《材料腐蚀与电化学测试技术》作业-论文翻译

《材料腐蚀与电化学测试技术》作业

材料与能源学院材料工程xxx学号xxx

文章来源Materials Science and Engineering C 57 (2015) 309-313

Microstructure and corrosion behavior of laser processed NiTi alloy

激光加工镍钛合金的微观结构与腐蚀行为

Jithin J. Marattukalam a, Amit Kumar Singha, SusmitDatta b, MitunDasb, Vamsi Krishna Balia b,*, SrikanthBonthac, Sreeram K. Kalpathya,**

a Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar P.0.575 025, India

bBioceramks Coating Division, CSIR-Central Class and Ceramic Research Jnstitute, 196 Raja S.C Mullick Road, Kolkata 700 032, India

c Department of Mechanical Engineering, National Institute ofTechnology Karnataka, Surathkal, Srinivasnagar P.0.575 025 India

ABSTRACT

Laser Engineered Net Shaping (LENS?), a commercially available additive manufacturing technology, has been used to fabricate dense equiatomicNiTi alloy components. The primary aim of this work is to study the effect of laser power and scan speed on microstructure, phase constituents, hardness and corrosion behavior of laser processed NiTi alloy. The results showed retention of large amount of high-temperature austenite phase at room temperature due to high cooling rates associated with laser processing. The high amount of austenite inthese samples increased the hardness. The grain size and corrosion resistance were found to increase with laser power. The surface energy of NiTi alloy, calculated using contact angles, decreased from 61 mN/rn to 56 mN/m with increase in laser energy density from 20 J/mm2 to 80J/mm2. The decrease in surface energy shifted the corrosion potentials to nobler direction and decreased the corrosion current. Under present experimental conditions the laser power found to have strong influence on microstructure, phase constituents and corrosion resistance of NiTi alloy. 摘要

激光近净成型（LENS?），一种用于商业上的的加工制造技术，已被用来制备致密的NiTi合金。这项工作的主要目的是研究激光功率和扫描速度的对显微结构、相组成、激光加工NiTi合金的硬度和腐蚀行为的影响。结果表明，在室温下，大量的高温奥氏体相由于激光处理的高冷却速率被保留下来。在样品中，大量的奥氏体增加了合金的硬度。晶粒尺寸和耐腐蚀性发随着激光功率的增加而提高。利用接触角计算可以发现，当增加激光的能量密度从20 J /平方毫米到80j /平方毫米，NiTi合金表面能从61mN/rn降至56 mN/m。表面能的降低使腐蚀电位更高、腐蚀电流降低。在本实验条件下，激光功率对NiTi合金的相组成、耐蚀性、显微结构组织有强烈影响。

1. Introduction

The expanding range of applications of near-equiatomicNiTi alloys in biomedical and other industrial sectors is now well accepted [1-4]. In particular, the commercial variant known as Nitinol (Ni50.8Ti49.2 in atomic proportions) has gained preference over standard engineering alloys like stainless steel and Ti for medical devices. The most perceptible attributes of

Nitinolinclude in vivo and in vitro biocompatibility [5-7], shape memory effect [8], superelasticity [9], and corrosion resistance [1,9]. Especially, the biocompatibility of the alloy has been found to be superior in both dense and porous forms [7]. The use of porous NiTi alloys also reduces the modulus mismatch (between the implant and the natural bone tissue) and allows living tissue to grow inside the pores thus improving overall implant stability [10,11].Manetal. [11] have used NiTi components and demonstrated improvement in osteointegration by depositing porous surface layers on them. Bernard et al [12] studied the anodization of the NiTi alloy surfaces using H2S04 electrolytes with varying pH with an aim to enhance the biocompatibility and bone-cell materials interaction of the alloy. The use ofNitinol foams for bone implants has been reviewed comprehensively by Bansiddhi et al. [6]

