformation. This work established a theoretical basis for microstructure control through W-diffusion engineering and provided a preliminary quantitative assessment of the associated strengthening effects，offering valuable guidance for the microstructural design of TiAl alloys.