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Abstract The decomposition of metastable β phase and microstructural evolution of a forged Ti-22Al-25Nb orthorhombic alloys were investigated by the electron microscopes and phase identification. For the near-Ti 2 AlNb alloy, the processing temperatures were super-transus (over the transition temperature of the β phase). β phase was ordered before its decomposition and cooling rate could strongly influence the precipitation of the explored Ti-22Al-25Nb alloy. Water quenching could prevent the transformation related to diffusion and air cooling hindered the occurrence of fine lenticular O phase in the matrix. Comparison with previous researches and selected area diffraction patterns revealed that there did exist a displacive precursor that provided nucleation sites for the α 2 phase during B2 → α 2 transition. Due to the transient structure and diffusive gradient of Nb component during the transformation, a "cellular structure" was assembled and quantities of equiaxed α 2 precipitates were acquired. Simultaneously allotriomorph α 2 precipitates inclined to grow into coarse plates on the boundaries, which would have a detrimental effect on the mechanical properties of the alloy. Widmanstätten structure was formed during the (B2+α 2 +O) field and massive transformation of lenticular O phase occurred in the (B2+O) field. Highlights • Effect of cooling rates on the decomposition of B2 phase in the Ti 2 AlNb-based alloy is presented. • Precipitation and microstructural evolution during slow-cooling process are discussed in detail. • The mechanism of the formation of cellular structure was put forward and verified. [ABSTRACT FROM AUTHOR]

Mo-modified Ti-22Al-25Nb alloys are sintered at 900 °C, 1000 °C, 1070 °C, and 1150 °C, followed by quenching and furnace cooling, respectively. We made comparisons of phase composition and microstructure between quenched and furnace-cooled alloys to interpret the phase transformation and microstructure evolution during cooling. The phase transformation was B2→O for the alloy cooled from B2+O region, and was B2→B2+O for those cooled from α 2 +B2+O, α 2 +B2, and B2 phase regions. Moreover, the morphology was B2+O colonies for those cooled from B2+O and α 2 +B2+O phase regions, and was Widmanstätten structure for those cooled from α 2 +B2, and B2 phase regions. Kinetics analysis indicated that the activation energy for the phase transformation of α 2 →B2 was 170 kJ mol−1 for Ti-22Al-25Nb, while it was 330 kJ mol−1 for Mo-modified Ti-22Al-25Nb. The retarded phase transformation in Mo-modified Ti-22Al-25Nb further led to curvy Widmanstätten structure when the alloy was furnace-cooled from single B2 phase region. The hardness of the Mo-modified Ti-22Al-25Nb alloys is up to 488 HV, while that of the Ti-22Al-25Nb alloys without Mo addition is less than 400 HV. • Diverse B2+O structures are observed in Mo-modified Ti 2 AlNb alloys. • Mo-rich kinky Widmanstätten structure corresponds to high Vickers hardness. • Kinetic analysis shows Mo addition inhibits the phase transformation of α 2.→B2. • The phase transformation during different cooling routes is reported. [ABSTRACT FROM AUTHOR]

Abstract The effect of dual aging treatment on phase morphology, composition, and microstructure evolution of a pre-deformed Ti 2 AlNb-based alloy were investigated in this study. The 1040 °C-pre-deformed alloys were dual aged at different temperature in O + β /B2 region followed by 750 °C-aged treatment. The experimental results show that coarse O lath and lamellar O + β /B2 were formed when the aging temperature is lower than 950 °C while globular O appeared when aging temperature is higher than 950 °C. The size and content of the O phase increase and decrease with the aging temperature, respectively. Widmanstätten O + β /B2 laths were obtained after secondary aging treatment at 750 °C when the first aging temperature is higher than 910 °C. Complete lamellar O + β /B2 precipitation and coarse O lath, in particular, were obtained in the 910 °C and 750 °C dual aged alloy with no Widmanstätten O + β /B2 laths appeared. The phase content is closely related to the aging temperature. When the 970 °C-aged alloy put into the secondary aging at 750 °C, coarse O laths which were fragmented or partially spheroidized and globular O during 970 °C-aged process were dissolved and the Widmanstätten O began to precipitate inside. Highlights • Effect of dual aging treatments on phase transformation and morphology of pre-deformed Ti 2 AlNb-based alloy was presented. • No Widmanstätten O + β /B2 laths appeared in the 910 °C and 750 °C dual aged alloy. • Dissolution and regeneration of O phase during aging treatment was discovered. [ABSTRACT FROM AUTHOR]

The effect of (6 wt%) Cr and (6 wt%) W on the oxidation behavior of Ni–8%Al alloy at 1000 °C in air was investigated. The oxidation-affected zones of Ni–8%Al–6%Cr/W alloys consist of outer oxidation zone (NiO layer) and mixed internal oxidation zone. The addition of Cr promotes the formation of the intermediate Ni(Al, Cr) 2 O 4 layer and internal Al 2 O 3 in internal oxidation zone, which provides a blocking effect and reduces the oxidation rate of Ni–8%Al–6%Cr alloy. However, the addition of W element facilitates the development of non-protective NiO and NiWO 4 oxides, which accelerates the oxidation of alloy. As a result, Ni–8%Al–6%W alloy shows poor oxidation performance. • The oxide films of Ni–8%Al–6%Cr/W consist of outer oxidation zone and mixed internal oxidation zone. • The addition of Cr promotes the formation of Ni(Al, Cr) 2 O 4 and Al 2 O 3 , providing a blocking effect. • NiWO 4 in Ni–%8Al–6%W provides diffusion paths for ions and accelerates the oxidation process. [ABSTRACT FROM AUTHOR]

