冷加工态Ti6Al4V合金的回复和再结晶行为研究

对冷拉拔变形量为60%的钛合金进行700～880℃,1～240min再结晶退火,利用金相显微镜、X射线衍射仪和透射电镜等手段分析不同状态下的组织演变、织构组成和位错组态。结果表明:冷变形后的Ti6Al4V合金经完全再结晶后α晶粒呈等轴状,β相在α相周围以条状沿α晶界析出或以小晶粒形式存在。计算表明,经60%冷变形量的钛合金再结晶激活能为107kJ/mol,较相同变形量的纯钛再结晶激活能高约50%。钛合金的再结晶分为回复、形核和晶核长大阶段,包括位错胞向亚晶转变、回复亚晶通过合并或长大形核、形核诱导高角度晶界形成而长大成新晶粒。经过冷拉拔后的丝材,存在着较强的100织构,而在再结晶过程中,沿100方向上产生的回复亚晶优先形核并长大形成新的晶粒。这导致在初始再结晶阶段,再结晶织构与冷变形织构取向一致,而在晶粒长大阶段,原先取向不利的晶粒吞并周围小晶粒长大,形成新的织构组元使原来的织构被弱化。

Recovery and Recrystallization Behavior of Ti6Al4V Alloys after Cold Deformation

The microstructure evolution,crystallographic texture and dislocation configuration during recrystallization of the cold-drawn Ti6Al4V alloy were investigated by optical microscope(OM),X-ray diffraction(XRD) and transmission electron microscopy(TEM).Recrystallization treatment in the 60% cold worked samples was carried out at various temperatures between 700 ℃ and 880 ℃ and holding time from 1 min to 240 min.The results demonstrate that recrystallization of the 60% cold-drawn alloy and subsequent grain growth have led α phase to fine equiaxed grains,while β phase distribute along grain boundaries of the α-phase in strip shape or small grains.The value of recrystallization activation energy of the alloy after 60% cold deformation is 107 kJ/mol,which is about 50% higher than that of the deformed pure titanium experienced the equal deformation.The microstructure evolution at different grain stages can be classified as recoveration,nucleation and grain growth stages.It is shown that the recovery involves rearrangement of dislocation cells into subgrains,formation of nuclei through growth or coalescence of subgrains,and growth of nuclei by high angle boundary migration during the annealing of the Ti6Al4V alloy.The as-cold drawn wires present strong <100> fiber texture.The recovery subgrains with particular crystal orientations provide preferential sites for the nucleation during subsequent recrystallization annealing and grow to be new grains.The texture at the end of primary recrystallization is consistent with the deformed state depending on the crystallographic orientation;while in the grain growth stage,misoriented grains resulting from the recrystallization mechanisms would subsequently grow by consuming neighboring small grains,so the original texture is weakened.