【作者】 张向东；

【导师】 董闯； Grosdidier Thierry； 郝胜智；

【作者基本信息】 大连理工大学 ， 材料物理化学， 2011， 博士

【摘要】 强流脉冲电子束材料处理是近十几年来发展起来的一种表面改性技术。强流脉冲电子束能在较短的时间内将大量的能量沉积在样品表层,在样品表层引起快速的加热,熔化甚至汽化等过程,快速加热引起的动态应力场可以在材料内部数百微米深度引发强烈改性,材料摩擦磨损和耐蚀性能获得显著提高。钛及钛合金因为比强度高,耐蚀性能好和良好的生物相容性,广泛应用于航空航天、汽车制造、生物医药等行业。但钛及钛合金面临表面硬度不高,耐磨性差,并且在某些介质中耐蚀性能较差等与表面性能相关的问题,制约了其更广泛的应用。因此有必要对其进行表面改性,以提高其整体性能。另外,钛及钛合金凝固时会发生同素异构转变,并且根据其β相稳定元素含量可能发生亚稳相变生成亚稳α’和α’’马氏体相,因此强流脉冲电子束钛及钛合金表面改性不但可以加深对强流脉冲电子束改性机理(如固态相变方面等)的认识,也可以探索强流脉冲电子钛合金表面改性潜在的工业应用意义。镍基单晶合金结构简单,不存在大角晶界,选择镍基单晶材料进行强流脉冲电子束处理,有助于加深对强流强流脉冲电子束表面和深层改性与初始晶体取向之间关系的认识,从而对推动对强流脉冲电子束改性机理的认知有重要意义。基于上述目的,采用强流脉冲电子束对钛合金(商业纯钛,TA15近α钛合金和亚稳β钛合金β-Cez)和不同取向的几种镍基单晶高温合金(AM1(100),AM1(111)和CMSX-2(100))进行了表面改性处理,对处理后试样的微观组织结构及性能进行了分析,主要结论如下：(1)揭示了近α钛合金TA15的强流脉冲电子束处理后的表层组织转变机理,发现最终表面组织为均匀单相六方α’马氏体。原始近α钛合金TA15组织含有α相和少量β相,经过强流脉冲电子束处理后样品表面熔化并伴有元素蒸发,冷却过程中发生了如下相变：α+βresidual→β→L→β→α’。而对于未发生熔化的热影响区来说,相变顺序为α+βresidual→β+βresidual→α’+βresidual。由于表面形成均匀单相α’马氏体组织,样品在5 wt% NaCl水溶液中腐蚀速率降低和腐蚀电阻升高。(2)β单相和α+β双相β-Cez钛合金表面最终均生成了由应力诱发的正交α’’马氏体组织。利用应力诱发马氏体相变的触发应力与温度的关系对强流脉冲电子束处理产生的应力的级别进行了估算,结果显示强流脉冲电子束处理产生的应力在GPa量级。(3)纯钛样品表面亦生成单相细晶α’马氏体,表面硬度升高,磨损性能提高,并由于去除表面夹杂,样品在5 wt％NaCl水溶液耐蚀性能显著改善。4)发现了镍基单晶合金的表面熔层的单晶外延凝固现象。由于强流脉冲电子束处理引起的应力作用,样品表面出现了许多滑移线。截面透射电镜分析和电子背散射衍射标定分析结果显示,熔化层与未熔基体晶体取向差呈线性连续增长,但是AM1(111)的斜率是AM1(100)的2倍。这是由于立方晶体择优生长方向为<100>,沿<111>则会引入较多的晶体缺陷。两种取向的样品截面硬度显现出明显差异。对AM1(100)样品来说,次表层出现了一个硬化区,这是由于处理产生的大量位错强化所致：而对AM1(111)样品来说,次表层出现了一个软化区,这是由于处理使析出相粗大所致。5)总结出了镍基单晶合金表面熔坑的形成及演化规律,发现CMSX-2镍基单晶合金表面熔坑密度随轰击次数的增加而降低,且CMSX-2单晶合金熔坑密度比普通多晶金属的熔坑密度低一到两个数量级。根据熔坑的形态提出了熔坑形成过程的不同阶段：首先在材料亚表层生成熔化液滴,然后液滴向表层膨胀、溢出乃至喷发,形成中心有孔的熔坑。更多还原

