再结晶组织对TA2纯钛绝热剪切行为的影响

Effects of recrystallized structures on adiabatic shear behaviors of TA2 pure titanium

使用二辊轧机对TA2工业纯钛进行多道次大应变冷轧处理,制备了冷轧总变形量为70%的TA2纯钛板。通过对冷轧TA2纯钛板进行500℃加热、不同保温时间的退火处理,获得了具有不同再结晶组织的钛板。基于帽形试样和限位环变形控制技术,在分离式霍普金森压杆装置上对不同再结晶组织的试样进行动态冲击冻结实验,结合光学显微镜和扫描电子显微镜表征试样冲击前后微观组织的变化,研究了再结晶组织对TA2纯钛绝热剪切行为的影响。结果表明,随着退火保温时间的延长,试样再结晶晶粒占比逐渐增大,晶粒分布由分散向局部聚集转变;在相同应变和应变率下,在所有试样中都观察到了绝热剪切带,再结晶晶粒占比高的试样更易诱发绝热剪切带中裂纹形核扩展。对比变形前后试样再结晶组织和几何必需位错变化,结合剪切区整体温升分析发现,再结晶晶粒作为材料软化点能够诱发剪切带的形成,而剪切带发展后期产生的绝热温升会促进剪切带内材料发生二次再结晶,提高剪切带内材料的韧性,延缓剪切裂纹的形成。

Adiabatic shear is a common form of deformation and failure of materials under high-speed impact loading.It generally exists in high-speed deformation processes such as high-speed impact,stamping forming,projectile penetration,high-speed cutting,and explosive crushing.A TA2 pure titanium plate with a total deformation of 70%was obtained by multipass large strain cold rolling on a two-high mill.By heating cold rolled TA2 pure titanium plates at 500℃and annealing at varying holding times,titanium plates with different recrystallization structures were produced.Based on a hat-shaped specimen and a limit-ring deformation control approach,dynamic impact freezing experiments were carried out on the specimens with different recrystallized structures by using a split Hopkinson pressure bar.The microstructure changes of the specimens before and after impact were characterized by using an optical microscope and a scanning electron microscope.The effects of recrystallized structures on adiabatic shear behaviors of TA2 pure titanium were studied,showing that with the increase of annealing holding time,the proportion of recrystallized grains increases gradually,and the grain distribution changes from dispersion to local aggregation.Under the same strain and strain rate,adiabatic shear bands were observed in all specimens.The specimens with high proportion of recrystallized grains are more likely to induce crack nucleation and propagation in adiabatic shear bands.The changes of recrystallization structures and geometric necessary dislocations before and after deformation were compared.Combined with the analysis of the overall temperature rise in the shear area,the recrystallized grain as the material softening zone can induce the formation of shear band.The adiabatic temperature rise effect mainly occurs in the later stage of the development of shear band,which promotes the secondary recrystallization of materials in the shear band,improves the toughness of materials in the shear band and delays the formation of shear cracks.