工业纯钛板激光冲击形变的特征微结构

用输出波长为1064nm、脉冲宽度为20ns的调Q钕玻璃激光器对TA2工业纯钛板料进行了激光连续冲击无模弯曲形变试验,用扫描电镜(SEM)和透射电镜(TEM)分析了激光冲击变形全断面的特征微结构。根据变形区的应力状态,观察到3种主要的特征微结构。一是位于压缩应变区域的近纳米级微孪晶栅,认为是由接近同一方向的高密度层错聚集的产物;同时由于新生微结构之间的交互作用而诱发的第三类微观内应力,在基体间形成高密度的位错网络和位错胞。二是同在压缩应变区域,在超高应变率和强大的冲击能量作用下局部切变诱发的α→α′的逆相变。三是在激光冲击超高速形变条件下,受高度约束的HCP晶系材料塑性变形阻力增大,在拉伸变形区域诱发沿解理方向的局部层状集群滑移现象。上述3种现象源于激光冲击形变时材料微观约束条件和形变方式,造成形变区域微结构和硬度的不均匀性,在重复冲击条件下,不利于钛板的均匀变形。

Deformation Microstructure Characteristics of Commercial Pure Titanium Sheet Induced by Laser Shock Forming

The commercial pure titanium sheet TA2 is continually mould-free laser shock deformed by means of a Ndglass Q-switched laser setup with 1064 nm wavelength and 20 ns short pulse. The microstructure characteristics of the deformed sheet on whole section are analyzed with the thermo-field emission scanning electron microscope (SEM) and transmission electronic microscope (TEM) respectively. Three main microstructure characteristics are observed based on the stress state in deformed zone. The first is the near nanometer micro-twin gate in the compression strain region, which is thought to be a product composed of high density of stacking faults in the same direction. Due to the inter-actions between neonatal microstructures and the induced third type of internal stress, high density of dislocation networks and/or dislocation cells formed in laser shocked substrate. The second is the inverse-transformed martensite, which is induced by partial shear deformation in the compression zone of laser shock deformed sheet. The third is the local stratified cluster slips along the cleavage direction, induced by the ultra high energy and ultra-high strain rate of laser shock, in the tensile deformed region owing to the increasing of deformation resistance of highly constrained hcp crystalline material. The above-mentioned phenomena can be attributed to the confinement conditions and deformation mode of micro-zone upon laser shock forming, and will result in microscopically inhomogeneous of microstructures and hardness in the deformed section. Therefore, repeated shocking is not conductive to the uniformity of deformation of titanium sheet.