【作者】 牛文娟；

【导师】 白晨光；

【作者基本信息】 重庆大学 ， 冶金工程， 2010， 博士

【摘要】 多孔钛及其合金综合了钛及钛合金与泡沫金属的特性,是一种新型的结构、功能材料。其潜在的应用领域包括作为催化剂载体、航天飞行器的夹层板、热交换器等,良好的耐腐蚀性、生物相容性及特有的多孔结构特性可以为骨组织细胞的生长提供优越的环境,为其在生物医学方面的应用提供广泛的前景。从目前的发展状况来看,由于经济上和技术上的原因,多孔钛及其合金的应用仍然限制在部分特殊领域,另外许多开发应用只限于实验室范围内,离工业应用的要求仍有一段距离。因此对于多孔钛及其合金系统性的研究对其发展应用起着决定性的作用。本文利用混合元素粉末冶金法成功制备了具有可控多孔结构的钛及其合金。通过原料和工艺参数的改变得到孔隙率在55%-75%多孔钛。采用正交实验法研究了原料和工艺参数对孔隙率的影响规律,方差分析的结果表明:压制压力对烧结坯的孔隙率影响最大,其次是造孔剂尺寸。随着造孔剂尺寸的减小、粘结剂含量及烧结温度的增加,孔隙率均呈减小趋势。而孔隙率随压制压力的增加先下降后上升。为了实现对多孔结构的准确表征,利用图像处理技术(Image-Pro Plus)分析了多孔钛不同的结构参数,包括孔尺寸及尺寸分布,孔壁厚度和圆度值。结果表明多孔钛的平均孔尺寸为600μm,其孔壁厚度为100~200μm,90%以上的孔圆度值超过了0.5,平均值为0.72。通过正交实验法研究了工艺过程参数对多孔钛性能的影响规律。压缩实验测得的多孔钛的杨氏模量在1.38-6.90GPa范围内,平台应力在4.51-24.70MPa范围内。方差分析的结果表明在本实验的参数选择范围内,造孔剂尺寸和工艺参数对杨氏模量没有明显的影响。而压缩平台应力随着压制压力的增加先上升后下降,与孔隙率随压制压力的变化规律正好相反。对于多孔钛压缩断面形状分析的结果表明:压制过程中的应力分布决定了材料的密度分布,进而决定了材料的强度分布,因此多孔钛的压缩断裂面呈现倒V型形状。通过添加合金元素铁和镍,研究了合金元素对多孔钛合金组织及性能的影响规律。添加合金元素铁之后,多孔钛铁合金的基体由α-Ti相、α-Ti和β-Ti的共析体组成。铁含量较少时,由于第二相的强化,多孔钛的杨氏模量从3.01GPa分别提高到4.27GPa(Ti-2wt.%Fe)和4.11GPa(Ti-4wt. %Fe),然而由于孔壁上孔洞的存在,其压缩强度保持在72MPa左右。当铁含量为6wt. %时,Kirkendall效应致使孔壁上的部分微孔体积增大,导致材料杨氏模量和压缩强度均明显下降。镍的加入使α-Ti基体得到一定细化。当镍的添加量较少时,材料中形成α-Ti相及α-Ti与β-Ti的共析体,但镍含量达到6wt.%时,材料中出现了Ti2Ni金属间化合物。镍含量为2wt.%和4wt.%时,多孔材料的压缩强度分别提高至112.59MPa和96.31MPa,杨氏模量提高到4.26GPa和3.73GPa,但镍含量为6wt.%时,由于烧结过程中液相过多,材料压缩强度和弹性模量反而减小。利用烧结机制的概念,研究了合金元素对多孔钛合金烧结机制的影响。通过分析材料的膨胀/收缩曲线发现多孔钛、多孔钛铁及钛镍合金的烧结过程约在1200℃的时候存在一个转折点,在低温烧结阶段,少量的铁和镍均会降低线收缩速率,但高温烧结阶段,铁和镍的加入明显加快了线收缩速率。然而,其烧结机制较为复杂,不能用单一的烧结机制来描述此过程,烧结过程不仅受到粉末颗粒性能的影响,还受到多孔结构的影响。利用有限单元法模拟了不同孔结构对多孔材料性能的影响。二维模拟结果说明:随着相对密度的增加,有效杨氏模量呈上升趋势;相对密度相同时,孔洞沿压缩方向上的尺寸减小会降低有效模量;同种孔形状的不规则排列方式也会降低多孔材料的有效性能。另外根据多孔材料孔结构及孔壁结构特征,分别引入了3D多孔模型和3D颗粒模型。简单立方(SC)排列方式下材料的相对杨氏模量最高,而面心立方(FCC)和体心立方(BCC)排列方式会降低相对杨氏模量,另外椭圆形孔也会降低有效杨氏模量;相对密度与相对杨氏模量符合幂函数关系。与多孔模型不同的是,颗粒模型都存在一个相对密度范围,杨氏模量与相对密度呈线性变化。根据本文实验数据建立了多孔材料弹性模量的预测模型。定义了宏观和微观相对密度两个状态参数,建立了双尺度模型。微观模型与实验数据吻合的很好,因为孔壁是由尺寸、形状类似的钛粉末颗粒烧结而成的,结构相对均匀;然而由于宏观孔洞结构不均匀性的影响,例如孔尺寸、形状和空间位置分布的不均匀性对材料性能的影响,宏观模型的预测值要比实验值高。利用COMSOL计算了密度不均匀材料中的应力分布,结果说明应力分布与密度分布无关,因此材料的破坏只取决于其本身的强度分布。更多还原

