室温ECAP变形纯钛孪生行为研究

Twinning Behavior of Pure Titanium Processed by ECAP at Room Temperature

室温下对纯钛进行多道次等径弯曲通道变形(ECAP),分别采用光学显微镜(OM)、透射电镜(TEM)、电子背散射衍射仪(EBSD)、室温拉伸和显微硬度观察,测试纯钛变形过程组织演变和力学性能变化规律,探讨纯钛室温变形机制和孪生行为。结果表明,纯钛ECAP变形过程中出现■拉伸孪晶和■压缩孪晶,随着挤压道次的增大,孪晶数量先增大后减小。孪晶的出现有效改变晶格取向,激发进一步位错滑移,辅助塑性变形过程,使纯钛显微组织有效细化,经过4道次ECAP变形,平均晶粒尺寸由约63.79μm细化至约2.81μm。1道次变形后晶粒细化效果最显著,平均晶粒尺寸比变形前减小约94%;随着变形道次的增加,晶粒细化效果减弱,4道次变形后平均晶粒尺寸累积减小约95.6%。同时,大量位错、孪晶和亚晶的形成,使得位错、孪晶以及亚晶之间的相互作用加强,显著提高了纯钛的屈服强度和显微硬度,4道次变形后,屈服强度从215 MPa增加到600 MPa,增幅为179%;显微硬度从HV 129增加到HV 200。由于1道次变形后晶粒细化效果最显著,并且出现大量孪晶和位错,屈服强度与硬度的增幅也最大。

Equal channel angular pressing(ECAP)is a kind of severe plastic deformation method which can refine grain size of metallic materials effectively and obtain bulk ultrafine grained materials by approximate pure shear deformation.Pure titanium with a composition of O 0.18,H 0.015,N 0.03,C 0.08 and Fe 0.2(%,mass fraction),and the rest was titanium,was processed by multi-passes of ECAP at room temperature.ECAP was performed at the ram speed of 2.5 mm·s~(-1),Route C,which required the sample rotation of180°around its longitudinal axis between adjacent passes.The internal channel angle and outer arc curvature angle of the die were135°and 20°correspondingly,which gave an equivalent strain of about 0.46 for each pass,and the cumulative equivalent strain of about 1.84 after 4 passes of ECAP.The microstructures including average grain size,dislocation density and deformation twinning of the longitudinal section of pure titanium during ECAP process were observed using optical microscope(OM),transmission electron microscope(TEM)and scanning electron microscope(SEM)equipped with an electron backscattering diffraction(EBSD),respectively.The mechanical properties were evaluated by tensile tests and Vickers micro-hardness tests at room temperature.It was evident from the high dislocation density in TEM images and lots of deformation twins in twinning statistics that the deformation mechanisms of pure titanium during ECAP process included dislocation slips and deformation twinning,and there were■tensile twins and■compression twins appeared during ECAP process of pure titanium.The twin type of pure titanium during ECAP process was affected by deformation temperature,strain rate,die angle,ECAP pass,grain size,impurity elements and so on.With the increase of ECAP passes,the area fraction of twins first increased and then decreased.Twinning would also affect the microstructure and mechanical properties of pure titanium during ECAP process,correspondingly.There were a few annealing twins in the original pure titanium,about 0.53%,which were concentrated in the interior of individual original equiaxed grains near the grain boundary.After 1 pass of ECAP,the number of deformation twins increased significantly,about 9.84%.After 2 passes,the number of twins continued increase to about 12.44%,which was mainly distributed in the strip and formed a certain angle with the strip boundary.Because the number of twins was very sensitive to grain size,with the increase of ECAP passes,the area fraction and size of twins decreased accordingly.After 4 passes,the number of twins decreased to about 8.68%,and the twin size decreased significantly and evenly distributed in the grain.Twinning could effectively change the lattice orientation,stimulate further dislocation slips,assist the plastic deformation process,and refine the microstructure of pure titanium during ECAP process.EBSD results showed that the average grain size before ECAP was about 63.79μm,and after 1 pass,2 passes and 4 passes of ECAP was 3.71,2.84 and 2.81μm,respectively.The grain refinement effect of pure titanium was the most significant after 1 pass of ECAP of which average grain size decreased by about 94%.With the increase of ECAP passes,the grain refinement effect decreased,and the cumulative average grain size decreased by about 95.6%after 4 passes.At the same time,the formation of a large number of dislocations,twins and subcrystals strengthened the interaction among dislocations,twins and subcrystals,which significantly improved the yield stress and microhardness of pure titanium during ECAP process.Before ECAP,the yield stress of pure titanium was about 215 MPa,and after 1 pass,2passes and 4 passes,the yield stress was about 435,500 and 600 MPa,respectively,a cumulative increase of 179%in 4 passes.The elongation decreased from 58%to 22%after 4 passes of ECAP,which was due to the high dislocation density in the microstructure after ECAP process,the reduction of plasticity of pure titanium by work hardening was more than the improvement of plasticity by grain refinement.The microhardness increased from HV 129 to HV 200 after 4 passes.Because the grain refinement effect was the most significant and there were a large number of twins and dislocations after 1 pass of ECAP,the increase of yield stress and microhardness was also the largest.With the increase of ECAP passes,on the one hand,a large number of twin interfaces hindered the movement of dislocations,a large number of dislocations piled up at the twin boundary,resulting in obvious stress concentration and significant hardening of the titanium.On the other hand,the appearance of twins contributed to the grain refinement,and the smaller the grain size was,the more the internal interface of the microstructure was,so the microhardness increased continuously.With further deformation,the number of twins decreased slightly,the increase of dislocation density decreased,and the increase of microhardness decreased.