【Abstract】 In recent decades, high current pulsed electron beam (HCPEB) has been developed as a new surface modification technique. HCPEB acting on a material can deposit abundant energy within a very short time and generate transient heating, melting and evaporation of the surface layer to which the energy has been delivered. The dynamic stress fields induced by the rapid heating process lead to intense modifications that can extend several hundreds of microns in depth in the material. Correspondingly, improvements of the wear and corrosion resistances can also be achieved.Titanium and titanium alloys, due to their good strength to density ratio, corrosion resistance and biocompatibility, are widely used in applications such as biomedical, aerospace and automotive fields. However, titanium and titanium alloys are all facing some serious surface-related disadvantages, such as soft surface, poor wear or corrosion resistance, that have strongly limited their further industrial applications. Therefore, surface modification techniques should be applied on titanium and titanium alloys in order to improve their global performance. On the other hand, titanium and titanium alloys exhibit allotropic phase transformation, and may undergo metastable phase transformation under rapid solidification leading to the formation of metastable martensitic phase a’or a" with respect to their content ofβstabilizers. Thus, we can not only get further understanding of the solid phase transformation bahaviors induced HCPEB treatment, but also can explore the potential industrial applications of HCPEB surface treatment on titanium and titanium alloys. Due to the simplicity of the single crystal structure and the elimination of large-angle grain boundaries, the HCPEB surface treatment on Ni-base single crystal alloys can help to understand the modification mechanisms both in the surface melted layer and in depth with respect to the initial crystal orientations. It is therefore of grate interest in achieving a better understanding of the HCPEB technique.To this end, titanium alloys (commercially pure Ti, a near a Ti alloy TA15 and aβmetastable Ti alloyβ-Cez) as well as Ni-base single crystal alloys with different orientations (AM1 (100), AM1 (111) and CMSX2 (100)) have been selected and investigated. The main conclusions are summarized as follows: 1) The transformation mechanism of the microstructure in the surface layer of the HCPEB treated near a titanium alloy TA15 has been examined and the results indicated that a single hexagonal a’martensite structure was formed in the surface melted layer.It was found that the top surface layer, that has been melted and partially evaporated during the HCPEB treatment, rapidly solidified asβand transformed into monolithicα’ martensite on following rapid cooling. The transformation sequence in the melted layer was determined to beα+βresidual→β→L→β→α’, while that in the unmelted HAZ layer was a +βresidual→β+βresidual→α’+βresidual. Due to the formation of uniform monolithic ultrafine phase in the treated surface layer, the corrosion rate decreased and the corrosion resistance increased in 5 wt% NaCl aqua solution.2) After HCPEB treatment, the orthorhombicα’’ martensite phase can be examined in the surface layer of both the single-β-phase andα+βdouble-phaseβ-Cez Ti alloys. The stress level induced by HCPEB treatment is experimentally estimated using the triggering stress for stress-induced martensitic phase transformation. The estimation indicated that the stress level induced by HCPEB irradiation can reach GPa magnitude.3) Singleα’ martensite phase was also achieved in the surface layer of the HCPEB treated pure Ti samples. The surface hardness and the wear resistance of treated samples were improved. The corrosion resistance in 5 wt.% NaCl aqua solution was also improved probably due to the removal of impurities under HCPEB.4) It was found that the surface melted layers of the HCPEB treated Ni-base single crystal superalloys solidified in an epitaxial manner. Many slip lines were present on the treated sample surfaces, which may be due to stresses generated by HCPEB treatment. TEM and EBSD cross section analysis revealed that, a linear increase of misorientation angles in the melted layer relative to the substrate were observed with the slope of for the treated AM1 (111) sample doubled that for the treated AM1 (100) sample. The faster increase of misorientation angle for the AM1(111) sample is due to that the preferred growth orientation for cubic metals is the<100> direction and more defects can be introduced when solidified along the <111> direction. Significant difference in cross section hardness between the AM1 (100) and AM1 (111) samples were also observed. For the AM1 (100) sample, a subsurface hardened zone was found, which is clearly due to the presence of high density dislocations induced by the stresses generated by the HCPEB treatment. For the AM1 (111) sample, however, a subsurface softened zone was observed. This subsurface softening can be attributed to the coarsening of the precipitates resulting from the HCPEB treatment.5) A morphological analysis of crater formation and evolution in the Ni-base single crystal alloy CMSX-2 induced by HCPEB treatment has been carried out. It was found that the crater density of the single-crystal superalloys decreased as the number of HPCEB pulse increased; moreover, the crater density of CMSX-2 single crystal alloy is one or two orders of magnitude than that of the previously studied polycrystalline metallic materials. The different stages of crater formation have been proposed according to the crater morphologies. Firstly, the melted liquid droplet formed beneath the surface; then the melted droplet expands, overflows and finally erupts towards the treated surface, causing the formation of craters with a hole in center.更多还原