激光冲击TC17钛合金诱生的微结构的特征及热稳定性

Gradient Microstructure and Thermal Stability of TC17 Titanium Alloy Induced by Laser Shock Peening

研究激光冲击(Laser shock peening,LSP)诱生TC17钛合金的表层梯度微结构特征及其热稳定性具有重要的理论与应用价值。TEM的研究结果表明:LSP处理后,靠近基体处以较高密度形变孪晶和位错为主;越接近冲击表面应变速率越高,发生孪生的原子来不及进行位置重排、孪晶数量较少,因而以高密度位错缠结、位错胞以及位错胞转变成的亚微米级亚晶为主;最表面晶粒平均尺寸由原始~43μm瞬间细化至~396nm,表面晶粒瞬间显著细化是旋转动态再结晶的结果。LSPed试样经573K/1h退火后各深度处的位错密度降低,孪晶密度/位错胞以及最表面晶粒尺寸变化不大;而LSPed试样经673K/1h退火后其各深度处的位错密度下降更明显,孪晶数量也有所下降,而位错胞和最表面晶粒平均尺寸长大明显;673K是激光冲击TC17钛合金诱生的微结构的热稳定临界温度,维氏硬度试验结果也表明当退火温度达673K后硬度值下降明显。

The gradient microstructure and its thermal stability of TC17 titanium alloy induced by ultra high strain rate deformation of LSP were systemically examined by transmission electron microscope(TEM).The microstructure near substrate consisted of high density deformation twins and dislocations;The microstructures featured of high density dislocation tangles,dislocation cells and subgrains closer to the surface and deformation twins became less prevalent due to the higher strain and strain rate which led to insufficient time for atoms to reposition to the twinned orientation to accomplish the twinning deformation;The original coarse grains with size of tens of micrometers(average grain-size 43μm)were refined instantly to hundreds of nanometers(average grain-size 396nm)in the top surface of TC17 titanium alloy after LSP.The quantitative calculation of recrystallization kinetics proved that the instant grain refinement of TC17 titanium alloy after LSP was the result of rotation dynamic recrystallization(RDR)mechanism.The LSPed TC17 titanium alloy samples were annealed at 573K,623K,673K,and 723K for 1h,respectively.Comparison of the gradient microstructures studied at different depths of the LSPed and LSP+573K/1h and LSP+673K/1h annealed specimens indicated that the dislocation density clearly decreased and the dislocation cells became clearer in the 573K/1h annealed specimen while there were no great changes in deformation twin density and the average grain size at the surface did not change greatly.After annealing at 673K for 1h,the dislocation density and deformation twins density both decreased greatly at different depths and the average grain size at the surface increased compared to those of the LSPed and 573K/1h annealed specimens.The average grain-sizes in the top surface layers of the LSPed and 573K/1h,623K/1h,673K/1h,and 723K/1h annealed samples were 396nm,422nm,493nm,1.04μm,and 2.46μm,respectively.The critical temperature at which the microstructures of the LSPed TC17 titanium alloy changed significantly was 673K;The fine gains at the top surface began to grow abnormally when the annealing temperature exceeds that temperature.There was a gradient distribution of hardness values of the LSPed surface layer.The hardness values decreased significantly when the annealing temperature reached 673K.It can be concluded that 673K was the critical temperature below which the microstructures induced by LSP were thermally stable in the TC17 titanium alloy.