高能喷丸制备具有表面梯度结构的Ti-6Al-4V合金的疲劳裂纹扩展行为

Fatigue crack propagation behavior in Ti−6Al−4V alloy with surface gradient structure fabricated by high-energy shot peening

通过高能喷丸(HESP)在Ti-6Al-4V合金表面成功制备梯度结构,并研究其对疲劳裂纹扩展的影响。采用光学显微镜、扫描电子显微镜、透射电子显微镜和X射线衍射仪对HESP过程中的显微组织和残余应力演变进行表征。结果表明,材料表面形成的梯度纳米结构具有220μm深度的残余压应力层。梯度纳米结构的产生能改善合金的强塑性匹配。最大残余压应力产生于次表层,且随着喷丸时间的增加逐渐增大。HESP处理有效降低裂纹扩展速率,提高疲劳裂纹扩展寿命。残余压应力可降低裂纹尖端的有效应力强度因子范围(ΔK’eff),从而产生裂纹闭合效应并延缓裂纹扩展。同时,晶粒细化引起的晶界增加、有效滑移长度减小和裂纹尖端塑性区的协同作用也会使裂纹扩展阻力增大。

A gradient structure was successfully fabricated on the surface of Ti−6Al−4V alloy by high-energy shot peening(HESP),and its effect on fatigue crack propagation was investigated.Optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffractometer were used to characterize the microstructure and residual stress evolution during HESP.The results show that a gradient nanostructure with residual compressive stress layer of 220μm in depth is formed.The generation of gradient nanostructures improves the strength−ductility combination of the alloy.Maximum residual compressive stress is generated on the subsurface,which gradually increases with the increase of shot peening time.HESP treatment effectively reduces the crack propagation rate and increases the fatigue crack propagation life.The residual compressive stress reduces the effective stress intensity factor range(△K′_(eff))at crack tip,thereby generating the crack closure effect and delaying the crack propagation.At the same time,the synergy effects of increase in grain boundaries,decrease in effective slip length and the plastic zone at the crack tip caused by the refinement of the surface grains can increase the crack propagation resistance as well.