Zhu Kai, Qu Shoujiang, Feng Aihan, Sun Jingli, Shen Jun
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China.
Shanghai Key Laboratory of Development and Application for Metal-functional Materials, Tongji University, Shanghai 201804, China.
Materials (Basel). 2019 Aug 6;12(15):2496. doi: 10.3390/ma12152496.
Multidirectional isothermal forging (MDIF) was used on a Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B (at. %) alloy to obtain a crack-free pancake. The microstructural evolution, such as dynamic recovery and recrystallization behavior, were investigated using electron backscattered diffraction and transmission electron microscopy methods. The MDIF broke down the initial near-lamellar microstructure and produced a refined and homogeneous duplex microstructure. γ grains were effectively refined from 3.6 μm to 1.6 μm after the second step of isothermal forging. The ultimate tensile strength at ambient temperature and the elongation at 800 °C increased significantly after isothermal forging. β/B2→α transition occurred during intermediate annealing, and α + γ→β/B2 transition occurred during the second step of isothermal forging. The refinement mechanism of the first-step isothermal forging process involved the conversion of the lamellar structure and discontinuous dynamic recrystallization (DDRX) of γ grains in the original mixture-phase region. The lamellar conversion included continuous dynamic recrystallization and DDRX of the γ laths and bugling of the γ phase. DDRX behavior of γ grains dominated the refinement mechanism of the second step of isothermal forging.
采用多向等温锻造（MDIF）工艺对Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B（原子百分比）合金进行加工，以获得无裂纹的饼坯。利用电子背散射衍射和透射电子显微镜方法研究了其微观组织演变，如动态回复和再结晶行为。多向等温锻造破坏了初始的近片层组织，产生了细化且均匀的双态组织。在等温锻造的第二步之后，γ晶粒从3.6μm有效细化至1.6μm。等温锻造后，室温下的抗拉强度和800℃时的伸长率显著提高。中间退火过程中发生β/B2→α转变，等温锻造第二步过程中发生α + γ→β/B2转变。第一步等温锻造过程的细化机制涉及原始混合相区域中片层结构的转变以及γ晶粒的不连续动态再结晶（DDRX）。片层转变包括γ板条的连续动态再结晶和DDRX以及γ相的凸起。γ晶粒的DDRX行为主导了等温锻造第二步的细化机制。
