700℃热暴露对β凝固γ-TiAl合金表面组织及形貌的影响

Influence of Thermal Exposure at 700℃ on the Microstructure and Morphology in the Surface of β-Solidifyingγ-TiAl Alloys

研究了700℃恒温热暴露对β凝固Ti-43.5Al-4Nb-1Mo-0.5B合金表面形貌与反应层组织的影响。结果表明:含有β0相的β凝固γ-TiAl合金热暴露反应产物、表面形貌和反应界面组织随着时间不断变化,TiO_(2)-R、α-Al_(2)O_(3)、Ti_(2)AlN和Nb_(2)Al等反应产物含量持续增多,初期出现的κ-Al_(2)O_(3)在热暴露200 h后转变为α-Al_(2)O_(3),而亚稳Ti_(4)O_(7)和TiAl_(2)O_(5)在200 h后出现。不同物相氧化行为差异导致反应界面组织明显不同,高Al含量γ相易局部氧化,形成连续Al_(2)O_(3)反应层,低Al含量β0相易内氧化而形成TiO_(2)弥散分布组织,TiO_(2)颗粒持续长大成连续反应层,其与局部氧化交替作用而形成TiO_(2)-Al_(2)O_(3)交替的层状组织。相比弥散反应层,连续反应层可阻碍O元素向基体扩散,降低TiO_(2)生长速率,因此,γ相表面反应物细小致密,α_(2)相反应物先迅速长大后缓慢生长,而β_(0)相表面反应物不断长大。

β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B has attracted considerable attention owing to its higher strength and excellent creep resistance at elevated temperature.Indeed,its application temperature is much higher than that of Ti-48Al-2Cr-2Nb.Becauseγ-TiAl alloys are exposed to air at elevated temperatures for a long time during application,an oxidation layer is formed in the surface.The oxidation layer,which is potentially harmful to the mechanical properties of the crack nucleation sites,was observed near the surface.Concerning theβ-solidifying Ti-43.5Al-4Nb-1Mo-0.5B,it has median Nb content and low Al content.Additionally,a considerable β_(0)-phase with lower Al content is retained.To better understand the influence of the composition and microstructure on the oxidation behavior ofγ-TiAl alloys,it is necessary to investigate the oxidation behavior and microstructure evolution in the surface ofβ-solidifyingγ-TiAl alloys during thermal exposure.In this study,samples ofβ-solidifying Ti-43.5Al-4Nb-1Mo-0.5B were obtained by investment casting and thermal exposure at 700°C for different times,and the oxidation behavior and microstructure of different phases in the surface were compared.The results showed that the constituents of the oxidation layer on the surface varied with the exposure time.The volume fractions of TiO_(2)-R,α-Al_(2)O_(3),Ti_2)AlN,and Nb_(2)Al increased by increasing the exposure time.Metastableκ-Al_(2)O_(3)was detected in the sample exposed for a short time,but it was transformed intoα-Al_(2)O_(3)after exposure for 200 h.Moreover,metastable Ti4O7 and TiAl_(2)O_(5) were detected in samples exposed for 200 and 500 h.The microstructures,morphologies,and heights of oxidations in the surface of a specific phase are different,varying by increasing the exposure time.These variations are related to the different oxidation behaviors during thermal exposure,i.e.,theγ-phase experienced selective oxidation after a short time exposure,α2-phase changed from internal oxidation to selective oxidation when the exposure time reached 200 h,while theβ0-phase suffered internal oxidation during the entire exposure.The different oxidation behaviors of each specific phase contributed to the different Al contents.Dispersed TiO_(2)was formed during internal oxidation,and it kept growing during thermal exposure,forming a continual layer at the end.The continual Al_(2)O_(3)layer was formed during selective oxidation,in which the Ti element was rejected in the reaction interface.When the content produced during the internal oxidation of the Ti element reached a critical value,dispersed TiO_(2)was formed and kept growing to form the continual layer.The alternating formation of continual Al_(2)O_(3)and TiO_(2)layers resulted in the layer structure observed in the surface.