超细晶/纳米晶γ-TiAl基合金的流变行为研究

Study on the flow behavior of ultrafine grain/ nanocrystalline γ-TiAl based alloys

摘要:

摘要：本文采用高能球磨及真空热压烧结方法制备超细晶/纳米晶双相γ-TiAl基合金。将Ti、Al、Nb单质粉末经25h高能球磨配制成名义成分为Ti-45Al-5Nb(at.%) 的纳米级混合粉末。球磨后的混合粉末经真空热压烧结（烧结温度1200℃，压力30MPa，保温保压1h），原位合成Ti3Al 及γ-TiAl双相等轴状合金组织，烧结组织由小于500nm的等轴γ-TiAl相和纳米晶Ti3Al相组成。利用Gleeble-1500D对合金进行热压缩模拟实验，变形温度为1100℃-1200℃、应变速率10-4-10-2s-1，研究该合金压缩组织及流变行为。研究结果表明：与γ-TiAl合金微米级晶粒组织相比，超细等轴双相TiAl+Ti3Al组织明显降低了流变峰值应力，使其在2-2.5%应变量时就达到最大，流变应力随应变速率的降低和温度的升高而降低。同时建立流变应力本构方程，反映一定条件下流变过程中材料的结构特性。随温度升高γ相的孪生倾向显著增加，形变主要发生在基体γ-TiAl相中,晶界滑移、位错及孪晶为等轴双相γ-TiAl合金的高温形变机制，动态回复和再结晶为其软化制。

Abstract:

Abstract: In this study, ultrafine grain/nanocrystalline two-phase γ-TiAl based alloy was prepared by high energy ball milling and vacuum hot pressing sintering. Ti, Al and Nb single powder were mixed together to make the nanocrystalline powder with a nominal composition of Ti-45Al-5Nb (at.%). After ball milling, the nanocrystalline powder was sintered at the temperature of 1200 °C with the pressure of 30 MPa. The sintered microstructure consisted of nanocrystalline α2-Ti3Al and the equiaxed γ-TiAl with grains less than 500 nm. The hot compressive flow behavior of Ti-45Al-5Nb alloy was studied using the Gleeble-1500D thermal simulator at deformation temperatures of 1100°C, 1150°C and 1200°C and strain rates of 1×10-4 s?1, 1×10-3 s?1and 1×10-2 s?1. The results reveal that the peak stresses of equiaxed ultrafine microstructure are significantly lower than the alloys with micron-scale structure. The value of peak stress is reached at the early stage of compression (2.5-3% strain), and the flow stress decreases with increasing temperature and decreasing strain rate. A constitutive equation was established based on experimental data which reflects the structural characteristics of the alloy during hot deformation. It shows that the deformation mechanisms are mainly intracrystalline dislocation in γ-TiAl phase and intergranular twins in γ/γ (001).