【作者】 胡伟；

【导师】 黄泽文；

【作者基本信息】 西南交通大学 ， 材料科学与工程， 2014， 硕士

【摘要】 本文系统研究了低铌无钨合金Ti-45Al-2Mn-2Nb-0.8vol.%TiB2和中铌低钨y-TiAl合金Ti-46Al-5Nb-0.5W-1.2B在大气环境中、700℃分别进行10000小时和5000小时热暴露处理后的热稳定性。采用扫描电镜和透射电镜等手段研究了此两种合金在整个热暴露过程中的微观组织变化规律,并在室温下进行了相应的拉伸和疲劳测试。研究发现：在长期的高温大气热暴露过程中,合金Ti-45Al-2Mn-2Nb-0.8vol.%TiB2主要通过渐进式的α2层片减薄模式实现α→γ相变,且α2层片的分解、转化、消失的趋势较弱。相对于高合金含量的TiAl合金,该合金的枝晶偏聚不严重,α2层片不平衡分布的趋势较小。除了极个别的α2→β外,没有发生融合型α2+γ→β的相变,在10000小时后其层片厚度约为热暴露前的一半,体积分数下降约为四分之一。合金Ti-46Al-5Nb-0.5W-1.2B在整个热暴露过程中,α2层片主要通过有限的平行分解模式和渐进式的减薄模式实现α2→γ相变。该合金并无残留β相,这说明只有超过一定量的Nb、W才能导致枝晶核心出现富含重金属元素的β相偏聚,在5000小时后,α2层片的厚度降低了20%,体积分数降低了19%,同时在α2板条端头和等轴α2晶粒处发生α2→β相变,该合金具有较高的组织热稳定性。对此两种合金的力学性能评估发现：合金Ti-45Al-2Mn-2Nb-0.8vol.%TiB2在10000小时、700℃的热暴露后,拉伸强度变化不大,维持在510MPa左右,条件屈服强度维持在480MPa左右,塑形降低不严重,这是由α2层片在热暴露全过程中的有限分解所致,相应的“释氧脆化”和“B2+ω共生脆化”的影响也较弱。热暴露后的S-N疲劳强度反而增加了30%。疲劳强度的热暴露增强现象是因为热暴露导致的应力弛豫效应所致。长期的热暴露导致样品表层的应力值降低,次表层的缺陷和微裂纹钝化,内部铸态α-γ层片间的应力集中降低,这些均有利于提高合金抵抗疲劳微裂纹萌生的抗力。合金Ti-46Al-5Nb-0.5W-1.2B在整个5000小时热暴露过程中,拉伸性能表现出良好的稳定性,这是因为α2层片分解有限,B2相的析出较少,且α2+γ层片晶团中无垂直分解现象。热暴露后的S-N疲劳强度稍有下降,没有出现上述的热暴露增强效应,这可能与该合金仅在700℃的热暴露5000小时有关。更多还原

【Abstract】 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 α2→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+γ→β occurs, except for occasional α2→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 α2→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 α2→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.更多还原