超细晶纯钛疲劳裂纹扩展及裂纹尖端组织演变

Fatigue Crack Growth and Crack Tip Microstructure Evolution of Ultrafine Grained Pure Titanium

采用疲劳裂纹扩展实验研究剧烈塑性变形制备的4种超细晶纯钛的疲劳裂纹扩展行为,通过观察不同组织超细晶纯钛的疲劳裂纹扩展路径,并对裂纹尖端塑性区进行显微硬度分布实验和显微组织观察,分析不同组织超细晶纯钛疲劳裂纹扩展的微观机制。结果表明,等径弯曲通道变形(equal channel angular pressing,ECAP)纯钛和旋锻后400℃退火纯钛的疲劳裂纹扩展路径曲折,而ECAP+旋锻变形纯钛和旋锻后300℃退火纯钛的疲劳裂纹扩展路径平直。循环载荷作用下,超细晶纯钛裂纹尖端区域的显微硬度和微观组织变化显著。其中ECAP+旋锻变形纯钛的组织位错密度和显微硬度降低,而ECAP变形和旋锻后退火纯钛的组织位错密度和显微硬度均升高。超细晶纯钛疲劳裂纹扩展过程中组织结构对位错运动的阻碍导致裂纹扩展方向改变,且裂纹尖端区域位错密度为显微硬度变化的主要影响因素。

Pure titanium with hexagonal close packed(hcp)structure is found to have more applications in the aerospace,transporta⁃tion,biotechnology and medical implants industries due to its low density,high biocompatibility and excellent corrosion resistance.However,conventional pure titanium is of relatively low mechanical strength.The ultrafine-grained(UFG)materials processed by se⁃vere plastic deformation(SPD)techniques have received enormous scientific attention due to their unusual mechanical properties.Equal channel angular pressing(ECAP)is a kind of severe plastic deformation method which can refine grain size of metallic materi⁃als effectively and obtain bulk ultrafine grained materials by approximate pure shear deformation.Four kinds of UFG pure titanium with different microstructures were fabricated by two passes of ECAP(with a die of 135°via route C at room temperature),two passes of ECAP+rotary swaging(RS),and annealing at 300 and 400℃for 1 h after two passes of ECAP+RS.Fatigue crack growth experi⁃ment was performed under sine wave loading frequency of 10 Hz at stress ratio of R=1 using Instron electro hydraulic servo fatigue test⁃ing machine at room temperature in air to study the fatigue crack growth behavior of four kinds of UFG pure titanium.The fatigue crack growth paths of UFG pure titanium with different microstructures were observed by optical microscope and the micro-hardness distribu⁃tion experiment and microstructure observation using transmission electron microscope(TEM)in the crack tip plastic area were car⁃ried out to analyze the micro mechanism of fatigue crack growth of ultrafine grained pure titanium with different microstructures.The re⁃sults showed that microstructure of UFG pure titanium was composed of bands with a width of about 1μm and high-density dislocations in the boundary of bands,and the grain size was not uniform after two passes of ECAP.Non-uniformity in microstructure of pure titani⁃um processed by ECAP was improved by subsequent rotary swaging.The grain size was refined to about 200 nm after rotary swaging.With the increase of annealing temperature,the grain size of UFG pure titanium after rotary swaging increased and the dislocation den⁃sity decreased.The fatigue crack growth rate test results of the standard single edge notched tensile specimen(SENT)showed that the microstructure had a significant effect on the fatigue crack growth of UFG pure titanium.In the stable propagation region,fatigue crack growth formulas of four kinds of UFG pure titanium were obtained by dealing with the experimental data with Paris equation,to evalu⁃ate the coefficient C and exponent m of the Paris relation.Curve fitting gave different values of the exponent m from 3.7773 to 6.9421 for four UFG pure titanium with different microstructures.The values of exponent m indicated that resistance of pure titanium annealed at 400℃for 1 h after ECAP+RS was the strongest,followed by pure titanium processed by ECAP and ECAP+RS,and the crack resis⁃tance of pure titanium annealed at 300℃for 1 h after ECAP+RS was the weakest.The fatigue crack growth paths of pure titanium pro⁃cessed by ECAP and annealing at 400℃after ECAP+RS were tortuous indicating higher energy for crack growth,which reduced the crack growth rate.However,the fatigue crack growth paths of pure titanium processed by ECAP+RS and annealing at 300℃after ECAP+RS were flat indicating lower energy for crack growth,which hardly reduced crack driving force and crack growth rate.In the early stage of crack growth of pure titanium processed by ECAP+RS,many small cracks were observed and only one of which could be⁃come the main crack,making pure titanium processed by ECAP+RS have the highest crack initiation life.Furthermore,pure titanium processed by ECAP+RS had the highest fatigue life.Under cyclic loading,the micro-hardness and microstructure of the crack tip area of UFG pure titanium changed significantly.The micro-hardness of the crack tip area of UFG pure titanium increased with increasing of distance to the crack tip.The micro-hardness of the crack tip area was higher than that of two sides of the crack.Among four kinds of UFG pure titanium,the dislocation density and micro-hardness of pure titanium processed by ECAP+RS decreased due to rearrange⁃ment and annihilation of the dislocations,while the dislocation density and micro-hardness of pure titanium processed by ECAP and annealing after ECAP+RS increased due to generation of dislocations from crack tip.In the process of fatigue crack growth of UFG pure titanium,the obstruction of the microstructure such as the grain boundary to the movement of dislocations caused the direction of crack growth to change,and crack growth and the dislocation density in the crack tip area was the main factor affecting the change of microhardness.It showed that there was obvious microstructure dependence in the crack paths and the resistance against fatigue crack growth in UFG pure titanium.