【Abstract】 Metallic foams are novel materials with extremely low densities and unique combination of excellent mechanical, thermal, electrical and acoustic properties. Due to the unique combination properties of porous materials and titanium alloys, the titanium-based porous materials can be used for structural and funtional applications. The potential application fields include catalyst substrate, sandwich core for aerospace vehicles and exchanger at elevated temperature up to 400℃. Furthermore, owing to the good corrosion resistance and wonderful biocompatibility, the porous titanium and its alloys can be used as medical implants, such as bone substitution to provide the porous structure to assist the growth of bone tissue. However, application fields of them are confined within some special areas until now because of economical or technical problems. And some research results only work in lab scale. So it’s important to make a fully understanding of these materials including preparation method, structure analysis as well as prediction of mechanical properties.In the present study, the powder metallurgy with space holder technique was used to produce porous titanium and its alloys with controlable porosity and stucture. Porous titanium with the porosity in the range of 55%~75% was fabricated by changing processing parameters. The orthoganal experiment was chosen to investigate the effects of manufacturing parameters on porosity. The most important factor for porosity is compaction stress, and the following is the size of space holder material. The porosity decreases with reducing of space holder size or increasing of the content of binder and sintering temperature. Initially, increasing the compaction stress can lead to lower porosity; however, the porosity will come up again with higher compaction stress.The image processing software Image-Pro Plus was used to identify the structure feature, such as pore size, pore wall and pore shape. And the results indicate that mean pore size is 410μm, thickness of the pore wall and the mean sphericity is about 100~200μm and 0.72, respectively.The orthoganal experiment was also used to study the effects of manufacturing parameters on mechanical properties. The Young’s modulus and plateau stress are in the range of 1.38-6.90GPa and 4.51-24.70MPa, respectively. Both of them increase with decreasing of porosity. The influence of different parameters on Young’s modulus is not so obvious. While the plateau stress rises and then decreases with compaction stress, which behaviors oppostie to the effect of porosity.Because of the pressure loss, the density distribution of the compact is un-uniform, so the strength of porous titanium has the same distribution with density. As a result, the typical repture section of compressed samples has inverse V-shape.The effects of alloys on microstructure and properties of porous titanium were also studied by adding iron and nickel. The matrix composition of porous titanium-iron alloy isα-Ti phase and eutectoid ofα-Ti andβ-Ti. The volume fraction of eutectoid increases with the addition amount of iron. There are two kinds of effect which dominate the mechanical property, one is second phase strengthening and another is pore weakening on the pore wall. The Young’s modulus increases from 3.01GPa to 4.27 GPa and 4.11GPa when adding 2wt.% and 4wt.% Fe, respectively, while the strength maintains at about 72MPa. When adding 6wt.% Fe, the Young’s modulus and strength are much lower bacause of more pores formed by Kirkendall effect. Nickel playes a key rule as a kind of grain refining element for porous titanium. The eutectoid is formed when adding small amount of nickel, while intermetallic compound is formed when adding 6wt.% nickel. The compression strength are increased from 72 MPa to 112.59MPa and 96.31MPa, respectively, while the Young’s modulus are 4.26GPa and 3.73GPa when additions are 2wt.% and 4wt.%, respectively. However, the compression strength and Young’s modulus can be deteriorated if adding 6wt.% nickel.Sintering mechanisum were investigated by adding iron and nickel. According to the expansion/shrinkage curves of porous titanium, Ti-Fe and Ti-Ni alloys, the sintering process can be separated into two parts. At low temperature sintering, sintering rate is reduced by adding iron and nickel. While at high temperature sintering, iron and nickel can accelerate the process significantly. However, the sintering mechanism can not be explained by only one mechanism because the sintering process is affected not only by particles themselves but also by porous structure.Furthermore, different porous structures were modelled to investigate their effects on effective properties. Finite element method is uesd to predict effects of porous structure on effective Young’s modulus. The Young’s modulus increases as the relative density increases. Crack-like pores penpendicular to the loading direction have the greatest effect in reducing the modulus. Moreover, the random distributions of pores decrease the moduli as well. According to the real structure of porous titanium (alloys) made by space holder technique, two kinds of model-3D porous model and 3D powder model are used, as well as three different spatial distributions of pores are considered: simple cubic (SC), face-centred cubic (FCC) and body-centred cubic (BCC). The SC array typically yields the highest modulus, while the rest two appeares similar. Changing from spherical pores to ellipsoidal pores reduces the modulus, in accordance with the predictions in 2D model. For porous model, power law relation can be used to fit all the relation between modulus and relative density. Different with porous model, powder models have relative density limitation obtained when there is no overlap between particles. The results indicate that modulus can be expressed by a linear function of relative density.Finally the predictive model for mechanical properties was proposed according to the current experimental data. Two-scale model is used by defining different variables: macro-relative density and micro-relative density. The results show that the micro-modulus fit very well with experimental data, because all the particles on the walls are roughly spherical and of similar size so the geometry is reasonably represented by the powder model. While the macro-modulus follows the same trend but lower than the simulated values, because the space holder technique generates a very random structure of different pore size, shapes and distributions within the material.The results of stress distribution in the material with non-uniform density indicate that the stress distribution doesn’t rely on density distribution. In this case the fracture of material only depends on their own strength.更多还原