The microstructure, element segregation and γ/γ′ interface characters of polycrystalline Ni 3 Al-based superalloys under conventionally solidified (~10 K/s) and rapidly solidified (~103 K/s) conditions were investigated. Cr and Fe are primarily partitioned into the γ phase while Al and Ni are into γ′ phase. However, rapid solidification rate of spray casting alloy improves the solubility of Cr and Fe elements at the expense of Al in γ′ phase, which decreases the lattice parameters of γ and γ′ phases. Moreover, it further increases the absolute value of γ/γ′ lattice misfit and contributes to the formation of (100) plane misfit dislocations in γ/γ′ interface. As a result, spray casting specimen possesses higher hardness (6.5 ± 1.0 GPa) and elastic moduli (166.4 ± 15.9 GPa) than that in as-cast alloy with lower solidification rate. • Rapid solidification rate of spray casting alloy improves the solubility of Cr and Fe elements in γ′ phase. • It further increases the absolute value of γ/γ′ lattice misfit. • The rapidly solidified spray casting alloy possesses high hardness and elastic moduli. [ABSTRACT FROM AUTHOR]

The microstructural evolution and phase transformation behavior of a designed multiphase Ni 3 Al-based superalloy in as-cast and spray casting states were investigated under thermal exposure at various temperatures. For the exposure temperatures range from 600 °C to 900 °C, the average size of γ′ precipitates in the dendritic dual (γ+γ′) areas of the two states alloys increases, and the morphology of γ′ changes to irregular shapes due to merging of neighboring particles and Ostwald ripening. After thermal exposure at 600 °C or 900 °C, bulky γ′ precipitates generate in the interdendritic regions of as-cast alloy, and the γ′ envelope develops a curved interface by aggregation. For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' from the preceding L1 0 -type martensite phases after thermal exposure at 600 °C. However, the γ′ precipitates without any microtwins form after thermal exposure at 900 °C. • For as-cast alloy, bulky γ′ precipitates generate in β phase after thermal exposure at 600 °C or 900 °C. • For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' after thermal exposure at 600 °C. • The γ′ precipitates without microtwins form in spray casting alloy after thermal exposure at 900°C. [ABSTRACT FROM AUTHOR]

Microstructure evolution, static coarsening mechanism and coarsening kinetics of a pre-deformed Ti 2 AlNb-based alloy during aging at 945 °C and 970 °C were investigated in this study. The corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results show that the pre-deformed alloy exhibited different morphologies aged at 945 °C and 970 °C. When the alloy was aged at 970 °C, the coarsening mechanism of the O precipitates was mainly controlled by the Ostwald ripening mechanism at first aging stage; after that, it was controlled by the boundary splitting mechanism and termination migration mechanism. Due to the difference in interface energy between the terminals and the long-axis direction of the O lath, diffusion process of solute atoms leads to the coarsening and fragmenting of O lath. The dimension of the O precipitates in the studied alloy continued to grow with the prolonged time during aging at 945 °C or 970 °C while the coarsening rate decreased. The static coarsening kinetic of the studied alloy was established in term of the modified Lifshitz-Slyozov-Wagner (LSW) theory. Static coarsening behavior of the pre-deformed alloy was mainly controlled by bulk diffusion at 945 °C and interfacial diffusion at 970 °C, respectively. • The thickness of O precipitates increased with aging temperature and time. • The microstructure consisted of lamellar O at 945 °C and changed from spherical to lath with aging time at 970 °C-aging. • Static coarsening behavior was controlled by Ostwald ripening, boundary splitting and termination migration mechanism. • Coarsening kinetics of the studied alloy can be established according to modified LSW theory. [ABSTRACT FROM AUTHOR]

Cyclic oxidation behavior of a polycrystalline Ni 3 Al-based superalloy under as-cast and solution treated conditions was investigated in detail. SEM equipped with energy dispersive X-ray spectrometer and high-resolution transmission electron microscopy was used for microstructure observation and mechanism analysis. Results showed that oxidation process occurred along eutectic area (γ-γ′) and dual phase area (γ+γ′)interfaces firstly for as-cast alloy and oxide scales along (γ-γ′)/(γ+γ′) interfaces spalled gradually with oxidation process proceeding. As for the solution treated alloy, though oxidation process also occurred along (γ-γ′)/(γ+γ′) interfaces, no obvious spalling of oxide scales was observed under the present conditions. Due to the precipitation of Hf-containing carbides which can increase the adhesion of scales and its segregation to Al 2 O 3 grain boundaries during cyclic oxidation to hinder outward migration of Al, leading to slower growth rate of scales. During the long-time (8–200 h) cyclic oxidation process of the solution treated alloy, O atoms continuously diffused towards the matrix along the pores in scales, resulting in thickening of Al 2 O 3 layer. Moreover, thickening of Al 2 O 3 layer also hindered the outward diffusion of Ni and a small amount of NiO was observed on surface of specimen after long-term cyclic oxidation, meaning the improvement of oxidation resistance. • The oxidation behavior occurs on the interface area firstly. • Hf-containing carbides increase the adhesion force of scale. • Solution elements and thermal expansion affect the oxidation behavior. [ABSTRACT FROM AUTHOR]