引言

原子比为1：1的NiTi合金在生物医学和其他工业领域的应用范围不断扩大，逐步为人们所接受[1-4]。特别是，商业上，镍钛记忆合金（NiTi原子比例50.8：49.2）已经像不锈钢和钛在医疗器械的应用一样，成为了优先选择的标准的工程合金。镍钛合金最明显的特点包括体内和体外生物相容性[5-7]、形状记忆效应[8] [9]、超弹性和耐腐蚀[1,9]。尤其是，不管是致密还是多孔的形式，该合金的生物相容性都是优越的。多孔的NiTi合金的使用减少了植入的物体与骨组织之间的弹性模量不匹配，允许活体组织在孔内生长，从而全面提高了孔内植入物的稳定性[10,11]。Manetal.[ 11 ]通过多孔表面层上的沉积和NiTi合金成分的改变，改善骨整合时的效果。伯纳德等[ 12 ]研究了不同pH值的硫酸电解液的使用对NiTi合金表面的阳极氧化的影响，目的是提高生物相容性和减少合金与骨细胞的排斥。多孔镍合金作为骨植入物已被bansiddhi等人详细评述[6]。

In spite of significant research on the functional properties of NiTi alloys, processing of these alloys economically to create near-net shape components is still a challenge. Conventional processing methods such as metal working and powder metallurgical routes are highly susceptible to oxygen and carbon contamination, and segregation of secondary phases [13,14]. Several alternative methods such as photolithographic techniques [15], thermal spray method for foils [13,16,17] have been used to produce NiTi alloys in different forms and shapes depending on the purpose and application. Some of these methods also involve certain handling difficulties. For example, in thin film fabrication techniques, homogeneity in the product cannot be obtained if elemental powders of Ni and Ti are used as starting materials [18]. Besides, the films deposited are generally amorphous and may require further heat treatments to recrystallize them [19]. Likewise the thermal spray method for fabrication of Nitinol films may require post-processing such as rolling or HIP to eliminate the porosity. In addition, these sprayed foils have been found to contain high oxygen contents [20].

在对镍钛合金功能特性的研究具有重要意义，如何用经济的方法加工这种合金的近净成形部件仍然是一个挑战。传统的处理方法，比如金属加工、粉末冶金，对氧和碳的污染高度敏感，易形成偏析 [13,14]。几种可供选择的方法，如光刻技术[15]、[ 13,16,17 ] 热喷涂技术，已用于根据不同的目的和应用生产不同形式和形状的镍钛合金。其中一些方法还存在一些的技术问题。例如，对于热喷涂技术，如果用Ni和Ti元素粉末作为原料，[ 18 ]得到的产品将会不均匀。此外，沉积的薄膜一般是非晶态的，可能需要进一步的热处理使之再结晶[19]。对于用热喷涂方法制备的镍钛合金薄膜的需要进一步的后处理，如清除气孔。此外，这些喷涂产生的薄膜已被发现有较高的氧含量[ 20 ]。

A promising alternative that could address many of the drawbacks of conventional processes is the Laser Engineered Net Shaping (LENS?) technique (an additive manufacturing technology)* This technique facilitates efficient realization of a 3-D CAD model into an actual three- dimensional object fabricated to near-net shape. Fabrication of chemically homogeneous bulk components of NiTi, without any undesirable secondary phases has been reported previously using this method [10]. In addition, the process offers the flexibility to deposit porouslayers of the alloy into the final structure for desired functional properties *11,21 +. As LENS? technology involves melting of metal using laser beam followed by rapid solidification, the final solidified structure would be very sensitive to the laser power and the scan rate of laser beam. Therefore, an investigation on the role of these process parameters on the microstructure of as-deposited alloys is essential. Recently Shiva et al. [22] have studied the influence of composition in fabrication of Ni-Ti structures using laser based rapid manufacturing. However，a systematic analysis of the effect of process parameters is not dealt with. In this present work, we have analyzed the microstructure of dense NiTi alloy as a function of the laser processing parameters. The corrosion behavior of the alloy has also been evaluated considering their biomedical applications.

