Enhancement of Strength and Fatigue Properties of Pure Titanium via Optimized ECAP Processing

采用多道次等径角挤压、锻造和拉拔工艺制备了单调强度超过1GPa的纯钛。研究了ECAP工艺路线、道次和ECAP后退火等工艺因素对金属和合金组织和力学性能的影响，特别是对疲劳性能的影响。通过TEM、EBSD和X射线分析对ECAP制备的材料的结构进行了评估。结果表明，不同途径制备的样品之间的结构差异随着压制次数的增加而减小。工艺路线对低周和高周疲劳性能几乎没有影响，而对晶粒形状的影响很大。因此，路线A（在后续道次之间不旋转工作坯料）可被推荐用于实际实施。在有效应变为8-30的剧烈塑性变形过程中，晶粒尺寸减小到150-300 nm的纳米尺度，并且根据材料、施加的应变量（ECAP压制次数）和ECAP后退火，hangangle晶界的分数达到65%。许多材料在高循环状态下的疲劳性能的改善已经被证明。对于纯钛，经过优化的剧烈塑性变形处理后，疲劳极限已达到550 MPa，并可以通过退火进一步提高。结果表明，除了Hall-Petch强化外，位错累积对力学性能的影响也很显著。此外，对于沉淀硬化材料，夹杂物颗粒的作用对于结构稳定和进一步提高机械性能是重要的。

Pure Titanium with the monotonic strength exceeding 1 GPa has been manufactured via multi-pass ECAP followed by forging and drawing. The role of processing factors such as ECAP route, number of passes and post-ECAP annealing on the structure and resultant mechanical properties and, particularly, in fatigue of metals and alloys has been investigated. The structure of ECAP-produced materials has been assessed by means of TEM and EBSD and X-ray analysis. It was shown that the structure difference between the samples produced by deifferent routes diminishes with increasing number of pressings. The processing route exerts little or no effect on both low-and high-cyclic fatigue properties, while the effect on the grain shape is very strong. Therefore, route A (no rotation of the working billet between subsequent passes) can be recommended for practical implementation. During severe plastic deformation to the effective strain of 8-30, the grain size is reduced to nanoscopic scale of 150-300 nm and the fraction of hghangle grain boundaries reaches of 65%, depending on the material, amount of imposed strain (number of ECAP pressings) and post-ECAP annealing. The improvement of fatigue properties in the high cycle regime has been demonstrated for many materials. For pure titanium the fatigue limit has reached 550 MPa after optimized severe plastic deformation processing and can be further improved by annealing. Hardening mechanisms were studied and it was shown that besides the Hall-Petch strengthening, the effect of dislocation accumulation on mechanical properties is significant. Furthermore, for precipitation hardenable materials the role of inclusion particles is important for structure stabilization and further enhancement of mechanical properties.