V和B元素对Ti-44Al-5Nb-1Mo合金显微组织及热变形机制的影响

Effects of V and B on the Microstructure Evolution and Deformation Mechanisms of Ti-44Al-5Nb-1Mo Alloys

为了探究V和B元素复合添加对β型γ-TiAl合金的显微组织和变形机制产生的影响,本工作针对Ti-44Al-5Nb-1Mo合金和Ti-44Al-5Nb-1Mo-2V-0.2B合金,进行了不同温度和应变速率条件下的高温热压缩实验,利用SEM-BSE和TEM对组织进行表征,对比分析了其变形后的显微组织,研究了添加V和B对Ti-44Al-5Nb-1Mo合金的显微组织及热变形机制的影响。结果表明,2种TiAl合金的显微组织差异较大,添加V和B可以显著改变TiAl合金对热变形的敏感性。Ti-44Al-5Nb-1Mo-2V-0.2B合金高温变形能力明显优于Ti-44Al-5Nb-1Mo合金。Ti-44Al-5Nb-1Mo合金的高温热变形以难变形片层团的偏转、变形带的产生为主,温度为1250℃时,其变形组织表现出较高的温度和应变速率敏感性,极易形成尺寸不均匀的近片层组织;对于Ti-44Al-5Nb-1Mo-2V-0.2B合金而言,升高变形温度或降低应变速率,既可以促进片层团内部的变形诱导L(α/γ)→α+γ+β/B2和γ→α相变,又可以促进α和β/B2相的球化/动态再结晶,从而大幅提高该合金的组织均匀性。基于变形参数对组织的影响规律,确定了控制Ti-44Al-5Nb-1Mo和Ti-44Al-5Nb-1Mo-2V-0.2B合金中近全片层、近双态显微组织的方法。

TiAl alloys are considered potential structural materials because of their low density,good high-temperature strength,and creep resistance.However,their low-temperature brittleness and poor high-temperature workability lead to narrow processing windows,which hinder their industrial applications,low pressure turbine blades of high-performance engines and thermal protection system for hyper-sonic space vehicles,for instance.Extensive studies on alloying and hot mechanical processes have been conducted to control the microstructure and then enhance the inherent ductility of TiAl alloys.Alloying is considered as an effective method to stabilize softening phases or refine grains to optimize microstructural homogeneity and hot workability.Thus,β-solidifyingγ-TiAl alloys represented by Ti-(40-45)Al-(2-8)Nb-(1-8)(Cr,Mn,V,Mo)-(0-0.5)(B,C)(atomic fraction,%)alloys were designed.Nb and Mo are added asβ-phase stabilizers.Meanwhile,B,C,and Y serve as grain refiners,and they are added to increase the hot workability.However,multiple phases,precipitation,and corresponding phase transformations are introduced,leading to complex flow localization and deformation incompatibility.Therefore,considerable effort has been exerted on thermo-mechanical processing to improve the microstructural homogeneity of these alloys.The interaction among work hardening,recovery,recrystallization,and multiphase transformation under different deformation conditions easily aggravates flow localization and deformation incompatibility,which are inadequately studied.Therefore,a comprehensive understanding of the deformation behavior among multiphaseβ-solidifyingγ-TiAl alloys is necessary.In this work,the uniaxial hot compressions of theβ-solidifyingγ-TiAl alloys with the nominal compositions of Ti-44Al-5Nb-1Mo and Ti-44Al-5Nb-1Mo-2V-0.2B were conducted.The microstructure of the alloys under different temperatures and strain rates was contrastively studied using SEM-BSE and TEM.The effects of V and B on the microstructural evolutions and deformation mechanisms were analyzed.The results indicated that the addition of V and B contributed considerable differences in microstructure and thermal mechanical sensibility.The Ti-44Al-5Nb-1Mo-2V-0.2B alloy showed high-temperature deformation ability.The deflection of residual lamellae and the formation of shear bands were the main deformation mechanisms,and a nearly lamellar microstructure with a nonuniform grain size was easily generated at 1250℃ for the Ti-44Al-5Nb-1Mo alloy.On the contrary,for the Ti-44Al-5Nb-1Mo-2V-0.2B alloy,the deformation-induced lamellae decomposition of lamellar(α/γ)(L(α/γ)for short)→α+γ+β/B2 andγ→αand the spheroidization or dynamic recrystallization ofαand B2 grains could be promoted with the increase of deformation temperatures and decrease of strain rates.Consequently,the microstructural homogeneity was greatly improved.Furthermore,specific deformation conditions of the microstructural control,including a nearly full lamellar and nearly duplex microstructure,were presented in this work.