激光近净成形（一种增材制造技术）可以克服许多传统工艺的缺点，是一个很有前途的替代技术。本技术促进了将三维CAD模型的转化成成一个真实的三维物体。用化学成分均匀的NiTi合金制造这种模型无任何不良的第二相。已经有人在实验室使用这种方法[ 10 ]。此外，该技术可以制备造型灵活的多孔的合金，使最终的结构能满足所需的功能特性[ 11,21 ]。LENS?技术就是使用激光束快速使金属熔化然后使之凝固，激光功率和激光束的扫描速度对最后的凝固的合金结构影响很大。因此，研究这些工艺参数对沉积的合金显微组织的影响是必不可少的。最近Shiva 等人[22]研究了成分对利用激光快速制造镍钛合金的结构的影响。然而，整个过程去缺少一个系统的工艺参数的分析。在这篇文章中，我们分析了激光加工致密NiTi合金的显微组织的工艺参数。考虑其生物医学应用，我们对合金的腐蚀行为也进行了评估。

2. Materials and methods

Net-shape cylindrical samples of ф12 mm and 40 mm in length were fabricated using Laser Engineered Net Shaping (LENS?, MR7, Optomec Inc. Albuquerque, NM) process equipped with Ytterbium-doped fiber laser. The feedstock was equiatomicNiTiprealloyed spherical powders (ATI Powder Metals, Oakdale, PA) with size range between 50 and 150 |лп. The composition of the powder was 55.2% Ni, 0.05% O, 0.006% N, 0.007% C and balance Ti. Several samples were deposited using a constant feed rate of 4.36 g/min at different laser powers (P) of 200 W, 300 W and 400 W and scan speed (v) of 10 mm/s and 20 mm/s. Argon controlled atmosphere where the oxygen content was less than 10 ppm was used to avoid oxidation of NiTi alloy samples. Table 1 shows process parameter combinations used in this study, and the corresponding values of laser energy density (Я), which were calculated as follows: where P is the laser power, v is the scan speed and d is the laser beam diameter (0.5 mm). 2材料和方法

研究人员采用光纤激光器用激光近净成形技术（LENS?, MR7, Optomec Inc. Albuquerque, NM）制备了直径为12毫米和40毫米的圆柱状试样。原料是 Ni和Ti原子数相等的合金粉末（ATI Powder Metals, Oakdale, PA），大小范围为50到150|лп。粉末含有55.2%镍，0.05% O，0.006% N，0.007% C，其余的物质是钛。在不同的激光功率（200 W，300 W和400 W）下，给料速度为4.36克/分钟，扫描速度（V）为10 mm / s和20mm/s，制备了几个样品。氩气产生的

保护气氛使得氧含量小于10 ppm，避免NiTi合金试样氧化。表1显示的是实验时的工艺参数。激光能量密度（Я）方法计算如下：

其中P为激光功率、V是扫描速度，D是激光束的直径（0.5毫米）。

The relative density of the samples was measured using Archimedes principle. The weight of the samples ( WT ) was measured in air at room temperature (RT). Then the samples were immersed in water and the weight (W2) was measured at RT. The relative density of the sample was calculated as follows:

利用阿基米德原理测定样品的相对密度。在室温下（RT）测定的样品的重量。然后计算室温下将样品浸泡在水中的重量（W2）。样品的相对密度的测定如下：

where density of water at RT is 1 g/cm3 and 6.45g/cm3 is the theoretical density of NiTi alloy used in the present work. The weight of the samples was measured using high-precision electronic balance with 0.1 mg resolution.

在室温下的水的密度为1 g/cm3，6.45g/cm3是NiTi合金在目前的工作环境中的理论密度。使用高精度的电子天平（分辨率为0.1毫克）进行样品的重量测定。

Thin disks of 3 mm thickness were sectioned from the cylindrical samples for testing and characterization. For microstructural observation, the disk samples were grinded using series of SiC emery papers of grades 1/0, 2/0, 3/0r and 4/0 respectively. The samples were then polished on a rotating disk covered with a velvet doth using aluminaslurry as an abrasive. These polished samples were etched using a reagent comprising 46 ml water, 3 ml HN03 and 1 ml HF for 1 min. The microstructures of the samples were observed using scanning electron microscope (jSM-6380LA,JEOL, Japan) equipped with energy dispersive X-ray spectrometer (EDAX) (JED-2300 Series, JEOL, Japan), The grain size of the samples was measured on microstructures taken using SEM, The microhardness of laser processed samples was measured using Vicker'smicrohardness tester (ESEWAY, W4303, UK) at a load of 300 g for 15 s. The hardness was measured at different locations on at least 3 identical samples and average of 10 measurements was reported. X-ray diffraction (XRD) was carried out using an X-ray diffractometer (X'Pert Pro MPDfPANalytical, USA) with Cu-Kot radiation, operating at 40 kV and 30 mA. The 2? angle was varied from 20° to 100° with a scan rate of 2° per minute.

