Thermal Stability Of TI-45AL-2MN-2NB-0.8vol%TIB₂and TI-46AL-5NB-0.5W-1.2B

The thermal stability of a low-Nb without W addition y-TiAl alloy Ti-45Al-2Mn-2Nb-0.8vol%TiB2and a medium-Nb with low-W addition y-TiAl alloy Ti-46Al-5Nb-0.5W-1.2B have been studied at700℃in air for up to10000h and5000h, respectively. The changes in microstructure and phase structure have been characterized using scanning electron microscopy and transmission electron microscopy. Tensile testing and S-N high cycle fatigue testing have been carried out corresponding for the two alloys at room temperature.It has been found that the phase transformation α2â†'γ occurs through gradual dissolution and thinning of α2lamellae in alloy Ti-45Al-2Mn-2Nb-0.8vol%TiB2during exposure, and the rate of the dissolution and thinning is slow. Compared with high-alloyed, high strength TiAl alloys, the degree of dendritic segregation and uneven distribution of a2lamellae are observed to be less severe. No combined phase transformation of α2+γâ†'β occurs, except for occasional α2â†'β The average thickness of a2lamellae reduced to a half and the volume fraction reduced by1/4after10000h exposure. For alloy Ti-46Al-5Nb-0.5W-1.2B, the phase transformation α2â†'γ occurs through limited parallel decomposition and gradual dissolution of a2lamellae during5000h-exposure. No (3phase retained after HIPping in the alloy5Nb-0.5W alloy, indicating that the dendritic core segregation of Nb/W-riched β phase occurs only in TiAl alloy with a certain amount of Nb and W (say8Nb and1W) during solidification. The average thickness of a2lamellae reduced by20%and the volume fraction reduced by19%after5000h exposure. The phase transformation α2â†'β occurs restrictly in the later of5000h exposure. This indicates that the medium-Nb with low-W addition y-TiAl alloy is relatively stable in terms of its resistance against thermal deterioration.The tensile properties and S-N fataigue strength have been assessed at room temperature. The effects of’oxygen-release embrittlement’ and ’B2+co-caused embrittlement’are found to be less significant in the two alloys subjected to long-term exposure because of the limited decomposition of a2lamellae and restricted formation of β(B2+ω). Accordingly, The tensile strength and ductility of alloy Ti-45Al-2Mn-2Nb-0.8vol%TiB2are barely changed during exposure. The tensile strength remains at about510MPa and0.2%proof stress at about480MPa. Moreover, the S-N fatigue strength is even increased by30%after the exposure. The "thermal exposure strengthening" phenomenon in S-N fatigue is attributed to a profound stress-relaxation effect. Immersion of individual fatigue samples into a long-term exposure actually reduces the surface and bulk stress concentration, and alleviate the deteriorated influence of subsurface defects. All of these are expected to increase the resistance to microcrack initiation under cyclic loading. The tensile properties of alloy Ti-46Al-5Nb-0.5W-1.2B also remains essentially unchanged during exposure. The high degree of stability is attributed to limited decomposition of α2lamellae limited formation of β(B2+ω) phase, and lack of perpendicular decomposition of α2lamellae. S-N fatigue strength is slightly decreased after the exposure. This suggest that the "thermal exposure strengthening" phenomenon is not considerable in the W-containing alloy since only5000h thermal-exposure was experienced.