【作者】 温爱玲；

【导师】 任瑞铭；

【作者基本信息】 大连交通大学 ， 材料加工工程， 2011， 博士

【摘要】 钛及其合金具有高的比强度、优良的耐蚀性和生物相容性等优异性能,被广泛地应用于航空航天、生物医学和化学工业等领域。但是,由于其疲劳强度低、耐磨性差等缺点,大大制约了其进一步的开发与应用。因此,如何提高钛及其合金的疲劳性能成为亟待解决的重要问题。自1999年卢柯院士等提出了表面剧烈塑性变形实现金属表面纳米化的基本方法以来,有关表面自身纳米化机理、表面身自纳米化技术及其工程应用等,一直是国内外十分关注的研究热点。因其工艺简单,而且通过这种方法获得的纳米表层与基体组织连续过渡,所以被认为极具开发应用潜力。本文通过高能喷丸方法实现钛及其合金的表面自身纳米化,系统研究了表面自身纳米化对钛及其合金疲劳性能的影响规律及机理,同时开辟出提高钛及其合金疲劳极限的表面纳米化新途径,并为表面纳米化技术的工程应用提供理论指导。首先采用履带式喷丸机对工业纯钛(TA2)及其合金(TC4)进行不同时间的表面高能喷丸处理,并采用X-射线衍射分析、透射电镜和扫描电镜进行表征分析。综合考虑表面纳米化程度、表面损伤及粗糙度等因素,TA2和TC4在弹丸直径1mm、弹丸速度50m/s、喷丸时间2h条件下的高能喷丸效果最佳,得到了晶粒尺寸约为30nm的纳米表层。通过金相观察分析等研究发现了层片状组织钛合金(TC4)在高能喷丸处理过程中存在“不均匀变形机制”,即主要是通过a-Ti层片沿着两相层片间的挤压塑性流变和剪切塑性流变来完成塑性变形,而p-Ti片层变形困难,由此又进一步加剧了不稳定剪切带的不均匀变形,此不均匀变形机制是造成层片状组织钛合金损伤的主要原因。对原始试样及不同喷丸时间处理后的试样进行了旋转弯曲疲劳试验研究。结果表明,高能喷丸表面纳米化处理使TA2的疲劳极限提高了34%,由退火态的220MPa提高到295MPa。等轴晶TC4疲劳极限提高了20%,由退火态的485MPa提高到580MPa。同时,高能喷丸造成表面机械损伤及表面粗糙度的增加又强烈地抑制了纳米表层发挥其对疲劳极限的提高作用。因此如何修复或减少表面损伤,充分发挥纳米晶体组织的作用,成为表面纳米化提高疲劳性能的关键问题。在以上研究结果的基础上,本文提出了“复合喷丸”表面纳米化的新工艺方法。即,在高能喷丸纳米化之后,再用直径0.5mm的小弹丸进行20min喷丸,以此修复表层显微损伤和降低表面粗糙度。通过这种复合喷丸,使TA2的疲劳极限进一步提高到了335MPa,比未喷丸提高了52%。而等轴晶TC4的疲劳极限进一步提高到了650 MPa,比未喷丸提高了34%。此结果显示了“复合喷丸”具有显著的提高疲劳极限的效果。分析认为,提高疲劳极限的主要原因是复合喷丸改善了表面质量,包括缓减微观局部变形的不均匀性和晶体内应力集中,有效地修复内部晶体缺陷、晶界及相界损伤、乃至表面细观损伤等微观缺陷。更多还原

【Abstract】 Titanium and its alloys are widely used in aerospace, biomedical and chemical industries and other fields due to the excellent properties as high specific strength, strong corrosion resistance and superior biocompatibility. However, they also have some disadvantages as low fatigue strength and poor wear resistance etc., which greatly restricted their further development and application. Therefore, it is very important to improve the fatigue property of titanium and its alloys.Since prof. K. Lu advanced the new concept of surface nanocrystallization (SNC) which induced by surface severe plastic deformation (SPD) in 1999, the research interests of the present authors currently focus on the mechanism, technique and the industrial application. This technique has been supposed that has vast development potential because its simple process and the nanostructured surface layer achieved by means of which continues to the substrate gradually.In this paper, the SNC of titanium and its alloys was carried out by high energy shot peening (HESP), then the effect of SNC on the fatigue of titanium and its alloys coupled with the intrinsic mechanism are studied systematically. Furthermore, a new approach that improves the fatigue strength of titanium and its alloys by SNC was proposed. Above mentioned study provides theoretical guidance for the engineering application of SNC.Surface anocrystallizationg on commercial pure titanium (TA2) and its alloy (TC4) were carried out by high energy shot peening (HESP) under the various shot time using crawler shot peening equipment. The microstructures on the surface teated by HESP have been characterized by X-ray diffraction analysis, transmission and scanning electron microscopy. Based the considering synthetically SNC degree, surface damage and roughness, both TA2 and TC4 achieved optimum peening result under the condition that the diameter of the shot is 1 mm, the peening velocity of the shot is 50 m/s and duration time is 2h. The average grain size in the as-treated surface layer is about 30 nm.It was found by observation of microstructure morphology using SEM that there is non-uniform deformation mechanism in the lamellar-structured titanium alloy (TC4) during HESP, i.e., the deformations are accomplished mainly through the extrusion plastic flowing and shear plastic flowing happened on a phase along the interface of a andβ, while theβlamellar deform hardly. Thus the strain concentration in unstable shear band is more exacerbated. The non-uniform deformation mechanism is the main reason that causes mechanical damage in lamellar-structured titanium alloy during HESP.The fatigue strength of original and as-peened samples was examined by rotary bending fatigue test. The results show that the fatigue limit of TA2 has been improved about 34% after SNC, namely improved from 220 MPa of fatigue limit of annealed specemen to 295 MPa. While the fatigue strength of equiaxial structured TC4 has been improved 20%, namely improved from 485 MPa (annealed fatigue strength) to 580 MPa. Meanwhile, HESP cause surface mechanical damage and surface roughness coarsening, which restrains the positive effect of surface nanostructure on the enhancement of fatigue resistance dramatically. Therefore, how to repair or reduce the surface damage so that let the nanocrystalline structure play full role on improvement of fatigue limit becomes a key factor.Based on the above mentioned results, a combined shot peening for surface nanocrystallization (CSPN) was presented as novel process of SNC, i.e. use smaller shots (0.5 mm in diameter) to peen the surface for 20 min after HESP, aiming at repairing the surface micro-damage and lowering the surface roughness. After CSPN, the fatigue limit of TA2 has improvee further to 335 MPa,52% higher than that of the anealled spcemen. While the fatigue limit of equiaxial structured TC4 increases further to 650 MPa,43% higher than the anealled spcemen. This result indicates that CSPN has marked effect on the fatigue limit enhancement, which is the most effective method that improves the fatigue limit of titanium and its alloy by means of SNC reported so far. The reason is due to CSPN may improve the surface quality, which inclose slowing down the inhomogeneity of local micro-deformation, reduction of the internal stress concentration in the crystal, and repairing crystal defects and damage in grain boundaries and phase boundaries as well as micro-damage caused by HESP.更多还原