研究人员从测试用的圆柱形样品切出3毫米厚的薄片。显微结构观察，将样品表面打磨光滑使用的SiC砂纸等级分别是1 / 0，2 / 0，3 / 0和4 / 0。然后将这些样品放在一个用天鹅绒覆盖的旋转盘上抛光，用氧化铝浆料作为研磨剂.。这些抛光样品用的试剂含有46毫升水、3毫升硝酸、1毫升HF。刻蚀时间为1分钟。用配有X射线能谱仪（EDAX） (JED-2300 Series, JEOL, Japan)的扫描电子显微镜（jSM-6380LA,JEOL, Japan）观察样品。使用SEM测定的样品的晶粒尺寸。用维氏硬度计 (ESEWAY, W4303, UK) 测量了激光处理后样品的显微硬度，负载为300克、15 s。至少有3个相同的样品被放在不同的位置上测量和。每10次测量的平均

硬度被记录下来。X射线衍射（XRD）是用X射线衍射仪 (X'Pert Pro MPDfPANalytical, USA) 进行，采用Cu Kot辐射，工作在40千伏和30毫安之下，2?角的范围从20°到100°之间，扫描速度为每分钟2°。

A three-electrode electrochemical system was used to study corrosion behavior of laser processed samples. Metallographically polished samples were used as working electrodes, while platinum and saturated calomel electrode (SCE) were used as auxiliary/counter and reference electrodes, respectively. The experiments were carried out using Ringer's physiological solution (NaCl: 9.0 g/L, CaCl2.2H20: 0.17 g/L, KC1: 0.4 g/L) buffered with NaHC03 (2.1 g/1). The pH of the electrolyte solution was maintained at 7.4 by adding NaOH as required. The input potential signal of the stationary working electrode was scanned linearly by means of a potentiostat/galvanostat (CHI604A, CH instruments, USA) and the resulting current was monitored. The potential was scanned between —1.0 V and H-1.0 V at the rate of 0.1 V/s. The corrosion current density (Ic〇rr), corrosion potential (Ecorr) and the corrosion rate were estimated using Tafel extrapolation of the cathodic part of the polarization curve.

采用一个三电极电化学系统研究激光加工样品的腐蚀行为。把金相抛光样品作为工作电极，铂电极和饱和甘汞电极（SCE）分别作为辅助/计数器电极和参考电极。试验在格氏生理溶液中进行（氯化钠：9克/ 升，二水硫化钙：0.17克/ 升，氯化钾0.4克/升），缓冲剂为碳酸氢钠（2.1 g / 升）。电解质溶液的pH值通过加入NaOH保持在7.4。固定的工作电极的输入电位信号采用恒电位/恒电流线性扫描方式（CHI604A, CH instruments, USA）。产生的电流被监测出来。扫描电位在- 1 V和0.1 V 之间，的腐蚀电流密度变化率（IC〇RR）为-1.0 V / S，腐蚀电位（Ecorr）和腐蚀速率采用极化曲线的阴极部分来进行塔菲尔外推估计。

The surface energy of laser processed NiTi alloy samples was calculated using experimentally determined contact angles. Metallographically prepared samples were used to determine the contact angles using two liquids with polar and non-polar characteristics (distilled water and diiodomethane, respectively). Average of 6 measurements on each sample is reported. The surface energy of the samples was calculated as follows using Fowkes equation [23]:

利用实验测定接触角计算激光加工NiTi合金样品的表面能。利用两种液体（蒸馏水和二碘甲烷）的极性和非极性决定金相制备的样品的接触角。把6个样品的平均值记录下来。样品

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